A microwave and quantum chemical study of allyltrifluorosilane

The structural and conformational properties of allytrifluorsilane, H₂CCH-CH₂-SiF₃, have been explored by microwave (MW) spectroscopy and high-level ab initio and density functional theory quantum chemical calculations. The microwave spectrum was investigated in the 18-62 GHz spectral regions. The a-type R-branch transitions of one conformer were assigned for the ground as well as for 10 vibrationally excited states. The CC-C-Si chain of atoms in this rotamer takes an anti-clinal (‘skew’) conformation, with a dihedral angle calculated to be 111.6° from the syn-periplanar (0°) conformation. The question whether a CC-C-Si syn-periplanar conformer exists as a high-energy form in the gas phase remains open. In most of the quantum chemical calculations this conformation is predicted to be a transition state. However, in the most advanced calculations (B3LYP/aug-cc-pVTZ level of theory) the syn-periplanar conformer is predicted to be a stable rotamer that is calculated to be 6.5 kJ/mol higher in energy than the anti-clinal form. Since there is no indication in the MW spectrum for the presence of high-energy form(s), it is concluded that the anti-clinal conformer is at least 4 kJ/mol more stable than any other hypothetical rotamer.     

Computational study of sulfoxides of thiacyclohexane, 4‐silathiacyclohexane, 4‐fluoro‐4‐silathiacyclohexane,and 4,4‐difluoro‐4‐silathiacyclohexane: Relative energies of conformations and sulfinyl oxygen stabilized pentacoordinate silicon in boat and twist structures

Second‐order Møller‐Plesset theory (MP2) has been used to calculate the equilibrium geometries and relative energies of the chair, 1,4‐twist, 2,5‐twist, 1,4‐boat, and 2,5‐boat conformations of thiacyclohexane 1‐oxide (tetrahydro‐2H‐thiopyran 1‐oxide), 4‐silathiacyclohexane 1‐oxide, cis‐ and trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide, and 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide. At the MP2/6‐311+G(d,p) level of theory, the chair conformer of axial thiacyclohexane 1‐oxide is 0.99, 5.61, 5.91, 8.57, and 7.43 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of equatorial thiacyclohexane 1‐oxide is 4.62, 6.31, 7.56, and 7.26 kcal/mol more stable (ΔE) than its respective 1,4‐twist and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of axial 4‐silathiacyclohexane 1‐oxide is 1.79, 4.26, 3.85, and 5.71 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The 2,5‐twist conformer of axial 4‐silathiacyclohexane 1‐oxide is stabilized by a transannular interaction between the sulfinyl oxygen and silicon, to give trigonal bipyramidal geometry at silicon. The chair conformer of equatorial 4‐silathiacyclohexane 1‐oxide is 2.47, 7.90, and 8.09 kcal/mol more stable (ΔE) than its respective 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The chair conformer of axial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 4.18 and 5.70 kcal/mol more stable than its 1,4‐twist conformer and 2,5‐boat transition state and 1.51 kcal/mol more stable than the chair conformer of equatorial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide. The chair conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 5.02 and 6.11 kcal/mol more stable than its respective 1,4‐twist conformer and 2,5‐boat transition state, but is less stable than its 2,5‐twist conformer (ΔE = ?1.77 kcal/mol) and 1,4‐boat transition state (ΔE = ?1.65 kcal/mol). The 2,5‐twist conformer and 1,4‐boat conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide are stabilized by intramolecular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The chair conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is 3.02, 5.16, 0.90, and 6.21 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 1,4‐boat conformers and 2,5‐boat transition state. The 1,4‐boat conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in a trigonal bipyramidal geometry at silicon. The relative energies of the conformers and transition states are discussed in terms of hyperconjugation, orbital interactions, nonbonded interactions, and intramolecular sulfinyl oxygen–silicon coordination. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Quantum Mechanical Study of the Syn and Anti Conformations of Solvated Cyclic GMP

