Enthalpies of formation and isomerization of cis‐ and trans‐decalin,and their oxa‐analogs by G3(MP2)//B3LYP calculations

G3(MP2)//B3LYP calculations have been carried out on trans‐ and cis‐decalin, and their mono‐, di‐, tri‐, and tetraoxa‐analogs. The main purpose of the work was to obtain enthalpies of formation for these compounds, and to study the relative stabilities of the cis–trans and positional isomers of the various (poly)oxadecalins. Comparison of the computational enthalpies of formation with the respective experimental ones, known only for the decalins and 1,3,5,7‐tetraoxadecalins, shows that in both cases the computational values are more negative than the experimental ones, the deviations being ?5 to ?7 kJ mol^?1 for the decalins and ?12 to ?17 kJ mol^?1 for the 1,3,5,7‐tetraoxadecalins. The respective computational enthalpies of cis–trans isomerization, however, are in excellent to satisfactory agreement with the experimental data. The cis–trans enthalpy differences vary from +11.0 kJ mol^?1 for decalin to ?15.4 kJ mol^?1 for 1,4,5,8‐tetraoxadecalin. Low relative enthalpy values were also calculated for the cis isomers of 1,8‐dioxadecalin (?3.7 kJ mol^?1), 1,3,6‐trioxadecalin (?4.6 kJ mol^?1), 1,3,8‐trioxadecalin (?9.7 kJ mol^?1), 1,4,5‐ trioxadecalin (?5.6 kJ mol^?1), 1,3,5,8‐tetraoxadecalin (?7.3 kJ mol^?1), and 1,3,6,8‐tetraoxadecalin (?14.5 kJ mol^?1). Copyright © 2009 John Wiley & Sons, Ltd.     

Computational study on structures,thermochemical properties,and bond energies of disulfide oxygen (S–S–O)‐bridged CH3SSOH and CH3SS(=O)H and radicals

Cleavage of disulfide bonds is a common method used in linking peptides to proteins in biochemical reactions. The structures, internal rotor potentials, bond energies, and thermochemical properties (Δ_fH°, S°, and Cp(T)) of the S–S bridge molecules CH₃SSOH and CH₃SS(=O)H and the radicals CH₃SS?=O and C?H₂SSOH that correspond to H‐atom loss are determined by computational chemistry. Structure and thermochemical parameters (S° and Cp(T)) are determined using density functional Becke, three‐parameter, Lee–Yang–Parr (B3LYP)/6‐31++G (d, p), B3LYP/6‐311++G (3df, 2p). The enthalpies of formation for stable species are calculated using the total energies at B3LYP/6‐31++G (d, p), B3LYP/6‐311++G (3df, 2p), and the higher level composite CBS–QB3 levels with work reactions that are close to isodesmic in most cases. The enthalpies of formation for CH₃SSOH, CH₃SS(=O)H are ?38.3 and ?16.6 kcal mol^?1, respectively, where the difference is in enthalpy RSO–H versus RS(=O)–H bonding. The C–H bond energy of CH₃SSOH is 99.2 kcal mol^?1, and the O–H bond energy is weaker at 76.9 kcal mol^?1. Cleavage of the weak O–H bond in CH₃SSOH results in an electron rearrangement upon loss of the CH₃SSO–H hydrogen atom; the radical rearranges to form the more stable CH₃SS· = O radical structure. Cleavage of the C–H bond in CH₃SS(=O)H results in an unstable [CH₂SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH₂ = S + HSO. The CH₃SS(=O)–H bond energy is quite weak at 54.8 kcal mol^?1 with the H–C bond estimated at between 91 and 98 kcal mol^?1. Disulfide bond energies for CH₃S–SOH and CH3S–S(=O)H are low: 67.1 and 39.2 kcal mol^?1. Copyright © 2011 John Wiley & Sons, Ltd.     

