Sang Joo Lee

A theoretical investigation of the nature of the π-H interaction in ethene–H2O, benzene–H2O, and benzene–(H2O)2

We have carried out a detailed investigation of the nature of the π-H interaction in the ethene–H2O, benzene–H2O, and benzene–(H2O)2 complexes using large basis sets (ranging from 6-31+G* to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Mo/ller–Plesset (MP2) level of theory. The binding energy of the ethene–H2O complex is only about 1 kcal/mol lower than that of the benzene–H2O complex. In the benzene–(H2O)2 complex, the interaction of benzene with the π-bonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interaction energies were decomposed into the individual interaction energy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies ...

Ab initio study of the complexation of benzene with ammonium cations

Abstract Complexes of benzene with ammonium cations (MenH(4−n)N+ for n=0−4) were studied using ab initio calculations with electron correlation included. The most stable structure and binding energies of the complexes are reported. The calculated binding energies are in good agreement with experiment. Two types of NH-aromatic π and CH-aromatic π interactions, which are important in biological systems, are responsible for the binding. From analysis of the structures and energies, the π-σ ∗ through-space interactions are seen to be significant in both types of NH-π and CH-π interactions.

Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, and equilibrium structures

High level ab initio calculations have been performed on the benzene-HCl and benzene-HF systems using the second-order Mo/ller-Plesset perturbation theory. In contrast to existing theoretical studies, the calculated binding energies indicate that HCl binds more strongly to benzene than HF. This is in accordance with the limited experimental data available on these systems. An explanation has been forwarded for the above observation by performing a molecular orbital analysis of both C6H6⋯HF and C6H6⋯HCl. In the global minimum of C6H6⋯HF, HF lies inclined to the benzene ring with the hydrogen atom pointing either towards a benzene carbon or the center of carbon-carbon bond. In the C6H6⋯HCl complex, HCl is found to lie along the C6 axis of the benzene ring for smaller basis sets, but it also tends to lie inclined to the benzene ring for a very large basis set. The quantum mechanical probabilistic characterization of the structure of the C6H6⋯HCl complex provides a more realistic description of the experiment...

Entropy-driven structures of the water octamer

Abstract Ab initio Hartree-Fock and Moller-Plesset second-order perturbation calculations have been performed on several structures of the water octamer which are expected to be in low-lying energy states. The ab initio results calculated with double zeta plus polarization basis sets predict that the global minimum energy octamer has a cubical structure with D 2d symmetry at 0 K, while the cubical structure with S 4 symmetry is more stable than the D 2d structure above ≈40 K. However, above ≈230 K, the cyclic octamers with S 8 and S 4 symmetries are more stable than the cubical species because of the entropy effect.

Ab initio study of water hexamer anions

Abstract The wet electron - an electron interacting with a small cluster of water molecules - is a simple yet fundamental system for understanding the behavior of electrons in complex molecular systems. A comprehensive post Hartree-Fock ab initio study is performed on the wet electron in various water hexamer clusters including the low-lying energy conformers of the neutral species. The predicted geometries, total energies, photoemission ionization energies, electronic structure and orbital character of the excess electron in ground and excited states are discussed. To understand the behavior of the excess electron in the clusters, the s-orbital-like character of the HOMOs and the p-orbital-like character of the LUMOs are investigated.

RAMAN INTENSITIES OF C=C STRETCHING VIBRATIONAL FREQUENCIES OF POLYENES : NODAL MODE ANALYSIS

It is not clearly understood how and why the Raman intensity increases drastically with an increasing chain length of polyenes. We therefore investigated the vibrational intensities of the C=C stretching vibrational modes of four polyene systems of X–(HC=CH)n–Y, where X/Y=H/H, H/NH2, H/NO2, and NH2/NO2. The investigation was done using nodal mode analysis (based on the number of nodes formed by the alternations of stretches and contractions) combined with ab initio frequency calculations. The C=C stretching/contracting mode without node is found to have the strongest Raman intensities regardless of polyene systems because of the long-range cooperation effect by the concurrent stretch/contraction motion of all C=C bonds. The corresponding IR spectra have also the strongest intensities for the nonsymmetric polyene systems, whereas are inactive for the symmetric polyenes (by the exclusion rule). The intensities of the nonconcurrent C=C stretching/contracting modes (particularly for the Raman spectra) tend to...