15627
J. Phys. Chem. 1995,99, 15627- 15632
Ion-Molecule Reaction between a Protonated Phenyl Ketone and an Alcohol Following Multiphoton Excitation in a Liquid Beam Jun-ya Kohno, Noriko Horimoto, Fumitaka MafunC, and Tamotsu Kondow* Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received: February 27, 1995; In Final Form: August 24, 1 9 9 9
An alcohol solution of phenyl ketone C6HsCOR1, where RI is CH3, C2H5, H, or C6H5, was introduced into a vacuum as a continuous liquid flow (liquid beam) and irradiated with a laser beam at wavelengths of 250, 280, and 355 nm. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by time-of-flight mass spectrometry. The mass spectra of the ions produced from various alcohol solutions of phenyl ketones at different excitation wavelengths indicate that a protonated phenyl ketone ion, C~HSC(OH)R~+, produced by laser irradiation reacts with an alcohol molecule, R20H, in the solution and C~H~C(OR~)R isI produced. + This reaction corresponds to the initial process for acetal formation from a phenyl ketone and an alcohol, and these product ions are identified to be its reaction intermediates. When an alcohol solution of benzophenone, C6H5COC6H5, was used, a pinacol ion, (C6H5)2C(OH)C(OH)(C6H5)2+ was produced in addition to C ~ H S C ( O R ~ ) C ~ H The ~ +appearance . of the pinacol ion suggests the presence of benzophenone dimers in the vicinity of the liquid surface.
1. Introduction Reactions on a surface of a liquid solution are expected to be specific and different from those occurring inside the solution.' Reactants on the liquid surface are not completely surrounded by solvent molecules although the density of the reactants is sufficiently high that the reactions proceed as efficiently as those occurring inside the solution. This insufficient solvation assists liberation of reaction intermediates during the course of the reactions, if they have sufficient kinetic energy to surmount the solvation energy. Especially for ionic products, they repel each other by Coulomb repulsion and can be ejected into a vacuum. In order to investigate ion-molecule reactions in the vicinity of the liquid surface, we have explored a technique of introducing a continuous liquid flow into vacuum (liquid bean^).^-^ Intensive investigations have been performed to characterize this beam surface: UPS,XPS, and Penning ionization electron ~pectrometry~-~ have shown that a clean liquid surface is exposed in the vacuum. A kinetic energy distribution of molecules evaporating from the liquid surface has revealed that the molecules evaporate directly from the surface into the vacuum without suffering any significant secondary collisions.'oJl Further, the liquid beam is laminar in the vacuum because the laser beam passes through the inside of the liquid beam without any observable scattering when the laser beam is introduced colinearly with the liquid beam.'O This characterization of the liquid beam, in combination with multiphoton ionization (MPI) by the laser, facilitates the application of the liquid beam technique to the studies of ion-molecule reactions on liquid surfaces. This method allows us to introduce ionic species into the liquid beam efficiently and selectively; one functional group of one species in the liquid can be activated by resonant MPI. The ions ejected into the vacuum are observed by means of time-of-flight (TOF) mass spectrometry, enabling mass selective detection of the product ions. By use of this method, the acetal formation reaction has been investigated in an alcohol solution of a phenyl ketone, C6H5COR', where R1 is CH3, C2H5, H, or C ~ H S When .~ an alcohol @
Abstract published in Advance ACS Abstracts, October 1, 1995.
XY fl Stage
Diffusion
Pump
Figure 1. Schematic diagram of the experimental setup.
solution of C&CORl is irradiated with unfocused ultraviolet laser light, it is excited to the TI state via an S, ~ t a t e ; ' ~C6H5-~~ COR' in the TIstate abstracts hydrogen from the alcohol to form the radical C&C(OH)Rl, and pinacol, (c&)(R~)c(oH)C(OH)(RI)(C~HS), is formed by the subsequent dimerization reaction. Under irradiation of focused laser light,2s.26on the other hand, it is expected that the radical C~HSC(OH)RIis ionized further by absorbing more photons before the dimerization, and C~H~C(OH)RI+, is formed; this ionic species is an intermediate of the acetal formation reaction. Accordingly, the reaction intermediate can be selectively produced by laser irradiation instead of by adding acid to the solution. In this paper, the reaction of C&C(OH)R'+ thus produced was studied and the formation mechanism was discussed together with the pinacol formation.
