Second-row anomeric interactions: The involvement of phosphorus

Proton NMR spectroscopy and x-ray crystallographic studies demonstrate the predominance of the axial conformer of 2-(diphenylphosphinoyl)-1,3-dithiane ( 1 ). Chemical equilibration of anancomeric models ( 3 ⇌ 4 ) allows quantitative determination of the conformational free energy in 1 , 1.0 kcal/mol, which corresponds to an anomeric effect of 2.64–3.74 kcal/mol, depending on the method used for estimating it. This value indicates that the anomeric effect operative in 1 may be among the largest yet measured. Five distinct rationalizations are considered to account for such a strong S-C-P(O) anomeric effect: (a) dipole-dipole interactions, (b) electrostatic, attractive interaction between the phosphoryl oxygen and the syn-axial hydrogens in 1 -axial, (c) delocalization of the lone pair on the endocyclic heteroatom into the antiperiplanar (axial) adjacent polar C-P bond, (d) through-space 3p-3d electron donation from sulfur to phosphorus, and (e) repulsive interaction between the lone pairs on sulfur and on the phosphoryl oxygen. All these hypotheses have some experimental and/or the-oretical support, but evidence contrary to expectation can be found also to argue against each one of them. The conformational preference of the diphenylthiophosphinoyl group in the 1,3-dithian-2-yl ring was determined by nuclear magnetic resonance (NMR) analysis and by chemical equilibration of diastereomeric models. The slight predominance of axial 2-(diphenylthiophosphinoyl)-1,3-dithiane ( 13 -ax) over 13 -eq reflects nonetheless the influence of a strong S-C-P anomeric interaction, worth ca. 2.3 kcal/mol. Spectroscopic evidence for the predominance of the equatorial conformers in 2-(diphenylphosphinoyl)-1,3-oxathiane ( 19 ) and 2-(diphenylphosphinoyl)-1,3-dioxane ( 22 ) was confirmed by the study of derivatives containing counterpoise substitutents, or by chemical equilibration on anancomeric models. ΔD°_27°C [P(O)Ph₂] = -3.23 kcal/mol was determined for the dioxane (chloroform solution), and ΔG°C_55°C [P(O)Ph₂] = -1.42 kcal/mol for the oxathiane (ethanolic solution). Nevertheless, evaluation of the different steric requirements in 1, 13 and 22 reveals that the magnitude of the O-C-P(O) and S-C-P(O) anomeric effects in the heterocycles is, in fact, quite similar, close to kcal/mol.     

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     

Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MI_ax) and equatorial (2MI_eq) conformers of 2MI have been observed. A 2MI_eq/2MI_ax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, \Updelta E = E _{2 \textMI\textax} - E _{2 \textMI\texteq} \Updelta E = E_{{ 2 {\text{MI}}_{\text{ax}} }} - E_{{ 2 {\text{MI}}_{\text{eq}} }} , of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: two equatorial conformers (2PI_eq0 and 2PI_eq90), and one axial conformer (2PI_ax). Only the axial conformer of 2PI (2PI_ax) was experimentally observed. The indan ring of 2PI_ax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular C_sp2–H/π interactions in 2PI_ax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial–equatorial interconversion. The barrier (2.33 kcal/mol) to axial–equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C–H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the C_sp2–H bond than that of C_sp3–H bond.     

Reversal of stability on metalation of pentagonal-bipyramidal (1-MB6H7(2-) 1-M-2-CB5H7(1-) and 1-M-2,4-C2B4H7) and Icosahedral (1-MB11H12(2-) 1-M-2-CB10H12(1-) and 1-M-2,4-C2B9H12) boranes (M = Al, Ga, In, and Tl): energetics of condensation and relationship to binuclear metallocenes

The usual assumption of the extra stability of icosahedral boranes (2) over pentagonal-bipyramidal boranes (1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of 1-M-2,4-C(2)B(4)H(7) varies from 14.44 kcal/mol (M = Al) to 15.30 kcal/mol (M = Tl). Similarly, M(2,4-C(2)B(4)H(6))(2)(1-) is more stable than M(2,4-C(2)B(9)H(11))(2)(1-) by 9.26 kcal/mol (M = Al) and by 6.75 kcal/mol (M = Tl). The preference for (MC(2)B(4)H(6))(2) over (MC(2)B(9)H(11))(2) at the same level is 30.54 kcal/mol (M = Al), 33.16 kcal/ mol (M = Ga) and 37.77 kcal/mol (M = In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and H(2)M-MH(2) (M= Al, Ga, and In).     

