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The decomposition reaction mechanism of the anions generated by atmospheric pressure chemical ionization mass spectrometry, in the negative mode, of nitroanilines and 2,4-dinitroanilines is probed using quantum chemical calculations. The result show that the effects of the amino and nitro groups as well as that of the negative charge on the nitro-nitric interconversion are small and do not qualitatively modify the landscape of the pertinent energy surface and thereby the NO-loss mechanism.

An experimental and computational study of hydrogen atom adducts to adenine is conducted. The result shows that the relative free energies of adenine radicals in aqueous solution point to the C-8 adducted as the most stable tautomer, which is predicted to be the predominating product at thermal equilibrium in solution at 298 K.

The dehydrobenzonitrilium distonic radical cations, isomers of benzonitrile radical cation, are prepared by collisional dehalogenation of protonated halogenobenzonitriles and characterized by tandem mass spectrometry making use of hybrid spectrometer of sector-quadrupole-sector configuration. The results show that although the distonic ions are less stable than the ion benzonitrile radical cation by ca. 45-50 kJ mol(super -1), they are protected against isomerization by relatively large energy barriers.

The first theoretical analysis of the interaction between the three thiouracils and one or three water molecules is presented. The geometries, relevant vibrational properties, and the binding energies with water are discussed.

The interaction of 2-hydroxy, 4-hydroxy, and 2,4-dihydroxy tautomers of uracil with a water molecule are studied at the B3LYP/6-31+G(d,p) computational level. Depending on the nature of the tautomers, cyclic or open structures are formed.

An analysis of electronic and structural features of p-phenylbisphosphinidene and the analogous p-phenylbisnitrene, p-phenylbismethylene, p-phenylbis(fluoromethylene) and p-phenylbis(chloromethylene) molecules is presented. P-phenylbisphosphinidene shows an open-shell singlet diradical ground state with a quinone-like structure.

Research is presented describing the use of density functional theory, together with the 6-31+G(d,p) basis set, to report on the energies, harmonic vibrational frequencies and the optimized geometries of the cyclic structures of monohydrated adenine and guanine complexes.

The potential energy surfaces of dithioformic acid (DTFA) and its functional isomer, dithiohydroxy carbene, in both neutral and ionized states were studied through ab initio molecular orbital computations at both (U)-MP2 and (U)QCISD(T) levels with the 6-31G(d,p) and 6-311++G(d,p) basis sets. In the molecular decomposition of DTFA, the primary channel was verified to be a dehydrogensulfidation (CH(=S)SH -> C=S + H2S) followed by a dehydrogenation.

A study on the pyrolysis of acetylene investigated the kinetics and the chemistry, particularly the potential energy surface, of a NO-C2H2 system in its electronic doublet ground state using electronic structure and rate theory calculations. On the doublet potential energy state, five intermediates were found to be involved in the formation of the most favorable dissociation product, HCO + HCN. Other experimental results are discussed.

A study was conducted to analyze the (C3,H5,N) potential energy surface. Total, relative and zero-point energies and isomers were examined. The products of methyl cation formation and methyl radical loss have also been analyzed. Results indicated that isomerization takes place before dissociation during high-energy CID experiments. Findings also showed that monotonic increases with potential energy.

Theoretical studies were conducted to examine the CH3CO + Cl reaction kinetics. The objective of the study is to determine the saddle points for the important channels and to gain an insight into the reaction kinetics. Higher-level calculations were used to reexplore the singlet ground-state potential energy surface including the missing portions. Results indicate that the reaction is a capture-limited association-elimination reaction with a very high rate coefficient.

Research was conducted to examine the CH2N system in both lowest lying doublet and quartet electronic stats. Ab initio molecular orbital measurements at the second-order perturbation theory and coupled cluster singles and doubles with corrected triples levels with 6-311++G(d,p) and 6-311++G(3df,3dp) basis sets were performed to construct the potential energy surfaces associated with the various reactions of the system. The quantum Rice-Ramsperger-Kassel theory was used to calculate rate constants using barrier heights, vibrational wavenumbers and moments of inertia.

A study was conducted to obtain the equilibrium geometries and binding energies of the 1:1 gas-phase complexes between CH4 and HX (X=f,Cl,Br). The geometries of the monomers were optimized at second-order Moller-Plesset perturbation theory level. Results indicated that the proton of H-X establishes a hydrogen bond with the center of one of the tetrahedral faces of methane supporting a near C(sub 3v) symmetry during equilibrium geometry.

A study was conducted to analyze the inversion processes in substituted phosphines and their radical cations by high-level ab initio molecular orbital and density functional theory computations. The various points of the potential energy surfaces were determined using the 6-311G(d,p) basis set. Experimental results indicated the existence of two distinct inversion pathways in substituted phosphines and their radical cations.

A stud was conducted to investigate the different aspects of cycloaddition reactions between a 1,3-dipole and a phosphorus-containing dipolarophile. Four 1,3-dipoles including H2CNN, HCNO, HNNN and H3CNNN and two dipolarophiles, namely, HCP and H3CCP were selected for the experiment which used the Gaussian 94 program package for all calculations and the MK option for the evaluation of the atomic charge. Results of the study are discussed.

