High resolution 13C NMR spectroscopy. IV—stereochemical assignments in butenedioic acids and 3-pentene-2-ones

Vicinal ¹³C, H coupling constants ³J(CO, H) for butenedioic acids and ³J(CH₃, H) for 3-pentene-2-ones have been determined and are correlated with the configuration of the corresponding C?C double bond. For both types the relationship ³J(CH) trans > ³J(CH)cis holds; in the case of the CH₃, H couplings, however, the ³J(CH₃, H) trans values are reduced because of steric reasons, so that configurational assignments seem possible only when both isomers are present. Additionally, the coupling constants ³J(COC H₃,H ) and the chemical shifts δ have been evaluated for the pentenones and it is shown that these parameters give information about the predominating conformation of α, β-unsaturated methyl ketones.     

NMR experiments on acetals—XXXIX: Conformational insights through NMR studies at 300 MHz of 2-methyl-4-halogenomethyl-1,3-dioxolanes

NMR spectra of cis- and trans-2-methyl-4-halogeno-methyl-1,3-dioxolanes have been analysed at 300 MHz. Some of the extracted parameters facilitate easy distinction between these 1,3-dioxolanes and the corresponding structurally isomeric 2-Methyl-5-halogeno-1,3-dioxanes. Criteria enabling configurational assignments to be made for the cis-trans isomers of the dioxolane series are tested. The Me-2 group causes an upfield shift (0·2 to 0·3 ppm) of a trans proton at position 5, but the reversed shift for the corresponding cis proton. This competes with, or even overwhelms the effect of the CH₂X-4 substituent, which by virtue of its pronounced preferential rotameric orientation and in comparison with a simple Me-group, has no large upfield effect on the shift of the syn-adjacent proton. Shift criteria and coupling constants J_{H-4, H-5} in cis- and trans derivatives allow further conformational insights into these 1,3-dioxolanes.     

Cationic polymerization of α,β-disubstituted olefins. IV. Stereospecific polymerization of propenyl alkyl ethers catalyzed by BF3·O(C2H5)2 and Al2(SO4)3–H2SO4 complex

The cis- and trans-propenyl alkyl ethers were polymerized by a homogeneous catalyst [BF₃·O(C₂H₅)₂] and a heterogeneous catalyst [Al₂(SO₄)₃–H₂SO₄ complex]. Methyl, ethyl, isopropyl, n-butyl and tert-butyl propenyl ethers were used as monomers. The steric structure of the polymers formed depended on the geometric structures of monomer and the polymerization conditions. In polymerizations with BF₃·O(C₂H₅)₂ at ?78°C., trans isomers produced crystalline polymers, but cis isomers formed amorphous ones except for tert-butyl propenyl ether. On the other hand, highly crystalline polymers were formed from cis isomers, but not from the trans isomers in the polymerization by Al₂(SO₄)₃–H₂SO₄ complex at 0°C. The x-ray diffraction patterns of the crystalline polymers obtained from the trans isomers were different from those produced from the cis isomers, except for poly(methyl propenyl ether). The reaction mechanism was discussed briefly on these basis of these results.     

Interactions intramoleculaires. XXI—Constantes de couplage et interactions gauches avec un groupement t-butyle (molécules cyclohexaniques deutériées)

Different isotopic modifications of deuterated products of 1-t-butlyl-4-methoxycarbonyl cyclohexene (d₄-3,3,6,6), cis- and trans-3-methyl-4-cyanocyclohexene (d₃-3,6,6), cis- and trans-3-methyl-4-cyanocyclohexene (d₃,6,6), cis- and trans-3-t-butyl-4-methoxycarbonylcyclohexene (d₃,-3,6,6) are shown by nuclear magnetic resonance spectral analysis. By comparison of ³J and ⁴J coupling constants of model molecules and molecules with large gauche interactions, we obtain proof that the latter are in a chair conformation with moderate cycle deformations.     

Angular dependence of the vicinal interproton spin–spin coupling in silacyclohexanes. The conformational energy term of the methyl group in 1-methyl-1-silacyclohexane

An approximate Karplus-Conroy relationship between the vicinal coupling constants J(H SiCH ) in a silaethane fragment and the corresponding torsional angles τ in silacyclohexanes is proposed. The relationship ³J(H SiCH ) VS τ is asymmetric. The ¹H NMR parameters and conformational features of cis- and trans-3,5-dimethyl-1-silacyclonexane and 3-silabicyclo[3.2.1]octane are discussed, and the conformational energy term for the methyl group in 1-methyl-1-silacyclohexane is found to be approximately 1.45 kJ mole^?1 in favour of the axial position for the methyl group, in good agreement with previously calculated values.     

