D. P. Lln and L. Kevan
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ordinate. In view of the difference between ET(anthracene) = 42 kcal/mol and ET(ll-cis-retinal) 5 38 kcal/mol> this would imply a relatively small gap (A& = 4-6 kcal/mol) between (11-cis) T1 and Tz. Unfortunately, it is difficult to test quantitatively such conclusions in terms of the published theoretical calculations of retinal triplet energy levels.l0J1These very substantially in the spacings between the Ti states and, more significantly, in the sign and size of their 11-12 rotational barrier. Qualitatively in keeping with the above interpretation is the prediction’l of four triplet levels (T1-T4)within the AETrange covered by the sensitizers of the present study. A more difficult task is that of suggesting a model accounting for the observed systematic decrease in $Eo at higher AET. It is possible that the population of higher vibronic levels in the triplet manifold leads to geometrical changes along other coordinates which prevent that corresponding to the 11-12 tortional motion. Such changes provide a deactivation path, back to the original 11-cis molecule, without leading to any net photochemical transformation. This approach seems also to provide the best explanation for the inefficiency of the reverse trans cis photoreaction. Thus, although the direct-excitation quantum yields of the all-trans (cis isomers) process are comparable to those of the 11-cis (all-trans) photoreaction (0.1 and 0.12 correspondingly, using 313-nm excitation), the photosensitized isomerization yields of the all-trans molecule measured by us with biphenyl (& < 0.01) or 1,2-benzanthracene = 0.02) as sensitizers are extremely low. Here too, the triplet sensitized photochemistry of the retinal model compound markedly differs
-
--
($E,
from that following direct excitation of the corresponding (transoid) visual pigment, bathorhodopsin.’ Acknowledgment. The authors are grateful to Professors R. Gerber, R. D. Levine, and B. Honig for valuable discussions. References and Notes (1) T. Rosenfekl, B. Honlg, M. Ottolenghl, J. Hurley, and T. 0. Ebrey, VII, J . Pure. Appl. Chem., In press (IUPAC Symposium on Photochemistry, Alx-en-Provence, France, July, 1976). (2) R. Hubbard, J. Am. Cbem. Soc., 78, 4662 (1956); A. Kropf and R. Hubbard, Pbofochem. Pbotoblol., 12, 249 (1970). (3) T. Rosenfeld, A. Alchalel, and M. Ottolenghl, J. Pbys. Cbem., 78, 366 (1974). (4) R. A. Raubach and A. V. Guzzo, J. Pbys. Cbem., 77, 889 (1973). (5) T. Rosenfeld, A. Alchalel, and M. Ottolenghi In “Excited States of Biological Molecules”, J. Birks and S. P. McGlynn, Ed., Wlley-Interscience Monographsin Chemlcal Physics, New York, N.Y., 1976, p 540. (6) T. Rosenfeld, A. Alchalel, and M. Ottolenghl, Pbofochem. Photobiol., 20, 121 (1974); A. Alchalel, 8. Honig, M. Ottolenghi, and T. Rosenfeld, J . Am. Cbem. Soc., 97, 2161 (1975). (7) E. L. Menger and D. S. Kliger, J. Am. Chem. Soc., 98, 3975 (1976). (8) See, for example, G. S.Hammond et al., J. Am. Chem. Soc., 86, 3197 (1964). (9) The latter value is e s t a b l i i by our & m a t h that the 11-cb-retlnal triplet does not transfer energy to azulene (ET= 38 kcal/mol), and is consistent with previously published estimates for the albtrans Isomer (ET= 36-38 kcallmol, see ref 4 and A. Guzzo and 0. L. Pool, J . Pbys. Chem., 73, 2512 (1969)). (10) J. R. Welsenfeld and E. W. Abrahamson, Photochem. Photobiol., 8,487 (1968); J. Langlet, B. Pullman, and H. Berthod, J. CMm. Pbys., 66, (1969); H. Suzukl, N. Takizawa, and T. Komatsu, Prog. Tbeor. Pbys. Suppl., 46, 16 (1970); R. S. Becker, K. Inuzaka, J. King, and D. E. Balke, J . Am. Chem. Soc., 93, 43 (1971). (11) L. J. Weimann, 0. M. Maggiora, and P. E. Blatz, Int. J. Quantum Chem., QBS2 9 (1975).
Second Moment Studies of the Electron Spin Resonance Line Shape of Trapped Electrons in Sodium-Ice Condensates. Relation to the Molecular Structure Around Trapped Electrons D. P. Llnt and Larry Kevan’ Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received December 8, 1976)
The second moment of the ESR line shape of trapped electrons produced by codeposition of sodium and water vapor at 77 K has been measured in H20and DzOat both 9 and 35 GHz microwave frequencies. The sodium concentration is less than 0.01 M in this matrix. By analyzing the difference in the second moments in the protiated matrix at 9 and 35 GHz the isotropic and anisotropic hyperfine constants for n nearest equivalent interacting protons are obtained. The structural results found are similar to those obtained previously for trapped electrons in 10 M NaOH glassy ice by analysis of second moment and electron spin echo data. Since the trapped electron solvation structure seems independent of the solute concentration,we suggest that this solvation structure is also characteristic of solvated electrons in liquid water.
