Submitted to: CHEMICAL PHYSICS
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/21/2007
Publication Date: 3/12/2008
Citation: Han, J., Heaven, M.C., Schnupf, U., Alexander, M.H. 2008. Experimental and theoretical studies of the CN-Ar van der Waals complex. Journal of Chemical Physics. 128(10):104308. Interpretive Summary: Radicals in gas phase and aqueous solutions play a significant role in radiation chemistry and biology, heterogeneous atmospheric processes, and are often implicated as enzymatic intermediates. However, the understanding of the electronic structure and spectroscopy of even simple radicals is very limited, due to the high reactivity of these radicals. Of interest are the specific effects of the radicals interacting with other compounds before they undergo chemical reactions. It is of high interest to understand how to activate chemicals, e.g. via laser excitations, so they will undergo selective chemical reactions. An excellent case in point is the cyano radical, CN, because of its high importance in combustion and atmospheric chemistry. Our study of the interaction of the CN radical with Argon (Ar) is a continuation of experimental and theoretical studies of a series of simple radicals. In this paper we present a structural and spectroscopic characterization of the CN-Ar interaction complex. These studies are carried out using powerful laser and computer methods. With these cutting-edge experimental and computational tools, we can relate previous information from structural/spectroscopic observations obtained by other researchers to details on the basic structure of CN and Ar-CN. This work has allowed us to better understand the energetics involved to precondition the CN radical via laser excitation to probe the interaction of Ar with CN. These studies will lead to more efficient designs for chemical reactions involving simple radicals used in numerous commercial applications.
Technical Abstract: The CN-Ar van der Waals complex has been observed using B2E+-X2E+ and A2II-X2E+ electronic transitions. The spectra yielded a dissociation energy of D0"=109+2 cm1 and a zero point rational constant of B0"=0.067+0.005 cm-1 for CN(x)-Ar. The dissociation energy for Cn(A)-Ar was found to be D0"=132+2 cm-1. Transitions to vibrationally excited levels of CN(b)-Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces for the X and B states. Ab initio potential energy surfaces were calculated for the X and B states. These were used to predict ro-vibrational energy levels and van der Waals bond energies (D0"=115 and D0"=184 cm-1). The results for the X state were in good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment.