Anal. Chem. 1991, 63, 2715-2718 (8) Ikonomou, M. 0.;Blades, A. T.; Kebarle, P. Anal. C t " . 1991,63, 1989-1998. (7) B l a h , A. T.; Ikonomou, M. Q.; Kebarle, P. Anal. Chem. 1991,63, 2109-2114. (8) Ikommou. M. 0.;Blades, A. T.; Kebarle. P. J . Am. Soc. Mass Spectrom.1991,2 , 497. (9) Bailey, A. G. Electrostatfc Spraying ofL@u&; John WHey: New York, 1988, (10) Pteifer. R. J.; Hendricks, C. D. AIAA J . 1968,6 , 496. (11) Smith, D. P. H. IEEE Trans. Ind. Appl. 1988. IA -22, 527. (12) Hayatl. 1.; Bailey, A. 1.; Tadros, 1.F. J . ColloM Intefface Science 1987, 117, 205,222. (13)Landoit. Btirnstein. Zahlenwerte und FunMfOnen; Springer Verlag: Berlin, 1980;Vol. 11, pp 366. 533. 651. (14) (a) Shedlovsky. T.; Kay, R. L. J . Phys. Chem. 1958. 60, 151. (b) Longsworth, L. G.; MacInnes, D. A. J . Phys. Chem. 1939,43, 239.
2715
(15) Johnson, J. B.; Funk, G. L. Anal. Chem. 1955,2 7 , 1464. (16) Hancbodr of ChemLsby and Physbs, 59 ed.: CRC Press: Boca Raton, FL, 1976. (17) Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarle, P. J . Chem. Phys . 1990,92, 5900;Int. J . Mass Spectrom . Ion Processes l9S0, 102, 251. (18) Iribarne. J. V.; Thomson, B. A. J . Chem. Phys. 1976,64, 2287. (19) Busman, M.; Sunner, J.; Vogei, C. R. J . Am. SOC.Mass Spectrom. 1991,2 , I. (20) Wlen, M. Annal. Phys. 1927,83,327; 1928,85,759. (21)Onsager, L.: Klm, S. K. J . Phys. Chem. 1957,6 1 , 198.
RECEIVED for review May 29, 1991. Accepted September 13, 1991.
Regioselective Chloride Ion Loss from Chlorine-37 Enriched Polychlorodibenzofurans by Negative=Ion Mass Spectrometry Yoon-Seok Chang, J a m e s A. L a r a m b , and Max L.Deinzer* Department of Agricultural Chemistry and Environmental Health Sciences Center, Oregon State University, Corvallis, Oregon 97331
Regkrpeclflc chlorine-37 enrlched dkhloro-, trlchloro-, tetrachloro-, and pentachlorodlbenzofuranIsomers were Investlgated unduf Wren capture negattvekn m a s spectromeiry (ECNIMS) conditions. The relative chbrlde ion loss from the enrlched podtiom, was determined from the chlorlde-37 and chlorlde-35 Ion abundances after correctlng for the Isotopic enrlchment. Regloselectlve chloride anion losses were observed wlth the carbon 3-cMorlne bond (@&krhre) generally found to be the most labile. The carbon-chlorine bond cleavages can be loosely assoclated wlth calculated carbon-chlorlno bond reddual charges.
