Ab initio molecular-orbital study of structures and energetics of Si3H3 neutral and anion

The geometric structures and isomeric stabilities of various stationary points in Si(3)H(3) neutral and its anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For geometrical surveys, the basis sets used are of the Dunning's correlation consistent basis sets of triple-zeta quality for the neutral. To the anions, the Dunning's correlation consistent basis sets of double-zeta quality with diffuse functions are applied. For the three lower-lying anion isomers, the Dunning's correlation consistent basis sets of triple-zeta quality with diffuse functions (aug-cc-pVTZ) are also used. The final energies for the optimized stationary points are calculated at the CCSD(T) level of theory with the aug-cc-pVTZ basis sets. The basis sets of 6-311++G(3df,2pd) were also used for the lower-lying anion isomers. The Gaussian-2 method was performed only for the lower-lying anion isomers to clarify the relative stabilities. The global minimum neutral 1 (C(1):(2)A) has an unsymmetrical hydrogen-bridged bond; the conformer 2 in C(s) symmetry is a saddle point connecting the two equivalent isomers 1. Two lower-lying isomers (3 and 4) are also predicted within the energy range of 20 kJmol. In the anion, however, the conformer 4 (C(s):(1)A(')) with five formal valence electrons is a global minimum. Two more isomers (2 and 3) lie within 20 kJmol as in the neutral; the conformer 1 converts to the isomer 2. The quartets for the neutrals and diradical triplets for the anions were further studied; lower-lying quartets and triplets, competing with the corresponding doublet and singlet, respectively, were not found in the present systems. The vertical and adiabatic electron affinities of the global minimum neutral 1, producing the second lowest-lying anion isomer 2, amount to 2.18 and 2.35 eV, respectively, at the CCSD(T)/aug-cc-pVTZ level of theory. The electron addition to the third lowest-lying neutral isomer 4 produces the largest vertical electron affinities of 2.48 eV. The D(3h) structure, being the global minimum in the corresponding Si(3)H(3) (+) cation (trisilacyclopropenyl cation), converts to the isomer 8 (C(s)) or 11 (C(2)) due to the Jahn-Teller effect in the Si(3)H(3) neutral.     

Theoretical study on isomeric stabilities of C2H2Si and its ionization potentials and electron affinities

The geometric structures and isomeric stabilities of various stationary points in C(2)H(2)Si neutral and its cation and anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For the geometrical survey, the basis sets used are of the Dunning's correlation consistent basis sets of triple-zeta quality (cc-pVTZ) for the neutral and cation. For the anions, the cc-pVTZ basis sets with diffuse functions (aug-cc-pVTZ) are used. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To lower lying neutrals and cations, the Dunning's correlation consistent basis sets of quadruple-zeta quality (cc-pVQZ) are also applied. Both the global minima of the C(2)H(2)Si neutral and cation, N-1 (C(2v):(1)A(1)) and C-1 (C(2v):(2)B(2)), respectively, are silacyclopropenylidene conformers, having a CCSi ring with a C[Double Bond]C double bond. No competitive stable isomers exist in the present C(2)H(2)Si neutral. In the cation, however, the second lowest lying isomer C-2 lies 10.8 kJ/mol above the most stable C-1. The vertical and adiabatic ionization potentials from the lowest lying neutral N-1 are 9.83 and 8.97 eV, respectively, at the CCSD(T)/cc-pVQZ level of theory. The electron addition to the N-1 does not result in the anion with positive (real) electron affinities. On the other hand, the electron addition to the N-2 isomer produces the global minimum anion A-1 (C(2v):(2)B(1)) with the positive electron affinities of 1.13 eV. The second lowest lying anion isomer A-2 with silylenylacetylene conformer, produced from an electron addition to the N-3 neutral, very well competes with the A-1 after the zero-point vibrational energy corrections. The energy difference between the two lowest lying isomers of the neutral and its anion, N-1 and A-1, is only 0.39 eV.     

