Molecular energetics of cytosine revisited: a joint computational and experimental study

A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T = 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p degrees = 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as -(221.9 +/- 1.7) kJ.mol(-1). This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ.mol(-1). Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer [J. Chem. Soc., Dalton Trans. 1984, 75], (155.0 +/- 3.0) kJ.mol(-1), results in a value for the gas-phase standard molar enthalpy of formation for cytosine of -66.9 kJ.mol(-1). A similar value, -65.1 kJ.mol(-1), has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as -60.7 kJ.mol(-1) and -57.2 kJ.mol(-1) for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C-H, N-H, and O-H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies.     

Structure and dynamics of binary and ternary lanthanide(III) and actinide(III) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA)-tributylphosphate (TBP) complexes. Part 3, the structure, thermodynamics and reaction mechanisms of 8- and 9-coordinated binary and ternary Y-TTA-TBP complexes studied by quantum chemical methods

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)(3)(OH(2))(2) and Y(TTA)(3)(TBP)(OH(2)) and intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(OH(2))(HTTA) and Y(TTA)(3)(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol(-1). 9-coordinated intermediates play an important role in all reactions. Y(TTA)(3)(OH(2))(3) has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol(-1), in good agreement with the experimental activation enthalpy of 19.6 kJ mol(-1). The mechanism for the intramolecular exchange between structure isomers in Y(TTA)(3)(OH(2))(2) involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol(-1), respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(HTTA) and Y(TTA)(3)(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol(-1). The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.     

Experimental and computational thermochemical study of 2-thiobarbituric acid: structure-energy relationship

This paper reports an experimental and computational thermochemical study on 2-thiobarbituric acid (2-thioxodihydropyrimidine-4,6(1H,5H)-dione), [CAS 504-17-6]. The value of the standard (p(0) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by bomb combustion calorimetry, using a rotatory bomb, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(396.8 ± 0.9) kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as (118.3 ± 2.2) kJ·mol(-1). From these results a value of -(278.5 ± 2.4) kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.     

Cooperative effect in hydrogen bonding of N-methylacetamide in carbon tetrachloride solution confirmed by NMR and IR spectroscopies

Recently we carried out ab initio molecular orbital calculations of the hydrogen bond energies in the dimers and trimers of N-methylacetamide (NMA), and found the existence of a cooperative effect in the hydrogen bonding, by which formation of one hydrogen bond in a hydrogen-bonded chain enhances the strength of another hydrogen bond. In order to confirm the existence of such a cooperative effect, we have determined experimentally the enthalpy changes occurring upon hydrogen bonding of NMA in CCl4 solution. First, following the method proposed by us, the population fractions of free (non-bonded) NH protons are obtained from the observed amide proton NMR chemical shifts and the IR intensities of the free NH stretching bands. Next, the enthalpy changes are evaluated by analyzing the equilibrium between the free and bonded states of an NH proton. In this analysis, the existence of the CCl4 solvent is taken into account. The stabilization energy of hydrogen bonds in a trimer, as compared with twice the hydrogen bond energy in a dimer, is 5.4 kJ mol(-1), in good agreement with the calculated value (5.9 kJ mol(-1)). This result provides experimental confirmation of the existence of a cooperative effect in hydrogen bonding.     

Calorimetric and computational study of thiacyclohexane 1-oxide and thiacyclohexane 1,1-dioxide (thiane sulfoxide and thiane sulfone). Enthalpies of formation and the energy of the S=O bond

A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, Delta(f)H(o) m(g) = -394.8 +/- 1.5 kJ x mol(-1). This value stands in stark contrast with the enthalpy of formation reported for thiane itself, Delta(f)H(o) m(g) = -63.5 +/- 1.0 kJ x mol(-1), and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, Delta(f)H(o) m(O,g) = +249.18 kJ x mol(-1), and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ x mol(-1), it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, Delta(f)H(o) m(5) = -174.0 kJ x mol(-1). The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, Delta(f)H(o) m(4) = -395.0 kJ x mol(-1) and Delta(f)H(o) m(5) = -178.0 kJ x mol(-1). Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ x mol(-1), respectively.     

Experimental and computational thermochemical study of barbituric acids: structure-energy relationship in 1,3-dimethylbarbituric acid

This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. The value of the standard (p° = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -639.6 ± 1.9 kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 ± 0.6 kJ·mol(-1). From these results a value of -547.3 ± 2.0 kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.     

