Photoionization and vibrational spectroscopy of the aniline-methanol clusters.

Aniline-methanol mixed clusters are ionized by single photon vacuum ultraviolet (VUV, 118 nm) radiation with which absorption to an excited intermediate S(1) state is not required. Aniline ion (An(+)), a series of (An)(n)(+)-(CH(3)OH)(m) (n = 1, 2) cluster ions, and their hydrogenated cluster ions, (An)(n)(+)-(CH(3)OH)(m)H (n = 1, 2) are observed by mass spectrometry. Infrared (IR) absorption spectra of aniline-methanol cluster cations and neutrals are measured through IR and VUV (118 nm) "ion dip" spectroscopy in the range 2500-4000 cm(-1). The observed mid-IR spectrum of the An(+)-CH(3)OH has two sharp absorption bands, at 3438 and 3668 cm(-1), which are assigned to the free NH stretch vibration of the aniline cation and the free OH stretch vibration of methanol, respectively. Calculations demonstrated that a change in the charges on the nitrogen atom of the amine group upon ionization of the neutral to the cluster cation alters the role of aniline from hydrogen acceptor to hydrogen donor in its interaction with methanol. Theoretical and experimental results suggest that a hydrogen bond forms between one of the H atoms of the aniline amine group and the lone pair of electrons of the methanol oxygen atom in the aniline-methanol cluster cation. Measured IR spectra and theoretical results for neutral clusters suggest that the H atom of the methanol OH moiety is bonded to the aniline amine group lone pair electrons for the neutral ground state aniline-methanol cluster.