1H NMR assignments for t-2-deuterio-r-1-methyl-1-phenylcyclopropane and c-2-deuterio-r-1-methyl-1-phenylcylclopropane. A literature correction

The synthesis and ¹H NMR spectra of the cis and trans monodeuterated isomers of 2-deuterio-1-methyl-1-phenylcyclopropane have been previously reported. We have prepared one of the isomers, c-2-deuterio-r-1-methyl-1-phenylcyclopropane, by a different, stereospecific, route. We now wish to correct the original ¹H NMR assignments which were in error.     

Nitrogen-15 coupling constants: Geminal coupling, 2J(15NH), in cis- and trans-aldonitrones and vicinal coupling, 3J(15NH), in cis-and trans-ketimines,oxaziridines and nitrones

Nitrogen-15-hydrogen ²J(¹⁵NH), ¹⁵N, ¹³C ¹J(¹⁵N¹³C) and ¹³C, H ¹J(¹³CH) coupling constants have been measured and their signs determined for cis-and trans-[9-anthryl(¹³C)methylene](²H₃)methylamine (¹⁵N)-oxide. Values of ²J(¹⁵NH) were of similar magnitude (～2 Hz) but were of opposite sign. The results are compared and contrasted with those reported for related imino and quaternary imino systems. Vicinal ³J(¹⁵NH) coupling constants have been measured in cis- and trans-¹⁵N-[1-(α-naphthyl)ethylidene]benzylamine and ¹⁵N-[1-(p-nitrophenyl)ethylidene]-t-butylamine and were found to be larger when the imino methyl group was cis to the nitrogen lone pair. The corresponding cis and trans ketonitrones formed by photoisomerization of the derived oxaziridines had ³J(¹⁵NH) values of c. 3.3 Hz. A study of the signs and magnitudes of observed and calculated ¹⁵N,H coupling constants for all of the ¹⁵N labelled imines, oxaziridines, imine N-oxides (nitrones) and similar model systems which were synthesized is described.     

Thermische Lactonisierung von Estern γ-Brom-α, β-ungesättigter Carbonsäuren zu Δα-Butenoliden. Direkte γ-Bromierung von α, β-ungesättigten Säuren

By heating with iron powder at 120–150° some γ-bromo-α, β-unsaturated carboxylic methyl esters, and, less smothly, the corresponding acids, were lactonized to Δ^7alpha;-butenolides with elimination of methyl bromide. The following conversions have thus been made: methyl γ-bromocrotonate ( 1c ) and the corresponding acid ( 1d ) to Δ^α-butenolide ( 8a ), methyl γ-bromotiglate ( 3c ) and the corresponding acid ( 3d ) to α-methyl-Δ^α-butenolide ( 8b ), a mixture of methyl trans- and cis-γ-bromosenecioate ( 7c and 7e ) and a mixture of the corresponding acids ( 7d and 7f ) to β-methyl-Δ^α-butenolide ( 8c ). The procedure did not work with methyl trans-γ-bromo-Δ^α-pentenoate ( 5c ) nor with its acid ( 5d ). Most of the γ-bromo-α, β-unsaturated carboxylic esters ( 1c, 7c, 7e and 5c ) are available by direct N-bromosuccinimide bromination of the α, β-unsaturated esters 1a, 7a and 5a ; methyl γ-bromotiglate ( 3c ) is obtained from both methyl tiglate ( 3a ) and methyl angelate ( 4a ), but has to be separated from a structural isomer. The γ-bromo-α, β-unsaturated esters are shown by NMR. to have the indicated configurations which are independent of the configuration of the α, β-unsaturated esters used; the bromination always leads to the more stable configuration, usually the one with the bromine-carrying carbon anti to the carboxylic ester group; an exception is methyl γ-bromo-senecioate, for which the two isomers (cis, 7e , and trans, 7d ) have about the same stability. The N-bromosuccinimide bromination of the α,β-unsaturated carboxylic acids 1b , 3b , 4b , 5b and 7b is shown to give results entirely analogous to those with the corresponding esters. In this way γ-bromocrotonic acid ( 1 d ), γ-bromotiglic acid ( 3 d ), trans- and cis-γ-bromosenecioic acid ( 7d and 7f ) as well as trans-γ-bromo-Δ^α-pentenoic acid ( 5d ) have been prepared. Iron powder seems to catalyze the lactonization by facilitating both the elimination of methyl bromide (or, less smoothly, hydrogen bromide) and the rotation about the double bond. α-Methyl-Δ^α-butenolide ( 8b ) was converted to 1-benzyl-( 9a ), 1-cyclohexyl-( 9b ), and 1-(4′-picoly1)-3-methyl-Δ^α-pyrrolin-2-one ( 9 c ) by heating at 180° with benzylamine, cyclohexylamine, and 4-picolylamine. The butenolide 8b showed cytostatic and even cytocidal activity; in preliminary tests, no carcinogenicity was observed. Both 8b and 9c exhibited little toxicity.     

