G.R. De Maré

Rate constants for the recombination of CCl3 radicals and for their reactions with Cl, Cl2 and HCl in the gas phase

The gas-phase photochlorination of CHCl3 has been investigated between 303.2 and 425.5° K. The reaction was followed both by a conventional and a mass spectrometric technique. Rate measurements in steady light lead to log10(k3/K½8)=–5,000/4.576 T+3.91, (i), and log10(k3k2/k7)½=–3,700/4.576 T+3.60 (ii), where k2, k3, k7 and k8 refer to Cl + CHCl3→ CCl3+ HCl (2), CCl3+ Cl2→ Cl + CCl4(3), Cl + CCl3→ CCl4(7), 2CCl3→ C2Cl6(8), Rate measurements in intermittent light give log10k3=–5,000/4.576 T+8.74, (iii) which combined with eqn. (i) and (ii), and the known value of k2 yields log10k7= 10.8 and log10k8= 9.6 6, indenpendent temperature. Using the values of K,2 and K,3 and thermodynamic data, one calculates log10K4=–11,300/4.576T+8.65 and log10k5=–20,000/4.576 T+10.93, for the reverse of reactions (2) and (3) respectively. All rate constants are given in mole–1 1. sec–1.

Thermal dimerization of 1,3-cyclohexadiene in the gas phase

The thermal dimerizations of 1,3-cyclohexadiene(Chd), 2 Chd→exo-dicyclohexadiene (D3)kD3(a) and 2 Chd→endo-dicyclohexadiene (D4)kD4(b) have been studied between 471.1 and 638.5 K at pressures ranging from 25 to 630 Torr. These reactions are second order and the rates are unaffected by the surface-to-volume ratio or by the presence of added propene. The rate constants are given by log10kD3(1.mol–1s–1)=–(25 100 ± 500)/4.576 T+(5.97 ± 0.25), and log10kD4(1.mol–1s–1)=–(24 300 ± 500)/4.576 T+(6.08±0.25).

Rate of recombination of trichloromethyl radicals in the gas phase

Abstract The rate constant for the recombination of trichloromethyl radicals has been measured in the gasphase photochlorination of chloroform at 344.7°K. The observed value is 109.62 mole-1 1 sec-1, much lower than the value predicted by transition state calculations using Gorins model.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethylcyclopropene. I. 3,3-Dimethyl-1,2-bis(tert-butyl)cyclopropene.

The geometrical parameters of 3,3-dimethyl-1,2-bis(tert-butyl)cyclopropene were optimised completely at the HF/6-31G* level. The HF/6-31G*//HF/6-31G* force field was calculated and scaled using Pulays scaling procedure. The set of 17 scale factors (for a 105-dimensional problem) was compiled from the sets obtained previously for 3,3-dimethyl-1-butene and 1-methyl-, 1,2-dimethyl-, and 3,3-dimethylcyclopropene. The vibrational problem was solved using the scaled quantum mechanical force field (QMFF) and assignments of the vibrational frequencies of 3,3-dimethyl-1,2-bis(tert-butyl)cyclopropene were considered in comparison with the known assignments of 3,3-dimethyl-1-butene and 3,3-dimethylcyclopropene. Assignments of four experimental IR bands of 3,3-dimethyl-1,2-bis(tert-butyl)cyclopropene given in the literature are suggested.

Advantages of scaled quantum mechanical molecular force fields

Abstract The main advantages of scaled quantum mechanical force fields in comparison with traditional force fields based on the solution of the inverse vibrational problem (even if the quantum mechanical force field is used as a starting approximation) are enumerated.

The vapour pressure of benzaldehyde between 273 and 376 K

The vapour pressure of benzaldehyde is given by the equation: log ⁡ 10 ( p / Torr ) = 8 . 3 5 1 5 - 2 4 5 5 . 4 ( T / K ) - 1 over the range 273 to 376 K. The enthalpy of vaporization is 11.2 kcal th mol −1 .

A theoretical study on the thermodynamic properties of the formation and decomposition of methyloxirane via triplet mechanisms

Abstract Molecular unrestricted Hartree-Fock calculations with geometry optimization have been carried out on eight triplet isomers of C 3 H 6 O considered to be possible intermediates in the addition of O( 3 P) atoms to propylene and in the Hg( 3 P) sensitization of methyloxirane. The computed thermodynamic stabilities reveal that four triplet states are available in the former and eight triplet states are available in the latter reaction. The isomer CH 3 ĊHCH 2 Ȯ is more stable than ȮCHCH 3 ĊH 2 . This latter feature gives a satisfactory explanation for the experimental observation that in the O( 3 P) + C 3 H 6 reaction propionaldehyde is the major carbonyl product.

Photosensitization by benzaldehyde in the gas phase. Cis- and trans-1,2-dichloroethylene

Abstract The benzaldehyde (3–9 torr) photosensitized cis - trans isomerizations of the 1,2-dichloroethylenes (20–450 torr) have been studied at 60°C (λ = 365.5 nm). The sum of the isomerization quantum yields is unity for dichloroethylene at pressures ⩾ 100 torr, indicating that the quantum yield for intersystem crossing in benzaldehyde is unity. Energy transfer from triplet benzaldehyde to cis -1,2-dichloroethylene, requiring an average of about 10 4 collisions, proceeds at about 1/40th the rate of transfer to trans -1,3-pentadiene.

Indo theoretical studies: torsional barriers of vinyl groups on cyclopropane rings

The magnitude of conjugation both into and through the cyclopropyl ring have been investigated by a large number of workers (see ref. 1 and references cited therein). For example, Kispert et al. ’ have used INDO calculations to attempt to assess the magnitude of “through-conjugation” in a series of trans-Z vinylcyclopropyl species by comparing calculated quantities of lb-f to those of la. These quantities included optimized bond lengths, spin densities, charge densities,

Kinetics and mechanism of the pyrolysis of cyclohexa-1,3-diene

The pyrolysis of cyclohexa-1,3-diene (C6H8) has been studied between 512·3 and 672·5 K at pressures between 10 and 500 Torr. The products are benzene (B), cyclohexene (C), a compound with formula C12H18(E), hydrogen, and traces of C2 compounds. The formations of benzene, cyclohexene, and C12H18 are all of the second order and their rates are unaffected by the surface-to-volume ratio of the reaction vessel or by the presence of added propene. The rate constants (in l mol–1 s–1) are given by equations (i)–(iii). The rate of formation of H2 is ca. 1/7 that of B. log10kB=–(35,500 ± 100)/4·576T+(10·13 ± 0·04)(i), log10kC=–(36,400 ± 1100)/4·576T+(10·25 ± 0·40)(ii), log10kE=–(35,600 ± 1800)/4·576T+(9·52 ± 0·67)(iii)The results are explained by a non-chain, radical mechanism where the initiation step is the bimolecular disproportionation (iv) to cyclohex-2-enyl and cyclohexadienyl radicals. The estimated value of the rate constant ka is [graphic omitted] (iv), log10ka=–(35,500 ± 1000)/4·576T+(10·04 ± 0·40) l mol–1 s–1.