Self-consistent Reaction Field (SCRF) computational methods have been applied to guanosine 3:5-cyclic monophosphate (cGMP) to determine the geometries and energetics of the syn and anti conformations of this cyclic nucleotide in aqueous solution. The syn conformation of cGMP has been predicted to be more stable in the gas phase due to an internal hydrogen bond. The syn conformation is observed in the crystal structure of the sodium tetrahydrate salt, although a bridging water molecule is present in lieu of the internal hydrogen bond. In the gas phase, we find from Hartree–Fock/6-31+G(d) optimizations that the syn conformation is more stable than the anti by about 4 kcal/mol. However, we report here that the anti conformation is more stable in aqueous solution, according to estimates based upon results from both the Onsager model and the Isodensity Polarized Continuum Method (IPCM). Our best estimate from single-point IPCM B3LYP/6-31+G(d) calculations has the anti conformation 19 kcal/mol lower in energy. For comparison purposes, we also present SCRF results for syn and anti adenosine 3:5-cyclic monophosphate (cAMP). For cAMP, we estimate the anti conformation to be more stable than the syn by about 6 kcal/mol. We suggest that the relative stability of the anti conformation of cGMP be considered in studies, such as, enzyme docking.     

Dissociation of diimide

Parts of the potential energy surface of N₂H₂ have been studied using CASSCF- and contracted CI-methods. Of particular interest was the concerted dissociation of cis- and trans-diimide into N₂ and H₂, since the trans-dissociation is symmetry allowed and the cis-dissociation forbidden. Three different saddle points were located, of which only one, of C ₂- symmetry, is a true transition state. Elaborate numerical gradient methods using exact Hessians and update procedures had to be used to find these saddle points on the unexpectedly complex N₂H₂-surface. The barrier height with respect to trans-diimide is 61 kcal/mol after vibration correction. Since this energy is higher than the barrier for interconversion, cis- and trans-diimide have the same transition state. It is further found that diimide preferably dissociates stepwise, by losing one hydrogen at a time, rather than in a concerted way. This conclusion is drawn basically because the geometry of the transition state for the concerted dissociation has a very long H-H distance of 5.6 a.u. The N-H bond energy in trans-diimide is 56 kcal/mol after vibration correction.     

A suggested revised conformation of methylthionitrite as seen by its spectra and preliminary ab initio calculations

A proof has recently been given that gaseous methylthionitrite, CH₃SNO, occurs exclusively (or mainly) in its anti conformation [1]. The present paper claims that existing spectral evidence and ab initio calculations now performed suggest that gaseous methylthionitrite is mainly the syn conformer, the extra stability being of the order 1–2 kcal mol^?1. Methyl group rotation in the syn conformer is hindered by a three-fold barrier of height 689 cm^?1 while the methyl group rotation in the anti conformer is hardly hindered. The syn/anti energy difference and the barriers hindering methyl group rotation closely parallel the corresponding measured quantities in methylnitrite, CH₃ONO.     

Die C8H12-Energiehyperfläche Thermolyse von syn- und anti-Tricyclo[4.2.0.02,5]octan. Experimentelle und theoretische Untersuchungen

The C₈H₁₂-Energy Hypersurface Thermolysis of syn- and anti-Tricyclo[4.2.0.0^2,5]octane. Experimental and Theoretical Studies The thermal behaviour of syn- and anti-tricyclo[4.2.0.0^2,5]octanes 9 and 10 in the gas phase as well as in solution is investigated. Two Mayn products are formed in parallel reactions: cis, cis-1,5-cyclooctadiene ( 11 ) and cis, trans-1,5-cyclooctadiene ( 15 ), the latter being partly isomerized to 11 under the reaction conditions. Minor products are cis-1,2-divinylcyclobutane ( 6 ), trans-1,2-divinylcyclobutane ( 16 ) and 4-vinyl-1-cyclohexene ( 17 ). Thermolysis of cis-1,2-divinylcyclobutane leads to small amounts of cis, trans-cyclooctadiene, presumably via a four-centre transition state. The tricyclics most likely prefer a stepwise isomerization. The decisive product-controlling factor seems to be the conformational mobility of intermediate diradicals. By comparison with the boat-Cope reaction of divinylcyclobutane the pericyclic six-centre transition state of this rearrangements is shown to lie energetically about 19 kcal/mol below the transition states in the thermolysis of 9 and 10 . The azo compound 12 on heating fragments predominantly in a concerted manner in contrast to the photolysis. Theoretical methods are applied to unveil structure and bonding in the supposed intermediate diradicals.     

