Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

Semiempirical and ab initio calculations on the alkaline hydrolysis of the β-lactam ring Influence of the solvent

We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol⁻¹ due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol⁻¹. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol⁻¹. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol⁻¹.     

Theoretical study of the N---H tautomerism in free base porphyrin

The N---H tautomerism of free base porphyrin is investigated at the semiempirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary structures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensure that realistic delocalized structures are obtained. Both the semiempirical and the ab initio calculations predict that the interconversion between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized as minima in the potential energy surface and are 8–10 kcal mol⁻¹ higher in energy. The activation energy for the trans → cis interconversion is calculated to be approximately 23 kcal mol⁻¹ at the 3-21G level. The activation energy for the synchronous trans → trans interconversion is higher and has a value of 30.5 kcal mol⁻¹. The activation energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values.     

MNDO study of the proton affinity of fluorinated formaldehydes and acetones

The MNDO molecular orbital method is employed to calculate the proton affinities of fluorinated formaldehydes and acetones. Agreement with experimentally reported proton affinities is good. In the acetone series a decrease in proton affinity is calculated for each successive fluorine substituent. The calculated strength of the intramolecular hydrogen bond in the protonated fluoro-formaldehydes and acetones is 0.6—2.7 kcal mol⁻¹, in good agreement with the experimental value of 2—3 kcal mol⁻¹ in the protonated fluoroacetones. Examination of the calculated charge distribution shows that the trends in proton affinity can be understood qualitatively both in terms of initial-state and final-state effects caused by the fluorine substituents. Protonation at the fluorine atom is less stable by about 25 kcal mol⁻¹ than protonation at the oxygen atom for monofluoroacetone.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

An ab initio MO study of allene episulfide, cyclopropanethione and thioxyallyl

The electronic structures of allene episulfide, cyclopropanethione and thioxyallyl were examined by ab initio MO calculations and were compared with those of the corresponding oxygen compounds, allene oxide, cyclopropanone and oxyallyl. The difference in reactivities of allene episulfide and allene oxide was also speculatively estimated from the calculated electronic structures. The lowest singlet state of thioxyallyl was predicted to be the B₂ state, which corresponds to the σ, π-diradical. A small activation energy is required for the cyclization of the B₂ state to give allene episulfide. The A₁ singlet state lies 11 kcal mol⁻¹ higher than the B₂ singlet state and undergoes the disrotatory rotation of methylene groups to give cyclopropanethione with no activation energy.     

Can chlorine anion catalyze the reaction of HOCl with HCl?

The reaction of HOCl + HCl → Cl₂ + H₂O in the presence of chlorine anion Cl⁻ has been studied using ab initio methods. The overall exothermicity is 15.5 kcal mol⁻¹ and this reaction has been shown to have a high activation barrier of 46.5 kcal mol⁻¹. Cl⁻ is found to catalyze the reaction via the formation of HOCl·Cl⁻, ClH·HOCl·Cl⁻ and Cl⁻·H₂) intermediate ion-molecule complexes or by interacting with a concerted four-center transition state of the reaction of HOCl + HCl.     

Thermochemistry of organosilicon compounds VII. Permethylcyclosilazanes and 1,1,3,3-tetramethyldisilazane

The enthalpy of formation (ΔH_f⁰), enthalpy of evaporation (ΔH_v⁰) and enthalpy of atomization (ΔH_a) of permethylcyclosilazanes (Me₂SiNH)_n (n = 3, 4) and 1,1,3,3-tetramethyldisilazane (Me₂SiH)₂NH have been determined. The enthalpies of formation of these compounds were compared with those calculated by the Benson-Buss-Franklin and Tatevskii additive schemes. In higher permethylcyclosilazanes the energy of the endocyclic Si---N bond is 306 ± 2 kJ mol⁻¹ (73 kcal mol⁻¹), that is 12 ± 2 kJ mol⁻¹ (3 kcal mol⁻¹) lower than the energy of the acyclic Si---N bond. The strain energy of the cyclotrisilazane ring is estimated to be 10.5 kJ mol⁻¹ (2.5 kcal mol⁻¹), whereas the energy of the ring Si---N bond is 295 kJ mol⁻¹ (70.5 kcal mol⁻¹).

The thermochemical data for permethylcyclosilazanes were compared with the corresponding values for permethylcyclosiloxanes calculated from the results of previously reported studies.     

