Pavel Hobza

Floppy structure of the benzene dimer: Ab initio calculation on the structure and dipole moment

The structure of the benzene dimer has aroused considerable interest due to recent experimental measurements and hence extensive theoretical calculation is topical. Nine structures of the benzene dimer were investigated using the second‐order Mo/ller–Plesset theory. The calculations were performed with smaller (MIDI‐1+s+p) and larger (6‐31+G*) basis sets. The T‐shape structure was found to be the most stable but with a very shallow minimum; the wagging motion around the lowest hydrogen in the range of ±10° is practically nonhindered. These results together are consistent with the structure found experimentally. The final binding energy for the T structure (distance of molecular centers equal to 5.0 A) is −2.7±0.4 kcal/mol, which is more than the value derived from experiments. The calculated dipole moment is in excellent agreement with experiment.

Abinitio second‐ and fourth‐order Mo/ller–Plesset study on structure, stabilization energy, and stretching vibration of benzene⋅⋅⋅X (X=He,Ne,Ar,Kr,Xe) van der Waals molecules

The C6v structure of benzene⋅⋅⋅X (X=He, Ne, Ar, Kr, Xe) complexes was investigated with second‐order Mo/ller–Plesset (MP2) theory; for the benzene⋅⋅⋅He the whole potential‐energy surface (PES) was also studied. The stabilization energy of the benzene⋅⋅⋅He was also determined at the fourth‐order Mo/ller–Plesset (MP4) level; the respective MP4 stabilization energy is almost identical with MP2 stabilization energy which is due to the compensation of MP3 and MP4 contributions. The ab initio MP2 intermolecular distances agree nicely for all the complexes studied with the experimental value. While the stabilization energy of benzene⋅⋅⋅He and benzene⋅⋅⋅Ne (67 cm−1; 99 cm−1) is considerably smaller than that of benzene⋅⋅⋅Ar (429 cm−1), the intersystem distance differs less (3.32 A, 3.50 A, 3.53 A). The stabilization energies and intersystem distances for benzene⋅⋅⋅Kr and benzene⋅⋅⋅Xe are 485 and 601 cm−1 and 3.71 and 3.89 A, respectively. The PES of benzene⋅⋅⋅He differs from that of benzene⋅⋅⋅Ar and can be charac...

Ab initio calculations on the structure, stabilization, and dipole moment of benzene⋅⋅⋅Ar complex

The potential energy surface (PES) of the benzene⋅⋅⋅Ar complex was investigated ab initio using the second‐order Moller–Plesset theory demonstrating the practical use of such calculations for these complexes. Among five structures studied, the highest symmetry C6v structure for the Ar appeared to be most stable (stabilization energy: 429 cm−1; distance of molecular centers: 3.526 A). The PES is much more isotropic than was found in previous papers using an empirical potential. The calculated intermolecular distance is in excellent agreement with recent high resolution measurements—also the dipole moment is in excellent agreement with known data.

Vibrational dynamics of the benzene…argon complex

Abstract A section of the potential energy surface of the benzene…argon complex corresponding to the argon versus benzene intermolecular vibrational modes has been determined by fitting the existing ab initio data with the use of two different empirical potentials. From these potential energy sections, using an approximate vibrational Hamiltonian, the intermolecular vibrational energy levels have been evaluated and compared with available spectral and previous theoretical data for C6H6…40Ar. The stretching fundamental frequency has been found to be in excellent agreement with the literature data. The bending (nearly degenerate) frequencies, however, have been predicted to be significantly higher than those from literature. A reassignment of the observed absorption based on our predictions has led to a complete agreement between theory and experiment. We feel that this casts doubt on the previous vibrational assignment of the observed transitions.

