Dynamical anharmonicity, hydrogen bonds

Our second example in Sect. 5 illustrates how theoretical dynamical spectroscopy is successful at capturing strong anharmonic hydrogen bonds of the type M - -solvent and the associated subtly enhanced solvent—solvent H-bonds of varying strengths in irniic clusters (where M is an imi and the solvent can be water or methanol). Strong anharmcmicities of 700-900 cm have been correctly captured by the dynamical spectra, in relation to IR-PD experiments conducted at 100 K temperature. 

Future applications by the VPIMD method include vibrational fluctuations of molecular clusters such as hydrogen bonded clusters. Molecular clusters characterized by weak intermolecular interactions are expected to have large anharmonicity of the potential energy surfaces. As demonstrated in the present study, the VPIMD method properly handles the anharmonicity including the case of multiple minima. Another important point is on the description of the adiabatic potential energy surfaces of molecular clusters. An improvement can be achieved by combining the VPIMD method with electronic structure calculations as in the case of the finite temperature path integral molecular dynamics [30-32], These issues will be addressed in the near future. 

V. Stefov, Lj. Pejov, B. Soptrajanov. The influence of NH... n hydrogen bonding on the anharmonicity of the n(N-H) mode and orientational dynamics of nearly continuously solvated indole. J Mol Struct 555 363-373, 2000. 

The reconstruction of the bandshape of the imidazole crystal was also performed using Car-Parrinello molecular dynamics (CPMD) simulation [73] of the unit cell of the crystal the results reproduce both the frequencies and intensities of the experimental IR spectrum of bands reasonably well, which we attribute to the application of dipole moment dynamics. The results are presented in Fig. 8 [70]. These and other recent CPMD calculations, on 2-hydroxy-5-nitrobenzamide crystal [71], oxalic acid dihydrate [72], and other systems [64-69], show that the CPMD method is adequate for spectroscopic investigations of complex systems with hydrogen bonds since it takes into account most of mechanisms determining the hydrogen bond dynamics (anharmonicity, couplings between vibrational modes, and intermolecular interactions in crystals). 

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