Modeling water exchange on an aluminum polyoxocation

For the first time, water exchange on a polymeric complex has been modeled using a combination of gas-phase ab initio calculations and molecular dynamics (MD) simulations. The GaO4Al12(OH)24(H2O)12(7+)aq ion (GaAl12) was chosen because high-quality experimental data exist, including an activation enthalpy (+63 +/- 7 kJ/mol) and an activation volume (+3 +/- 1 cm3/mol). We took a two-step approach. First, the local solvent structure and the initial states for reaction were inferred from the molecular dynamics simulations. Second, we used this information to evaluate initial-state structures in the ab initio calculations. The energy differences between the initial and transition states from the ab initio calculations varied from +59 kJ/mol to +53 kJ/mol depending upon details, closely approximating the activation enthalpy.     

Enthalpy of formation of the cyclopentadienyl radical: photoacoustic calorimetry and ab initio studies

The gas-phase C-H bond dissociation enthalpy (BDE) in 1,3-cyclopentadiene has been determined by time-resolved photoacoustic calorimetry (TR-PAC) as 358 +/- 7 kJ mol(-1). Theoretical results from ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)) are reported. The CCSD(T) prediction for the C-H BDE of 1,3-cyclopentadiene (353.3 kJ mol(-1)) is in good agreement with the TR-PAC result. On the basis of the experimental and the theoretical values obtained, we recommend 355 +/- 8 kJ mol(-1) for the C-H BDE of 1,3-cyclopentadiene and 271 +/- 8 kJ mol(-1) for the enthalpy of formation of cyclopentadienyl radical.     

Structural and spectroscopic study of the Br2...3-Br-pyridine complex by DFT calculations

The structure and the Raman vibrational spectrum of the complex Br(2)...3-Br-pyridine are determined by DFT calculations using different parametrizations. The calculations are performed taking into account the effects of the dichloromethane as solvent by the CPCM method. A value of 39 kJ mol(-1) for the formation enthalpy and of 1 kJ mol(-1) for the formation free energy at room temperature in presence of the solvent is found. The predicted Raman spectrum is compared with the experimental one and the essential features of the spectrum are well reproduced by the B3LYP parametrization. The intensity changes of the bands when going from the free moieties to the complex are also generally correctly predicted by the theoretical treatment.     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Solvation enthalpies of free radicals: O-O bond strength in di-tert-butylperoxide.

The photolysis reaction of di-tert-butylperoxide was studied in various solvents by photoacoustic calorimetry (PAC). This technique allows the determination of the enthalpy of this homolysis reaction, which by definition corresponds to the O-O bond dissociation enthalpy of the peroxide in solution, DHsin(degrees)(O-O). The derived value from these experiments in benzene, 156.7 +/- 9.9 kJ mol(-1), is very similar to a widely accepted value for the gas-phase bond dissociation enthalpy, DH(degrees)(O-O) = 159.0 +/- 2.1 kJ mol(-1). However, when the PAC-based value is used together with auxiliary experimental data and Drago's ECW model to estimate the required solvation terms, it leads to 172.3 +/- 10.2 kJ mol(-1) for the gas-phase bond dissociation enthalpy. This result, significantly higher than the early literature value, is however in excellent agreement with a recent gas-phase determination of 172.5 +/- 6.6 kJ mol(-1). The procedure to derive the gas-phase DH(degrees)(O-O) was tested by repeating the PAC experiments in carbon tetrachloride and acetonitrile. The average of the values thus obtained was DH(degrees)(O-O) = 179.6 +/- 4.5 kJ mol(-1), confirming that the early gas-phase result is a lower limit. More importantly, the present study questions the usual assumption that the solvation terms of homolysis reactions producing free radicals in solution should cancel, and suggests a methodology to estimate solvation enthalpies of free radicals.     