Direct ab initio dynamics study on the rate constants and kinetics isotope effects of CH(3)O+H-->CH(2)O+H(2) reaction

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of CH(3)O+H-->CH(2)O+H(2). The unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional with Dunning's correlation consistent polarized valence double-zeta basis set, the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set, and the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set methods were employed to optimize the structures and to calculate frequencies for all stationary points. Minimum energy paths were obtained by the unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional and the unrestricted quadratic configuration interaction calculation including single and double substitutions with the same Dunning's correlation consistent polarized valence double-zeta basis set levels of theory. No barrier is found at the unrestricted Becke's half-and-half hybrid functional using the Lee-Yang-Parr correlation functional with Dunning's correlation consistent polarized valence double-zeta basis set level of theory in contrast to a small barrier of 1.43 kcal mol(-1) at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set level of theory. In particular, the barrier vanishes as the energies along the minimum energy path MEP are refined at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set level of theory. Smaller barriers of 0.47 and 0.17 kcal mol(-1) were obtained at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set and the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set based on the geometries at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence triple-zeta basis set levels of theory, respectively. The forward rate constants are evaluated with the canonical variational transition state theory in the temperature range of 300-2500 K. The calculated forward rate constants at the unrestricted quadratic configuration interaction calculation including single and double substitutions with a triples contribution with Dunning's correlation consistent polarized valence triple-zeta basis set based on the geometries at the unrestricted quadratic configuration interaction calculation including single and double substitutions with Dunning's correlation consistent polarized valence double-zeta basis set level of theory are in good agreement with the available experimental data. The kinetic isotope effects are estimated.     

An ab initio MO study on structures and energetics of CH2Si2, CH2Si2, and CH2Si2

The geometric structures and isomeric stabilities of various stationary points in CH₂Si₂ neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory. For the geometrical survey, the basis sets used are of the cc-pVTZ for the neutral and cation. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To the competitive two-anion isomers, the aug-cc-pVTZ basis sets are applied. The global minimum (N-1) of the CH₂Si₂ neutral has a quite different framework from those of the C₃H₂ (cyclopropenylidene) and Si₃H₂ (trisilacyclopropenylidene) neutrals. No competitive low-lying isomers are found in the CH₂Si₂ neutral. The attractive conformer (C-1) is predicted for the most stable cation, where its framework is quite different from that of the neutral N-1. Both H atoms are connected to the same C atom, but each C–H bond length is different from each other. Two competitive anion isomers with positive (real) electron affinities are predicted. The framework of the most stable anion A-1 is quite similar to that of the cation C-1, whereas both H atoms are equally connected to the same C atom. The framework of the anion isomer A-2 is the same as that in the neutral N-1. The vertical and adiabatic ionization potentials from the most stable neutral N-1 are 9.02 and 8.71 eV, respectively. The adiabatic electron affinity of the lowest lying isomer N-1 is only 0.43 eV and the vertical electron detachment energy form the global minimum anion (A-1) is 2.02 eV. The multi-centered Si–H–Si bonds are found in the neutral, cation, and anion.     

Relative stability of planar versus double-ring tubular isomers of neutral and anionic boron cluster B20 and B20-

High-level ab initio molecular-orbital methods have been employed to determine the relative stability among four neutral and anionic B20 isomers, particularly the double-ring tubular isomer versus three low-lying planar isomers. Calculations with the fourth-order Moller-Plessset perturbation theory [MP4(SDQ)] and Dunning's correlation consistent polarized valence triple zeta basis set as well as with the coupled-cluster method including single, double, and noniteratively perturbative triple excitations and the 6-311G(d) basis set show that the double-ring tubular isomer is appreciably lower in energy than the three planar isomers and is thus likely the global minimum of neutral B20 cluster. In contrast, calculations with the MP4(SDQ) level of theory and 6-311+G(d) basis set show that the double-ring anion isomer is appreciably higher in energy than two of the three planar isomers. In addition, the temperature effects on the relative stability of both 10B20- and 11B20- anion isomers are examined using the density-functional theory. It is found that the three planar anion isomers become increasingly more stable than the double-ring isomer with increasing the temperature. These results are consistent with the previous conclusion based on a joint experimental/simulated anion photoelectron spectroscopy study [B. Kiran et al., Proc. Natl. Acad. Sci. U.S.A. 102, 961 (2005)], that is, the double-ring anion isomer is notably absent from the experimental spectra. The high stability of the double-ring neutral isomer of B20 can be attributed in part to the strong aromaticity as characterized by its large negative nucleus-independent chemical shift. The high-level ab initio calculations suggest that the planar-to-tubular structural transition starts at B20 for neutral clusters but should occur beyond the size of B20- for the anion clusters.     

