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Research is done on the electronic structures and vibrational frequencies of dimethyl-substituted carbonyl oxide and its cyclic isomer dimethyldioxirance have been investigated using high-level ab initio quantum mechanical technique with large basis sets. In conclusion, it is hoped that the theoretical frequencies for the dimethyl carbonyl oxide isomer will stimulate new experiments.

Issues concerning the prediction of the adiabatic electron affinities of C3O and C4O based on electronic structure calculations are discussed. There is strong evidence that the reported value for the electron affinity of C3O is rather high.

Research is presented demonstrating the single, double and triple bonds of binuclear homoleptic nickel carbonyls. Central bond distances are predicted and prospects for related dinickel compound synthesis are discussed.

The first study of the O-H stretching spectrum of gaseous ethanol and butanol clusters is presented. It is suggested that more than one dimer conformer is present in the molecular beam.

Research has been conducted on the relative energies of dihydroxycarbene, formic acid, dioxirane and hydroperoxycarbene. The use of BLYP and B3LYP density functions is discussed.

An investigation of the energetics and physical properties of the four low-lying electronic states of NH2 is presented. The energetics and physical properties of the three equilibrium states studied may be accurately obtained in the variational sense.

Research was conducted to predict the structures and energetics of molecules and anions in the series BrClF(sub n), where n = 1-5, using density functional methods. The vertical electron affinities, adiabatic electron affinities of the BrClF(sub n) molecules and the vertical detachment energies of the BrClF(sub n)- anions were predicted. Results demonstrate that the predicted EA(sub ad) for BrClF4 exceeds the corresponding values of BrF5 by 0.7-1.0 eV while BrF4 and particularly BrF6 have the highest values among the bromine halides.

Research was conducted to examine in detail the three different conformations of cyclopentasilane, Si5H10, by means of ab initio and density functional methods at the HF/B2, MP2/B2, CCSD/6-31G* and B3LYP/B2 levels of theory. The objective was to compute molecular structures containing second-row elements using high levels of theory. Results showed that the averaged Si-Si bond lengths revealed no significant differences between C(sub s) and C2 for the levels of theory employed.

The energies and structures of the three tetrahydrobenzene (C6H2) isomers (ortho, meta and para) were investigated through density functional theory, complete active space and the coupled-cluster technique with single, double and perturbative triple excitations in combination with basis sets of up to triple-zeta plus double polarization quality. Results showed that the meta isomer 1,2,3,5-tetrahydrobenzene is the most favorable cyclic isomer, but is 38 kcal/mol less stable than the hexatriyne C6H2 global minimum. All of the cyclic isomers have single ground states, but these are less stable for the ortho and para isomers. Thus, the single-triple energy splittings for 1,2,4,5 and 1,2,3,4-tetrahydrobenzene are smaller for ortho-benzyne.

The three lowest-lying electronic states of the aluminum hydride anion were investigated. Self-consistent-field, two-configuration self-consistent field, complete active space self-consistent-field, configuration interaction including single and double excitations and CASSCF-based second-order interaction levels of theory were used with five basis sets of triple-zeta quality. The total electronic energies, the dipole moments, harmonic vibrational frequencies and associated infrared intensities were determined for each electronic state.

Binuclear alkyne manganese carbonyls of the type (RC≡CR')Mn2(CO)n (R and R'=methyl or dimethylamino; n=8, 7, 6) and their isomers related to the experimentally known (MeC2NEt2)Mn2(CO)n (n=8, 7) structures have been investigated by density functional theory. The alkyne ligand remains intact in the only low energy (Me2N)2C2Mn2(CO)8 isomer, which has a central Mn2C2 tetrahedrane unit and is otherwise analogous to the well‐known (alkyne)Co2(CO)6 derivatives except for one more CO group per metal atom. The low‐energy structures of the unsaturated (Me2N)2C2Mn2(CO)n (n=7, 6) systems include isomers in which the nitrogen atom of one of the dimethylamino groups as well as the C≡C triple bond of the alkyne is coordinated to the central Mn2 unit. In other low‐energy (Me2N)2C2Mn2(CO)n (n=7, 6) isomers the alkyne C≡C triple bond has broken completely to form two separate bridging dimethylaminocarbyne Me2NC ligands analogous to the experimentally known iron carbonyl complex (Et2NC)2Fe2(CO)6. The (alkyne)Mn2(CO)n (n=8, 7, 6) systems of the alkynes MeC≡CMe and Me2NC≡CMe with methyl substituents have significantly more complicated potential surfaces. In these systems the lowest energy isomers have bridging ligands derived from the alkyne in which one or two hydrogen atoms have migrated from a methyl group to one or both of the alkyne carbon atoms. These bridging ligands include allene, manganallyl, and vinylcarbene ligands, the first two of which have been realized experimentally in research by Adams and coworkers. Theoretical studies suggest that the mechanism for the conversion of the simple alkyne octacarbonyl (MeC2NMe2)Mn2(CO)8 to the dimethylaminomanganaallyl complex Mn2(CO)7[μ‐η4‐C3H3Me2] involves decarbonylation to the heptacarbonyl and the hexacarbonyl complexes. Subsequent hydrogen migrations then occur through intermediates with C−H−Mn agostic interactions to give the final product. Eight transition states for this mechanistic sequence have been identified with activation energies of ∼20 kcal/mol for the first hydrogen migration and ∼14 kcal/mol for the second hydrogen migration. [ABSTRACT FROM AUTHOR]

Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H2O)3 → HCl + (H2O)2OH using a standard DFT method. As found for the analogous fluorine reaction, the geometrical and energetic results for the four chlorine pathways are closely related. However, the energetics for the Cl reaction are very different from those for fluorine. In the present paper, we investigate the lowest-energy chlorine pathway using the "gold standard" CCSD(T) method in conjunction with correlation-consistent basis sets up to cc-pVQZ. Structurally, the stationary points for the water trimer reaction Cl + (H2O)3 may be compared to those for the water monomer reaction Cl + H2O and water dimer reaction Cl + (H2O)2. Based on the CCSD(T) energies, the title reaction is endothermic by 19.3 kcal mol−1, with a classical barrier height of 16.7 kcal mol−1 between the reactants and the exit complex. There is no barrier for the reverse reaction. The Cl⋯(H2O)3 entrance complex lies 5.3 kcal mol−1 below the separated reactants. The HCl⋯(H2O)2OH exit complex is bound by 8.6 kcal mol−1 relative to the separated products. The Cl + (H2O)3 reaction is somewhat similar to the analogous Cl + (H2O)2 reaction, but qualitatively different from the Cl + H2O reaction. It is reasonable to expect that the reactions between the chlorine atom and larger water clusters may be similar to the Cl + (H2O)3 reaction. The potential energy profile for the Cl + (H2O)3 reaction is radically different from that for the valence isoelectronic F + (H2O)3 system, which may be related to the different bond energies between HCl and HF. [ABSTRACT FROM AUTHOR]

Aluminyl anions are low‐valent, anionic, and carbenoid aluminum species commonly found stabilized with potassium cations from the reaction of Al‐halogen precursors and alkali compounds. These systems are very reactive toward the activation of σ‐bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation⋯π interactions with nearby (aromatic)‐carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing K⋯H bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H2 molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H2 utilizing a NON‐xanthene‐Al dimer, [K{Al(NON)}]2 (D) and monomeric, [Al(NON)]− (M) complexes are studied using density functional theory and high‐level coupled‐cluster theory to reveal the potential role of K+ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H2. The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol−1). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H2 distorts the most, usually over 0.77 ΔEdist≠. Overall, it is found here that electrostatic and induction energies between the complexes and H2 are the main stabilizing components up to the respective transition states. The results suggest that the K+ atoms act as stabilizers of the dimeric structure, and their cooperative role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H2. Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H2 is wholly disengaged. [ABSTRACT FROM AUTHOR]

The molecular structures, electron affinities and dissociation energies of the BrF(sub n)/BrF(sub n)(super -) (n = 1-7) molecules were investigated by applying hybrid Hartree-Fock/density functional theory. The DZP BHLYP program was found to be most reliable technique for computing the electron affinities and molecular structures of the bromine fluorides. The neutral BrF(sub n) distances varied from 1.70 to 1.83 Angstroms.

The n-propylperoxy radical has been described as a molecule of critical importance to studies of low temperature combustion. Ab initio methods were used to study this three-carbon alkylperoxy radical, normal propylperoxy. Reliable CCSD(T) (coupled-cluster theory, incorporating single, double, and perturbative triple)/ANO0 geometries were predicted for the molecule's five rotamers. For each rotamer, energetic predictions were made using basis sets as large as the cc-pV5Z in conjunction with coupled cluster levels of theory up to CCSDT(Q). Along with the extrapolations, corrections for relativistic effects, zero-point vibrational energies, and diagonal Born-Oppenheimer corrections were used to further refine energies. The results indicate that the lowest conformer is the gauche-gauche (GG) rotamer followed by the gauche-trans (0.12 kcal mol-1 above GG), trans-gauche (0.44 kcal mol-1), gauche'-gauche (0.47 kcal mol-1), and trans-trans (0.57 kcal mol-1). Fundamental vibrational frequencies were obtained using second-order vibrational perturbation theory. This is the first time anharmonic frequencies have been computed for this system. The most intense IR features include all but one of the C-H stretches. The O-O fundamental (1063 cm-1 for the GG structure) also has a significant IR intensity, 19.6 km mol-1. The anharmonicity effects on the potential energy surface were also used to compute vibrationally averaged rg,0K bond lengths, accounting for zero-point vibrations present within the molecule. [ABSTRACT FROM AUTHOR]

The iron carbonyls Fe(CO)5, Fe2(CO)9 and Fe3(CO)12 were studied using hybrid Hartree-Fock/density functional theory. The geometries of the molecules were fully optimized at four levels of theory, namely, DZ BP86, DZP BP86, DZ B3LYP and DZP B3LYP. Vibrational frequences were gathered by analytically evaluating the second derivatives of energy with respect to the nuclear coordinates. The theoretical geometries and harmonic vibrational frequencies were compared with experimental results. In the case of Fe3(CO)12, most of its vibrational frequencies have never been assigned experimentally.