Theoretical study of B3O radical isomers and their interconversion pathways

Despite the fact that B₃O is the second simplest B _n O radical after BO, a controversy recently emerged concerning the molecular structure of its global minimum. Two recent theoretical groups predicted the linear quartet BBBO to be the ground isomer. By contrast, another recent theoretical group reported that B₃O has a doublet B₃-ring ground structure. Moreover, larger B _n O clusters usually have low-lying B₃-ring isomers. In order to determine the accurate energetic competition between linear and cyclic structures in both the doublet and quartet, and to understand the detailed isomerism between various isomers, which is vital for understanding the formation mechanism of B₃O, we report the first potential energy surface (PES) study of B₃O at various computational levels, including CCSD(T)/6-311+G(2df), CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ and G3B3 for the single-point energy, as well as B3LYP/6-311+G(d) and QCISD/6-311+G(d) for geometrical optimisation. It is shown that the isomers in the quartet state are all thermodynamically more stable than the corresponding doublet ones, and on both the quartet and doublet PESs, the linear form has the lowest energy. Therefore, our study on both linear and cyclic isomers shows that the linear quartet BBBO ⁴ 01 is definitively the ground isomer. Although being much less stable than the quartet linear BBBO global minimum by >20 kcal mol^?1, five cyclic isomers exist as local minima, with the bi-cyclic structure ⁴ 02 possessing the smallest barrier of around 15 kcal mol^?1. The dissociation energies for direct combination processes B₃ + O, B₂ + BO and B + B₂O are discussed. The present work may be helpful in obtaining a deep understanding of the doping and oxidation process of pure B _n clusters.     

Possible spectroscopic manifestation of the angular group induced bond alteration (AGIBA) effect in toluene

The most obvious consequence of the concept of aromaticity is the common confidence that in aromatic compounds, bond lengths do not alternate and are between typical to the single and double ones. However, in 1994, performing crystal structure investigations of substituted pyridines and their salts, Krygowski and co‐workers have discovered a very surprising angular group induced bond alteration (AGIBA) effect: It appears that some angular substituents, like methoxy or nitrozo groups, can induce bond alternation in aromatic rings. Crystal studies do not allow one to operate with liquids that are more common in organic chemistry. This paper presents the first possible evidence of spectroscopic manifestations of the AGIBA effect. Raman spectra of the liquid toluene are analyzed. It is found that instead of being single, the line corresponding to the ring breathing vibrations is clearly split by 1.0–1.4 cm^?1, thus indicating the presence of two (cis‐ and trans‐) AGIBA isomers. The energy difference between these isomers estimated in temperature dependent Raman studies is found equal to 6.68 kJ mol^?1. The low‐wavenumber line therefore corresponds to the cis‐AGIBA isomer and the high‐wavenumber line to the trans‐AGIBA isomer stabilized by the AGIBA effect. Copyright © 2007 John Wiley & Sons, Ltd.     

FC(O)O2的结构及其自由基与NO反应的微观机理研究

用密度泛函理论(DFT)和哈特里-福克(HF)从头计算方法和半经验势方法等研究了FC(O)O₂ 的结构和振动性质.在DFT中采用B3LYP方法,在6-311G(d)基组上对FC(O)O₂自由基与NO反应的微观过程进行了分析.首先给出了各反应物、中间体、过渡态和生成物的几何构型,然后计算了它们的能量和频率,通过频谱分析得到反应的中间体和过渡态信息,即FC(O)O₂与NO反应为多反应通道,势垒高度和反应速度给出主要通道是FC(O)O₂     

The heats of formation of the haloacetylenes XCCY [X,Y = H,F, Cl]: basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects. and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: δH₁ ²⁹⁸(HCCH) = 54.48 kcal mol^?1, δH_f ²⁹⁸(HCCH) = 25.15 kcal mol, δH_f ²⁹⁸(FCCF) = 1.38 kcal mol^?1, δH_f ²⁹⁸(HCCC1) = 54.83 kcal mol^?1, δH_f ²⁹⁸(CICCC1) = 56.21 kcal mol^?1, and δH_f ²⁹⁸(FCCC1) = 28.47 kcal mo1^?1. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the WI level of theory. So doing we find the following destabilization order for acetylenes: FCCF > ClCCF > HCCF > ClCCCl > HCCCI > HCCH. By a combination of WI theory and isodesmic reactions. we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to ?31.8 ± 0.6 kcal mol^?1, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes, such as G2, G3, G3X and CBS-QB3 theories, has been analysed.     