Introduction Solvated electrons are perhaps the most important reactive intermediates in the radiolysis of condensed systems.l In the past 12 years much has been learned of their reaction rates with a host of compounds, but their geometrical structure is only now being elucidated. Pulse radiolysis and 4 K radiolysis studies suggest that trapped electrons in organic and aqueous glassy matrices reorient their surrounding solvent molecules at 77 K to reach an Present Address: Edward Waters College, Jacksonville, F1. The Journal of Pbysical Chemistry, Vol. 81, No. 15, 1977
equilibrium solvated state. The most detailed picture of the arrangement of solvent molecules around a solvated electron has been obtained for electrons trapped at 77 K in a glassy ice composed of 10 M NaOH.2-4It is found that six H20 molecules are arranged equivalently (i.e., octahedrally) around the excess electron with one OH bond from each HzOmolecule oriented toward the electron. The nearest neighbor electron-proton distance is 2.1 f 0.1 A. This geometrical model has been obtained through a combination of electron spin resonance (ESR) techniques involving analysis of electron spin echo modulation
ESR Line Shape of Trapped Electrons in Sodium-Ice Condensates
patterns3 and second moment ESR line shape ana lyse^.^^^ It has been suggested that the structure deduced for the solvated electron in the 10 M NaOH glassy ice matrix is also applicable to the solvated electron in water: The spin echo results show that the electron in 10 M NaOH ice does have nearest neighbor water molecules and that the Na' ions are at least 5 A away. However, the high ion concentration in 10 M NaOH may impose structural constraints and it is desirable to study the geometrical structure of the electron in a less concentrated aqueous matrix. One cannot simply lower the NaOH concentration much because below about 5 M the glassy state is no longer produced by rapid freezing and the trapped electron yield drops concommitantly. However, Bennett et al.5 found that trapped electrons in reasonably high yield can be produced by codeposition of water vapor and sodium vapor at 77 K. With this technique the Na concentration is less than 0.01 M. In the present work we have studied the second moment ESR line shape of trapped electrons produced by codeposition of sodium and water vapor at 77 K in order to assess if the geometrical structure of this electron is significantly different from that produced in 10 M NaOH ice. Although the experimental uncertainty is larger than desirable, we find little significant difference between the solvated electron structure in these two matrices. Experimental Section Bennett et al.5 used a complex rotating cryostat arrangement for codeposition of sodium and water vapor. However Froben and Willard6 codeposited sodium and 3-methylpentane directly onto a pyrex cold finger and appeared to achieve a well-mixed, uniform deposit. We have used this latter, simpler method as adapted by Raitsimring et aL7 Water and purified sodium metal were contained in separate reservoirs attached to an evacuated flask containing a 8-mm 0.d. pyrex cold finger. The cold finger was filled with liquid nitrogen and could be removed from the flask by a ground glass joint. During deposition the sodium was gently heated and the water reservoir stopcock was opened; the total deposition time was 40-60 min. As deposition proceeds the matrix becomes blue and finally blue-black indicating spontaneous ionization of the sodium to produce trapped electron^.^ After deposition the cold finger is quickly removed from the flask and the blue color fades somewhat. The deposit is scraped off under liquid nitrogen and is transferred under liquid nitrogen into an ESR insertion dewar for X-band studies or directly into the ESR cavity for Q-band studies. Based on Bennett et al.5 we estimate that the Na concentration in our samples is less than 0.01 M. The ESR measurements were performed on a Varian V-line spectrometer at X-band and on a Varian E-line spectrometer at Q-band. The Q-band cavity was immersed in a cold gas flow dewar for temperature control. Most measurements were made at 100 K at Q-band and at 77 K at X-band. Warm-up measurements at X-band were made by depositing the sample on a 3-mm 0.d. Suprasil quartz cold finger which was then inserted into a flow dewar. Temperatures were measured with copperconstantan thermocouples with direct read out. ESR spectra were obtained with 100-kHz field modulation at a microwave power of about 20 pW at X-band and about 100 pW at Q-band. Based on earlier measurements these powers are low enough to exclude saturation broadening. Results 1. Spectral Characteristics. The X-band ESR spectra of the Na-ice condensates are shown in Figures 1 and 2
1409 l
10G
I
No/ H20
XXX"
,
AHpp
,
Figure 1. X-band ESR spectra of a Na-H,O condensate at 77 K before (-) and after (---) bleaching with visible light. The difference spectrum (XXX) corresponding to the bleachable component is also shown.
-v H
XXX
Figure 2. X-band ESR spectra of a Na-D20 condensate at 77 K before (-)and after (---) bleaching with visible light. The difference spectrum ( X X X ) corresponding to the bleachable component Is also shown.
for H 2 0 and D20 matrices, respectively. The solid lines represent the observed spectra, with care being taken not to expose the sample to bright room lights. The H 2 0 spectrum is quite asymmetric. The reason for the asymmetry is revealed by visible bleaching. The dashed line is the spectrum observed after 30 s bleaching by a 500-Wslide projector. At this point the sample is opaque white so we assume that all trapped electrons (e;) have been optically bleached. The difference spectrum is shown by the crosses; it is fairly symmetric and is attributed to trapped electrons. Bennett et aL5did not report any background spectrum after optical bleaching and their original spectra are symmetric. Our background spectra are characterized by g = 2.007 and peak-to-peak line widths (AHpp)of 17 G in HzO and 5.5 G in D20. At present we are unsure of the identity of our background spectra, however, this is not critical since our main focus in this paper will be on the trapped electrons. The difference spectrum in HzO clearly shows some structure and is similar to that reported by Bennett et al.5 They were able to improve their resolution quite considerably by thermal annealing at 173 K. In one early experiment we were also able to improve the resolution in this manner, although not nearly as dramatically as shown by Bennett et al. However, in all subsequent experiments on a different apparatus, thermal annealing only led to electron decay and loss of color by the sample. This is the behavior Bennett et al. found for 10-fold lower Na concentrations (
