Dipole moment vibrational modes, hydrogen-bonded

Closing naphthyl ring. The induced dipole moment is then chirally disposed in relation to the inducing NH2 dipole moment. This mechanism, referred to as the dynamic polarization model (45), is shown to explain most of the observed VCD intensity in the synunetric NH2 stretching mode, >>nh2- The anisotropy ratio for this VCD band is ca. 10. Since an NH. . . x type hydrogen bond is possible in this molecule, a description based on vibrationally induced charge flow (currents) may also be riuitfiil, similar to that proposed for a-phenyleth-ylamine. Sect. FV-B-2. 

In the case of cyclopropane, the vibrational force field and infrared intensities have been examined in some detail both experimentally and via calculation. One of the more interesting observations is that the dipole moment derivatives for the C-H stretching modes are relatively small (corresponding to their low intensities). There is a relationship between the derivatives and the bond dipoles The low value for cyclopropane is a reflection of the smaller than normal electron population at its hydrogens (Table 3). 

In this account we have attempted to provide a brief overview of the concepts of first-principles methods tailored for the calculation of structures, energetics, and properties of supramolecular assemblies. The presentation of the theory focussed on the most essential building blocks in order to provide a general frame to interrelate the various methods available. Thereafter, we discussed the relation of these methods to experiment and to well-known concepts for the description of typical interaction patterns. Also, new methods tailored for tackling problems specific to supramolecular chemistry have been discussed (like the calculation of local dipole moments in CPMD simulations, the Mode-Tracking protocol for the selective calculation of vibrational frequencies and intensities, or the SEN method for the calculation of hydrogen bond energies). 

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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