Nitrosation of dialkylhydroxylamines,and computational and NMR investigations of the interconversion of conformational isomers of N‐nitroso‐dimethylhydroxylamine

The effect of acidity upon the rate of nitrosation of N‐benzyl,O‐methylhydroxylamine ( 3 ) in 1:1 (v/v) H₂O/MeOH at 25 °C has been investigated. The pseudo‐first‐order rate constant (k_obs) for loss of HNO₂ as the limiting reagent decreases as [H₃O⁺] increases. This is compatible with two parallel reaction channels (Scheme 2 ). One involves the direct reaction of the free hydroxylamine with HNO₂ (k₁ = 1.4 × 10² dm³ mol^?1 s^?1, 25 °C) and the other involves the reaction of the free hydroxylamine with NO⁺ (k₂ = 5.9 × 10⁹ dm³ mol^?1 s^?1). In contrast, there is only a very slight increase in k_obs with increasing [H₃O⁺] for nitrosation of N,O‐dimethylhydroxylamine ( 4 ) in dilute aqueous solution at 25 °C to give N‐nitroso‐dimethylhydroxylamine, 5 . This also fits a two‐channel mechanism (Scheme 3 ). Again, one involves the nitrosation of the free base by NO⁺ (k₂ = 8 × 10⁹ dm³ mol^?1 s^?1, 25 °C) but the other channel now involves catalysis by chloride (k₃ = 1.3 × 10⁸ dm³ mol^?1 s^?1). Arising from these results, we propose an estimate of pK_a ～ ?5 for protonated nitrous acid, (O = N? OH), which is appreciably different from the literature value of +1.7. The interconversion of cis and trans conformational isomers of 5 has been investigated by temperature‐dependent NMR spectroscopy in CDCl₃, methanol‐d₄, toluene‐d₈ and dimethyl sulfoxide‐d₆. Enthalpies and entropies of reaction and of activation have been determined and compared with computational values obtained at the B3LYP/6‐31G* level of theory. The cis form is slightly more stable at normal temperatures and no solvent effects upon the thermodynamics or kinetics of the conformational equilibrium were predicted computationally or detected experimentally. In addition, key geometric parameters and dipole moments have been calculated for the cis and trans forms, and for the lowest energy transition structure for their interconversion, in the gas phase and in chloroform. These results indicate electronic delocalisation in the ground states of 5 which is lost in the transition structure for their interconversion. Copyright © 2010 John Wiley & Sons, Ltd.     

Explicitly correlated benchmark calculations on C8H8 isomer energy separations: how accurate are DFT,double-hybrid,and composite ab initio procedures?

Accurate isomerization energies are obtained for a set of 45 C₈H₈ isomers by means of the high-level, ab initio W1-F12 thermochemical protocol. The 45 isomers involve a range of hydrocarbon functional groups, including (linear and cyclic) polyacetylene, polyyne, and cumulene moieties, as well as aromatic, anti-aromatic, and highly-strained rings. Performance of a variety of DFT functionals for the isomerization energies is evaluated. This proves to be a challenging test: only six of the 56 tested functionals attain root mean square deviations (RMSDs) below 3?kcal?mol^?1 (the performance of MP2), namely: 2.9 (B972-D), 2.8 (PW6B95), 2.7 (B3PW91-D), 2.2 (PWPB95-D3), 2.1 (ωB97X-D), and 1.2 (DSD-PBEP86) kcal?mol^?1. Isomers involving highly-strained fused rings or long cumulenic chains provide a ‘torture test’ for most functionals. Finally, we evaluate the performance of composite procedures (e.g. G4, G4(MP2), CBS-QB3, and CBS-APNO), as well as that of standard ab initio procedures (e.g. MP2, SCS-MP2, MP4, CCSD, and SCS-CCSD). Both connected triples and post-MP4 singles and doubles are important for accurate results. SCS-MP2 actually outperforms MP4(SDQ) for this problem, while SCS-MP3 yields similar performance as CCSD and slightly bests MP4. All the tested empirical composite procedures show excellent performance with RMSDs below 1?kcal?mol^?1.     

Catalytic effect of water,water dimer,water trimer,HCOOH, H2SO4, CH3CH2COOH and HN(NO2)2 on the isomerisation of HN(NO2)2 to O2NNN(O)OH: a mechanism study

