J. Phys. Chem. 1992,96,715-726 pattern. In our case it would appear that the through-bond forces are of negligible intensity and through-space interactions are dominant. An explanation that occurs to us derives from the fact of the flexibility of our spacer system. Cave et al.23have calculated the electronic matrix element for the electron transfer between a pair of porphyrins. They find that the mutual orientation of the donor and acceptor states has severe consequences on the magnitude of the coupling which reduces to zero in some orientations. In their review article Class and Millerlb put forward a pictorial description of how coupling between the LUMO's of donor and acceptor is achieved via the LUMOs of the spacer groups. Extending the conclusions of Cave et al.23the magnitude of such couplings will be affected by the orientations of the intervening bonds. In a rigid system the interbond alignments are presumably optimal whereas in flexible spacers such as ours the molecules will visit a multitude of conformations only some of which will be capable of providing effective LUMO coupling. In this way through-bond interactions may be diminished. In summary, we conclude that the intramolecular electron transfer between reduced viologen radicals and a quinone occurs with an efficiency that increases as the number of intervening bonds increases. This is antisense to what is required for any (23) (a) Cave, R. J.; Siders, P.; Marcus, R. A. J. Phys. Chem. 1988, 90, 1436-1444. (b) Siders, P.; Cave, R. J.; Marcus, R. A. J . Chem. Phys. 1984, 81, 5613-5624.
715
mechanism (e.g., through bond or through space) that has a diminishing interaction with donor-acceptor distance. Such an effect can be understood if the intervening chain has a degree of flexibility that allows conformations to occur that place D and A residues in conjunctions that allow close through-space (i.e., collisional) interactions. The longer the chain, the more such interactions are favored, i.e., flexibility increases with chain length. However, our simple calculations on what intramolecular rates would be expected show that the probability of these close encounters occurring between D and A pairs is very small.
Acknowledgment. The electron pulse radiolysis experiments were carried out at the Center for Fast Kinetics Research, which is supported jointly by the Biotechnology Program of the Division of Research Resources of NIH (RR 00886)and by the University of Texas at Austin. Partial support for this work came from NIH grant GM 31603 (to M.A.J.R.). We are very grateful to B. K. Naumann for his expert technical assistance in operating the electron accelerator. We thank Mark Meier and Tom Mallouk from the Chemistry Department, University of Texas at Austin, for their advices with the synthesis of the molecule MVQ, and A. J. Bard and G. Denuault from the Chemistry Department, University of Texas at Austin, for assistance in the electrochemical measurements. Supplementary Material Available: Synthesis of the molecules MVQ and MVnnQ ( n = 3, 5 , 7 and n' = 2, 3) (12 pages). Ordering information is given on any current masthead page.
Sodium/Sulfur Chemical Behavior in Fuel-Rich and -Lean Flames Keith Schofield* and Martin Steinberg Department of Chemistry, University of California. Santa Barbara, California 931 06 (Received: February 28, 1991)
An experimental and analytical program examining sodium/sulfur chemistry has been conducted in a series of fuel-rich and -lean H2/02/N2flames, with and without added sulfur, and covering a wide range of temperatures and stoichiometries. Fluorescence measurements of OH and Na downstream profiles and sodium line reversal temperatures provided a broad data base for kinetic modeling. Analysis indicated NaS02to be the only significant sodium/sulfur product formed in the lean flames. Even so,its concentrationsremain an extremely small fraction of the total sodium. The more important perturbation of the distribution of sodium over its molecular forms results from the catalytic effect of sulfur on the flame radical concentration levels rather than from the formation of additional species. A bond dissociation energy of Doo(Na-S02) = 197 20 kJ mol-' is derived assuming a nonplanar structure or 210 f 20 kJ mol-' if the molecule is planar. NaOS is dominant in the rich flames, coupled with small contributions from NaS02, NaSH, NaS, and NaS2. Together, these can constitute from about 10 to 20% of the total flame sodium and do represent in this case an enhancement of molecular formation. Preliminary data in fuel-rich C2H2/02/N2flames are consistent with this model. This further illustrates the general insensitivity of alkali-metal chemistry to fuel type. When coupled to other available data, the low levels of NaS02 estimated for the present fuel-lean flames tend to suggest that Na$04 formation in the gas phase appears to be unlikely under most practical conditions. However, this still cannot be rigorously ruled out for all situations and requires further study to resolve the critical dependences that control high-temperature Na2S04corrosion.
