Anharmonicity and hydrogen bonding. III. Analysis of the near infrared spectrum of water trapped in argon matrix

Abstract

The infrared spectra of H 2 16 O and H 2 18 O trapped in solid argon were recorded in the range 8000–15 cm −1 at Ar/H 2O molar ratios between 2000 and 20. At low concentration in water the quasifreely rotating monomer predominates, giving rise to relatively narrow rovibrational signals for the transitions involving exclusively the J=0 and 1 rotational levels, i.e. the R(0)-, Q(1)- and P(1)-type transitions. For the dimer most of the one and two quanta transitions of both proton acceptor (PA) and proton donor (PD) subunits were identified on the basis of 16 O/ 18 O isotopic substitution and of the results previously obtained in nitrogen matrix [Chem. Phys. 266 (2001) 109]. A new assignment for ν 3 of PA is proposed, involving the internal rotation of PA around its symmetry axis, as observed in the gas phase and in He clusters. As in N 2 matrix the 2 ν 1 band of PD has not been observed, which confirms the intensity weakening of the first overtone of a hydrogen-bonded OH oscillator (OH b). The same phenomenon occurs for larger polymers (H 2O) n , n>2. The data analysis is focused on three points: rovibrational analysis and determination of the rotational parameters of some A 1 vibrational levels; role of Fermi resonance as shaping mechanism of the νOH b band of water polymers; determination of electrooptic parameters from intensity measurements for both monomer and dimer.