A Gaussian-2 ab initio study of [C2H5S] ions: III. H2 and CH4 eliminations from CH3CHSH and CH3SCH2

Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis-CH₃CHSH⁺ (1⁺), trans-CH₃CHSH⁺ (2⁺), and CH₃SCH₂⁺ (3⁺): 1⁺→CH₃⁺+trans-HCSH (1); 1⁺→CH₃+trans-HCSH⁺ (2); 1⁺→CH₄+HCS⁺ (3); 1⁺→H₂+c-CH₂CHS⁺ (4); 2⁺→H₂+CH₃CS⁺ (5); and 3⁺→H₂+c-CH₂CHS⁺ (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol⁻¹, respectively. Loss of CH₄ from 1⁺ (reaction (3)) proceeds through proton transfer from the S atom to the methyl group, followed by cleavage of the C–C bond. The reaction pathway has an energy barrier of 292 kJ mol⁻¹ and a transition state with a wide spectrum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol⁻¹ and it also proceeds through the same proton transfer step as reaction (3), followed by elimination of H_2. Formation of CH₃CS⁺ from 2⁺ (reaction (5)) by loss of H₂ proceeds through protonation of the methine (CH) group, followed by dissociation of the H₂ moiety. Its energy barrier is 276 kJ mol⁻¹. On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H₂ 1,1-elimination from 3⁺ (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH₃SCH₂⁺ and has a critical energy of 269 kJ mol⁻¹.     

An ab initio and density functional theory study of the dimethylzinc-hydrogen selenide adduct: (CH3)2Zn:SeH2

The molecular structure (equilibrium geometry) and binding energy of the dimethylzinc (DMZn)-hydrogen selenide (H₂Se) adduct, (CH₃)₂Zn:SeH₂, have been computed with ab initio molecular orbital and density functional theory (DFT) methods and, where possible, compared with experimental results. The structure of the precursors DMZn and H₂Se are perturbed to only a small extent upon adduct formation. (CH₃)₂Zn:SeH₂ was found to be 3 kcal mol⁻¹ less stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation, indicating that the (CH₃)₂Zn:SeH₂ adduct is unlikely to be a stable gas-phase species under chemical vapour deposition conditions. Further calculations at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation suggest that the 1:2 adduct species, (CH₃)₂Zn:(SeH₂)₂, is much less stable than the 1:1 adduct and consequently the precursors by 19 kcal mol⁻¹.     

Ab initio molecular orbital study on three feasible mechanisms for substitution of the vinylic carbon in F2C=C(OMs)BMe3−

A substitution on 2,2-difluorovinylic carbon was investigated by using ab initio molecular orbital calculations. Three feasible mechanisms, which are the S_N1-like, the S_N2-type and the addition-elimination mechanisms, were ex- amined for a model borate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borate. Four TSs were obtained depending on the position of Li⁺ around the vinylborate although activation energies in the gas phase are rather high (ca. 30–40 kcal mol⁻¹) in comparison with that expected from the experimental conditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the acti- vation energy of one S_N2-type mechanism very much (4. l kcal mol⁻¹ at the B3LYP/6-31+G^*//RHF/6-31+G^/s* level of theory) while those for the other mechanisms do not change very much. Therefore, the S_N2-type mechanism is applicable to the substitution reaction observed for the vinylborate.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Kinetic study of the reaction H2O2+H→H2O+OH by ab initio and density functional theory calculations

Theoretical investigations on the kinetics of the elementary reaction H₂O₂+H→H₂O+OH were performed using the transition state theory (TST). Ab initio (MP2//CASSCF) and density functional theory (B3LYP) methods were used with large basis set to predict the kinetic parameters; the classical barrier height and the pre-exponential factor. The ZPE and BSSE corrected value of the classical barrier height was predicted to be 4.1 kcal mol⁻¹ for MP2//CASSCF and 4.3 kcal mol⁻¹ for B3LYP calculations. The experimental value fitted from Arrhenius expressions ranges from 3.6 to 3.9 kcal mol⁻¹. Thermal rate constants of the title reaction, based on the ab initio and DFT calculations, was evaluated for temperature ranging from 200 to 2500 K assuming a direct reaction mechanism. The modeled ab initio-TST and DFT–TST rate constants calculated without tunneling were found to be in reasonable agreement with the observed ones indicating that the contribution of the tunneling effect to the reaction was predicted to be unimportant at ambient temperature.     

