Bridged structures in multiply bonded silicon compounds: Disilyne,protonated disilyne and disilene

Ab initio SCF and electron correlation calculations are reported for the singlet ground state of the title compounds. These calculations confirm earlier findings that non-planar bridged Si₂H₂ is the most stable structure. For protonated disilyne (Si₂H³⁺) a bridged D_3h structure is the global mimimum. Two bridged structures of C_2v and C_2h symmetry are found in the case of disilene (Si₂H₄) which are only 14–17 kcal/mol above the D_2h structure.     

The nature of the bonding of Li+ to H2O and NH3; A3 initio studies

Ab initio wavefunctions have been calculated for the complex of Li⁺ with NH₃ and H₂O in order to better characterize the nature of the bonding. Hartree—Fock and generalized valence bond calculations were performed using a double zeta basis plus polarization functions. The binding energies obtained at the GVB level are D_e (Li⁺ — NH₃) = 40.4 kcal/mol and D_e (Li⁺ ? H₂O) = 37.6 kcal/mol, in reasonable agreement with experimental values. Model calculations indicate that the Li⁺ ? base bond is basically electrostatic. Small basis sets were found to lead to D_e as large as 75 kcal/mol for Li⁺ — NH₃, a significant overestimation. Repulsions due to the Li⁺ core are responsible for keeping the Li⁺ too far away for significant relaxation effects.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

A theoretical study of the sulphonium cation SH+3

Ab initio Hartree—Fock calculations with STO-3G functions have been performed to determine the structure (1.371 Å and 95.33°) of SH⁺₃ and the proton affinity (≈196 kcal/mol) of H₂S. Inclusion of a sulphur 3d function in the basis has been found essential to give a better geometry of SH⁺₃.     

Gaseous cations (carbonium ions) : Mindo/3 calculations on singly positively charged propyl ions

MINDO/3 calculations have been used to study processes involving n-propyl cations and sec-propyl cations. The calculations show that the former is interconverted into the more stable sec-C₃H₇⁺ cations with no localised minima. The lowest energy process is shown to be the production of H₂ and C₃H₅⁺ ions from sec-C₃H₇⁺. The results of the calculations of the decomposition of n-C₃H₇⁺ to CH₃⁺ + C₂H₄ and methylene + C₂H₅⁺ are given. The calculations are generally consistent with experimental studies.     

A theoretical study of the chemical reaction between ethylene and N2H2

The interaction between the molecules ethylene and cis-N₂H₂ has been studied using a gaussian basis in a series of ab initio SCF calculations. The results obtained indicate that the synchronous hydrogen transfer reaction is a one-step reaction having an activation energy of around 60 kcal/mol. Our results do not lend support to the hypothesis that the rate of the overall reaction between C₂H₄ and N₂H₂ is controlled by the rate of isomerization of trans-diimide to the cis form.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

Ab initio calculation of the electronic structure and the barrier to pyramidal inversion for H3S+

The electronic structure of H₃S⁺ is examined by ab initio MO calculations (STO 3G) and is compared with those of other XH₃ type molecules. The barrier to pyramidal inversion and the proton affinity of H₂S are calculated to be 34.8 and 225.05 kcal/mol respectively. Computations are made with respect to a model A₃S⁺ which is employed to discuss the barrier to pyramidal inversion of H₃S⁺, where A corresponds to an electromagnetive substituent or an electron donating one.     

Jahn-Teller distortion in the SnH4+ ion

The extension of the Jahn-Teller distortion of the SnH₄⁺ structure is estimated from ab initio SCF CI calculations using pseudopotential atomic approximation with and without relativistic correction. These calculations show the lowest minima of SnH₄⁺ to have C_2v symmetry and are very close to the C_3v minima. The calculated photoelectron spectrum of SnH₄ is in good agreement with the experimental values.     

