The heats of formation for 19 molecules have been calculated with PM3 and AM1 semiempirical methods. The values obtained have been compared with experimental heats of formation. With PM3 and AM1 the average differences between calculated and experimental heats of formation are 8.45 and 12.34 kcal mol?1 respectively. There are significant differences when large molecules are considered: this suggests that the parameterization should be done including larger molecules. 相似文献
The geometric structures of the conformers of 2,3,4-trimethyl-7-methylidene-1,5-di-(thiophen-2-yl)-6,8-dioxabicyclo[3.2.1]octane were studied by the MP2/6-311++G**// B3LYP/6-31G* method. The average deviations of the calculated bond lengths from those determined by X-ray diffraction are no larger than 0.02 Å, and the deviations of the bond angles and dihedral angles are 0.7 and 3.5°, respectively. In the gas phase, the chair conformation is thermodynamically ~7.3 kcal mol?1 more favorable than the boat conformation, and, taking into account the solvent effect of DMSO, the chair conformation is ~9.4 kcal mol?1 more favorable. For formaldehyde and a series of ketones, including alkyl hetaryl ketones, the enthalpies of competitive formation of acetylenic alcohols and 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes were calculated. The formation of the latter compounds is characterized by a considerable decrease in the enthalpy (to ?91 kcal mol?1). 相似文献
By using a set of model reactions, we estimated the heat of formation of gaseous UO22+ from quantum‐chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO22+, UO2, UF6, and UF5. We used two gradient‐corrected exchange‐correlation functionals (revised Perdew–Burke–Ernzerhof (PBEN) and Becke–Perdew (BP)) and we accounted for spin‐orbit interaction in a self‐consistent fashion. Indeed, spin‐orbit interaction notably affects the energies of the model reactions, especially if compounds of UIV are involved. Our resulting theoretical estimates for Δf (UO22+), 365±10 kcal mol?1 (PBEN) and 370±12 kcal mol?1 (BP), are in quantitative agreement with a recent experimental result, 364±15 kcal mol?1. Agreement between the results of the two different exchange‐correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well‐established data for other compounds of the element in question. 相似文献
The heat of reaction for SnJ2 (c)+J2 (c)+4045 CS2 (l)=[SnJ4; 4045 CS2] (sol) has been determined to be (?41.12±0.55) kJ mol?1, [(?9.83±0.13) kcal mol?1] by isoperibol solution calorimetry. Combining this result with the heat of formation of SnJ4 in CS2 determined in a previous investigation11 the value (?153.9±1.40) kJ mol?1, [(?36.9±0.33) kcal mol?1] has been derived for the heat of formation, ΔHfι (SnJ2;c; 298.15 K), of tin diiodide. 相似文献
The ionization and [C4H7]+ appearance energies for a series of C4H7CI and C4H7Br isomers have been measured by dissociative photoionization mass spectrometry. Cationic heats of formation, based on the stationary electron convention, are derived. No threshold ion is observed with a heat of formation corresponding to the trans-1-methylallyl cation, although there is evidence for formation of the less stable cis isomer. A 298 K heat of formation of 871 kJ mol?1 is obtained for the cyclopropylcarbinyl cation, with the cyclobutyl cation having a higher value of 886 kJ mol?1. At the HF/6-31G** level, ab initio molecular orbital calculations show the 2-butenyl, isobutenyl and homoallyl cations to be stable forms of [C4H7]+, being less stable than the trans-1-methylallyl cation by 101 kJ mol?1, 159 kJ mol?1 and 164 kJ mol?1, respectively. However, threshold formation is not observed for any of these ions, the fragmentation of appropriate precursor molecules producing [C4H7]+ ions with lower energy structures. 相似文献
The kinetics of the reaction between CH3 and HCl was studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (296–495 K) and were fitted to an Arrhenius expression: k1 = 5.0(±0.7) × 10?13 exp{?1.4(±0.3) kcal mol?1/RT} cm3 molecule?1 s?1. This information was combined with known kinetic parameters of the reverse reaction to obtain Second Law determinations of the methyl radical heat of formation {34.7(±0.6) kcal mol?1} and entropy {46(±2) cal mol?1 K?1} at 298 K. Using the known entropy of CH3, a more accurate Third Law determination of the CH3 heat of formation at this temperature was also obtained {34.8(±0.3) kcal mol?1}. The values of k1 obtained in this study are between those reported in prior investigations. The results were also used to test the accuracy of the thermochemical information which can be obtained from kinetic studies of R + HX (X = Cl, Br, I) reactions of the type described here. 相似文献
