Theoretical & Experimental Studies of Small Molecules in Helium Nanodroplets: Anharmonic Modeling and Infrared Spectroscopy
Peter R. Franke and Gary E. Douberly
Various small, organic molecules of interest to atmospheric and combustion chemistry are interrogated with infrared action spectroscopy and ab inito computations. Molecules are solvated by helium nanodroplets, at temperatures of 0.4 K, and their mid-infrared spectra are measured with species-selective droplet beam depletion spectroscopy. In some cases, multiple cold reactants are brought together in the droplets, and the products are investigated. Infrared spectra in the CH stretching region display rich structure arising from strong couplings between CH stretch fundamentals and HCH bend overtones and combinations. These highly anharmonic spectra are modeled with second-order vibrational perturbation theory with resonances (VPT2+K), based on quartic expansions of the potential energy surface. Effective application of this model is stressed, and various cost-saving approximations for accurate quartic force fields are discussed. Particular attention is given to systems that possess several low-energy rotamers. And in many cases, VPT2+K simulations allow for rotamer-specific assignments. For certain radicals that undergo large amplitude motion (LAM), their average structures are rationalized by solving the vibrational Schrödinger equation in vibrationally adiabatic potential energy surfaces, which describe the effective potential felt by the molecule during the LAM.