Ab initio calculations of small hydrides including electron correlation

Five different structures of CH₅ ⁺ and one structure of CH₅ ^– are calculated using a gaussian basis both in the SCF approximation and with the inclusion of electron correlation in the independent electron pair approximation (IEPA). While on SCF level the C _sstructure of CH₅ ⁺ has to lowest energy, the energy difference between the C _sand C _2vstructures becomes negligible if correlation is included. In contrast to this the approach of a proton to CH₄ at large and intermediate distances is most favorable towards a corner of the CH₄ tetrahedron which means a structure. The decomposition of CH₅ ⁺ into CH₃ ⁺ and H₂ requires 20kcal/mol on SCF level and 40 kcal/mol if correlation is included.     

Eckenprotoniertes cyclopropan in der gasphase: kopplung von unimolekularer [1.1]-H2-eliminierung aus C3H7+ und ringöffnung des cyclopropylkations

Based on energetic data (activation energy), kinetic isotope effects and MINDO/3 calculations it is suggested that unimolecular loss of H₂ from C₃H₇⁺ proceeds via a transition state (10) involving synchronous loss of a hydrogen molecule (symmetry allowed [1.1]-elimination) and ring opening of the cyclopropyl to the allyl cation. Alternative mechanisms are discussed.     

Proton affinity correlations for alkyl chlorides

Ab initio SCF calculations with minimal STO-3G and extended 44-31G basis sets have been performed on the simple alkyl chlorides, HCl to t-BuCl, and their protonated analogs. MINDO/3 calculations are also reported for these species and a variety of cyclic and bicyclic chlorides. The much closer agreement with experiment for STO-3G proton affinities than for 44-31G values is in sharp contrast to the results for first-row bases. An excellent correlation is found between both the STO-3G and MINDO/3 proton affinities and the charge on the CIH fragment in RCIH⁺. For the acyclic chlorides, correlations of PA's with the polar substituent constant, σ*, and IP's are also reasonable. In addition, the calculated carbonium ion-HCl interaction energy for t-BuClH⁺ indicates that protonated tertiary chlorides are no more than marginally stable in the gas phase.     

A low-energy passage for C+ + H2 → CH+(1Σ+) + H

Ab initio (SCF and CI) calculations have been performed for the linear approach (C_∝v) of C⁺ to H₂. For the ² Σ⁺ surface the saddle point and barrier height were determined. The interaction of the ²Σ⁺ and ²Π surfaces was investigated, leading to the conclusion that in near-C_∝v symmetry a low-energy path exists by which CH⁺¹ Σ⁺ can be formed, with negligible barrier in excess of endothermicity.     

Structure and properties of H2CN,H2CC−, H2BO and H2CO+

Ab initio SCF—MO calculations are presented for H₂CN, H₂CC^?, H₂BO and H₂CO⁺, including geometry optimization. One-electron properties are presented and compared with experiment where possible, particularly ESR hyperfine data.     

Structure of tetracoordinated sulphur compounds. Model calculations on SH4 and SF2H2

The structure of the hypothetical sulphuranes SH₄ and SF₂H₂ is investigated through ab initio LCAO MO SCF calculations. In contrast to a recent report the structure of lowest energy for SH₄ is of C_4V symmetry. For SF₂H₂ a structure of C_2V symmetry with axiai fluorine atoms is predicted. These results have been rationalized on the basis of perturbation theory.     

Structures of gas-phase C2F 7 + ions

In an ion cyclotron resonance spectrometer, less than 96% of the C₇F ₇ ⁺ cation formed on electron ionization of perfluorotoluene reacts with hexamethyldisilazane. In contrast, the C₇F ₇ ⁺ from perfluoronorbornadiene or perfluorobicyclo[3.2.O]hepta-2,6-diene is nonreactive with hexamethyldisilazane. Collision-induced dissociation results support this dichotomy, although the evidence is not as clear-cut. The reactive ion is assigned the benzyl structure and the nonreactive ion the tropyl structure, on the basis of analogy with the protio cases. By AM1 calculations, the perfluorobenzyl ion is 25 kcal/mol more stable than the perfluorotropyl ion, the opposite of the situation for the protio analogs (? 12 kcal/mol). Ab initio calculations at the 3–21G level agree with the semiempirical energy difference to within 0.4 kcal/mol; at the more appropriate 6–31G*/MP2 level, the perfluorobenzyl cation is 9.7 kcal/mol more stable than the perfluorotropyl cation.     

