The SO2(3B1) photosensitized isomerization of cis- and trans-1,2-dichloroethylene

The photolysis of SO₂ at 3712 Å in the presence of the 1,2-dichloroethylenes has been investigated at 22deg;C. The data are consistent with the SO₂(³B₁) photosensitized isomerization of the 1,2-dichloroethylene isomer. A kinetic treatment of the initial quantum yield data was consistent with the formation of a polarized charge-transfer intermediate whenever SO₂(³B₁) molecules and one of the 1,2-dichloroethylene isomers collide which ultimately decays unimolecularly to the cis-isomer with a probability of 0.70 ± 0.26 and to the trans-isomer with a 0.37 ± 0.16 probability. Quenching rate constants for removal of SO₂(³B₁) molecules by cis- and trans-1,2-dichloroethylene have been estimated from quantum yield data and from laser excited phosphorescence lifetimes using an excitation wavelength of 3130 Å. Estimates of the quenching rate constant (units of 1./mole ± sec) are for the cis-isomer, (1.63 ± 0.71) × 10¹⁰, quantum yield data, and (2.44 ± 0.11) × 10¹⁰, lifetime data; and for the trans-isomer,(2.59 ± 0.09)×10¹⁰, lifetime data, and (2.35 ±0.89) × 10¹⁰, quantum yield data. An experimentally determined photostationary composition,[cis-C₂Cl₂H₂]/[trans-C₂Cl₂H₂] = 1.8 - 0.1, was in good agreement with a value of 2.00 - 1.15 which was predicted from rate constants derived in this study.     

The photolysis of SO2 at 3080 Å in the presence of cis- and trans-1,2-difluoroethylene

The photolysis of SO₂ at 3080 Å, FWHM = 150 Å, and 22°C has been investigated in the presence of cis- and trans-C₂F₂H₂. Quantum yield measurements for the photosensitized isomerization of cis-C₂F₂H₂ to trans-C₂F₂H₂ have been made for a variation in the [SO₂]/[cis-C₂F₂H₂] ratio from 0.992 to 253. The results fit a mechanism which is consistent with the SO₂(³B₁) state being the reactive excited state of sulfur dioxide. A mechanism employing only the SO₂(¹B₁) and SO₂(³B₁) excited states is quite satisfactory to rationalize the data. A value for the SO₂ collisionally induced intersystem crossing efficiency from SO₂(¹B₁) to SO₂(³B₁) of 0.35 ± 0.14 was estimated while the cis-C₂F₂H₂ efficiency was found to be 0.030 ± 0.012. The rate constant at 22°C for the removal of SO₂(³B₁) molecules by cis-C₂F₂H₂ was found to be (1.43 ± 0.13) × 10¹⁰1./mole · sec. A photostationary composition, [cis]/[trans] = 1.0 ± 0.1, was found from prolonged irradiations of SO₂ in the presence of the cis and trans isomers.     

Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.     

A kinetic study of the chemistry of the SO2 (3B1) reactions with cis- and trans-2-butene

The chemical reactions of SO₂(³B₁) molecules with cis- and trans-2-butene have been studied in gaseous mixtures at 25°C by excitation of SO₂ within the SO₂(³B₁) → SO₂(+, ¹A₁) ‘forbidden’ band using 3500–4100-Å light. The initial quatum yields of olefin isomerization were determined as a function of the [SO₂]/[2-butene] ratio and added gases, He and O₂. The kinetic treatment of these data suggests that there is formed in the SO₂(³B₁) quenching step with either cis- or trans-2-butene, some common intermediate, probably a triplet addition complex between SO- and olefin. It decomposes very rapidly to form the 2-butene isomers in the ratio [trans-2-butene]/[cis-2-butene] = 1.8. In another series of experiments SO₂ was excited using a 3630 ± 1-Å laser pulse of short duration, and the SO₂(³B₁) quenching rate constants with the 2-butenes were determined from the SO₂(³B₁) lifetime measurements. The rate constants at 21°C are (1.29 ± 0.18) × 10¹¹ and (1.22 ± 0.15) × 10¹¹ l/mole·sec with cis-2-butene and trans-2-butene, respectively, as the quencher molecule. Within the experimental error these quenching constants equal those derived from the quantum yield data. Thus the rate-determining step in the isomerization reaction is suggested to be the quenching reaction, presumably the formation of the triplet SO₂-2-butene addition complex. In a third series of experiments using light scattering measurements, it was found that the aerosol formation probably originates largely from SO₃ and H₂SO₄ mist formed following the reaction SO₂(³B₁) + SO₂ → SO₃ + SO(³Σ^?). Aerosol formation from photochemically excited SO₂-olefin interaction is probably unimportant in these systems and must be unimportant in the atmosphere.     