Structure and reactivity of aquacarbonylruthenium(II) complexes. An X-ray and oxygen-17 NMR study

The water exchange on [Ru(CO)(H2O-eq)4(H2O-ax)](tos)2 (1), [Ru(CO)2(H2O-eq)2(H2O-ax)2](tos)2 (2), and [Ru(CO)3(H2O)3](ClO4)2 (3), the 17O exchange between the bulk water and the carbonyl oxygens have been studied by 17O NMR spectroscopy, and the X-ray crystallographic structures of 1 and 2 have been determined. The water exchange of equatorially and axially coordinated water molecules on 1 and 2 follow an Id mechanism and are characterized by keq298 (s-1), delta H++ (kJ/mol), and delta S++ (J/(mol K)) of (2.54 +/- 0.05) x 10(-6), 111.6 +/- 0.4, and 22.4 +/- 1 (1-eq); (3.54 +/- 0.02) x 10(-2) and 81 (1-ax); (1.58 +/- 0.14) x 10(-7), 120.3 +/- 2, and 28.4 +/- 4 (2-eq); and (4.53 +/- 0.08) x 10(-4), 97.9 +/- 1, and 19.3 +/- 3 (2-ax). The observed reactivities correlate with the strength of the Ru-OH2 bonds, as expressed by their length obtained by X-ray studies: 2.079 (1-eq), 2.140 (1-ax), 2.073 (2-eq), and 2.110 (2-ax) A. 3 is strongly acidic witha pKa of -0.14 at 262 K. Therefore, the acid-dependent water exchange can take place through 3 or Ru(CO)3(H2O)3OH+ with an estimated keq298 of 10(-4)/10(-3) s-1 and kOH262 of 0.053 +/- 0.006 s-1. The 17O exchange rate between the bulk water and the carbonyl oxygens increases from 1 to 2 to 3. For 1 an upper limit of 10(-8) s-1 was estimated. For 2, no acid dependence of kRuCO between 0.1 and 1 m Htos was observed. At 312.6 K, in 0.1 and 1 m Htos, kRuCO = (1.18 +/- 0.03) x 10(-4). For the tricarbonyl complex, the exchange can proceed through 3 or Ru(CO)3(H2O)2OH+ with kRuCO and kRuOHCO of, respectively, 0.003 +/- 0.002 and 0.024 +/- 0.003 s-1, with a ruthenacarboxylic acid intermediate.     

Antibody-catalyzed oxy-cope rearrangement: mechanism and origins of catalysis and stereoselectivity from DFT quantum mechanics and flexible docking

Density functional theory using B3LYP and flexible ligand docking methods were used to investigate the complete potential surface for the uncatalyzed and the AZ28 antibody-catalyzed oxy-Cope reaction of 2,5-diaryl-1,5-hexadien-3-ol derivatives. The reaction mechanism is stepwise, involving a cyclohexane diyl intermediate. Theoretical deuterium isotope effects match well with those from experiment. Gas-phase transition structures show mixed preferences for the axial vs equatorial hydroxyl group, while solvation favors the axial isomer. Specific phenyl group conformations are shown to be critical to transition-state stabilization (by up to 15 kcal/mol), and the effective conformation is not that found in the hapten used to generate the germline and affinity-matured AZ28 catalytic antibodies. Docking studies support greater transition-state binding than reactant binding. Docking studies also predict the S stereoselectivity of mature AZ28 and suggest that binding modes quite different from those of the hapten might play a role in catalysis, with specific focus on ligand hydrogen bonding to Asp(H101).     

