........ published in NEWSLETTER # 49
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........ published in NEWSLETTER # 49
RECENT EXPERIMENTAL AND COMPUTATIONAL ADVANCES IN MOLECULAR SPECTROSCOPY
by Professor R. Fausto, University, Coimbra (Portugal) and Dr. J.M. Hollas, University, Reading (U.K.)
The related fields of molecular spectroscopy and ab initio calculations have been the subject of many recent and spectacular advances. In molecular spectroscopy one such advance is in the use of a supersonic jet to produce extremely cold molecules. This technique has opened up many branches of molecular spectroscopy to the study of the structures of very much larger molecules than was previously possible, for example molecules of biological importance.
In parallel with these experimental advances there have been developments in computer technology which have made possible ab initio calculations relating to the properties of molecules which can rival experiments in the level of accuracy which can be achieved.
The NATO Advanced Study Institute, held in Ponta Delgada, in the Azores, in August/September 1992, brought together these two approaches to the determination of molecular structures and properties. Before this meeting, many scientists working in these related fields had not been sufficiently aware of important modern developments and advances.
This book (NATO ASI SERIES C406) reflects the interdisciplinary nature of the ASI in chapters relating to lectures given by the invited speakers. On the theoretical side, ab initio methods applied to the protonation of unsaturated organic compounds confirm experimental findings. Frontier orbital theory allows the prediction of chemical reactivity. The energy, properties and reactivity of molecules in excited electronic states can now be calculated using the configuration interaction `singles' method. The strengths and weaknesses of semiempirical methods are now more apparent. Molecular properties can also be calculated by the novel method of atoms in molecules which relates the dynamics of the molecule to the contributions from each consistent atom. The consistent force field method of calculation is particularly suited to conformational analysis of flexible molecules.
The vibrational energy levels of small molecules in their ground electronic states can be calculated accurately to very high energies using an anharmonic force field. For large molecules, such as pyrimidine bases, which are important in the genetic code, vibrational spectra can now be calculated by ab initio methods. The results have been shown to compare well with matrix_isolation spectra.
The conformation and geometry of quite large organic molecules can be determined from their electronic spectra either in the gas phase or a supersonic jet. Two approaches, from the rotational fine structure or the vibrational coarse structure, have proved very powerful. Raman spectroscopy can also provide information on molecular conformation for which ab initio calculations give a valuable comparison. Both infrared and Raman spectroscopy are also used to probe the structures of polyconjugated polymers. Pressure_tuning infrared and Raman spectroscopy is a powerful tool for studying structural changes at the molecular level in human diseases. The resolution in vibrational spectra can be improved by Fourier deconvolution or Fourier derivation. Neutron and electron diffraction are powerful structural techniques while surface spectroscopies probe the behaviour of adsorbed molecules. Molecular size presents no barrier to using NMR spectroscopy for structural studies.
This book covers all these topics in detail. It provides an excellent, up_to_date and timely overview of computational and experimental methods and results. It will be useful to theoreticians who are out of touch with recent experimental results and to experimentalists who are unaware of the rapid advances being made by theoreticians.
Reference books: C234, C406, C410