SCF,MP2, and CEPA-1 calculations on the OH ‥ O hydrogen bonded complexes (H2O)2 and (H2O-H2CO)

Counterpoise corrected ab initio calculations are reported for (H₂O)₂ and H₂O-H₂CO. Geometry searches were done in the moment-optimized basis DZP' at the SCF, MP2, and CEPA-1 levels of theory, followed by more accurate single-point calculations in basis ESPB, which includes bondfunctions to saturate the dispersion energy. The final equilibrium binding energies obtained are ?4.7 ±0.3 kcal/mol for a near-linear (H₂O)₂ structure and ?4.6 ±0.3 kcal/mol for a strongly bent HOH ‥ OCH₂ structure. The energy difference between these systems is much smaller than in all previous ab initio work. Cyclic (C_2h) and bifurcated (C_2v) transition structures for (H₂O)₂ are located at 1.0 ±0.1 kcal/mol and 1.9 ±0.3 kcal/mol above the global minimum, respectively. A new partitioning scheme is presented that rigorously partitions the MP2 correlation interaction energy in intra and intermolecular (dispersion) contributions. These terms are large (up to 2 kcal/mol) but of opposite sign for most geometries studied and hence their overall effect upon the final structures is relatively small. The relative merits of the MP2 and CEPA-1 approaches are discussed are discussed and it is concluded that for economical reasons MP2 is to be preferred, especially for larger systems.     

Energy surface calculations for the reaction HF + H+ ⇌ HFH+

Various GTO basis sets were investigated for their effectiveness in determining the SCF energy and geometry of the HFH⁺ molecule. A double zeta set augmented with a p_z function on each H atom was used to calculate the potential energy surface for the collinear protonation of HF. Limited configuration interaction calculations gave an energy of ?100.27365 E_a for an HF separation of 1.819 a₀ and a bond angle of 118.1°, and an energy of protonation of 119.5 kcal/mol.     

Theoretical study of the molecular ions SiH−5 and SiH−3

Ab initio HF and Cl calculations were performed to determine the equilibrium geometry of SiH^?₅ and SiH^?₃, the barrier for internal rotation (SiH^?₅) and inversion (SiH^?₃) and the stability of SiH^?₅ and further to study the effect of electron correlation on reaction energies. The gaussian-type basis included d and f functions on Si and a p set on II. The D_3h structures of SiH^?₅ is lower in energy than the C_4v structure by 2.9(3.2) kcal/mol (corresponding HF results in parentheses). SiH^?₃ has C_3v structure, the inner-ion barrier computed is 26.2 (27.3) kcal/mol. SiH^?₅ turns out to be stable with respect to SiH₄ + H^? by 20.3 (13.8) kcal/mol, but it is unstable with respect to SiH^?₃ ← H₂ by 6.3 (5.6) kcal/mol. These results show that electron correlation has a small effect on barriers of inversion (SiH^?₃) or pseudorotation (SiH^?₅), but may have a pronounced effect on reaction energies even if all systems involved have closed shells. The correlation energy contributions are analyzed in terms of intrapair and interpair terms in order to get a better understanding of the influence of correlation on reaction and activation energies.     

The C2H 3 + cation and its interaction with HF

The structure and stability of classical and bridged C₂H ₃ ⁺ is reinvestigated. The SCF and CEPA-PNO computations performed with flexibles andp basis sets including twod-sets on carbon confirm our previous results. We find the protonated acetylene structure to be more stable than the vinyl cation by 3.5–4 kcal/mol. The energy barrier for the interconversion of these two structures is at most a few tenths of a kcal/mol. The equilibrium SCF geometries of Weberet al. [15] are affected insignificantly by further optimization at the CEPA-PNO level. Several structures for the interaction of C₂H ₃ ⁺ with HF have been investigated at the SCF level. With our largest basis set which includes a complete set of polarization functions we find a remarkable levelling of the stabilities of most of the structures. In these cases the stabilization energy ΔE ranges from −10 to −13 kcal/mol.