- Email: [email protected]
13C NMR spectroscopy
117
trends in amlytical chemistry, vol. 8, no. 3,1989
books === A well-researched text 13 NMR Spectroscopy, by H. Kalinowski, S. Berger, and S. Braun, Wiley, 1988, f 75.00 (xvi + 776 pages) ISBN O-471-91306-j
This large volume is an English translation of ‘3C-NMR-Spektroskopie, originally published in German in 1984 by Georg Thieme Verlag. The book is addressed to organic chemists and others interested in applications of 13CNMR to complex organic and biochemical systems, and it should serve well both as a reference and text. Of greatest value, perhaps, is the extensive compilation and discussion of chemical shifts and coupling constants, which amounts to over 75% of the printed pages. Supplementing the abundant tables of data are ample references to the original literature, provided not only in extensive numbered lists by chapter but in separate bibliographies for each subsection as well. There is also an index of compounds with over 3000 entries and a general bibliography at the end. Beyond merely tabulating the data, however, the authors explain the various rules and trends from the perspective of the organic chemist, and briefly describe the physical origins of chemical shielding and spin-spin coupling. For this latter purpose they use some results from elementary quantum mechanics and molecular orbital theory. The book is introduced by two chapters dealing with fundamental principles and techniques of Fourier transform NMR. The authors begin with the quantization of nuclear spin and the basics of resonance, go on to discuss NMR instrumentation and the mathematics of the numerical Fourier transform, and finish the introduction by surveying various experimental methods. Among these are many recently developed techniques such as polarization transfer,
double-quantum filtering, and twodimensional spectroscopy. In addition to these chapters and the two extended chapters on chemical shifts and J couplings, there are briefer discussions of spin-lattice relaxation, the nuclear Overhauser effect, dynamic exchange, lanthanide shift’reagents, and techniques for studying reaction mechanisms, This book is concerned mostly with applications of i3C NMR, so perhaps it is not surprising that its explanations of why certain techniques work are sometimes less than satisfying. Where the workings of a particular method are discussed, the picture invariably used is the semiclassical vector model of the nuclear magnetization. Usually the explanations are accurate and should satisfy the reader mostly interested in applications, but still there can be problems. First, descriptions of multipulse experiments by such pictures tend to become a bit labored even when essentially correct. Second, there are many instances when the simple model fails -with coupled spins, for example, and most notably with multiple-quantum coherence. Thus a reader is left without any real understanding of why the ‘INADEQUATE‘ method works, or even an indication of what double-quantum coherence is and why it has the properties it does. Third, unless there is an honest assessment of the limitations of the vector magnetization picture, a student may find it hard at some later time to accept more involved descriptions of modern NMR experiments. The danger is that a novice may assume that all resonance phenomena are reducible to the simplest model and that any embellishments are probably unnecessary. A book of this sort need not and should not treat the quantum mechanics of NMR in detail, but it can go further in pointing out where and why the semiclassical interpretation
of the magnetization fails. Pedagogical problems of this sort surely will increase as more and more two-dimensional and multiple-quantum techniques become routine in 13C spectroscopy. These comments nothwithstanding, one must remember that such topics constitute only a small portion of the book and that, in general, the chapters dealing with mathematics, quantum mechanics, and pulse sequences do serve their purpose. Though sometimes brief and without elaboration, these sections clearly give the reader some idea of what is involved and where to look for more information. There are, however, a few mistakes. On p. 7, for example, the authors claim that the spin of a nucleus with well-defined quantum numbers I and m precesses about an external magnetic field B,, maintaining an angle 8 = cos-l [mlVI(I + l)] with respect to B,. Yet, for a single spin-l/2 eigenstate, the expectation value of the magnetization remains either parallel or antiparallel to B, while vanishing in the transverse plane. Larmor precession of the sort illustrated in Fig. 1.3 is possible only when states with m = Y2 and m = -l/2 are superposed, and for that the angle 8 depends primarily on the relative difference in population. 8 may assume any value between 0 and JCnot just ‘up’ or ‘down’. This confusion between stationary states (100% spin polarization) and timedependent superpositions has arisen before in other elementary books on NMR. Another troublesome part of the present book is the explanation on p. 47 of why the scalar coupling vanishes under heteronuclear decoupling - an explanation that improperly treats the role of the Hamiltonian in the dynamics and that is highly misleading at best. But, overall, these should be regarded as small flaws in what generally appears to be a well researched text. M. MUNOWITZ M. Munowitz is withAmoco Technology Company, Naperville, IL 60566, U.S. A.
