The OH vibrational spectrum of liquid water from combined a b i n i t i o and Monte Carlo calculations

Abstract

The infrared vibrational OH stretching spectrum of isotopically isolated HDO molecules in liquid water has been calculated by ab initio methods at the MP2 level for a number of geometrical configurations taken from a Monte Carlo simulation. Each vibrating water molecule with its environment was described by a pentamer supermolecule, surrounded by a large number of point charges representing polarized water molecules. The anharmonic stretching potentials (MP2 force constants up to fifth order) for 40 uncoupled OH water vibrators were calculated. The average computed re distance found for liquid water is 0.01 Å longer than the free-water value. The frequencies were obtained by solving the one-dimensional Schrödinger equation variationally for each OH potential curve. Using the squared dipole moment derivatives, which vary by a factor of 7 over the frequency band, the density-of-states histograms were converted to intensities. The resulting computed average frequency downshift is ∼260 cm−1, compared to ∼310 cm−1 (experimental), with a bandwidth in good agreement with experiment. The remaining discrepancy between theoretical and experimental frequency shifts is to a large part due to the charge transfer within the water clusters. This charge transfer gives rise to an electrostatic field which, at the site of the vibrating H atom, counteracts the downshift induced by the other environmental effects. The agreement between experiment and theory is very satisfactory when this charge transfer effect is corrected for or when point-charge embedded heptamer clusters are considered.

Accurate theoretical prediction of vibrational frequencies in an inhomogeneous dynamic environment: A case study of a glutamate molecule in water solution and in a protein-bound form

“…Both continuum and molecular solvent models as well as various hybrid continuum/ molecular approaches were employed. [30][31][32][33][34][35][36][37][38] One important conclusion which resulted from the prior theoretical analysis of the zwitterionic l-alanine, l-glycine, and l-glutamine in water solution was that both local hydrogen bonding and long-range electrostatic solvent effects must be taken into account in order to correctly predict their vibrational frequencies. [39][40][41] While the reaction field approach (PCM) 42 in which the solvent is represented by dielectric continuum and the shape of the solute is taken into account is more computationally efficient than explicit solvent models, it cannot be applied in an inhomogeneous environment such as a protein binding site.…”

Hydrogen bonding and Raman, IR, and 2D-IR spectroscopy of dilute HOD in liquid D 2 O

“…The resulting dynamic frequency fluctuations, also known as spectral diffusion, can be measured by transient vibrational hole-burning and three-pulse echoes (5,6,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). In particular, these experiments provide information about both the short-time (stretching) and longtime (making and breaking) aspects of intermolecular hydrogen bonds (23-35).We and others have developed methods for the theoretical calculation of steady-state and ultrafast vibrational spectroscopy observables (7,(23)(24)(25)(26)(27)(36)(37)(38)(39)(40). In our approach, the single vibrational mode of interest, for example, the OH stretch of HOD (when it is immersed in D 2 O), is treated quantum mechanically, whereas all other degrees of freedom (the bath) are treated classically.…”

“…We and others have developed methods for the theoretical calculation of steady-state and ultrafast vibrational spectroscopy observables (7,(23)(24)(25)(26)(27)(36)(37)(38)(39)(40). In our approach, the single vibrational mode of interest, for example, the OH stretch of HOD (when it is immersed in D 2 O), is treated quantum mechanically, whereas all other degrees of freedom (the bath) are treated classically.…”

Chemical Accuracy Obtained in an Ab Initio Molecular Dynamics Simulation of a Fluid by Inclusion of a Three-Body Potential

“…If this could be generalised for the structure of molecular liquids it would be of great help for the calculation of accurate solvent effects as performed by several authors for infrared absorption, [17] nuclear quadrupole couplings, [18±22] and NMR shifts. [23,24] Pressure: As mentioned above and shown in Table 1 and Figure 3, [25] the pressure simulated with pair potentials deviates increasingly from experimental values at higher densities.…”