NOTES
1395
Y
Gas-Phase Photolysis a t 1470 A of Mixtures of Cyclohexane with Benzene and with
0.8.
Nitrous Oxide a t 750 Torr 0.6 .
by Robert R. Hentz and D. B. Peterson Department of Chemistry and the Radiation Laboratory,1 University of Notre Dame, Notra Dame, Indiana /6666 (Receiued September I&,1969)
h
I"
0.4.
Y
8
Holroyd and coworkers2 have observed that both benzene and NZO as solutes reduce cb(H2) from photolysis of liquid cyclohexane a t 1470 A and have attributed this reduction to energy transfer from excited cyclohexane to the solute. Hentz and Knighta studied hydrogen quantum yields in the 1470-A photolysis of c-C~HIZ-C~H~ and c-C6Hlz-N~O mixtures in the gas phase at total pressures of 1 and 70 Torr. They concluded that the results show no evidence of energy transfer from that state of cyclohexane initially excited. Consequently, an upper limit of 0.2 nsec was estimated for the lifetime, 7 , of that state of cyclohexane initially excited a t 1470 A in the gas phase. It was noted3 that for T < 0.2 nsec, the results of Holroyd, et require specific rates of transfer from cCeH12*in the liquid to benzene and NzO of k > 1.6 X 10'0 M-' sec-l and k > 3.4 X 1O'O M-l sec-', respectively. Thus, results of the gas-phase photolyses at 70 Torr do not permit an unambiguous decision as to whether or not H2 formation occurs from the same excited state of cyclohexane in both the gas and liquid phases. Consequently, a closer estimate of the gasphase T was sought in the present work by extension of the previous study of 6(Hz) in the 1470-A photolysis of c-C~HI~-C&&, and C-CeHlz-NzO mixtures to a total pressure of 750 Torr at 95". Sources and methods of purification of cyclohexane, benzene, and N20 have been d e s ~ r i b e d . As ~ ~ ~in previous worksn4 the light source was a microwavepowered Xe resonance lamp but with a sapphire window. The only significant emission below 2500 A was at 1470 A. The lamp was sealed into a 600-ml Pyrex reaction cell in such a way that the window extended well into the interior of the cell. The cell contained a magnetically driven stirrer to provide efficient mixing of gases near the window. Gases were introduced or collected through a stainless steel valve which connected the cell to a vacuum manifold. The cell and adjoining valve were enclosed in a small oven with which the system was maintained at 95" during photolysis. Collection and measurement of gas noncondensable a t 77"K314 and gas chromatographic analysis of such gas for H+ have been described. Pure N2O at 20 Torr and room temperature was used as an actinometer with ~ ( N z 02) = 1.6.6
+
Fc Figure 1. $(Ha) in 1470-A photolysis of cyclohexane mixtures a t 750 Torr and 95' os. Fa, the fraction of light absorbed by cyclohexane: 0, c-CaHl&Ha; 0 , c-C~HIZ-NZO. The broken curve is calculated as described in the text.
Intensities varied somewhat throughout the course of the work but were usually about 5 X lOI4 quanta sec-'. Actinometry immediately preceded each photolysis. There was no significant decrease in intensity during any one photolysis. A slight intensity decrease did occur after a number of photolyses owing to buildup on the lamp window of polymer which was removed periodically by photolysis of oxygen in the cella4 Reduced polymer deposition on the window in this work may be a result of stirring (absent in the earlier work804) or the higher temperature. The maximum conversion in pure cyclohexane was ~ l O - ~ o J o . Conversions in all experiments were such that less than 10% of the H atoms produced would be scavenged by unsaturated products. Values of +(HJ were calculated without correction for an estimated decrease of -6% in transmission of the sapphire window at 95".' Such values of cb(H2) are presented in Figure 1 as a function of the fraction of light Fo absorbed by cyclohexane in the mixtures with benzene or NzO at 95" and a total pressure of 750 Torr. As in the previous study,$values of Fa were calculated (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC Document No. COO-38-695. (2) R. A. Holroyd, J.Phys. Chem., 72,759 (1968): J. Y. Yang, F. M . Servedio, and R. A. Holroyd, J . Chem. Phys., 48,1331 (1968). (3) R. R. Hentz and R. J. Knight, J . Phys. Chem., 72,4684 (1968). (4) R. R. Hentz and S. J. Rzad, ibid., 71,4096 (1967). (5) R.R. Hentz and R. J. Knight, ibid., 72,1783 (1968). (6) J. Y. Yang and F. M. Servedio, J . Chem. Phys., 47, 4817 (1967). (7) A. H. Laufer, J. A. Pirog, and J. R. McNesby, J . Opt. SOC.Am., 55, 64 (1965). Volume 74, Number 6 March 19, 1970
1396
NOTES
using 408,8 15gj8 and 1109 atm-' cm-l as decadic extinction coefficients (measured at 25') for cyclohexane, benzene, and N20, respectively. The solid line in Figure 1 indicates the behavior expected of cp(H2) in the absence of any kind of interaction between the mixture components. Values of 4(H2) for c-C&t12-C6H6 mixtures fall below the straight line expected for no interaction between the components. The broken curve in Figure 1 is based on the assumption that energy transfer from cd6H12* to C6H6 is solely responsible for the deviac tion from linearity. The curve was calculated by use of T = 4.7 x sec in eq 1
