Near-infrared spectroscopy of micro-solvated protonated methanol.

Gas phase near-infrared (NIR) spectra of micro-solvated protonated methanol, MeOH2+⋯Xn (X = Ar, N2, and CO; n = 1 and 2), in the region of the first overtones of OH stretches were obtained via infrared photodissociation spectroscopy. Spectral details were analyzed with the support of high-precision ab initio anharmonic simulations. The free OH stretching overtones and combination bands of OH stretch and bend are the main features in this region. The hydrogen-bonded OH stretching overtone transitions exhibit weak and broad characteristics across all solvents (X). This trend increases with the proton affinity of X. Hot bands, intramolecular vibrational energy redistribution, and band congestion due to strong anharmonic couplings are proposed to have played crucial roles in this observation. Meanwhile, perturbations from low frequency modes, such as CH/OH rocking, CH bending, and solvent intramolecular vibrations, should not be overlooked in this cluster system. Our anharmonic algorithm has demonstrated its potential in reproducing precise NIR spectra, and the present system can serve as a benchmark for theoretical anharmonic computations in the NIR region.