Introduction Sodium sulfate was identified 45 years ago as a serious corrmive agent in the ash deposits on power plant boiler tubes.' Subsequently, the surprising corrosive character of sodium sulfate also was noted in deposits on oxidation resistant alloys in combustion driven gas t~rbines.2~Whether it is formed through homogeneous gas-phase chemistry in the combustion gases or is produced heterogeneously on the surfaces has remained an unanswered question. There is no chemical kinetic information for modeling (1) Reid, W. T.; Corey, R.C.; Cross, B. J. Trans. ASME 1945, 67, 279. Corey, R. C.; Cross, 9. J.; Reid, W. T. Trans. ASME 1945, 67, 289. (2) Simons, E. L.; Browning, G. V.; Liebhafsky, H. A. Corrosion 1955, 11, 505. (3) de Crescente, M. A.; Bornstein, N . S. Corrosion 1968, 24, 127.
this aspect of the problem. Consequently, past efforts to analyze the gas/surface interactions have been forced to invoke limiting conditions of frozen chemistry or chemical equilibrium for the sodium/sulfur system.4~~ Although an extensive literature has developed concerning sodium sulfate in combustion systems, only a few efforts have been made to understand the underlying sodium/sulfur flame chemistry. Fenimore6 reported a decrease of Na with the addition of SO2 (4) Fryburg, G. C.; Miller, R. A.; Stearns, C. A.; Kohl, F. J. High Temperature Metal Halide Chemistry. The Electrochemical Society Softbound Proceedings, Hildenbrand, D. L., Cubicciotti, D. D., Eds.; Princeton, NJ; 1978; Vol. 78-1, p 468; (also N A S A TM-73794, 1977). (5) Rosner, D. E.; Chen, B.-K.; Fryburg, G. C.; Kohl, F. J. Combust. Sci. Technol. 1979, 20, 87. (6) Fenimore, C. P. Symp. (Int.) Combust. [Proc.] 1973, 14, 955.
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716 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
I
to rich and lean H2/air flames. He assumed the flame chemistry to be fully equilibrated and attributed the sodium decay to reaction 1. By further supposing that the sodium was distributed solely NaOH + SO2 = NaS02 + OH (1) between its Na, NaOH, and NaS02 forms, he derived an equilibrium constant value of K1 = 5 exp(-71 kJ mol-’/R7‘). Durie et ai.,’ in a study in rich and lean C3H8/O2/N2flames, assigned instead the corresponding sodium decays to the equilibration of reactions 2 and 3, respectively, and derived equilibrium constants NaOH + S = NaS + OH (2) NaOH
MONOCHROMATOR
+ SO3 = NaS03 + OH
(3) K2 = 8 X lo5 exp(-243 kJ mol-’/RT) and K3 = 2 X lo6 exp(-172 kJ mol-’/RT). The flame chemistry in this study was also assumed to be fully equilibrated. They felt that Fenimore’s leanflame data were better correlated by reaction 3 than by reaction 1. Fryburg et al.4 identified NaS02, NaS03, and Na2S04 as gas-phase products in a mass spectrometric study of lean CH4/02 flames seeded very heavily with sodium and sulfur. They reported clogging of the sample inlet orifice, which limited data gathering and interpretation. Thus it is not possible to conclude whether the observed species are in fact homogeneous gas-phase flame products. The current study is an extension of two earlier investigations in this laboratory, the first on sulfur chemistry8in rich H2/02/N2 flames and the other considering sodium oxidation chemistry9J0 in lean H2/02/N2flames. The conclusions of each of these now have been confirmed elsewhere.’1-13 It is well established that the radicals H, 0, and OH in the hot burnt gases of H2/02/N2flames can sharply overshoot their equilibrium levels through rapid bimolecular chain reactions 4-6 H + 0 2 = OH + 0 (4)
0 + H2 = OH + H H2
TUNGSTEN LAMP
+ OH = H20 + H
AMPLl F I ER
INTEGRATOR
Figure 1. Optical arrangment.
