HYBRID DFT-MD SIMULATIONS OF GEOMETRY AND HYPERFINE-STRUCTURE OF THE CCH RADICAL IN ARGON MATRICES AT LOW-TEMPERATURES

The thermal motion of the CCH radical embedded in a matrix of solid ar gon is simulated at 4 and 40 K, using a hybrid density functional theo ry-molecular dynamics (DFT-MD) approach. The DFT calculations are perf ormed at the B3LYP/6-311G(d,p) level. It is concluded that the CCH mol ecule when embedded in the Ar matrix favors an oscillating, slightly b ent geometric structure, whereas in vacuum the molecule is linear. In the matrix at 4 K, the oscillations lie centered at a CCH bond angle o f 170+/-5 degrees. At 40 K far larger oscillations are noted (up to +/ -19 degrees bending motion, centered at a 154 degrees CCH angle), due to the increased thermal energy. As a consequence of the vibrational m otion, the radical hyperfine structure becomes significantly modified, and agree far better with experimental data than do the linear optimi zed vacuum geometry results. The B3LYP/6-311G(d,p) computed vibrationa lly averaged isotropic couplings in an ordered Ar matrix at 4 K are 93 5, 173 and 42 MKz for C-C-H, respectively, to be compared with the exp erimental values (Ar matrix, 4 K) 902, 156 and 44 MHz, and the data fo r the B3LYP/6-311G(d,p) optimized linear structure in vacuum: 1043, 22 4 and 54 MHz. The present hybrid DPT-MD results also agree well with p revious vibronically corrected MRDCI data. (C) 1996 American Institute of Physics.