Ab initio MO study on the global minimum structure of C3H3 anion: propynl-1-yl versus allenyl anions

The geometrical structures of the C₃H⁻₃ anion are surveyed at the coupled-cluster doubles (CCD) level of theory with the aug-cc-pVDZ basis set. To clarify the CCD geometries, the stable two isomers -- propynl-l-yl 1 and allenyl 2 anions -- are further optimized at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory both with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The final energies are calculated at the CCSD(T) and the complete active space self-consistent field (CASSCF) multi-reference internally contracted CI (MRCI) levels of theory with the aug-cc-pVTZ basis set. At the MRCI level of theory including both the corrections due to the cluster energies (MRCI+Q) and the zero-point vibrational energies, the allenyl anion 2 is about 1.3 kcal mol⁻¹ lower in energy than the propynl-l-yl anion 1. These results contrast with the previous theoretical estimates, where the propynl-l-yl anion 1 is 2-3 kcal mol⁻¹ lower in energy than the allenyl anion 2. The activation energies of the intramolecular hydrogen transfer in the 1 → 2 conversion reactions are 63.5 kcal mol⁻¹ at the MRCI+Q level of theory with the aug-cc-pVTZ basis set including the zero-point energy corrections. The adiabatic electron affinity of the planer propargyl (H₂CCCH) radical, which is the global minimum of the C₃H₃ radical, is calculated to be 0.976 eV (after correction for the zero-point energy changes) at the CCSD(T) level of theory with the aug-cc-pVTZ basis set. The present electron affinity is in fairly good agreement with the experimental one (0.893 eV) observed by Oakes and Ellison.     

Multireference configuration interaction studies on the ground and excited states of N2O2: the potential energy curves of N2O2 along N-N distance

In this paper, the ground and excited states of N2O2 were studied at the multireference configuration interaction (MRCI) level of theory with Dunning's [J. Chem. Phys. 90, 1007 (1985); 96, 6796 (1992)] correlation consistent basis sets augo-cc-pVDZ and aug-cc-pVTZ. The geometry optimizations were performed for the ground state of N2O2. The vertical excitation energies and transition moments were calculated for the low-lying singlet states of N2O2 including the lowest three 1A1 states, two 1B1 states, one 1B2 state, and two 1A2 states at the MRCI level of theory with Dunning's correlation consistent basis sets aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ. Furthermore, for the first time, the potential energy curves were calculated at the complete active space self-consistent-field and MRCI levels of theory for as many as 12 N2O2 singlet electronic states along the N-N distance. The dissociation asymptotes of these 12 N2O2 singlet electronic states were discussed.     

Structures and infrared spectra of fluoride-hydrogen sulfide clusters from ab initio calculations: F(-)-(H2S)n, n = 1-5

Clusters formed between a fluoride anion and several hydrogen sulfide molecules have been investigated via ab initio calculations at the MP2 level of theory, using Dunning's augmented correlation consistent basis sets. Optimised geometries, vibrational frequencies, and enthalpy changes for the ligand association reactions are presented for clusters with up to five H2S ligands interacting with a F- anion. The minimum energy structure for the 1:1 F(-)-H2S complex features proton transfer from the H2S to the F- anion, forming a planar C(s) symmetry FH...SH- structure. For the F(-)-(H2S)2 cluster, the FH...SH- core remains and is solvated by a perturbed H2S ligand. For the larger F(-)-(H2S)(3-5) clusters, in addition to the FH...SH(-)-(H2S)n cluster forms, other minima featuring a 'solvated F-' anion are predicted. Calculated infrared spectra for the minima of each cluster size are presented to aid in assigning spectra from future experimental studies.     

