One and Two Dimensional NMR Spectroscopy

576 Pages·1989·24.414 MB·English

by  Atta-ur- Rahman (Auth.)

ONE AND TWO
DIMENSIONAL NMR
SPECTROSCOPY
Atta-ur-Rahman
H.E.J. Research Institute of Chemistry,
University of Karachi,
Karachi 32, Pakistan
EIS Eli ER
Amsterdam — Oxford — New York — Tokyo 1989
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First edition 1989
Second impression (with corrections) 1991
Library of Congress Cataloging-imPublication Data
Rahman, Atta-ur-, 1942-
One and two dimensional NMR spectroscopy.
Includes index.
1. Nuclear magnetic resonance spectroscopy. I. Title.
QD96.18R35 1989 543'.0877 88-33605
ISBN 0-444-87316-3 (U.S.)
ISBN 0-444-87316-3
© Elsevier Science Publishers B.V., 1989
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V
PREFACE
If you have just picked up this book from the shelf and are wondering if you should
buy it, I would like to draw your attention to some features of this book which may
help you decide. I suggest that you carefully go through the "contents" section which
will indicate the extent of its coverage of modern 1D and 2D NMR techniques.Most
chapters end with several problems and solutions which should be useful to
newcomers to 2D NMR spectroscopy. The discussions of solutions, and the inclusion
of practical examples should also help the readers in developing a practical feel of the
subject. I have assumed that the reader is unfamiliar with the newer developments in
the field, and have therefore begun the book with basic aspects of pulse NMR, and
included the "nuts and bolts" of modern NMR spectrometers, particularly the
principles governing their optimum operation, such as shimming, tuning the probe
etc. which are so important for recording a good spectrum. The casual, often
"conversational", tone which I have adopted will hopefully make the reading of this
book an enjoyable proposition.
I have avoided discussing such routine subjects as factors governing chemical
shifts and coupling constants, or including tables of chemical shift/coupling constant
values as there are many NMR textbooks published in 1970's and 1980's (including
mine published by Springer-Verlag in 1986) which have covered these topics
comprehensively. The emphasis is therefore to describe the recent developments,
and on the practical applications to solving the structures of complex organic
molecules. I have tried to show the "real life" situation in chapter 13 by describing the
various modern 2D NMR techniques as applied to solving the structure of a new
natural product isolated in my laboratory, including COSY, hetero COSY, COLIC,
NOESY and INADEQUATE. Most of the problems are also taken from the
ongoing work of my research group, hence their bias towards new natural products.
I felt the need for writing an uptodate text on nuclear magnetic resonance
spectroscopy because of the explosive growth in this field during the last ten years.
This has been brought about by several factors. The advent of pulsed Fourier
transform NMR spectrometers with mini-computers has allowed the manipulation of
nuclear spins in various predetermined ways by the application of appropriate pulses,
and the storage of the resulting data in the computer memory. The powerful
superconducting magnets now available provide excellent field stability with good
dispersion , thereby allowing the long term collection of data with little variation in
the recording conditions. This has triggered the development of two-dimensional
NMR spectroscopy which has now come to be routinely used in its various forms as
a powerful method for structure elucidation of complex organic molecules and created
the need of a text book which will present these developments in a readily
ni
understandable form to chemists and biochemists. Every attempt has been made to
avoid a complex mathematical treatment, and to emphasise how each technique can
be applied to solve practical problems. The text is confined to NMR spectroscopy in
the liquid state and primarily concerned with 1H and 13C NMR spectroscopy, since
these are the areas of interest to most organic chemists and biochemists, although it
may also be found useful by persons working in other areas since the general
principles are of universal application. It is hoped that the book will be stimulating
and enjoyable to the readers and that it will meet the undergraduate and postgraduate
level requirements of courses of organic chemistry, medicinal chemistry, biochemistry
and pharmaceutical chemistry.
I am grateful to a number of persons for their assistance in the preparation of this
manuscript. They include Mr. Abdul Hafeez for the drawing of diagrams, and Mr.
Shabbir Ahmed, Mr. Asif Mehmood Raja, Mr. Habib Alam and Mr. Wajihul Husnain
for typing the manuscript. I am particularly indebted to Miss Anis Fatima whose help
has been invaluable in the checking of the manuscript. I am also grateful to a number
of my students particularly Mr. Habib-ur-Rehman, Mr. Muzaffar Alam, and Mr.
Habib Nasir for preparing some of the diagrams and Miss Khurshid Zaman, Mr.
Zahir Shah, Mr. Dildar Ahmed and Mr. Syed Safdar Ali for proof-reading of the
manuscript.My thanks also go to Mr.Ejaz A.Soofi for his assistance in the
arrangement of the Index.
Most of this book was written in the evenings and summer "holidays" during a
two-year period -- I wish to express my heartfelt thanks to my wife Nargis for her
enduring patience for this time which in all fairness belonged to my family.