Conformations and electronic structures of axially coordinated fullerene-porphyrin-fullerene triad (C60CHCOO)2-Sn(IV) porphyrin

The conformational (cis and trans) stability and electronic structures of (C(60)CHCOO)(2)-Sn(IV) porphyrin, recently synthesized as a novel fullerene-porphyrin-fullerene triad linked by metal axial coordination, have been studied by ab initio calculations. The cis conformer was found to be slightly more stable than the trans by 1.38 kcal/mol in the neutral compound. Upon the addition of an electron to the triad, the relative stability of the cis conformer was found to be higher (3.29 kcal/mol) than that in the neutral one. From the investigation of frontier molecular orbitals, for the cis conformer, it was found that the electrons are localized in HOMO of the porphyrin, while the electrons are localized in LUMO of the syn-fullerene. For the trans conformer, it was found that the electrons are localized in HOMO of the porphyrin, while the electrons are localized in LUMO of one of the two fullerene moieties, and the electrons are localized in LUMO2 of the other fullerene moiety, but the LUMO and LUMO2 have the same orbital energy. Thus, the PET may take place unidirectionally in the cis conformer from the porphyrin to the syn-fullerene, while it is bidirectional from the porphyrin to both of the fullerene moieties.     

Molecular structure and conformational preferences of 1-bromo-1-silacyclohexane, CH2(CH2CH2)2SiH-Br, as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH₂(CH₂CH₂)₂SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and C _s symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (G _ax ^?? ? G _eq ^?? ) = ?0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.     

Conformational analysis of cis-2-halocyclohexanols; solvent effects by NMR and theoretical calculations

Conformational problems often involve very small energy differences, even low as 0.5 kcal mol(-1). This accuracy can be achieved by theoretical methods in the gas phase with the appropriate accounting of electron correlation. The solution behavior, on the other hand, comprises a much greater challenge. In this study, we conduct and analysis for cis-2-fluoro-, cis-2-chloro-, and cis-2-bromocyclohexanol using low temperature NMR experiments and theoretical calculations (DFT, perturbation theory, and classical molecular dynamics simulations). In the experimental part, the conformers' populations were measured at 193 K in CD(2)Cl(2), acetone-d(6), and methanol-d(4) solutions; the preferred conformer has the hydroxyl group in the equatorial and the halogen in the axial position (ea), and its population stays at about 60-70%, no matter the solvent or the halogen. Theoretical calculations, on the other hand, put the ae conformer at a lower energy in the gas phase (MP2/6-311++G(3df,2p)). Moreover, the theoretical calculations predict a markedly increase in the conformational energy on going from fluorine to bromine, which is not observed experimentally. The solvation models IEF-PCM and C-PCM were tested with two different approaches for defining the atomic radii used to build the molecular cavity, from which it was found that only with explicit consideration of hydrogens can the conformational preference be properly described. Molecular dynamic simulations in combination with ab initio calculations showed that the ea conformer is slightly favored by hydrogen bonding.     

Why Doesn't all-trans-1,2,3,4,5,6-Hexaspiro(THF)cyclohexane complex metal ions?

Despite the inherent preference for placing alkyl substituents, rather than alkoxy substituents, in equatorial positions, all-trans-hexaspiro(THF)cyclohexane strongly favors the all-O-equatorial conformer. Ab initio and density functional calculations on a series of cyclohexane derivatives containing one, two, or three spiro(THF) units demonstrate that this preference results from at least two important factors. First, when oxygen atoms are attached to adjacent carbons, the gauche effect favors the di-O-equatorial arrangement. In trans-1,2-dispiro(THF)cyclohexane, the single gauche interaction overcomes the inherent steric preference for projecting the two oxygen atoms axially. Similarly, in the all-trans-hexaspiro(THF)cyclohexane the six gauche interactions in the all-O-equatorial conformer overpower the inherent conformational biases of the six isolated spiro(THF) moieties. Nevertheless, the gauche effect only partially accounts for the more than 20 kcal/mol conformational bias calculated for this molecule. There is also another factor, the high energetic cost associated with projecting multiple alkoxy substituents axially on the same face of a cyclohexane scaffold. The calculations find the energetic cost of each 1,3-diaxial interaction is about 2 kcal/mol larger between alkoxy substituents than between alkyl substituents.     

