Multiple intermolecular bend vibrational excitation of a hydrogen bond: An extended infrared study of OCOHF

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Abstract
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 warming of the slit jet expansion to 16 K, we observe bending ‘‘hot band’’ transitions built on the fundamental vHF=1←0 HF stretch from the lowest five internally excited bending states (i.e., vlbend=00←00, 11←11, 20←20, 22←22, and 31←31) which correspond to low-frequency, skeletal bending of the intermolecular hydrogen bond. In addition, much weaker parallel (Δl=0) combination band transitions (vlbend=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 00←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 (vHF=1) and ground-state (vHF=0) complexes. The bending frequencies are surprisingly low (∼10 cm−1 ) and exhibit positive anharmonicity (i.e., the energy level spacings increase with vlbend ). The results suggest nearly unrestricted bending of the hydrogen bond in a very flat, highly anharmonic angular potential. In contrast with many other weakly bound complexes, the lowest bending frequency decreases dramatically upon HF excitation, which signals a vHF vibrationally induced shift from a linear to a nonlinear equilibrium geometry for the vHF=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 (vlbend=00) to 650 ps (vlbend=31) as a function of intermolecular bending excitation.