ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
We employ laser double resonance techniques to measure the rates of gas-phase collisional deactivation of vibrationally excited HF by NO molecules, in order to compare the relaxation efficiency of this free radical species with CO and other closed-shell molecules whose vibrational dynamics are well known. Although the near-resonant energy gaps for HF collisional energy transfer are less favorable for NO than for CO, we find that NO is as much as an order of magnitude more efficient than CO in relaxation HF vibrations. Since the NO and CO collision partners have rather similar dipole moments (0.153 vs 0.112 D), rotational constants (1.17 vs 1.93 cm−1), and molecular weights, the disparity in vibrational relaxation efficiency may come from chemical factors, particularly the open- vs closed-shell electronic character, associated with long-range interactions. Ab initio calculations and natural bond orbital (NBO) analysis of the structure and energetics of NO:HF and CO:HF complexes indicate that the NO monomer is better able to form effective n→σ* donor–acceptor H bonds to HF over a wide range of nonlinear "acceptance angles.'' Compared to CO, NO presents a significantly attractive potential to HF over an appreciably wider range of collision orientations, leading to structures in which HF stretching couples to other internal modes of the transient complex and vibrational excitation is efficiently quenched. Our results strongly suggest the important role that chemical factor can play in the dynamics of fast vibrational relaxation processes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.457076
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