2. Experimental Section Figure 1 shows a schematic diagram of a liquid beam apparatus and a reflectron TOF mass spectrometer. A continuous laminar liquid flow (liquid beam) of an alcohol solution of a phenyl ketone was introduced into a vacuum chamber from a nozzle having an aperture with a diameter of 20 pm. A constant
0022-3654/95/2099- 15 627$09.00/0 0 1995 American Chemical Society
Kohno et al.
15628 J. Phys. Chem., Vol. 99, No. 42, 1995
liquid flow was supplied by a Shimadzu LC-6A pump designed for a liquid chromatograph. The flow rate was maintained at 0.2 mUmin with a pressure of typically 30 atm inside the nozzle; the flow rate was optimized for the viscosity of a given liquid and the diameter of the nozzle aperture, so that the Reynolds number of the flow and the stagnation pressure inside the nozzle were within suitable ranges. The liquid beam was trapped at 5 cm downstream from the nozzle by a cylindrical cryopump cooled by liquid N2. Its effective pumping speed was estimated to be -5000 L s-' for a condensable gas. The liquid beam chamber was evacuated by a 1200 L s-' diffusion pump. The ambient pressure was typically to Torr during injection of the liquid beam. Commercial benzaldehyde, acetophenone, propiophenone, and benzophenone (Wako Pure Chemical Industries Ltd., 99% purity) were used without further purification. Traveling a distance of 5 mm from the nozzle, the liquid beam was crossed with a UV laser beam in the first acceleration region of the TOF mass spectrometer. The UV laser beam was obtained by frequency-doubling of the output of a Quanta-ray PDL-3 dye laser pumped by the third harmonics of a Quanta-ray GCR-3 Nd:YAG laser. The laser power (-40 pJlpulse) was monitored by a photodiode. A 355 nm laser beam was obtained directly from the third harmonics of the Nd:YAG laser. The laser beam was focused into the liquid beam by a lens with a focal length of 450 mm. The mass-to-charge ratio, mlz, of each ion produced by laser photoionization was analyzed by the reflectron TOF mass spectrometer. The ions ejected from the liquid beam were accelerated by a pulsed electric field in the fist acceleration region in the direction perpendicular to both the liquid and the laser beams. The delay time from the ionization to the ion extraction was varied in the range 0-1.6 ps in order to obtain a higher resolution for the mass election.^ The ions were then steered and focused by a set of vertical and horizontal deflectors and an einzel lens. The first and the third plates of the einzel lens were grounded to prepare a field-free region of 1.5 m beyond the einzel lens. The reflectron provided a reversing field tilted by 2" off the beam axis. After traveling a 0.5 m fieldfree region, the ions were detected by a Murata EMS-6081B Ceratron electron multiplier. Signals from the multiplier were amplified and processed by a Yokogawa DL 1200E transient digitizer based on an NEC 9801 microcomputer. The mass resolution, defined as m l h , was 300 under the present experimental conditions.
3. Results Figure 2 shows a typical TOF mass spectrum of ions produced from a 0.5 M solution of acetophenone, C6H5COCH3, in ethanol by irradiation with a 250 nm laser. In this spectrum, one intense peak appears at mlz = 149 together with other small peaks. The mass number of this intense peak agrees with the sum of those of acetophenone (mlz = 120) and an ethyl group (mlz = 29), while the small peaks are assignable to C6H5C(OH)CH~+-(C~HSOH)~ (n 2 0). In order to identify this intense peak, acetophenone was replaced with benzaldehyde, C&COH, or propiophenone, CsHsCOC2H5, in ethanol, and the mlz values were observed. As shown in Figure 3, the most intense peaks shift by mlz = 14 with increases in the chain length of the functional group, RI,in C&CORl. The peak shifts correspond to the mass differences between benzaldehyde and acetophenone, and acetophenone and propiophenone. On the other hand, when the ethanol solution of acetophenone was replaced with a methanol and a propanol solution of acetophenone, the most intense peaks shift similarly by mlz = 14, with increases in the chain length of the functional group, R2, in the solvent R20H
;I
- I .-S 149
v
50
150
100
250
200
300
350
Mass Number (miz) Figure 2. TOF mass spectrum of ions produced from a 0.5 M solution of acetophenone in ethanol by irradiation with a 250 nm laser. The peak with mlz = 149 is assigned as C6H5C(OC2H5)CHj+ and the other (n 2 0), as shown peaks are assigned as C6H5C(OH)CH3*(C2H5OH),++ in the figure.
r-----l
50
100
150
200
250
300
350
Mass Number (m/z) Figure 3. TOF mass spectra of ions produced from 0.5 M solutions of benzaldehyde, acetophenone, and propiophenone (C6H5CORI; R, = H, CH3, C2H5) in ethanol by irradiation with a 250 nm laser. The most intense peak shifts by mlz = 14 with an increase in the chain length of RI in C6H5COR1.
