Rotational Isomerism in Acrylic Acid

15 Pages·2003·2.44 MB·English

View Online / Journal Homepage / Table of Contents for this issue
J. CHEM. SOC. FARADAY TRANS., 1995, 91(11), 1571-1585 1571
Rotational Isomerism in Acrylic Acid
A Combined Matrix-isolated IR, Raman and ab initio Molecular Orbital Study
Anatoly Kulbida
Institute of Physics, St. Petersburg University, Peterhof, 198904 St. Petersburg, Russia
Mozart N. Ramos
Departamento de Quimica Fundamental, Universidade Federal de Pernambuco, Recife-PE, Brazil
Markku Rasanen and Janne Nieminen
Department of Physical Chemistry, P.O. Box 55, (A.I. Virtanens Square I), 00014 University of
Helsinki, Finland
Otto Schrems
Alfred-Wegener lnstitut fur Polar und Meeresforschung, Postfach 120161, D-27515 Bremerhaven,
1 Germany
7
15 Rui Fausto*
0
91 Departamento de Quimica, Universidade de Coimbra , P-3049 COIMBRA , Portugal
5
9
9
1T
1F
ecember 20oi:10.1039/ Taishdoedla itrtieeosdnu , llotbswo ot-htfe amR acpmoemraanbtu inareen ddI RvIR ibs rpsaeptcieotcrnotarsalc aoonpf dyl iqsautnriuddc ataubcr ariyln liisctti oua dcSyi Cdo Ffa -tHnhdFe taahncedr y RlMicaP ma2ac niMd sOmp ecocantlorcumumlea rtio ouf nntsdh eear rtcaerk yepsnrtea bsl eya nrmtee adat.rl sixion-
Dd
on 06 c.org | rmeipxoturtreed o af ntwdo i nctoenrpforermteedr.s Iot fi ss ismhiolawrn e tnheartg iine sb, odtihff earrignogn b ayn tdh ek rryeplatotinv em oartireicnetast iaocnr yolfi cth aec Cid= Cm-oCn=oOm er eaxxiisst.s U apso an
mbra ubs.rs iirnrga dtoia tthioen c aotn 1fo =rm 2a4t3io nnmal bgyro au nxedn sotant ela, mcopn, vtheert ss ctois t hfoer sm-t r(aCn-sC -fCor-m0 (C=Cd-iChe=dOr al andgihlee derqaul aaln tgol e0 "e),q cuoarlr teos p1o8n0d")-.
de de Coin http://p tcIinory ntshateal lls iqotanutlieyds tp hrheea ftesherer,e rdmdim otode yraincba osmvtreiuc acctlaluynr e masl osssotrt osbnteag bliynle fpe frroerredmdo mf(rsoincmaist et)h, e ebx uicstot tsrhr.e eR seepxsoiusntldtesinn ocgfe a vibinb itrnhaiittsiio opn hSaaCl sFsep- HoeFfc t tahrane.d tIw nMo tP uc2ron mn, fooinlrem tchaue--
sida95 o lar orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmo-
Univerary 19 dneicp feonrcdee nficeeld osf, sfoorm teh er erleelveavnatn ts tcrouncftourrmala ptiaornaaml esttaetress i so fu ascerdy ltioc cahcaidr aacrtee raislseo thpere smeonstet dim apnodr ttahnet cinotnrfaomrmolaetciounlaarl
y nu interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of
ded b01 Ja cah naorrgmeaclh-marogdee aflunax-lyosvies rlbaaps e(dC oCnF Oth)e mthoedoerel twicearl eh aursmeodn itco fohreclep fiienltdesrp arentd tohf eI Re ixnpteenrismiteyn statul dvieibsr abtaiosenda lo nd athtae,
wnloaed on enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.
oh
Dblis
u
P
Acrylic-based materials (polymers) have a wide range of prac- not analy~ed,'o~n ly a few more recent structural and/or
tical commercial applicatiorls and have been widely vibrational experimental studies on monomeric acrylic acid
studied.'-3 However, despite the relevance of these systems, have been reported. In a first study, the microwave spectrum
to date their precursors (monomers) have neither been of acrylic acid was analysed, and two different conformers
studied systematically in detail or characterised well, either differing in the relative position of the acrylyl group were
structurally or spectroscopically. In addition, acrylic-like found to exist in the gaseous phase (the s-cis and s-trans
molecules have also been proved to be of fundamental impor- forms; Fig. l).14 In that study, however, a unique set of
tance to the study of some catalytic reactions involving serine assumed geometrical parameters taken from related mol-
proteases (e.g. ~hymotrypsin),~s-i~n ce they can be suc- ecules was used to describe the structure of both conformers.
cessfully used as in situ resonance Raman spectroscopic Thus, it was not possible to analyse the influence of the con-
probes of the enzyme-substrate transient complexes formed formation on the molecular geometry and, consequently, also
within the enzyme's active site during Indeed, in to determine, from the conformational dependence of the
this field of research, a detailed knowledge of both conforma- geometrical parameters, which are the main factors
tional preferences and vibrational properties of simple mol- responsible for the relative stability of the observed con-
ecules containing the acrylyl moiety assumes particular formers. A second study used the semiempirical Neglect of
relevance, as stressed el~ewhere.~.' Diatomic Differential Overlap (NDDO) method to investi-
Among the simplest molecules containing the acrylyl frag- gate the conformational stabilities, dipole moments and ion-
ment, acrylic acid monomer naturally appears as one of the isation potentials of the two most stable conformers of acrylic
most fundamental systems to*b e studied, but it also rep- acid." Reasonable agreement between the calculated and
resents a challenge to fundamental research owing to the available experimental data could be reached, despite the low
practical difficulty of avoiding its polymerisation under theoretical level of the calculations undertaken. A more
current experimental condition^.^ This may explain why recent study considered the structures of acrylic acid and
neither the precise molecular structure nor the vibrational complexes of this molecule with Lewis acids (e.g. H+, Li',
spectra of the monomeric acrylic acid have been reported, BH,), in order to rationalize the stereoselectivity of catalysed
though several studies have already looked at its dimeric Diels-Alder reactions of chiral acrylates.I6 SCF-HF calcu-
structure^.^-' lations, undertaken using both the 3-21G and 6-31G* basis
To the best of our knowledge, besides an earlier electron sets at the 3-21G optimised geometries, confirmed the slightly
diffraction study in which the conformational isomerism was lower energy of the s-cis conformer of acrylic acid
View Online
1572 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91
molecule and to study the rotamerization processes taking
I place upon sample irradiation by UV light at /z = 243 nm. To
lP help interpret the experimental data, a series of SCF-HF and
HJ
\c- c MP2 ab initio molecular orbital calculations have been per-
I +
formed using both the 6-31G* and 6-311 G** basis sets
H
H' which include, respectively, one set of polarisation functions
in all non-hydrogen atoms and polarisation functions in all
s-cis s-trans atoms plus one set of diffuse functions in non-hydrogen
atoms. Besides the molecular structures and relative energies
H' of the various stable conformations, theoretical calculations
of the vibrational spectra of the individual conformers have
also been carried out, providing us with very valuable infor-
-C mation for the assignment of the experimental spectra. These
I \
results are complemented by normal-mode calculations based
H/ O
on the SCF-HF 6-31G* theoretical harmonic force fields and
1
7 by IR intensity studies based on the CCFO Finally,
5 s-trans (C-0)
01 the experimental vibrational spectra (Raman and IR) of
1
59 Fig. 1 Conformers of acrylic acid (monomer) acrylic acid in both liquid and crystalline phases are reviewed
99 considering the results obtained for the isolated molecule.