Synthesis and 13C NMR spectra of cis- and trans-{2-(haloaryl)-2-[(1H-imidazol)-1-yl]methyl]}-1,3-dioxolane-4-methanols

Syntheses and ¹³C nmr spectra of a number of cis and trans 2-(haloaryl)-2-[(1H-imidazol-1-yl)rnethyl]-4-(hydroxymethyl)-1,3-dioxolanes are described. The haloaryl groups are 2,4-dichloro, 2,4-difluoro-, 4-chloro-and 4-bromophenyl. In these series, some of the cis compounds become available through crystalline bromo benzoates 5 . Separations of some trans isomers are achieved through fractional crystallizations of imidazolyl benzoate nitrates 6 . Stereochemical assignments are based primarily on one major ¹³C chemical shift difference, namely that of C-4 of the 1,3-dioxolane ring, the chemical shift of the trans isomers being 1.0-2.5 ppm downfield from that of the cis isomers.     

Stereospecific elimination of acetic acid in 3- and 4-arylcyclohexyl acetates under electron impact

The elimination of CH₃COOH from the molecular ions of trans-3- and 4-arylcyclohexyl acetates takes place to a greater extent than in the cis isomers. Deuterium labelling shows that the elimination involves mainly the benzylic hydrogen in the trans-acetates, but not in the cis isomers. This behaviour is similar qualitatively to that of the corresponding alcohols and methyl ethers, but entirely different from that of t-butylcyclohexyl acetates, which do not exhibit any stereospecificity. Substituent effects on the elimination for both cis and trans isomers are discussed.     

Interet de la deuteriation des molecules organostanniques en rmn de 119Sn : I. Acces aux constantes de couplage nJ(SnD) et application a l'etude structurale de vinyl et d'allyletains

The selective deuteration of organotin compounds gives FT ¹¹⁹Sn NMR a new proficiency, since it allows direct access to ⁿJ(SnD) coupling constants. Usually ⁿJ(SnD) is easily apparent for n  1, 2, 3 and sometimes 4 (allyltin compounds). This criterion has been successfully applied for qualitative and subsequently quantitative analysis of vinyltin and allyltin compounds.In the first series, the stereochemistry of alkyne deuterostannation has been easily explored, the assignments of configurations being made directly from ³J(SnD) on the basis of ³J(SnD)_trans > ₃J(SnD)_cis. In the second series, the method appears to be the best tool for simultaneous study of regiochemistry and stereochemistry in the substitution of 3-deutero-5-methyl-2-cyclohexenyl toluene sulphonates by trialkylstannyl anions. The choice between regioisomers was made on the basis of ₂J(SnD) >- ⁴J(SnD).A further aspect of this work is the observation of isotopic effects on ¹¹⁹Sn chemical shifts which were tentatively rationalized considering inductive, steric or hyperconjugative contribution of deuterium.     