(see Figure 4). The peak shifts correspond to the mass differences between methanol and ethanol, and ethanol and 1-propanol. These results indicate that the ionic species associated with the most intense peak in the mass spectrum includes RI from the phenyl ketone and R2 from the alcohol. Figure 5 shows typical TOF mass spectra of ions produced from the solution of acetophenone in ethanol by laser irradiation at wavelengths of 250 nm (panel a), 280 nm (panel b), and 355 nm (panel c), at which the S3, S2, and SIstates, respectively, are expected to be excited. The spectrum does not change significantly with the excitation wavelength, although the intensities of the small peaks increase only slightly with the excitation wavelength. This finding indicates that each ion is produced through the same excited state. It is highly likely that a ketyl radical, C&C(OH)C&, is produced from an acetophenone molecule in the T I state by hydrogen abstraction
Reaction between C6HsCOR and an Alcohol
J. Phys. Chem., Vol. 99, No. 42, 1995 15629
CH3OH
135
h
.-VIc 4-
CH3CHzOH
149
=! n
2 .-x v 3
3!
Y
CH~CHZCH~OH
0
163
?J 30l 40
10
50
60
Laser power (pJ / pulse) Figure 6. Abundances of (C&)2COH+ and (C&)~CO(CZH~)+ plotted as a function of the laser power. The solid lines in this figure are drawn as an eye guide. 50
100
150
200
250
300
350
f i
Mass Number(m/z) Figure 4. TOF mass spectra of ions produced from 0.5 M solutions of acetophenone in methanol, ethanol, and n-propanol (R2OH; RZ = CH3, CZH5, C3H7) by irradiation with a 250 nm laser. The most intense peak shifts by mlz = 14 with an increase in the chain length of RZ in R2OH.
8
3
I
1
I
100
200
I
300 Mass (miz)
I
I
400
500
Figure 7. TOF mass spectrum of ions produced from 0.5 M solutions of benzophenone in ethanol by irradiation with a 250 nm laser. The ions Cd-I5C(OCzH5)CsH5+*(C2H5OH),,+(n 2 0) and C ~ H ~ C ( O H ) C ~ H P (CzH50H),+ (n 2 0). together with pinacol ions, ((C&)~COH)Z+,are observed in the spectrum.
Mass Number (m/z) Figure 5. TOF mass spectra for a 0.5 M solution of acetophenone in ethanol after irradiation with a (a, top) 250 nm, (b, middle) 280 nm, and (c, bottom) 355 nm laser. The peak assignments are given in the caption of Figure 2.
from ethanol and is ionized into C,&C(OH)CH3+ by absorbing photons before the dimerization reaction. In order to elucidate the formation mechanism, the abundances of the ions were measured as a function of the laser power. Figure 6 shows the abundances of (C&5)2COH+ and (C&)2CO(C2&)+ as a function of the laser power when a solution of benzophenone (R1 = C6H5) in ethanol is irradiated with a 250 nm laser. The abundances of all the ions increase similarly with the laser power. Figure 7 shows a TOF mass spectrum of ions produced from a 0.5 M solution of benzophenone in ethanol by irradiation with a 250 nm laser. In this spectrum, (C6H5)2COH+(EtOH), and (C6H5)2COC2H5+(EtOH), are observed in a low mass region.