INTRODUCTION One-electron reduction of halogenated aromatic compounds occurs under ECNIMS conditions to give long-lived (>lo%) radical anions (I). The radical anion can undergo subsequent carbon-chlorine bond cleavage to give (M - XI- or X- ions (2). More often, halogenated aromatics capture an electron which leads immediately to loss of a halogen atom or anion. This dissociation is viewed as a single-step process; i.e. electron transfer and bond breaking are synchronous (3). Structural differences between parent molecules, however, lead to a wide range of fragmentation rates in the radical anion (4), thereby indicating that the single-step dissociative electron capture event is an over-simplified view of the process. Branching ratios, log [(M - Cl)-]/ [Cl-1, of polychlorinated dibenzo-p-dioxins (PCDDs) under ECNIMS conditions had previously been measured and correlated with CNDO-calculated total energies for the two dissociative electron capture pathways (4). Similar results were observed for polychlorinated dibenzofurans (PCDFs) (5). Implicit in these studies was the assumption that chlorine loss from the molecule under ECNIMS conditions is regioselective. This assumption has now been tested directly with the aid of regiospecific chlorine-37 enriched polychlorodibenzofuran isomers. Some useful generalizations concerning reductive dehalogenations of chlorodibenzofurans in the gas phase also *Corresponding author. 0003-2700/91/0363-2715.$02.50/0
can be made from the results reported in this study. The numbering scheme for these compounds is
EXPERIMENTAL SECTION Molecular Orbital Calculations. Most MO calculationswere performed using a newly developed AM1 (Austin Model 1)method (6). The AM1 method is known to be generally superior to MNDO (modified neglect of diatomic overlap) (7) especially in calculations of radical species (8). Some calculations were carried out by MNDO or by CNDO (complete neglect of differential overlap) (9) methods for comparison purposes. The AM1 version used in this study was adapted to a FPS (floating point system) supercomputer interfaced to a VAX 11/780. The MNDO and CNDO versions were adapted to an IBM-PC and parameterized to include the second-row elements. Accelerator hardware and software (Microway, Inc.) was used to decrease the program run time. Closed-shell systems (neutrals or anions) were calculated using the RHF (restricted H a r t r e e Fock) method whereas open-shell calculations (radicals or radical anions) were carried out using the UHF (unrestricted HartreeFock) method. Mass Spectrometry. Measurements were made on a Fmigan 4023 (4500 ion source) instrument employing a quadrupole electric field or on a Kratos MS50RF spectrometer using magnetic deflection analysis. The quadrupole instrument was tuned in the positive-ion mode using perfluorotributyl amine (FC 43) to give maximum resolution and a peak height ratio of m/z 219 to m / z 502 of 1O:l at the best overall sensitivity. A minimum of four measurements were recorded on two or more different occasions. The results typically agreed to within better than *5%. Isotope enrichments were measured on the Finnigan 4023 instrument under 70-eV electron impact conditions and 140 "C ion source temperature, and the mass resolution was set to the maximum capability of the instrument with the FWHM of two adjacent high mass peaks roughly equal to half their separation. Data were acquired by scan and MID modes and processed as centroid and raw data. Chloride losses were measured on the Kratos MS-50 instrument under ECNIMS conditions with methane at 0.6-Torr pressure BS reagent gas (IO).The temperature of the ion source was 200 "C, and the mass resolution was 1600. Source temperatures were varied from 120 to 250 "C. Peak sizes varied, but there was no effect on the isotopic ratios. Raw data 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 23, DECEMBER 1, 1991
Table I. Regioselective Chloride Ion LOSSby Electron Capture Negative-Ion Mass Spectrometry from Chlorine-37 Labeled Dichlorodibenzofurans (DCDF) %
I
I
I
200
100
compd no.
DCDF
1
2,8(37C1)-
2 3 4 5 6 7 8
1(37C1),6-
I
300
MASS Flgure 1. Electron capture negative-ion mass spectrum of 2,6,7(37CI)-trichlorodibenrofuranobtained from a Kratos MS-50 RF instrument (methane reagent gas at 0.6 Torr pressure, with ion source temperature at 200 OC). were acquired at 3 s/decade scan rate. Samples were introduced into the instrument by splitless injection through a DB-5 30 M x 0.25 mm i.d. GC column. The chloride background signal was of the signal being measured. Comnever more than 4 X parative studies between the two instruments were made with the results agreeing to better than 12%. Chemicals. Regiospecific chlorine-37 enriched dichloro-, trichloro-, tetrachlore, and pentachlorodibenzofuransused in this study were synthesized as described elsewhere (11). The strategy for introducing chlorine-37 isotope regiospecifically was to reduce a nitro derivative to the corresponding amine and converting the amine to the diazonium salt with tert-butyl nitrite. This product was then converted to the chloro compound via the Sandmeyer reaction using chlorine-37 labeled copper(1) chloride. The final products and all reaction intermediates should be handled with due respect to their toxicity. Purities of the polychlorodibenzofurans used ranged from a low of 10% in the case of 2,3,7,8-TCDF to 99% for 2,8-DCDF; all but two compounds were above 50%. Capillary column gas chromatographic delivery was optimized to assure only the isomer of the compound of interest was recorded in the analysis. A more polar DB-23 column (50% cyanopropyl; cf. 68% cyanopropyl for SP-2330) was also used to assure that only pure isomers had been recorded. A final check of the integrity of the results was made on the least pure samples using a 30 M SE-54 column-no interfering isomers were found. Isotope Enrichment Determination. Isotope enrichments were calculated by using an in-house program written for the HP-41 calculator (code available on request). The algorithm seeks to minimize the x2 between a synthetically generated isotopic pattern based on a modified binomial expansion and the experimental data. A new residual is calculated for each increment of chlorine37 abundance until a minimum is found which is within a tolerance of 10.05%. The synthetic isotope pattern is generated from the equation: (x + y)"'(A + B ) where x and y are natural abundances of chlorine-35 and chlorine-37 and m is the number of natural chlorines present. A and B are the chlorine-35 and chlorine-37 residuals due to the enrichment. Precisions are calculated by the propagation of error and reported at the 99% confidence level. Chlorine-37 enrichments generally were in excess of 90%.