4 = 4OFc/(l
+ 27)
(1)
in which r$o represents the quantum yield for pure cyclohexane (Fc = 1) and 2 the frequency of collision of c-C6H12*with the energy acceptor,1° in this case C6H6. The value 7 = 4.7 X 10-l' sec is necessarily an upper limit because (1) the collision diameters are probably a lower limit for excitation-transfer diameters" and (2) H-atom scavenging by CsH6, in competition with abstraction from c-c~H12, is expected to contribute to deviation of 4(H2) from linear it^.^ Indeed, with the assumption that H atoms contribute -20% to 4O((H2)I2 and with a reasonable estimate of the specific rate of scavenging relative to that of abstraction at 95') a curve can be calculated (for no energy transfer) that fits the data about as well as the broken curve in Figure 1. As noted in the earlier complications associated with H-atom reactions should be absent in the c-C&l12-N20 mixtures because all H are expected to react with c-ci"~ over the range of mixture compositions used. The results presented in Figure 1 for such mixtures at 750 Torr and 95" show no evidence of energy transfer from c - C ~ H ~to~the * potential acceptor N20.2 An upper limit for 7 can be estimated with the reasonable assumption that a deviation from linearity corresponding to 4/cp°Fc < 0.9 at Fc = 0.4 should have been manifest (ie., not obscured by experimental errors) in a plot such as that of Figure 1. Thus, from eq 1 with 4/4OFc > 0.9 and 2 = 5.1 X lo9 sec-' at FC = 0.4, T = 2 X lo-" sec is obtained as an upper limit. We conclude from the results shown in Figure 1 that absorption of a 1470 - A photon by cyclohexane in the gas phase produces an excited state with a lifetime that is certainly less than 4.7 x lo-" sec and probably less than 2 X lo-" sec. It is clear from present results that Hz formation cannot occur from the same excited state of cyclohexane in both the gas and liquid photolyses at 1470 8. For example, if the results in Figure 1 are attributable to energy transfer alone, then the excited state involved must transfer energy more efficiently to benzene than to N2O; however, such behavior is contrary to that observed in the liquid phase.2 Furthermore, even for T = 4.7 X lo-" sec, T h e Journal of Physical Chemistry
the results of Holroyd and coworkers2 require specific rates of transfer from c-C6&* in the liquid to CBHG and N2O of 6.8 X 1OO ' M-' sec-l and 14.5 X 1Olo M-' sec-l, respectively. The simplest interpretation of present results is that (1) Hz formation in the gas phase occurs from that state of cyclohexane produced by absorption of a 14708 photon and (2) Hz formation in the liquid phase occurs from a lower, longer-lived state of excitation. Such an interpretation is consistent with recent conclusions of Hirayama and Lipskyla from their observations on the fluorescence of saturated hydrocarbons. Fluorescence was observed from alkanes, including cyclohexane, when excited as neat liquids at wavelengths in the range 1470-1720 A. With excitation in the liquid at 1470 8, sensitization of benzene fluorescence was shown to accompany quenching of cyclohexane fluorescence. However, no fluorescence was observed on excitation of alkane vapors at 1470 A. Excitatitn of the vapor at wavelengths longer than -1600 A produced an emission which, except for a slight blue shift, was identical with the liquid fluorescence. The authors conclude that both liquid and vapor emissions are of molecular origin with the upper state being strongly predissociated in the vapor above -7.7 eV . (8) Values for cyclohexane and benzepe were obtained by S. Lipsky with a high-pressure Ar lamp at 1467 A and were privately communicated. (9) M.Zelikoff, K.Watanabe, and E. C. Y. Inn, J . Chem. Phys., 21, 1643 (1953). (10) Collision frequencies were calculated using 6.1, 5.3, and 3.9 k as the collision diameters of c-CeH~z,C&, and NzO, respectively; cf., J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids," John Wiley & Sons, Inc., New York, N. Y., 1954, p 1111. (11) T.Watanabe, Advances in Chemistry Series No. 82, American Chemical Society, Washington, D. C., 1968, p 176. (12) Hentz and Rzad (ref 4) have made a rough estimate of -10% for the H-atom contribution. (13) F. Hirayama and S. Lipsky, J . Chem. Phys., 51,3616 (1969).
Nuclear Magnetic Resonance Study of Solvent Effects on Hydrogen Bonding in Methanol1
by William B. Dixon2 Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received October 26,1989)
For the past decade or more, nmr techniques have been very fruitfully applied to the study of hydrogenbonded systems in solution.3~4 This success is due to (1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) Resident Research Associate from Wheaton College, Wheaton, Ill. Department of Chemistry, State University College, Oneonta, N.Y. 13820.