Quantitative measure for the "nakedness" of fluoride ion sources

A quantitative measure for the donor strength or "nakedness" of fluoride ion donors is presented. It is based on the free energy change associated with the transfer of a fluoride ion from the donor to a given acceptor molecule. Born-Haber cycle calculations were used to calculate both the free energy and the enthalpy change for this process. The enthalpy change is given by the sum of the fluoride ion affinity of the acceptor (as defined in strict thermodynamic convention) and the lattice energy difference (DeltaU(POT)) between the fluoride ion donor and the salt formed with the acceptor. Because, for a given acceptor, the fluoride affinity has a constant value, the relative enthalpy (and also the corresponding free energy) changes are governed exclusively by the lattice energy differences. In this study, BF(3), PF(5), AsF(5), and SbF(5) were used as the acceptors, and the following seven fluoride ion donors were evaluated: CsF, N(CH(3))(4)F (TMAF), N-methylurotropinium fluoride (MUF), hexamethylguanidinium fluoride (HMGF), hexamethylpiperidinium fluoride (HMPF), N,N,N-trimethyl-1-adamantylammonium fluoride (TMAAF), and hexakis(dimethylamino)phosphazenium fluoride (HDMAPF). Smooth relationships between the enthalpy changes and the molar volumes of the donor cations were found which asymptotically approach constant values for infinitely large cations. Whereas CsF is a relatively poor F(-) donor [(U(POT)(CsF) - U(POT)(CsSbF(6))) = 213 kJ mol(-)(1)], when compared to N(CH(3))(4)F [(U(POT)(TMAF) - U(POT)(TMASbF(6))) = 69 kJ mol(-)(1)], a 4 times larger cation (phosphazenium salt) and an infinitely large cation are required to decrease DeltaU(POT) to 17 and 0 kJ mol(-)(1), respectively. These results clearly demonstrate that very little is gained by increasing the cation size past a certain level and that secondary factors, such as chemical and physical properties, become overriding considerations.     

Conformational analysis of indole alkaloids corynantheine and dihydrocorynantheine by dynamic 1H NMR spectroscopy and computational methods: steric effects of ethyl vs vinyl group

1H NMR (400 MHz) spectra of the indole alkaloid dihydrocorynantheine recorded at room temperature show the presence of two conformers near coalescence. Low temperature 1H NMR allowed characterization of the conformational equilibrium, which involves rotation of the 3-methoxypropenoate side chain. Line-shape analysis yielded enthalpy of activation DeltaH(double dagger) = 71 +/- 6 kJ/mol, and entropy of activation DeltaS(double dagger) = 33 +/- 6 J/mol.K. The major and minor conformation contains the methyl ether group above and below the plane of the ring, respectively, as determined by low-temperature NOESY spectra, with free energy difference DeltaG degrees = 1.1 kJ/mol at -40 degrees C. In contrast to dihydrocorynantheine, the corresponding rotamers of corynantheine are in the fast exchange limit at room temperature. The activation parameters determined for corynantheine were DeltaH(double dagger) = 60 +/- 6 kJ/mol and DeltaS(double dagger) = 24 +/- 6 J/mol.K, with DeltaG degrees = 1.3 kJ/mol at -45 degrees C. The difference in the exchange rates of the rotamers of corynantheine and dihydrocorynantheine (respectively, 350 s(-1) and 9 s(-1) at 0 degrees C) reflects the difference in the steric bulk of the vinyl and the ethyl group. The conformational equilibria involving the side chain rotation as well as inversion of the bridgehead nitrogen in corynantheine and dihydrocorynantheine was studied by force-field (Amber and MMFF) and ab initio (density-functional theory at the B3LYP/6-31G level) computational methods, the results of which were in good agreement with the 1H NMR data. However, the calculations identified the rotamers as essentially isoenergetic, the experimental energy differences being to small to be reproduced exactly by the theory. Comparison of density-functional and force-field calculations with experimental results identified Amber as giving the most accurate results in the present case.     

Conformational studies in the cyclohexane series. 2. Phenylcyclohexane and 1-methyl-1-phenylcyclohexane

The structures and relative energies of the conformers of phenylcyclohexane, and 1-methyl-1-phenylcyclohexane have been calculated at theoretical levels including HF/6-31G, B3LYP/6-311G, MP2/6-311G, MP2/6-311(2df,p), QCISD/6-311G, and QCISD/6-311G(2df,p). The latter gives conformational enthalpy (DeltaH degrees ), entropy (DeltaS degrees ), and free energy (DeltaG degrees ) values for phenylcyclohexane that are in excellent agreement with the experimental data. The calculations for 1-methyl-1-phenylcyclohexane find a free energy difference of 1.0 kcal/mol at -100 degrees C, favoring the conformation having an axial phenyl group, that is in only modest agreement with the experimental value of 0.32 +/- 0.04 kcal/mol. The origin of the phenyl rotational profiles for the conformers of phenylcyclohexane and 1-methyl-1-phenylcyclohexane is discussed.     

