The electronic Hamiltonian describes the mixing of the proton vibrational states of the dimer, belonging to different irreducible representations of the C i group. The purely electronic wave functions and may be treated as the developing coefficients of vibra tional functions in eq 30. On the other hand, aromatic carboxylic acid dimers should be characterized by stronger vibronic coupling efects of the Herzberg_Teller type. Therefore, in their IR spectra the forbidden transition spectrum, activated via the vibronic promo tion mechanism, should be more intense than the intensity of the corresponding spectrum of aliphatic carboxylic acids. This con From our analysis of polarized IR spectra of the PAM crystal it results that centrosymmetric dimeric N_H 3 3 3 O hydrogen bond systems are the bearers of the crystal spectral properties.
This is due to the fact that the strongest vibrational exciton couplings involve the closely spaced hydrogen bonds, each from a diferent chain of the PDE Inhibitors associated molecules in the lattice. In the crystalline spectra the lower frequency branch of the N_H is attributed to the forbidden transition leading to the A g excited state of the dimer. The transition is activated by the vibronic promotion mechanism presented above involving nonadiabati cally coupled proton vibrations and the electronic motions in the hydrogen bond centrosymmetric dimeric systems in the crystal. Consequently, the normal vibrations of the protons in the dimers exhibit no precisely defined symmetry properties. Therefore, the dipole selection rules become weakened and the forbidden vibrational transition in IR is activated.
From our previous studies it results that the integral intensity of the lower frequency branches Pelitinib of the X H bands in IR spectra of centrosymmetric hydrogen bond dimeric systems strictly depends on the electronic structure of the associated molecules. In the case of the polarized IR spectra of the PAM crystal the efect of the selection rule breaking seems to be strong since the lower frequency branch of the N_H band is extremely intense in comparison with the corresponding spectra of other amide crystals. This spectral branch intensity is most probably the result of the coupling of the protonic motions with electrons of not only the hydrogen bridge atoms but also those of the substituent groups linked to the amide fragment.
In the case of amide crystals the linking of the acryl group to the carbonyl group significantly enhances the polarization properties of the proton OdC hydrogen caspase bonds. They reach the SdC hydrogen bonds found level characteristic for the N_H 3 3 313 The mechanism of the PAM crystal spectra generation, including the anomalous H/D isotopic efect in the crystalline spectra, fairly resembles the mechanism of the spectra generation of some rare molecular system cases, e. g., 2 mercaptobenzo thiazole and N methylthioacetamide crystals. Thus the above evidence seems to point to the fact that the spectral properties of the PAM crystals result from the strong in uence of the electro nic efects on the mechanisms of the generation of the centro clusion is supported by experiment.
acceptor in the N_H 3 3 3 in N methylthioacetamide crystals. symmetric dimer system IR spectra of the N_H 3 3 3 bonds Ponatinib in the crystal lattice. O hydrogen derivative of the compound. From our model calculations aiming at reproducing the N_H and N_D band shapes it results that the forbidden transition band intensity in the small. The N_D N_D band is negligibly band is practically formed by the allowed transition band. The explanation of this efect can also be found in our model. The promotion mechanism is strongly hydrogen atom mass dependent since the deuteron vibrations in the N_D 3 3 3 O deuterium bonds are characterized by a lower anharmonicity than the proton vibration anharmonicity in the N_H 3 3 3 O hydrogen bonds in the crystal. The magnitude of this efect depends on the potential energy surface shape of the proton stretching vibrations in the crystal.
PARP This shape is formed by the vibronic coupling mechanism. Similar H/D isotopic efects were observed in the IR spectra of the hydrogen bond in molecular crystals with the N_H 3 3 3 S bonds in their lattices. They characterize, for instance, the IR spectra of 2 mercaptobenzothiazole 56 and N methylthioacetamide 31 crystals. On the other hand, the identical H/D isotopic efect is the attribute of the spectra of 2 hydroxybenzothiazole crystals. Such a nonrandom arrangement of protons and deuterons in the lattice is isotopic dilution prove the in uence of the dynamical cooperative interactionsinhydrogenbondsystemsonthehydrogenbondenergy of molecular complexes. In this case the strongest dynamical cooperative interactions involve the closely spaced translationally nonequivalent hydrogen bonds. Moreover, each moiety belongs to a diferent chain of the associated molecules of PAM penetrating a unit cell of the lattice.