Edet F. Archibong

Vibrational deactivation of N2(v=1) by inelastic collisions with He3 and He4: An experimental and a theoretical study

A new N2–He intermolecular potential with vibrational coordinate dependence is presented. Rate constants for the vibrational deactivation of N2(v=1) by He in the gas phase have been calculated over the temperature range 5–300 K. Accurate values of the rate constants for this process are known down to 100 K. We have now extended these measurements down to 70 K for the deactivation of 14N2(v=1) by 4He and down to 50 K for the deactivation of 15N2(v=1) by 3He. Agreement between the theoretically calculated and the experimentally determined rate constants is excellent with the calculated values reproducing the experimental measurements within their error bars. An investigation of the low impact energy regime is also presented. While this focuses on collision energies of less than 20u2009cm−1 and yields rate constants which are in a temperature region inaccessible to our experimental method, it gives further insights into the influence of the attractive well on vibrational energy transfer processes.

QUANTUM MECHANICAL AND 13C DYNAMIC NMR STUDY OF 1,3-DIMETHYLTHIOUREA CONFORMATIONAL ISOMERIZATIONS

Abstract We present 13 C dynamic NMR results for relative free energies of equilibrium structures and free energies of activation for conformational transformations of 1,3-dimethylthiourea in aqueous solution, and we compare the results to theoretical predictions. The latter are based on ab initio gas-phase electronic structure calculations of the geometries, dipole moments, and energies combined with semiempirical molecular orbital calculations of the free energies of solvation in three different solvents. The gas-phase electronic structure calculations were performed using Moller-Plesset second-order (MP2) perturbation theory with a correlation-consistent polarized valence-double-zeta basis set; we calculated relative energies for the three minima Z , Z , E , Z , and E , E and for three transition states on the potential energy surface. The solvation energy calculations were carried out using the SM5.4/AM1-aqueous, -chloroform, and -organic solvation models; these solvation models are based on semiempirical molecular orbital theory with class-IV charges and geometry-based first-solvation-shell effects. The relative energies of the conformers and transition states are compared to experiment in water and five organic solvents.

Abinitio prediction of the structure, harmonic vibrational frequencies, and dissociation energy of the H2–GeH+3–H2 cluster ion

Ab initio predictions of the molecular geometry, harmonic vibrational frequencies, and dissociation energies are reported for the germanium hydride cluster ion GeH+7. Seven stationary points were located on the potential energy surface (PES) of GeH+7 using the self‐consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double‐ζ plus polarization (DZP) and a triple‐ζ plus polarization TZ(3d1f,1p) quality basis set. The most stable structure has a C2 symmetry with the two H2 subunits rotating freely about the symmetry axis of the GeH+3 fragment. Our best estimate of the dissociation energy for GeH+7, taking into account the zero point vibrational energy (ZPVE) is 3.10 kcal/mol compared to 4.6 and 1.2 kcal/mol obtained, respectively, for the SiH+7 and CH+7 cluster ions.

Quantum chemical conformational analysis and X-ray structure of 4-methyl-3-thiosemicarbazide

Abstract We present X-ray crystallographic results and gas-phase electronic structure calculations of the geometry of 4-methyl-3-thiosemicarbazide. Using the Hartree-Fock theory with a 6-31G ∗ basis set, we calculated relative energies for eight different conformations. For the lowest-energy conformations of each of the four possible combinations of rotamers about the two Cue5f8N bonds, we also included electron correlation by Moller-Plesset second-order (MP2) perturbation theory with the same basis set. From these calculations, we selected the lowest-energy structure and calculated structural parameters at the MP2 level of theory with the larger correlation-consistent cc-pVDZ basis set. The geometry of the minimum-energy gas-phase structure is in good agreement with the structure observed experimentally in the crystal.

Cyclic-AlO2 revisited

Abstract In this report, we present the results of ab initio electronic structure calculations of the structures, harmonic vibrational frequencies and the dissociation energy of cyclic AlO2. At the highest level of theory employed [CCSD(T)/6-311++G(3df)] the 2A2 state is lower than the 2A1 state by roughly 7 kcal mol−1. The molecular structure for the 2A2 state is predicted to have r e (Alue5f8O) = 1.940 A and r e (Oue5f8O) = 1.354 A . The Alue5f8O2 dissociation energy is computed to be 65 kcal mol−1 using full counterpoise corrections. The ω2 mode (Alue5f8O symmetric stretch) of 529 cm−1 calculated at the CCSD(T) level is about 6% higher than the experimental value of 496 cm−1.

Ab initio predictions of the molecular structures and harmonic vibrational frequencies of Ga2O2 isomers

The potential energy surface of Ga2O2 is examined at the SCF and MP2 levels employing basis set of triple-ζ plus double polarization quality. Four stationary points located at the SCF level are characterized via their Hessian index. Electron correlation is important for the energy ordering and splitting of the isomers. For example, two minimum energy structures, a cyclicD2h form and a linear Ga-O-Ga-O, separated by 25.69 kcal/mol at the SCF level have an energy difference of only 1.70 kcal/mol at the MP2 levels. Our computed harmonic vibrational frequency at 962 cm−1 for the Ga-O-Ga-O minimum structure in in good agreement with the experimental predicted value of 967 cm−1.