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The Gaussian-2 computational method has been applied to study the following two reactions: (1) dihydrogen exchange between methanol and formaldehyde, and (2) hydrogen transfer in formic acid. In the investigation of these two reactions, the cases of both having and not having a catalytic water molecule have been examined. It has been found that, for the first reaction, the transition structures (TS) with and without the water molecule entail essentially the same barrier (117 kJ mol–1). On the other hand, for the second reaction, the process involving the formic acid dimer TS has a barrier of 13 kJ mol–1, while that involving a HCOOH+H2O TS has a barrier of 53 kJ mol–1. The G2 method is suggested as a reliable tool to study reactions of this type.

Ab initio MO calculations predict the activation energies for the insertions of difluorocarbene into substituted ethenes C2H3X (X = F, Cl, Me) to be 61,68 and 52 kJ mol-1, respectively; these results indicate that the monosubstitution on the ethene has no significant effects on the activation energy of the reaction.

The anion CH2N02“ has been studied with ab initio molecular orbital calculations. The structures of various CH2N02“ isomers and other related species have been determined by full geometry optimization at the Hartree-Fock level with the diffuse-function augmented basis sets 3—21+ G and 6—31+ G*. In addition, improved energy comparisons have been obtained at the MP3/6—31++G** level, and v.brational frequency analysis of the various species have been carried out in order to characterize the stationary points, as well as to determine the zero-point vibrational energies of these species. The rotation around C-N bond for CH2N02" has been calculated to have a barrier of 128 kJ mol-1. The transition structure of this process is found to have Cs symmetry. In addtion, the electron affinity of CH2N02' is calculated to be 168 kJ mol"1, not in very good agreement with the experimental estimate. The C-proton and O-proton affinities of CH2N02~ have been calculated to be 1579 and 1512 kJ mol-1, respectively. The former is in very good agreement with the experimental result.

The cyclobutyl anion has been studied with ah initio molecular orbital calculation. The structures of two cyclobutyl anion conformers and some related species have been determined by full geometry optimization at the Hartree-Fock level with the diffuse-function augmented basis sets 3-21 +G and 6—31+ G*. In addition, improved energy comparisons have been obtained at the MP3/6—31+ G* level, and vibrational frequency analysis of the various species has been carried out in order to characterize the stationary points as well as to determine the zero-point vibrational energies of these species. It has been found that the cyclobutyl anion conformer with the lone hydrogen at the anionic center occupying the equatorial position (1) is more stable than the one with hydrogen occupying the axial position (2) by 13 kJ mol-1. The transition structure for the rearrangement 2-^1 has an almost completely flattened four • -membered ring and a nearly planar geometry at the anionic center. The barrier of the rearrangement is calculated to be 24 kJ mol"1. In addition, the calculated proton affinities for 1 and 2 are comparable to those of CH3 , C2H3', and C2H5-. Calculated electron affinities for the cyclobutyl radical indicate that, even if conformers 1 and 2 are stable towards spontaneous electron loss, their ionizations potentials should be very low.

The dimerization of borane has been investigated with the MNDO method. Three pathways have been studied by imposing different symmetry restrictions: C2h symmetry, least-motion, and with no symmetry at all. The activation energies of the three pathways are 3.8, 31.5, and 2.7 kcal/mol, respectively. In view of the known tendency of MNDO to yield high energy for transition states, the low activation energy of the la'St pathway probably means that the dimerization has no or a very small energy barrier. Moreover, it turns out that C, symmetry is retained automatically for this pathway.

The method of Murrell as modified by Golebiewski has been applied to determine the maximum overlap hybrid orbitals for PF5 and SF4• For the former system, three cases have been considered: cr overlap integrals of the central atom valence and F 2p orbitals are (i) taken as angular parts of P atomic orbitals projected in the bond directions, (ii) evaluated using SCF atomic wave functions with equidistant F ligands, and (iii) calculated using experimental bond lengths. The resultant overall hybridizations are compared and discussed. For SF4, only case (iii) has been carried out. In addition, the wave function of the lone pair orbital is obtained with symmetry consideration and orthonormality relationships among all the hy- . brids. This appears to be an inexpensive way to determine the form of the lone pair and to estimate its size.

It is found that path (i), which is concerted and stereospecific, is favored both thermodynamically and kinetically, in agreement with exp~'rimental results. In addition, paths (ii) and (iii) are found to be competitive with each other and both are highly energetic due to steric effects, with activation energy being about 50 kcal/mo!.

Applying MNDO approximatlon, the heats of formation (LlHt) and structures of the navel compounds helvetane (1) and israelane (2) have been calculated. The MNDO sn, for 1 and 2 are 336 and 719 kcal/mol, respectively, in agreement with the previous prediction that 1 is more stable than 2. In addition, in view of the structures calculated, the large positive LlHt values of 1 and 2 are most likely the result of these malecules' angular strain.

The spin-allowed transitions in the visible spectra of some rr.onoacidopentaammine and diacidotetraammine complexes of Co(III), with the acido groups being C03z--, H20, N02-, 02C3H5-, p-, No3-, ci-, and Br-, have been resolved and the relevant crystal field and Racah parameters determined with a modified version of the model originally proposed by Wentworth and Piper. The tesults appear to be consistent within the series and agree well with those reported in the literature for structurally related systems.

The lowest singlet and triplet states of cyclopentadienylidene (1) have been studied with the MIND0/3 method. It is found that · the ground state of 1 is a spin triplet, which is supported by experimental finding and some previous calculations. The relationship between the geometrical and electronic structures for these states is also discussed. In studying the least-motion and non-least-motion paths of the hydrogen •insertion of 1, it is found that the latter is preferred energetically. In addHion, as in the cases of other similar carbenehydrogen reactions, the concerted non-least-motion path consists of, in order, electrophilic and nucleophilic steps. However, for the present reaction, in order to distinguish the two steps, the charges on the hydrogens, rather than that on the carbenic center, should be monitored.

Applying the maximum overlap criterion, the localized molecular orbitals of square pyramidal (CH)/ cation have been determined. The resulting multi-center bonding orbitals are examined in detail. In addition, the basal C-C and all C-H bonds are compared with those of the more familiar systems, e. g., cyclobutane and cyclobutene.

The various minima and some of the saddle points on the C3H3- potential energy surface have been studied with ab initio molecular orbital calculations. The structures of these species have been determined by full geometry optimizations at the Hartree- -Fock level with the diffuse-function augmented basis sets 3-21+G and 6-31+G*. In addition, improved energy compairsons have been obtained at the MP3/6-31 + + G** level, and vibrational frequency analysis of the various species have been carried out in order to characterize the stationary points as well as to determine zero-point vibrational energies of these species. Eight stable species have been found on the potential energy surface of C3H3-. In descending order of stability, they are: l-propynyl (1), allenide (3), cyclopropen-l-ide (5), prop-2-en-l-yliden- I-ide (6), three isomers (EZ, EE, ZZ) of prop-l-en-3-yliden-l-ide (7, 8, 9) and cyclopropen-3-ide (10) ions. The transition structures found are those for the methyl migration of 1, the inversion at the anionic center of 3, and the pseudorotation of 10. In addition, for all the equilibrium structures found, the electron affinities of the corresponding free radicals have been calculated. From these results, it is deduced that, among the eight anionic equilibrium structures, only 10 is unstable toward spontaneous electron loss.