In addition, ((CsH5)2COH)2+(EtOH), are observed in a high mass region, being produced by a dimerization reaction of ketyl radicals followed by photoionization in the vicinity of the liquid surface. Parts a and b of Figure 8 show the abundance of the pinacol ion as a function of the irradiation laser power and the concentration of benzophenone in the ethanol solution, respectively. As the laser power increases, the abundance of the pinacol ion starts to rise at a threshold laser power and tends to level off after passing through a slight maximum. On the other hand, the abundance of the pinacol ion does not change with the concentration. These tendencies are contrasted with those of the protonated benzophenone; the abundance of the protonated benzophenone ion continues to increase with an increase in the laser power and the concentration of benzophenone. 4. Discussion
4.1. Product Ions and Their Formation Processes. When an alcohol solution of an aromatic compound is irradiated with a W laser, cluster ions which consist of the solute and the solvent molecules are ejected into the v a c u ~ m . ~Similarly, -~ van der Waals cluster ions are expected to be ejected into the vacuum following multiphoton ionization in the liquid beam of the alcohol containing phenyl ketones. However, the mlz value of the dominant ionic species does not correspond to any
Kohno et al.
15630 J. Phys. Chem., Vol. 99,No. 42, 1995 I
I
I
I
I
I
1
1
Benzaldehyde1ethanol
h
VJ
I ._ E
?
Propiophenone /ethanol
n
.-m
e
e, E
8 0
10
20
30
40
50
60
Laser power (pJ / pulse)
SO
100
150
200
250
300
350
Mass Number (mlz) Figure 9. TOF mass spectra of ions produced from 0.5 M ethanol solutions of benzaldehyde, propiophenone, and benzophenone (CasCOR]; RI = H, CzH5, C a s ) by irradiation with a 250 nm laser. The reactant ions, C6H&(OH)RI+(marked with *), are observed in the spectra together with the product ions, C~H~C(OR~)RI+ (marked with +). The abundance of C&C(OR2)R~+relative to that of C&C(OH)R I + decreases drastically with the geometrical size of RI.
SCHEME 1 a{,R' 0
0.1
0.2 0.3 0.4 0.5 0.6
0.7
Concentration 1 M Figure 8. (a) Abundances of (C&)ZCOH+ and ((CsH&COH)2+ plotted as a function of the laser power. (b) Abundances of (C&)zCOH' and ((C~HS)ZCOH)Z+ plotted as a function of the concentration of benzophenone. The solid lines in the figure are drawn as an eye guide.
?
H
-
-
hv
a;,R~
I
?
1
H
-
Gas Phase
R2OH
a;,R1 0
__t
Gas Phase
R2
simple combination of the mass numbers of the solute and the solvent molecules. As shown in section 3, the dominant ionic species should include RI and R2 (see also Figures 3 and 4). By taking the findings mentioned above into consideration, it is concluded that the most intense ion is assignable to C&C(ORz)Rl+. In the conventional acetal formation reaction from a phenyl ketone and an alcohol, a solution of a phenyl ketone in an alcohol ought to be acidic, so as to prepare C6H5C(OH)Rl+ as a reaction precursor of the acetal formation. This precursor ion reacts readily with the alcohol to produce C&C(OR2)Rlf. In analogy with the conventional acetal formation reaction, C~H~C(OH)RI+ prepared by the laser irradiation should react with the alcohol RzOH, and C&C(ORz)Rl+ is considered to be formed. The subsequent ionization of the ketyl radicals under the intense laser irradiation is proved by the observation of the emission spectra of ketyl radicals under intense nanosecond UV laser irradiati~n.~'.~~ The results of the emission spectra indicate that the rates of intersystem crossing from the SIstate to the TI state of the phenyl ketone and the hydrogen abstraction reaction are sufficiently fast that the product ketyl radicals can further absorb photons in a nanosecond time range. When the ketyl radical C~H~C(OH)RI is ionized by absorbing more than one photon, a protonated phenyl ketone, C~H~C(OH)RI+, is produced in the ethanol solution. In C~H~C(OH)RI+, the carbon atom bonded to the OH group is positively charged and is readily
t H20
attacked by the oxygen atom of an alcohol molecule having a lone pair electron as a nucleophile, and C ~ H ~ C ( O R ~ ) RisI + generated as a result. In the mass spectra (see Figures 2 and 7), the reactant C~H~C(OH)RI+ is comparable in abundance to the product C&iC(OR)Rl+. As these two ions depend similarly on the laser power, both the ions require the same number of photons for their formation from the parent solute molecules (Figure 6 ) . This dependence is consistent with that predicted from the proposed formation mechanism (Scheme 1). In the reaction of protonated phenyl ketones with alcohols, the reactivity is expected to decrease with an increase of the geometrical size of R1. In order to elucidate the relation of the reactivity with the geometrical size of R,, the mass spectra of the ions produced from C ~ H ~ C O R(RI I = H, CH3, and C6H5) are shown in Figure 9. As shown in this figure, the relative abundance of the product ion C&C(OR2)Rt+ compared to that of the reactant ion C~H~C(OH)RI+ decreases with the geometrical size of RI. Therefore, the relative abundance can be used as a measure of the reactivity. 4.2. Formation of Pinacol Ion. It is well-known that the ketyl radical C~H~C(OH)RI in the excited TI state is dimerized to pinacol, ( C ~ H ~ C ( O H ) R IThough )~. the ketyl radical must be present under these experimental conditions, the pinacol ion