Structure and conformation of cyclobutylsilane: an electron diffraction and ab initio study

A gas electron diffraction study of cyclobutylsilane results in a mixture of equatorial and axial conformers, with the equatorial confomer slightly more stable (Δ G = 0.8 ± 0.4 kJ mol⁻¹). The cyclobutyl ring is distorted with the adjacent bonds longer (C₁---C₂ = 1.573 (4) Å) than the opposite bonds (C₂---C₃ = 1.557 (4) Å). The experimental values for the energy difference between the two conformers and for the geometric parameters are reproduced very well by ab initio calculations. The importance of silicon 3d orbitals in the interpretation of ring distortion is ambiguous, but on the basis of the ab initio calculations the participation of silicon 3d functions is negligible.     

Ab initio calculations of the rotational barriers in formamide and acetamide: The effects of polarization functions and correlation

Ab initio calculations have been used to determine the gas-phase rotational barrier about the CN bond in formamide and acetamide. The results indicate that the inclusion of polarization functions in the basis set leads to a substantial decrease (ca. 5 kcal mol⁻¹) in the calculated barrier height at the SCF level. Electron correlation effects decrease the barrier by less than 1 kcal mol⁻¹, while the addition of zero point energy corrections changes the barrier height only slightly. Based upon the current calculations, the 0 K rotational barriers for isolated formamide and acetamide are predicted to be 14.2 and 12.5 kcal mol⁻¹, respectively.     

Barrier to internal rotation in 2,3- and 3,3-difluoropropenal and their methyl derivatives

The conformational stability and structure of 2,3-dimethylpropenal, 2,3-difluoropropenal and their 3,3-dimethyl and 3,3-difluoro derivatives were investigated utilizing ab initio calculations with 3-21G and 6-31G basis sets. For 2,3-dimethylpropenal and 3,3-difluoropropenal the s-trans was predicted to be the low-energy form. In the case of 3,3-dimethylpropenal and 2,3-difluoropropenal the s-cis was predicted by both levels of calculation to be the more stable conformer. Full optimization was performed at the transition states and the barriers to internal rotation were calculated. Methyl and fluorine substitution were found to significantly increase the barrier to interconversion in propenal. The relative change in the barrier depends on the position and the type of the substituent. The trans to cis barrier in 2,3-dimethylpropenal was calculated to be about 3 kcal mol⁻¹ greater than that in 3,3-dimethylpropenal, while the cis to trans barrier in 2,3-difluoropropenal was predicted to be about 7 kcal mol⁻¹ higher than the corresponding one in 3-3-difluoropropenal.     

Can triorganoboroxins exist in a “monomeric” R---B=O form? MNDO calculations and ebulliometric molecular weight determination

MNDO calculations were made for triethylboroxin (EtBO)₃ and triphenylboroxin (PhBO)₃ using both X-ray determined and optimized geometry of these molecules. The results were compared with hypothetical “monomeric” molecules R---B=O. Calculated energies of trimerization are about −200 kJ mol⁻¹ for both compounds and confirm the much higher stability of the “trimer”. Ebulliometric determination of molecular weight of triphenylboroxin in 2-pentanone confirms its trimeric character.     

On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm⁻¹) of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm⁻¹ less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r₀ parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm⁻¹ with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm⁻¹ and that of the outer one of 1662 cm⁻¹ from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm⁻¹, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′₃₃ (sin 3τ₀ sin 3τ₁ term) value which ranged from 190 to 232 cm⁻¹ is in reasonable agreement with the experimental value of 268±3 cm⁻¹ but the predicted V₃₃ (cos 3τ₀ cos 3τ₁ term) value of −73 to −139 cm⁻¹ is 25% smaller and of the opposite sign of the experimental value of 333±22 cm⁻¹. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.     

Effect of small cations on the hydrogen bond between an N-aromatic heterocycle and amine

The influence of an Li⁺ ion on the structure and bonding of the H-bond interaction of an N-aromatic heterocycle and a singly bonded N-amine is studied by ab initio SCF calculations with imidazole (Imh) and NH₃ serving as models of the two families. Full optimization of structures have been carried out for DZ, DZP and TZP basis sets. The computed H-bond energy for Imh/ NH₃ of −6.4 kcal/mol is in close agreement with a recent experiment. An appreciable three-body interaction of −3.8 kcal/mol is found for the complex Imh/NH₃/Li⁺.     