Structure and bonding in Hartwig stretched eta(3)-hydridoborate sigma-complex of niobium(III)

Ab initio calculations at the SCF, MP2, CASSCF, and CASPT2 levels of theory with basis sets using atomic pseudopotentials have been carried out for the stretched eta(3)-hydridoborate sigma-complex of niobium, [Cl2Nb(H2B(OH)2)], in order to investigate the nature and energetics of the interaction between the transition metal and the eta(3)-hydridoborate ligand. The geometry of the complex [Cl2Nb(H2B(OH)2] and its fragments [Cl2Nb](+) and [H2B(OH)2](-) were optimized at SCF and CASSCF levels. These results are consistent with [Cl2Nb(eta(3)-H2B(OH)2)] being a Nb(III) complex in which both hydrogen and boron of the [eta(3)-H2B(OH)2](-) ligand have a bonding interaction with the niobium preserving stretching B-H bond character. The calculated values of DEF (energy required to restore the fragment from the equilibrium structure to the structure it takes in the complex) for [Cl2Nb](+) are 5.35 kcal/mol at SCF, 3.27 kcal/mol at MP2, 4.80 kcal/mol at CASSCF, and 2.82 kcal/mol at CASPT2 and for [H2B(OH)2](-) 21.13 kcal/mol at SCF, 23.85 kcal/mol at MP2, 20.69 kcal/mol at CASSCF, and 23.48 kcal/mol at CASPT2. Values of INT (stabilization energy resulting from the coordination of distorted ligand to the metal fragment) for the complex [Cl2Nb(H2B(OH)2)] are -239.35 kcal/mol at SCF, -260.00 kcal/mol at MP2, -230.76 kcal/mol at CASSCF, and -252.60 kcal/mol at CASPT2. For the complex [(eta(5)-C5H5)2Nb(H2B(OH)2)], calculations at the SCF and MP2 levels were carried out. Values of INT for [(eta(5)-C5H5)2Nb(H2B(OH)2)] are -169.93 kcal/mol at SCF and -210.62 kcal/mol at MP2. The results indicate that the bonding of the [eta(3)-H2B(OH)2](-) ligand with niobium is substantially stable. The electronic structures of [Cl2Nb(H2B(OH)2)], [(eta(5)-C5H5)2Nb(H2B(OH)2)], and its fragments are analyzed in detail as measured by Mulliken charge distributions and orbital populations.     

Synthesis,polarography and herbicidal activity of quaternary salts of 2-(4-pyridyl)-1,3,5-triazines, 5-(4-pyridyl)pyrimidine, 2-(4-pyridyl)pyrimidine and related compounds

2-(4-Pyridyl)-1,3,5-triazine, 2-(4-pyridyl)-4-methyl-1,3,5-triazine, 2-(4-pyridyl)-4,6-dimethyl-1,3,5-triazine and 2-(4-pyridyl)pyrimidine have been prepared by modification of established triazine and pyrimidine syntheses. These compounds and some of their relatives have been converted to quaternary pyridinium salts. The polarographic reduction potentials of the salts in aqueous solution are pH dependent. The activity of the salts as post-emergent herbicides is reported.     

Conformational equilibria of trans -3-X-cyclohexanols (X = Cl, Br, CH3 and OCH3). A low temperature NMR study and theoretical calculations

The integration of 1H and 13C NMR spectra, at - 90 degrees C in CS2/CD2Cl2 (9:1), for the trans-3-chlorocyclohexanol (1), trans-3-bromocyclohexanol (2), and trans-3-methoxycyclohexanol (4) showed that the equatorial-axial (ea) conformer occurs as ca 63, 63, and 69% in the conformational equilibrium, respectively. This corresponds to the following DeltaG(ea-ae) values (from (1)H spectrum): - 0.32 +/- 0.01, - 0.32 +/- 0.04, - 0.48 +/- 0.05 kcal mol(-1); and to (from 13C spectrum): - 0.31 +/- 0.04, - 0.35 +/- 0.05, and - 0.44 +/- 0.01 kcal mol(-1), respectively, in very good agreement within both series. Thus, although bromine is bulkier than chlorine, the 1,3-diaxial steric effects are similar in these equilibria. However, the integration of (1)H NMR spectrum for the trans-3-methylcyclohexanol (3) gave 90% of the 3ae conformer in the equilibrium, at - 90 degrees C on CS2/CD2Cl2 (9:1), corresponding to a DeltaG(ea-ae) value of 1.31 +/- 0.02 kcal mol(-1). The values obtained through the additivity rule, with data from monosubstituted cyclohexanes (DeltaG(Ad) = DeltaG(X) + DeltaG(OH)), for compounds 1, 2, and 4 (-0.37 +/- 0.15, - 0.34 +/- 0.09, and - 0.46 +/- 0.04 kcal mol(-1), respectively) are in very good agreement with the experimental values, but it is significantly smaller for compound 3 (0.79 +/- 0.02 kcal mol(-1)). Theoretical calculations through different levels of theory (HF/6-311 + g**, B3LYP/6-311 + g**, MP2/6-31 + g**, and CBS-4M) showed that CBS-4M is the best method for the study of conformational equilibria for these systems, since it provides DeltaG(ea-ae) values similar to the experimental values.     

