Ab initio study of hydrazoic acid

SCF and CI calculations were carried out on the ground^1A state of HN₃. The equilibrium geometry and vibration frequencies were computed. The results point to a planar structure (groupC _s) but to a non-linear (170 °) N-N-N conformation. The calculated vibration frequencies are in fair agreement with experimental assignments.The dissociation path of the molecule to NH and N₂ products was investigated and compared to the isoelectronic reaction of diazomethane. The dissociation energy of hydrazoic acid is estimated to be about –8 kcal/mole, with a potential barrier to dissociation of about 30 kcal/mole.Boursier IRSIA     

The electronic structure and one-electron properties of BH computed from SCF and CI wave functions

The SCF and CI wave functions for BH, obtained in calculations described in detail elsewhere [2], are compared through their electron distributions and electron moments.Taken in part from a Ph.D. thesis submitted to the University of Toronto in 1971.     

Geometry optimization inab initio SCF calculations

The floating orbital geometry optimization (FOGO) described previously [1, 2] for atoms without polarized inner-shell electrons, is extended to the general case. Instead of the Hellmann-Feynman force a special gradient is calculated analytically and utilized in a variable metric procedure simultaneously with the ordinary energy gradient. Test calculations on a sample of 12 molecules were performed to check the efficiency of the method. The geometries obtained are better than those obtained with the corresponding double-zeta basis set. The most striking results, however, are excellent dipole moments.     

Ab initio calculation of some low-lying electronic excited states of methane

The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions.     

Clusters of hydrogen molecules: Ab initio SCF calculations corrected semiempirically for correlation energies

Stabilization energy of the (H₂) _n clusters (n = 2–8) was calculated as a sum of the SCF interaction energy and the semiempirical interaction correlation energy estimated according to Sinanolu and Pamuk. Optimum successive attachment of hydrogen molecules leads to the formation of a gas-phase solvation shell consisting of seven hydrogen molecules. Basis set effect has been found to be important with all clusters under study. The non-additivity effect was investigated with the (H₂)₄ cluster. Vertical ionization potentials of the clusters considered are predicted to be 0.4–0.6 eV lower than the ionization potential of the parent H₂ molecule.     

Ab initio Studies on polymers

Ab initio crystal orbital calculations have been performed on the infinite all-trans polyene. A structure with r _c=c= 1.346 Å, r _c-c = 1.446 Å, r _c-h = 1.08 Å, and CCC = 125.3 ° was found to be most stable. The most important force constants, the band structure and the density of states were determined as well.     

Theoretical assignments of the electronic spectrum of acetylene

Non empirical calculations of energies and properties of some excited states of acetylene are presented. A frozen core approximation is used and excitations to , and MO's are taken into account. Both valence and Rydberg states are considered. Assignments of the UV and electron impact spectra are proposed and some questions are raised.     

Theoretical study of the electronic structure of diazomethane

The least-energy dissociation path of the ground state of CH₂N₂ was determined fromab initio calculations using in a complementary way basis sets of minimal size (STO-3G) and double-zeta (DZ) quality. The results indicate that the least-energy point of attack of the N₂ molecule on CH₂ (¹ A ₁) is roughly perpendicular to the molecular plane (93 °), the C and N atoms being almost co-linear (angle C-N-N203 ° with outermost N atom pointing away from CH₂). The potential barrier of 1.2 eV found previously on theC _2v dissociation path, disappears completely along the least-energy dissociation path (point groupC _s (out-of-plane)). These findings corroborate the Woodward-Hoffman rules for this process since the outermost orbitals of the two intersecting states found in point groupC _2v (...2b ₁ and ...8a ₁) both correlate to the same irreducible representation (10á) in point groupC _s (out-of-plane).Larger basis set calculations (DZ + polarization functions on all centers, 3d _c and 3d _N developed here), were also carried out on CH₂N₂ (¹ A ₁,³ A ₂ and¹ A ₂) at the¹ A ₁ equilibrium geometry and on CH₂ (³ B ₁) and N₂ (¹ _g ⁺ ) at their respective equilibrium geometries. These calculations, together with consideration of correlation energy differences, yieldD ₀ ⁰ (CH₂N₂,¹ A ₁) = 19 kcal/mole and vertical excitation energies of 67 and 73 kcal/mole for the³ A ₂ and¹ A ₂ states respectively. The latter value is in good agreement with the measured experimental value: 72.4 kcal/mole corresponding to the maximum of intensity in the¹ A ₂¹ A ₁ absorption band.     

An ab initio study of formamide

Calculated energy and molecular properties of the ground and low-energy excited states of formamide are presented at the ground state geometry. Satisfactory results are obtained except for the ¹* energy which remains too high by 1 eV (which is nevertheless a large improvement over previous calculations). The predicted triplet energies lie at 5.4 eV (³ n*) and 5.8 eV (³*).     

SCF MO CI perturbation calculations of inner shell and valence shell vertical ionization potentials

A CI method for calculating inner and valence shell vertical ionization potentials is presented. It is based on ab initio SCF MO calculations for the neutral closedshell ground state followed by CI perturbation calculations for the ground and ion states including all spin and symmetry adapted singly and doubly excited configurations with respect to the main configurations of the state of interest. The state energy is computed by performing a CI calculation for a set of selected configurations, and then adding the contributions of the remaining configurations as estimated by second order Brillouin-Wigner perturbation theory. The use of the same set of MO's for all states together with the CI perturbation method makes the method rather rapid. The numerical results are, in spite of the limited Gaussian basis sets used, in good agreement with experiment.     