Equilibrium structure of the carbon dioxide-water complex in the gas phase: an ab initio and density functional study

High-level ab initio (MP2/6-311++G(2d,2p) geometry, Gaussian-2, MP4(SDTQ) and QCISD(T) binding energies) and density-functional (Becke3LYP/6-311++G(2df,2pd)) calculations have been performed on the charge-transfer complex between water and carbon dioxide. The complex appears to have two equivalent non-planar minima of C_s symmetry. Minima are separated by transition states with C₁ symmetry, whereas the totally planar structure with C_2v symmetry is a second-order transition state. All the critical points lie at approximately the same energy (less than 0.05 Kj mol⁻¹ difference). Therefore, the experimentally observable structure should be planar. The best equilibrium intermolecular distance for this complex calculated at the MP2/6-311++G(2d,2p) level is 2.800 Å. Our best estimate of the observable intermolecular distance (corrected for anharmonicity) is 2.84 Å, in agreement with the experimentally derived value of 2.836 Å. Our best estimate of the binding energy at the QCISD(T) level, taking into account the variation of the distance owing to anharmonicity and the use of more sophisticated theoretical treatments, is −12.0 ± 0.2 kJ mol⁻¹. Our best estimate of the barrier to internal rotation, also at the MP2/6-311++G(2d,2p) level, is 4.0 kJ mol⁻¹, outside the error limits of the experimental determination (3.64 ± 0.04 kJ mol⁻¹). Density functional theory at the level employed here gives an equilibrium intermolecular distance that is too large (2.857 Å), a binding energy that is too small (8.1 kJ mol⁻¹), attributable neither to geometry nor to the basis set, and also a barrier to internal rotation that is slightly too small (3.39 kJ mol⁻¹). The overall picture is, however, reasonably good.     

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.     

Ab initio study of sulfur isotope fractionation in the reaction of OCS with OH

The fractionation of sulfur isotopes in the gas-phase reaction of OCS with OH was calculated by ab initio methods. The first reaction step occurs through a carbon bonded OH–OCS transition state. The activation barrier is 18.4 kJ mol⁻¹ (CCSD(T)/aug-cc-pVDZ level plus zero-point energy) relative to the reactants. Fractionation constants are −1.4, −2.6 and −6.3‰ for ³³S, ³⁴S and ³⁶S respectively. These results bring new insight regarding the origin of stratospheric sulfate aerosols, including the identification of characteristic isotopic fractionations in the lower and middle stratosphere.     

Negative hyperconjugation in methyl and silyl anions and its effect on the acidities of CH(4-n)Fn CH(4-n)C1n SiH(4-n)Fn and SiH(4-n)Cln

Group separation reactions calculated using an ab intio molecular orbital calculation at the MP4/6-31 + + G(d,p) level of theory, show the negative hyperconjugation between fluorine atoms to be larger in methanes than in silanes. Stabilisation due to negative hyperconjugation is larger in anions than in identically substituted neutral molecules, e.g. 43.1 kcal mol⁻¹ in CF₃⁻ compared with 26.7 kcal mol⁻¹ in CHF₃. By contrast, in chloro-substituted methanes, silanes, methyl anions and silyl anions, group separation energies are approximately zero, indicating no appreciable negative hyperconjugation. An -chloro substituent is more effective than an -fluoro one at delocalising the negative charge of an anion and, as a consequence, the chloromethanes and chlorosilanes are all more acidic than the identically substitued fluoromethanes and fluorosilanes. For chloro-substituted molecules the acidity is linearly dependent on the number of chlorine atoms; for fluoro-substituted molecules stabilisation by negative hyperconjugation results in each additional fluorine atom increasing the acidity by larger increments.     

Das perfluortriazinium-kation als oxidationsmittel in der metallorganischen synthese—ein neuer weg zur darstellung von [Cp2MCl2](M = Mo