1T
1F
mber 200.1039/ 3[A1EG+*t raanmd) -3(s--2c1isG)= )J 2.H86o wkJe vmero, ln-'o g(3e-o2m1Get)r;y 2 o.5p1t imkJi smatiooln-' w(a6s- Experimental and Computational Methods
eceoi:1 carried out at the highest level of calculations used (SCF-HF Acrylic acid (99+% purity) was obtained from Fluka and
Dd 6-3 1G*), and electron correlation effects were not analysed. purified by several freeze-pumpthaw cycles. Ar and Kr were
on 06 c.org | tIank aend.d1i6ti oFni,n ianl ltyh, att hsetu dlyat, ensot vsitburdayti ocnoanl csetrundeide s twheer em uantdriexr-- toibvtealyin, e9d9 .9fr9o9m% Manedss e9r9 .9G9r8ie%s hpeiumri tyG. mAbllH m aantrdic ehsa dw, erree spperec--
ade de Coimbra on http://pubs.rs stcSihsaoCoemml Fadppt-ilelHfeedf meF irr ee4srnnap-t2tdee 1icidanGt rttiae bo nlyens vo ietfaby l .y "b at hecUehrIyonValrv ietcihtlo iiecugsaarhe clt o ildfaso tstbuhtta deastyi tnou ebd2cds5iaee rsrrfv,iKr eoebd,dmaI s7b eo aaduna tdnbm sdoae ttudh piw utoohamnnes TspmRiahlOiirxeceK todudn re eipd1n o0i oos-an3id t0 eico0 aon a n ngv(tdoeLe mlnedaty p-ipboDelonraRalatdtlCeu -dHr w e9easc3r,yoC a pme bputeyeesa mr)s dupecmrepleoriodrasrs teoiuadtrrti-,oe ct nyhcc ceoool oenfm l tertirdohrer lefolru erisgrg i awen(srgLieat ahotoak urnaes.
sid95 pressure mercury lamp and on the comparison between the Shore Cryotonics) were 14 or 18 K (Ar) and 25 K (Kr). The
by Univeranuary 19 pooobbssseeedrrvv. eeAdd lbtahanondud gs hct aolt chthuelesae ts e-sdcti usd saipenesdc thsra-at,vr aean nsg rceoaasntslfyiog rnmimmepersrno tw veaodsf potrhoue-r mmh-aa tt'.rr iixxF u:g rsatoshl uefrtleo wdre atctaiaio.l ss5 .ow0 nex re th eca .s a1m(5Ap0rl0)e : o 1pr r ae1np.5da rxat htieo nr atpe(rsKo croe)f d muthroeel
ded 01 J knowledge of the acrylic acid monomer structural and vibra- can be found in ref. 23.
wnloaed on ctioounladl nporto apnesrtwieesr, stohme er iemsuplotsr taonbtt aqinueedst iionn st:h e earlier studies cmT-h' e rIeRso lsupteiocntr)a owne rae Brercuokredre dIF iSn 6th6e Vr efFloecutriioenr mtraondsef o(r0m.5
oh
Dublis of (tih) eF tiwrsot, oitb swearvs endo ct opnofossrimbleer st oa nsde,p tahruaste, scolmeaer loyf tthhee sapsesicgtrna- tsepre catnrodm weittehr ae qDuiTppGeSd dweittehc tao rg ewrmitha nCiusmI woinn dCoswI sb. eTamhe siprrliat--
P
ments made in ref. 17 were only tentative; diation of the samples was carried out using a UV-light
(ii) The force field scaling used to fit the calculated spectra source (L.0.T.-Oriel GmbH) whose main component is a 500
to the experimental frequencies was made by transferring dif- W Xe arc lamp. The collimated UV beam irradiated the
ferent scale factors previously obtained for other mol- sample through the Suprasil window of the cryostat after
ecule~.'~*H'o~w ever, the use of several scaling factors may being filtered by both a water filter and interference filter of
lead to significant changes in the potential-energy distribu- the FS10-50 type with a bandwidth of ca. 8-10 nm (L.0.T.-
tion (PED), thus often introducing undesirable a posteriori Oriel GmbH).
distortions in the force fields. The method of transferring the Raman spectra were obtained using a SPEX 1403 double
scale factors from different molecules naturally strongly monochromator spectrometer (focal distance 0.85 m, aperture
increases the probability of obtaining physically meaningless f/7.8),e quipped with holographic gratings with 1800 grooves
results. Moreover, the theoretical results were obtained at the mm-' (1800-1SHD). The 514.5 nm argon laser (Spectra-
SCF-HF 4-21G level," and it is nowadays well known that, Physics, model 164-05) line, adjusted to provide 220 mW
for systems containing conjugated double bonds, reliable power at the sample, was used as the exciting radiation.
structural and vibrational results can in general only be Detection was effected using a thermoelectrically cooled
achieved if at least polarisation functions on all non- Hamamatsu R928 photomultiplier. Spectra were recorded
hydrogen atoms are added to the basis using increments of 1 cm-' and integration times of 1 s.
(iii) Finally, no discussion was presented concerning both Under these conditions, the estimated errors in wavenumbers
the differences in the molecular geometries of the two con- are cm-'.
formers studied and the factors which may explain their rela- The ab initio SCF-HF and MP2 MO calculations were
+
tive stability, and no attempt was made to correlate the carried out with both the 6-31G* and 6-311 G** basis
relevant geometrical parameters with either vibrational fre- set^,^^,^' using the GAUSSIAN92 program system.26 The
quencies or intensities. 6-31G* calculations were performed on a DEC ALPHA 7000
In this work, considering all the open questions above, the computer, at the Centre of Informatics of the University of
+
molecular structure and vibrational properties of acrylic acid Coimbra (CIUC), Portugal, while the 6-311 G** calcu-
conformers have been studied using a more systematic and lations were carried out on CRAY-X MP/EA 432 and on
precise approach. Since matrix-isolation spectroscopy pro- CONVEX C3840 computers at the Centre for Scientific Cal-
vides a unique way of studying the monomer of acrylic acid, culations, Espoo, Finland. Molecular geometries were fully
this technique was used to obtain the IR spectrum of the optimised by the force gradient method using Berny's algo-
View Online
J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1573
rithm.27 The largest residual internal coordinate forces were P
+
always less than 3 x lop4 Eh a, ' or Eh rad-', for bond H2C=CH-C * HC-CH-C * H2C ----CH=C
stretches and angle bends, respectively.? The force constants *+
'OH OH 'OH
(symmetry internal coordinates) to be used in the normal
coordinate analysis were obtained from the ab initio Cartesian
0) (11) fW
harmonic force constants using the program TRANS-
Fig. 2 Canonical forms of acrylic acid showing (11) mesomerism
FORMER.28 This program was also used to prepare the
within the carboxylate group and (111) mesomerism associated with
input data for the normal coordinate analysis programs used the two double bonds
in this study (BUILD-G and VIBRATz9). The calculated
force fields were then scaled down by using a simple linear
regression for each symmetry class in order to adjust the cal-
culated to the experimental frequencies (two scale factors are bility of this conformation in molecules, such as aldehydes
obtained: one related to the in-plane vibrations, and the and ketone^:*^^-^^ where mesomerism involving the X group
other to the out-of-plane modes). Unobserved bands (or is absent.