Electronic and infrared spectroscopy of jet-cooled (+/-)-cis-1-amino-indan-2-ol hydrates

The role of conformational isomerism in molecular interaction has been studied using the example of jet-cooled complexes of (+/-)-cis-1-amino-indan-2-ol with water. The two formerly evidenced conformers of (+/-)-cis-1-amino-indan-2-ol easily form hydrates and dihydrates, which have been studied by means of laser-induced fluorescence and IR/UV double resonance spectroscopy, as well as ab initio calculations. All the 1 : 1 and 1 : 2 complexes with water evidenced in this work involve "ring" structures, in which the water monomer or dimer acts as an acceptor from the NH(2) and a donor to the OH groups of (+/-)-cis-1-amino-indan-2-ol. However, the water lies externally to the indan frame in the hydrates of conformer I of (+/-)-cis-1-amino-indan-2-ol, which possesses axial NH(2) and equatorial OH groups, and above it for the hydrates with the less stable conformer II, with equatorial NH(2) and axial OH groups. Consequently, the different steric constraints which exist in the two conformers result in different hydrogen bond topologies, with an additional OH[dot dot dot]pi interaction for the hydrates of conformer II.     

Quantum chemical study of adsorption and dissociation of H2S on the gallium-rich GaAs (001)-4 x 2 surface

H(2)S adsorption and dissociation on the gallium-rich GaAs(001)-4 x 2 surface is investigated using hybrid density functional theory. Starting from chemisorbed H(2)S on the GaAs(001)-4 x 2 surface, two possible reaction routes have been proposed. We find that H(2)S adsorbs molecularly onto GaAs(001)-4 x 2 via the formation of a dative bond, and this process is exothermic with adsorption energy of 6.6 kcal/mol. For the first reaction route, one of the H atoms from the chemisorbed H(2)S is transferred to a second-layer As atom and the dissociated SH is inserted into the Ga-As bond with an activation barrier of 8.2 kcal/mol, which is found to be 29.3 kcal/mol more stable than the reactants. For the second case, the dissociated species may insert themselves into the Ga-Ga dimer resulting in the Ga-H-Ga and Ga-HS-Ga bridge-bonded states, which are found to be 29.8 and 22.2 kcal/mol more stable than the reactants, respectively. However, the calculations also show that the activation barrier (16.1 kcal/mol) for chemisorbed H(2)S dissociation through the second route is higher than the transfer of one H atom into a second-layer As atom. As a result, we conclude that sulfur insertion into the Ga-As bond is more kinetically favorable.     

Computational study of conformations and of sulfinyl oxygen-induced pentacoordination at silicon in 4-chloro-4-silathiacyclohexane 1-oxide and 4,4-dichloro-4-silathiacyclohexane 1-oxide

