Quantum Chemical Investigation of the Cold Water Dimer Spectrum in the First OH-Stretching Overtone Region Provides a New Interpretation

Abstract

Intramolecular vibrational transition wavenumbers and intensities were calculated in the fundamental HOH-bending, fundamental OH-stretching, first OH-stretching-HOH-bending combination, and first OH-stretching overtone (Δv OH = 2) regions of the water dimer’s spectrum. Furthermore, the rotational–vibrational spectrum was calculated in the Δv OH = 2 region at 10 K, corresponding to the temperature of the existing jet-expansion experiments. The calculated spectrum was obtained by combining results from a full-dimensional (12D) vibrational and a reduced-dimensional vibrational–rotational-tunneling model. The Δv OH = 2 spectral region is rich in features due to contributions from multiple vibrational–rotational-tunneling sub-bands. Origins of the experimental vibrational bands depend on the assignment of the observed sub-bands. Based on our calculations, we assign the observed sub-bands, and our reassignment leads to new values for the vibrational band origins of the free donor and antisymmetric acceptor OH-stretching first overtones of ∼7227 and ∼7238 cm–1, respectively. The observed bands with origins at 7192.34 and ∼7366 cm–1 are assigned to the symmetric acceptor OH-stretching first overtone and the OH-stretching combination of the donor, respectively.

Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer

“…For floppy systems (systems with low-frequency vibrations) like hydrogen-bound bimolecular complexes, the comparison of cold and warm spectra is complicated and even the interpretation of cold spectra requires advanced ro-vibrational models. − For a bimolecular complex, six low-frequency, large-amplitude intermolecular vibrations are formed upon complex formation. At ambient temperatures, the thermal energy (∼200 cm –1 ) is typically comparable to the energy scale of these intermolecular vibrations, and therefore, many excited vibrational states are populated.…”