trends in amlytical chemistry, vol. 8, no. 3,1989
books === A well-researched text 13 NMR Spectroscopy, by H. Kalinowski, S. Berger, and S. Braun, Wiley, 1988, f 75.00 (xvi + 776 pages) ISBN O-471-91306-j
This large volume is an English translation of ‘3C-NMR-Spektroskopie, originally published in German in 1984 by Georg Thieme Verlag. The book is addressed to organic chemists and others interested in applications of 13CNMR to complex organic and biochemical systems, and it should serve well both as a reference and text. Of greatest value, perhaps, is the extensive compilation and discussion of chemical shifts and coupling constants, which amounts to over 75% of the printed pages. Supplementing the abundant tables of data are ample references to the original literature, provided not only in extensive numbered lists by chapter but in separate bibliographies for each subsection as well. There is also an index of compounds with over 3000 entries and a general bibliography at the end. Beyond merely tabulating the data, however, the authors explain the various rules and trends from the perspective of the organic chemist, and briefly describe the physical origins of chemical shielding and spin-spin coupling. For this latter purpose they use some results from elementary quantum mechanics and molecular orbital theory. The book is introduced by two chapters dealing with fundamental principles and techniques of Fourier transform NMR. The authors begin with the quantization of nuclear spin and the basics of resonance, go on to discuss NMR instrumentation and the mathematics of the numerical Fourier transform, and finish the introduction by surveying various experimental methods. Among these are many recently developed techniques such as polarization transfer,
double-quantum filtering, and twodimensional spectroscopy. In addition to these chapters and the two extended chapters on chemical shifts and J couplings, there are briefer discussions of spin-lattice relaxation, the nuclear Overhauser effect, dynamic exchange, lanthanide shift’reagents, and techniques for studying reaction mechanisms, This book is concerned mostly with applications of i3C NMR, so perhaps it is not surprising that its explanations of why certain techniques work are sometimes less than satisfying. Where the workings of a particular method are discussed, the picture invariably used is the semiclassical vector model of the nuclear magnetization. Usually the explanations are accurate and should satisfy the reader mostly interested in applications, but still there can be problems. First, descriptions of multipulse experiments by such pictures tend to become a bit labored even when essentially correct. Second, there are many instances when the simple model fails -with coupled spins, for example, and most notably with multiple-quantum coherence. Thus a reader is left without any real understanding of why the ‘INADEQUATE‘ method works, or even an indication of what double-quantum coherence is and why it has the properties it does. Third, unless there is an honest assessment of the limitations of the vector magnetization picture, a student may find it hard at some later time to accept more involved descriptions of modern NMR experiments. The danger is that a novice may assume that all resonance phenomena are reducible to the simplest model and that any embellishments are probably unnecessary. A book of this sort need not and should not treat the quantum mechanics of NMR in detail, but it can go further in pointing out where and why the semiclassical interpretation
of the magnetization fails. Pedagogical problems of this sort surely will increase as more and more two-dimensional and multiple-quantum techniques become routine in 13C spectroscopy. These comments nothwithstanding, one must remember that such topics constitute only a small portion of the book and that, in general, the chapters dealing with mathematics, quantum mechanics, and pulse sequences do serve their purpose. Though sometimes brief and without elaboration, these sections clearly give the reader some idea of what is involved and where to look for more information. There are, however, a few mistakes. On p. 7, for example, the authors claim that the spin of a nucleus with well-defined quantum numbers I and m precesses about an external magnetic field B,, maintaining an angle 8 = cos-l [mlVI(I + l)] with respect to B,. Yet, for a single spin-l/2 eigenstate, the expectation value of the magnetization remains either parallel or antiparallel to B, while vanishing in the transverse plane. Larmor precession of the sort illustrated in Fig. 1.3 is possible only when states with m = Y2 and m = -l/2 are superposed, and for that the angle 8 depends primarily on the relative difference in population. 8 may assume any value between 0 and JCnot just ‘up’ or ‘down’. This confusion between stationary states (100% spin polarization) and timedependent superpositions has arisen before in other elementary books on NMR. Another troublesome part of the present book is the explanation on p. 47 of why the scalar coupling vanishes under heteronuclear decoupling - an explanation that improperly treats the role of the Hamiltonian in the dynamics and that is highly misleading at best. But, overall, these should be regarded as small flaws in what generally appears to be a well researched text. M. MUNOWITZ M. Munowitz is withAmoco Technology Company, Naperville, IL 60566, U.S. A.