Sequential Hydrogen Tunneling in o-Tolylmethylene

o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniques were used to identify two conformers of 1 (anti and syn) in noble gas matrices and in frozen organic solutions. Conformer-specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti 1 and syn 1 rapidly interconvert via efficient quantum tunneling forming a rotational pre-equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti 1 to 2 , is rate-limiting for the formation of 2 . We here present an efficient strategy for the study of such tunneling equilibria.     

A Computational Study of Chair–Twist Energy Differences and the Chair–Chair Conformational Interconversion in Tetrahydro-2H-thiopyran (Thiacyclohexane,Thiane)

Ab initio molecular orbital theory with the LANL2DZ, 3-21G, 6-31G(d), 6-31+G(d), 6-31+G(d,p), 6-311+G(d,p),6-31G(2d), 6-31G(3d), and 6-311G(d,p) basis sets and density functional theory (B3P86, B3LYP, B3PW91) have been used to calculate the structures, relative energies, enthalpies, entropies, and free energies of the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran (tetrahydrothiopyran, thiacyclohexane, thiane, pentamethylene sulfide). All levels of theory calculated similar energy values and the effect of basis sets on the calculated energies was small. The chair conformer of tetrahydro-2H-thiopyran was 5.27 kcal/mol more stable than the 1,4-twist conformer, which was slightly more stable (0.81 kcal/mol) than the 2,5-twist conformer. The chair–1,4-twist and chair–2,5-twist free energy differences ( G°_{c – t}) were 5.44 and 5.71 kcal/mol, respectively. Intrinsic reaction coordinate [IRC, minimum-energy path (MEP)] calculations connected the transition state between the chair and the 2,5-twist conformers. This transition state is 9.73 kcal/mol higher in energy than the chair conformer and the energy differences between the chair and the 1,4-boat and 2,5-boat transition states were 8.07 and 6.38 kcal/mol, respectively. Stereoelectronic hyperconjugative interactions were observed in the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran. The stereoelectronic hyperconjugative effects in the chair conformer of tetrahydro-2H-thiopyran have been compared to those in the respective chair conformers of tetrahydro-2H-pyran, tetrahydro-2H-selenane, and tetrahydro-2H-tellurane.     

Cyclic-tri(N-methyl-meta-benzamide)s: substituent effects on the bowl-shaped conformation in the crystal and solution states

Cyclic trimers of 3-(N-alkylamino)benzoic acid (calix[3]amides) with various substituents at the meta position of the phenyl rings were synthesized and the effects of the substituents on the crystal structures and energy profiles in solution were examined. The calixamides existed in a syn conformation in the crystal state, and this was also the major conformation in solution, especially in polar solvents. The energy barrier between syn and anti conformers in the solution was not significantly affected by substituents (12.7-14.0 kcal/mol). The effect of the substituent on the temperature dependence of the syn/anti ratio are discussed.     

Transition structures for the reaction of difluorocarbene with propene

Two $\text{ab}$$\text{initio}$ transition structures for the reaction of difluorocarbene with propene have been located with gradient techniques and the 3-21G basis set. The activation energy is 1.3 kcal/mol lower than for the reaction with ethylene. Two transition structures with the fluorines approaching $\text{syn}$ or $\text{anti}$ to the methyl group are identical in energy     