The synthesis of cis- and trans-1,2-dibromocyclopropane

A preparative route to cis- and trans-1,2-dibromocyclopropane ($\text{1}$) was developed via the Hunsdiecker reaction of silver cyclopropane-1,2-dicarboxylate ($\text{2}$). Cis- and trans-$\text{2}$ gave the same ratio of cis- and trans-$\text{1}$ (1:3.2). The mechanism of this reaction is briefly discussed.     

Vibrational spectra of cis- and trans-1,2-dimethyl-3-bromocyclopropane

The infrared and Raman spectra of cis- and trans-dimethylbromocyclopropane have been recorded from 4000 to 50 cm⁻¹. An assignment of the majority of the fundamentals is proposed and compared to those of related molecules. Definite and consistent trends in a number of normal modes of the ring and the methyl groups with the nature, position and number of the substituents have been found. Clear evidence has been obtained for steric interaction between the three substituents in cis-position.     

Prototropic isomerization of trans-4-benzamido-3-hydroxy-2-(4-carbomethoxybutylidene)thiophan to cis- and trans-4-benza mido-3-oxo-2-(4-carbomethoxybutyl) thiophan

trans-4-Benzamido-3-hydroxy-2-(4-carbomethoxybutylidene)thiophan (I) was prototropically isomerized in the presence of hydrogen chloride. It is shown that the two possible isomers cis- and trans-4-benzamido-3-oxo-2-(4-carbomethoxybutyl)thiophan — are formed in the process.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 2, pp. 175–177, February, 1973.     

Solid-state structures of cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines

Solid-state structures have been determined for cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines using single-crystal X-ray diffraction techniques. The cis isomer crystallizes in the centrosymmetric monoclinic space group P2₁/c (No. 14), with a = 18.669(3)Å, b = 5.709(1)Å, c = 17.412(2)Å, β = 96.29(1)° and Z = 4; the trans isomer crystallizes in the noncentrosymmetric orthorhombic space group Pna2₁ (No. 33), with a = 17.089(2)Å, b = 18.729(3)Å, c = 5.749(1)Å and Z = 4. Full-matrix least-squares refinement of the structural parameters led to the following final agreement factors: R₁ (unweighted, based on F) = 0.040 and R₂ (weighted, based on F) = 0.054 for the 2592 independent reflections of the cis isomer having 2θ_MoK¯α <55° and I>3σ₁, and R₁ = 0.033 and R₂ = 0.031 for the 1504 independent reflections of the trans isomer having 2θ_MoK¯α <55° and I>3σ₁. The statistically significant differences that exist between the two isomers for two bond lengths and ten bond angles (p < 0.05) appear to be the direct result of the p-toluyl group orientation with respect to the cyclohexyl and phenyl substituents. In the cis isomer it is anti with respect to the N-cyclohexyl group and cis with respect to the phenyl group, whereas in the trans isomer it is syn with respect to the N-cyclohexyl and trans with respect to the phenyl group. Three-ring to carbonyl hyperconjugation is correlated with stereoelectronic interactions in the trans isomer. Bonding, determined by X-ray and nmr studies, is discussed for the three-membered aziridine ring proper; while bonding, determined by X-ray studies, is discussed for substituents of the aziridine ring. These aziridinyl ketone compounds are of importance as potential mammalian DNA alkylating anti-tumor agents in solid-state solid-state systems. To date only a trans isomer has demonstrated this biological activity in tumor-bearing rats.     

Radical ions in photochemistry 10. The photosensitized (electron-transfer) isomerization of 1,2-diphenylcyclopropanes.

The photoisomerization of the 1,2-diphenylcyclopropanes in polar solvent sensitized by 1-cyano- and 1,4-dicyanonaphthalene proceeds via an electron-transfer mechanism. An analysis of photo-CIDNP results provide details regarding the intermediates.     

H2S-promoted thermal isomerization of cis-2-pentene to 1-pentene and trans-2-pentene around 800 K

H₂S accelerates the thermal isomerization of cis-2-pentene (P2c) to 1-pentene (P1) and trans-2-pentene (P2t) to around 800 K. This effect is interpreted on the basis of a free radical mechanism in which 2-pentenyl and thiyl radicals are the main chain carriers. P1 formation is essentially explained by the competing processes: P2t formation is due to addition-elimination processes: the importance of which has been evaluated against process (?4μ): The following ratios of rate constants have been measured and are discussed: (RT in cal mol^?1).     

High-resolution rovibrational analysis of the Nu4 band of cis-1,2-difluoroethylene

The diode laser spectrum of cis-1,2-CHF=CHF has been measured and analyzed in the nu4 fundamental region near 1016 cm(-1). This vibration of symmetry species A1 corresponds to the C-F symmetric stretching motion and gives rise to a strong b-type band. The rovibrational analysis, extended to the P, Q, and R branches, led to the identification of 2800 lines with J < or = 62, Ka < or = 18, Kc < or = 62. The assigned transitions free of major resonance contributions, fitted using Watson's A-reduction Hamiltonian in the Ir representation, yielded a set of spectroscopic parameters up to the quartic coefficients for the V4 = 1 state. Several perturbation effects occur throughout the band, mainly caused by the first-order c-type Coriolis interaction with the nu5 + nu11, vibrational state. Even though no transitions to the perturbing level were observed, the band orign and the rotational constants for the perturber were determined from a dyad model which includes the Coriolis interaction term.     

