An ab initio study of the relative stabilities of the isomers of CH2N2 and SiH2N2

Ab initio SCF calculations of the equilibrium geometries have been carried out on nine possible isomers of MH₂N₂, where M = C or Si, and compared with the results of MNDO calculations. The results for the carbon compounds are in good agreement with available experimental data, but in the case of the silicon compounds, the molecules are predicted to be unstable with respect to decomposition to SiH₂ and N₂. The inclusion of electron correlation at the MP3 level does not alter the order of the relative stabilities, although the importance of the correlation contribution varies substantially between the different isomers.     

Electron donor-acceptor complexes: Evaluation of MNDO as a computational tool to probe intermolecular interactions

Donor-acceptor pairs form EDA complexes that exist as conformational isomers exhibiting different ground-state and photochemical properties. We have sought a rapid, general, and accurate quantum mechanical computational method to generate potential energy surfaces that are representative of the donor-acceptor intermolecular interactions at the self-consistent field (SCF) level. The semiempirical molecular orbital (MO) method MNDO has been compared to ab initio methods to assess its behavior with respect to energy, dipole moment and ionization potential shifts. MNDO correctly distinguishes between repulsive and bound EDA complex states at the SCF level and produces potential curves that are smooth and free of spurious minima or cusps. MNDO curves are systematically more repulsive than those for ab initio STO-3G calculations; calculated interaction energies exhibit a mean absolute deviation of 2.90 kcal/mol. MNDO appears to provide a reliable qualitative estimate of the nondispersion portion of the interaction energy. Limitations and errors arising from minimal basis sets, single determinants, and neglect of dispersion are discussed.     

Theoretical Modeling of the Stereoisomerization of Tetracoordinated Be(II) Bis(chelate) Complexes

Quantum-chemical (semiempirical MNDO and ab initio RHF SCF with an STO-3G basis set) calculations were performed for stereoisomerization of tetracoordinated Be(II) bis(chelate) complexes with BeN₂O₂ coordination unit. The most probable pathway for their monomolecular isomerization was found to involve, in agreement with experimental data, cleavage of a Be–N coordination bond to form a tricoordinated Be intermediate (dissociation–recombination mechanism) rather than a competitive polytopal rearrangement. The planar structure of the intermediate detected upon thermo- and photoexcitation of the complexes was predicted from our calculations.     

Ab initio Calculations of the potential energy surface of the reaction of singlet methylene with the hydrogen molecule

The potential energy surface for the insertion of singlet methylene into H₂ has been computed on theab initio SCF level as well as with inclusion of electron correlation by means of the CEPA method. The results are compared with those of previous semiempirical,ab initio SCF and CI calculations. The system is a prototype of a reaction where an allowed and a symmetry-forbidden path can compete. The electron correlation energy was found to be very different for different regions of the surface, but did not have much influence on the optimum reaction path. From the computed heat of the reaction, the heat of formation of singlet methylene was estimated to be 101.5 kcal/mol. According to the calculations the reaction does not need any activation energy.     

Improvement of the hydrogen bonding correction to MNDO for calculations of biochemical interest

Recent suggestions for correcting H? Acceptor interactions within MNDO, together with results of crystallographic analysis, were used to modify this SCF semiempirical calculation for multiple hydrogen bonded associations. Thermodynamic profiles for model systems of biochemical interest such as H₂O? H₂O, hydration of neutral and charged molecules, dimerizations and proton transfers show the advantages of this method. Its treatment of charges, bonding, geometries, energetics and vibrational frequencies is shown to be comparable to ab initio calculations with various basis sets. However, basic MNDO deficiencies and criteria applied for H-bonding result in some too high barriers for proton transfers.     

Isolobal analogy between trivalent boron and divalent silicon

Singlet organosilylenes with a lone pair and an emptyp orbital are isolobal to trivalent borane if a B-H is equated to the lone pair on Si. Using this analogy, a particular isomer of CSi₂H₂ (24) is predicted to be a stable structure. MNDO calculations on24 and many of its possible isomers suggest that24 is at global minimum on the potential energy surface of CSi₂H₂.Ab initio calculations using a, minimal STO-3G basis set, on some selected structures also support these results.     

Relative stability of the2 A 1g and2 E g states of the C2H 6 + ion

Anab initio study of the relative stability for the states² A _1g and² E _g of C₂H ₆ ⁺ has been carried out. The results of the Open Shell Restricted Hartree-Fock calculations lead to assign the² A _{1 g} as the ground state of the molecule in agreement with previous SCF calculations.The correlation energy associated to both states has been calculated within the correlation hole model and the results, contrary to those obtained from Configuration Interaction calculations, do not alter qualitatively the conclusions from SCF.     

