Structures and Conformations of Non-Rigid Molecules

From the beginnings of modern chemistry, molecular structure has been a lively area of research and speculation. 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 experimental 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. The thirty chapters in this volume contributed by the key speakers at the Workshop are divided over the various areas. Both vibrational and rotational spectroscopy have been effective at 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 of the ARW related to the question of when an ab initio is reliable. 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. The complexities of non-rigid molecules are many, but major strides have been taken to understand their structures and conformational processes.

Inhalt

Theory.- 1. The Symmetry Groups and Rotation-Vibration Energy Levels of Some Very Nonrigid Molecules.- 2. Vibration-Inversion-Torsion-Rotation Energy Levels and Vibrational Resonances in the Methylamine Molecules.- 3. The Excited State Dynamics of Conjugated Molecules: Theoretical Studies of Slow Vibrations.- Vibrational Spectroscopy.- 4. Vibrational Potential Energy Surfaces of Non-Rigid Molecules in Ground and Excited Electronic States.- 5. Structural Information on Large Amplitude Motions.- 6. Conformational Stability of Five-Membered Ring Molecules.- 7. The Structure and Dynamics of Van Der Waals Molecules.- 8. Applications of Asymmetric Top Contour Simulation to Conformational Equilibria.- Microwave Spectroscopy.- 9. Large Amplitude Motions in Two Ring Molecules.- 10. Rotational Studies of Four-and Five-Membered Ring Molecules.- 11. Large Amplitude Motions in Molecular Complexes.- 12. Accurate Structures of Non-Rigid Molecules by Microwave Spectroscopy.- 13. Tunneling Motions in Sulfur Dioxide Complexes.- 14. Recent Gas-Phase Studies of Intermolecular Hydrogen Bonding.- 15. Millimeter- and Submillimeter-Wave Spectroscopy of Non-Rigid Transient Molecules: Analysis of Silane and Acetylene Plasmas.- Ab Initio Calculations.- 16. The Cis Monobridged Equilibrium Geometries of Si2H2, Ge2H2, Al2H2, and Ga2H2: A Fundamentally New Type of Molecular Structure.- 17. Ab Initio Anharmonic Vibrational Analyses of Non-Rigid Molecules.- 18. The Use of Natural Coordinates in Molecular Geometry Optimizations.- 19. Ab Initio Molecular Orbital Calculations of the Infrared Spectra of Interacting Water Molecules. Complexes of Water With Carbon Dioxide and Nitrous Oxide..- 20. Large Amplitude Motion In Oxalyl Chloride.- Electron Diffraction.- 21. Gas-Phase electron Diffraction Applied to Molecules Undergoing Large-Amplitude Motion.- 22. Conformational Analysis By Gas Electron Diffraction.- 23. Linear, Bent, and Quasilinear Molecules.- 24. Structures and Conformational Analysis of Monosubstituted Cyclopropanes of the Type: c-Pr-XY and c-Pr-CH -XY (X = Si,Ge; Y = H,F,Cl).- 25. Small Forces and Large Molecular Distortions.- 26. Gas-Phase Structures and Conformational Properties of Malonic Acid Derivatives, {CY2(COX)2}, as Studied By Electron Diffraction and ab Initio Calculations.- Electronic Excited States, Fluorescence Spectra, and Dynamics.- 27. Molecular Conformations in Electronic Excited States.- 28. A Comparison of the Methyl Torsion and Aldehyde Wagging Dynamics in the Ground and First Excited States of Acetaldehyde and Thioacetaldehyde.- 29. Isolation and Assignment of Organic Molecular Conformers via Laser Induced Fluorescence Spectroscopy of Jet-Cooled Molecular Beams.- 30. The Structure and Dynamics of Van Der Waals Clusters as Studied by Formation of Negative Cluster Ions.