Molecular dynamics in hydrogen‐bonded interactions: A preliminary experimentally determined harmonic stretching force field for HCN‐‐‐HF

Observation of the 2ν₁ overtone band in the hydrogen‐bonded complex HCN‐‐‐HF permits evaluation of the anharmonicity constant X₁₁=−116.9(1) cm⁻¹ and determination of the anharmonicity corrected fundamental frequency ω₁. This information, and available data from previous rovibrational analyses in the common and perdeuterated isotopic species of HCN‐‐‐HF, offer an opportunity for calculation of an approximate stretching harmonic force field. With the assumptions f₁₂=f₂₄=0.0, the remaining force constants (in mdyn/Å) are evaluated as: f₁₁=8.600(20), f₂₂=6.228(9), f₃₃=19.115(40), f₄₄=0.2413(39), f₁₃=0.000(13), f₁₄=0.0343(2), f₂₃=−0.211(6), f₃₄=0.000(2). These compare to f₁₁=9.658(2) in the HF monomer and f₁₁=6.244(3) and f₃₃=18.707(16) in the HCN monomer. These results provide the information necessary to quantitatively assess the applicability of the Cummings and Wood approximation in this hydrogen‐bonded complex and also give an estimate of D^e_j, the equilibrium distortion constant in the harmonic limit. Comparisons of these experimentally determined parameters with the predictions of ab initio molecular orbital calculations at several levels of approximation are presented.