A study was conducted on the benzene potential energy hyperface was explored using second-order perturbation theory, hybrid Hartree-Fock/density functional theory and the coupled-cluster method with single, double and perturbative triple excitations. Results revealed that there are three competing mechanisms for the high-temperature intramolecular topomerization of benzene. However, it was unlikely that benzene topomerizes at 1373 K exclusively via one mechanism since these mechanisms were all within 3.2 kcal mol.

The title electronic states of the nitrenium ion have been investigated by ab initio molecular electronic structure calculations. The equilibrium geometries, harmonic vibrational frequencies, infrared intensities and dipole moments were determined by self-consistent-field, configuration interaction with single and double excitations and complete active space wave functions. The results showed that the ground, first and second excited states are bent while the third excite state is linear in geometries.

The ring of the model compound (AlH-NH)2 can be seen in a plane where aluminum and nitrogen bond lengths are found. Although the model compound and the aluminum and nitrogen bonds are of the same plane, no significant phi-interaction occurs. The nitrogen atom remains planar because of the large aluminum and nitrogen charge transfer. Furthermore, the planar nitrogen permeates some phi-back-donation to the aluminum.

The characteristic of the triple bonding between gallium (Ga) atoms was determined through a theoretical analysis of a gallyne complex's model electronic structure and related molecules. The existence of the triple bond was doubted since the bond length was shorter than known Ga-Ga single bonds. It was observed that bent structures of model molecules contained triple bonds composed of two dative and a pi bond although the dative bonds in the molecules were weak. The nature of the chemical bond was determined mainly by the electronic structure and not by the molecular geometry.

Quantum mechanical and molecular mechanical studies were performed to investigate the potential energy surfaces of all eight D-aldohexoses and four D-ketohexoses. All of the hexoses studied exhibited anomeric preferences for the axial OH positions. The gas-phase potential energy surfaces of the cyclic hexoses were heavily influenced by the anomeric effect and intramolecular hydrogen bonding. It was also observed that the furanose forms were more stable than the pyranose forms for all of the hexoses except for galactose, idose, psicose and sorbose.

A study was conducted to characterize the ring opening of trans- and cis-cyclopropylidenes to cyclic allenes using ab initio quantum mechanical computations and density functional theory. Second-order Moller-Plesset perturbation theory was also carried out to determine the geometry optimizations of cyclic allenes. Results suggested two possible disrotatory motions for the ring opening of the cyclopropylidenes.

Density functional, many body perturbation and coupled-cluster theories were used to study the possible energy surfaces for methylene and fluorinated methylenes insertion reactions into the Hsub2, HF, and Fsub2 molecules. The barrier for the CFsub2 insertion was predicted to be lower than previous estimates, and a new transition state was observed for the reaction path leading to CHF+HF.

The disagreement between the stability of carbocation substrates and the energies of solvolysis transition states leading to the formation of epimeric norbornyl derivatives was analyzed. Analysis of the stabilization of endo and exo reactions revealed that an energetic bias existed favoring either endo or exo reaction in the paucity of a bridging reaction. The increased stabilization in the nonclassical 2-carbonyl cation resulted to the bridging reaction.

A theoretical research project to analyse the behaviour of the unstrained carbene ethylmethylcarbene and the strained carbenes cyclobutylidene, 2-norbornylidene and 2-bicyclohexylidene in various chemical reactions establishes that the simple acyclic dialkyl carbene normally uses the 1,2-H-migration pathway with a deltaG activation energy of around 5 kcal per mole. Upon rearrangement, a primary or secondary alkene has a deltaG activation energy of around 3 kcal per mole. Other observations are made.

The characteristics of AlCl3NH3 adduct and its dissociation products AlCln, NHn(n=1-3), ring and cluster compounds [(Cl2AINH2)n(N=1,2), (ClAlNH)n(n=1,2,3,4,5,6)] are investigated. The coordination numbers of the aluminum and nitrogen centers appear to influence the length of the Al-N bonds. Research figures show that 2.0 A lengths are apparent in 4-coordinated Al/N centers while 1.68 A lengths are apparent in 2-coordinated Al/N centers. The research describes AlN deposition involving cluster formation in the gas phase.

The naphthylcarbene potential energy surface (PES) was analyzed ab initio, using self-consistent field (SCF), second-order perturbation (MP2) and density functional (Becke3LYP) techniques in conjunction with 6-31G*, DZ, DZP and 6-311+G* basis sets. Both 1- and 2-naphthylcarbene (1 and 2) have triplet ground states but the delta-E(sub ST)'s are small, similar to phenylcarbene. The triplet rotational barrier for the exo-methylene in 2 is relatively small because of comparable electronic interactions in the ground and transition structures. An in-plane triplet transition structure is a stationary point of Hessian index two, but is also low-lying.