Tae-Kyu Ha

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 ...

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

The Fermi‐resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3‐hexafluoro‐2‐trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF‐CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD‐CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.

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.

Ethylene glycol: Infrared spectra, AB initio calculations, vibrational analysis and conformations of 5 matrix isolated isotopic modifications

Abstract An extension of earlier ab initio calculations is reported which consists in a more detailed structure optimization of conformations investigated earlier, using the previously described as well as a more flexible basis set. The measurement and analysis of infrared spectra of five isotopic modifications, isolated in Ar and Xe matrices, are described. The analysis of the spectra, carried out within the framework of two H-bonded conformations r e1 and r e2 predicted by ab initio calculations to be the most stable ones, is complicated by the occurrence of unusually large matrix splittings and irreversible changes of the spectra under irradiation by infrared light. The spectra are interpreted in terms of essentially two slightly different conformers of type r e1 , deformed by the matrix field. The 18 O isotope shifts prove to be important for correct assignment and the least squares fit. A valence force field is given, whose dependence on the conformation of the OH groups and especially on H-bonding is discussed. According to normal coordinate analysis, the effect of the internal H-bond is most pronounced in the COH bend and OH torsion region. The differences of the potential constants of free and bonded OH groups are responsible for a zero point energy differences of about 20 cm −1 of two isomers of the modification CH 2 ODCH 2 OH. From experimental evidence for at least a partial equilibration of these two isomers at liquid helium temperature, internal rotation of the OH groups in an Ar matrix may be deduced. Thermodynamic functions of the r e1 conformation of glycol in the rigid rotor-harmonic oscillator approximation are reported.

Decomposition mechanism of the polynitrogen N5 and N6 clusters and their ions

Abstract The decomposition mechanism of the pentanitrogen ( N 5 , N 5 − , N 5 + ) and hexanitrogen ( N 6 , N 6 + , N 6 − ) clusters and their ions was studied using ab initio MO calculations up to the CCSD(T) level with 6-311+G(3dp) and aug-cc-pVTZ basis sets and also DFT/B3LYP method. While the cyclic anion N5− and open-chain cation N5+ are relatively stable with respect to elimination of N2, the neutral N5 radical does not exist as a discrete species. The elimination of two N2 molecules in the neutral N6 is found to be a concerted process, whereas that in the ionized N6+ cluster is stepwise due to the existence of a N4+ ion. The fragmentation of a N6 cluster into two azide (N3) fragments is, irrespective of its charge state, also a facile process, making them, at most, metastable molecules. Different thermochemical parameters of these Nn species have also been evaluated.

N-methylmethyleneimine and ethylideneimine: Gas- and matrix-infrared spectra, AB initio calculations and thermodynamic properties

Abstract Gas- and matrix spectra of CH 3 N:CH 2 (I) produced by controlled pyrolysis of N-trimethyl hexahydro-s-triazine and matrix spectra of CH 3 CH:NH (II) and its d s modification produced by photolysis in Ar matrices have been measured. A set of ab initio SCF LCAO calculations for both the cis and trans isomer of II, of I and of the isomers CH 2 :CHNH 2 and CH 2 NHC H 2 is given. For I and II assignments of the vibrational spectra, sets of harmonic force constants as well as statistical thermodynamic functions and thermodynamic data of their interconversion are presented.

A theoretical study of conformations and rotational barriers in dihydroxybenzenes

Abstract Results of ab initio SCF calculations of o -, m -, and p -dihydroxybenzenes (catecol, resorcinol and p-hydroquinone) are reported. The relative stabilities of the structural isomers of each compound and their transition states were determined. The internal rotation of the OH group, the harmonic force fields and vibration frequencies, and the molecular electronic properties were calculated and compared with each other with reference to those of the monosubstituted analogue, phenol. The calculated values are also compared with recent results obtained from molecular beam experiments.

Overtone intensities and dipole moment surfaces for the isolated CH chromophore in CHD3 and CHF3: Experiment and ab initio theory

The band strengths of fundamentals (N=1) and overtones (up to N=6) of the strongly coupled CH stretching and bending vibrations in CHD3 and CHF3 are calculated using high level ab initio (SCF‐CI) dipole moment functions and potential surfaces in one and two (three) dimensions. The calculations are performed in approximate normal coordinate and internal coordinate subspaces, the former giving generally superior results. The overall prediction of relative and absolute intensities ranging over many orders of magnitude is often accurate to within a factor of 2, but not to within experimental accuracy. Different dipole model functions and potential surfaces are investigated and an empirical adjustment of the dipole function to experiment is proposed for CHF3. The comparison of experimental and ab initio overtone intensities for the Fermi resonance system is discussed in some detail, as well as the importance of the results for IR spectroscopy and IR multiphoton excitation.

Ab initio calculation of the thermochemical properties of polysulphanes (H2Sn)

Abstract Thermochemical properties of gaseous polysulphanes (H 2 S n ) were calculated using the G2 and CBS-Q ab initio theoretical methods. The temperature dependence of the heat capacities and entropies for the polysulphanes are computed between 300 and 1000 K. The difference between the available experimental heat capacity of disulphane (298 K) and the calculated one is found to be quite small (0.7 cal/mol K). The latter is indicative of hindered internal rotation in polysulphanes. Ab initio calculations of rotational barriers in polysulphanes show that the relatively high barriers of internal rotations can be due to the interplay of attractive stabilization, π-character of the S–S bonds (due to hyperconjugation effects) and the participation of the sulphur 3d orbitals in bonding.

IR induced conversion of rotamers, matrix spectra, AB initio calculation of conformers, assignment and valence force field of trans glycolic acid

Abstract Glycolic acid isolated in Ar has been found to undergo conformer isomerization by broad band IR irradiation. Analysis of the spectra of CH 2 OHCOOH, CH 1 2 8 OHCOOH CH 2 OHC 1 8 OOH, CH 2 OHCO 1 8 OH, CH 2 OHC 1 8 O 1 8 OH and 13 C isotopic modifications based on a two-conformer model allows the determination of the structure of a new conformer produced by photoconversion as For this result, observation of 1 8 O isotope shifts is essential. Ab initio calculations on the 6–31G* level are reported for eight conformers with a planar skeleton and for some non-planar conformations possibly involved in the reaction path. Furthermore a constrained valence force field for trans glycolic acid based on certain transferability rules is reported.