2812
G. Lessard and G. Durocher
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978
importance of magnetic effects for this reaction. The absence of oscillations in our data leads us to speculate that the results reported by Krinchik et al. are not inherent in the carbonylation reaction itself. The failure to detect foreign gases with the mass spectrometer indicates that there were no significant impurity gases in the reaction cell. Our use of high purity Ni and our ion bombardment treatment of the Ni sample should leave an atomically clean surface with no foreign solid impurities. Therefore we believe that our data reflects only the basic carbonylation reaction with no competing reactions involving impurity gases or solids. We have no explanation for the oscillations obtained by Krinchik and Shvartsman.
Conclusion We have studied the rate of carbonylation of Ni powder with CO gas as functions of temperature and pressure. We find an approximately linear dependence of the rate on pressure below 1 atm and a monotonically increasing dependence on temperature between 25 and 90 "C. The temperature dependence is consistent with an activation energy of 0.27 eV. The field dependence of the rate was studied on single crystal slabs with either (100) and (110)
faces exposed for the field along (100). No evidence of an oscillatory field dependence was observed. We do not believe such oscillations are an intrinsic property of the carbonylation reaction. Acknowledgment. We acknowledge useful conversations with Dr. S. Susman, Mr. R. T. Kampwirth, Dr. B. M. Abraham, Dr. L. Iton, Dr. L. Falicov, Dr. H. Suhl, and Mr. R. Mehta.
References and Notes (1) A. Ya. Kipnis, N. F. Mikhailova, and R. A. Shvartsman, Kinet. Katal., 15, 1328 (1974). (2) G. S. Krinchik, R. A. Shvartsman, and A. Ya. Kipnis, JETP Lett., 19, 231 (1974). (3) G. S. Krinchik and R. A. Shvartsman, Sov. Phys. JETP, 40, 1153 (1975). (4) W. Goldberger, Ph.D. Thesis, Polytechnic Institute of Brooklyn, 1961. (Available from University Microfilms, Ann Arbor, Mich.) (5) H. Trivin and L. Bonnetain, C . R . Acad. Sci. Paris, Ser. C , 270, 13 (1970). (6) R. S. Mehta, M. S. Dresselhaus, G. Dresselhaus, and H. J. Zeiger, Bull. Am. Phys. SOC.,23, 418 (1978), Surf. Sci., to be published. (7) J. C. Measor and K. K. Afzulpurkar, Phil. Mag., 10, 817 (1964). (8) H. Uhlig, J. Pickett, and J. Magnairn, Acta Met., 7, 111 (1959). (9) B. C. Sales and M. B. Maple, Phys. Rev. Lett., 39, 1636 (1977). (10) C. Herring in "Magnetism", Vol. IV, G. T. Rado and H. Suhl, Ed., Academic Press, New York, 1966, pp 143-145.
Singlet and Triplet Quenching of Indole by Heavy Atom Containing Molecules in a Low Temperature Glassy Matrix. Evidence for Complexation in the Triplet State Ginette Lessard and Gilles Durocher" D6partement de Chimie, Universit6 de Montrsal, Montrgai, Qdbec, Canada H3C 3V1 (Received April 18, 1978; Revised Manuscript Received August 21, 1978) Publication costs assisted by Le Ministere De L ' Education Du Quebec (FCAC)
The absorption, fluorescence, and phosphorescence spectra along with the phosphorescence decay function of indole perturbed by various amounts of halocarbons have been studied in a 1:l ethanol-ether glass at 77 K. For the cases of carbop tetrachloride, chloroform, halothane, and methylene chloride used as perturber molecules, the biexponential nature of the phosphorescence decay prompted us to propose a kinetic model which assumes the formation of a triplet state complex responsible for the fast phosphorescence decay component. The equilibrium constants of these complexes were estimated to vary between 0.1 and 9 L/mol. On the other hand, propyl bromide gives rise to a nonexponential phosphorescence decay when added to indole. This and the fact that the phosphorescence intensity increases regularly with the amount of quencher added suggest the existence of a normal external spin-orbit coupling interaction in which the exchange mechanism seems to be operative.
Introduction In the last 10 years, the photoionization of a number of aromatic amines in low temperature matrices has been extensively studied, both experimentally and theoretially.'-^ Molecules containing heavy atoms have been used as chemical traps for the photoejected electrons and the mechanism of electron trapping has been studied re~ e n t l y . ~ On - ~ the other hand, the spectroscopic and photophysical properties of aromatic hydrocarbon molecules in solutions are modified by the presence of heavy atoms in the s o l ~ e n t . ~ -The l ~ effect of external heavy atoms on the photophysical properties of some aromatic amines in low temperature matrices has also been investigated. Meye@ and Willard et al.I9 have studied the perturbation of tetramethyl-p-phenylenediamine (TMPD) by some chloro and bromo compounds, and Bagdasar'yan e t aL20 have studied the effect of propyl bromide on the phosphorescence of carbazole. 0022-3654/78/2082-2812$01 .OO/O
In view of the biological importance of heavy atom quenching molecules such as chloroform and others,21it appeared to us worthwhile to investigate the perturbation of the fluorescence and phosphorescence emission spectra of the tryptophan chromophore indole in the presence of varying amounts of halocarbons in rigid ethanol-ether (1:l) solutions a t 77 K. We arrived at the conclusion that a triplet state complex does explain the biexponential phosphorescence decay observed in the case of some of the halocarbons studied. The ratios (of complexed to uncomplexed indole) of the first singlet and triplet state decay rate constants have been calculated along with the equilibrium constants of the complexes. On the other hand, one of the perturbers studied (propyl bromide) behaves like a normal external atom perturber.