bands doubtfully ascribable using only a pure empirical Table 2 shows the calculated molecular geometries, rota-
1
7 approach) were then calculated from the corresponding force tional constants and energies for the various possible con-
5
01 fields by interpolation using the straight lines obtained pre- formers of acrylic acid. At the SCF-HF level of theory, the
1
59 viously. While very simple, this scaling procedure preserves s-cis form is predicted to be more stable than the s-trans con-
99 the PEDs as they emerge from the ab initio calculations, thus former by 1.95 and 1.09 kJ mol-', respectively, using the
1T +
1F having an important advantage over the more elaborate force 6-31G* and the 6-311 G** basis sets (the corresponding
mber 200.1039/ ffioerl de ascchal isnygm pmroectreyd ucrlaesss w whhicichh u usseu malolyre g tivhea nri soen eto s cimalep ofartcatnotr eAnEe(rsg-irersa nr-es (sd)- cuiscf e vslailguhetsl y ctoor r1e.c8t6e da ndby 1.z0e3r koJ-p moionlt- ')v. ibTrahteio cnaall-
eceoi:1 PEDs distortions from the ab initio calculated values. culations carried out at the MP2 level of theory yield slightly
on 06 Dc.org | d poIlRar tienntesnosri t(yA PpTar)a fmoremtearlsi smwe3r' ea nexda tmhein CedC FuOsin ~g othdee lat.o~m'i,c~ ' hS3i1Cg1hF +e-rH GAF*E *(l seav-tre rla th-n(e (sMs -)cSi sP)C 2F/v6-a-Hl3uF1e/Gs6 *-t3h: 1a1n2 .+8th1 G os*ke*J uonmpdtioemrlt-ia's;ke edn M gaePto 2mt/h6ee--
ade de Coimbra on http://pubs.rs GSeevoemraelt rieasc arynldi cE nRearcegisdiue sl tsd aernidv aDtivisecsu ssoifo n general formula ecoatrfulbisfelteoaasc t:iet ntshd1e et .d5e n1vsm daakm liJuct oeremo sfwo oarlcvad-ovo e')meur, rrp toehsafsl uruiemgsl thai tnf(gla0dyni.i r6ictlt0hyua e td&i enws 0g-eca. 2litslsh4 a tckwhto Jeine tlfmhceo ocortnmtlrh-fo eoenrr . m c1pT4o)e rhrrera eveensilneoad etu cirasoagrlnlyye-
sid95 H,C=CHC(=O)X, including a~rolein,'~m.~et~hy,l~ v~in yl difference found for methyl acrylate (2.40 kJ mol-'; ab initio
Univerary 19 kbeeetonn teh,3e 4s uacbrjyeclyt lo fh daelitdaei^le,^d- ^s^tr uacntdu rmale athnydl vai~brraytiloantael, ~st'u*hd~ai'ev se, SblCe FS-CHFF-/4H-F3 l dGa*ta 4,0 )t.h eT askliignhgt liyn tloo wceorn sviadleureast ifoonu nthde fcoor macprayrlaic-
by anu and it has been shown that the relative order of stability of acid when compared with those obtained for methyl acrylate
ownloaded hed on 01 J aTtnhtaaoetb musler-.ce i 1Ison faf onftrdah c east ,-s struetahrbniese sts ir cteouolfena ntfaiotcv,r remyX lA,ei crEbs oa( iscus-i ntdsrdt ar dnotesnor()gsi --vl tcyhais te)di vveceataeslr urbcmeoasinn ny seblhd eo c bwaiynrn bt teohirnne- cdfrauaneg mtboee nteht xeip sml amiensoeordme ebirymi spmcoo rnwtsaiintdhte irnii nnt ght eht heC a(et= sOtther)e O tsXh-ac ins( X is nt=a b tHhile,i s CaatHcioi,dn),
Dublis preted by considering that when conjugation is possible mowetihnygl tgoro uthpe toawddaritdiso nthael eeslteecrt rooxny gcehna argtoem r.e lease from the
P between the carbonyl and the X substituent (canonical form
Note that the proposed relative importance in the two con-
I1 in Fig. 2), the s-cis conformer is considerably stabilised
formers of the stabilising intramolecular interactions due to
relative to the s-trans form. This justifies the relative values
(i) mesomerism involving the carboxylate group (which
observed for the halides, where the extent of the conjugation
favours the s-cis form) and (ii) mesomerism within the
between the halogen and the carbon atoms follows the well
C=C-C=O fragment (which favours the s-trans form) is
known order F > C1 > Br. Moreover, the results point also
consistent with the geometrical changes observed upon con-
to a particularly important s-cis stabilisation in carboxylic
former interconversion. Thus, the C-0 bond is shorter in
compounds. On the other hand, the greater importance of the
the s-cis than in the s-trans form, while the C-C bond is
mesomerism within the C-C-C-0 moiety (canonical form
longer, clearly showing that canonical form I1 of Fig. 2 is
111 in Fig. 2) in the s-trans form may explain the greater sta-
more important in the s-cis conformer and canonical form I11
is more important in the s-trans form.
The calculated changes in C-(2-0, C-C-0, C=C-C,
Table 1 AE(s-r,on-s ()s -cis) (kJ mol - I) for several molecules of general H-C-C and H'-C-C angles with conformation reveal
formula H,C=CHC(=O)X
the presence of stronger steric repulsions between the vinyl
molecule AEa ref. group and the carboxylic oxygen atom in the s-trans con-
former, when compared with the vinyl-carbonyl interaction
H,C=CHC(=O)H - 6.99 33 present in the s-cis form. In fact, the C-C=O and H-C-C
H,C=CHC(=O)CH, - 4.48 34 angles decrease in going from the s-cis to the s-trans con-
H,C=CHC(=O)Br - 1.89 38 former, while the remaining angles increase. It is obvious that
H ,C=CHC( -0)Cl - 1.76 38
these changes, in particular those observed in the H-C-C
H,C=CHC(=O)F 0.95 38
H,C=CHC(=OfOCH , 2.40 40 and H'-C-C angles, constitute strong evidence indicating
H,C=CHC(=O)OH 1.95 this work the greater importance of the CH2=CH/-O-- repulsions.
- More important steric repulsions between a-substituents and
Ii Results from SCF-HF ab initio calculations carried out at the split the -0- atom, compared with repulsions involving the car-
valence or split valence plus polarisation levels of theory.
bony1 oxygen, have been previously found in other a-
substituted carbonyl corn pound^^^-^^ and seem to be a
general structural trend which usually has important geo-
t 1 E, a;' x 8.23873 x N. metrical implications.
