Optimal structures, interaction energies, and harmonic vibrational frequencies of the NH 3 ‚‚‚H 2 O complex have been determined from the second-order Møller-Plesset perturbation theory and DFT/B3PW91 calculations with the aug-cc-pVTZ basis set. Some properties of the complex have been calculated at the MP4 and CCSD-(T) levels. The tunneling motion of the water molecule around its c inertial axis was studied, and the barrier to the exchange of the bound and free hydrogen atoms was determined as 1267 cm -1 . The nature of the intermolecular interactions in the complex was investigated by symmetry-adapted perturbation theory (SAPT). As revealed by the SAPT analysis the main binding contributions are the electrostatic and induction components. The calculations of the vibrational frequencies and infrared intensities for this complex are presented to facilitate the frequency assignments of the experimental spectra.

No abstract

The effect of heteroatom or heteroatomic group incorporation into unsaturated five- and six-membered cyclic systems was studied by means of DFT/B3LYP/aug-cc-pVDZ calculations. Two descriptors of the incorporation effect, sEDA(II) and pEDA(II), reflecting the influence of the incorporated atom or group on the population of the σ and π valence electrons, were constructed on the basis of natural bond orbital analysis. The sEDA(II) and pEDA(II) descriptors were shown to be linearly independent; the former correlated very well with electronegativity scales, whereas the latter correlated with NICS(1)(ZZ) and HOMA(CC) aromaticity indices. The two descriptors seem to be universal tools for analyzing different chemical and physicochemical effects occurring in unsaturated heterocyclic systems.

New descriptors of the double bonded substituent effect, sEDA(=) and pEDA(=), were constructed based on quantum chemical calculations and NBO methodology. They show to what extent the σ and π electrons are donated to or withdrawn from the substituted system by a double bonded substituent. The new descriptors differ from descriptors of the classical substituent effect for which the pz orbital of the ipso carbon atom is engaged in the π-electron system of the two neighboring atoms in the ring. For double bonded substituents, the pz orbital participates in double bond formation with only one external atom. Moreover, the external double bond forces localization of the double bond system of the ring, significantly changing the core molecule. We demonstrated good agreement between our descriptors and the Weinhold and Landis' "natural σ and π-electronegativities": so far only descriptors allowing for evaluation of the substitution effect by a double bonded atom. The equivalency between descriptors constructed for 5- and 6-membered model structures as well as linear dependence/independence of the constructed parameters was discussed. Some interrelations between sEDA(=) and pEDA(=) and the other descriptors of (hetero)cyclic systems such as aromaticity and electron density in the ring and bond critical points were also examined.

No abstract