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A new method for calculating nuclear magnetic resonance shielding constants of relativistic atoms based on the two-component (2c), spin-orbit coupling including Dirac-exact NESC (Normalized Elimination of the Small Component) approach is developed where each term of the diamagnetic and paramagnetic contribution to the isotropic shielding constant σiso is expressed in terms of analytical energy derivatives with regard to the magnetic field B and the nuclear magnetic moment μ. The picture change caused by renormalization of the wave function is correctly described. 2c-NESC/HF (Hartree-Fock) results for the σiso values of 13 atoms with a closed shell ground state reveal a deviation from 4c-DHF (Dirac-HF) values by 0.01%-0.76%. Since the 2-electron part is effectively calculated using a modified screened nuclear shielding approach, the calculation is efficient and based on a series of matrix manipulations scaling with (2M)3 (M: number of basis functions). [ABSTRACT FROM AUTHOR]

The analytical energy gradient and Hessian of the two-component Normalized Elimination of the Small Component (2c-NESC) method with regard to the components of the electric field are derived and used to calculate spin-orbit coupling (SOC) corrected dipole moments and dipole polarizabilities of molecules, which contain elements with high atomic number. Calculated 2c-NESC dipole moments and isotropic polarizabilities agree well with the corresponding four-component-Dirac Hartree-Fock or density functional theory values. SOC corrections for the electrical properties are in general small, but become relevant for the accurate prediction of these properties when the molecules in question contain sixth and/or seventh period elements (e.g., the SO effect for At2 is about 10% of the 2c-NESC polarizability). The 2c-NESC changes in the electric molecular properties are rationalized in terms of spin-orbit splitting and SOC-induced mixing of frontier orbitals with the same j = l + s quantum numbers. [ABSTRACT FROM AUTHOR]

An algorithm is developed that leads to the correct correlation of the vibrational modes of two isotopomers using the concept of a mass reaction path. The mass reaction path is defined by weighted incremental changes of the masses that convert the 'mass reactant' into the 'mass product'.

The study was conducted to investigate the mechanism of the reaction between ethene and 1,-3-butadiene using the unified reaction valley approach (URVA). The results were analyzed by characterizing normal modes, the reaction path vector, and the curvature vector in terms of generalized adiabatic modes associated with internal parameters were used to describe the reaction complex.

A theoretical description of primary kinetic isotope effects for adiabatic proton transfer reaction was presented utilizing a nontraditional picture in which the proton nuclear motion was treated quantum mechanically but was not tunneling. A detarted comparison between the present picture and the standard description was presented.

A thorough evaluation of the estimated power indicates that the gas-phase mechanism of the reaction is governed by radical or biradical mediators, whilst the solution-phase mechanism is distinguished by hydride transfer and the creation of a transitional ion pair which comprises of HOOO- anion. Finally, the amount of ozonation of alcohols and toxicity of ozone consumed in the blood can be determined.

A method has been developed to determine the most stable conformations of puckered rings by a comparison of measured and calculated spin-spin coupling constants (DORCO: determination of ring confirmations). The DORCO method uses the ring puckering coordinates to express the properties of a pseudorotating puckered ring.

The nuclear magnetic resonance (NMR) spectroscopy was used to investigate the (super 13)C, (super 15)N-labeled DNA hairpin molecule of the sequence d(GCGAAGC) = d(G1C2G3A4A5G6C7), and to determine the one-bond and two-bond NMR spin-spin coupling constants. The result shows that one-bond and two-bond coupling constants are very sensitive to the position of the C and N nucleus in the pyrimidine or purine rings.

A research work is presented which determines whether the quantum chemical methods based on single-determinant theory can provide a reliable description of the equilibrium geometry of difluoro peroxide or not. It was found that the molecular dipole moment is a sensitive antenna for probing the quality of the quantum chemical description of FOOF.

Research is done on sixth order Moller-Plesset perturbation theory (MP6) in connection with correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ that was used to calculate accurate equilibrium geometries for 14 molecules. The experiment reflects the importance of infinite order effects as provided by couple cluster methods some of which are often superior to certain MPn methods.

A study looking into the production of methyl-enecyclopropene from the photochemistry of matrix-isolated butatriene at 10 K is presented.

Issues concerning the r(sub e) structure of cyclopropane are discussed. It was established that reliable results are gained for cyclopropane only if coupled-cluster methods with inclusion of triple excitations are used in combination with large basis sets.

Labile species were isolated after 2,6-dimethylcyclohexa-2,5-dien-1-on-4-ylidene underwent ultraviolet photolysis and matrix isolation in argon at 10 K. The species were found to be alpha,3-Didehydro-5-methyl-6-hydroxybenzene after comparing the results to computations at the ROSS-BLYP/6-31G(d,p) level of theory. Photochemical reactions caused the ring opening of the diradical to produce 3-hydroxy-4-methylhepta-1,2,4-trien-6-yne, thereby indicating Myers cyclo-reversion.

The mechanism of the photodecarboxylation of carbene and the electronic structure of 2,4-didehydrophenol by high level ab initio theory were studied. The thermodynamic role of carbon dioxide formation in the decarboxylation process was emphasized by estimating the heat of formation of triplet carbene. The presence of an intramolecular hydrogen bridge was observed in the x-ray structure analysis of diazo compounds. OH stretching vibrations of free carboxylic acids was also seen by matrix isolation spectroscopy.

Kinetic measurements and B3LYP/ and MP2/6-31G(d,p) calculations show that carbonyl oxides are a significant source of OH radicals when formed i the gas-phase ozonolysis of alkylated alkenes. An investigation into whether carbonyl oxides directly decompose to yield OH radicals was undertaken, using high level ab initio methods. CH200 was found to decompose after H migration and intermediate forming of carbene to OH and formyl radicals. OH radicals were generated during the gas-phase degradation of alkenes by ozonolysis.

Dimesityldioxirane was synthesized via oxidation of dimesitylcarbene. The colorless, cyrstalline dioxirane was entirely stable at -20 deg C and spectroscopic methods, X-ray crystallogaphy and DFT calculations were used to characterize it. Locl C2 symmetry was shown with the mesityl rings twisted by ca. 60 deg. The experimental structural parameters agreed with the theoretical values.

The unexpectedly high stability of the dioxirane CF2O2, its unique geometry, its infrared spectrum and NMR chemical shifts were determined and studied on the basis of extended ab initio computations including seven different techniques and nine different basis sets. Results indicated that the unique electronic structure of the said dioxirane is caused by CF,CF bond interactions which produce a relatively small FCF angle and enhanced pi-donation from the F atoms to the antisymmetric Walsh MO.

NMR/ab initio/IGLO techniques show that silylium cations, R3Si+ and R2HSi+, (R = methyl, ethyl, butyl), form complexes with solvents, (S), in different ratios at varying temperatures. Complex formation depends upon the steric effects of R and S groups, internal stabilization of silyl cation by alkyl group via hyperconjugation, and external stabilization by S-coordination. A tetracoordinate Si is formed with solvents more nucleophilic than methylene chloride in R3Si(S)+ and R2HSi(S)+ complexes. R(sub 3-n)HnSi+, n > or = 1, complex has a pentacoordinate Si and two S molecules.

The stability and structure of trishomocyclopropenylium cations are examined. The magnetic properties and rearrangement possibilities of these cations are also considered. These compounds are investigated with the help of a 6-13G(d) basis set that allows various perturbation levels to be analyzed. Results show that trishomoaromaticity is best indicated by chemical shifts in nuclear magnetic resonance tests and by magnetic susceptability.