Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect

Abstract

Intriguing vibrational features of solvated protonated methanol between 2400−3800 cm −1 are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initio anharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl−hydroxyl rocking modes and their participation in FR.

Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations

“…However, owing to the non-negligible magnitude of coupling constants, these overtone and combination tone states become visible in the spectra through effective intensity borrowing from the stretching fundamental states which carry the major oscillator strength. 38,40,48…”

“…However, owing to the non-negligible magnitude of coupling constants, these overtone and combination tone states become visible in the spectra through effective intensity borrowing from the stretching fundamental states which carry the major oscillator strength. 38,40,48 We have found that the anharmonic couplings are intrinsic properties of these cations. The solvation environments would alter the magnitudes of these FR couplings but to a limited extent.…”

“…A similar feature has been reported for protonated methanol clusters. 48 It is also worth mentioning that while the methyl CH stretching modes are always found to carry insignificant IR intensities compared to the NH stretching modes in the protonated amine species, 42,[53][54][55] such rockingmode-related bands show that the methyl groups are not outsiders in the IR spectra. Some related questions would arise: can the bright states light up any dark states nearby?…”

“…In addition to the expected FR bands associated with specific generalized bending modes like NH 2 scissoring and NH 3 umbrella, we found that the alkyl backbone can also participate in the coupling mechanism through CH–NH rocking modes and generates additional minor FR bands. 40,48…”

“…In addition to the expected FR bands associated with specific generalized bending modes like NH 2 scissoring and NH 3 umbrella, we found that the alkyl backbone can also participate in the coupling mechanism through CH-NH rocking modes and generates additional minor FR bands. 40,48 If different cluster systems present similar spectral features of the H-bonded bands, it implies that these systems may have very similar vibrational coupling schemes. 40,49 In this work, we applied the ab initio anharmonic algorithm to simulate the anharmonic vibrational spectra of ammonium and aminium ions solvated by multiple water molecules and bisulfate ions.…”