Mechanismus der photochemischen Methanol-Addition an 2-allylierte Aniline

Mechanism of the Photochemical Addition of Methanol to 2-Allylated Anilines We studied in methanol the photoreaction of the 2-allylated anilines, given in Scheme 3 (cf. also [ 1 ]). Irradiation of N-methyl-2-(1′-methylallyl)aniline ( 15 ) with a high pressure mercury lamp yielded trans- and cis-1,2,3-trimethylindoline (trans- and (cis- 34 ) as well as erythro- and threo-2-(2′-methoxy-1′-methylpropyl)-N-methylaniline (erythro- and threo- 35 ; Scheme 7). When the corresponding aniline d₃- 15 , specifically deuterated in the 1′-methyl group, was irradiated in methanol, a mixture of trans- and cis-d₃- 34 , and of erythro- and threo-d₃- 35 was obtained. Successive dehydrogenation of the mixture of cis/trans-d₃- 34 by Pd/C in boiling xylene and by MnO₂ in boiling benzene lead to the corresponding indole d₃- 36 (cf. Scheme 9), the ¹H- and ²H-NMR. spectra of which showed that both cis-d₃- and trans-d₃- 34 had bound the deuterium labeled methyl group exclusively at C(3). The ¹H- and ²H-NMR. analyses of the separated methanol addition products revealed that erythro-d₃- 35 contained the deuterium label to at least 95% in the methyl group at C(1′), and threo-d₃- 35 to 50% in CH₃? C(1′) and to 50% in CH₃? C(2′) (cf. Scheme 9). To confirm these results 2-(1′-ethylallyl)aniline ( 16 ) was irradiated in methanol, whereby a complex mixture of at least 6 products was obtained (cf. Scheme 11). Two products were identified as trans- and cis-3-ethyl-2-methylindoline (trans- and cis- 37 ). The four other products represented erythro- and threo-2-(1′-ethyl-2′-methoxypropyl)aniline (erythro- and threo- 39 ) as major components, and erythro- and threo-2-(2′-methoxy-1′-methylbutyl)aniline (erythro- and threo- 40 ). These results clearly demonstrate that the methanol addition products must arise from spirodienimine intermediates of the type of trans- 9 and cis- 11 (R¹ = CD₃ or C₂H₅, R² = CH₃ or H; Scheme 2) which are opened solvolytically with inversion of configuration by methanol. Thus, cis- 11 (R¹ = CD₃, R² = CH₃) must lead to a 1:1 mixture of threo- 13 and threo- 14 (i.e.) a 1:1 distribution of the deuterium labelled methyl group between C(1′) and C(2′) in threo- 35 ) The formation of erythro-d₃- 35 with at least 95% of the deuterium label in the methyl group at C(1′) indicates that trans- 9 (R¹ = CD₃, R² = CH₃) reacts with methanol regioselectively (> 95%) at the C(2), C(3) bond. Similarly, the formation of the methanol addition products in the photoreaction of 16 (Scheme 11) can be explained. Since the indolines, formed in both photoreactions, show no alteration in the position of the subsituent at C(1′) with respect to the starting material we suppose that the diradical 7 (R¹ = CD₃ or C₂H₅, R² = CH₃ or H; Scheme 2) is a common intermediate which undergoes competetive 1.3 and 1.5 ring closure yielding the spirodienimines and the indolines. This conception is supported by irradiation experiments with N, 3,5-trimethyl-2-(1′-methylally)aniline ( 17 ) and 2-(2′-cyclohexenyl)-N-methylaniline ( 18 ) in methanol. In the former case the formation of spirodienimines is hindered by the methyl group at C(3) for steric reasons, thus leading to a ratio of the indoline to the methoxy compounds of about 6.3 as compared with ca. 1.0 for 15 (cf. Scheme 12). On the other hand, no methoxy compounds could be detected in the reaction mixture of 18 (cf. Scheme 13) which indicates that in this case the 1.3 ring closure cannot compete with the 1.5 cyclization in the corresponding cyclic diradical of the type 7 (R¹–C(1′)–C(2′) is part of a six-membered ring; Scheme 2). We suppose that the diradicals of type 7 are formed by proton transfer in an intramolecular electron-donor-acceptor (EDA) complex arising from the excited single state of the aniline chromophor and the allylic side chain. This idea is supported by the fluorescence specta of 2-allylated N-methylanilines (cf. Fig.1-4) which show pronounced differences with respect to the corresponding 2-alkylated anilines. Furthermore, the anilines 18 and 20 when irradiated in methanol in the presence of an excess of trans-1,3-pentadiene undergo preferentially an intermolecular addition to the diene, thus yielding the N-(1′-methyl-2′-butenyl)anilines 52 and 51 , respectively (Scheme 15), i.e. as one would expect the diene with its low lying LUMO is a better partner for an EDA complex than the double bond of the allylic side chain.     

