Probing Potential Surfaces for Hydrogen Bonding: Near-Infrared Combination Band Spectroscopy of Van Der Waals Stretch (Ņ4) and Geared Bend (Ņ5) Vibrations in (HCl)2
M Farnik, S Davis, M D. Schuder, David Nesbitt
High resolution infrared spectra of the two low frequency intermolecular modes - van der Waals stretch (Ņ4) and geared bend (Ņ4) and geared bend (Ņ5) of (HCI)2 - have been characterized in HCI-stretch excited states using a slit jet spectrometer. In a high resolution high sensitivity search coveringthe range between 2880 cm-1 and 3070 cm-1 four (HCl)2 combination bands associated with in-plane vibrations Ņ4, and Ņ5 have been observed. The vibrational assignment of these bands is based on comparison between observed intermolecular mode energies and predictions from recent6D QM calculations, though additional confirmation is provided by ancillary spectroscopic information such as rotational constants, predissociation linewidths, and 35CI/37Cl isotopic band shifts. The experimentally observed intermolecular energies agree with theoretical predictions to = 2-4 cm-1 out of 60-90 cm-1 out of 60-90 cm-1, suggesting that the 6D PES can describe combination band excitation in these lower frequency intermolecular coordinates fairly well. Three of the four observed combination bands arise from the upper tunneling level (B*), and all four bands are built exclusively on bound HCI stretch (Ņ2). To account for these striking intensity anomalies, a simple model for 3D QM calculation of transition moments is introduced, which correctly reproduces the experimental trends. In this model, the propensity for Ņ2 based combination bands arising from upper tunneling levels can be successfully ascribed to the unusually floppy nature of the intermolecular vibrations, whichresults in a harmonic oscillator δŅ=+1 propensity for excitations between tunneling levels along the geared bend coordinate.
Journal of Chemical Physics
combination band spectroscopy, hydrogen bonding, low-frequency intermolecular modes
, Davis, S.
, Schuder, M.
and Nesbitt, D.
Probing Potential Surfaces for Hydrogen Bonding: Near-Infrared Combination Band Spectroscopy of Van Der Waals Stretch (Ņ<sub>4</sub>) and Geared Bend (Ņ<sub>5</sub>) Vibrations in (HCl)<sub>2</sub>, Journal of Chemical Physics
(Accessed June 9, 2023)