Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: New experimental results

Abstract

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.

Geometries and energetics of methanol–ethanol clusters: a VUV laser/time-of-flight mass spectrometry and density functional theory study

“…Not surprisingly the moststable neutral mixed clusters are monocyclic, similar to that found for neat-alcohol clusters (4). These structures are also consistent with recent infrared studies that suggest that the OH groups of the monomers involved in neat methanol and ethanol clusters larger than the dimer are all involved in hydrogen bonding (22)(23)(24). The protonated trimer ions correspond to the most abundant species detected by VUV laser/time-of-flight mass spectrometry.…”

“…The methanol protonated trimer in particular is a "magic" structure, as it can be regarded as a proton with a complete first-solvation shell (11, 26). Infrared spectroscopy reveals that the free and hydrogen-bonded OH stretching frequencies of the neat cluster ions blue-shift with increasing N (22,24). The mixed protonated ions are expected to exhibit a similar spectroscopic signature.…”

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization

“…26 Recently, several new vibrational spectroscopic techniques based on the VUV one-photon ionization detection have been developed and they enable us to apply IR spectroscopy to unstable cations such as the unprotonated cluster cations of the protic molecules. [27][28][29][30][31] We observed the infrared ͑IR͒ spectra of the unprotonated cluster cations of ammonia and methanol as the first examples of those of protic molecules. 29,[31][32][33][34] The structures of the unprotonated cluster cations were determined to be the proton-transferred type, where the dimer cation core of the protonated cation and counter radical are formed.…”

Infrared absorption of methanethiol clusters (CH3SH)n, n = 2–5, recorded with a time-of-flight mass spectrometer using IR depletion and VUV ionization

“…[1][2][3] Methanol clusters, (CH 3 OH) n , have been much investigated 4,5 through their infrared (IR) spectra. [6][7][8][9][10][11][12][13] The published experimental results indicate that (CH 3 OH) 2 has an open-chain structure, whereas cyclic hydrogen-bonded structures dominate in larger clusters of CH 3 OH. [11][12][13] Compared with oxygen, sulfur is less electronegative, and hence a poor proton acceptor in hydrogen bond.…”