Experimental Section (1)Products. "Certified quality" indole from Eastman 0 1978 American Chemical Society
Singlet and Triplet Quenching of Indole
Kodak was vacuum sublimed and kept in the dark. In all our experiments the solvent was an equal volume mixture of ethanol and diethyl ether. Absolute ethanol was refluxed over concentrated sulfuric acid for at least 48 h and fractionally distilled. The purest fraction in emission was conserved and used. “Reagent grade” diethyl ether from Baker was refluxed over sodium, distilled, and kept in the dark. Halothane (CF3CHBrC1) was from Ayerst Laboratories Inc., chloroform (CHCl,) and methylene chloride (CH2ClZ)were from Fisher Laboratories, certified quality ACS. “Reagent grade” carbon tetrachloride (CC14) was from A & C American Chemicals. Propyl bromide (CH3CH2CH2Br)was from Eastman Kodak. All these heavy atom compounds were purified by distillation over sodium and all were checked for any possible interfering emissions when excited a t 287 nm either a t room temperature or a t 77 K. (2) Sample Preparation. The concentration of indole was fixed a t M a t room temperature for all the experiments performed in absorption and emission. The concentrations of some of the perturbers varied between 0 and 4 M a t room temperature, which means between 0 and 5 M a t 77 K taking into account a 25% volume contraction upon freezing of the solvent. In order to avoid cracking in these low temperature glasses, it was necessary to degas all solutions before freezing them by the wellknown freeze-thaw cycle technique. (3) Apparatus. All absorption spectra were recorded on a Cary 14 spectrophotometer using a 1-mm optical path cell immersed in a liquid nitrogen dewar. The emission spectra were recorded on an Aminco spectrophotofluorometer already described2 without using the phosphoroscope attachment. The excitation wavelength was always fixed a t 287 nm, and the emission wavelength for the observation of the phosphorescence decay was at 440 nm. After the phosphorescent intensity had reached a steady state, the exciting radiation was cut off and the dark decay was then followed on the XY recorder. (4) Method. After degassing the solutions contained in a 3-mm diameter cylindrical cell, the cell was rapidly immersed in the Aminco liquid nitrogen dewar. The temperature measured in the bulk of the glass was fixed a t 77.4 K. The fluorescence and phosphorescence spectra were recorded a t least 5 min after the formation of the glass. Since we were interested in the fluorescence and the phosphorescence ratios I F / I F oand I p / I p owith and without a quencher it appeared unnecessary to correct for the variable response of the detection system with the emission wavelength. All the intensities reported in this work represent an average over two different glasses with at least three different trials on each of them, the reproducibility criterion was a 5% variation in all the measurements done. In order to be able to analyze the phosphorescence decay of the free indole molecules in mixtures of perturbed and unperturbed indole, it was necessary to record the decay over 4 or 5 orders of magnitude. This was done by varying the series of excitation and emission slit widths and also by varying the sensitivity of the photometer. A series of overlapping decays in time was then recorded, replotted, and studied by a linear regression analysis computer program.
Results No gross changes in the absorption spectra of the indole molecule were observed upon addition of the external heavy atom perturbers. Figure 1shows a set of absorption spectra of indole in various environments typical of those studied in emission. The spectrum of indole in the ethanol-ether mixture is typical of those indole spectra
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978 2813 370
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published in polar low temperature mat rice^.^^^^^ In din-propyl ether at -184 “C it was suggested that an indole-ether complex exists and is responsible for the decrease in the ‘La band intensity compared to that of the ‘Lb band, when the temperature is lowered. The same behavior was observed in e t h a n 0 1 . ~ ~The - ~ ~0-0 ‘Lb band intensity a t 287 nm is not affected by the external heavy atom perturbers which do not absorb a t this wavelength in the concentration range used here. At wavelength lower than 280 nm, the perturbers start to absorb slightly and it has been verified that this small absorption can explain the loss of vibrational structure below 280 nm in some of the spectra shown in Figure 1. In conclusion, the absorption spectra of indole in the presence of the heavy atom molecules studied here failed to provide any evidence for a ground state complex between indole and these heavy atom perturber molecules. In the same way, not much change in the shape of the fluorescent and the phosphorescent emission spectra were observed upon addition of external heavy atom perturbers. Figure 2 shows the emission spectra of indole perturbed by halothane. The spectra are nearly the same as far as the shape is concerned but the intensities of both
2814
G. Lessard and G. Durocher
The Journal of Physical Chemistry, Voi. 82, No. 26, 1978
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