RESULTS Regiospecific chlorine-37 enriched dichloro- (DCDF), trichloro- (TrCDF), tetrachloro- (TCDF), and pentachloro dibenzofuran (PCDF) isomers were ionized under ECNIMS conditions. The DCDF isomers yield (M- H)- clusters and intense C1- peaks, while the TrCDFs and higher chlorinated congeners typically give molecular ion cluster peaks, and C1(Figure 1). The (M- C1)- ion cluster also is observed for the TCDF and PCDF isomers. The relative chloride ion loss from the positions was determined from the observed [36Cl-]/[37C1-] ratios by solving the simultaneous equations ax + cy = I. bX + dy = IB where Z, is the relative intensity of the %C1- ion and ZB is the relative intensity of the 37Cl- ion obtained from the mass spectrum. The coefficient c is the residual of chlorine-35, and
2(37C1),63(37C1),62,7(37C1)3(37C1),42,3(37C1)-
37c1
enrichmenta
o/c excess 37C1-lossb
85.7 94.6 93.7 92.1 90.9 94.5
-0.7 f 1.8
73.1
95.1
1,2(37c1)-
3.6 f 1.0 1.7 f 1.9 18.0 6.5 14.3 1.8 33.2 f 1.6 9.2 f 5.0 12.5 f 3.7
* *
chargec 0.150 0.182 0.151 0.242 0.240 0.240 0.234 0.114
Values obtained from comparison of the electron impact mass spectrometrically derived molecular ion cluster with those calculated from ( x + y)"(A + B ) (see text). bValuesshown are the percent loss relative to statistical loss of total residual chlorine-35and excess chlorine-37 from the labeled position. Electron density less valence of labeled carbon-chlorine position. Table 11. Regioselective Chloride Ion Loss by Electron Capture Negative-Ion Mass Spectrometry from Chlorine-37 Labeled Trichlorodibenzofurans (TrCDF) % 37c1
compd no. 9
10 11 12 13 14
TrCDF
Enrichmenta
2,3,7(37C1)1,2(37C1),32,3(37C1),81,4,7(37C1)2,6,7(37C1)2,3(37C1),4-
90.1 90.7 90.8 90.0 91.9 93.2
% excess 37Cl-lossb
-10.0 f 2.0 -11.6 3.8 11.1 13.4
*
-0.2
* 2.2
19.8 f 2.3 -5.9 f 3.2
chargec 0.220 0.0816 0.215 0.218 0.219 0.214
Values obtained from comparison of the electron impact mass spectrometrically derived molecular ion cluster with those calculated from (x + y)"(A + B ) (see text). bValuesshown are the percent loss relative to statistical loss of total residual chlorine-35and excess chlorine-37 from the labeled position. Electron density less valence of labeled carbon-chlorine position. (I
d is the relative excess of chlorine-37 from regiospecific labeling. In most cases the chlorine-37 enrichment was >go%. The value x is the fraction of chlorine lost from the unenriched position(s), and y is the fraction of chlorine loss from the position enriched with chlorine-37. Natural abundance of the chlorine isotopes 35C1/37C1was determined by mass spectrometry to be 0.7555 and 0.2445 f 0.0022 (99'70 confidence limit). TrCDF values for a and b, respectively, are 2 times these values and for TCDFs they are 3 times these values, etc. For ease of comparison between isomers with different degrees of chlorination, the loss of chlorine is presented (Tables 1-111) relative to the statistical value. A positive value, therefore, means more chlorine than a statistical amount is lost from that position and a negative value means less than a statistical amount is lost. In this form comparisons can be made directly between isomers and congeners of differing isotopic enrichments and degrees of chlorination. Loss of chlorine from several unlabeled positions in polychlorodibenzofurans, of course, cannot be distinguished using a single chlorine-37 label in the same molecule, but in several cases the same isomer had been synthesized with the label regiospecifically placed in two different positions in separate molecules. The relative losses of two chlorines in two polychlorinated dibenzofuran isomers could, thus, be compared. The equations for determining the regiospecific chlorine loss from position 7 of 2,6,7-trichlorodibenzofuran(Figure l),are then 0 . 7 5 5 5 ~ 0 . 0 7 8 ~= 0.3955 0 . 2 4 4 5 ~ 0 . 9 2 2 ~= 0.6045
+ +
and yield a y value of 53.13% which corresponds to an excess
ANALYTICAL CHEMISTRY, VOL. 63,NO. 23, DECEMBER 1, 1991
Table 111. Redoselective Chloride Ion Loss by Electron Capture Negative-Ion Mass Spectrometry from Chlorine-37 Labeled Polychlorodibenzofurans yo 37c1