View Online
Downloaded by Universidade de Coimbra on 06 December 2011Published on 01 January 1995 on http://pubs.rsc.org | doi:10.1039/FT9959101571 Table Calculated optimised geometries, rotational constants, energies and electric dipole moments for the relevant forms of acrylic acid“ 2 -- - - (C-0) s-trans s-trans s-cis SCF-HF SCF-HF SCF-HF MP2 MP2 MP2 + + + parameter 6-31G* 6-311 G** 6-31G* 6-31G* 6-311 G** 6-31G* 6-31G* 6-311 G** 6-31G* c=-0 1 18.99 18.46 1 122.05 1 18.94 122.15 117.78 1 18.47 118.36 121.45 c-0 136.10 132.97 132.82 133.13 132.89 133.42 136.21 133.30 136.55 c-c 148.32 148.10 148.13 148.47 147.79 148.24 149.25 149.43 149.23 c-c 133.87 131.89 131.91 131.97 131.89 133.96 131.89 131.96 133.82 107.38 107.42 C,-H 108.50 107.38 107.42 107.73 108.52 107.75 108.83 C,-H 108.52 107.45 107.53 107.48 107.55 107.44 108.40 107.52 108.54 C,-H’ 108.44 107.42 107.53 107.36 107.44 107.40 108.50 107.51 108.44 0-H 97.95 95.18 95.21 94.59 94.72 97.89 94.12 97.37 94.56 0-c-0 122.84 122.43 122.30 122.56 120.46 122.57 122.38 120.54 120.00 c-c-0 125.77 126.20 123.63 125.75 123.47 123.94 124.16 124.03 124.29 c-c-0 110.96 1 13.27 114.07 111.80 11 1.69 114.15 115.60 11 5.43 H-C-C 1 17.42 116.87 116.85 113.86 118.57 114.24 118.47 113.85 119.83 C-C-c 119.98 120.54 120.65 123.92 123.93 120.54 123.73 120.67 119.44 121.63 121.49 H-C*C 120.97 121.22 121.58 120.92 120.76 121.31 119.91 H’-C=C 120.29 120.88 121.88 120.87 121.74 120.57 121.52 120.60 121.74 C-0-H 108.12 105.50 107.73 108.74 105.01 1 12.43 108.44 112.44 109.97 - c- C( 180.00 180.00 180.00 180.00 0) 0 180.00 180.00 180.00 180.00 180.00 H-C-C=O 180.00 180.00 180.00 180.00 180.00 180.00 0.00 0.00 0.00 c==c-c-o 180.00 180.00 180.00 0.00 0.00 0.00 0.00 0.00 0.00 180.00 H-C-C-C 180.00 180,OO 180.00 180.00 180.00 180.00 180.00 180.00 H’-C=C- C 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0-C-0-H 180.00 180.00 180.00 0.00 0.00 0.00 0.00 0.00 0.00 1 1484.18 10913.58 11 442.80 11032.77 A 11047.66 10672.38 11 195.10 11237.43 10775.19 11078.80“ 10716.11b B 4253.96 436 1.47 4301.50 4302.23 4435.62 4256.25 4443.37 4328.34 4326.37 4251.94b 4388.28b C 3060.88 3096.16 3129.37 3126.67 3163.69 3121.48 305 1.07 3 168.85 3123.74 307 3.39b 3114.31b - - - AEc 1.946 31.20 29.08 2.807 30.78 1.088 - - - (1.860) (1.028) (29.96) (27.79) 1.718 1.745 2.376 2.524 4.978 1.636 2.489 4.763 4.846 IPI 0.05b 2.02 O.lOb 146 f ” MHz, Bond lengths in pm, angles in degrees, rotational constants in energies in mol-’, dipole moments in D. Experimental value obtained by microwave kJ spectro~copy.’~ Relative energies to the most stable conformer, values presented in parentheses are relative energies including zero-point vibrational energy + corrections. The total energies for the most stable form calculated with the and basis sets are, respectively, and 6-31G* 6-31 1 G** -265.654347 1 s-cis + E,. The total energy for this conformer calculated at the level is calculations performed at the -265.7349242 MP2/6-31G* -266.380 565 MP2/6-311 G** E,. 5 optimised geometries obtained at the SCF-HF level using the same basis set yield mol-’ and AEfs-rr~ns~C-O)l-~s-cis) mol-’. 1.514 26.27 AE~s-rrons)-(s-cis) kJ kJ = =
View Online
J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1575
In summary, the principal factors which determine the available experimental data obtained by microwave
relative stability of the s-cis and s-trans conformers of acrylic ~pectroscopy's~h ow rather large differences. On the other
acid are (i) mesomerism within the carboxylate group hand, the MP2/6-3lG* optimised structures are clearly differ-
(canonical form I1 in Fig. 2), (ii) mesomerism involving both ent from those obtained at the SCF-HF level, and lead to
C=C and C-0 double bonds (canonical form I11 in Fig. 2) rotational constants that show a very good agreement with
and (iii) steric repulsions between CH,=CH and the oxygen the experimental data. Besides the C-H bond lengths, which
atoms. The first factor is the most important and, together are calculated to be ca. 1 pm longer at the MP2 level, the
with the third, favours the s-cis conformer; the second factor main differences between the MP2 and SCF-HF geometries
favours the s-trans conformer. On the whole, these inter- occur for the C=O, C-0, C=C and O-H bond lengths
actions make the s-cis form more stable than the s-trans con- (which are calculated, respectively, to be ca. 3, 3, 2 and 3 pm
former. longer at the MP2 level) and for the C-0-H angles (which
The SCF-HF/6-3 lG* calculated potential-energy profile decrease by 2-3" when electron correlation effects are
for internal rotation about the C-C bond is shown in Fig. 3, considered).
together with the results of the quantitative potential-energy Acrylic acid monomer possesses an additional degree of
deconvolution conformational freedom, which is related to the internal rota-
1
7 tion about the C-0 single bond. In general, carboxylic acids
5
101 v = I/" + -1 1 v,[1 - cos(n.r)] adopt the s-cis conformation about this bond (O=C-0-H
959 n=l,3 dihedral angle equal to OO), the energy difference between this
9 conformation and that of the second stable form (the s-trans
ecember 2011oi:10.1039/FT eTawnnhheTdeer h rgceecyo o rzccrr oareielrsscrs peuptoslhoapnentode dnsdCi dn t=iegonCn gea-e r CtxgCopEy =ea Or CbCi am--rCCrei=n-edCt=rai Oh=la eO mvd aroadluuli nheatseasn dn gargtalroleele 3aao01nnf .6gd40.l" 0ek.I k J/o" f m6 i9.so02 l .-t4hk',' eJ. ecabqonetundwaf ol4e re0mtn ok e tJr1h, 8emc0soo e"r lCr-te'w),s paoon rnefdod~ rtipmhneegs c tetuonis veaurenagll ylOyy ~= bb~Caer--irO~Tnieg'h-)rH e .v f meorrya iinnlda tierfhgareccedot orn(aorv lve waersnhr igio2clnh0e
on 06 Dc.org | d mlatoiol-n' s caonnds id8e9r a&b l4y0 .1o4v eTrehsutism, athtee thSeC eFn-eHrgFy abba rriineirt,i oa craelscuul-t daaxxetiiess rwmis hintehenes ctpohrmee smpeanurcceehd ilwno witthhee r t ehlnaaettt regoryf otohff ett hhsee- t rssa-tcrnoissn OOgl==y CCs--t00ab--HHili sing
de de Coimbra n http://pubs.rs eptwMacahnurPictlc2ieeah/s 6l o -ffwd 3oeoh1llluiGoecbwch*tl s reco a tanl hbls ceoocu n otledrrarxe^tein.ho^ldia,nb- ts^iipto ^ rwn ec Ievoonrinneo f ouoatrshlrdlsmeyoe arc oc taibatloocrsnrue iaerlealvvd teae idlodsu o uafemtotn erfee orrtoirghs tyehmth eb eirmaa rrmbrpoiootoeaurrl---,t gspthcearonroapeulirlcgaeahll,l - lassyl-pi tgauracnnnemd sefe ri(ne Clttd h-o 0efi )n tpthereerc asoCcentn=iftooO nrem xrapeensersudr il amtOirnee-gHn n t faorlto bmcooob nntshdde eirdtv inioepednoa srls,el pysue .n4catln2re tIosin--s
ao particular specific intramolecular stabilising interactions are
Universidary 1995 tatioto tn2ha6el. 9St rCkaJnF sm-iHtiooFln- l e's,vt aealtn eo .f i Tmthhpeero ocrvayel.c muelantte do veenre rthgey breasruriletsr oabmtaoiunnetds oliinpk ecerh aclotoirnnogfao c(reamtsi,ac tf ioaocrn iidsn smotfa onacn,ep,~- uinmntsraeatmurro~alet~Hecdou' ~wlacer~a vrh)beyro.,dn sry-otlrgsa ennhs a bv(Coe n-0bde)in-e gn
y nu Comparison of the calculated structural data given in
ba recently proposed as catalytically important conformational
ownloaded hed on 01 J TrbSoaCatsbaFiltsei- oH sne2Fat s l l.se chvTooenhwl sisosta f nltetthhsaae dtwos r,h ysifi cmouhrs,i ilinainnrg s ttutrahernesncu ,e 6l,wt- s3 ht1oeaG nr ie*dc eoaonmnbtdiptca aa6irln-e 3edc1d a1 wl c+aiutthl Ga ttt*hhed*ee cecounlfeo, rtmheat hteiuonsne jarugl syts itdfayitfiefnesg re otnfh ceae ci bnryetetlwircee saet ncii ndt h seatu ssd iwynigenllgle. ssF--ttorraar nntssh i((sCC -m-00o)) l -
Dblis conformer predicted by the SCF-HF calculations (in this con-
u former the C-C-C=O axis adopts the s-cis conformation;
P
see Fig. 1) and the most stable conformer (s-cis), calculated
""T +
using the 6-31G* and 6-311 G** basis sets amounts,
30i respectively, to 31.20 and 29.08 kJ mol-' (if the zero-point
251
vibrational energy corrections are considered, the calculated
values are reduced to 29.96 and 27.79 kJ mol-'). Consider-
ation of electron correlation does not significantly affect this
5 20 energy difference, though slightly lower values were obtained
+
/ T \ (MP2/6-31G*: 30.78 kJ mol-'; MP2/6-311 G** at the
+
E SCF-HF/6-311 G** optimised geometries: 26.27 kJ
mol-'). Note that, as previously found for methyl acry-
late,40,48t he s-trans (C-C)/s-trans (C-0) conformation was
not found to correspond to a minimum in the potential-
energy surface of acrylic acid, pointing to the existence of
"t/ very strong steric interactions between the carboxylic hydro-
gen atom and the vinyl group in this conformation.
