3774
J. Phys. Chem. 1988, 92, 3174-3118
teracting with HF. These matrix infrared observations predict that the hydrogen-bonded Br2-HF species is the more stable form, and as such it should be observed in a gas-phase nozzle beam experiment. Complementary experiments in neon matrices show small (20-30 cm-I) differences between argon and neon matrix absorptions and predict gas-phase fundamentals just above the solid neon values. The matrix infrared experiment has isolated and
characterized both hydrogen-bonded and fluorine-bonded forms of molecular halogen-hydrogen fluoride complexes and provided information on their relative stabilities. Acknowledgment. We gratefully acknowledge financial support from National Science Foundation Grant C H E 85-1661 I . Registry No. C12, 7782-50-5; HF, 7664-39-3; DF, 14333-26-7; F2, 7782-41-4; Br,, 7726-95-6; BrF, 13863-59-7; CIF, 7790-89-8; Ar, 744037-1; Ne, 7440-01-9.
Stimulated Emission Spectroscopy of Jet-Cooled Polyatomlcs: S, Ionization Dlp Spectra of m-Fluorotoluene and Aniline
-
So Two-Color
Toshinori Suzuki, Masayuki Hiroi, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: October 12, 1987; In Final Form: January 26, 1988)
The ground-state vibrational levels of jet-cooled m-fluorotolueneand aniline have been observed by stimulated emission from the first excited singlet state of each molecule. By use of the two-color ionization dip method, 200 vibrational levels of m-fluorotoluene and 200 of aniline have been clearly observed below the vibrational energy of 2800 and 3300 cm-l, respectively. The results proved that stimulated emission spectroscopy using the two-color ionization dip method is a powerful tool for the study of the ground-state vibrational states over a wide energy range.
Introduction Stimulated emission spectroscopy,Id which utilizes the stimulated emission from an electronically excited state (usually SI), is a new powerful means for the study of high vibrational states of an isolated molecule in the ground state. The advantages of the stimulated emission spectroscopy are as follows: (1) No severe selection rule such as Av = f l in vibrational spectroscopy exists, because the method utilizes electronic transitions. (2) High resolution can be achieved because the instrumental resolution is determined only by the laser resolution. (3) High detection sensitivity can be obtained. Since the stimulated emission to a selected vibrational level in So is induced by intense laser light, even a level having a small transition probability from SI can be detected. In a previous paper," we reported the stimulated emission spectra of jet-cooled trans-stilbene using the two-color ionization dip method. We pointed out there that the two-color ionization dip method can remove experimental difficulties such as the temporal and spatial mismatch of two laser beams which is inherent in any optical-optical double resonance experiment. As a result of the experimental feasibility, we could observe as many as 500 vibrational levels of trans-stilbene in the energy region below 3300 cm-I. On the basis of the observed results, we discussed the molecular structure, large-amplitude torsional vibration, and IVR dynamics in So of the molecule. The present paper represents a further application of the two-color ionization dip method to jet-cooled benzene derivatives, which have been extensively studied by conventional vibrational spectroscopies. m-Fluorotoluene and aniline were selected because they have large-amplitude vibrations such as the internal rotation (1) (a) Abramson, E.; Field, R. W.; Imne, D.; Innes, K. K.; Kinsey, J. L. J . Chem. Phys. 1985,83,453. (b) Sundberg, R. L.; Abramson, E.; Kinsey, J. L.; Field, R. W. J. Chem. Phys. 1985,83,466. (c) Hamilton, C. E.; Kinsey, K. L.; Field, R. W. Annu. Rev. Phys. Chem. 1986, 37,493, and references therein. (2) (a) Cooper, D. E.; Wessel, J. E. J . Chem. Phys. 1982, 76, 2115. (b) Cooper,D. E.; Klimcak, C. M.; Wessel, J. E. Phys. Rev. Lett. 1981, 46, 324. (3) Murakami, J.; Kaya, K.; Ito, M. Chem. Phys. Lett. 1982, 91, 401. (4) (a) Suzuki, T.; Mikami, N.; Ito, M. J. Phys. Chem. 1986, 20, 6431. (b) Suzuki, T.; Mikami, N.; Ito, M. Chem. Phys. Lett. 1985, 120, 333. ( 5 ) Xie, J.; Sha, G.; Zhang, X.; Zhang, C. Chem. Phys. Lett. 1986, 124, 99. (6) Celii, F. G.; Maier, J. P.; Ochsner, M. J . Chem. Phys. 1986.85, 6230.
of the C H 3 group and the inversion of the N H 2 group. It is of particular interest to examine the intermode coupling between the ring vibrations and these large-amplitude anharmonic vibrations from the viewpoint of intramolecular vibrational redistribution (IVR). Hundreds of vibrational levels in the ground state of m-fluorotoluene and aniline were clearly observed by the two-color ionization dip method, proving the general utility of this method.