The oxidation of Cp₂MCl₂ (M= Mo, W) with perfluortriazinium tetrafluoroborate, [(FCN)₃F]⁺[BF₄]⁻, in the presence of a flouride ion acceptor (BF₃ or PF₅) in SO₂ solution yielded the cationic metallocene complexes [Cp₂MCl ₂]²⁺[BF₄]⁻ or [C_p2MCl₂] ²⁺[BF₄]⁻[PF₆]⁻ (M = Mo, W), respectively. In these reactions, for the first time the perfluortriazinium cation has proved to be easy to handle and a useful oxidizer in organometallic chemistry. The oxidizer strength of three fluorotriazinium cations, [(XCN)₃F]⁺ (X = F, Cl, H), has been computed ab initio (HF/6 − 31 + G) and calibrated on literature data which were obtained by local density functional calculations. It was anchored to its F⁺ zero point by an experimental value for KrF⁺. ab]Die Oxidation von Cp²MCl₂ mit (M = MO, W) Perfluortriaziniumtetrafluoroborat, [(FCN)₃F]⁺[BF₄]⁻, in Anwesenheit eines Fluoridionenakzeptors (BF₃ oder PF₅) führte in SO₂-Lösung zur Bildung der kationischen Metallocen-Komplexe [Cp₂MCl₂₊]²⁺[BF₄]₂⁻ bzw. [Cp₂MCl₂]²⁺[BF₄]⁻ [PF₆]⁻ (M = Mo, W). In diesen Reaktionen konnte erstmals gezeigt werden, daß Perfluortriazinium-Kationen einfach zu handhabende und nützliche Oxidationsmittel im Bereich der metallorganischen Synthese darstellen. Das (Mdationsvermögen von drei Fluorotriazinium-Kationen, [(XCN)₃F]⁺(X = F, Cl, H), wurde ab initio berechnet (HF/6 − 31 + G) und mit Hilfe von Literaturdaten, die mittels local density functional-Berechnungen erhalten und am experimentellen Wert von KrF ⁺ bezüglich des F⁺ Nullpunktes verankert wurden, kalibriert.     

An ab initio study of the Cl(P)+C2H6→C2H5+HCl abstraction reaction

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(³P)+C₂H₆→C₂H₅+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6.     

Ab initio study of some peroxides. HOOH, CH3OOH and, CH3OOCH3

The geometries of HOOH, CH₃OOH, and CH₃OOCH₃, were optimized with different basis sets (3-21G, 6-31G^*(*) and D95^**) at different levels of theory (HF, MP2, MP4, and CI). HF/3-21G optimizations result in planar trans conformations for all three peroxides. HF/6-31G^** calculations predict skew conformations for HOOH and CH₃OOH, but a planar trans struture for CH₃OOCH₃. For the larger basis set the calculated bond lengths, especially the O-O bonds, are too short. Optimizations for HOOH including electron correlation at the MP2, MP3, MP4, CI, and CCD level improve the agreement for bond lengths and the OOH angle, but result in dihedral angles Which are too large by 3– 8°. In the case of CH₃OOCH₃, similar calculations at the MP2 and CI level predict planar trans structures instead of the experimentally observed skew conformation. On the other hand, MP4 single point calculations at MP2 optimized parameters result in a correct skew structure. For all three peroxides a computationally “economic” method, i.e., single point calculations at MP2 or MP4 level with HF/3-21G optimized parameters, result in close agreement between calculated and experimental structures.     

Selective IR-induced isomerization of 1,2-dichloropropane isolated in xenon matrix

The conformations of monomeric 1,2-dichloropropane isolated in low temperature xenon matrix were characterized experimentally using FTIR spectroscopy. The interpretation of the experimental spectra was aided by ab initio MP2/6-311++G(3df,3pd) calculations. Relative energies of three structures, differing by the orientation of the Cl−C−C−Cl dihedral angle, fall within 0–6 kJ mol⁻¹ range. The spectral signatures of all three conformers were identified experimentally. Infrared irradiation of the matrix isolated compound resulted in selective conversion of the most stable Trans conformer into the higher energy Gauche+ rotamer, while the Gauche− conformer remained unaffected. This result was interpreted in terms of calculated energy barriers and dipole moments.     

Structures and energetics of C3H6S isomers: a Gaussian-3 ab initio study

Twenty-two isomers/conformers of C₃H₆S^+√ radical cations have been identified and their heats of formation (ΔH_f) at 0 and 298 K have been calculated using the Gaussian-3 (G3) method. Seven of these isomers are known and their ΔH_f data are available in the literature for comparison. The least energy isomer is found to be the thioacetone radical cation (4⁺) with C_2v symmetry. In contrast, the least energy C₃H₆O^+√ isomer is the 1-propen-2-ol radical cation. The G3 ΔH_f298 of 4⁺ is calculated to be 859.4 kJ mol⁻¹, ca. 38 kJ mol⁻¹ higher than the literature value, ≤821 kJ mol⁻¹. For allyl mercaptan radical cation (7⁺), the G3 ΔH_f298 is calculated to be 927.8 kJ mol⁻¹, also not in good agreement with the experimental estimate, 956 kJ mol⁻¹. Upon examining the experimental data and carrying out further calculations, it is shown that the G3 ΔH_f298 values for 4⁺ and 7⁺ should be more reliable than the compiled values. For the five remaining cations with available experimental thermal data, the agreement between the experimental and G3 results ranges from fair to excellent.

Cation CH₃CHSCH₂^+√ (10⁺) has the least energy among the eleven distonic radical cations identified. Their ΔH_f298 values range from 918 to 1151 kJ mol⁻¹. Nevertheless, only one of them, CH₂=SCH₂CH₂^+√ (12⁺), has been observed. Its G3 ΔH_f298 value is 980.9 kJ mol⁻¹, in fair agreement with the experimental result, 990 kJ mol⁻¹.