Ring opening of the molecular ion of 5(4H)-oxazolone

Quantum chemical calculations were carried out at several theoretical levels (semi-empirical, MNDO; ab initio, 3–21G SCF, 6–311G** SCF and DZP CISD) to investigate the ring-opening process of and the loss of CO from the molecular ion of 5(4H)-oxazolone. The ring-opening process is predicted to be slightly endothermic and the loss of CO from the open-ring molecular ion to be slightly exothermic. Detailed population analysis calculations suggest the weakening of the lactonic C? O bond in the closed-ring molecular ion and weak carbon—carbon and nitrogen—(formy)—carbon bonds in the open form. Both the open-ring molecular ion and the [M–CO]^+· ion are suggested to be of distonic type.     

Ab initio SCF calculations on low-energy conformers of cyclohexaglycine

Results from ab initio self-consistent field (SCF) calculations with a 3-21G and a double-zeta-plus polarization (DZP) basis set on four low-energy conformations of cyclohexaglycine are reported. In agreement with results from semiempirical and molecular mechanics force field calculations, the lowest-energy conformation found at the DZP level is a conformation forming six C₇ turns. However, the energy difference to the β-turn conformers is significantly smaller at the ab initio DZP level than calculated by the other methods. In contrast to the results obtained with some of the other methods, the present ab initio calculations show that both the double-type-I β turn and the double-type-II β-turn conformer of cyclohexaglycine are stable low-energy structures. © 1995 by John Wiley & Sons, Inc.     

Comparative theoretical study of the dissociation process of the isoelectronic molecules BH3CO,CH2CO,HNCO, CO2 and BH3N2, CH2N2, HN3, N2O

Anab initio study of the electronic structure of several 22-electrons molecules is presented. The equilibrium geometries of their ground state are calculated at the SCF level using the 6–31G basis set and are found to be in good agreement with the experimental geometries. The dissociation process of these molecules leading to the isoelectronic products CO or N₂ on the one hand and BH₃, CH₂, NH and O on the other hand is studied. The least-energy dissociation paths of the ground states determined at the SCF level are compared on the basis of electron density interactions. The dissociation energies corresponding to the two lowest dissociation channels are calculated. In these calculations, the correlation energy is taken into account using a non-variational method developed previously. The calculated values of dissociation energies are in good agreement with the existing experimental values. The results permit to predict values for HNCO, BH₃CO and CH₂N₂ and to confirm the instability of BH₃N₂.Aspirant du Fonds National Belge de la Recherche Scientifique.     

On the electronic structure of N2H2. A possible triplet ground state in diazines

Within the frame of closed-shell and restricted open-shell ab initio SCF calculations 1,1-dihydrodiazine H₂N=N has a triplet ground state, ³A₂. This result, though not unsuspected from simple valence theory, is critically discussed and possible chemical implications are briefly mentioned.The ab initio calculations were performed at the IBM Research Center, San Jose, California.     

Three electron bonds. I. The H2SSH radical cation

Ab initio molecular orbital calculations are reported for H₂S, its radical cation, and the H₂SSH radical cation. At the MP 2/4-31G level the S? S three-electron bond is 2.85 Å long, and has a dissociation energy of 31.2 kcal mole^?1. The performance of MNDO semiempirical molecular orbital theory is compared with the ab initio results.     

Infrared experimental and ab initio quantum mechanical studies of 2-mercaptopyrimidine tautomers

Results are presented from ab initio SCF(3-21G*) calculations for the geometries and vibrational spectra (wavenumbers and absolute intensifies) of the thiol and thione tautomers of 2-mercaptopyrimidine. The results of calculations are compared with available experimental data, particularly with the reported vibrational spectra of the molecule isolated in inert gas matrices (Ar, N₂) and in crystalline state. The calculations of the normal modes predicted the experimental spectrum close enough to allow reliable assignment of most of the bands. The thiol⇌⇌thione tautomerism of the molecule is discussed. Matrix isolated monomers were observed in the thiol form only. That agrees with the results of ab initio calculations of internal energies of the tautomers [SCF(6-31Gu*) + MBPT(2)(6-31G*) + vib(0)(3-21G*); at the SCF(3-21G*) geometries] which predict the energy of thiol form to be ≈33 kJ mol⁻¹ lower than that of thione form. In the crystalline state the hydrogen-bonded associations in the thione form dominate while in disordered amorphous layers, in matrices with a high guest-to-host ratio and in annealed matrices the associations both in thiol and thione form were observed.     

Theoretical study of “protonated pyruvate”: A methylhydroxycarbene—carbon dioxide complex—implications for the decarboxylation of pyruvic acid

