The effect of isotopic substitution and detailed balance on the infrared spectroscopy of water: A combined time correlation function and instantaneous normal mode analysis

We have recently demonstrated that simple classical molecular dynamics meth ods are capable of nearly quantitatively reproducing most of the intermolec ular and intramolecular infrared (IR) spectroscopy of water [H. Ahlborn, X. Ji, B. Space, and P. B. Moore, J. Chem. Phys. 111, 10622 (1999)]. Here it is demonstrated that the result is robust by quantitatively reproducing exp erimentally measured D2O IR spectroscopy utilizing the same models. This su ggests that the quantum effects associated with light atom motion are relat ively unimportant. Instantaneous normal mode (INM) theory and the time corr elation function (TCF) methodology are used in a complimentary fashion to a nalyze the molecular origin of the IR spectroscopy of deuterated water (D2O ). The TCF methods demonstrate that our models of the dynamics and the syst em dipole are reasonable by successful quantitative comparison of the theor etical spectrum with experimental results. INM methodology is then employed to analyze what condensed phase motions are responsible for the observed O -D stretching line shapes. It is surprising that classical models can repro duce the complex spectroscopy of both liquid H2O and D2O, and this result i mplies that the motions responsible for the signal must be effectively harm onic in nature. This assertion is supported by the drastic impact that is s een on both the intensity and line shape through the choice of detailed bal ance correction factor that is used to quantum correct the classical vibrat ional line shape. (C) 2000 American Institute of Physics. [S0021-9606(00)50 518-2].