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Development of Atom-Atom Pair Potentials for Fluorine-Substituted Benzene Clusters. Approximately 1700 single point energy calculations were carried out on cluster dimers at the MP2/6-31+g (2d, p) level of theory. Systems specifically evaluated in this work include: neat benzene (B₂), fluorobenzene (F₂), and 1,3,5-triflourobenzene (T₂); and mixed BF, BT, and FT dimers. Several initial dimer structures were generated on the basis of published nearest-neighbor configurations of optimized (C₆H₆)₁₃ and (C₆H₆)(C₆H₁₂)_n clusters [J. Phys. Chem. A 2003, 107, 7733; 2005, 109, 673). Each of the initial configurations was systematically modified by adjusting each of the relevant variables, one at a time, generating a library of dimer structures. The resulting structures and their calculated MP2 single-point energies were analyzed via nonlinear regression to obtain the optimized atom-atom pair parameter set for a 12-9-6-4-1 potential.

Minimum-Energy Dimer Structures. Optimized 12-9-6-4-1 parameters were used to identify low temperature minimum-energy structures of each dimer type via a Monte Carlo Simulated Annealing algorithm. Preliminary results indicate that all dimers have parallel-displaced structures. Because the neat benzene dimer is widely believed to have a different (T-shaped) structure, we are undertaking further analysis of our set of potential energy parameters. To this end we have begun a series of MP2 optimization and frequency calculations, the results of which will be directly compared to predicted Monte Carlo structures. Results of the ongoing MP2 frequency calculations will also be used to interpret the experimental ultraviolet spectra of neat fluorobenzene and mixed benzene-fluorobenzene dimers and trimers, described below.

Ultraviolet Spectroscopy of neat fluorobenzene and mixed benzene-fluorobenzene clusters. Neat fluorobenzene (F_n) and mixed benzene-fluorobenzene (B_mF_n) clusters were interrogated via resonant two photon ionization (R2PI) spectroscopy. One-color spectra were collected through fluorobenzene's B₂ ← A₁ 0⁰₀ vibronic transition at 37813 cm^-1 over a spectral shift ranging from -200 cm^-1 to +80 cm^-1 (relative to 37813 cm^-1). For all dimers and trimers, the R2PI spectra reveal reproducible sharp spectral features. Spectra of F, F₂, and F₃ are shown below. Preliminary analysis suggests the possibility of four different van der Waals progressions in the fluorobenzene dimer spectrum, with fundamental frequencies of 4.1, 6.1, 14.6, and 16.2 cm^-1 respectively. If the preliminary analysis is correct, it is possible that the sharp features in the ultraviolet spectrum may originate from two distinct F₂ isomers, each having two separate van der Waal's modes with transitions near 5 and 15 cm^-1 respectively. This hypothesis is under continuing investigation via MP2 calculations that will identify optimized local-minimum isomeric structures and the corresponding van der Waals frequency fundamentals.