In this fleeting existence I have lived with a constant sense of awe of the wondrous
facets of nature, the magnetic properties of nuclei being one such facet. I hope I can
succeed in passing some of these feelings of wonder, admiration and excitement over
to the readers. I rejoice, for! have had this joyful moment to learn and to teach. And
in years to come - - - - - - although I shall disperse with the wind and grow in the
fields, and rain from the clouds and glisten as dewdrops on flowers - - - - - - -but my
spirit in heavens yonder shall always rejoice in wondrous admiration of the Lord
- - - - - -reminding me of the Quranic verse (Surah Rahman):
Then what other manifestations of the Lord will you deny
PROF. ATTA-UR-RAHIAN
Ph. D. (Cantab.), Sc. D. (Cantab.)
VII
FOREWORD
In the period immediately following the second world war,, organic chemists
became devotees of u.v./visible and i.r. spectroscopic methods for the determination
of structures of organic molecules, in general, and natural products in particular. Many
workers in that period actually committed to memory large numbers of characteristic
i.r. frequencies and l max values for electronic transitions in conjugated systems. It is,
therefore, not surprising that, with the arrival of commercial CW 1H NMR
spectrometers in the late 50's, these chemists avidly adopted this new technique and
were soon quoting proton chemical shifts and coupling constants with the same facility
as with i.r. and u.v. data. The CW experiment is conceptually simple, to the point where
it was soon included in the standard text books of organic chemistry. The advent of
Fourier transform NMR in 1966 and of commercial FT spectrometers in the early 70's
heralded a new dimension in NMR spectroscopy, one of much greater versatility but
of considerable greater complexity. Still, for the most part, structural organic chemists
were content to take the transform from the time to the frequency domain for granted
13C
and busily set about learning characteristic chemical shifts while solving structural
problems of ever increasing complexity.
The 1980's have seen yet another revolution in NMR spectroscopy, namely
development and deployment of a whole galaxy of iD and 2D multipulse techniques
which, in some cases at least, can lead to self-contained structure proofs for quite
complex molecules. For the first time, the organic chemist is being compelled to seek
a truly in-depth understanding of the theory and practice of modern NMR
methodologies, a problem which is exacerbated by the fact that more and more
organic chemists find themselves sitting at the consoles of NMR spectrometers or data
stations in order to acquire or process their own data.
As a result of these recent developments, there has arisen a need for an
authoritative text on the theory and practice of multipulse experiments which at the
same time is intelligible to the organic chemist and which illuminates the enormous
power of the new techniques. Professor Atta-ur-Rahman has written a text which
admirably fills these needs. He is a noted natural product chemist and, as such, he has
recognized the overwhelming power of modern NMR spectroscopy. He has,
therefore, undertaken the daunting task of acquiring, from the primary literature, a
proper understanding of the subject. Consequently, it is not surprising that his book
is written at a level which will retain its audience. At the same time, the reader is not
denied the opportunity to proceed to an even deeper level of understanding should
his ability and inclination so dictate. Thus, while vector pictures are used to describe
most experiments, treatments employing spin product operators are also presented
and the reader is further directed to references dealing with full density matrix
VIII
methods. Organic chemists will be grateful to Professor Rahman for passing on to
them his hard won understanding and appreciation of the use of 1D and 2D NMR
spectroscopy in structural organic chemistry.
Lloyd M. Jackman,
The Pennsylvania State University,
University Park.
1
Chapter 1
Basic Principles of Modern
NMR Spectroscopy
1.1 INTRODUCTION
Nuclear magnetic resonance spectroscopy relies on the fact that certain nuclei
possess a spin angular momentum with a corresponding magnetic moment. When
placed in a magnetic field, they can adopt one of a number of quantized orientations,
each orientation corresponding to a particular energy level. The magnetic moment of
the nucleus is most closely aligned with the external magnetic field in the orientation
corresponding to the lowest energy, and least closely aligned with the external
magnetic field in the orientation corresponding to the highest energy. Transitions
between these levels can be induced by absorption of radiofrequency radiation of the
13C
correct frequency. In the case of a proton or the isotope of carbon, the nuclei can
exist in two orientations, and the energy difference D E between the two energy levels
is proportional to the external magnetic field. Since the nucleus is spinning on its axis,
the external magnetic field causes it to "precess" i.e. the spinning nuclear axis rotates
in a circular motion, analogous to the rotation of the axis of a spinning gyroscopic top
before it topples. If the radio-frequency field is applied perpendicular to the magnetic
field, and at a frequency which exactly matches the frequency of the precessional
motion (Larmor frequency), absorption of energy will occur causing the nucleus to
"flip" to a higher energy orientation (i.e one aligned against the external magnetic
field). This results in a change in the impedance of the oscillator coils which can be
measured on a recorder in the form of an NMR signal.
There are two main types of relaxation processes by which. nuclei in the higher
energy state can relax to the lower energy state. The first, termed spin-lattice relaxation
(Ti), corresponds to the transfer of energy from the nucleus to the "lattice" i.e. the
assembly of molecules around it. The rapid rotational and translational motions of
molecules result in varying magnitudes of magnetic fields which may be considered
to have a large number of oscillating components. When these components are
correctly oriented, and when their frequencies exactly match the precessional
frequency of the nucleus, transfer of energy from the nucleus to the lattice can occur,
and the nucleus will thus be able to relax to its lower energy state.