In this article, the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH without and with catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂) have been investigated theoretically at the CBS-QB3 level of theory. Our results show that the catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄ and CH₃CH₂COOH) shows different positive catalytic effects on reducing the apparent activation energy of the isomerisation reaction processes. Such different catalytic effects are mainly related to the number of hydrogen bonds and the size of the ring structure in X (X = H₂O, (H₂O)₂ and (H₂O)₃)-assisted transition states, as well as different values of pK_a for H₂SO₄, HCOOH and CH₃CH₂COOH. Very interesting is also the fact that H₂SO₄-assisted reaction is the most favourable for the hydrogen transfer from HN(NO₂)₂ to O₂NNN(O)OH, due to the smallest pK_a (?3.0) value of H₂SO₄ than H₂O, HCOOH, H₂SO₄ and CH₃CH₂COOH, and also because of the largest ∠X???H???Y (the angle between the hydrogen bond donor and acceptor) involved in H₂SO₄-assisted transition state. Compared to the self-catalysis of the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH, the apparent activation energy of H₂SO₄-assisted channel also reduces by 9.6 kcal?mol^?1, indicating that H₂SO₄ can affect the isomerisation of HN(NO₂)₂ to O₂NNN(O)OH, most obvious among all the catalysts H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂.     

Formic acid in deuterium and hydrogen matrices

ABSTRACT

Formic acid (HCOOH, FA) was studied experimentally, by infrared spectroscopy, in H₂ and D₂ matrices, with focus on the preparation and characterisation in these matrix media of structures containing the higher-energy (cis) conformer. The cis-FA monomer and the cis-FA^?…?N₂ complex were successfully produced by selective vibrational excitation of corresponding trans-FA based species, and vibrationally characterised. The tunneling-induced conversion of the cis-FA^?…?N₂ complex in the studied matrices into the corresponding trans-FA complex was also investigated, and the found tunnelling properties discussed, in particular in comparison with those observed for the spontaneous conversion of cis-FA monomer into trans-FA. This article constitutes the first report on the infrared spectrum of FA conformers and stability of cis-FA monomer in a D₂ matrix, and on the structure, spectroscopy and stability of the cis-FA^?…?N₂ complex in both H₂ and D₂ matrices. Different attempts to prepare the cis-FA^?…?H₂O complex in the two investigated matrices are also described in detail, both from previously in situ generated cis-FA monomer followed by thermal mobilisation and by direct selective vibrational excitation of the trans-FA-H₂O complex.     

Substituent effects on the conformational stability of allyl fluorides

By the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCH?CHCH₂F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for Y?H, Cl, Me, Vinyl, CF₃, CN, CHO, and NO₂, whereas the gauche is favored for Y?OMe, OH. A correlation of ΔH with the substituent constants σ⁺(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH₂?CHCH₂F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C₂–C₃ vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C₂–C₃ vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive π_C?C→σ*_{C? F} interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermochemistry,bond energies and internal rotor barriers of methyl sulfinic acid,methyl sulfinic acid ester and their radicals

Sulfur–Oxygen containing hydrocarbons are formed in oxidation of sulfides and thiols in the atmosphere, on aerosols and in combustion processes. Understanding their thermochemical properties is important to evaluate their formation and transformation paths. Structures, thermochemical properties, bond energies, and internal rotor potentials of methyl sulfinic acid CH₃S(?O)OH, its methyl ester CH₃S(?O)OCH₃ and radicals corresponding to loss of a hydrogen atom have been studied. Gas phase standard enthalpies of formation and bond energies were calculated using B3LYP/6‐311G (2d, p) individual and CBS‐QB3 composite methods employing work reactions to further improve accuracy of the ${\Delta} _{{\bf f}} H_{{\bf 298}}^{{\bf o}} $ . Molecular structures, vibration frequencies, and internal rotor potentials were calculated. Enthalpies of the parent molecules CH₃S(?O)OH and CH₃S(?O)OCH₃ are evaluated as ?77.4 and ?72.7 kcal mol^?1 at the CBS? QB3 level; Enthalpies of radicals C^?H₂? S(?O)? OH, CH₃? S^?(?O)₂, C^?H₂? S(?O)? OCH₃ and CH₃? S(?O)? OC^?H₂ (CBS‐QB3) are ?25.7, ?52.3, ?22.8, and ?26.8 kcal mol^?1, respectively. The CH₃C(?O)O—H bond dissociation energy is of 77.1 kcal mol^?1. Two of the intermediate radicals are unstable and rapidly dissociate. The CH₃S(?O)? O. radical obtained from the parent CH₃? S(?O)? OH dissociates into methyl radical (${\bf CH}_{{\bf 3}}^{{\bf .}} $ ) plus SO₂ with endothermicity (ΔH_rxn) of only 16.2 kcal mol^?1. The CH₃? S(?O)? OC^?H₂ radical dissociates into CH₃? S^?=O and CH₂=O with little or no barrier and an exothermicity of ?19.9 kcal mol^?1. DFT and the Complete Basis Set‐QB3 enthalpy values are in close agreement; this accord is attributed to use of isodesmic work reactions for the analysis and suggests this combination of B3LYP/work reaction approach is acceptable for larger molecules. Copyright © 2010 John Wiley & Sons, Ltd.     