A study was conducted to compare the optimized geometries, the characteristic vibrational modes and the energies of the uracil- and thymine-water complexes and to discuss these parameters as a function of the basicity of the O atoms and the proton donor ability of the NH groups. The most interesting results obtained in this theoretical study are that the most stable cyclic complexes between uracil or thymine with one water molecule are formed on the O atom having the lowest proton affinity and the NH site having the highest acidity.

Mechanisms of the reaction happening at the multichannel lowest doublet potential energy surface of the [H2NO] system containing H2NO, HNOH and NOH2 intermediates and including H2 + NO, NH2 + O, NH + OH, and H + HNO entry channels were investigated by applying unrestricted coupled-cluster formalism. Direct hydrogen abstraction reaction was determined to be the primary channel of the H2 + NO process and it contributes mainly to the total rate constant of the reaction.

The influence of the solvent on the thiol-thione tautomeric equilibrium of monomeric thioformic acid was studied. Results showed that both forms convert into each other by a 1,3-hydrogen shift. Specific solvent effects were evaluated by constructing supermolecules formed by a solute molecule and a model solvent molecule, namely, dimethyl ether. It was also found that in polar and aprotic solvent, specific interactions between solute and solvent molecules resulting in strong molecular complexes play a significant role in the control of the thermodynamic equilibrium.

The electronic structure calculations are used to show that the borane molecule (B[H.sub.3]) act as an efficient bifunctional acid-base catalyst in the [H.sub.2] elimination reactions of X[H.sub.n]Y[H.sub.n] systems (X, Y=C, B, N). It is found that the B[H.sub.3] acts as a bifunctional catalyst facilitating H-transfer leading to [H.sub.2] bond formation.

The coupled cluster theory CCSD(T) calculations are used to reevaluate the thermochemical parameters of three [C.sub.2][H.sub.5]O radicals derived from ethanol. Evidence is provided that heavy atom tunneling can play a role in the rate constant of beta-C-C scission in alkoxy radicals.

Proton affinities (PAs) of a series of H-X compounds and the corresponding methyl-(H3C-X) and phenyl -(Ph-X) substituted derivatives with a variety of proton acceptor atoms, such as C, O, N, Si, P, S Cl was determined by density functional theory. The results indicate a substituent effect on PAs and this substituent effect on the PAs of the ring carbon atoms follow a good Hammett-type correlation.

The potential energy surface for the O((super 3)P) + C2H4 reaction, which plays an important role in C2H4/O2 flames and in hydrocarbon combustion in general, was theoretically reinvestigated using various quantum chemical methods, including G3, CBS-QB3, G2M(CC,MP2), and MRCI. The product yields are found to show a monotonic dependence on temperature.

The electronic structure aspects of M[(super 1)S,(super 3)P] + N2O(X(super 1)sigma(super +) reactions is investigated using the CASSCF/MRMP2 computational methodology. The results reveal that the techniques based on the multiconfigurational wave functions are unavoidable if a correct topology of the potential energy surfaces (PESs) governing these reactions is needed.

The basicity of phosphinine in reevaluating its proton affinity (PA) and gas-phase basicity and the pKa value of its protonated form is investigated. The reevaluated pKa value could be used as a reference on the studies of phosphinines.

Combined experimental and computational study of adenine ion dissociations to shed some light on the formation, structures, and stabilities of distonic and ylid ions derived from adenine. The deprotonation of the amino group in the ionized adenine moiety in DNA is expected to produce a highly reactive radical that is able to abstract a hydrogen atom from thymine and amino acid residues exothermically.

The electronic mechanisms of 1,3-dipolar cycloaddition reactions of fulminic acid (ONCH) and diazomethane [NNCH(sub 2) with HC triple bond CH, N triple bond CH, and P triple bond CH are studied using ab initio molecular orbital methods. The 1,3-dipolar cycloaddition reaction mechanisms of ONCH and NNCH(sub 2) with HCCH, NCH and PCH dipolarophiles could be classified using the CiLC-IRC analysis on the basis of ab initio multireference wave functions.

The reactions of nitrous oxide with alkali-metal atoms was examined by means of CASSCF (11/12/) MR-MP2 calculations. The results provide clarifying insights into the nature of a number of previous kinetic and spectroscopic observations.

The reactivity of different radicals and substrates towards hydrogen abstraction and radical addition reactions is critically analyzed by using density functional theory based reactivity descriptors, namely, local softness and electronegativity. The regiochemistry of the radical addition reaction is explained from the local softness values of the potential additional sites.

Some spectral and thermochemical properties of 1H-phospholes are reinterpreted. The site of protonation and the basicity of 1H-phospholes in both the gas phase and acidic solution is investigated.

Ab initio calculations determining structures of the tetranitrogen N4 (super .+1) / N4 system and mass spectrometric experiments were carried out to understand the process occurring in neutralization-reionization mass spectrometric (NRMS) experiment starting from a linear N4 (super .+1) radical cation. The results suggest that although azido-nitrene was apparently generated in NRMS experiments, only a very small fraction of the N4 neutral could effectively be reionized, and the resulting spectra could not be reproduced easily, when changing even slightly the experimental conditions.