Experimental Section The experimental setup for the measurement of the two-color ionization dip spectrum of a jet-cooled molecule was described e l ~ e w h e r e . ~A XeCl excimer laser (Lambda Physik EMG 102MSC) was used to pump two dye lasers (Molectron DL-14 for v1 and Lambda Physik FL2002E for v 2 ) , which produce the pulses of 14-ns duration. The output of the first dye laser (Coumarin 540A for m-fluorotoluene and Rhodamine 6G for aniline) was frequency doubled by a KDP crystal to produce UV light, vl. v2 was the second harmonic of the second dye laser (Coumarin 540A for m-fluorotoluene, Rhodamine 610 and Sulforhodamine 640 for aniline). The stable v2 output was obtained by a KDP crystal autotracking system (Inrad autotracker 11). The two laser beams were introduced coaxially into a vacuum chamber and crossed the jet 30-40 mm downstream of a nozzle. u2 was focused by a lens of 25-cm focal length to obtain sufficient photon density. An optical time delay of 1-5 ns relative to v i was given to v2. The resolutions of vi and v 2 were 1 and 0.2 cm-' (fwhm), respectively. The first laser beam, v I , pumps the jet-cooled molecule to the Oo level in SI (intermediate state), and the second laser beam, v2, induces the transitions from the intermediate state to the vibrational state in So and also to the ionization continuum. The intensity of the v, beam was weakened to avoid the one-color ionization caused by v l alone. Under this condition, all the ion signals occurred only in the presence of both v i and v2 (two-color ionization). When the stimulated emission was induced by v 2 beam, the ion signal produced by two-color ionization suddenly decreased (two-color ionization dip). The stimulated emission was detected by this two-color ionization dip. The ion produced was pushed into a detector chamber by a repeller at an appropriate voltage (20-40 V/cm) and was detected by an electron multiplier (Murata Ceratron). The ion signal was
0022-3654/88/2092-3774$01 .SO10 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3775
Jet-Cooled m-Fluorotoluene and Aniline
TABLE I: Frequencies of Combinations and Overtones of Ring Modes of m-Fluorotoluene vib freq/cm-' vib freq/cm-'
-3.8-05.0
37400 EXCITATION ENERGY
37500 (cm-1)
Figure 1. Fluorescence excitation spectrum of jet-cooled m-fluorotoluene in the vicinity of the 0; band. Frequencies measured from the 0: band
are shown. amplified by a current amplifier (Keithley 427). The ion current was averaged by a boxcar integrator system (PAR 440214420). The dispersed fluorescence spectra were also measured for comparison with the dip spectra. For the measurement of the dispersed fluorescence spectrum, the exciting light was fixed to the 0; band of each molecule and the fluorescence was dispersed by a 75-cm monochromator (Nalumi RM21-750) in the second order. The dispersed light was detected by a photomultiplier (Hamamatsu R 928), and the photocurrent was averaged by a boxcar integrator system. The spectral resolution was about 10 cm-I . The sample vapor was seeded in 3 atm of H e (or Ar) gas at room temperature and expanded into a vacuum chamber through a pulsed nozzle with a 400-pm-diameter orifice. The backing Torr. pressure in the expansion chamber was about 1 X m-Fluorotoluene (Tokyo Kasei) and aniline (Tokyo Kasei) were purified by vacuum distillation.