Reaction between C&COR and an Alcohol
J. Phys. Chem., Vol. 99, No. 42, 1995 15631
is hardly observed when a solution of benzaldehyde, acetophenone, or propiophenone in alcohol is irradiated with a W laser. This probably arises from the small probability of a bimolecular encounter of the ketyl radicals in the time window of the photoionization ( 5 10 ns). The pinacol ion could be observed when the time window of photoionization is sufficiently long or the probability of the bimolecular encounter is sufficiently large. In contrast, a pinacol ion, (C&C(oH)C&)2+, was detected in an alcohol solution of benzophenone. Since the time window of the photoionization is less than 10 ns as well, the probability of a bimolecular encounter of C&C(OH)C.& must be larger than those of other radicals. The abundance of the pinacol ion was measured as a function of laser power (see Figure 8a). As the laser power increases, the abundance of the pinacol ion starts to rise at a threshold laser power and tends to level off, while the abundance of the protonated benzophenone ion continues to increase. This laser power dependence is explained in terms of a simple kinematic model: (1) A benzophenone molecule (B) is excited into the TIstate (B*) by absorbing one photon. ( 2 ) B* abstracts one hydrogen atom from an alcohol molecule (ROH) to form a ketyl radical (BH). (3) BH either is dimerized to pinacol and ionized or is ionized before the dimerization. The whole process is expressed as B+hv-B* B* -I- ROH-BH BH
+ 2hv -BH+
2BH (BH),
+ (ROH - H)
-
(BH),
+ 2hv - (BH),'
(1)
dt
d[BHl = K,[B*][ROH] dt
the laser power dependences can be reduced as
[(BW2+1 = a[BlI (12) On the other hand, if the laser power is sufficiently large that the relation
bZ2 >> 4a~,[BlZ
(3)
(13) is satisfied, the laser power dependence can be further simplified as
(4)
[BH'] = a[B]Z
(2)
(14)
(5)
Assuming the steady state condition for [B*], [BHI, and [(BHhI, one obtains the relations
-d[B*l - aZ[B] - K,[B*][ROH] = 0
SCHEME 2
(6)
- bZ2[BH] - K,[BH]~= 0 (7)
where brackets designate the number density of the species associated with the reactions, a is the absorption coefficient of benzophenone, Z is the laser power, K, is the rate constant for the hydrogen abstraction reaction (process 2), b is the coefficient for the photoionization of ketyl radical (process 3), K~ is the rate constant for the dimerization reaction (process 4), and c is the coefficient for the photoionization (process 5) (see Scheme 2). By solving these relations, [Bl, [BHI, and [(BH),] are obtained. Since [BH+] and [(BH)2+] are proportional to [BH] and [(BH)2], respectively, the laser power dependences of [BH+] and [(BH)2+] are obtained as
where [BH+] and [BH2+] are the number densities of BH+ and BH2+, respectively. If K~ is sufficiently larger than bZ/4a[B],
The I-dependences derived from the model reproduce the experimental dependences as shown in Figure 8a. The decrease of [(BH)2+] with the laser power is explained as follows: The number density of the ketyl radical [BH] is proportional to the laser power, and the probability of the bimolecular encounter of BH is proportional to that for [BHI2. The increasing photoionization probability of BH with the laser power implies that the two competing paths operate for consumption of BH. When the laser power is relatively small, the probability of the bimolecular encounter is larger than that of the photoionization, whereas the probability of the photoionization surpasses that of the bimolecular encounter as the laser power increases, and as a result, [(BH)2+] decreases with the laser power. The pinacol ions were not produced from the alcohol solutions of benzaldehyde, acetophenone, and propiophenone but was produced from the alcohol solution of benzophenone. If the ketyl radicals are homogeneously distributed in the vicinity of the liquid surface, the probability of the bimolecular encounter is considered to be almost the same for all the ketyl radicals studied. In addition, [BH2+] is expected to be proportional to [B],, as shown by eq 15, but in reality, it remains constant regardless of [B]. These two findings lead us to conclude that benzophenone dimers are present initially in the vicinity of the liquid surface and serve as precursors of the pinacol formation reaction. The concentration of the dimer precursor seems to be invariable with the solute concentration. Seemingly, a high dimer concentration on the liquid surface results from the fact that the benzophenone molecules in the vicinity of the alcohol surface tend to orient with the two hydrophobic phenyl groups outside the surface because of the hydrophilic nature of the alcohol.
