5552
J. Phys. Chem. 1987, 91, 5552-5554
Investigatlon of the Solute- Induced Phase of trans ,trans-4'-n-Butyl-l ,l'-bicyclohexyl-4-carbonitrile by Deuterlum Magnetic
Resonance and Solute Photoreactivity Parameters' Richard L. Treanor and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D C 20057 (Received: July 6, 1987)
A comparison between our previously determined photoproduct ratios from four alkylphenones and their quadrupolar deuterium splittings in the mesophases of trans,trans-4'-n-butyl- 1,1'-bicyclohexyl-4-carbonitrile (BCCN) shows that the unusual phase of the solvent exists as pools of ordered aggregated solvent molecules around solute molecules. The pools, moving as a unit, cause the solutes to appear to be in a disordered environment according to deuterium magnetic resonance parameters. The photochemical results affirm that this is not the case. A model which is compatible with both results is discussed.
Introduction During our investigations of the influence of liquid-crystalline solvent order on the dynamics of reacting solutes,' we found that the Norrish I1 photoprocesses (Scheme I) of each of the alkylphenones 1-4 are affected differently by the mesophases of trans,truns-4'-n-butyl- 1,l'-bicyclohexyl-4-carbonitrile(BCCN) .2 Work by others and us with BCCN2*3and related solvents4 has emphasized the close structural relationship that must be maintained between a solute and the solvent molecules in order that mesophase order lead to reaction selectivity.
Recently, Fung and Gangoda5 reported that small amounts of a solute, like benzene, can induce a plastic-crystalline6" or cubic-crystalline phase6b (p phase) of BCCN. Its p phase occurs in the temperature range where the neat material is smectic and solid. Usually, plastic or cubic phases consist of molecules which are individually disordered rotationally or translationally.6 This does not appear to be the case for solutions of BCCN. Initially, we did not believe that formation of p phases was induced by 1-4 in BCCN since no discontinuity in their E / C product ratios could be detected at the temperatures where the phase change was expected to occur. However, upon reexamination of 1-4 (deuteriated at the carbons cy to the carbonyl groups) (1) Part 28 in our series, "Liquid-Crystalline Solvents as Mechanistic Probes". For part 27, see: Nuiiez, A,; Weiss, R. G.J . Am. Chem. SOC.,in press. (2) Zimmermann, R. G.;Liu, J. H.; Weiss, R. G.J. Am. Chem. Soc. 1986, 108, 5264. (3) Leigh, W. J.; Jakobs, S . Tetrahedron 1987, 43, 1393. (4) (a) Samori, B.; De Maria, P.; Mariani, P.; Rustichelli, F.;Zani, P. Terruhedron 1987, 43, 1409. (b) Solladie, G.; Gottarelli, G.Ibid. 1987, 43, 1425. (c) Treanor, R. L.; Weiss, R. G. Ibid. 1987, 43, 1371. (d) Anderson, V. C.; Weiss, R. G . J. Am. Chem. SOC.1984, 106, 6628. ( 5 ) (a) Fung, B. M.; Gangoda, M. J . Am. Chem. SOC.1985, 107, 3395. (b) Gangoda, M.; Fung, 8. M. Chem. Phys. Lett. 1985, 120, 527. (6) (a) Dunning, W. J. In The Plastically Crystalline Stare; Sherwood, J. N., Ed.; Wiley: Chichester, 1979 p 1. (b) Charvolin, J. J. Phys. (12s Ulis, Fr.) 1985, C3, 173.
SCHEME I: Norrish I1 Reactions of an Alkylphenone Showing Elimination (E) and Cyclization (C) Products
in BCCN by deuterium magnetic resonance (2H N M R ) spectroscopy, we have discovered that p phases do form and that they retain control over intramolecular solute motions.
Experimental Section BCCN (Licristal ZLI-1538) was used as received from Merck. By gas chromatographic analysis, it was >99.5% pure. pCyclohexylacetophenone (5, Aldrich, 99%) was recrystallized from ethanol. Ketones 1-4 were available from our previous work., They and 5 were deuteriated at the positions LY to their carbonyl groups (>90% by 'H N M R analysis) by treatment with D 2 0 / Na2C03in monoglyme.' n-Dodecane-d,, was used as received from MSD Isotopes. Ketone solutions in BCCN for photochemical2and spectroscopic studies were usually ca. 2% (w/w) and gave no evidence for solventsolute phase separations or the presence of microcrystallites by optical microscopy (Kofler hot-stage microscope with light polarizers): patterns observed at nontransition temperatures were homogeneous and representative of the neat solvent. Phase transition temperatures for the solutions are summarized in Table I. They were measured on the hot-stage microscope and are corrected. 2H N M R and proton-decoupled I3C spectra were recorded on a Bruker AM-300 WB spectrometer with an Aspect 3000 computer. The details of the 2H N M R pulsing sequence are described elsewhere.8 All samples with BCCN were aligned rapidly (