-
Results and Discussion
In Figure 1 the SI So fluorescence excitation spectrum of jet-cooled m-fluorotoluene is shown in the vicinity of the 0; band (37 386 cm-I). The low-frequency vibrational structure near the 0; band has been assigned to the internal rotational levels of the methyl group in SI by Okuyama et al.' In the measurement of the stimulated emission spectrum, ul was fixed to the 0; (Oa, Oal; 37 386 cm-I) band of the SI So transition. As is seen from the figure, the 0; (Oal Oal) band is very close to the hot band of the l e l e transition, the separation between the two bands being only 3.8 cm-I. Since the Oa, and l e levels in So are almost degenerate, the hot band cannot be removed, even under the jet-cooling condition. Hence, much effort was made to resolve the two nearby bands by cooling the rotational temperature of the molecule as much as possible. Ar gas was found to be more efficient for extensive rotational cooling than He gas. As a result of the complete resolution of the two bands, pumping to the single vibronic level SI O0(0aI) was accomplished. Figure 2a shows the two-color ionization dip spectrum of jetcooled m-fluorotoluene measured after pumping the molecule to SI OO(Oal)with u l . It is seen that a large number of sharp dips appear throughout the spectrum. All the dips reflect the depletion of the population of the SI Oo level due to the stimulated emission induced by u2. Even very weak dips were found to be quite reproducible. As a result, about 200 vibrational levels were observed in the vibrational energy region less than 2800 cm-'. Figure 2b shows the corresponding dispersed fluorescence spectrum of jet-cooled m-fluorotoluene obtained after exciting the molecule to SI Oo. A comparison between spectra a and b of Figure 2 shows that the weak bands in the dispersed fluorescence spectrum generally appear with fairly deep dips in the stimulated emission
- -
-
+-
(7) Okuyama, K.; Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89, 5617.
vib level
obsd
11 121 131 141 12 1,121 1,13,
728 1004 1252 1271 1457 1733 1979
calcd
vib level
obsd
calcd
lI14,
2000 2186 2257 2274 2462 2738
1999 2184 2256 2275 2460 2736
13
12,13, 12,14, 1212, 11122
1456 1732 1980
spectrum. Many of the fluorescence bands having the intensity smaller than 10% of the intensity of the 0; band give, in the stimulated emission spectrum, dips of as large as 20-40% of the ion signal. The deep depth for the weak transition comes from saturation of the stimulated emission process. The stimulated emission efficiency is determined by the product of the radiative cross section (u) and the u2 laser power (I). Therefore, a small cross section for the weak band in the fluorescence spectrum can be compensated by the intense laser light. This is one of the greatest advantages of stimulated emission spectroscopy, because the overtone and combination levels having small cross sections from S, can be easily observed. It is noted in Figure 2a that the depths of the strong bands at 728, 1004, and 1733 cm-I become deeper in the order of increasing vibrational energy. This is just the opposite order for the intensities of the corresponding bands in the dispersed fluorescence spectrum. The growth of the depth comes from the increase of the IVR rate in So vibrational levels with increase of the excess vibrational energy, which will be discussed elsewhere. The improvement in spectral resolution is another advantage of stimulated emission spectroscopy. Figure 3 shows two small regions of the stimulated emission spectrum on an expanded scale. It is seen that a single peak in the dispersed fluorescence spectrum is clearly resolved into two or more dips in the stimulated emission spectrum. Figure 4 shows a part of the two-color ionization dip spectrum of jet-cooled aniline measured after pumping the molecule to the S I 0' level (34029 cm-I) with uI. The corresponding dispersed fluorescence spectrum is also shown in the figure. Again, a large number of sharp dips appear throughout the spectrum. Although the vibrational levels lying above 2000 cm-I are very weakly observed in the dispersed fluorescence spectrum, they clearly show up in the two-color ionization dip spectrum. As a result of the high detection sensitivity and high resolution, 200 vibrational levels in So were found in the vibrational energy region less than 3300 cm-I. General features of the stimulated emission spectrum of aniline are similar to those of m-fluorotoluene; the weak bands in the dispersed fluorescence spectrum are exaggerated in the stimulated emission spectrum. The frequencies of combination and overtone levels of a molecule are in general slightly shifted from those calculated from the fundamental frequencies of modes involved in the combinations and overtones. This frequency shift gives a measure of the anharmonicity of the mode and the anharmonic coupling between the different modes. The spectral resolution of stimulated emission spectroscopy is high enough to discuss this kind of small shift. In Table I the assignments of the overtones and combinations of several modes of m-fluorotoluene are given. The assignments have been made by referring to ref 7. It is seen from the table that ring vibrations are very harmonic in this energy region; no shift larger than 2 cm-' from the harmonic frequency is recognized. Next, we shall see the combinations involving the internal rotation of the methyl group which is a large-amplitude motion. (Internal rotation of the C H 3 group of So m-fluorotoluene is almost free rotation.') In the case of trans- tilb bene,^ a large frequency shift due to intermode coupling was found between the large-amplitude torsional mode 37 (fundamental 8 cm-') and the in-plane ethylene bending mode 25 (fundamental 202 cm-I). A similar intermode coupling is expected for the internal rotation of the methyl group of m-fluorotoluene. In the SI So spectrum of m-fluorotoluene,
-
3176
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Suzuki et al.
