CH3OH⋯(H2O)n [n = 1-4] clusters in external electric fields

Abstract

For hydrogen-bonded neutral molecular clusters, response to an externally applied electric field can critically affect molecular cooperativity. In this light, response of dilute methanol-water admixtures to an external, perturbative electric field is studied at the simplest molecular level in the cluster configurations CH3OH⋯(H2O)n with "n" chosen to range from 1 to 4, employing the M06-2X hybrid functional in conjunction with the 6-311++G(2d,2p) basis set, well-suited for hydrogen bonding. Methanol is seen to favorably bond with the water molecules at its hydroxyl end up to certain characteristic maximum threshold field strengths beyond which the HOMO-LUMO energy-gap abruptly drops to zero culminating into a complete breakdown of the cluster. In the interim regime prior to breakdown, the electric field significantly alters the hydrogen bonding pattern primarily by elongating the cluster, resulting in a marked enhancement in its electric dipole moment leading to alterations in the molecular electrostatic potential. With the application of electric field, certain "exotic" O-H vibration bands appear that at the threshold field fall in the frequency range of 2510 cm(-1)-1880 cm(-1) in the IR spectra, in contrast with their normal (zero-field) counterparts that occur in the range of ∼3300-3900 cm(-1).