MULTIPLE INTERMOLECULAR BEND VIBRATIONAL-EXCITATION OF A HYDROGEN-BOND - AN EXTENDED INFRARED STUDY OF OCOHF

We report the use of near infrared tunable difference frequency laser absorption methods to investigate low-frequency bending of the intermolecular hydrogen bond in OCOHF complexes. By deliberate thermal wanning of the slit jet expansion to 16 K, we observe bending "hot band" transitions built on the fundamental υHF = 1←0 HF stretch from the lowest five internally excited bending states (i.e., υbendl = 00←00, 11←-11, 2 0←-20, 22←22, and 3 1←31) which correspond to low-frequency, skeletal bending of the intermolecular hydrogen bond. In addition, much weaker parallel (Δl = 0) combination band transitions (υbendl = 20←00 and 31←11) are observed at ≲5% of the 00←00 intensity. Furthermore, measurements of the extremely weak 11←00 perpendicular (Δl = 1) band are obtained at ≲1% of the 0 0←00 intensity. The fundamental, hot band, and combination band data permit quantitative measurement of the absolute vibrational energies of all vibrational levels for the l = 0 and 1 manifolds in both HF excited (υHF = 1) and ground-state (υHF = 0) complexes. The bending frequencies are surprisingly low (∼10 cm -1) and exhibit positive anharmonicity (i.e., the energy level spacings increase with υbendl). The results suggest nearly unrestricted bending of the hydrogen bond in a very flat, highly enharmonic angular potential. In contrast with many other weakly bound complexes, the lowest bending frequency decreases dramatically upon HF excitation, which signals a υHF vibrationally induced shift from a linear to a nonlinear equilibrium geometry for the υHF = 1 excited OCOHF potential surface. Excess Lorentzian line widths are observed in all OCOHF transitions, attributable to vibrational predissociation lifetimes that vary smoothly from 1.2 ns (υbendl = 0 0) to 650 ps (υbendl = 31) as a function of intermolecular bending excitation. © 1990 American Institute of Physics.