Mass Spectrometry of Organic Ions

726 Pages·1963·9.223 MB·English

by  F. McLafferty (Eds.)

M a ss S p e c t r o m e t ry
of O r g a n ic I o ns
Edited by
F. W. McLafferyt
Eastern Researc hLaboratory
The Dow Chemica Clompany
Framingham M, assachusetts
Academci Press, New York and London 1963
COPYRIGHT © 1963, BY ACADEMIC PRESS INC.
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United Kingdom Edition
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Library of Congres sCatalo g Card Numbe r 62-22357
PRINTED IN THE UNITED STATES OF AMERICA
Contributosr
Κ. BIEMANN, Departme notf Chemistr yM,assachuse tItsnstitu toef Technology,
Cambridg, eMassachuse. ttPsage 529
VERNON H. DIBELER, Physica Cl hemistr yDivision ,Nationa Blureau of Standards,
Washingto, nD.C. Page 111
HENRY M. GRUBB, Researc hand Developme nDtepartmen At,merica nOil
Company W, hitin,g Indian. aPage 453
A. G. HARRISON, Departme notf Chemistr yU,niversit yof Toronto T,oront,o
Canad.a Page 207
A. HOOD, Exploratio nand Productio nResearc hDivision ,Shel lDevelopment
Compan y(A Division of Shel lOil Company )H, ousto,n Texa.s Page 597
P. F. KNEWSTUBB, Departme notf Physica lChemistr yU, niversit yof Cambridge,
Cambridg, eEnglan.d Page 255
MORRIS KRAUSS, Physica lChemistr Dyivision ,Nationa lBureau of Standards,
Washingto, nD.C. Page 1, 117
F. W. MCLAFFERTY, Eastern Researc hLaborator yT,he Dow Chemica Cl ompany,
Framingha, mMassachuse. tPtsage 309
CHARLES E. MELTON, Chemistr yDivision ,Oak Ridge Nationa Llaboratory,
Oak Ridge, Tennesse. ePage 65 ,163
SEYMOUR MEYERSON, Researc hand Developme nDtepartmen tA,merica nOil
Company W, hitin,g Indian. aPage 453
R. IVOR REED, Chemistr Dyepartmen Tt,he Universit yG,lasgwo Scotlan. dPage 637
Η. M. ROSENSTOCK, Physica Cl hemistr yDivision ,Nationa Blureau of Standards,
Washingto, nD.C. Page 1
RAGNAR RYHAGE, Laborator yfor Mass Spectromet rKy,arolinsk aInstitutet,
Stockhol,m Swede.n Page 399
R. A. SAUNDERS, Imperia lChemica Ilndustrie Lstd . (Dyestuf fsDivision),
Manchest,e Ernglan.d Page 343
EINAR STENHAGEN, Institu toef Medica lBiochemistr Uy,niversit yof Goteborg,
Swede.n Page 399
A. E. WILLIAMS, Imperia lChemica Ilndustrie Lstd . (Dyestuf fsDivision),
Manchest,e rEnglan.d Page 343
ν
Preface
The study of organci moleculse by mass spectromeyt rhas led, on the
one hand, to the exciting new field of the chemistyr and physics of
noncondendse organci ions—theri formatio,n decompositio, nreaction,s
and physicla propertie. sOn the other hand, much of the rapid growht
of the method has been due to the surprising varieyt of unique applica­
tions found for it across the areas of physic,s chemistr,y and the biolo­
gical science. s
The first purpose of this book is to give comprehensi,v ecritica,l and
up-to-daet reviews for the student of particulra fields of mass spectr­o
metry. The secon,d and possibyl more importan, tpurpose is to provide
scientisst in other specialitise a referenec to the underlying theorise
and major applicatiosn of the method so that they can assess its usefulness
in theri own researc.h
A comprehensei vreview of the quasi-equilibrmiu theory, the classicla
generla theoyr for the formatino and decompositnio of organci ions, is
especiayll timeyl becaues of severla recent detailde investigatios nwhich
indicaet that major revisions are necessar. yThe extremeyl rapid
reactions of organci ions with moleculse is a fascinatign new area of
scienec whoes major significanec in upper atmospheer physic,s radiation
chemistr,y and similar high energy reactiosn has only recentyl been
appreciate. dA review of the study of appearanec potentias lfor the
formation of organci ions, basci values which have been well recognizde
for the determinatnio of ionizatino potentias l and bond dissociatino
energise of organci molecule, sis especiayll pertinetn becaues of recent
theoreticla developmesn twhich reveal basci difficultise in the conven­
tional interpretatnio of appearanec potentila data. Mass spectrometcr i
studies in the past have been limited almots exclusivey l to positive
ions, a situatino shown by recent researhc to be unwarrantde in view
of the unique applicatiosn and important roles played by negatiev ions
in a number of vital fields . Mass spectromeyt r has provided a
unique tool for the identificatnio and direct study of the reactiosn and
propertise of organci radical,s whose importanec as intermediast ein
many chemicla reactiosn is now well recognize. dThe presenec of ions
in electrci discharge, sflames ,and similar high energy reactiosn has been
known for years, but it has only been the recent applicatino of the mass
vii
viii PREFACE
spectromert eto theri study that has yielded any detailde picture of