By comparing the molecular geometries of the s-trans
(C-0) form with those of the two s-cis (C-0) conformers
0 30 60 90 120 150 180 (s-cis and s-trans forms) the following conclusions can also be
s-cis s-trans drawn:
(i) The O-H bond is shorter in the s-trans (C-0) con-
Cy C-C-0 dihedral angle/degrees
former than in the two s-cis (C-0) conformers. This is a
Fig. 3 SCF-HF/6-3l G* optimised conformational energy profile for direct consequence of the presence in these latter conformers
internal rotation about the C-C bond of acrylic acid (0-C-0-H of the above mentioned C=O- - -0-H through-space field
axis in the s-cis conformation). The energy components resulting
interaction which leads to an increase in the O-H bond
from the quantitative potential-energy deconvolution, V = V"
+ +En= length by inducing an electronic charge migration from the
1,3 V,[l - cos(nz)], where z is the C-C-(2-0 dihedral
angle and V" is the energy corresponding to a C=C-C=O angle of overlap O-H region towards the -0- atom;
O", are V, = 2.43, V, = 29.60 and V, = - 0.81 kJ mol-". The (ii) The C-0 and C-0 bond lengths are, respectively,
unfilled circle represents experimental data.14 smaller and larger than in the two s-cis (C-0) conformers,
View Online
1576 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91
indicating that the mesomerism within the carboxylate group The corrected Mulliken charges are given by
has a reduced importance when the O=C-0-H axis
assumes the s-trans conformation; Ccorr(a) = + A:; , 4
(iii) Though they are less strong than in the s-trans (C-C)/ where x is the Cartesian axis perpendicular to the molecular
s-trans (C-0) conformation (which, as pointed out before, plane, and A::,, is a specific element of the overlap tensor
corresponds to a conformational transition state, saddle derived from the CCFO model for IR in
point) the OH. . .vinyl repulsive steric interactions are still order to reproduce the SCF dipole moments from atomic
very important in the s-trans (C-0) conformer (the charge^.^^-'^ It has been sho~n~’-’th~a t this correction is
C-C-0, C-0-H and H-C-C angles are very large in useful to reduce the large basis set dependence of Mulliken
this form). They are not, however, strong enough to put the charge~~an~d yto~ im~p rove the description of intramolecular
energy of the planar conformation above that of non-planar interactions.
structures, where the mesomerism involving the The atomic charge partitionings given by the corrected and
C.-C-C-O moiety and/or the carboxylic group cannot standard Mulliken charges show important differences. In
operate efficiently; particular:
(iv) The OH- -vinyl steric interactions are also responsible, (i) The bonds involving all but the hydrogen atoms are pre-
1 *
57 at least in part, for the presence of a longer C-C bond in the dicted to be considerably less polarised when the corrected
1
0 s-trans (C-0) conformer, though this result may also be par- charges are considered, in particular the C-0 single bond,
1
59 tially due to a reduced importance in this form of the mesom- whose associated bond-dipole moment is calculated to be ca.
9
1T9 erism involving the C-C-C=O moiety. 3.5 Dt when corrected Mulliken charges are used and ca.
mber 2010.1039/F twhhaIitnc has ludl methtmeer amfroyiun, rei t m thcoea snrt ebilema tipcvooenr tecanlnuetdr geinidet srf aroomfm ot hlteehc eud lisaftfrre uricnenttuet rrcaaocl tndifoaontras- 4ar.8e(i Dic) o Twnhsheied nec rosartbarenlycd taelerdsd s a Mtpooumslliiitcki vecenh acthrhgaaenrsg etohsn ea rtceho ecr orhenysspdidoroenrgdeeidnn.g a stotamns-
eceoi:1 mations of acrylic acid [C=O*.-O-H bond dipolar dard charges. This feature has also been noted in other
mbra on 06 Dubs.rsc.org | d (dubtotoheuxr obytulloeag thsebo- ogsmnprdoaesc u ee;px ;vf tiieemnnlydetl )s. o tionm.ct eeairnrraibcscormtexiao yisnnlia;nv tgome l vsethtsieneorg mi cete nhrreeeirsp gmCuyl = swioCoift n hstaih]nn e dc tho sCe-nt tr=craiaObnr-s- gprHeelCdannu(ea=crratX ilo )crnYau rlHobef o; fnotXhyr el, t hYahinsy = ddk r i0otn hgd ieo onorc f a aSrmtbSooo2mln)e]yic,c lau nclmahdroa sltrehgycuesutssel emuaspp. po[Tfenoha rer c sooe rxbtroase emcbrtvpeio leenad,
de de Coin http://p (CCh-a0r)g e Dcoisntfroibrmuteiorn w Aithn arleysspise ct to the most stable forms. aoafmn dvo asultnuaetnssd tbaoer dicn agM. 00u..0l1l7i3 ke-e0 (n.11 c 6eh a=er a g1ne.s6d,0 r02e. s12p80e9-c02t.i 2vx4e lleyo, ( -fs’oe’re tFChi)eg, .tc h4oe)r. rreacntgeed
y Universidanuary 1995 o aFMsig uc.l ali4lkc eupnlra eatseteodnm wtsii ctvh ca hltuhaeergs 6 eo-s3f f1osGrt a*tnh bdeaa tsrhidsr es(eeC Mti,s) oaamtn tedhr esc SoofrC raeFcc-rtHyeldFic ()(,, , ,laecviedl., tnrieevvg(eiea iritts)ih veaUden pci noht nahar elg lc eCocsro,,r. bn eufcIottn irdotmheniee,s rd srt,h,e detbh uoepct hotCi lo,a cnraai tirtsybo ocmoonf n bsateithdcoeoem rmaCsbi n=lrygeC dml uesocbsreo e nn tpedhgr eoaii-r-s
ded b01 Ja onfo uthnec eadb oinv et-hme ecnatsieo noef dC g,.e nTehrauls r, etdhuisc trieosnu lotf isp oas ictoivnes ecqhuaergnecse
ownloahed on Con, hhyads rojugsetn so nuep ohny dcroorgreecnt)i.o nH (oCw,e hvaesr , twthoe hpyidctruorgee nosb, twaihnielsdt
Dblis ,C (--00..137358 ) 0 (-0.572) using the corrected charges is the one which is consistent
Pu H ’ Nc -c 40.577 (wdietshc rthibei nrge lethvea nmt iemsopmoretrainsmce ionfv coalvnionngi cbaol tfho rmth eI 1C1 =ofC F iga.n d2
0.133
(0.201) - C=O double bonds) to the structure of the molecule. Very
0.500 interestingly, the considerable reduction upon correction of
0.164 (-0.705)
(0.224) the charge of the -0- atom in all conformers (much more
pronounced than that of the carbonyl atom) leads to a charge
distribution picture which is in much better agreement with a
relevant contribution of canonical form I1 of Fig. 2
(representing the mesomerism within the carboxylate group)
\ to the molecular structure of acrylic acid.
c-c 40.566
\( 0.750) In general terms, it can be concluded that the standard
4-0.226 H, 0.388
H-c (-0.258) 0 (0.466) Mulliken partitioning of charges overestimates the polarity of
(O .133 \” -( -- 00..139129 ) (-- 00.4.87171 ) the various bonds, in particular those of the C-0 bonds in
all conformers. Since the IR intensities of well localised
0.155
(0.225) stretching vibrations in general increase with the polarisation
of the associated bond,55 it could then be expected that IR
intensities predicted using the Mulliken standard charges
would also show a general overestimation. In turn, as it will
0.154
(0.238) - be stressed in the next section, the corrected charges can be
H 0.480 successfully used to estimate the IR intensities of several
important vibrations of acrylic acid.