The potential-energy surface of the uracil tautomerization was compared to twelve different tautomeric forms, except uracil itself. The most stable uracil structure is the 2-hydroxy-4-oxo form 3,2(sub 1)-U with the tautomerization energy E(sub rel)=45.6 kJ mol (super -1), where the zero-point vibrational energies (ZPVE) contribution provides only 1.8%.

The energies of minima and TS as well as characteristic NMR chemical shifts are reported for the parent unsaturated phospho-organic molecules with CP2 backbone. The two most stable structures out of the fourteen relevant isomers are 1,2-diphospha-1-propene, and diphosphirane. The calculated energy barriers of CH3 - P = PH indicate that this isomer should be kinetically stable as isolated molecule.

Protonation of weak piH-bonded benzene-water complex is studied. Experiments prove that proton affinity (PA) of ammonia is more basic than benzene and therefore protonation of benzene-ammonia complex is a single stage with the excess proton attaching to ammonia.

The first experimental and theoretical results are reported concerning the reaction of [1,3-butadiene] (super +) radical cation. Experiments are conducted in the gas phase and under low pressure.

Two theories for investigating the potential energy surface of the [C2,H3,N,O] system in its electronic singlet ground state are explored. Reactive intermediates are identified.

The efficient formation of HCN/HNC fromformamide (FM) combiningthe advantages of water-assistance, self-catalyzed reactions, andthe mineral surfaces was investigated. Periodic density functionaltheory calculations with plane-wave pseudopotential basis sets wereperformed to study the interaction of FM with pyrite (100) ideal anddefect surfaces. Effects of sulfur vacancy defect and water on tautomerizationand rearrangement barriers of FM on the (100) surface were evaluated.Calculated results show that FM adsorbs more strongly on the defectsurface than on the ideal surface, with the lowest adsorption energyon the defect surface being −22 kcal/mol. The energy barriersfor rearrangements of FM on these two surfaces being close to eachother suggests that the adsorptions on the surfaces have small effectson the energy barriers. The energy barriers for formimic acid isomerformations are 44.5 and 46.0 kcal/mol, and those of aminohydroxymethyleneformations are 72.6 and 71.9 kcal/mol on the ideal and defect surfaces,respectively. A reduction of ∼30 kcal/mol in tautomerizationenergy barriers is observed in water-assisted process on the defectsurface. Because this reduction is close to that of the gas-phasereactions, the catalytic effect is clearly due to the presence ofwater molecule instead of the interaction with the surface. In thiscase, the pyrite surfaces with the ability to accumulate reactivespecies only play the role of connecting bridges between the two stepsof the proposed reaction mechanism: the water-assisted rearrangementand the self-catalyzed dehydration. [ABSTRACT FROM AUTHOR]

The potential energy surfaces of acetyl cyanide and its functional isomer acetyl isocyanide in their electronic ground state were studied through ab initio molecular orbital MP2(FU)/6-311G** and QCISD(T)/6-311G** computations. The lower well depth of acetyl isocyanide suggested that the isomerization is a reversible process and the equilibrium is more toward acetyl cyanide. The single-step unimolecular dissociation of CH3C(O)CN into HCN and ketene has an activation barrier around approximately 65 kcal/mol and is expected to contribute in the unimolecular dissociation kinetics of CH3C(O)X at high temperatures.

A comprehensive theoretical investigation on structuresand propertiesof niobium clusters in the range from 13 to 20 atoms, in three differentcharged states, is performed by using the BPW91 and M06 functionalsand the cc-pVDZ-PP basis set. These species are predicted to preferlow spin ground state, i.e., singlet (for even electron) and doublet(for odd electron) systems. In terms of growth mechanism, a compactstructure with one Nb encapsulated by a cage formed from five andsix triangles is found to be favored over an icosahedral evolution.Unlike many 3d metals, whose volumes are much smaller, 13 and 19 Nbatoms clusters do not exist as icosahedra and double-icosahedra. Adistinct case is Nb15as it bears a slightly distortedbcc structure. For some systems, several lower lying isomers are computedto be so close in energy that DFT computations cannot clearly establishtheir ground electronic states. The existence of structural isomerswith comparable energy content is established for Nbnspecieswith n= 13, 18, 19, and 20 in both neutral and chargedstates. The vibrational (IR) spectra are also calculated. While thespectra of smaller systems are strongly dependent on addition or removalof an electron from the neutral, the spectra of the larger size clustersare mostly independent of the charged state. The neutrals and theircorresponding ions usually have a quite similar IR pattern. Electronaffinities (EA), ionization energies (IE), average binding energies,dissociation energies, and frontier orbital energy gaps are evaluated.The computed EAs and IEs are generally in fair agreement with experiment.The Nb15system is observed to be stable and it can forma highly symmetric structure in all charged states with both openand closed electron shells. [ABSTRACT FROM AUTHOR]