theri identitie, sconcentratio, nand propertie.s
An area of very broad usefulness and potentila which is just being
fully recognizde is the determinatnio of the moleculra structuer of
organci compounsd by mass spectromet.r yFundamentla to this use is
a knowledeg of the mechanisms by which organci ions can decompoes
and rearrang,e and a generalizde concept in terms of physical-orgacn i
chemistyr is presente. dHigh resolutino mass spectromeyt rhas provided
a very powerflu new tool in such structuer determinatnio by the elucida­
tion of the empiricla formulas of organci ions. Detailed correlatiosn and
mechanisms of the mass spectar of long-chani ester,s alkylbenzen,e s
a varieyt of natural product,s aliphatci compound, sand terpense not
only provide fundamentl areferenec data for the interpretatnio of such
spectra, but also point to the broad applicatino which can be made with
further study to the structurla elucidatino of even more complex mole­
cular system. sIllustratiev of the power of this method is the amazingyl
detailde picture of the structuer of petroleum, hitherot thought of as a
hydrocarbno mixtuer of almots hopeless complexit.y
Despiet the implicatiosn of the above statemen,t sthe techniqeu of
mass spectromeyt rfinds much of its value in increasin,g not replacin,g
the effectivense sof other technique, ssome of which have also shown a
spectaculra rise in application. sThe editor is especiayll prejudice,d
howeve, rin favor of the uniqueness of the informatino that can be
gained in a wide varieyt of fields by mass spectromet.r yThis shoudl
logicalyl suggets that many new, equalyl valuabel applicatiosn of the
method may yet be introducde by further researhc in many areas of
physics and the biologicla science, sand in most areas of chemistr.y
January 1963 F. W. MCLAFFERTY
1
Quasi-Equilibrimu
Theory of Mass Spectra
Η. M. Rosensto ckand M. Krauss
Physica lChemistr yDivision ,Nationa lBureau of Standards,
Washingto n,D.C.
I. INTRODUCTION 2
1. Characteristsic of Mass Spectar 2
2. A Theory of Mass Spectar 3
II. AN ABSOLUTE REACTION RATE THEORY APPROPRIATE TO MASS
SPECTRA 4
1. Fundamentla Assumptiosn 4
2. Rate Processse in the Mass Spectromert e 5
3. Comparisno with Ordinary Gas Kinetics 6
4. The Problem of Effective Oscillatosr 8
5. Fluctuatino Effects 12
6. Quantum Corrections 14
7. Current Form of the Theory 16
III. INITIAL PREPARATION AND VALUES OF PARAMETERS 16
1. Energy Transfer Functions 16
2. Electronci States and Ionizatino Processse 22
3. Reaction Mechanisms 27
4. Parametesr of the Activated Complex and Activation Energies . 32
5. Separaet Electronci States 35
IV. CONSEQUENCES OF THE QUASI-EQUILIBRIUM THEORY 38
1. Breakdown Curves 39
2. Total Mass Spectar and Isotope Effects 40
3. Metastabel Transitions and Reaction Competitino 43
4. Appearanec Potentias l 46
5. Temperatuer Effects 48
6. Variation of Initial Preparatino 50
V. FOUNDATIONS OF THE QUASI-EQUILIBRIUM THEORY 55
1. Nature of Equilibrium 55
2. Possibel Approach to Equilibrium 56
3. Initial Preparatino and Intramolecurl aEnergy Transfer . . .. 56
4. Competitino and Rate Theories 59
VI. CONCLUSION 60
REFERENCES 61
1
2 Η. Μ. ROSENSTOCK AND Μ. KRAUSS
/ . Introduction
The essentila featurse of the ionizatino and dissociatino of diatomci
moleculse by electrno impact are well understoo. dThe process of
ionizatino producse a vertical or Franck-Condno transitino to one or
anothre of the potentila curves of the ion. Fragmentatnio occurs in that
fraction of the ions initialyl formed above the dissociatino limits of the
potentila curve. With knowledeg of the shapes and location of the
potentila curve,s and knowledeg of the transitino probabilitise to the
discreet and continumu levesl of each potentila curve, it is possibel in
principel to predict the experimenlt aresulst concernign the extent of
fragmentatio, nthe form of the ionizatino efficienyc curves of parent
ions and fragment, sthe kinetci energy distributino of the fragment, s
and isotope effecst and temperatuer effect.s In practice our knowledeg
of the potentila curves is incomplet, eand of the transitino probabilitise
to various curve,s almots nonexiste.n tNeverthele,s sthe meaningflu
comparisosn that have been made betwene experimetn and theoyr
indicaet the essentila correctness of this picture. Compared to the mass
spectromert etime scale, ~10~6 second, sthe dissociatino process is
instantaneo,u s~10~13 second.s
It is not surprising that the situatino in regard to polyatomc imoleculse
is less satisfactor. yThere are a greatre varieyt of fragmentatnio product,s
a greatre number and varieyt of potentila surface sand new types of
behaviro as the parametesr of the experimens tare varied.
