........ published in NEWSLETTER # 50
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........ published in NEWSLETTER # 50
STRUCTURE AND CONFORMATIONS OF NON_RIGID MOLECULES
by Professor J. Laane, Texas A&M University, College Station/TX (U.S.A.)
For more than half a century spectroscopy and other methods have been available to characterize the structures and shapes of molecules, particularly those that are rigid. However, most molecules are at least to some degree non_rigid and this non_rigidity plays an important role in such diverse areas as biological activity, energy transfer, and chemical reactivity. In addition, the large_amplitude vibrations present in non_rigid molecules give rise to unusual low_energy vibrational level patterns which have a dramatic effect on the thermodynamic properties of these systems. Only in recent years has a coherent picture of the energetics and dynamics of the conformational changes inherent in non_rigid (and semi_rigid) molecules begun to emerge. Advances have been made in a number of different experimenal areas: vibrational (infrared and Raman) spectroscopy, rotational (microwave) spectroscopy, electron diffraction, and, most recently, laser techniques probing both the ground and excited electronic states. Theoretically, the proliferation of powerful computers coupled with scientific insight has allowed both empirical and ab initio methods to increase our understanding of the forces responsible for the structures and energies of non_rigid systems. The development of theory (group theoretical methods and potential energy surfaces) to understand the unique characteristics of the spectra of these floppy molecules has also been necessary to reach our present level of understanding.
Since the research on non_rigid molecules has involved many diffeent specialized areas, the leading scientists doing the work have seldom had the opportunity to meet and discuss the ramifications of their investigations and to incorporate the findings from other disciplines. In September of 1992 a NATO Advanced Research Workshop (ARW) on `Structures and Conforma on Non_Rigid Molecules' was held at Castle Reisensburg and at the University of Ulm in Germany. The workshop brought together leading experts from the various areas focusing on non_rigid molecules.
The thirty chapters in the resulting volume (NATO ASI SERIES C410) contributed by key speakers at the Workshop are divided among various different areas. Both vibrational and rotational spectroscopy have been effective in determining the potential energy surfaces for non_rigid molecules, often in a complementary manner. Recent laser fluorescence work has extended these types of studies to electronic excited states. Electronic diffraction methods provide radial distribution functions from which both molecular structures and compositions of conformational mixtures can be found. Ab initio calculations have progressed substantially over the past few years, and, when carried out at a sufficiently high level, can accurately reproduce (or predict ahead of time) experimental findings. Much of the controversy in the ARW related to the question of when an ab initio calculation is reliable. Clearly, low level calculations can produce meaningless results. Since the computer programs are readily available many poor calculations have been carried out. However, excellent results can be obtained from computations when properly done. A similar situation exists for experimental analyses. Careful consideration of vibrational models, selection rules, mean vibrational amplitudes, and spectroscopic and quantum theory must be applied in order to correctly interpret the data. The complexities of non_rigid molecules are many, but major strides have been taken to understand their structures and conformational processes.
Reference books: C182, C406, C410