. Can. J . Chem. 70, 272 ( 1992). Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane ( l b cis); 1,4-dicyanobenzene (2), an electronaccepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies >99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of l b cis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion 2;, and reprotonation of the benzylic anion to give both the cis and the tratls isomers of lb. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of l b cis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon-hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 3 1 % effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trcrns isomer. This proposed mechanism is supported by Stern-Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile-methanol-0-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trat~s isomer is stable under these conditions.Key words: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3). conduit a une isomerisation configurationnelle du cyclopentane; 1'Ctat photostationnaire favorise I'isorqere trans i plus de 99%. Le mecanisme propose pour cette reaction implique la formation du cation radicalaire du produit l b cis par le biais d'un transfert d'electron photo-induit vers 1'Ctat excite singulet du produit 2, une deprotonation du cation radicalaire assistee par la base 3 , la reduction, par I'anion radicalaire 2;; du radical benzylique qui en resulte et un reprotonation de l'anion benzylique qui fournit les isomeres cis et trans du produit l b . L'etat photostationnaire est contr61C par les vitesses relatives de deprotonation des cations radicalaires des produits l b cis et trans; ces vitesses dependent du degre de recouvrement de la liaison carbone-hydrogene benzylique et des orbitales molCculaires SO du cation radicalaire qui sont trks associkes au noyau phknyle. On a fait appel a des calculs de mkcanique molCculaire (MM3 et MMP2) pour calculer les conformations privilCgiCs des isomkres. Le recouvrement orbitalaire requis est efficace a 3 1% dans la conformation globale minimale de I'isomkre cis et est ...

The photosensitized (electron transfer) bond cleavage of some β-phenylethyl ether radical cations has been investigated. In previous studies the feasibility of the bond cleavage was thought to depend on the bond dissociation energy (BDE). However, this simple hypothesis led to several incorrect predictions and therefore additional criteria, conformational effects, were added to the hypothesis. This study has now been extended and additional examples of the importance of the conformation on the carbon–carbon bond cleavage of radical cations are provided. The four β-phenylethyl ethers studied are 2-methoxy-3-phenylbutane (9, both diastereomers), and cis- and trans-2-methyl-3-phenyltetrahydropyran (10c, 10t). Generally, bond cleavage will occur if the (calculated) BDE in the radical cation is less than 55 kJ/mol, and if there is significant overlap between the singly occupied molecular orbital (SOMO) and the vulnerable (C—C or C—H) bond. In the case of a β-phenylethyl ether radical cation, the alkoxy group must also be oriented so that an oxygen lone pair of electrons can overlap with the C—C sigma antibonding (σ*) orbital. The calculated BDE values of the vulnerable C—C bond in the radical cations of the four ethers studied here are well below the threshold value, 55 kJ/mol, and C—C cleavage will therefore be governed by conformational effects. Molecular mechanics (MM3) calculations were used to identify the most stable conformers of the neutral molecules. Based on the calculated angles and overlap between orbitals it was predicted that the global-minimum conformers of the ethers 9 and 10c would not give C—C bond cleavage products or deprotonation to any significant extent. The global minimum of ether 10t is well oriented for C—C cleavage but not for deprotonation. Irradiation of an electron-accepting photosensitizer, 1,4-dicyanobenzene (2), in the presence of the ethers showed that the ethers 9 did not cleave efficiently; no deprotonation or isomerization was observed. This is in good agreement with the predictions based on the MM3 calculations. Both ethers 10c and 10t gave reasonable yields of the C—C cleavage products; in fact, ether 10c cleaved more efficiently than 10t. This can be explained by the fact that a conformer of 10c, only 4.35 kJ/mol higher in energy than the global minimum, is perfectly aligned for cleavage. Ether 10t did not show any evidence for deprotonation whereas 10c did. This is also in good agreement with the calculations. Keywords: photoinduced electron transfer, radical cations, bond cleavage, conformation, molecular mechanics calculations.