Conformational analysis of 3‐methyl‐3‐silathiane and 3‐fluoro‐3‐methyl‐3‐silathiane

The conformational equilibria of 3‐methyl‐3‐silathiane 5 , 3‐fluoro‐3‐methyl‐3‐silathiane 6 and 1‐fluoro‐1‐methyl‐1‐silacyclohexane 7 have been studied using low temperature ¹³C NMR spectroscopy and theoretical calculations. The conformer ratio at 103 K was measured to be about 5 _ax: 5 _eq = 15:85, 6 _ax: 6 _eq = 50:50 and 7 _ax: 7 _eq = 25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol^?1) is much less than in 3‐methylthiane 9 (1.40 kcal mol^?1) but somewhat greater than in 1‐methyl‐1‐silacyclohexane 1 (0.23 kcal mol^?1). Compounds 5–7 have low barriers to ring inversion: 5.65 (ax → eq) and 6.0 (eq → ax) kcal mol^?1 ( 5 ), 4.6 ( 6 ), 5.1 (Me_ax → Me_eq) and 5.4 (Me_eq → Me_ax) kcal mol^?1 ( 7 ). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6 , or different conformer ratio for 5 and 7 . Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si–X and adjacent C–H, C–S, and C–C bonds proved responsible. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical study on C100 fullerenes and C96X4 (X=N,P, B,Si)

The geometrical structures and electronic properties of six fullerene isomers of C₁₀₀ were studied at the HF/6-31G^? and B3LYP/6-31G^? levels, respectively. The results of the fully optimized calculations show that three C₁₀₀ isomers 449:D₂, 425:C₁ and 442:C₂ are near isoenergetic isomers. The energies and properties of C₁₀₀ hexaanions were calculated. The C₁₀₀^6? (450:D₅) isomer is predicted to be the most stable isomer at the B3LYP/6-31G^? level, and the C₁₀₀^6? (449:D₂) isomer is 44.1 kcal/mol higher in energy. The heterofullerenes C₉₆X₄ (X=N, P, B, Si) formed from the initial C₁₀₀ (449:D₂) have also been investigated at the B3LYP/6-31G^? level. The HOMO–LUMO gaps and aromaticities show that the replacement of fullerene carbon atoms with four heteroatoms can enhance the electronic stabilization of C₁₀₀ (449:D₂).     

Isomerization of C3H3NO isomers: ab initio study

The structures and isomerization process of C₃H₃NO species have been explored at the MP2/6–311++G(d,p) level of theory of the ab initio method. Eleven minima and four interconversion transition states have been identified. The zero-point vibrational energy corrections were made to predict reliable energies. We predict a five-membered ring-like structure to be the lowest energy isomer, which is 177.73?kcal?mol^?1 more stable than the least stable isomer X found on the potential energy surface. The transition states and minima isomers were verified by frequency calculation. Intrinsic reaction coordinate (IRC) calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The isomer stabilities have been studied using the relative energies, chemical hardness and chemical potential. The MHP principle could not predict the order of stability for C₃H₃NO isomers as arrived at with the relative energies. The role of intramolecular hydrogen bonds on the equilibrium structure has been discussed. The energy barrier and reaction enthalpy have been calculated during isomerization.     

Calculation of anharmonic effects in the unimolecular dissociation of M2+(H2O)2 (M = Be,Mg, and Ca)

The anharmonic and harmonic rate constants of the unimolecular dissociation of M²⁺(H₂O)₂ (M = Be, Mg, and Ca) were calculated using the Rice–Ramsperger–Kassel–Marcus theory. The anharmonic effects of the reactions were investigated. The results show that the energy barrier of the dissociation of Be²⁺(H₂O)₂ is 68.47 kcal/mol, and the anharmonic (T_4000K = 4.28×10⁸ s^?1) and harmonic (T_4000K = 4.22×10⁸ s^?1) rate constants were close in value in both the canonical and microcanonical systems. The energy barriers of the two steps for the dissociation, Mg²⁺(H₂O)₂ → MgOH⁺+H₃O⁺, were 37.41 and 11.39 kcal/mol, and those for the dissociation, Ca²⁺(H₂O)₂ → CaOH⁺+H₃O⁺, were 21.15 and 26.42 kcal/mol. The anharmonic effect of the two reactions is significant and cannot be neglected in both the canonical and microcanonical systems. The comparison also shows that the rate constants of the dissociation of Ca²⁺(H₂O)₂ have the maximum values, while those of Be²⁺(H₂O)₂ have the minimum values in the three reactions; however, the anharmonic effect also shows the similar trend in the comparison.     