The mass spectar of polyatomc i moleculse have been describde in
detali in other chaptesr of this book and elsewheer [1, 2]. In the present
context the following few observatiosn are relevan.t
1. CHARACTERISTICS OF MASS SPECTRA
(a) Mass spectar present a "chemica"l appearanc. eFragment ions
are found which can be formed from parent ions by bond rupture and
rearrangemet nprocessse closeyl resemblign those which occur in
reaction mechanims of thermalyl excitde neutral molecule. sIsomesr may
have radicalyl different mass spectr.a
(b) The effecst of source temperatuer are far more pronouncde in
polyatomci mass spectar than in diatomci mass spectr.a Parent ion
intensitise show a pronouncde negatiev temperatuer coefficietn wheresa
fragment ion intensitise may show a positiv,e negativ,e or zero coefficien. t
(c) Isotopci substitutino can either hinder or enhanec particulra
fragmentatnio processe. s
1. QUASI-EQUILIBRIUM THEORY OF MASS SPECTRA 3
(d) The kinetci energise of fragmetn ions, where measure,d are very
small (severla tenths of a volt or less) with some exceptiosn such as some
methyl and ethyl ions.
(e) Metastabel transitiosn are observe,d i.e,. unimolecurla decomp­o
sition reactiosn occurring with a rate of roughyl 10esec_1. These
transitiosn correspodn to either parent or fragmetn ions forming other
ions by spontaneos udecompositio. nIn some cases severla metastabel
transitiosn can be found which form a sequenec of decompositnio steps
connectign important ions in the mass spectrum. One ion may decompoes
according to one or more metastabe ltransitiosn and, in some case,s a
particular ion may be formed by different metastabe ltransitiosn from
different precursro ions.
(/) Ionizatino efficienyc curves for various ions do not all have the
same shape, especiayll near the threshol.d
(g) Some mass spectr,a particularyl of oxygen compound, sshow the
presenec of negatiev ions; they are generayll in smaller abundanec than
positive ions.
(A) There is little evidenec for multipyl charged ions in the mass
spectar of aliphatci compound, swheresa with some aromatci compounsd
multipel ionizatino becomse very significan. t
2. A THEORY OF MASS SPECTRA
The generla nature of the above observatiosn led to the suggestnio
that the fragmentatnio processse leading to the formatino of mass spectar
could be viewed as rate processse quite similar to those occurring in
ordinary chemicla reaction. sThis viewpoitn led to the formulatino of
the quasi-equilibrmiu theoyr of mass spectr.a At the present time the
status of the problem is essentiayl las follow.s The theoyr has proved
to be of considerabe lqualitatiev value in understandgin many of the
qualitatiev featurse listed above. Although the original applicatino of
the theoyr containde a mathematicl aapproximatnio which was recentyl
shown to be invalid, earyl applicatiosn to calculatiosn of total mass
spectar led to quite encouragign result.s Recen,t more detailde compar­i
sons betwene theoyr and experimetn using' electrno impac,t photo-
ionizatio,n and charge exchaneg techniquse indicatde that the earlier
form of the theoyr was not applicabel unless one assumde that the
effectiev number of degrese of freedom to be used in the rate equatiosn
was consideraby lless than the actual number given by the structuer
of the ion. An improved mathematicl aapproximatnio has largeyl
4 Η. Μ. ROSENSTOCK AND Μ. KRAUSS
removed this difficult.y More detailde consideratios nof the foundatiosn
of the theoyr have now placed new emphass ion the importanec of the
primary ionizatino and subsequet nenergy transfre processse and has
revealde new questiosn concernign the validiyt of the equilibrium
hypothes.i s
All in all, the theoyr at present has been neithre proved nor disprove,d
although applicatino of the theoyr has led to a number of encouragign
result.s In recent years the many problems involved in understandgin
mass spectar have become much more clearyl defined as have the limita­
tions of existing theory. These mattesr will be discussde below under
five heading:s
(i) an absoluet rate theoyr appropriat eto mass spectr;a
(ii) initial preparatino of ions and the values of parameter; s
(iii) consequensc eand tesst of the theory;
(iv) foundatiosn of the quasi-equilibrmiu theory;
(v) conclusion. s
// . An Absolut eReactio nRate Theory Appropria teto Mass Spectra
1. FUNDAMENTAL ASSUMPTIONS
There are two principal assumptiosn of the quasi-equilibrmiu theoyr
of mass spectar [3, 4]. First, it is postulatde that the moleculra processse
leading to the formatino of a mass spectrum consits of a series of com­
peting, consecuteiv unimolecurla decompositnio reactiosn of excitde
parent ions. Second, it is postulatde that the rate constanst for each of
these reactiosn can be calculatde by means of an appropriaet form of
absoluet reaction rate theory.