Finally, it should be noted that the change in conformation
9-trans (C-0)
does not produce large changes in the atomic charges. This is
true for both standard and corrected charges (see Fig. 4), and
0.157 is a very interesting result since, on the contrary, in some
(o.lw) 0.387
(0.464)
Fig. 4 SCF-HF/6-3l G* calculated standard and corrected Mulli-
ken atomic charges for the various conformers of acrylic acid. The
standard values are given in parentheses. t 1 D z 3.33564 x C m.
View Online
J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1577
cases, the intensities of the IR bands ascribable to the same place at ca. 253 nm (filter 250FS10-50; peak transmission
normal mode in different conformers show remarkable differ- 19% at 253 nm; bandwidth 8.8 nm). Under irradiation in the
ences (this is particularly evident in the case of stretching 243 nm region (filter 240FS10-50; peak transmission 24% at
vibrations involving non-hydrogen atoms). Thus, the theo- 243 nm; bandwidth 9.4 nm), the changes were found to be
retical results indicate that the relative contribution of the several times faster than during the 253 nm irradiation,
charge-flux terms” to the total IR intensities of these bands though weak traces of CO, CO, and H,O appear in the spec-
are dominant. A systematic analysis of the importance of trum, indicating the beginning of the acrylic acid photo-
charge fluxes to the relative intensities of the IR bands associ- dissociation processes. Fig. 5(b) shows the differential
ated with the most important normal modes originating in spectrum after irradiation in the 243 nm region over 140 min
the various conformers of acrylic acid is presented in the next (in hexane solution at room temperature, the A,,, of the
section. acrylic acid absorption band is 223 nm, and the half band-
width 30 nm; thus, the irradiation was carried out in the
long-wave wing of the absorption band). The changes in the
Vibrational Studies
spectrum shown in Fig. 5(b) are close to the saturation point
Assignment of Vibrational Spectra of the Monomer (except in the case of the bands due to the products of photo-
1
57 The IR spectra of acrylic acid isolated in Ar and Kr matrices dissociation, CO, CO, and H,O, which were still slowly
101 are essentially the same; the observed frequencies and inten- increasing in intensity). Under irradiation by non-filtered UV
59 sities do not differ by more than 0.5%. The following dis- light, the observed changes in the spectrum were found to be
9
1T9 cussion will therefore focus on the results obtained in the Ar the same, but the CO, : CO band intensity ratio increases
mber 2010.1039/F m18aT tKrhi exd .I eRp osspieticotnru mte mofp earcarytulirce aicsi dsh torawpnp eidn iFni ga.n 5 A(ar ).m Inat raidxd ai-t tfarono mbAe r cFcaa .e . x11c :i0m 3 t eitmro l eacssl oehrsi eg( 1ht9oe3r u ntnmhiat)yn, . t htIhnea attu mornfo ,uC nuOnt d.o feT rC hiOerr ,pa drwoiaadtsui ofcontsu nboydf
de de Coimbra on 06 Decen http://pubs.rsc.org | doi:1 imctatniao oiianuUntntiolrs andti tlixd slocy oebe : mreta ps h boireeirlel dre ulsaoeb etdrenwane itntrad i adfitiinitinsseiott oderntdsin h b.ubse wueiy tjti yeitthor oteb n ucUa tsohnoVerefdd d sb lim intadgogonhu ndptet o r hoetimfenop ted aetsrerrnspeac,sec ritceetthihtaseree sas oi s nfsba wgpecm eigywtcpihcntall reisvudcs e mti,dl fowefi emncthrgaoieekctnnrhhes-t, hdoiaaCnfcaneH trvnAdaey,eccn sClH er isbyctH io,hte lOi,ineee ca n. s p Uca+ WoriocVedf ib Cced tsu ih Oearirpenrrrszhva e op modeo rcrdti reauoac ctCtredoriurxiinoeHsrccfnnsseoeCoo tsplrlrc,Hyme t ii aban tsad+htatctisunoiol o uHdsdnonedys n,. i Oi lncnayco glg ror + tetrtuohahe lC desedap y O pp orrpeh,nbo drobdedeitu,issou nttedcr gnnifte b tocot yurooi i ntne miC elotedwphnOx o e dab,so em sifCapt pnipetOoedhlncnese,-- ,
ao trum (two sets of bands showing different behaviour upon
sid95 irradiation were found) and, in particular, since no new bands
y Univernuary 19 I I crootualmd ebriez aotbiosne rvreeda,c ttihoen obbestewrveeedn cthhean sg-ecsi s maunsdt bs-et radnuse ctoon a-
ba formers, the two forms being significantly populated at the
ded 01 J deposition temperature. Indeed, as indicated by the calcu-
wnloaed on flaotrimonesr (osef e aTcarbylleic 2 )a,c tihde r[ethlaet ivse-t reannesr gy(C o-f 0t)h e rfeomrmai]n inisg choignh-
oh 4000 3000 1500 1000 500
Dblis wavenumber/cm- ’ enough to avoid this form being significantly populated even
u I at considerably high temperatures. The comparison between
P (b)
1 the difference spectra shown in Fig. 5(b) and the calculated
spectra [Fig. 5(c)] unequivocally proves that the upwards
bands originate in the s-trans conformer, while the down-
wards bands are due to the s-cis form and, thus, that irradia-
tion promotes (s-cis) (s-trans) isomerization.
--.)
Ij Considering the ratio of the s-cis and s-trans rotamer
t 1 absorption coefficients taken from the differential spectrum
4000 3000 1500 1000 500
’ [Fig. 5(b)],i t can be estimated from the spectrum shown in
wavenu mber/cm -
Fig. 5(a) that the (s-cis)/(s-trans)c oncentration ratio in the
.>- non-irradiated sample is ca. 1.4. Thus, assuming that the con-
4-
v) centration ratio did not change appreciably during the depo-
Q,
.4-- sition process, this value leads to an estimate of
AEb-trons)-(s-ciso)f ca. 0.8 kJ mol- I, which is in good agree-
ment with the energy difference found in the gaseous phase
by microwave spectroscopy (0.60 & 0.24 kJ mol-’ On the
14).
other hand, since the annealing of the matrices up to 32 K
4000 3000 1500 1000 500 does not change the spectra of either irradiated or non-
wavenumber/cm- ’ irradiated samples, it can be concluded that the barrier for
internal rotation about the C-C bond must be larger than
Fig. 5 (a) IR spectrum of acrylic acid isolated in an argon matrix
12 kJ mol-’. This lower limit also agrees fairly well with the
after deposition at 18 K; (b) difference spectrum obtained by subtrac-
ting (a) from the spectrum obtained after UV-irradiation in the gas-phase value, obtained from microwave spectroscopy
243 nm region over 140 min [scales of (a) and (b) are related by a (16.0 k 6.0 kJ mol-’ 14).