Photochemistry of the SO2, 2-pentene system at 3660 Å

The photolysis of SO₂ in the presence of cis- and trans-2-pentene has been investigated at 3660 Å and 22°C. Quantum yield measurements of the SO₂ photosensitized conversion of one isomer into the other are consistent with a mechanism in which the only participating excited electronic state of SO₂ is the SO₂(³B₁) state. Quantum yield measurements were made for a variation in P_SO2/P_isomer reactant ratios of 4.01–283 and 57.5–351 for the cis and trans isomers, respectively. The data are consistent with a mechanism in which a (SO₂-olefin)³ collision intermediate is the precursor to the photosensitized isomeric products. The intermediate undergoes unimolecular decay to yield the cis and trans isomers with probabilities of 0.26 ± 0.05 and 0.69 ± 0.04, respectively. Estimates of the quenching rate constants at 22°C for removal of SO₂(³B₁) molecules by cis- and trans-2-pentene are (0.633 ± 0.125) × 10¹¹ l./mole/sec and (1.00 ± 0.27) × 10¹¹ l./mole/sec, respectively. An experimentally determined photostationary composition, [trans-2-pentene]/[cis-2-pentene] = 2.3 ± 0.1 was in fair agreement with that of 1.7 ± 0.7 as predicted from kinetic data derived in this study.     

Synthesis, 1H- and 13C-NMR spectra,crystal structure and ring openings of 1-methyl-6,9-epoxy-9-aryl-5,6,9,10-tetrahydro-1H-imidazo [3,2-e] [2H-1,5] oxazocinium methanesulfonate

The methanesulfonic acid catalyzed reaction of 1-(4-chloro- and 2,4-dichlorophenyl)-2-(1-methyl-2-imida-zolyl)ethanones 1a and 1b with glycerol produced cis- and trans-{2-haloaryl-2-[(1-methyl-2-imidazolyl)methyl]-4-hydroxymethyl}-1,3-dioxolanes 2a and 2b with a 2:1 cis/trans ratio. Besides these five-membered ketals, the reaction of 1a with glycerol afforded a small amount of trans-{2-(4-chlorophenyl)-2-[(1-methyl-2-imidazolyl)methyl]-5-hydroxy}-1,3-dioxane ( 3a , 7%). The reaction of methanesulfonyl chloride with cis-1 formed the corresponding methanesulfonates, cis- 4 , which rapidly cyclized to the title compounds 5 . Base-catalyzed ring opening of 5 furnished 1-methyl-5,6-dihydro-6-hydroxymethyl-8-(4-chloro- and 2,4-dichlorophenyl)-1H-imidazo[3,2-d][1,4]oxazepinium methanesulfonates 7 . Acid-catalyzed hydrolyses of 5 or 7 provided 1-methyl-2-[(4-chloro- and 2,4-dichloro)phenacyl]-3-[(2,3-dihydroxy)-1-propyl]imidazolium salts 12 . Structure proofs were based on extensive ¹H and ¹³C chemical shifts and coupling constants and structures of 3a and 5a were confirmed by single crystal X-ray crystallography.     

A study of the protonation and alkylation oft-butyl isocyanide intrans-[M(CNBu-t 2(Ph2PCH2CH2PPh2)2] (M = Mo or W): Formation of carbyne-type complexes and theirtrans- tocis-isomerization

Summary Treatment of complexestrans-[M(CNBu-t)₂(dppe)₂][(1) M = Mo or W, dppe = Ph₂PCH₂CH₂PPh₂] with protic acid gives a mixture of the aminocarbyne complexestrans- pluscis-[M(CNHBu-t)(CNBu-t)(dppe)₂]⁺ (2) and the hydridocompounds [MH(CNBu-t)₂(dppe)₂]⁺ (3), whereas reaction with an alkylating agent (R⁺) appears to give the dialkylaminocarbyne compounds [M(CNRBu-t)(CNBu-t)(dppe)₂]⁺ (4) also as a mixture of thetrans andcis isomers.     