Dehydrohalogenations of cis- and trans-1-bromo-2-flourocyclohexanes

Reactions of both diastereomeric vicinal bromofluorocyclohexanes with sodamide, sodium methoxide, potassium tert-butoxide, and triethylamine were studied. cis-1-Bromo-2-fluorocyclohexane eliminated almost exclusively hydrogen bromide giving mainly 1-fluorocyclohexene and a small amount of 3- fluorocyclohexene. trans-1-Bromo-2-fluorocyclohexane eliminated preferentially hydrogen fluoride on treatment with sodamide. Potassium tert- butoxide eliminated hydrogen bromide and yielded 3-fluorocyclohexene and a small amount of 1,3-cyclohexadiene, whereas sodium methoxide converted trans-1-bromo-2-fluorocyclohexane to 3-fluorocyclohexene and 3-methoxycyclohexene. Possible mechanisms and stereochemistry of the preferential elimination of hydrogen fluoride are discussed and new interpretations are offered.     

Pressure saturation of SO2(3B1 0,0,0) lifetimes

Lifetime data have been obtained for the decay of SO₂(³B₁ 0,0,0) at 25°C over the pressure range of 1–762 torr. The ³B₁ state was populated by direct absorption to eliminate any possible complications in interpretation due to the participation of excited-singlet manifolds. At pressures greater than about 10 torr, the measured lifetimes are longer than predicted from low-pressure Stern–Volmer parameters. This deviation can be interpreted in terms of Freed's theory on collisionally induced intersystem crossing and provides unequivocal evidence to support earlier speculations that the lengthening of the lifetimes at high pressures is due to saturation in depopulation of the ³B₁ state.     

Thermal isomerization of cis- or trans-2-butene. The unimolecular elimination of hydrogen from cis-2-butene around 500°C

An analytical and kinetic study of the thermal reaction of cis- or trans-2-butene has been performed in a static system over the temperature range of 480–550°C and at a low extent of reaction and initial pressures of 10–100 torr. The rate constant of the unimolecular cis–trans isomerization of cis-2-butene, determined under the conditions (2.3 RT in cal/mole) is in good agreement with previous measurements made at lower pressures. A comparison between the formation rates of hydrogen from the thermal reactions of cis- and trans-2 butene around 500°C leads to the rate constant value (2.3 RT in cal/mole) for the unimolecular 1,4? hydrogen elimination from cis? 2? butene.     

Boron fluorides of cis- and trans-2-aryl-3-aroylaziridines

The reaction of boron trifluoride etherate with cis- and trans-2-aryl-3-aroylaziridines yields the respective complexes in high yield. According to PMR spectral data the methyl group of boron trifluoride trans-1-methyl-3-aroylaziridines is located in syn-position, and in the cis-aziridine complexes in anti-position, to the carbonyl group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1047–1049, August, 1987.     

The photolysis of SO2 at 3130 Å in the presence of cis- and trans-2-pentene

The photolysis of SO₂ at 3130 Å, FWHM = 165 Å, and 22°C has been investigated in the presence of cis- and trans-2-pentene. Quantum yields for the SO₂ photosensitized isomerization of one isomer to the other have been made for a variation in the [SO₂]/[C₅H₁₀] ratio of 3.41–366 for cis-2-C₅H₁₀ and of 1.28–367 for trans-2-C₅H₁₀. A kinetic analysis of each of these systems permitted new estimates to be made for the SO₂ collisionally induced intersystem crossing ratio at 3130 Å from SO₂(¹B₁) to SO₂(³B₁). The estimates of k_1a/(k_1a + k_1b) obtained are 0.12 ± 0.01 and 0.12 ± 0.02 (two different kinetic analyses in the cis-2-C₅H₁₀ study) and 0.20 ± 0.05 and 0.20 ± 0.04 (two different kinetic analyses in the trans-2-C₅H₁₀ study). Collisionally induced intersystem crossing ratios of k_2a/(k_2a + k_2b) = 0.51 ± 0.10 and k_3a/(k_3a + k_3b) = 0.62 ± 0.12 were obtained for cis- and trans-2-pentene, respectively. Quenching rate constants at 22°C for removal of SO₂(³B₁) molecules by cis- and trans-2-C₅H₁₀ were estimated as (1.00 ± 0.29) × 10¹¹ l./mole·sec and (0.857 ± 0.160) × 10¹¹ l./mole/sec, respectively. Prolonged irradiations, extrapolated to infinite irradiation times, for mixtures initially containing SO₂ and pure isomer, either the cis or trans, yielded a photostationary composition of [trans-2-pentene]/[cis-2-pentene] = 2.1 ± 0.1.