Absoluet reaction rate theoyr is based on the assumptino that the
reaction rate is determinde by the concentratnio and propertise of
activatde complexe, ssuitabyl define,d and that the activatde complexse
are in equilibrium with the reactant specie.s This concentratnio can
then be calculatde from an equilibrium constatn by the methosd of
statisticla mechanisc [5]. It shoudl be mentionde at this point that there
are other theoreticl aapproachse to reaction kinetics using the methosd
of statisticla mechanisc and that the distinctiev featuer of the one under
discussino is the assumptino of equilibrium and not the use of statisticla
method.s Thus the term "quasi-equilibrmiu theory" is to be preferre,d
rather than "statisticla theory". At this point the generla problem
divides into two parts—firs,t the developmet nof quantitatiev rate
1. QUASI-EQUILIBRIUM THEORY OF MASS SPECTRA 5
equatiosn in accordanec with the two basci assumption, sand secon,d
the justificatino of the assumption. sIn this section we discuss the first
problem.
2. RATE PROCESSES IN THE MASS SPECTROMETER
From the kinetics viewpoitn adoptde here the sequenec of evenst in
the mass spectromert eis describde in the following terms. An assembyl
of noninteractgin moleculse in the source chambre is ionized and excitde
by electrno (or photon or ion) impac.t The excitatino process and sub­
sequetn internal energy transfre processse are assume dto lead very
rapidly to an essentiayl luniform distributino of excitde ions among all
accessibe lquantum states of the ions compatibel with energy and angular
momentmu restriction. sA certani fraction of the quantum states
correspodn to so-callde activatde complexe. sThe equilibrium postulaet
now permist one to assume that the fraction of ions which are activatde
complexse is at all time sequal to the fraction of quantum states which
represetn activatde complex state.s The rate of decompositnio of the
ions is then given by the concentratnio (or fraction) of activatde com­
plexes multiplide by the average rate at which the activatde complexse
travel over the potentila barrier. In the case of further decompositnio
reaction,s i.e,. A->B—*C etc,. the intermediast eare also assumde to
be formed in a uniform distributino among theri accessibe l quantum
states so that the propertise of the activatde complex and the equilibrium
assumptino suffice for calculatino of these rate constanst as wel.l
With the values of the rate constanst assumde known, the extent of
fragmentatnio is determinde by the time availabel for decompositnio to
occur. In a magnetci sector mass spectromert ea typical ion will spend
about one microsecodn in the ionizatino chambre betwene the instant
of formatino and its departuer through the exti slit. It is acceleratde to
an energy of three kilovolst in the next seven microsecond, sspends
about four microseconsd traversign the fieldfre eregion and the magnetci
field ,and in anothre two microseconsd it arrives at the collecto. rThese
times depend, of course, on the mass of the ion and on the geometyr
and voltagse of the instrumen. t(More detailde analysse will be found in
referencse [6-8]) .For fragmetn ions to be formed in the ion source it is
necessayr that the decompositnio reaction() sproceed with a rate of
10e sec-1 or faste.r If the rate of the reaction is in the neighborhodo
of 105 sec-1 the decompositnio will occur in transi,t and that part
occurring in the field-fre eregion betwene the electrostact iacceleratign
and the magnetci deflectign fields producse the metastabe lions [6, 9, 10].