compd no. 15
16 17 18
furan 1,2(37C1),3,4-TCDF 1,2,3(3'C1),4-TCDF 2,3(37C1),7,8-TCDF
enrich-
%
menta
37Cl- lossb
49.3
85.6 92.5
excess
-15.4 f 6.2
15.6 f 7.6 2.3 f 2.9 3.1 f 4.8
chargeC 0.062 0.202 0.196
0.123
1,2,3,4,9('"Cl)-
95.0
19
PCDF 1,2,3,4,6(37C1)PCDF
96.0
-16.3
f 4.6
0.109
20
1,2,3,4,8(37C1)-
95.0
-12.8 f 2.8
0.139
PCDF 21
1(37C1),2,3,4,5,6-
91.7
4.11 f 7.2
0.196
hexachlorobenzene a Values obtained from comparison of the electron impact mass spectrometrically derived molecular ion cluster with those calculated from ( x + y)"(A + E) (see text). *Valuesshown are the percent loss relative to statistical loss of total residual chlorine-35 and excess chlorine-37 from the labeled position. CElectrondensity less valence of labeled carbon-chlorine position.
chlorine-37 loss of 19.80% above the statistical loss of 33.33% for a trichloro compound. Effects of pressure and temperature in the ECNIMS determinations of [35C1-]/ [37C1-]ratios were tested with several of the isomers. The sensitivity and the ratio of chloride ion versus the molecular ion vary significantly with variation in ion source pressure and temperature, but the [35C1-]/[37C1-] ratio, Le., the relative bond cleavage rate, changes by less than 5% within the range 0.3-1.2-Torr pressure and 100-250 "C source temperature. The symmetrical 2,8(37C1)-DCDF(Table I) shows no detectable chlorine37 isotope effect when analyzed by ECNIMS. Chlorine comes off equally from each of the two positions. A regiospecific chlorine-37 enriched hexachlorobenzene also showed that 16.6% of the chloride ion comes from the labeled position under ECNIMS conditions. This is in sharp contrast to other systems in which very large chlorine isotope effects have been observed (12). In each of the unsymmetrical DCDF isomers (Table I), however, loss of one of the chlorines is preferred over the other. Chlorine at position 3 (or 7), i.e. the @-position,is most labile, as shown by comparison of the results for 2,7(37C1)-and 3(37C1),6-DCDF.When both chlorines are in the same ring, the @-positionstill is most labile, as shown by the results for the 2,3(37C1)-and 3(37C1),4-DCDFisomers. The y-position is more reactive than the &position, as shown by analysis of 1,2(37C1)-DCDFs. However, the y-position is very close in reactivity to the a-position, as shown by the results for 2(37C1),6-DCDF. The @-chlorineof TrCDFs (Table 11) is most labile in instances in which there is an adjacent chlorine and a third one in the other ring, as shown by the 2,3(37C1),8-and 2,6,7(37Cl)-TrCDFisomers. But, surprisingly, chlorine-3(7) is not particularly labile in 1,4,7(37C1)-and 2,3(37C1),4-TrCDFs.The results for the latter isomer shows there is not much of an effect arising from steric relief. There is also no rate enhancement that could be attributed to steric relief in the reductive dechlorination of the 1,2(37C1),3-TrCDFisomer as the middle carbon-chlorine bond is ruptured only 22% of the time or 12% less frequently than would be expected on a purely statistical basis. The results from the analysis of 2,3,7(37C1)-TrCDFin comparison to 2,3(37C1)-DCDFindicate that an isolated chlorine in the @-positiondoes not compete with a &chlorine that has an adjacent chlorine atom. Two tetrachloro isomers, 1,2(37C1),3,4-TCDFand 1,2,3(37C1),4-TCDFshow the @-positionis very labile in comparison
2717
to the y-position (Table 111). The 2,3(37C1),7,8-TCDFisomer shows cleavage of the 3-chlorine to be nearly statistical. By symmetry it is concluded that the relative loss of the @chlorines is 55% and the loss of y-chlorines is 45%. In comparison of these values with the 2,3-DCDF isomer, it is noted that 59% of the chlorine comes from the @-position. The difference can be ascribed to resonance interaction between the rings; chlorines in one ring influence those in the other. The pentachloro isomers further show the unreactive nature of the y- as well as the a-positions. The &position shown by loss of the 9-chlorine from 1,2,3,4,9(37C1)-PCDFgave a statistical loss from this position, even where large steric interactions would be expected to enhance the loss of chloride ion from the polychlorinated ring.
