High Resolution Vibration - Rotation - Tunneling Spectroscopy of Weakly Bound Complexes

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In this project in the Physical Chemistry program of the Chemistry Division, Prof. Geoffrey A. Blake of the California Institute of Technology will engage in detailed spectroscopic studies of intermolecular forces as revealed by weakly bound, i.e., van der Waals and hydrogen-bonded, complexes. These studies will encompass experimental, computational, and theoretical investigations. The complexes are formed in supersonic expansions of the component gases into a high vacuum, and the studies will focus on the far infrared region where most of the telltale intermolecular vibrational frequencies of the complexes are located. Hybrid laser spectrometers that operate throughout the microwave, millimeter, and far-infrared regions will be used to obtain the spectra, with resolution appropriate to resolve the rotational, tunneling, and hyperfine structures of the vibrational bands. The experimental efforts are coupled to computational and theoretical studies to extract from the experimental data reliable intermolecular potential energy surfaces. Systems to be investigated include bi-, tri, and higher-order complexes composed of monomer molecules, such as benzene-water, ammonia-water, nitrogen-water, and carbon monoxide-water; argon-water in which the several argon atoms and several water molecules form a complex; dimer complexes composed of water with hygrogen fluoride, hydrogen chloride, hydrogen cyanide, and acetylene; and larger size complexes such as glycine-water and others of biological importance such as those involving amino acids and purines/pyrimidenes. Accurate descriptions of intermolecular forces play a pivotal role in chemistry and molecular biology. Despite long term interests in the weak, i.e., van der Waals and, in particular, hydrogen-bonded, interactions there is a paucity of reliable quantitative data that describe these effects. Traditional methods for obtaining information on the intermolecular interactions were based on the determination of the coefficients in the virial expansion of the equation of state of a gas, pressure broadening of spectral lines, and other classical properties. More recently molecular beam scattering experiments have yielded useful results. The couplings of molecular beam techniques with those of high resolution microwave and laser spectroscopies have provided tools by means of which the most detailed and exact data on weakly bound complexes composed of two or more individually strongly bound monomer molecules are obtained. The investigations pursued in this project will contribute accurate structures, lifetimes and other dynamical parameters of weakly bound molecular complexes as well as detailed quantitative information on their intermolecular potential energies. This information will lead not only to a better understanding about the cohesion of matter in general, but also for those systems that are of practical importance in environmental and biological chemistry.