An alternative relaxation pathway involving spin-spin relaxation (T2) results from
an exchange of energy between neighbouring nuclei. This will occur when the rotating
2
component of a magnetic vector of a neighbouring nucleus is in a plane at right angles
to the external field, and when the frequency of the rotating component of that vector
exactly matches the precessional frequency. The process of spin-spin relaxation
results in a broadening of the resonance signals.
Hence by absorption of energy from the radiofrequency source, transitions of the
nuclei occur to a higher energy state, while by the relaxation processes mentioned
above, the nuclei relax back to the lower energy state, and an equilibrium state is
established in which the population of the lower state is slightly in excess (Boltzmann
excess) of the population in the upper state. Thus on a 100 MHz instrument, for every
one million nuclei existing in the lower energy state there will be 999,987 nuclei in the
upper state, leaving an excess of only 13 nuclei in the lower state. It is this tiny excess
which is responsible for the NMR signal. Processes which increase the ground state
population of the nuclei (e.g. positive nuclear Overhauser effect, polarisation transfer
etc.) will result in an enhancement of the signal.
A detailed discussion of the various factors which affect the chemical shifts and
coupling constants of the nuclei is presented in another text by the author (ref.1).
1.2 SOME FUNDAMENTAL CONSIDERATIONS IN NMR
SPECTROSCOPY
1.2.1 Instrumentation
1.2.1.1 The Magnet
Modern high field NMR spectrometers are now available with oscillator
frequencies of upto 600 MHz. These instruments have superconducting magnets
which operate at liquid helium temperature (-259°C). The solenoid in these magnets
is constructed from a wire made of niobium alloy, which is dipped in liquid helium.
The helium is contained in an inner chamber, while the outer chamber is cooled with
liquid nitrogen to reduce the evaporation rate of helium.Special Dewars may be
ordered with the instruments so that helium refilling is required much less frequently
than in the past. On a 300 MHz instrument fitted with one of these special Dewars,
the helium is required to be refilled only once in 10 months, making it much easier
to operate these instruments in cities where liquid helium is not readily available. The
distinctive feature of superconducting magnets is their extremely high stability over
long periods of time. Fig. 1.1 shows a 400 MHz NMR spectrometer system.
1.2.1.2 The Probe
The bore of the magnet contains the field gradient coils, and inside these sits the
probe. This is a cylindrical metal tube (Fig. 1.2) from which the pulses are transmitted
to the sample, and the resulting NMR signals are received. The probe is inserted
3
i:__
4j„I~~ • ,
i
-9
I
—
i
Fig. 1.1: A 400 MHz superconducting h R spectrometer.
into the area between the magnet from the bottom of the cryostat, while the tube
containing the sample is gently lowered (over a cushion of air) from the top so that it
is exposed to the upper region of the probe. The sample tube, which is spun on its
axis by a stream of air, is normally kept at room temperature. The probe and the
gradient coils are also at room temperature. A number of different types of probes
1H
are available, depending on the types of measurements required. For -NMR
spectra, the maximum sensitivity is usually obtained if a dedicated proton probe is
used. However in most laboratories, a considerable number of 13C-NMR spectra are
also recorded, and it is therefore more convenient to use a dual (1H-13C) probe. While
this means that there is some sacrifice in sensitivity (10-20%), but the bother of
changing probes, retuning and reshimming (see sections 1.2.1.3 and 1.2.1.4) is thus
1H 13C
19 31
avoided. If one wishes to study other nuclei in addition to and (e.g. F P
etc.) then broadband multinuclear probes may be employed, but they possess an even
lower sensitivity (by a factor of 2 approx.) than the dedicated probes for a given
nucleus.
Probes are available in various diameters (e.g. to take 5 mm,10 mm, 15 mm sample
tubes). In wide bore magnets, the probes are of even larger diameter allowing whole
animals e g cockroaches or even mice to be inserted into the sample tubes. Normally
one uses the 5 mm probe, the larger diameter probes (e.g. 10 mm) being preferred
only when the solubility of the sample is a critical problem, making it desirable to
4
n
~
Fig. 1.2: A probe assembly.
subject as much solution as possible to the NMR experiment for obtaining a good
signal. Often the problem is not one of solubility but of the small quantity of the
substance available. In such situations it is preferable to use the smallest diameter
probe which will give a better signal-to-noise ratio than a larger probe, given that the
sample quantity remains the same in both cases. However if the quantity of material
available is not a limiting factor, then one obviously wishes to subject as much of it as
conveniently possible to the NMR experiment in order to obtain a good spectrum in
the shortest possible time. In such situations the use of a 10 mm or an even larger
sample tube is advisable.
The dramatic improvements in sensitivity of NMR spectrometers during the last
few years has been mainly due to improved probe design. Apart of the probe is very