Theoretical reinvestigation of the ozonolysis mechanism of allyl alcohol

Allyl alcohol (AA) is the simplest unsaturated alcohol. Ozonolysis is one of the key removal processes for AA in the atmosphere. However, a recent theoretical study suggests that the ozonolysis of AA cannot feasibly occur in atmospheric conditions because of the high barrier (~96 kcal/mol) involved in the primary ozonide (POZ) decomposition. In this work, the ozonolysis mechanism of AA was reinvestigated theoretically. The computed barrier for POZ decomposition is only ~20 kcal/mol. Therefore, the AA ozonolysis can take place in the atmosphere, consistent with the experimental conclusions. Moreover, two new Criegee intermediates (syn- and anti-AA-CI) were found to be produced in this reaction. The wave function analyses indicate that there exists an intermolecular hydrogen bond in syn-AA-CI, which significantly affects its unimolecular decomposition and reactions with H₂O. Compared with the normal reactions of anti-CI-AA, the stabilized syn-AA-CI has two distinct isomerization channels: (i) addition of OH group to the reactive sites of CI forming an ethylene oxide (HOOCH₂OCH₂) and (ii) double H-transfer producing HOOCH₂CHO. Meanwhile, the addition of H₂O in syn-AA-CI also exhibits two different pathways. One is the unique addition-coupled double H-transfer, and the other is the addition-coupled single H-transfer, both leading to the formation of CH₂(OH)CH(OH)OOH.     

Mechanism and structural aspects of thermal Curtius rearrangement. Quantum chemical study

The electronic and geometric structures of formyl, acetyl, and benzoyl azides were studied and fragments of the potential surfaces for the thermal Curtius rearrangement of these azides into the corresponding isocyanates were calculated by density functional theory at the PBE/TZ2P level. Acyl azides adopt two stable, conformations syn and anti, with respect to the C-N bond. The syn conformers are more stable than their anti analogs. The activation energies of the syn-anti isomerization in the series under study are 9.4, 7.0, and 9.2 kcal mol⁻¹, respectively, and the activation energies of the reverse reaction are 8.5, 6.1, and 2.5 kcal mol⁻¹. The rearrangement of syn-acyl azides is a one-step process, in which elimination of N₂ occurs synchronously with the rearrangement of atoms and bonds to form isocyanates. The activation energies of the rearrangements of syn-HC(O)N₃, syn-MeC(O)N₃, and syn-PhC(O)N₃ are 28.0, 32.9, and 34.5 kcal mol⁻¹, respectively. The rearrangement of the anti conformers of the above-mentioned azides involves the formation of singlet acylnitrene. The activation energies of the latter process are 34.6, 32.9, and 32.3 kcal mol⁻¹, respectively. The activation energies of the rearrangement of acylnitrenes into isocyanates are 20.9, 18.9, and 13.6 kcal mol⁻¹, respectively. The energy characteristics of the process and the structural data for the starting compounds, final products, and transition states provide evidence that the thermal Curtius rearrangement occurs predominantly by a concerted mechanism. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2200–2209, October, 2005.     

A High‐Barrier Molecular Balance for Studying Face‐to‐Face Arene–Arene Interactions in the Solid State and in Solution

An atropisomeric molecular balance was developed to study face‐to‐face arene–arene interactions. The balance has a large central 1,4,5,8‐naphthalene diimide surface that forms intramolecular arene–arene interactions with two pendent arms. The balance adopts distinct syn and anti isomers with varying numbers of intramolecular interactions. Thus, the strength of the arene–arene interaction could be quantitatively measured by NMR spectroscopy from the anti/syn ratios. The size of the arene arms was easily varied, which allowed examination of the relationship between arene size and strength of the interaction. A nonlinear size dependence was observed in solution with larger arene arms having a disproportionately stronger arene–arene interaction. The intramolecular arene–arene interactions were also characterized in the solid state by X‐ray crystallography. These studies were facilitated by the kinetic stability of the syn and anti isomers at room temperature due to the high isomerization barrier (ΔG=27.0 kcal mol^?1). Thus, the anti isomer could be selectively isolated and crystallized in its folded conformation. The X‐ray structures confirmed that the anti isomers formed two strong intramolecular arene–arene interactions with face‐to‐face geometries. The solid‐state structure analysis also reveals that the rigid framework may contribute to the observed nonlinear size trend. The acetate linker is slightly too long, which selectively destabilizes the balances with smaller arene arms. The larger arene arms are able to compensate for the longer linker and form effective intramolecular arene–arene interactions.     