Sequence dependent intrinsic deformability of the DNA base amino groups. An ab initio quantum chemical analysis

Abstract An accurate description of the interactions between DNA bases is of fundamental importance in the theoretical analysis of DNA structure, flexibility and dynamics. These base-base interactions are significantly influenced by the properties of the DNA base amino groups. In the present paper we show that, in constrast to the empirical force fields, ab initio calculations predict non-planar geometries for the DNA base amino groups. We compare the amino group non-planarity of cytosine, adenine, guanine, the guanine amino tautomer, 2-aminoadenine, 5-methylcytosine and the guanine formamidine hydrogen bonded complex, at the HF/6–31G(NH 2 ∗ ) level of theory. In addition, the geometry of cytosine is optimized at the MP2/6–31G ∗ , MP2/6–31G(NH ∗ 2 ), HF/6–31G ∗ , HF/4–21G(NH ∗ 2 ) and HF/6–31G levels of theory to estimate the role of electron correlation and polarization functions in the amino group geometry. It is shown that the gradient geometry optimization with inclusion of electron correlation significantly increases the non-planarity of the cytosine amino group compared to the HF level of calculations. The influence of non-planar DNA base amino groups on the conformational variability of DNA is briefly discussed.

Ground state potential surface for van der Waals complexes: Abinitio second‐order Mo/ller–Plesset study on benzene...N2 van der Waals molecule

The potential energy surface of the benzene...N2 was investigated ab initio using second‐order Mo/ller–Plesset (MP2) theory. The calculations were performed with 6‐31+G*/[5s4p2d] and 6‐31+G*/[5s4p3d] basis sets. Among five structures studied the sandwich structure (N2 lies in the plane parallel to the benzene ring) appeared to be most stable. The calculated intermolecular distance (3.46 A after vibrational averaging) is in excellent agreement with recent high resolution measurements. Calculated stabilization energy (591 cm−1) and intermolecular vibrational frequencies (stretching, 57 cm−1; torsion, 73 cm−1) are compared with available experimental results.

Properties of fluorobenzene⋅⋅⋅Ar and p‐difluorobenzene⋅⋅⋅Ar complexes: Ab initio study

The potential energy surfaces of fluorobenzene...Ar and p‐difluorobenzene...Ar were studied by ab initio calculations with inclusion of second‐order Mo/ller–Plesset correlation energy. The optimal structures of both complexes agree nicely with experimental data. The theoretical stabilization enthalpy for the second complex (294 cm−1) agrees well with the experimental data (190–242 cm−1). The fluorine substitution of the benzene molecule does not influence the characteristics of the respective complex. The dipole moment of both complexes was calculated; in both cases the electron transfer from fluorobenzene to Ar was found.

Applicability of the supermolecule MP2 approach to intermolecular interactions: He2 and Ne2

Abstract Arguments are given for applying the full counterpoise correction to eliminate properly the basis set superposition error in the correlation energy. Comparison is made with the intermolecular perturbation theory and more complete treatments (MP4, CEPA). The results for the dimers studied suggest that MP2 is a poor approximation. Even if very extended basis sets are used, hardly more than 65% of the stabilization energy is recovered.

MP4 Interaction energies and basis set superposition errors for the (H2)2dimer

Abstract The energy of formation of the T-shaped (H 2 ) 2 dimer is calculated using different basis sets and Moller-Plesset perturbation theory up to fourth order. The roles of the second-, third- and fourth-order contributions as well as that of the different excitations at the fourth-order level were investigated. The results suggest that a correction for basis set superposition error should be included at both the SCF and post-SCF levels.

Ab initio calculations on the structure, vibrational frequencies, and valence excitation energies of the benzene…Ar and benzene…Ar2 cluster

Abstract Excitation spectra of van der Waals (vdW) clusters show spectral shifts due to change in the stabilization energy in the ground and excited state. Ab initio calculations are applied to determine the structure and vibrational frequencies of vdW clusters like benzene…Ar and benzene…Ar2 in the electronic excited state. The calculation gives values for the valence excitation energy and intermolecular vibrations which are in good agreement with experimental data.