DISCUSSION In studying the reduction mechanism of haloaromatics, it is important first to establish which orbital ( u or A ) receives the ionizing electron. In this regard it has been demonstrated that there is a relationship between the A LUMO energies and the product distribution, Le., the branching ratios of chloride and (M - C1)- ions versus the molecular radical anion abundances (2). But a possible relationship between the u LUMO energies and these branching ratios was not investigated. In studies of the reduction of organohalogen compounds using a simple MO method, Fukui and co-workers (13) suggested that the LUMO for conjugated organic polyhalogenated compounds might be a u rather than a A orbital since the u LUMO energy levels decrease with increasing chlorine substitution and the A LUMO levels do not change significantly. There seems not to be a connection between the half-wave reduction potentials and the A LUMO states, either. Beland and coworkers (14) also suggested on the basis of CNDO/2 calculations that the ionizing electron is accepted into the u-orbital of chlorobenzenes. If electron capture is represented as the addition of an electron to a u antibonding orbital of the molecule, then a given carbon-chlorine bond surely would be weakened and bond cleavage should proceed quickly. The electron density in the antibonding orbital is localized in the carbon-chlorine bonds. In this case there may exist meaningful correlations between the u LUMO electron densities and the observed distribution of products. The electron density distribution in the u LUMO calculated for the neutral chlorobenzenes correctly predicted the major electrochemical reduction products and gave reasonable estimates of the amount of each isomer formed (14). However, this argument is unsatisfactory when a more sophisticated MO calculation method, Le. the MNDO method, is used. MNDO calculations predict almost all chlorobenzenes to have lower A LUMO energy states (15). The AM1 method ( 6 ) , which generally gives closer approximations to experimental values than do either the MNDO or CNDO method, was used to calculate the LUMO energies for polychlorodibenzofurans. The results show that with increasing chlorine substitution both u and A LUMO energies decrease, as expected, but the average energy differences between the two states remains about 1.8 eV, the A LUMO energy being lower. Open-shell calculations by the AM1 method were used to obtain energy estimates of the singly occupied molecular orbital (SOMO). Each SOMO was found to be a A orbital with average energy ( ~ 7 . eV) 3 much lower relative to that of the corresponding 0 LUMO. These results strongly suggest that ionizing electrons enter the A antibonding orbital in chlorodibenzofurans. If this view is adopted, an explanation is required for the bond cleavage process since the transition from the A* state to the u* state necessary for bond cleavage to occur is nonallowed. We have previously invoked a ubentn bond model to describe the bond cleavage process (Scheme
2718
ANALYTICAL CHEMISTRY, VOL. 63,NO. 23, DECEMBER 1, 1991
Scheme I a
6
a
9
1
1
2
:
A
v)
3 = 0
302010-
tended delocalization, to be sure, is gained at the expense of aromaticity in the participating ring. Even without considering it, the intermediate radical anions resulting in @-, 6carbon-chlorine bond cleavage should be more stable because a resonance contributor proceeding to cleavage of the a-or y-chlorines places the negative charge next to the oxygen, which is highly undesirable. The high degree of reactivity of the @-chlorinemay be due in part to a resonance form which places the charge on the benzylic carbon and provides maximum charge separation in the radical anion. This scheme is only reasonable if the dissociative electron capture reaction, in fact, involves the A orbital electrons. Several congeners, for example, compounds 8,9, and 14 are far from the band of data established by the other 19 data points (Figure 2). It is entirely possible that higher energy states are populated by ionizing electrons which lead to dissociative electron capture events. We have constructed an electron monochromator which allows electron energy scanning of higher negative-ion resonance states populated by low-energy electrons. This instrument should provide the tools to monitor the appearance energies for the dissociative electron capture processes and give some clues on the nature of the negative-ion resonance states that are responsible for the ejection of chloride ions from different isomers (21). Finally, the calculations also must be recognized as being limited. Higher level calculations would be desirable.
