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The FH···NCH hydrogen-bonded complex, which is a reference structure, has been reviewed from an electronic and vibrational point of view. Ab initio calculations have been made, using a large basis set and thus providing first structural and electrical properties on the subunits when the electronic correlation effects are taken into account in very close agreement with the experimental data. The structure and dipole moment of the complex are well represented. The νFH and νF···N stretching modes are calculated by means of the variational method, taking into account the mechanical anharmonicity and anharmonic coupling between both vibrations. It is shown that the νF···N stretching frequency, which is larger than the experimental value at the self-consistent-field level and especially when electronic correlation effects are taken into account, is strongly decreased, and so approaches the experimental value, when corrected for basis set superposition error. [ABSTRACT FROM AUTHOR]

This paper presents the first ab initio attempt to reconstruct the observed band profile of the stretching fundamental vFX (X=H,D) in the FX···O(CH3)2 hydrogen-bonded system. The two-dimensional potential energy surface V(rFH,RF···O) is evaluated by means of large basis set SCF calculations. The related force constants up to the fourth order are obtained via the analytical fit to a polynomial expansion. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The side bands of the stretching fundamental vFX are described in terms of the vFX ±nvFX···O combination bands in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations on the Li–C2H4 complex have been performed with coupled-cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be 2B2, with a C2v symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including 13C/12C, 7Li/6Li, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li–C stretching mode. These calculations also allow to propose an assignment for the observed C–H/C–D stretching modes. The observed blue-shift of the symmetric CH2 bending mode as well as the red-shift of the antisymmetric CH2 bending, CD2 bending, and C–C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na...C2H4 complex has been found to be unstable in its 2B2 electronic state. The study of the 2A1 electronic state for both Na...C2H4 and K...C2H4 complexes show that they are at most very weak van der Waals complexes. This result confirms the conclusions of matrix isolation experiments. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]