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 argon is simulated at 4 and 40 K, using a hybrid density functional theory-molecular dynamics (DFT-MD) approach. The DFT calculations are performed at the B3LYP/6-311G(d,p) level. It is concluded that the CCH molecule when embedded in the Ar matrix favors an oscillating, slightly bent geometric structure, whereas in vacuum the molecule is linear. In the matrix at 4 K, the oscillations lie centered at a CCH bond angle of 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 motion, the radical hyperfine structure becomes significantly modified, and agree far better with experimental data than do the linear optimized vacuum geometry results. The B3LYP/6-311G(d,p) computed vibrationally averaged isotropic couplings in an ordered Ar matrix at 4 K are 935, 173 and 42 MKz for C-C-H, respectively, to be compared with the experimental values (Ar matrix, 4 K) 902, 156 and 44 MHz, and the data for the B3LYP/6-311G(d,p) optimized linear structure in vacuum: 1043, 224 and 54 MHz. The present hybrid DPT-MD results also agree well with previous vibronically corrected MRDCI data. (C) 1996 American Institute of Physics.