factor of three]; (c) SCF-HF/6-31G* calculated IR spectra of s-cis The experimental and 6-3 lG* calculated frequencies and
and s-truns conformers. In (b) and (c), the upwards going bands are intensities, as well as the PED for the s-cis and s-trans con-
due to the s-trans conformer while the downwards going bands orig-
formers, are shown in Tables 3 and 4. Table 5 presents the
inate from the s-cis form. Relative intensities in (b) and (c) correspond
calculated values obtained for the higher energy s-trans
to normalised intensities to the most intense band (calculated) or
group of bands (experimental) assigned to the v(C=O) vibration in (C-0) form. These tables also include the frequencies
the s-trans form. obtained with the scaled force fields. The latter show a
View Online
1578 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91
Table 3 Experimental and calculated vibrational wavenumbers and intensities for acrylic acid (s-cis form)"
exp (IR, Ar matrix) calc (6-31G*)
approximate
description symmetry v I ,,gc b potal c 1 ,,scaled PEDd
A' 3566.7 16.0 3564.8 30.4 4059 24.1 3557
3564.2 7.2
3561.3 7.2
A' no 3445 0.7 3021
A' no 3397 0.7 2980
A' no 3353 1.4 2941
A 1764.1 44.0 1762.4 80.0 2017 67.4 1777
1762.5 31.2
1746.0 4.8
+ +
A' 1663.0 2.4 1638.8 15.2 1865 10.8 1645 s,[73] s7[18] s9[10]
1635.0 4.8
1 1633.9 8.0
57 A' 141 1.6 20.8 1410.8 47.2 1588 24.1 1403
1
0 1410.7 20.0
1
9 1408.2 6.4
5 +
99 S(C-0-H) v(C-0) A 1323.4 4.0 1323.4 4.0 1497 6.8 1324
mber 20110.1039/FT 6v((CC--H0)) + 6(C-0-H) AA' 1111 121135992240....2004 5145...862 11123582..70 " 91.2 11341197 760..52 11126594
eceoi:1 1131.1 29.6
Dd A 1060.7 4.8 1060.7 4.8 1176 3.0 1044
on 06 c.org | A 882380..27 21..64 829.3 4.0 915 3.2 817
mbra ubs.rs A' 661260..35 23..42 618.1 5.6 682 6.8 614
ade de Coion http://p AAA' ' 499n989o164.. .890 435...286 499901..68 130..60 1251935831 406...512 942798964
sid95 A" 973.4 3.6 972.5 6.8 1122 4.7 972
y Univernuary 19 AA" 968171012...144 1938...822 680192..84 218..68 692910 2114..18 680164
ba 607.0 2.4
ded 01 J A 448.8 4.4 488.8 4.4 529 4.7 481
Downloablished on aT aWblaev e6n fuomr cboeorsr dinin camtes- ';d evfi, nsittrieoAtnc". h Einxgp;e r6i,m nbeoenn tdailn ign;t eon,siw tiaegs gpirnegs;e ant,e odu at-roef n-polr1am2n2ae l;i s5e, 0dt.o v3r sailoune1s; 4tn3oo ,t hneo tto otabls einrvteends; isty, soyfm thmee gtrriocu; pa so, fa bsyamndms eatsrsicig. nSeede
u to v(C=O) in the s-trans conformer, which corresponds to the group of bands ascribed to a single vibration having the largest total intensity in
P
the observed spectrum. Calculated intensities are normalised values to the calculated intensity of the band associated with the same vibration
[v(C=-0) in s-trans form, whose non-normalised ab initio intensity is 526.9 km mol-'1. The calculated intensities should be multiplied by 1.38 if
a correction related with the relative population of the s-cis form to the s-trans form at room temperature, obtained from assuming a Boltz-
mann distribution and AE(s-rrarrr)-(s=-c i0r.f8 kJ mol-', is considered. vgxc ic orrespolnod btos )thEe iw avenumbers of the gravity centres of the groups
of bands ascribed to the same normal mode; they are calculated as vgc = (vobs x lobwsh, ere i is the number of total components of the
group of bands. ' 1'"'"' is the total intensity of the bands ascribed to the same normal mode. PEDs < 10% are not presented in the table.
" Value not used in the force field scaling.
general agreement with the experimental values to within 2% lations predict that these modes should give rise to very low
for the two experimentally observed conformers (see also Fig. intensity IR bands in both the s-cis and s-trans conformers.
6). In turn, their Raman activities were predicted to be rather
All the conformers of acrylic acid belong to the C, point large [the 6-31G* calculations predict that, together with the
group and thus their 21 normal modes span the irreducible v(0-H) and v(C=C) modes, the v(CH) vibrations are those
+
representations, 15A 6A. Table 6 shows the C, symmetry giving rise to the most intense Raman bands], and in the
coordinates used in the normal coordinate analysis. Raman spectra of acrylic acid in diluted CCl, solutions,
Most of the observed IR bands ascribable to individual prominent bands appear at 2996,3041 and 31 10 cm-', which
conformers appear as close doublets resulting from molecules can be ascribed to the v(CH,),,, v(C-H) and v(CH,),,
trapped in different matrix environments. Thus, for these respectively (Fig. 7). The predicted (scaled) frequencies (see
cases, in order to make a more direct comparison with the Tables 3 and 4) are somewhat lower than those observed in
calculated values, the gravity centres of the experimental CCl, solution, but it is possible that, at least in part, this may
bands were obtained [vgC = xAvobs x Iobs)/~iIobsI;' = result from solvent effects or be due to the involvement of the
with i being the number of components resulting from matrix v(CH) fundamentals in Fermi resonance interactions, as sug-
site effects]. gested by the appearance in the v(CH) spectral region of a
2800-3600 cm- Region. This spectral region includes the number of additional lower intensity bands.
v(0-H) and V(CH) [v(CH,),,, v(CH,), and v(C-H)] vibra- The experimental and calculated frequencies and intensities
tions. The bands ascribable to the v(CH) modes could not be associated with the v(0-H) vibration agree very well. In par-
observed experimentally in either Ar or Kr matrices. Indeed, ticular, in agreement with the results of the calculations, the
in consonance with the experimental findings, the calcu- intensities of the IR bands originating in the two conformers
View Online
J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1579
Table 4 Experimental and calculated vibrational wavenumbers and intensities for acrylic acid (s-trans form)"
exp. (IR, Ar matrix) calc. (6-31G*)
approximate
description symmetry v I vgc ltota: v I ,,scaled PEDd
A 3575.9 8.0 3572.8 26.8 4063 24.3 3560
3571.5 18.8
A' no 3445 0.9 3021
A' no 3400 0.5 2982
A no 3355 1.4 2943
A' 1759.9 24.0 1757.6 100.0 2029 100.0 1788
1757.4 52.0
1755.9 24.0
v(C=C) A' 1651.4 2.0 1631.0 9.2 1853 1.9 1634
1625.7 5.6
1623.8 1.6
1 WH2) A' 1416.0 9.6 1416.0 9.6 1587 8.7 1403
7 +
5 v(C-0) 6(C-0-H) A 1329.7 8.8 1329.7 8.8 1510 16.1 1335
1
0 6(C-H) A no 1428 0.6 1264
591 6C-0-H) + v(C-0) A 1187.0 6.4 1185.0 36.8 1352 39.5 1198
99 1184.6 30.4
11FT A' 1019.2 18.4 1018.3 30.0 1140 11.4 1013
mber 200.1039/ ~v(Co-C=) c-o) AA'' 1580281506...599 1271...206 858205..59 72..20 693182 120..30 587164
on 06 Decec.org | doi:1 Qs6((( cCc-H- cc 2)- - 0C ) ) AAA''" 995n992o815 ...062 042...800 959265..10 06..80 1531701854 041...399 925829530
mbra ubs.rs R(C-H) A 'I 997680..16 55..22 968.7 14.4 1130 7.0 979
ade de Coion http://p tt((CC=-0=)C ) AA"" 985561667687....1240 2114974....6064 856176..52 4147..06 961495 2113..87 857074
sid95 A'' 475.9 4.0 474.0 8.0 526 0.4 478
Univerary 19 z(C-C) A" 4n7o2. 0 4.0 122 0.0 143
y nu
ded b01 Ja inSteene sTityab olfe th3e f ogrr oduepf ionfi tbioannsd.s S aeses iTgnaebdle t o6 vf(oCr =cOoo),r dwinhaicthes c doerfriensiptioonnd. sE tox ptehrei mgreonutapl oinf tbeannsidtise as spcrreibseendt teod aa rsein gnloer mviablriasteido nv ahlauveisn gto t hteh ela rtgoetaslt
wnloaed on stoatmale ivnitbenrastiitoyn in [ vth(Ce 5o0b)s]e,r vewdh sopsee cntrounm-n. oCramlcaulilsaetedd a ibn tiennistiioti eins taernes nitoyr mis a5l2is6e.d9 vkamlu meso tlo- '.t he cvgacl ccuolrarteesdp ionntedn tsoit yt hoef twhaev beannudm absesros coiaf tethde w gitrha vtihtey
Doblish ctoetnatle rcso omf pthoen egnrtosu opf st hofe bgarnodusp aosf crbiabnedds t.o tIhteo tsaiasml t eh en otormtaal li nmteondsei;t yth oefy tharee b caanldcus laatsecdri base dv gtco = th ceJ vsaombsex n Ioorbmsa)l~ miIoodbesw., hePrEe Di iss -=t h1e0 n% umarbee rn ooft
Pu presented in the table.