Calorimetric and computational study of 1,3,5-trithiane

To understand the differences in conformational behavior and reactivity of oxygen- and sulfur-containing 1,3,5-heterocyclohexanes, the enthalpies of formation and sublimation of 1,3,5-trithiane, 1, have been measured. The numerical value of the enthalpy of formation for this compound in the solid state is -8.6 +/- 2.6 kJ mol(-1), while the corresponding value in the gaseous state is 84.6 +/- 2.6 kJ mol(-1). The value for the enthalpy of sublimation is 93.2 +/- 0.2 kJ mol(-1). Standard ab initio molecular orbital calculations at the G2(MP2), G2, and G3 levels were performed, and the calculated enthalpies of formation are compared with the experimental data. These experimental and theoretical studies support the relevance of through-space lone pair-lone pair electronic repulsion in the sulfur heterocycle.     

Experimental and computational study on the molecular energetics of indoline and indole

Static bomb calorimetry, Calvet microcalorimetry and the Knudsen effusion technique were used to determine the standard molar enthalpy of formation in the gas phase, at T = 298.15 K, of the indole and indoline heterocyclic compounds. The values obtained were 164.3 +/- 1.3 kJ x mol(-1) and 120.0 +/- 2.9 kJ x mol(-1), respectively. Several different computational approaches and different working reactions were used to estimate the gas-phase enthalpies of formation for indole and indoline. The computational approaches support the experimental results reported. The calculations were further extended to the determination of other properties such as bond dissociation enthalpies, gas-phase acidities, proton and electron affinities and ionization energies. The agreement between theoretical and experimental data for indole is very good supporting the data calculated for indoline.     

Thermochemistry of acetonyl and related radicals

Density functional and ab initio calculations at CBS-QB3 levels of theory were employed with a series of isodesmic reactions to determine the thermochemistry of the 2-oxopropyl or acetonyl radical (CH(3)COC*H2). In turn, this was used to determine formation enthalpies of 2-oxoethyl or formylmethyl (C*H(2)CHO), 2-oxobutyl (C*H(2)COC(2)H(5)), 1-methyl-2-oxopropyl or methylacetonyl (C*H(CH(3))COCH(3)), 1-methyl-2-oxobutyl (C*H(CH(3))COC(2)H(5)), and 3-oxopentyl (C*H(2)CH2COC(2)H(5)). Our computed standard enthalpy of formation of -34.9 +/- 1.9 kJ mol-1 and a resonance stabilization energy of approximately 22 kJ mol(-1) for acetonyl are in good agreement with recent re-determinations, which have indicated a substantial lowering in the long-established value for DeltaH(f)o (298.15 K). A bond dissociation energy of 401 kJ mol(-1) is suggested for the C-H bond in acetone with consistent values for the others. The calculations support the enthalpy of formation of acetaldehyde obtained from combustion experiments of -166.1 kJ mol(-1) rather than the figure of -170.7 kJ mol(-1) extracted from enthalpies of reduction and, in addition, serve to reduce the uncertainty in DeltaH(f)o the 2-oxoethyl radical to +13 +/- 2 kJ mol(-1).     

Chair, boat and twist conformation of dodecamethylcyclohexasilane and undecamethylcyclohexasilane: a combined DFT and Raman spectroscopic study.

The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.     

Ab initio study on the mechanism of reaction HNCO+NH_2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH2+ HNCO→NH3+NCO (1) and NH2+HNCO-N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300-2700 K, transition theory rate constant for reaction (1) is 1.68 × 1011- 3.29 × 1011 mL · mol-1· s-1, which is close to the experimental one of 5.0 ×1011 mL× mol-1· s-1 or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes, cis and trans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (for cis-mode) and 147.43 kJ/mol (for trans-mode), respectively, which is much higher than     

Insight into the Detailed Mechanism of HRh（CO）（PPh3）3 Catalyzed Hydroformylation of 1-Phenyl-1-（3-pyridyl）ethene：A DFT Study

Density functional calculations have been used to study the mechanism of 1-phenyl-1-(3-pyridyl)ethene hydroformylation using rhodium catalyst.Our calculations reveal that the rate-determining step is the oxidative addition of hydrogen molecule and the preferred path is the one involving ts3ans for the lowest activation free-energy (ΔrGa),63.8 kJ/mol.This reaction is demonstrated to be strong exothermic by-96.6 kJ/mol of branched products and-98.2 kJ/mol of linear products.And the predominant product is the linear 3-phenyl-3-(3-pyridal)propanal (pr-ns) determined by both thermodynamics and kinetics.These results are in agreement with the practicality experimental studies.