Kinetic study of trifluoromethylation with s-(trifluoromethyl) dibenzothiophenium salts

The kinetic parameters were determined for C-trifluoromethylation of aniline with S-(trifluoromethyl)dibenzothiophenium triflate (1), its 3,7-dinitro derivative (2) and S-(trifluoromethyl)diphenylsulfonium triflate (3) in DMF-d₇. The higher reactivity of heterocyclic 1 compared with non-heterocyclic 3 could be explained on the basis of its greatly enhanced activation entropy (ΔS^≠: −11.2 cal mol ⁻¹ K⁻¹ for 1; −47.1 for 3), but not its enhanced activation enthalpy (ΔH^≠: 21.2 kcal mol⁻¹ for 1; 12.1 for 3). The aromatic delocalization of the heterocyclic ring may thus be only slightly disturbed by the S-trifluoromethyl substituent. The high reactivity of 2 was attributed to the great electron deficiency caused by two nitro groups in addition to the heterocyclic salt system (ΔH^≠ 17.0 kcal mol⁻¹, ΔS^≠ −9.1 cal mol⁻¹ K⁻¹ for 2). The reaction mechanism is discussed; the conventional S_N2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF₃-S bond may possibly be applicable.     

A model potential for the internal rotation of neighbouring rings of bithiophene and bipyrrole

Based upon ab initio Hartree-Fock calculations we propose a model potential for simplifying the study of the internal rotation of neighbouring rings. The quality of the proposed analytic potential was numerically verified in the case of bithiophene and bipyrrole by determining the deviations of the predicted energy values along the torsional angle with respect to 19 independent reference values. It is found that our model potential describes correctly the torsional motion the average deviations of the predicted energies along the torsional angle with respect to the ab initio reference energies are 0.20 and 0.22 kJ mol⁻¹ for bithiophene and bipyrrole, respectively. In both molecules, when going from a coplanar to a perpendicular conformation, two stable isomers have been detected. These isomers are separated by a barrier height of about 4 kJ mol⁻¹. Full optimization of the structural parameters along the torsional angle shows that they remain almost constant along the isomerization process.     

Ab initio studies on the electronic structures of certain 10π-electron six-membered ring compounds

By using ab initio methods of all-electron or effective core potential calculations, the electronic structures and the possible aromaticity of some 10π-electron systems, C₆H₆⁴⁻ (1), N₆⁴⁻ (2), P₆⁴⁻ (3), S₆²⁻ (4), Te₆²⁻ (5) and S₃N₃⁻ (6), have been studied at the SCF levels using 4-31G//4-31G and 6-31G^*//6-31G^* basis sets. The bonding characteristics of these systems are analysed in terms of the canonical molecular orbital and the Foster-Boys localized molecular orbital results. The application of the second-order Jahn-Teller theorem to the stability of these diamagnetical planar species is presented.     

Pentacoordinate silicon compounds: Stereochemical non-rigidity of chelates formed by intramolecular ring-closure. Crystal structure of 8-dimethylamino-1-trifluorosilylnaphthalene

The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me₂ iXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me₂NC₁₀H₆SiF₃ has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 Å (average of two distinct but closely related molecular conformations), which is less than the C¹---C⁸ distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me₂N group, and where possible (¹⁹F, ¹H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol⁻¹ [SiH₃, Si(OR)₃], which is below the limit of direct measurement, to 13 kcal mol⁻¹ for Me₂NCH(Me)C₆H₄SiF₃; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol⁻¹. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.     

Structures and relative stabilities of [C2H6N] ions: A non-empirical and MNDO study

Non-empirical molecular orbital calculations have been performed on the seven possible [C₂H₆N]⁺ isomers at different levels of sophisitcation. The results suggest that the ions possessing a resonance contribution with a filled octet are the most stable ones. Barriers to CH₃ — and NH₃ —rotation have also been computed and are generally quite low ( 5 kJ mol⁻¹).     

Structure and bonding studies of ClX-C2H4 (X = H or Br) van der Waals complexes

High-quality ab initio calculations at the MPn (n = 2, 3 or 4) levels for the π-donor complex formed by HCl and ethene, and the recently characterized ClBr-ethene complex are presented. Interaction energies were calculated with the inclusion of both basis-set superposition error and zero-point energy corrections, resulting in values of about 1 kcal mol⁻¹ for both complexes. The total charge densities for both complexes yielded molecular graphs indicative of the weak binding in these molecules, and correspondingly, calculated charges showed that the XCl moiety has an overall slightly negative charge, while the ethene moiety is slightly positive. Analysis of the Laplacian of the charge density showed that the geometry of the ClBr- C₂H₄ complex may be understood in terms of the VSEPR model as an example of an AXYE₃ molecule.