Table 5 Calculated vibrational wavenumbers and intensities for acrylic acid [s-trans (C-0) formy
calc. (6-31G*)
approximate
description symmetry V I ,,scaled PED~
$0--H) A 4120 16.0 3610
v(CH2) aP A 3448 0.4 3024
V(CH 2h A 3361 2.4 2948
4C-W A 3341 1.8 293 1
v(C=O) A 2049 61.4 1804
v(C=C) A 1860 11.3 1640
WH,) A 1580 25.1 1344
+
v(C-0) 6(C-0-H) A' 1472 36.0 1302
&C-W A 1447 69.6 1280
+
&C-0-H) v(C-0) A' 1283 4.5 1138
o(CH2) A' 1173 0.2 1042
v(C-C) A' 927 2.3 827
d(O=C-O) A' 694 2.1 624
6(c - c=0 ) A' 542 0.1 492
S(C=C-C) A 310 1.8 290
WH2) A 1156 6.3 lo00
W-W A 1101 4.7 955
z(C=C) A" 903 10.2 79 1
n(C-=O) A" 542 2.8 517
r(C-0) A" 450 24.6 415
z(C-C) A" 99 0.2 124
a See Table 3 for definitions. See Table 6 for coordinates definition. Calculated intensities are normalised values to the calculated intensity of the
band associated with v(C=O) in the s-trans conformer. PEDs < 10% are not presented in the table.
View Online
1580 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91
hn I
t
-2.5
-3.51 '
vexP/cm-
1
57 Fig. 6 Differences between the SCF-HF/6-31G* calculated (scaled)
01 wavenumbers and the experimental values [lo0 x ( v ~- ~vex~P)/ ' ~ ~ 1
9591 vDex, Poo/u^]t.- o0f-,pI lna-npel asn-tera sn-sc.i sT; h0e ,sicna -lpelda nvea lsu-tersa wnse;r em c, aolcuut-laotfe-dp lafrnoem s- cthise; 2850 Raman shi3ft0/0cm0 - ' 31 50
9
11FT optimised straight lines obtained by fitting calculated to experimental Fig. 7 v(CH) stretching region of the Raman spectrum of acrylic
on 06 December 20c.org | doi:10.1039/ swv----tirab av00riea g 88nth72iuot19m n sb, e,,l,,rauainsbnb e duiisnn si irtiiitnooh g++e o t bw4o12et9oh.y.2 7e d81ri73f f f(etoRrhr(ee2 Rn o=tz ul =0itne-. 9oe0qa9f.u-9r8pa9 7rlt9)aie.o3ng n)re;se :sm svioodnvesSs,~C o*(nlsC(eeod e~f u ottr(-e ioxinn~ft--)-pp.p lllTaaa'nnnheeee)) ~ wa~chide nin Y d iilus tae dh CydCr1o4{ gsCoelonuA taiYoton)m +(r)o Ccoamang tc ebome~ prp(eYrroa)p/taueRrrel,y-) Y dIeRs$c-ryib)Ze d55 by
de de Coimbra n http://pubs.rs ttwvio(oe0 nrt-e hH wef)o a uss- ntsfrdtoar uetnontsc d hbf ieotno rsgm i bmm.e i Tolsadlhrie ega hincntlda yl t cthlhuoeelw a sft-eritroer qantunhs sea ann(lc Csytoh - oa0pft) r techdoecir ocrsnte- sfcotiphsro mavntide birtrn haiges- aRtwhn,hed e- re etqh bue[oi: lenicrbdrhr iafaurrnmogdme - M f(ilt~usuxc le loiqakrusersin/loRi bccxiroai-utryermed)c R ptwe(o~adsiybt iht bci horetanhvr,eg iRa et$sse t-dro e yntX,c -hrtYheisne p gea -+ct ootfi)m v eta hlrYyee.
ao larger than in both s-cis and s-trans forms (see Tables 3-5). Since the corrected Mulliken charges on the carboxylic
sid95 This result can be correlated with the absence in the s-trans hydrogen atom are very similar in the three conformers (ca.
y Univernuary 19 (thCro-0u)g h-sfpoarcme fieolf d inthteer acatbioonv et hamt epnotliaorniseeds thCe =0O-H... Ob-oHn d f0l.u3x9 ete;r mse et hFaigt . m4)u, sitt cbaen cboen sciodnecralubdlye ds mthaaltl eirt iins tthhee csh-atrragnes-
ba in the most stable s-cis (C-0) conformers (s-cis and s-trans) (C-0) form than in the two observed conformers. Indeed,
ded 01 J leading to reduced k,, force constants and, consequently, to the charge-flux reduces from ca. - 0.80 e A-', in both s-cis
wnloaed on lowNeort ve( 0th- atH th) ev ipbrreadtiiocnteadl firnetqeunesnitcyi eosf. the v(0-H) IR band (aCnd-0 s)- trfaonrms .f orms, to about - 0.72 e A-', in the s-trans
oh
Dblis of the s-trans (C-0) conformer is considerably lower than in In the case of the v(CH) stretching modes, their average
u the two observed conformers. In general, the intensity of a low intensity correlates very well with the corresponding
P
well localised v(X-Y) stretching vibration (in particular average corrected Mulliken charges on hydrogens and
Table 6 Definition of the internal symmetry coordinates used in the normal-coordinatea nalysis"
approximate
coordinate description symmetry definition
v(0-H) A v(0-H)
V(CH,),, A' v(C-H) - v(C-H')
v(C-H) A' v(C,-H)
+
V(CH 2)s A' v(C-H) v(C-H')
v(C=O) A v(C-0)
v(C=C) A' v(C=C)
WH2) A' 26(H-C-H') - G(H-C--C) - G(H'-C=C)
S(C- 0- H) A' 6(C-0-H)
6(C-H) A' S(C-C-H) - S(C=C-H)
v(C-0) A v(C-0)
o(CHZ) A' S(H-C=C) - S(H'-C=C)
v(C-C) A' v(C-C)
s(O=C-O) A' 26(O-C-O) - S(C-c-0) - qc-c-0)
6(C- c =0 ) A' S(C-c-0) - S(C-c-0)
6(c =c - C) A' 2S(CsC-C) - S(C-C-H) - 6(C=C-H)
WH2) A" RCH-C( =C)- H']
R(C-H) A" Q[C=C( -H )- C]
+ + +
z(C=C) A z(H--C=C-C) z(H'-C=C-C) z(H-C=C-H) z(H'-C=C-H)
+
z(c-0) At' z(H-O--C=O) z(H-0-C-C)
Q(C=O) A" atc- C( =0 )- 0+1 + +
7(C-C) A" z(H-C-C=O) z(H-C-C-0) z(C=C-C=O) z(C=C-C-O)
" Normalization constants are not given here; they are chosen as N = (IC')w-h'e're~ ci, a re the coeficients of the individual valence coordi-
nates; see Table 3 for definitions.