100 -
l a ) TWO-COLOR
-d -m
IONIZATION
DIP
L
120
+ c al L
1004
I
I
1270
1457
a
v
1733
h
c 0 I
Ib)
I
DISPERSED
FLUORESCENCE
0-
1000
"
1500
VIBRATIONAL
So
ENERGY
(cm-11
I
I
( b l DISPERSED
I
FLUORESCENCE
I
I
\
h
OJ
J j L A
I
I
I
I
2000 So VIBRATIONAL
I
I
I
I
I
I
2500
ENERGY
(cm-11
Figure 2. (a) Two-color ionization dip spectrum of jet-cooled m-fluorotoluene and (b) corresponding dispersed fluorescence spectrum. In the measurement of both spectra, the molecule was pumped to the SIOolevel. The step-up of the total ion current at the points shown by arrows comes from the resonance of total photon energy [q v2] with the vibrational states of m-fluorotoluene cation. This step-up simply changes the reference level of the ion signal but does not change the dip depth measured as the reduction rate of total ion current.
+
TABLE II: Frequencies of Methyl Torsion in Vibrationally Excited States of Ring Modes of m-Fluorotoluene
vib level (frea 1cm-l) ef
1, (728) 12, (1004) 18al (1078) 131 (1252) 14, (1271) 1 2 (1457) 17b2 (1712) 1, (2186) 12,131 (2257) 12,14, (2274)
h
d
b c
I1h l
1200
"
"
1
'
1300 So VIBRATIONAL
*
I
l
l
1
1960 2000 ENERGY ( c m - ' )
Figure 3. Details of the parts of (a) two-color ionization dip spectrum
of jet-cooled m-fluorotolueneand (b) corresponding dispersed fluorescence spectrum. In the measurements,the molecule was pumped to the SIOolevel. The vibrational frequencies of the indicated bands are (a) 1213, (b) 1240, (c) 1252, (d) 1271, (e) 1296, (f) 1302, (9) 1315, (h) 1320, (i) 1969, Q) 1979, (k) 1984, (I) 2000, (m) 2008, (n) 2015, (0)2024, and (p) 2028 cm-I. a satellite band due to the internal rotation of the methyl group can be found for each of main vibrational bands. It appears at about 52 cm-' on the higher energy side of the vibrational band and is assigned to the 3a1 level of the internal rotation of the methyl group in S,. (3al is a notation of the methyl internal rotation level under the partial symmetry of C3, of the methyl group; see ref
torsional freq/cm-' (Oal-3al separation) 51 50 52 50 50 52 52 53 51 51
7.) However, no appreciable frequency difference was found for 3al, when it forms the combination with various ring modes of 11, 121,etc. (Table 11). It is concluded therefore that no strong coupling exists between the methyl internal rotation and the ring mode. This result may be natural, because the frequency of the internal rotation is much smaller than those of ring modes. Recently, the intermode coupling between CH3 internal rotation and another low-frequency vibration (1 18 cm-l) was found in SI acetophenone.* The difference between the cases of acetophenone and m-fluorotoluene is that in m-fluorotoluene the CH, internal rotation is well-isolated in frequency from other vibrations. The assignments of some combination and overtone levels of the ring modes of aniline are tabulated in Table 111. Most of the combinations and overtones involving the ring modes have (8) Ohmori, N.; Suzuki, T.; Ito, M. J . Phys. Chem. 1988, 92, 1086.