A couple of reactions involving C₃H₆S^+√ isomers CH₂=SCH₂CH₂^+√ (12⁺) and trimethylene sulfide radical cation (13⁺) have also been studied with the G3 method and the results are consistent with experimental findings.     

Rate constants for the reaction of OH radicals with CH3CN, C2H5CN AND CH2=CH-CN in the temperature range 298–424 K

Rate constants for the reactions of OH with CH₃CN, CH₃CH₂CN and CH₂=CH-CN have been measured to be 5.86 × 10⁻¹³ exp(−1500 ± 250 cal mole⁻¹/RT), 2.69 × 10⁻¹³ exp(−1590 ± 350 cal mole⁻¹/RT and 4.04 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively in the temperature range 298–424 K. These results are discussed in terms of the atmospheric lifetimes of nitrfles.     

A theoretical study of the reaction of SiH2 with C2H2 and C2D2

Potential energy surfaces of the reaction of SiH₂ and C₂H₂ (and C₂D₂) have been calculated by means of ab initio molecular orbital theory at the QCISD/6-311G++(2df, 2p)//MP2/6-31G(d, p) level with corrections for the triple excitations to the QCISD energies. The barrier heights for the two reaction channels of the adduct, thus calculated, were further utilized for the dynamical calculation of the rate constants in the framework of quantum statistical Rice-Ramsperger-Kassel theory. Contributions of the rate constants of the various pathways to the total rate constant (K_T) for the disappearance of the reactants are critically examined and compared with experiment. The pressure dependence of K_T(C₂H₂) is primarily due to the formation of silirene. K_T(C₂D₂) is consistently higher than K_T(C₂H₂). The standard heat of formation of silirene is predicted to be 72.1 ± 3 kcal/mol. Rearrangement of silirene to vinylsilylene requires an activation energy smaller than that to silylacetylene.     

Ab initio quantum chemical studies of reaction mechanism for CH2CO with NCO

An extensive quantum chemical study of the potential energy surface (PES) for all possible isomerization and dissociation reactions of CH₂CO with NCO is reported at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVDZ//B3LYP/6-311++G(d,p) levels of theory. For the CH₂CO+NCO reaction, the formation of CO+CH₂NCO via an addition–elimination mechanism is the dominant channel on the doublet surface. While the formation of CO+CH₂OCN via bimolecular substitution reaction is in the secondary. Meanwhile, the isomerization and dissociation reactions of the products, CH₂NCO and CH₂OCN, also have been investigated using the same theoretical approach. It can be concluded that these reaction channels are not feasible kinetically at low or fairly high temperatures. On the basis of the ab initio data, the total rate constants for the CH₂CO+NCO reaction in the T=296–560 K range have been computed using conventional transition state theory with Wigner tunneling correction and fitted by a rate expression as k=2.14×10⁻¹² (cm³ molecule⁻¹ s⁻¹) exp(654.29/T). The calculated total rate constants with Wigner tunneling correction for the CH₂CO+NCO reaction are in good agreement with the available experimental values.     

Reaction of NS with N3

Thionitrosyl hexafluoroarsenate, [NS][AsF₆], reacted with caesium azide, [Cs][N₃], to form S₂N₂ which polymerized to give (SN)_x. The structures of the following likely intermediate species were calculated at correlated MP2/6–31G(d,p) level of theory (relative energies in kcal mol⁻¹): thionitrosyl azide, N₃---N=S (37.8), thiazyl azide, N₃---SN (39.2) and cyclic N₄S (0.0).^‡     

Synthesis, characterization and antitumor activity of some arylantimony triphenylgermanylpropionates and crystal structures of Ph3GeCH(Ph)CH2CO2SbPh4 and [Ph3GeCH2CH(CH3)CO2]2Sb(4-ClC6H4)3

A series of novel arylantimony(V) triphenylgermanylpropionates with the formula (Ph₃GeCHR¹CHR²CO₂)_nSbAr_(5−n) (R¹=H, Ph; R²=H, CH₃; n=1, 2) were synthesized and characterized by elemental analysis, IR, ¹H-NMR, ¹³C-NMR and mass spectroscopy. The crystal structures of Ph₃GeCH(Ph)CH₂CO₂SbPh₄ and [Ph₃GeCH₂CH(CH₃)CO₂]₂Sb(4-ClC₆H₄)₃ were determined by X-ray diffraction. The in vitro antitumor activities of some selected compounds against five cancer cells are reported.