This paper is dedicated to Professor Ronald J . Gillespie on the occasion of his 65th birthday BRIAN J. FAHIE and WILL~AM J. LEIGH. Can. J. Chem. 67, 1859Chem. 67, (1989. The photochemistry of 1,3,3-trimethylcyclopropene and 1-tert-butyl-3,3-dimethylcyclopropene has been investigated in hydrocarbon, methanol, and 1-hexene solution with far-ultraviolet (185-228 nm) light. Direct photolysis of the two compounds affords allene, alkyne, and 1,3-diene derivatives, formally as a result of initial bond cleavage to yield vinylcarbene intermediates. Products derived from cleavage of the most substituted (Cl-C3) cyclopropene bond account for 60-80% of the observed mixture in each case. Results from the photolysis of 1 ,3,3-trimethylcyclopropene-1-13C suggest a second pathway for formation of alkyne products in cyclopropene photochemistry, which occurs in competition with (or by exclusion of) the vinylcarbene

. Can. J. Chem. 65, 2734Chem. 65, (1987.The photosensitized (electron transfer) reactions of 3-phenyl-2.3-dihydrobenzofuran @a), 5-methyl-3-phenyl-2,3-dihydrobenzofuran (8b), cis and trans-2-methoxy-1-phenylindane (9, cis and trans), 3,3-diphenyltetrahydrofuran (lo), and 22-diphenyl-1-methoxycyclopentane (11) have been studied using 1,4-dicyanobenzene as an electron-accepting photosensitizer and acetonitrile-methanol (3: 1) as solvent. These reaction conditions cause carbon-carbon bond cleavage of analogous acyclic P,P-diphenylethyl ethers to give products derived from the diphenylmethyl radical and the a-oxycarbocation intermediates. The purpose of this study was to determine if this reaction could be applied to five-membered cyclic derivatives to give 1,5-radical cations.The primary products from 8 a and 8b were the dehydrogenated, aromatized 3-phenylbenzofurans 140 and 14b. These products react further; continued irradiation gave the methanol adducts, cis and tmns-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a and 15b, cis and trans). The only observed reaction of the indanes (9, cis and trans) was cis-trans isomerization. Deuterium was incorporated at the bis-benzylic position of 8 and 9 when the irradiation was carried out in acetonitrilemethanol-0-d. These results are consistent with reversible deprotonation from the radical cations. 'There was no evidence for carbon-carbon bond cleavage with either 8 or 9. The relative rate, deprotonation faster than carbon-carbon bond cleavage, is explained in terms of the conformation of the bond that cleaves in relation to the singly occupied molecular orbital (SOMO) of the radical cation. Oxidation potential measurements support the conclusion that the SOMO of 8 and 9 is largely associated with the fused phenyl ring and is therefore orthogonal to the benzylic carbon-carbon bond. Irradiation of cis or trans-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a, cis or trans), under these conditions, leads to cis-trans isomerization. The mechanism in this case involves the reversible loss of methanol. There is evidence that the addition of methanol to 14 involves the sensitizer radical anion -14 radical cation pair.In contrast with the fused bicyclic systems, the monocyclic tetrahydrofuran 10 and the methoxycyclopentane 11 both cleave under these conditions; the products are the expected acetals 22 and 29 formed from the intermediate 1,5-radical cations. In 10 and 11 the SOMO, which is largely associated with the diphenylmethyl moiety, can overlap with the adjacent carbon-carbon bond and cleavage occurs as in analogous acyclic systems. Both 10 and 11 are relatively stable to irradiation under conditions that are identical except with acetonitrile as solvent (without methanol). We found no evidence for cyclization of the intermediates (1,hadical cation or 1,5-diradical) into the terminal phenyl ring. DONALD R. ARNOLD, BRIAN J. FAHIE, LAURIE J. LAMONT, JACEK WIERZCHOWSKI et KENT M. YOUNG. Can. J. Chem. 65, 2734Chem. 65, (1987.Utilisant le dicyano-1,4 benzkne comme photosen...