The specific heat, the melting heat and entropy, the vaporization heat of naphtalene disulfide (C10H6S2) and of diphenylene disulfide (C12H8S2) have been determined by differential scanning calorimetry (DSC).Over the temperature range examined the specific heat may be represented as follows: where T is the temperature in degrees Kelvin, while melting heat, vaporization heat, melting entropy are for naphtalene disulfide: 3.10 kcal mol?1, 6.42 kcal mol?1, 7.87 cal deg? mol?1 and for diphenylene disulfide: 4.62 kcal mol?1, 6.90 kcal mol?1 and 11.87 cal deg?1 mol?1. 相似文献
A modified version (MM 2′) of the Allinger's 1977 force field is checked against cycloheptane and cyclooctane. Cycloheptane is characterized by two pseudorotating itineraries, chair/twist-chair and boat/twist-boat, separated by a barrier of 8.5 kcal mol?1. The activation energy in the C/TC pseudorotation is estimated to be 0.96 kcal mol?1, while B and TB transform into each other freely at an energy level 3.8 kcal mol?1 above the global energy minimum (TC). With cyclooctane the lowest energy is calculated for the boat-chair form which participates in a pseudorotational process with TBC through a saddle point lying 3.5 kcal mol?1 above BC. The chair/chair and boat/boat families contain only one local minimum, crown and BB, respectively, on the MM 2′ surface. The results are presented as an illustration for quick coverage of torsional energy surface by two-bond driver calculation with the block-diagonal Newton–Raphson minimization, followed by the force search of stationary points by full-matrix Newton–Raphson optimization. 相似文献
The solid-state kinetics for the olation reactions of [Co(NH34(OH)(H2O)]X2 (where X Cl?, Br?, or SO2?4 were determined by several different methods using dynamic and isothermal thermogravimetric data. For the reduced-time plot method, E values were 20, 43, and 25 kcal mol?1 for the chloride-bromide, and sulfate complexes, respectively. For the Jacobs and Kureishy method, E values of 21, 37, and 17 kcal mol?1, were obtained for the above three complexes, respectively. A possible reaction pathway is suggested for the olation reaction. 相似文献
The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG≠, ΔH≠, ΔS≠ parameters related to the corresponding hindered rotation have been determined by 13C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG≠ values decreases from 19.1 kcal mol?1 (79.9 kJ mol?1) to 12.6 kcal mol?1 (52.7 kJ mol?1) according to the following sequence for the R-4 substituents: -C6H5, -CH3, -OCH3, -N(CH3)2. 相似文献
From measurements of the heats of iodination of CH3Mn(CO)5 and CH3Re(CO)5 at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔHof[CH3Mn(CO)5, c] = ?189.0 ± 2 kcal mol?1 (?790.8 ± 8 kJ mol?1), ΔHof[Ch3Re(CO)5,c] = ?198.0 ± kcal mol?1 (?828.4 ± 8 kJ mo?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn2(CO)10 and Re2(CO)10, values are derived for the dissociation energies: D(CH3Mn(CO)5) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol?1 and D(CH3Re(CO)5) = 53.2 ± 2.5 kcal mol?1. In general, irrespective of the value accepted for D(MM) in M2(CO)10, the present results require that, D(CH3Mn) = D(MnMn) + 18.5 kcal mol?1 and D(CH3Re) = D(ReRe) + 30.8 kcal mol?1. 相似文献
Azoethane was irradiated in the presence of carbon monoxide in the temperature range of 238 to 378 K. Kinetic parameters for the addition of ethyl radicals to carbon monoxide and for the decomposition of propionyl radicals were determined. The rate constants were found to be log k(cm3 mol?1 sec?1) = 11.19 - 4.8/θ and log k(sec?1) = 12.77 - 14.4/θ, respectively. Estimated thermochemical properties of the propionyl radical are ΔHf0 = -10.6 ± 1.0 kcal mol?1, S0 = 77.3 ± 1.0 cal K?1 mol?1, and D(C2H5CO? H) = 87.4 kcal mol?1. 相似文献
The electronic structure and redox properties of the highly oxidizing, isolable RuV?O complex [RuV(N4O)(O)]2+, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol?1, respectively. The overall free energy activation barrier (ΔG≠=25.5 kcal mol?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [RuIII(N4O)(HOCl)]2+ (ΔG=15.0 kcal mol?1) and the substitution of HOCl by a water molecule (ΔG≠=10.5 kcal mol?1). For water oxidation, nucleophilic attack on RuV?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [RuV(N4O)(O)]2+ leads to the [(N4O)RuIV? O2? RuIII(N4O)]4+ complex with a calculated free energy barrier of 12.0 kcal mol?1. 相似文献
Effects of Substituents on the Strength of C - C Bonds, 81. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol -1, s. d. in parentheses) ΔH°c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔHv = 14.3 (± 0.3) and the heat of sublimation of 2 ΔHsub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (s) of 15.9 and 8.0 kcal mol-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group. 相似文献