Raman and ab initio study of the conformational isomerism in the 1‐ethyl‐3‐methyl‐imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid

The calculated and experimental Raman spectra of the (EMI⁺)TFSI⁻ ionic liquid, where EMI⁺ is the 1‐ethyl‐3‐methylimidazolium cation and TFSI⁻ the bis(trifluoromethanesulfonyl)imide anion, have been investigated for a better understanding of the EMI⁺ and TFSI⁻ conformational isomerism as a function of temperature. Characteristic Raman lines of the planar (p) and non‐planar (np) EMI⁺ conformers are identified using the reference (EMI⁺)Br⁻ salt. The anion conformer of C₂ symmetry is confirmed to be more stable than the cis (C₁) one by 4.5 ± 0.2 kJ mol⁻¹. At room temperature, the population of trans (C₂) anions and np cations is 75 ± 2% and 87 ± 4%, respectively. Fast cooling quenches a metastable glassy phase composed of mainly C₂ anion conformers and p cation conformers, whereas slow cooling gives a crystalline phase composed of C₁ anion conformers and of np cation conformers. Copyright © 2006 John Wiley & Sons, Ltd.     

Time resolved measurements of methanol decomposition on Ni(100) utilizing laser induced desorption

A new method utilizing laser induced desorption (LID) is used to study the decomposition of methanol on Ni(100) in real time. The dependence of the rate of decompositition on surface coverage and on surface temperature is measured. The decomposition rate decreases during reaction in a manner characteristic of a self-poisoned reaction. The rate data are fit to a model in which the energy barrier to reaction increases in proportion to the coverage of the CH₃O product. The energy barrier obtained is 9 kcal mol^?1 plus 4 kcal mol^?1 monolayer^?1 of CH₃O. The frequency factor of 2 × 10⁹ s^?1 suggests there is significant entropy barrier to decomposition. Substitution of deuterium for the alcoholic hydrogen alters de decomposition rate appreciably and identifies the breaking of the OH bond as the rate determining step.     

Raman and infrared spectra,conformational stability,vibrational assignment,barriers to internal rotation and ab initio calculations of but-2-enoyl chloride

The Raman (3500–10 cm⁻¹) and infrared (3200–50 cm⁻¹) spectra were recorded for the fluid and solid phases of but-2-enoyl chloride (crotonyl chloride), trans-CH₃CHCHCClO, where the methyl group is trans to the CClO group, and a complete vibrational assignment is proposed. These data were interpreted on the basis that the s-trans (anti) form (two double bonds oriented trans to one another) is the most stable form in the fluid phases and the only conformer remaining in the solid state. The asymmetric torsional fundamental of the more stable s-trans and the higher energy s-cis (syn) form were observed at 97.5 and 86.9 cm⁻¹, respectively. From these data the asymmetric potential function governing the internal rotation about the C C bond was determined. The potential coefficients are V₁ = −111 ± 2, V₂ = 1860 ± 48, V₃ = 6 ± 2, V₄, = −43 ± 24 and V₆ = −22 ± 6. The s-trans to s-cis and s-cis to s-trans barriers were determined to be 1890 and 1785 cm⁻¹, respectively, with an enthalpy difference between the conformers of 105 ± 52 cm⁻¹ [300 ± 149 cal mol⁻¹ (1 cal = 4.184 J)]. Similarly, the barrier governing internal rotation of the CH₃ group for the s-trans conformer was also determined to be 912 ± 30 (2.61 ± 0.09 kcal mol⁻¹) from the torsional fundamental observed in the far-infared spectrum of the gas. All these data were compared with the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing the RHF/3–21G*, RHF/6–31G* and/or MP2/6–31G* basis sets. These results were compared with the corresponding quantities for some similar molecules.