and its concentration can be assumed to be in steady state through reactions 7-9. SO2 + 0 M = SO3 + M (7) SO3 + 0 = SO2 O2 (8)
+
+ SO3 + H = SO2 + OH
(9) The oxidation of sodium in fuel-lean, sulfur-free, H2/02/N2 flames was found to exhibit an interesting c h e m i ~ t r y . ~The J~ performance of the total model, consisting of 17 possible reactions, is approximated by reactions 10-14. The dominant oxidation Na O2 + M = N a 0 2 + M (10) N a 0 2 + OH = NaOH + O2 (1 1)
+
+ OH NaO + H 2 0 = NaOH + OH Na + H 2 0 = NaOH + H N a 0 2 + H = NaO
(5)
(6)
which are balanced (equilibrateddue to their large kinetic fluxes in an otherwise nonequilibrated flame environment). From measures of OH, temperature, the overall flame stoichiometry, and the equilibrium constants for reactions 4-6, the nonequilibrium concentrations of H, 0, H2, 02,and H 2 0 in rich and lean flames are calculable, using such partial equilibrium concepts. The radicals recombine through relatively slow three-body reactions (H + H, 0 + 0, H + OH, and H + 02).With the addition of sulfur to these flames, radical recombination is c a t a l y ~ e d , ’ and ~-~~ in turn the decaying radical concentrations control the partial equilibrium balances known to exist among SO3,SO2,SO, H2S, SH, S2,and S.8 As with the flame radicals, the concentrations of these sulfur species are coupled and can be calculated from measured OH, temperature, total sulfur content, and the equilibrium constants for the appropriate coupling reactions. SO3 is exceptional, being formed only by a slow termolecular reaction,”J8 (7) Durie, R. A.; Johnson, G. M.; Smith, M. Y. Symp. (Int.) Combust. [Proc.] 1974, 15, 1123. (8) Muller, C. H., 111; Schofield, K.; Steinberg, M.; Broida, H. P. Symp. (Int.) Combust. [Proc.] 1978, 17, 867. (9) Hynes, A. J.; Steinberg, M.; Schofield, K. J . Chem. Phys. 1984,80, 2585. (10) Steinberg, M.; Schofield, K. The High Temperature Chemistry and Thermodynamics of Alkali Metals (Lithium, Sodium, Potassium) in Oxygen Rich Flames. Paper 3A-042 presented at the Fall Joint Meeting of the Western States and Japanese Sections of the Combustion Institute, Honolulu, HI, Nov 1987. (11) Zachariah, M. R.; Smith, 0. I. Combust. Flame 1987, 69, 125. (12) Kasparov, M. G.; Mokhov, A. V.; Nefedov, A. P. High Temp. USSR 1988, 26, 463.
(13) Slack, M.; Cox, J. W.; Grillo, A.; Ryan, R.; Smith, 0. Combust. Flame 1989, 77, 3 1 1. (14) Fenimore, C. P.; Jones, G. W. J . Phys. Chem. 1965,69, 3592. (15) Durie, R. A.; Johnson, G. M.; Smith, M. Y. Combust. Flame 1971, 17, 197. (16) Kallend, A. S. Combust. Flame 1972, 19, 227. (17) Astholz, D. C.; Glanzer, K.; Troe, J. J. Chem. Phys. 1979,70,2409.
(12) (1 3)
(14) path occurs through a surprisingly fast 3-body process, reaction 10, the kinetic nature of which is now well The Na02 reacts with the radicals, H and OH, and can produce a very large overshoot in NaOH concentrations, which are balanced with small amounts of NaO in the very fast reaction (13). The free sodium profiles closely track the H atom profiles in the lean flames. This is mainly a consequence of the low H atom concentrations in these flames limiting the conversion of NaOH back to Na via reaction 14 and is the major factor in controlling the overshoots of the NaOH. In rich flames, reaction 10 is negligible and sodium oxidation is slight, controlled predominantly by reaction 14. On the basis of this information, the concentration profiles for Na, Na02, NaOH, and NaO in the sulfur-free flames can now be calculated from measured OH, temperature, flame stoichiometry, and the total sodium concentrati~n.~J~ The complementary nature of the rich- and lean-flame systems is of great value to this study. In lean flames, the sodium chemistry is complex and that of the sulfur is quite simple, and under rich conditions, exactly the opposite is found. When the two are present simultaneously, the possible sodium/sulfur chemistry is superimposed on their individually complex yet reasonably well understood systems. What is found is that sodium sulfidation chemistry competes with sodium oxidation, but both are seen to be heavily (18) Smith, 0.I.; Tseregounis, S.;Wang, S.-N. Int. J. Chem. Kinet. 1982, 14, 679.