Second-order Møller-Plesset theory (MP2) and density functional theory (B3LYP) with the 6-311G(d,p) and 6-311+G(d,p) basis sets have been used to calculate the equilibrium geometries and relative energies of the chair, twist, and boat conformations of 4-chloro-4-silathiacyclohexane 1-oxide and 4,4-dichloro-4-silathiacyclohexane 1-oxide. The chair conformers of the axial sulfoxides are lower in energy than the chair conformers of the corresponding equatorial sulfoxides. MP2/6-311+G(d,p) predicted the chair conformer of axial trans-4-chloro-4-silathiacyclohexane 1-oxide (4a) to be 6.12, 0.44, and 0.45 kcal/mol, respectively, more stable than the corresponding 1,4-twist (4b), 2,5-twist (4c) and 1,4-boat (4d) conformers and 6.93 kcal/mol more stable than the 2,5-boat transition state ([4e]^‡). Structures 4c and 4d are stabilized by intramolecular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The 1,4-boat conformer (7d) of axial 4,4-dichloro-4-silathiacyclohexane 1-oxide is also stabilized by transannular coordination of the sulfinyl oxygen with silicon. The energy difference (E_rel = 4.23 kcal/mol) between the chair conformer (7a) and 7d is larger than that between 4a and 4d. The relatively lower stability of the 1,4-boat conformer (7d) of axial 4,4-dichloro-4-silathiacyclohexane 1-oxide (7a) may be due to repulsive interactions of the axial halogen and sulfinyl oxygen atoms. The relative energies and structures of the conformers and transition states of cis- and trans-4-chloro-4-silathiacyclohexane 1-oxide and 4,4-dichloro-4-silathiacyclohexane 1-oxide are discussed in terms of hyperconjugative interactions, orbital interactions, nonbonded interactions, and intramolecular sulfinyl oxygen-silicon coordination.     

Synthesis and equilibrium of stereoisomers of 5-phenyl-5-thio(seleno)-2-phenyl(ethoxy)-1,3, 5-dioxaphosphorinanes

Conclusions Individual stereoisomers of 2,5-diphenyl-5-thio-1,3,5-dioxaphosphorinane, 5-phenyl-5-thio-2-ethoxy-1,3,5-dioxaphosphorinane, and 5-phenyl-5-seleno-2-ethoxy-1,3,5-dioxaphosphorinane, having the chair conformation with the axial orientation of the phenyl group on the phosphorus atom and the equatorial and axial orientation of the substituent in the 2-position, have been isolated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 640–643, March, 1986.     

Computational study on kinetics and mechanisms of unimolecular decomposition of succinic acid and its anhydride

The mechanisms and kinetics of unimolecular decomposition of succinic acid and its anhydride have been studied at the G2M(CC2) and microcanonical RRKM levels of theory. It was shown that the ZsgsZ conformer of succinic acid, with the Z-acid form and the gauche conformation around the central C-C bond, is its most stable conformer, whereas the lowest energy conformer with the E-acid form, ECGsZ, is only 3.1 kcal/mol higher in energy than the ZsgsZ. Three primary decomposition channels of succinic acid producing H2O + succinic anhydride with a barrier of 51.0 kcal/mol, H2O + OCC2H3COOH with a barrier of 75.7 kcal/mol and CO2 + C2H5COOH with a barrier of 71.9 kcal/mol were predicted. The dehydration process starting from the ECGCZ-conformer is found to be dominant, whereas the decarboxylation reaction starting from the ZsgsZ-conformer is only slightly less favorable. It was shown that the decomposition of succinic anhydride occurs via a concerted fragmentation mechanism (with a 69.6 kcal/mol barrier), leading to formation of CO + CO2 + C2H4 products. On the basis of the calculated potential energy surfaces of these reactions, the rate constants for unimolecular decomposition of succinic acid and its anhydride were predicted. In addition, the predicted rate constants for the unimolecular decomposition of C2H5COOH by decarboxylation (giving C2H6 + CO2) and dehydration (giving H3CCHCO + H2O) are in good agreement with available experimental data.     

A carbon-13 cp/mas NMR investigation of the conformation of substituted cyclohexanes in thiourea inclusion compounds

Inclusion compounds of several monosubstituted cyclohexanes and thiourea have been studied using high-resolution ¹³C CP/MAS NMR spectroscopy. The ¹³C NMR chemical shifts of the substituted cyclohexane ring are sensitive to the conformation of the substituent and allow one to predict qualitatively the relative populations of the axial and equatorial conformers. For methylcyclohexane trapped in thiourea the methyl substituent prefers the equatorial conformation while for the cyclohexyl halides (Cl, Br and I) the axial conformer is preferred. In the case of fluorocyclohexane the equatorial conformer appears to predominate; however, this conformer is in rapid equilibrium with the axial conformer.     

Off-center oxygen-arene interactions in solution: a quantitative study

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.