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3777
Jet-Cooled m-Fluorotoluene and Aniline
(a) TWO- COLOR IONIZATION DIP
100 -
n
L
m
c
1706
823
2075
C
a J L L
3 U
c
(b) DISPERSED FLUORESCENCE
I
I 1000
I
I
1 I 1500 So VIBRATIONAL ENERGY ( cm-') I
I
I
I
I
I
I
I
2000
(a) TWO- COLOR IONIZATION DIP
n
m
I
I
I
c
2229
2525
2700
L
c
2902
a J L L
3 U
c 0 Y
( b ) DISPERSED FLUORESCENCE 0
I
1
I
I
I
I
2000
1
1
I
I
SO VIBRATIONAL
I
I
2500
1
I
3000
ENERGY ( cm-'1
Figure 4. (a) Two-color ionization dip spectrum of jet-cooled aniline and (b) corresponding dispersed fluorescence spectrum. In the measurements, the molecule was pumped to the S,Oo level. TABLE III: Frequencies of Some Combinations and Overtones of Ring Modes of Aniline vib freq/cm-l vib level
obsd
6al 11 12, 18a, 71 6a2 1,6a, 12,6a,
528 823 1004 1031 1282 1059 1352 1532 1646
I2
calcd
1056 1351 1531 1646
vib freq/cm-' vib level
obsd
calcd
716a1 12111 18alll 116az 122 18a112, 7111 12112
1807 1826 1854 1882 2009 2033 2102 2648
1810 1827 1854 1880 2008 2035 2105 2650
frequencies expected from the harmonic approximation. In this sense, the ring modes of aniline are quite harmonic. Aniline has the anharmonic N H 2 inversion in So; the vibrational frequencies of its overtones are 40.8 ( u = l), 423.8 ( u = 2), and 700.1 cm-' (v = 3).9 Since the overtones of the NH2 inversion have frequencies close to those of some ring modes, intermode coupling (9) (a) Larsen, N. W.; Hansen, E. L.; Nicolaisen, F. M. Ckem. Phys. Lerr. 1976, 13,584. (b) Kydd, R. A,; Krueger, P. J. Chem. Phys. Lett. 1977, 49, 539.
TABLE IV: Frequencies of NH2 Inversion in Vibrationally Excited States of Ring Modes of Aniline vib level (freq/cm-')
inversion freq/cm-l (u" = 2 u u" = 0)
6aI (528) 1, (823) 12, (1004) 18al (1031) 6az (1059) 7, (1282) l16al (1351) 12,6a1 (1533) 18a,6a1 (1559) 12 (1646) 7,6a, (1807) 12111 (1827) 1 8 a l l , (1854) 7,1, (2105)
422 425 423 423 417 424 422 422 425 426? 422? 424? 423? 420?
is anticipated. We examined the inversion frequencies of u = 2 accompanied by the ring modes. However, no large frequency difference was found from its original value of 423.8 cm-I (Table IV). In conclusion, we could observe 200 vibrational levels of mfluorotoluene lying below the vibrational energy of 2800 cm-I and
3778
J . Phys. Chem. 1988, 92, 3778-3781
200 levels of aniline below 3300 cm-l by stimulated emission spectroscopy using the two-color ionization dip method. The result proves the utility of the method for the study of the detailed ground-state vibrational level structure of a polyatomic molecule over a wide energy region. The method particularly has a great
advantage in the detection of combination and overtone levels which are hardly observed by IR, Raman, and fluorescence spectroscopies. Registry No. m-Fluorotoluene, 352-70-5; aniline, 62-53-3.
13C Hyperfine Constants of Allyl Radical Hugh J. McManus, Richard W. Fessenden,* and Daniel M. Chipman Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 29, 1987)
The 13Cisotropic hyperfine constants of the allyl radical have been measured by in situ radiolysis ESR experiments carried out on aqueous solutions of propene that was labeled at either the 1- or 2-position. The values are 21.93 and 17.21 G for the end and center carbons, respectively. Ab initio calculations have been used to calculate the equilibrium geometry and IH and 13Cisotropic and anisotropic hyperfine constants. The calculated isotropic hfc are in reasonable agreement with the experimental values. Values calculated from the Karplus-Fraenkel equation with standard values of the Q parameters are also in accord with the experimental values.
The allyl radical has long been used as a prototype in discussions of electronic structure and spin density in conjugated radicals.'-2 It is the simplest example of an odd alternant radical and as such shows sign alternation of the a spin density on the carbons, with negative spin density at the central p ~ s i t i o n . ~It has played an important conceptual role in formulating the linear relationship which is found to hold between proton hyperfine constant (hfc) and a spin density on the adjacent carbon The I3C hfc for the central carbon is unusual in this radical because it receives negative contributions from both the negative a spin density on that carbon and by means of spin polarization of the u bonds from both positive spin densities on the end carbons. Because of the small number of atoms, it is often used to test theoretical calculations. Even though accurate 'H hfc have been known for a long time,5 no values of the 13C hfc have been measured for the unsubstituted radical. It is desirable to have these parameters to evaluate the accuracy of calculations. Values have been determined for several allyl radicals which are substituted with bulky groups such as tert-butyl to make the radicals much longer lived.6 However, it is always unclear in such cases whether the substitutions significantly affect the values of the hfc by, for example, preventing the two ends of the radical from being coplanar. This paper presents the results of experiments designed to measure the I3C hfc of allyl radicals formed from enriched precursors and of calculations designed to give the geometry and iso- and anisotropic hfc of this radical.