(19) Husain, D.; Plane, J. M. C. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 163.
(20) Silver, J. A.; Zahniser, M. S.;Stanton, A. C.; Kolb, C. E. Symp. (Znt.) Combust. [Proc.] 1984, 20, 605. (21) Husain, D.; Marshall, P.; Plane, J. M. C. J. Chem. SOC.,Faraday Trans. 2 1985,81, 30 1. (22) Husain, D.; Marshall, P.; Plane, J. M. C. J. Photochem. 1986, 32, 1. (23) Plane, J. M. C.; Rajasekhar, B. J . Phys. Chem. 1989, 93, 3135. (24) Marshall, P.; Narayan, A. S.; Fontijn, A. J. Phys. Chem. 1990,94, 2998.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 717
Sodium/Sulfur Chemical Behavior in Flames TABLE I: E x p e h t d Flame Matrix rad Burnt Gls Roperties H21021N2
temp, K
gas flow, cm3/scm2 133
[H2Olb
[021b
------
W21b
----------
PHIb
0.611 11 1906-1 929 9.9 (17)c 1.2 (18) 5.2 0.3 (1 5)” 4.0 0.9 (16) 2066-2 100 160 9.9 (17) 4.8 (17) 7.4 0.9 (15) 3.7 1.3 (16) 11112 4.7 (17) 23 0.2 (15) 4.2 0.4 (16) 1667-1730 71 9.3 (17) 11113 9.9 (17) 2.0 (1 7) 18 2.9 (15) 4.0 1.6 (16) 2125-2197 142 1.41113 3.8 0.6 (16) 1.41114 1810-1847 89 9.7 (17) 2.0 (17) 45 0.8 (15) 1.4/1/5 1654-1 669 58 9.0 (17) 1.9 (17) 75 0.4 (15) 3.8 0.3 (16) 1.1 (18) 5.9 (16) 48 13 (15) 4.4 2.1 (16) 1.81113 2280-2405 196 54 8.8 (1 5) 3.8 1.5 (16) 5.3 (16) 1.81114 2060-2228 133 9.8 (17) 1.81115 1825-1916 98 9.8 (17) 5.6 (16) 65 4.4 (1 5) 3.3 0.8 (16) 1.81116 1695-1726 62 9.5 (17) 5.5 (16) 72 2.5 (15) 3.0 0.5 (16) 187 1.0 (18) 20 2.5 (15) 1.1 (17) 3.8 1.3 (16) 2.21114 2160-2308 2.21115 1975-2 106 124 9.5 (1 7) 23 1.7 (15) 1.0 (17) 3.2 0.8 (16) 9.9 (16) 7.0 0.5 (16) 98 9.3 (17) 65 1.2 (15) 2.21116 1780-1897 2050-2167 21 1 9.7 (17) 3.2 0.05 (15) 4.8 (17) 2.3 0.3(16) 31114 9.2 (17) 3.6 0.03 (15) 4.6 (17) 2.0 0.2 (16) 152 1883-201 0 31115 1775-1857 120 8.8 (17) 4.0 0.03 (15) 4.4 (17) 1.8 0.2 (16) 31116 1898-2042 225 9.1 (17) 4.2 0.08 (14) 9.1 (17) 1.0 0.2 (16) 41114 8.7 (17) 0.8 0.1 (16) 1767-1900 161 8.8 (17) 4.1 0.08 (14) 41115 5.0 0.07 (14) 8.2 (17) 0.7 0.1 (16) 1650-1788 134 8.4 (17) 411 16 molecules/cm’. CConccntrations;read as 9.9X lo”, average downstream “Gas flow rate to the burner per cm2 of burnt flame gases at 298 K. value. “Concentrations: read as 5.2 X lOI5 at 0.25 ms downstream from the reaction zone, decaying to 0.3 X 10” at 4.0 ms.
