Vibration Rotational Spectroscopy And Molecular Dynamics

The book reviews the results of vibration-rotational spectroscopy of molecules obtained recently by combining modern computational methods of quantum chemistry with the new techniques of high-resolution rotational and vibration-rotational spectroscopy. It shows for example that the tunneling vibration-rotational spectroscopy of the van der Waals complexes provides a new look at intermolecular forces while the high precision and sensitivity of the submillimeter-wave and Fourier transform microwave spectroscopy make it possible to study complex rotational spectra of molecules in excited vibrational states. New results of high level ab initio quantum chemical computations of vibrational and rotational energy levels and dipole moment functions of unusual molecules will be discussed together with the recent discovery of clustering of energy levels in asymmetric tops. Group theoretical analysis of floppy molecules, especially the tunneling effects in nonrigid molecules, will also be discussed.

Vibrational Dynamics of Molecules represents the definitive concise text on the cutting-edge field of vibrational molecular chemistry. The chapter contributors are a Who's Who of world leaders in the field. The editor, Joel Bowman, is widely considered as one of the founding fathers of theoretical reaction dynamics. The included topics span the field, from fundamental theory such as collocation methods and vibrational CI methods, to interesting applications such as astrochemistry, supramolecular systems and virtual computational spectroscopy. This is a useful reference for theoretical chemists, spectroscopists, physicists, undergraduate and graduate students, lecturers and software developers.

Since the first stimulated emission pumping (SEP) experiments more than a decade ago, this technique has proven powerful for studying vibrationally excited molecules. SEP is now widely used by increasing numbers of research groups to investigate fundamental problems in spectroscopy, intramolecular dynamics, intermolecular interactions, and even reactions. SEP provides rotationally pre-selected spectra of vibrationally highly excited molecules undergoing large amplitude motions. A unique feature of SEP is the ability to access systematically a wide variety of extreme excitations localized in various parts of a molecule, and to prepare populations in specific, high vibrational levels. SEP has made it possible to ask and answer specific questions about intramolecular vibrational redistribution and the role of vibrational excitation in chemical reactions.

The pure rotational and vibrational-rotational absorption bands for a diatomic are calculated directly from classical molecular dynamics, classical linear response theory and classical statistical mechanical ensemble averaging with the use of simple quantum corrections. The experimental spectral band intensities and contours are well reproduced for CO from dilute gas phase through solution in compressed Ar to solution in liquid Ar by these 'Newtonian' classical spectral calculations. The typical evolution seen in vibrational spectra from multiple-peaked gas phase bands to single-peaked solution bands is observed. The 'Newtonian' gas phase calculations also match quantum and correspondence principle classical spectral calculations. This molecular dynamic approach may be applied to compute the spectra of complex molecules or liquids for which a normal mode analysis may be impractical, and may also be extended to nonequilibrium systems, for example to compute transient vibrational spectra during chemical reactions. (Author).

This work had its beginnings in the early 1980s at the University ofWollongong, with significant contributions from Dr. Margret Hamilton, Professors Peter G. Burton and Greg Doherty. The emphasis was to develop computer code to solve the nuclear Schrodinger problem. For bent triatomic molecules the project was fmally realized at the University of Newcastle a decade or so later, with the contribution from Ms. Feng Wan g. Aspects of this work are now taught in the quantum mechanics and electron spectroscopy courses at The University of Newcastle. Even now "complete" ab initio solutions of the time-independent SchrOdinger equation is not commonplace for molecules containing four atoms or more. In fact, when using the Eckart-Watson nuclear Hamiltonian a further restriction needs to be imposed; that is, the molecule is restricted to undergoing small amplitudes of vibration. This Hamiltonian is useful for molecules containing massive nuclei and moreover, has been extremely useful in interpreting the rovibrational spectra of small molecules. Nevertheless, a number of nuclear Hamiltonians that do not embed an equilibrium geometry have become well established and are extremely successful in interpreting rovibrational spectra of floppy molecules. Furthermore, solution algorithms vary greatly from research group to research group and it is still unclear which aspects will survive the next decade. For example, even for a triatomic molecule a general form of a potential function has not yet been uncovered that will generally interpolate with accuracy and precision ab initio discrete surfaces.

The definitive text on the rotational spectroscopy of diatomic molecules.

Due to the sensitivity of vibrational chromophores to their local environments, linear and ultrafast vibrational spectroscopy have proven to be very useful techniques for studying the structure and dynamics of condensed phases. Because spectroscopic techniques encode information related to the time-dependent configuration of an entire system into spectra resolved over at most a few dimensions, however, it is very difficult to interpret vibrational line shapes in a detailed and unambiguous manner. One approach to surmounting this difficulty is to calculate vibrational line shapes from molecular dynamics (MD) simulations by employing vibrational response theory and spectroscopic maps. (The maps relate observables in classical MD simulations to quantum spectroscopic quantities.) Once validated by comparison of experimental and theoretical line shapes, MD simulations can be used as an unequivocal basis for the interpretation of vibrational spectra. Here, we employ this approach in order to gain insight into small-molecule solutions and proteins. After sketching the theoretical formalism underlying the calculations of vibrational spectra (Chapter 2), vibrational spectroscopic analysis of the urea/water (Chapter 3) and cyanide/water (Chapter 4) solutions is presented. Analysis of linear infrared (IR) line shapes provides information concerning the local solvation structure of these molecules, while analysis of two-dimensional IR and anisotropy decay yields insight into frequency and rotational dynamics. The remainder of this work concerns the vibrational spectroscopy of the amide I (mostly CO-stretch) band of proteins. After presenting additional theoretical formalism and maps for protein spectroscopy (Chapter 5), the maps are evaluated by examining IR spectra for a single conformation of an alpha-helical model peptide in the gas phase (Chapter 6). These methods are then applied to evaluate the 2D IR spectra of two important biological systems: polyglutamine (Chapter 7) and the potassium ion channel KcsA (Chapter 8). Notably, these studies employ isotope-labeling techniques to isolate the vibrational response of a subset of amide I modes in a non-perturbative fashion. Finally, extensions to the theory are presented to enable the computation of amide I vibrational sum-frequency generation spectra (Chapter 9), which are expected to be sensitive to the structures of interfacial proteins.