Potential energy surface and MULTIMODE vibrational analysis of C2H3+

A full dimensional, ab initio-based semiglobal potential energy surface for C(2)H(3) (+) is reported. The ab initio electronic energies for this molecule are calculated using the spin-restricted, coupled cluster method restricted to single and double excitations with triples corrections [RCCSD(T)]. The RCCSD(T) method is used with the correlation-consistent polarized valence triple-zeta basis augmented with diffuse functions (aug-cc-pVTZ). The ab initio potential energy surface is represented by a many-body (cluster) expansion, each term of which uses functions that are fully invariant under permutations of like nuclei. The fitted potential energy surface is validated by comparing normal mode frequencies at the global minimum and secondary minimum with previous and new direct ab initio frequencies. The potential surface is used in vibrational analysis using the "single-reference" and "reaction-path" versions of the code MULTIMODE.     

Rotational spectroscopic and ab initio studies of the Xe-H2O van der Waals dimer

An ab initio potential energy surface of the Xe-H(2)O van der Waals dimer was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. For the Xe atom, the small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ-PP) basis set was used. Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was chosen for O and H atoms. Midbond functions were used to supplement the atom-centered basis sets. Rotational spectra of the Xe-H(2)O van der Waals dimer were recorded with a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the 0(00) and 1(01) states, were measured and assigned. Nuclear quadrupole hyperfine structures due to the (131)Xe (I = (3)/(2)), D (I = 1) and (17)O (I = (5)/(2)) nuclei were also observed and analyzed. Information about the molecular structure and the H(2)O angular motions was extracted from the spectroscopic results with the assistance of the ab initio potential.     

Searching of potential energy curves for the benzene dimer using dispersion-corrected density functional theory

The present work aims to establish the utility of dispersion-corrected density functional theory for potential energy curves of the benzene dimer, a problem that has received significant attention for a long time. The interaction energies of parallel-stacked, T-shaped and parallel-displaced benzene dimer configurations have been evaluated using both dispersion- and normal gradient-corrected Perdew-Burke-Ernzerhof functionals along with Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis functions and compared with explicit correlation methods. The potential energy curves for the parallel-stacked and parallel-displaced benzene dimers are in excellent agreement with highly accurate coupled cluster (CCSD(T)) results, while for the T-shaped benzene dimer the dispersion-corrected results show a distinct deviation, being closer in that case to the MP2 level of results. The overestimation of interaction energy in the T-shaped dimer may be attributed to the presence of a permanent dipole moment in this configuration and indicates a structural dependence of the dispersion-corrected density functional method.     

Auxiliary basis sets for density fitting-MP2 calculations: Nonrelativistic triple-zeta all-electron correlation consistent basis sets for the 3d elements Sc-Zn

Auxiliary basis sets for density fitting second-order Moller-Plesset perturbation theory (DF-MP2) have been optimized for use with the triple-zeta nonrelativistic all-electron correlation consistent orbital basis sets, cc-pVTZ-NR and aug-cc-pVTZ-NR, for the 3d elements Sc-Zn. The relative error in using these auxiliary basis sets is found to be around four orders of magnitude smaller than that from utilizing triple-zeta orbital basis sets rather than corresponding quadruple-zeta basis sets, in calculation of the correlation energy for a test set of 54 small to medium sized transition metal complexes.     

Ab initio investigation of the NH(X)-N2 van der Waals complex

The NH-N(2) van der Waals complex has been examined at the CCSD(T) level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The full basis set superposition error correction was applied. Two minimum energy structures were located for the electronic ground state. The global minimum corresponds to a linear geometry of the complex (NH-N-N), with D(e)=236 cm(-1) and R(c.m.)=4.22 A. The secondary minimum corresponds to a T-shaped geometry of C(2v) symmetry, where the nitrogen atom of the H-N moiety points toward the center of mass of the N(2) unit, aligned with the a-inertial axis of the complex. The binding energy and R(c.m.) value for the secondary minimum were 144 cm(-1) and 3.63 A, respectively. This potential energy surface is consistent with the properties of matrix-isolated NH-N(2), and it is predicted that linear NH-N(2) will be a stable complex in the gas phase at low temperatures.     