ACKNOWLEDGMENT
U
0.0 0.1 0.2 0.3 CHARGE AT CARBON - CHLORIDE37 CLEAVAGE SITE Figure 2. Regioselective chloride ion loss as a function of charge on the separating carbon-chlorine centers. Compound identification numbers are those referred to in Tables 1-111. Error bars are at the 99 % confidence level.
We thank Brian Arbogast and Donald Griffin for assistance in obtaining mass spectrometry data. We also thank Peter Freeman for helpful technical discussions. This paper is issued as Technical Paper No. 9251 from the Oregon Agricultural Experiment Station.
I) (16). As the carbon-chlorine bond stretches in this model, the chlorine "bends" toward the plane of the A system and the unpaired electron becomes increasingly localized in the sp2orbital while the system rearomatizes. The energy for this transition is provided by resonance stabilization in the 7 system and by the high electron affinity of the chlorine atom. Localized charges have been calculated for carbon-chlorine bonds by subtracting the number of valence electrons from the charge densities obtained by the AM1 method for the fully geometry optimized chlorodibenzofuran radical anions. A subtle, but definite correlation was found between the degree of regioselectivity of chloride ion loss (from the labeled position) and the carbon-chlorine charges at the chlorine-substituted position in the radical anion (Figure 2). A linear regression analysis gives a correlation coefficient of 0.64, which is highly significant at the 99% confidence level. The carbon-chlorine bond with the highest charge is the one which preferentially cleaves. Thus, at least some localization of charge accounts for the preferred fragmentation. If the results are viewed in terms of an electron capture process leading to a stabilized Meisenheimer-type intermediate (17,18) from which dissociation occurs (Scheme I), a reasonable explanation for the preferred bond cleaved can be given. From the results of the DCDF isomers it is observed that the @-chlorine(Table I) is considerably more labile than the others. The &chlorine is next most reactive, followed by the y-chlorine. The a-chlorine is least reactive. It is apparent (Scheme I) that the intermediate radical anions from which the @- and &chloride ions arise, according to the mechanism proposed above, are stabilized better through delocalization of the charge over both aromatic rings. This principle has been invoked previously to account for the relative stabilities of isomeric oxygenated chlorodibenzofuran molecular ions (19) and intermediates (20) under ECNIMS conditions. The ex-
Johnson, J. P.; McCorkle, D. L.; Chrlstophorou, L. G.; Carter, J. 0.J . Chem. Soc., Farehy Trans. 2 1975, 7 1 , 1742. b r a & , J. A,; Chang, Y . 4 . ; Arbogast, B. C.; Delnzer. M. L. Blamed. Environ. Mass Spectrom. 1988. 17, 63. Naff,W. T.; Cooper, C. 0.;Compton, R. N. J . Chem. phys. 1968,49,
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2359. Fukul. K.; Mordcuma, K.; Kato. H.; Yonezawa, T. BUU. Chem. Soc. Jpn. 1963,36, 47. Beland, F. A.; Farwell. B. S. 0.;Callls, P. R.; Qeer, R. D. J . E l e c l r w M I . Chem. Interfac&l Electrod". 1977, 78. 145. OHdewell. C. Chem. Scr. 1985, 20, 142. Freeman, P. K.; Srinlvase, R.; Campbell, J.-A.; Deinzer, M. L. J . Am. Chem. Soc.1986, 108, 5531. Strauss, M. J. Acc. Chem. Res. 1974,7 , 181. Castelhano, A. L.; Griller, D. J . Am. Chem. Soc. 1962, 104. 3655. Delnzer, M. L.; Grlffln, D.; MIHer, T.; Lamberton. J.; Freeman, P.; Jonas, V. B(0med. Mass Spectrom. 1982. 9 , 85. Campbell, J. A. B.; Griftin, D. A.; Delnzer. M. L. Urg. Mass Spectrom. la65 2 0 , 122. LaramBe. J. A.; Mahiou, B.; Delnzer, M. L. I n hcedhgs oftho 39th ASMS Conference on Mass Spectrometty and A W Topics, 1991.
RECEIVED for review June 20,1991. Accepted September 13, 1991. This work was supported by the National Institutes of Health (Grants NIEHS ES00210 and NIEHS ES00040).