Delocalization corrections to the strictly localized molecular orbitals: A linearized SCF approximation

An explicit formula is derived for calculating the delocalization corrections (tails) to be added to the strictly localized bond orbitals. It was obtained by solving analytically the SCF problem for the interbond interactions in a linearized approximation. The model calculations at the CNDO/2 level show that this simple approach is sufficient to account for the molecular conformations.     

Ab initio crystal orbital studies on linear chains of H atoms

Anab initio crystal orbital method is used to calculate the energies of an infinite chain of H atoms and of linear arrangements of H₂ molecules with different interatomic distances. The H₂ arrangements are not stable in respect to isolated molecules. The cohesive energy of an optimized arrangement of H atoms chain is 0.0354 a.u.     

SCF perturbation theory for unimolecular reactions

A CNDO/2 SCF perturbation theory is presented for interpreting the form of CNDO/2 potential energy surfaces of unimolecular reactions. The analysis is performed by calculating the energy change E arising from a distortion of the molecular geometry along the reaction coordinate. E is decomposed into different perturbational contributions which are appropriate for an interpretation of the perturbation energy E. Moreover, E is resolved into energy parts arising from a single occupied orbital and contributions due to pairwise orbital interactions. In this way one evaluates numerically how the form of the occupied and unoccupied orbitals determines the magnitude of E. If the distortion occurs along a definite symmetry coordinate, group-theoretical arguments can be applied to discuss the magnitude of characteristic components of the perturbation energy. The SCF perturbation theory is used to analyze the isomerization of ethylene, cis-2-butene and cis-2-butenenitrile.This work was partially supported by Nato-Grant No. 1072     

Conformation analysis of formic acid. Extended basis set SCF and CEPA calculations

SCF computations using extended DZP and TZP basis sets have been performed to determine the structure of syn and anti formic acid and the transition state for rotation of the OH group. Effects of electron correlation were accounted for by means of CEPA calculations which predict the anti conformer to be 5 kcal/mol and the transition state 14.7 kcal/mol higher in energy than the syn conformer, with probable error estimates of 0.7 kcal/mol and 2 kcal/mol respectively.     

Ab initio calculation of the band structure of some boron-nitrogen polymers

Ab initio calculations using a Gaussian orbital basis set were performed on the two boron-nitrogen polymer systems polyaminoborane and polyboronimide. For the polyaminoborane system an alternating B-N bond model appears to be more stable than a symmetric B-N bond model. An electron drift from the NH₂ group to the BH₂ moiety was calculated for both models although the nitrogen atom was found to possess a negative charge stemming from polarization of the N-H bonds. The energy band diagrams derived from both models show rather featureless bands indicative of weakly interactive systems although that of polyboronimide indicates that it is a more delocalized system than its saturated counterpart. The conduction and valence bands at the X-point are composed of orbitals and the lowest electronic transition is predicted to be —* in nature. The electron distribution of polyboronimide indicates a movement of -electrons from the boron to the nitrogen coupled with a smaller -electron drift from the nitrogen to the boron.     

A non-empirical molecular orbital study on the relative stabilities of adenine and guanine tautomers

The relative stabilities of a series of adenine and guanine tautomers have been calculated using anab initio Hartree-Fock-Roothaan SCF MO method. The calculated relative stabilities agree in general with the results of earlier semiempirical studies. According to the present study, tautomeric forms with regular Kekulé structure for the six-membered purine ring are the most stable. The amine-imine tautomerization of purine bases is not likely to be responsible for spontaneous mutations in DNA.     

A theoretical study of electrophilic reactions

The electronic structures of protonated formyl and acetylium cations and their deprotonation paths leading to HCO⁺, COH⁺ and CH₃CO⁺have been studied by means of ab initio calculations. The results support Olah's theory that dipositive species can be the de facto reagents in electrophilic reactions.     

Theoretical interpretation of the valence X-ray photoelectron spectrum of TiC

The valence X-ray photoelectron spectrum of TiC is interpreted in terms of the density of states and in terms of a simple model for transition matrix elements. The conceptual difficulties arising with both the initial and the final state are discussed.     

Ab initio studies of the low-lying π-states of borazine

Self-consistent-field and configuration-interaction studies were performed on borazine, using a double-zeta basis set augmented by six diffuse -functions. Low-lying singlet and triplet states of the A ₁ , A ₂ and E species were calculated, corresponding to ^* excited valence and Rydberg states. A selection out of singly and doubly excited configurations relative to a set of reference configurations was made for each species. Calculated vertical excitation energies (in eV) are 7.12 for ¹ A ₂(V), 8.45 for ¹ E (3p-R), 8.57 for ¹ A ₁(V), 8.9 for ¹ E(V-R), 9.55 for ¹ E(3d-R), 6.98 for ³ A ₁(_V), 7.27 for ³ E(V), 7.82 for ³ A ₂(V), 8.30 for ³ E(3p-R), and 9.5 for ³ E(3d-R), where V and R refer to valence or Rydberg character. The results are compared with experimental excitation energies, previous ab initio studies of borazine, and with recent ab initio studies of benzene.Dedicated to Professor Dr. H. Hartmann on the occasion of his 65th birthday.