Nucleophilic Substitution at Tetracoordinate Phosphorus. Stereochemical Course and Mechanisms of Nucleophilic Displacement Reactions at Phosphorus in Diastereomeric cis- and trans-2-Halogeno-4-methyl-1,3,2-dioxaphosphorinan-2-thiones: Experimental and DFT Studies

Geometrical cis- and trans- isomers of 2-chloro-, 2-bromo- and 2-fluoro-4-methyl-1,3,2-dioxaphosphorinan-2-thiones were obtained in a diastereoselective way by (a) sulfurization of corresponding cyclic P^III-halogenides, (b) reaction of cyclic phosphorothioic acids with phosphorus pentachloride and (c) halogen–halogen exchange at P^IV-halogenide. Their conformation and configuration at the C₄-ring carbon and phosphorus stereocentres were studied by NMR (¹H, ³¹P) methods, X-ray analysis and density functional (DFT) calculations. The stereochemistry of displacement reactions (alkaline hydrolysis, methanolysis, aminolysis) at phosphorus and its mechanism were shown to depend on the nature of halogen. Cyclic cis- and trans-isomers of chlorides and bromides react with nucleophiles (HO⁻, CH₃O⁻, Me₂NH) with inversion of configuration at phosphorus. DFT calculations provided evidence that alkaline hydrolysis of cyclic thiophosphoryl chlorides proceeds according to the S_N2-P mechanism with a single transition state according to the potential energy surface (PES) observed. The alkaline hydrolysis reaction of cis- and trans-fluorides afforded the same mixture of the corresponding cyclic thiophosphoric acids with the thermodynamically more stable major product. Similar DFT calculations revealed that substitution at phosphorus in fluorides proceeds stepwise according to the A–E mechanism with formation of a pentacoordinate intermediate since a PES with two transition states was observed.     

Interactions intramoléculaires: XXIII—Constantes de couplage et interactions gauches avec un groupement t-butyle (cyclohexanones disubstituées 3, 4)

The ¹H NMR spectra of a series of cis and trans-3R,4 X-cyclohexanones (-2,2,6,6-d₄) are analysed. By comparison of their ³J coupling constants with those of cyclohexane homologues we obtain information about the chair–chair equilibrium constants for R = CH₃, X = CN, the chair structure of cis isomers with an equatorial t-butyl group, and a conformational heterogeneity with trans (CH₃)₃C and CN groups. This latter situation is analysed by means of a simplified but controlled Karplus relationship, on the basis of a mixture of two conformers; this involves a diequatorial chair and a boat form with a dihedral angle Φ₃₄ of about ?6°.     

Direct and indirect substituent effects on 1J(CH) in vinyl derivatives and related compounds

The variation in the one–bond couplings ¹J(CH) in vinyl derivatives with substituent has been examined. For the geminal proton ¹J correlates very badly with substituent electronegativity but extremely well with σ_I, if conjugating substituents are excluded. In the case of halogen substituents the marked stereospecificity of ¹J(CH) for the cis and trans protons can be rationalised in terms of an intrinsic dependence of π_CH on the dihedral angle between the coupling atoms and the perturbing substituent, with an additional positive increment to the cis coupling due to direct interaction of the substituent non-bonding electrons or to orbital circulation of the substituent electrons. The intrinsic specificity of β-substituent effects on ¹J(CH) is also found in analogous compounds containing C?N and C?O bonds.     

Kinetics of the reactions of O3 and OH radicals with a series of dialkenes and trialkenes at 294 ± 2 K

Rate constants for the gas phase reactions of O₃ and OH radicals with 1,3-cycloheptadiene, 1,3,5-cycloheptatriene, and cis- and trans-1,3,5-hexatriene and also of O₃ with cis-2,trans-4-hexadiene and trans -2,trans -4-hexadiene have been determined at 294 ± 2 K. The rate constants determined for reaction with O₃ were (in cm³ molecule^-1s^?1 units): 1,3-cycloheptadiene, (1.56 ± 0.21) × 10^-16; 1,3,5-cycloheptatriene, (5.39 ± 0.78) × 10^?17; 1,3,5-hexatriene, (2.62 ± 0.34) × 10^?17; cis?2,trans-4-hexadiene, (3.14 ± 0.34) × 10^?16; and trans ?2, trans -4-hexadiene, (3.74 ± 0.61) × 10^?16; with the cis- and trans-1,3,5-hexatriene isomers reacting with essentially identical rate constants. The rate constants determined for reaction with OH radicals were (in cm³ molecule^?1 s^?1 units): 1,3-cycloheptadiene, (1.31 ± 0.04) × 10^?10; 1,3,5-cycloheptatriene, (9.12 × 0.23) × 10^?11; cis-1,3,5-hexatriene, (1.04 ± 0.07) × 10^?10; and trans 1,3,5-hexatriene, (1.04 ± 0.17) × 10^?10. These data, which are the first reported values for these di- and tri-alkenes, are discussed in the context of previously determined O₃ and OH radical rate constants for alkenes and cycloalkenes.     

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.