Diastereoselectivity in theLewis acid mediated aldol reaction of chiral α, β-epoxyaldehydes with a ketene silyl acetal

Summary The Lewis acid mediated aldol reaction of chiral , -cis andtrans epoxyaldehydes1 and2 withtert-butyl ketene silyl acetal proceeds mainly withanti diastereofacial preference. The best results were obtained forcis epoxyaldehyde1 in the presence of catalytic amounts of BiCl₃·1.5 eq. ZnI₂ (anti:syn 13:1), whereas the poorest stereoselectivity was observed when an excess of LiClO₄ was used (anti:syn 1:1). The more stable epoxyaldehyde conformers were determined and the diastereofacial preference was found to be in agreement with a nucleophilic attack on the energetically more favoured conformers.

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Diastereoselektivität derLewis-Säure-katalysierten Aldolreaction zwischen chiralen , -Epoxyaldehyden und einem Ketensilylacetal

Zusammenfassung DieLewis-Säure-katalysierte Aldolreaktion der chiralen , -cis- und -trans-Epoxyaldehyde1 und2 mittert-Butylketensilylacetal verläuft stereoselektivanti. Die besten Ergebnisse wurden für dencis-Epoxyaldehyd1 in Gegenwart katalytischer Mengen BiCl3·1.5 eq. Znl₂ erhalten (anti:syn 13:1). Die geringste Stereoselektivität trat auf, wenn LiClO₄ im Überschuß eingesetzt wurde (anti:syn 1:1). Das beobachtete Verhalten steht mit einem nucleophilen Angriff am energetisch günstigeren Konformeren im Einklang.

     

Die Dienol-Benzol-Umlagerung von Allyldienolen: aromatische [1,2]-, [3,3]- und [3,4]-sigmatropische Umlagerungen

The dienol-benzene rearrangement of syn and anti-4-allyl-4-methylcyclohexa-2,5-dien-1-ol (syn and anti 15) occurs by formation of a benzonium ion intermediate in p-toluene-sulphonic acid in ether below 0° and leads to a mixture of 2-, 3- and 4-allyltoluenes in the ratio 54:10:36. By the introduction of ¹⁴C-, D- and methyl labelled dienols it is shown that only the allyl group migrates and that this rearrangement is an intramolecular, one-step process. The formation of 2-allyltoluene occurs with retention, whereas the 3- and 4-allyltoluenes are formed by inversion of the carbon skeleton of the migrating allyl group. These rearrangements can be therefore classified as suprafacial, aromatic sigmatropic reactions of the order [1,2], [3,3] and [3,4]. The transition state can be postulated as representing a positively charged complex consisting of interacting allyl and tolyl radicals. The interaction of the two parts is controlled by the symmetry of the highest occupied π-orbitals (ψ₃ for toluene and ψ₂ for the allyl group) in agreement with the Woodward-Hoffmann rules. The better “distribution” of the charge in the transition state of these reactions in comparison to the ground state is chiefly responsible for the CoPE-like [3,3] sigmatropic reaction occurring at low temperatures. In general, sigmatropic reactions in charged systems are faster. The rearrangement of syn and anti 2-allyl-2-methylcyclohexa-3,5-dien-1-ol (syn and anti 28) gives results similar to those obtained with the para-allyldienols. The thermal rearrangement of 15 and 28 gives 3-allyltoluene by a [3,3] sigmatropic Cope rearrangement followed by elimination of water.     

Theoretical studies on the conformations of selenamides

Ab initio HF/6-31+G*, MP2/6-31+G*, B3LYP/6-31+G* level calculations have been performed on HSe-NH₂ to estimate the Se-N rotational barriers and N-inversion barriers. Two conformers have been found withsyn andanti arrangement of the NH₂ hydrogens with respect to Se-H bond. The N inversion barriers in selenamide are 1.65, 2.47, 1.93 kcal/mol and the Se-N rotational barriers are 6.58, 6.56 and 6.12 kcal/mol respectively at HF/6-31+G*, MP2/6-31+G* and B3LYP/6-31+G* levels respectively. The n_N →Σ *_Se-H negative hyperconjugation is found to be responsible for the higher rotational barriers.     