Asymmetrically solvated anion with both kinetic and thermodynamic stabilities: Theoretical studies on the cluster anions (HF)n - (n=3-6)

At the level of MP2 with the aug-cc-pVDZ and aug-cc-pVTZ basis sets supplemented with diffuse bond functions, the authors searched the potential energy surfaces of (HF)(n) (-) (n=3-6). In accordance with the literature, they found that the symmetrically solvated-electron anion (3(FH){e}) possesses the largest vertical detachment energy (VDE), while the dipole-bound anion ((FH)(3){e}) is the lowest isomer in energy for (HF)(3) (-). Their calculations demonstrated that, with the increase of the cluster size, the asymmetric (FH)(a){e}(HF)(b) cluster is stabilized with a simultaneously increased VDE. Thus they predicted that, for (HF)(6) (-), the (FH)(4){e}(HF)(2) cluster is both kinetically and thermodynamically most stable, possessing the largest VDE and being the global minimum at the same time.     

Mono- and dibridged isomers of Si2H3 and Si2H4: the true ground state global minima. Theory and experiment in concert

Highly correlated ab initio coupled-cluster theories (e.g., CCSD(T), CCSDT) were applied on the ground electronic states of Si(2)H(3) and Si(2)H(4), with substantive basis sets. A total of 10 isomers, which include mono- and dibridged structures, were investigated. Scalar relativistic corrections and zero-point vibrational energy corrections were included to predict reliable energetics. For Si(2)H(3), we predict an unanticipated monobridged H(2)Si-H-Si-like structure (C(s), (2)A') to be the lowest energy isomer, in constrast to previous studies which concluded that either H(3)Si-Si (C(s), (2)A') or near-planar H(2)Si-SiH (C(1), (2)A) is the global minimum. Our results confirm that the disilene isomer, H(2)Si-SiH(2), is the lowest energy isomer for Si(2)H(4) and that it has a trans-bent structure (C(2)(h), (1)A(g)). In addition to the much studied silylsilylene, H(3)Si-SiH, we also find that a new monobridged isomer H(2)Si-H-SiH (C(1), (1)A, designated 2c) is a minimum on the potential energy surface and that it has comparable stability; both isomers are predicted to lie about 7 kcal/mol above disilene. By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, the rotational spectrum of this novel Si(2)H(4) isomer has recently been measured in the laboratory, as has that of the planar H(2)Si-SiH radical. Harmonic vibrational frequencies as well as infrared intensities of all 10 isomers were determined at the cc-pVTZ CCSD(T) level.     

正、负和中性TiP10团簇结构与电子性质的密度泛函研究

采用密度泛函理论的B3LYP方法研究了正、负和中性TiP10团簇的几何构型和电子结构. 计算结果表明, 中性TiP10团簇的基态构型为金属夹心结构, 正、负离子团簇同样具有该基态稳定结构. 通过对基态稳定结构的分子轨道分析表明, δ键对形成夹心结构起到重要作用. 理论计算得到的中性TiP10团簇的垂直和绝热电离能分别为7.84和7.68 eV, 垂直和绝热电子亲和势分别为3.18和3.35 eV.     

Gold as hydrogen. An experimental and theoretical study of the structures and bonding in disilicon gold clusters Si2Au(n)- and Si2Au(n) (n = 2 and 4) and comparisons to Si2H2 and Si2H4