15632 J. Phys. Chem., Vol. 99, No. 42, 1995
5. Conclusion In a conventional acetal formation reaction, acids must be added to a solution of acetophenone in ethanol so as to prepare the reaction intermediate C&C(OH)CH3+. However, the acids very often disturb other reaction processes involved and are not selective for the protonation of acetophenone. In this regard, the photochemical reaction under intense laser light is particularly advantageous for dealing with a compound which has functional groups vulnerable to the acids. This new method of inducing and probing multiphoton chemical reactions on a liquid surface allows selective ionization of a given functional group of a molecule in solution and initiation of specific ion-molecule reactions.
Acknowledgment. The authors are grateful to Professor K. Narasaka for helpful discussion on the reaction mechanism. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. References and Notes (1) Eisenthal, K. B. Acc. Chem. Res. 1993, 26, 636. (2) Mafunb, F.; Takeda, Y .; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1992, 199, 615. (3) Mafunt, F.; Kohno, J.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 7 . (4) Mafunt, F.; Takeda, Y.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 234.
Kohno et al. (5) Kohno, J.; Mafunt, F.; Kondow, T. J . Am. Chem. SOC.1994.116, 9801. ( 6 ) Siegbahn, H. J . Phys. Chem. 1985, 89, 897. (7) Keller, W.; Morgner, H.; Muller, W. A. Mol. Phys. 1986,57,623. (8) Delahay, P. Acc. Chem. Res. 1982, 15, 40. (9) Ballard, R. E.; Jones, J.; Inchley, A.; Cranmer, M. Chem. Phys. Lett. 1988, 149, 29. (10) Faubel, M.; Schlemmer, S . ; Toennies, J. P. Z. Phys. D 1988, 10, 269. (1 1) Faubel, M.; Kisters, Th. Nature 1989, 339, 527. (12) Yang, N. C.; McClure, D. S . ; Murov, S. L.; Houser, J. J.; Dunsenbery, R. J . Am. Chem. SOC. 1967, 89, 5466. (13) Yang, N. C.; Dusenbery, R. L. J . Am. Chem. SOC. 1968, 90,5900. (14) Cohen, S. G.; Green, B. J . Am. Chem. SOC. 1969, 91, 6824. (15) Lewis, F. D. J . Phys. Chem. 1970, 74, 3332. (16) Cohen, S. G . ;Parola, A.; Parsons, G. H., Jr. Chem. Rev. 1973, 73, 141. (17) Anderson, R. W., Jr.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. Chem. Phys. Lett. 1974, 28, 153. (18) Topp, M. R. Chem. Phys. Lett. 1975, 32, 144. (19) TUITO, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, NJ, 1978. (20) Schaefer, C. G.: Peter, K. S . J . Am. Chem. SOC. 1980, 102, 7566. (21) Encinas, M. V.; Scaiano, J. C. J . Am. Chem. SOC. 1981,103,6393. (22) Simon, J. D.; Peters, K. S. J . Am. Chem. SOC. 1982, 104, 6543. (23) Manring, L. E.; Peters, K. S . J . Am. Chem. SOC.1985, 107, 6452. (24) Devadoss, C.; Fessenden, R. W. J . Phys. Chem. 1990, 94, 4540. (25) Warren, J. A.; Bemstein, E. R. J . Chem. Phys. 1986, 85, 2365. (26) Robbin, M. B.; Kuebler, N. A. J. Am. Chem. SOC. 1975, 97,4822. (27) Naqvi, K. R.: Wild, U. P. Chem. Phys. Lett. 1976, 39, 423. (28) Baumann, H.: Schumacher, K. P.; Timpe, H. J.; Rehik, V. Chem. Phys. Lett. 1982, 89, 315. JF950556M
