Colorado State University

The properties and behavior of isolated molecules are major themes of chemistry; nonetheless, most chemistry occurs in dense, crowded, condensed phases and thus, intermolecular interactions are an ever-present factor for chemistry. Our research focuses on the study of intermolecular interactions and their effects on molecular properties and behavior. Our investigations include both experimental and theoretical exploration of species isolated in the gas phase and solvated in small clusters by other molecules. The experiments center around various laser optical spectroscopy and mass spectroscopy techniques. The theoretical investigations encompass both ab initio and density functional theory quantum chemistry approaches and empirical potential energy calculations on isolated species and small clusters, as appropriate. Given this general area of interest, we will address a number of specific topics over the next few years: the properties of non-rigid molecules and the effects of solvation on the structure, energy levels, and dynamics of these molecules; generation and study of reactive intermediates with specific interest in solvation, dynamics, and reactions; and neutral metal oxide and nitride cluster catalytic reactions in the gas phase. All these species can be generated in a supersonic expansion and isolated and cooled so that they can be studied spectroscopically without the perturbation of any intermolecular interactions. Molecules are placed into the expansion by heating or laser ablation. Radicals are generated in the expansion by pyrolysis or photolysis of appropriate precursors, and metal oxide and nitride clusters are generated by laser ablation and reaction. Clustering occurs in the expansion as appropriate species are added to the expansion gas. Once the systems have been created in the supersonic expansion, we can use laser induced fluorescence, dispersed emission, IR/UV and UV/UV double resonance, mass-resolved excitation, photoelectron spectroscopy and time-resolved spectroscopies to probe their properties and behavior. The non-rigid molecule phenethylamine (C₆H₅CH₂CH₂ NH₂) is an important model neurotransmitter, and we have shown that it has five low energy conformations in isolation in the gas phase. These conformers arise from alkylamine chain conformations and amino group spatial orientations. Current studies of clusters show that not all conformations are sustained under solvation conditions. Continued research in this area is underway with neurotransmitters such as amphetamine, serotonin, dopamine, histamine, adrenaline, noradrenaline, and saccharides. Radical reactions in isolated clusters are of fundamental interest because one can study reactivity under “entrance channel” control of geometry, kinetics, and energy. We have investigated photo-initiated, excited state radical addition reactions and radical hydrogen abstraction reactions thus far. Methyl and benzyl radical addition to ethylene has been studied by optical spectroscopy and ab initio calculations. We know these reactions occur for the electronically excited radicals, and the potential energy surfaces for the reactions have been explored at a high level of ab initio theory. Presently, we are investigating reactions of small radicals (e.g., CH₃, CH₃O, NCO, NO₃) with hydrogen containing small molecules to obtain product radicals (e.g., OH, CH₃O, CN, CH₃) that can be detected with state and time resolution. We are also pursuing IR/UV double resonance spectroscopy for the peroxy radicals CH₃OO and CH₃(O)OO. We have found that covariance mapping is a powerful technique for the study of cluster growth and fragmentation. This statistical approach to data analysis allows one to determine correlations between different features in a spectrum and to determine how such features are related. Thus, we can evaluate if species of clusters are related to one another by growth or fragmentation. We have determined the kinetics and mechanisms for the generation of cluster distributions of aniline/argon and fluorostyrene/argon in a supersonic expansion. We presently are investigating the kinetics and mechanisms of growth and fragmentation of metal oxide and nitride clusters generated in a supersonic expansion employing different lasers (193, 118, 46.5 nm) for neutral cluster ionization. The chemistry explored with these characterized neutral clusters includes CO/NO, CO₂/N₂, SO₂/SO₃, H₂O/H₂ + O₂, and CO/H₂/CH₃OH. We thus are studying the effects of intermolecular interactions on molecular structure, dynamics, and reactions at many different levels and on a number of different systems. In the process, we learn about molecular properties and behavior under isolated and solvated conditions and what are the essential features of intra- and intermolecular effects in chemistry. Clearly our research has both fundamental and practical components.