In a previous communication, we showed that a single Au atom behaves like H in its bonding to Si in a series of Si-Au clusters, SiAu(n) (n = 2-4) (Kiran et al. Angew. Chem., Int. Ed. 2004, 43, 2125). In this article, we show that the H analogy of Au is more general. We find that the chemical bonding and potential energy surfaces of two disilicon Au clusters, Si(2)Au(2) and Si(2)Au(4), are analogous to Si(2)H(2) and Si(2)H(4), respectively. Photoelectron spectroscopy and ab initio calculations are used to investigate the geometrical and electronic structures of Si(2)Au(2)(-), Si(2)Au(4)(-), and their neutral species. The most stable structures for both Si(2)Au(2) and Si(2)Au(2)(-) are found to be C(2)(v), in which each Au bridges the two Si atoms. For Si(2)Au(4)(-), two nearly degenerate dibridged structures in a cis (C(2)(h)) and a trans (C(2)(v)) configuration are found to be the most stable isomers. However, in the neural potential energy surface of Si(2)Au(4), a monobridged isomer is the global minimum. The ground-state structures of Si(2)Au(2)(-) and Si(2)Au(4)(-) are confirmed by comparing the computed vertical detachment energies with the experimental data. The various stable isomers found for Si(2)Au(2) and Si(2)Au(4) are similar to those known for Si(2)H(2) and Si(2)H(4), respectively. Geometrical and electronic structure comparisons with the corresponding silicon hydrides are made to further establish the isolobal analogy between a gold atom and a hydrogen atom.     

Ab initio and analytic intermolecular potentials for Ar-CH(3)OH

Ab initio calculations at the CCSD(T)/aug-cc-pVTZ level of theory were used to characterize the Ar-CH(3)OH intermolecular potential energy surface (PES). Potential energy curves were calculated for four different Ar + CH(3)OH orientations and used to derive an analytic function for the intermolecular PES. A sum of Ar-C, Ar-O, Ar-H(C), and Ar-H(O) two-body potentials gives an excellent fit to these potential energy curves up to 100 kcal mol(-1), and adding an additional r(-n) term to the Buckingham two-body potential results in only a minor improvement in the fit. Three Ar-CH(3)OH van der Waals minima were found from the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ calculations. The structure of the global minimum is in overall good agreement with experiment (X.-C. Tan, L. Sun and R. L. Kuczkowski, J. Mol. Spectrosc., 1995, 171, 248). It is T-shaped with the hydroxyl H-atom syn with respect to Ar. Extrapolated to the complete basis set (CBS) limit, the global minimum has a well depth of 0.72 kcal mol(-1) with basis set superposition error (BSSE) correction. The aug-cc-pVTZ basis set gives a well depth only 0.10 kcal mol(-1) smaller than this value. The well depths of the other two minima are within 0.16 kcal mol(-1) of the global minimum. The analytic Ar-CH(3)OH intermolecular potential also identifies these three minima as the only van der Waals minima and the structures predicted by the analytic potential are similar to the ab initio structures. The analytic potential identifies the same global minimum and the predicted well depths for the minima are within 0.05 kcal mol(-1) of the ab initio values. Combining this Ar-CH(3)OH intermolecular potential with a potential for a OH-terminated alkylthiolate self-assembled monolayer surface (i.e., HO-SAM) provides a potential to model Ar + HO-SAM collisions.     

Ab initio study of the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2) and related C(4)H(10)(2+) isomers.

Ab initio calculations at the MP4(SDTQ)/6-311G//MP2/6-31G level were performed to study the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2), and related C(4)H(10)(2+) isomers. Two doubly hydrogen bridged diborane type trans 1 and cis 2 isomers were located as minima. The trans isomer was found to be more favorable than cis isomer by only 0.6 kcal/mol. Several other minima for C(4)H(10)(2+) were also located. However, the global energy minimum corresponds to C-H (C(4) position) protonated 2-butyl cation 10. Structure 10 was computed to be substantially more stable than 1 by 31.7 kcal/mol. The structure 10 was found to be lower in energy than 2-butyl cation 13 by 34.4 kcal/mol.     

The C4H7+ cation. A theoretical investigation

The potential energy surface of the C4H7+ cation has been investigated with ab initio quantum chemical theory. Extended basis set calculations, including electronic correlation, show that cyclobutyl and cyclopropylcarbinyl cation are equally stable isomers. The saddle point connecting these isomers lies 0.6 kcal/mol above the minima. The global C4H7+ minimum corresponds to the 1-methylallyl cation, which is 9.0 kcal/mol more stable than the cyclobutyl and the cyclopropylcarbinyl cation and 9.5 kcal/mol below the 2-methylallyl cation. These results are in excellent agreement with experimental data.