Experimental Section Several routes to the allyl radical are possible. The one chosen here involves abstraction of an H atom from propene by radiolytically produced 0- in basic aqueous solution. A main advantage of this method is the commercial availability of propene labeled a t either the 1- or 2-position. Design of an apparatus that allowed dissolution of 100 cm3 of propene in a fixed volume of solution so that little gas remained (1) McConnell, H . M.; Chesnut, D. B. J . Chem. Phys. 1958, 28, 107. (2) McConnell, H. M. J . Chem. Phys. 1958.28, 1188; Ibid. 1959, 29,244 (3) The term spin density is conventionally used in this context. What is meant is the spin population of the pI atomic orbital on the specific carbon. A negative density arises if the spin orientation is opposite to that on the radical as a whole. (4) McConnell, H. M. J . Chem. Phys. 1956, 24, 764. Weissman, S. I. J . Chem. Phys. 1956, 25, 890. Bersohn, R. J . Chem. Phys. 1956, 24, 1066. (5) Fessenden, R. W.; Schuler, R. H . J . Chem. Phys. 1963, 39, 2147. (6) Ahrens, W.; Schreiner, K.; Regenstein, H.; Berndt, A. Tetrahedron Lett. 1975, 50, 45 1 1 .
0022-3654/88/2092-3778$01 .50/0
in any dead space was a major challenge. The idea was to provide an adjustable volume by use of a syringe and to circulate the solution through a tower filled with glass beads to provide extra surface for more rapid dissolution of the propene. A diagram of the apparatus finally used is shown in Figure 1. There were two distinct sections to the equipment. The first section was a loop which allowed recirculation of the solution through the cavity. During the experiment, the solution was extracted from the bottom of the tower, pumped through the ESR cell, and reinjected through the top of the tower. The second section involved the gas-handling apparatus. It consisted of a gas bulb of 100-cm3 volume, a Teledyne Hastings-Raydist 0-1000-Torr vacuum gauge, and a peristaltic pump. When a gas sample was to be injected into the recirculation loop, the output of the peristaltic pump was connected to the top of the tower. In a typical experiment, N,O was flushed through the recirculation system for about 1 h. Two liters of > 18 Mohm cm water were purged with N20, and 25 g of KOH was added. The basic solution was then pumped into the system which had a capacity of 1 L. The second section of the apparatus was then pumped down to -6 Torr with the peristaltic pump and filled with N2 by reversing the pump. This process was repeated twice more. After the third evacuation, the lecture bottle was opened and the 100-cm3volume filled with propene at 1 atm. The propene gas was then admitted into the cylinder by simultaneously opening the taps on the pump side of the bulb and on the cylinder. The syringe slowly filled with solution. When it had reached 100 cm3, the tap on top of the cylinder was closed. The solution was recirculated with pressure periodically applied to the plunger of the syringe to coax the gas into solution. Dissolution of the gas typically took around 1 h. In experiments with the I3C-enriched samples, their containers took the place of the 100-cm' bulb. ESR spectra at X band were taken with the in situ radiolysis apparatus described previously.' The magnetic field was computer controlled by means of a field-tracking N M R unit and frequency counter which also read the microwave frequency. In this way, a field/frequency lock was maintained and compensation for any drifts in microwave frequency could be provided; very accurate and slow field scans could be made. The system time constant was 18 s, and repetitive scans of the same field region were used in some cases for signal averaging for additional signal-to-noise ratio improvement. The g factors were determined by reference to the line of SO3'- at g = 2.003 06.* Radiolysis was with a 2.5-1A
-
~~~~
~~
~~~~~
~~
~
(7) Jinot, C.; Madden, K. P.; Schuler, R. H . J . Phys. Chem. 1986, 90,
4979.
0 1988 American Chemical Society
