Resonant ion-dip infrared spectroscopy of benzene–(methanol)m clusters with m=1–6

Abstract

Resonant ion-dip infrared spectroscopy has been employed to record cluster-size-specific spectra of C6H6–(CH3OH)m with m=1–6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogen-bonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm−1 due to π hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm−1. The lower frequency peak is redshifted from the free monomer value by 175 cm−1 and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm−1 due to a strengthened π hydrogen bond. In benzene–(CH3OH)3, three sharp OH stretch transitions are observed at 3389, 3435, and 3589 cm−1. The comparison of these absorptions with ab initio calculations and with experiments on the pure methanol trimer leads to a structure for benzene–(CH3OH)3 which incorporates a π hydrogen-bonded methanol trimer chain, confirming the earlier assignment based on its ultraviolet spectrum. The 3589 cm−1 transition, due to the π hydrogen bond of the terminal methanol, is redshifted from the free monomer by 93 cm−1, a value approaching that of the donor methanol in methanol dimer (−107 cm−1). The lower frequency transitions in the m=3 spectrum arise from the donor–acceptor and donor OH stretches in the methanol trimer chain. The spectral characteristics change when m=4. The OH stretch transitions are all located in a region around 3320 cm−1 and are significantly broadened compared to the smaller clusters. By comparison with ab initio calculations, the methanol tetramer structure in benzene–(CH3OH)4 is deduced to be a cyclic methanol tetramer. The spectra for m=5 and 6 are slightly redshifted but similar to m=4 and point toward cyclic structures as well.