Two conformers of protonated pyruvate, CH₃C⁺(OH)COO, with the OH group either trans or cis to the methyl group and the carboxylate group in the C? C? C plane have been studied using the ab initio SCF/3-21G method, as well as by some semiempirical AM1 calculations. Both ab initio SCF and AM1 curves for the potential energy as a function of the C? COO distance exhibit a minimum corresponding to a complex of methylhydroxycarbene, CH₃COH, associated with carbon dioxide, but only the AM1 curves predict an inner minimum corresponding to a covalently bonded protonated pyruvate molecule with a C? COO distance of 1.6–1.7 Å. The two models also disagree on the dissociation pathway for pyruvic acid, with the AM1 calculations predicting formation of acetyl and HOCO radicals while the ab initio method predicts dissociation into methylhydroxycarbene and carbon dioxide following an initial intramolecular proton transfer. The weakly bound complexes of methylhydroxycarbene and carbon dioxide have been studied in some detail using ab initio SCF and MP2 methods in conjunction with 6-311G** basis sets, obtaining equilibrium geometries and vibrational frequencies. In addition, the lactone-type isomer of protonated pyruvate, which contains a C? C? O ring, was also studied. The conclusions of these calculations are consistent with those from earlier work using the smaller 3-21G basis set. The most stable complex is predicted to occur between trans-methylhydroxycarbene and carbon dioxide where substantial stabilization is provided by an OH ? OC hydrogen bond. © 1993 John Wiley & Sons, Inc.     

Ab initio study,including electron correlation,of the electronic structures,the dipole moments,the static polarizabilities and of the harmonic force fields of H2CO,H2CS and H2SiO

Ab initio calculations including electron correlation (on the PNO-CI and CEPA-PNO levels) are carried out for the isovalence electronic molecules H₂CO, H₂CS and H₂SiO, and for comparison also for H₂O and CO. The CEPA equilibrium distances are accurate to within 0.003 Å, while SCF results show significantly larger errors. The harmonic force constants on CEPA level are satisfactory as well, but for stretching of double or triple bonds inclusion of singly substituted configurations is imperative. Dipole moments were obtained with an error of 0.1 Debye from CEPA calculations with sufficiently large basis sets and inclusion of singly substituted configurations. The dipole polarizabilities are less sensitive to correlation effects but require larger basis sets.The population analysis reveals that the SiO bond in H₂SiO is highly polar and thatd-AO's cannot be regarded as valence AO's in any of the molecules of this study. The binding energy of H₂SiO (with respect to H₂Si(¹ A ₁) + O(³ P)) is predicted as 140 ± 5 kcal/mol. The contributions of different pairs in terms of localized orbitals to the correlation energy of the molecules of this study are analyzed.     

Ab initio and pseudopotential calculations on the singlet and triplet states of the disilyne isomers

Ab initio SCF as well as pseudopotential calculations were performed for determining equilibrium structures and relative stabilities of several disilyne isomers. For the singlet state there are only two structures, the bridged and the silavinylidene carbene, which correspond to minima on the energy hypersurface. The most stable of the six isomeric structures investigated is the bridged conformer in the ¹A₁ electronic state, followed by the silavinylidene carbene in the ¹A₁ and ³A₂ electronic states. Inclusion of electron correlation by MRD-CI calculations has no qualitative influence on the relative stabilities found in the SCF calculations.     

Optimized Structures and Relative Stabilities of the Carboranes from Ab Initio Calculations

Ab initio calculations at the STO-3G level were performed on almost all of the possible isomers for the entire series of closo-carboranes, C₂B_n-2H_n, 5 ? n ? 12. Geometry optimizations using the gradient method were also included in all calculations. We report here the relative energies obtained for the various isomers as well as the optimized structures. These calculations confirm our previous predictions of relative stabilities obtained from topological charge stabilization. Comparisons of our structures with those from experimental data provide us with a measure of reliability for bond distances obtained using ab initio SCF MO calculations at the STO-3G level. Results from the geometry optimization substantiated the experimentally known fluxional behavior of the 8 and 11 atom polyhedra.     

The instability of the planar structure of carbanion ⊖:CH2-CN

Ab initio SCF calculations using uniform quality extended basis set supplemented with polarization functions predict a slightly pyramidal carbanion centre in :CH₂-CN. This finding is in contrast to the usual finding of planar geometry of carbanion centres generated next to other conjugative groups. Although the predicted barrier to pyramidal inversion is very small (0.1 kcal/mole) it is estimated that correlation energy contributions, neglected in any SCF calculations, may be too small to remove the barrier. The present results confirm earlier experimental findings that counter-ion and solvent effects can have a dominant role in determining the geometry of this unusual ion in actual chemical systems.     

Potential energy surface of the system CO + O: An ab initio study on the O?CO bond formation

Quantum mechanical computations on the potential energy surface of the system CO + O at an SCF –MO ab initio level are presented and discussed. The calculations are performed on a minimal basis set of atomic functions. Comparison with results obtained with extended basis sets are also presented.     

Ab initio study of the He(1S)-Li2(X̃, 1∑) interaction by the SCF and MP2 methods

Ab initio electronic structure calculations have been carried out for the He(¹S)-Li₂ (X?, ¹∑) interaction both by the single-configuration SCF and correlated second-order MP2 methods using an extended basis set. From these calculations, an estimate of the isotropic (V₀) and first two anisotropic (V₂ and V₄) terms of the He-Li₂ potential surface has been obtained. An assessment of the leading induced-dipole-induced-dipole dispersion energy is presented from the MP2 energies. Where possible, a comparison is made with previous unpublished ab initio calculations by Staemmler and Stahl using the CEPA method.