. Can. J. Chem. 69, 1189 (1991). In many applications of NMR spectroscopy to chemistry, quantitation plays a key role. This paper argues that peak heights not only can be used for quantitative work in NMR but that for careful work they should be used. Integrals are accurate, but because they depend on many subjective factors, their precision is suspect. Peak heights are very reproducible on modem spectrometers and so, given this precision, the accuracy can be obtained as well by using calibration methods. An example of the application of these methods is given in the case of a 13c labelling experiment that elucidates a photochemical transformation. Introduction NMR is certainly well known as one of the best methods for qualitative analysis: the identification of a molecule. Analysis of 13C and 'H spectra can often solve a structure completely, and the development of two-dimensional NMR methods has dramatically advanced this process (1, 2). These methods are based on spectral analysis, i.e., the positions of the lines in the spectrum, but quantitative analysis has also been an important part of NMR spectroscopy (3, 4). Counting protons using integrals is a very familiar part of proton NMR. Theory says that in the absence of saturation, the integral over a spectral line is directly proportional to the number of spins and the proportionality is the same for all lines in the spectrum. This simple quantitation can be extended and applied so that accurate and precise integration of NMR spectra is important in many fields (5-10). Measurements of enantiomeric and diastereomeric excess are crucial in the design of chiral synthesis. Particularly in isotopic labelling studies, such as the one described in this work, NMR is one of the few methods than can give a detailed picture of the mechanism of the reactions. The important data in this case are relative enrichments at different sites in the molecule (6, 7), so separations are impossible, and mass spectrometry is not selective enough to identify individual sites. However, the validity of these results depends crucially on the precision and accuracy of the quantitation of the NMR data.Normal methods of quantitation using integration are not without problems (3,4,11,12). In practice, the process involves several subjective decisions on the part of the operator. The choice of integration region, the phasing of the spectrum, and the correction of the baseline are all under the control of the operator, and each may profoundly affect the value of the integral. Careful work with good signal-to-noise ratios and standard conditions can minimize the variation of the results, but there will still be a random component due to the operator's particular choices as well as possible biases between operators (12). Typical estimates of the reliability of an integral are 3-5%

. Can. J. Chem. 69, 1435 (1991).The photochemistry of P-phenyl-4-methoxypropiophenone (1) in the isotropic and liquid crystalline (nematic and smectic B) phases of trans,trans-4'-alkyl-(l,l'-bicyclohexyl)-4-carbonitrile (CCH-n) and trans-4-alkylcyclohexyl-trans-4'-alkylcyclohexylcarboxylate (0s-nm) mesogens has been investigated using nanosecond laser flash photolysis techniques. Solubility limits of 1 as a function of temperature in the smectic phases have been rigorously determined by 'H NMR spectroscopy, using a-, methoxy-, and P-phenyl-deuterated analogs of the ketone. Triplet decay of 1 in the smectic (Crystal B) phase of CCH-4 is multiexponential, suggesting that the ketone is solubilized in a distribution of conformations in the highly-ordered liquid crystalline lattice. The average triplet lifetime of 1 at various temperatures throughout the smectic phase is about a factor of three longer than that in homologous nematic and isotropic phases at the same temperature. The differences in the Arrhenius parameters for triplet decay in the smectic and nematiclisotropic phases are very small, however. The present results for 1 in the smectic phase of the CCH-n mesogens correct the interpretation of previously reported data obtained with higher concentration mixtures, and afford an accurate indication of the true effects of smectic B liquid crystalline order on the conformation-dependent triplet state behavior of this molecule. In 0s-nm liquid crystals, triplet decay follows clean first order kinetics in all phases, but the variations in Arrhenius dependence with solvent phase are similar to those observed in the CCH-n liquid crystals.Key words: smectic, liquid crystals, 'H NMR, nanosecond laser flash photolysis, intramolecular triplet quenching, photochemistry. On a CtudiC la photochimie de la P-phknyl-4-mCthoxypropiophCnone (1) dans les phases isotropes et cristalline liquide (nkmatique et smectique) des mCsogbnes trans,trans-4'-alkyl-(1,1-bicyclohexyl)-4-carbonitrile (CCH-n) et trans-4-alkylcyclohexyl-trans-4'-alkyl-cyclohexanecarboxylate (0s-nm) en utilisant la technique de photolyse Cclair avec un laser ?i nanoseconde. On a dCterminC rigoureusement par la rmn du 'H, la limite de solubilitC du composC 1 en fonction de la tempkrature dans la phase smectique, en utilisant les analogues a-mCthoxy et P-phCnyl deutCriCs de la cCtone. La dtcroissance du triplet du compost 1 dans la phase smectique (cristal B) du CCH-4 est multiexponentielle, suggkrant ainsi que la cCtone est solubiliske avec une distribution de conformations dans le rCseau cristallin liquide fortement ordond. La durCe de vie moyenne du triplet du compost 1 ?i diffkrentes temptratures ?i travers la phase smectique est environ trois fois plus longue que celle du triplet dans les phases homologues ntmatique et isotrope i la m&me temptrature. Cependant la diffkrence dans les parambtres d'ArrhCnius pour la dCcroissance du triplet dans les phases smectique et nCmatiquelisotrope, est trks faible. Les rksultats actuels relatifs au compost 1 dans la phase s...

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