. Can. J. Chem. 69, 225 (1991). The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-l,4-dioxaspiro[4,5]decane (8), methyl cis-and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-l,4-dioxaspiro[4.5]decane (lo), generated by photosensitized (electron transfer) irradiation, has been studied. Solutions of the ethers and acetals in acetonitrile-methanol (3:1), with 1,4-dicyanobenzene (2) serving as the electron acceptor, were irradiated with a medium-pressure mercury vapour lamp through Pyrex. The diphenyl derivatives 7 and 8 were reactive; 7 gave 6,6-diphenylhexanal dimethyl acetal(11) and 8 gave 2-methoxy-2-(5,5-diphenylpenty1)-1,3-dioxolane (12). These are the products expected from the intermediate 1,6-radical cation, formed upon carbon-carbon bond cleavage of the cyclic radical cation.The monophenyl derivatives 9cis and trans and 10 were stable under these irradiation conditions. The mechanism for the carbon-carbon bond cleavage and for the cis-trans isomerization is discussed. An explanation, based upon conformation, is offered for the lack of reactivity of 9 and 10. Molecular mechanics (MM2) calculations were used to determine the preferred conformation of 9cis and trans, and 10.Key words: photosensitization, electron transfer, radical cation, carbon-carbon bond cleavage, conformation. Chem. 69, 225 (1991). On a CtudiC la rCactivitC des cations radicalaires du mkthoxy-2,2,-diphCnylcyclohexane (7), du 6,6-diphCnyl-l,4-dioxaspiro [4,5] Introduction Photosensitized (electron transfer) carbon-carbon bond cleavage of five-membered cyclic compounds generates 1,5-radical cations (2). When the irradiation is carried out in the presence of a nucleophile, often the solvent, further reaction of these intermediates proceeds by nucleophilic attack on the cationic site, and reduction of the radical site by electron transfer from the radical anion of the ~hotosensitizer. followed by protonation of the resulting anion 12). A typical example is shown in reaction [I]: irradiation of an acetonitrile-methanol (3: 1) solution of methyl 2,2-diphenylcyclopentyl ether (1) and 1,4-dicyanobenzene (2) (an electron-accepting photosensitizer)

The heat capacities of KOH and its deuterated analog KOD have been measured over the temperature range 17<T<342 K and 32<T<295 K, respectively. The thermodynamics of the phase transformations to the low-temperature antiferroelectric phases have been characterized: Ttr (KOH)=226.7±0.2 K; Ttr (KOD)=253.1±0.1 K; ΔStr (KOH)=0.121±0.002 R; ΔStr (KOD)=0.1258±0.0007 R. The thermodynamic changes are consistent with tunneling of the hydroxide (deuteroxide) ions between two minima in a double-well potential.

This paper is dedicated to Professor James A. Morrison MARY ANNE WHITE and ALLYSON PERROTT. Can J. Chem. 66, 729 (1988) The heat capacity of tert-butylaldehyde, (CH3)3CCH0, has been measured as a function of temperature from 29 to 298 K, to investigate solid-state polymorphism. There are two solid-solid phase transitions in this material, T,, = 158.5 and 183.9 K, AS,, = 0.402R and 3.153R respectively. The thermodynamic parameters for the transitions are consistent with mechanisms proposed on the basis of recent nuclear magnetic resonance investigations of this material.MARY ANNE WHITE et ALLYSON PERROTT. Can J. Chem. 66, 729 (1988) Afin d'ttudier le polymorphisme a l'ttat solide, on a mesurt la capacitt calorifique du tert-butylaldthyde, (CH3)3CCH0, en fonction de la temptrature de 29 a 298 K. Avec ce produit, on a observt deux transitions de phase solide-solide, T,, = 158,5 et 183,9 K, AS,, = 0,402R et 3,153R. Les paramktres thermodynamiques pour les transitions sont en accord avec les mtcanismes proposts en se basant sur des etudes rtcentes en resonance magnttique nucltaire de ce compost.[Traduit par la revue]