Methyl Fluorocarbonyl Disulfide (FC(O)SSMe): A Theoretical Study on the Structural and Conformational Properties of Its Neutral Ground State and Lowest‐Lying Cationic State

In this work, we present a theoretical study on the structural and conformational properties of FC(O)SSMe in its neutral and cationic ground states. The structure of the neutral molecule, as deduced from Hartree? Fock (HF), Density Functional Theory (DFT), and Møller? Plesset (MP2) methods, agrees with the experimentally determined value for the CSSC dihedral angle (C? S bonds gauche with respect to each other) and with the syn preference of the SSCO dihedral angle (C?O bond syn with respect to the S? S bond). The calculated values for these two dihedral angles are 81.9 and 4.2 degrees, respectively. From the energy difference of the anti vs. syn conformer computed at the CCSD(T)/6‐311++G** level of theory, a 3% contribution of a less‐stable conformer at room temperature is proposed. The potential barrier of rotation about the S? S bond is 5.7 kcal/mol (B3PW91/6‐311++G** approximation). The FC(O)SSMe molecule adopts a planar structure after ionization, the anti conformer (CSSC dihedral angle 180°) being the most‐stable form. For the first ionization of the title compound, the adiabatic ionization potential (IP^ad) derived from the three mentioned theoretical methods (using the 6‐311++G** basis sets) is 8.48, 9.06, and 8.99 eV, whereas the vertical ionization potential (IP^ver) is 8.96, 9.79, and 9.62 eV, respectively (experimental value: 9.0 eV). The results are compared with previous experimental studies carried out for the neutral and charged species interpreted on the basis of the Natural Bond Orbital (NBO) analysis. From these calculations, the importance of the anomeric and mesomeric effects becomes evident. The preferred conformation can be quantitatively explained by evaluation of donor/acceptor interaction energies.     

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions,Ab Initio Calculations,and r 0 Structural Parameters of the Propenoyl Halides (F,Cl, Br)

Variable temperature (–100 to –150°C) studies of the infrared spectra (3500–400 cm^–1) of propenoyl bromide, CH₂=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm^–1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm^–1, which is in excellent agreement with the experimental results. The r ₀ adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

The Structure and Conformation of (CH3)3CSNO

The gas‐phase molecular structure of (CH₃)₃CSNO was investigated by using electron diffraction, allowing the first experimental geometrical parameters for an S‐nitrosothiol species to be elucidated. Depending on the orientation of the ?SNO group, two conformers (anti and syn) are identified in the vapor of (CH₃)₃CSNO at room temperature, the syn conformer being less abundant. The conformational landscape is further scrutinized by using vibrational spectroscopy techniques, including gas‐phase and matrix‐isolation IR spectroscopy, resulting in a contribution of ca. 80:20 for the anti:syn abundance ratio, in good agreement with the computed value at the MP2(full)/cc‐pVTZ level of approximation. The UV/Vis and resonance Raman spectra also show the occurrence of the conformational equilibrium in the liquid phase, with a moderate post‐resonance Raman signature associated with the 350 nm electronic absorption.     

Molecular organization of reactants in the kinetics and catalysis of liquid phase reactions: XII. Mechanism of the boron trifluoride-catalyzed Curtius thermal rearrangement of benzoyl azide into phenyl isocyanate

The kinetics of the thermal decomposition and rearrangement of benzoyl azide into phenyl isocyanate was studied in n-heptane in the presence of boron trifluoride etherate as the catalyst. The apparent activation energy of the noncatalytic reaction is 28.0 kcal/mol, and that of the catalytic reaction is 11.0 kcal/mol. The electronic structure and geometry of various complexes between benzoyl azide and BF₃ were studied using the PBE/TZ2P density functional method, and fragments of the potential energy surface were calculated for the catalytic rearrangement. Comparatively stable 1: 1 and 1: 2 complexes between the syn conformer of benzoyl azide and the catalyst can form in the system by coordination to the oxygen and nitrogen atoms of the acyl azide group. The heats of formation of these complexes are between ?1.7 and ?6.4 kcal/mol. The main consequence of the formation of these complexes is that the acyl azide group comes out of the benzene ring plane and thus becomes more reactive. The effective activation energies calculated for the catalytic rearrangement involving complexes of different compositions are 12–15 kcal/mol lower the effective activation energy of the noncatalytic reaction. Information has been obtained about the structure of the transition state of the catalytic reaction, in which a nitrogen molecule is abstracted from benzoyl azide with a synchronous rearrangement of other atoms, resulting in the formation of the ultimate product.