----+
+
influenced, if not controlled, by the flame radicals and the radical/sulfur chemistry interaction. Although potentially very complex, as will be seen, the system has been amenable to a reasonable analysis.
surements are absolute values calibrated against a high-temperature flame (H2/02/N2 = 4/ 1/2, T = 2500 K) in which O H is at thermodynamical equilibrium.28
Experimental Conditions Measurements are made in the laminar, premixed, one-dimensional flow of the post-flame g a m above a Padley-Sugdenzs burner of bundled hypodermic tubing with an overall diameter of 2.2 cm. Sodium is introduced as an aqueous NaN03 aerosol generated in an ultrasonic nebulizer26 and transported by a portion of the gas flow to a 1.1-cm diameter central core of the burner. Sulfur is added as H2S to rich and as SO2 to lean flames to minimize the thermal contributions these additives make to flame properties. Except for the small change in stoichiometry and the accompanying thermal effect, the additions of H2S or SOzto a given H2/02/N2flame are equivalent sources of sulfur yielding approximately the same distribution of sulfur among the species SO3,SO2,SO,H2S, SH, S2,and S8 The central burner core is surrounded by an annular shield section which bums the same basic flame but without added sodium or sulfur. A schematic of the optical system is shown in Figure 1. Temperatures are determined by using the sodium line reversal method. They were obtained at various points (0.254 ms downstream) through the burnt gases region for all the flame conditions studied. Sodium and OH concentrations are measured by laser-induced fluoresc e n ~ e ~ .utilizing ~ ’ . ~ a YAG-pumped dye laser source beam passed through the flame parallel to the burner surface. The fluorescence, from a slice of the laser-excited flame volume at flame center, is collected by a 6-in mirror and focused with unit magnification through an image rotator into the vertical entrance slit of the monochromator. The burner is mounted on a motor driven table programmed to collect data as a function of distance (and time) above the burner surface. The monochromator output is detected with a photomultiplier; the signal is amplified, processed in a boxcar integrator, and recorded. The sodium 589.0-nm transition was pumped, and the fluorescence was detected via the collisionally coupled 589.6-nm transition. Saturation excitation was employed to eliminate quenching uncertainties which may be significant for atomic species. The OH(A2Z+-X211)(l,O), R1(6) transition at 281.14 nm was excited, and fluorescence was detected at 314.69 nm from the ( l , l ) , Ql(7) transition. The previously validated OH mea-
Measurements have been made in 19 lean and rich H2/02/N2 flames containing sodium with and without added sulfur. The flame matrix is listed in Table I. Measured Na and OH concentration profiles are plotted for a lean flame in Figure 2. The sodium experimental data have been scaled to refer to the same amount of total sodium in each flame. Sodium concentrations are the order of 10” cm-’ (e0.1ppm); the radicals are at least a few orders of magnitude greater, with total sulfur at the 1-2% level. Thus all species can influence the concentrations of sodium species, but sodium has no effect on other atomic or molecular concentrations. With the addition of sulfur to any given flame, the free sodium concentration is depressed by roughly the same fraction over its whole profile, as illustrated in Figure 2. The magnitude of the sodium depletion increases with decreasing temperature and varies only slightly with stoichiometry. This constant fractional depression of sodium with sulfur addition in a given flame is of itself quite informative. This implies that species whose concentration profiles vary markedly with time probably can play no role in the dominant sodium sulfidation processes. As will be seen, the decreases in [Na] with added SO2,illustrated in Figure 2, result from two effects. The first, which is dominant in lean flames, is the increased sodium oxidation resulting from the sulfur-catalyzed flame radical decay while sodium/sulfur chemistry plays a relatively small role. On the other hand, sodium oxidation is minor in the rich flames, and sodium/sulfur compound formation makes a significant contribution.
(25) Padley, P. J.; Sugden, T. M. Proc. R. Soc. London, A 1958,248,248. (26) Denton, M. B.; Swartz, D. B. Reo. Sci. Instrum. 1974, 45, 81. (27) Schofield, K.; Steinberg, M. Opr. Eng. 1981, 20. 501. (28) Muller, C. H., III; Schofield, K.;Steinberg, M. ACS Symp. Ser. 1980, 134, 103.
Modeling Overview The experimental data base consists of the measured nonequilibrium flame profiles of temperature, OH concentrations, and relative sodium concentrations,all with and without the addition of H2S or SO2. From the temperature and O H measurements, along with the monitored sulfur addition to the flames, one can calculate the corresponding profiles of H20, the coupled radicals H and 0,the trace s p e c k H2 in lean flames and Ozin rich flames, and the concentrations of the sulfur species S,S2,SH,H2S, SO, SO2,and SO3. Thcse are plotted for a lean flame in Figure 3 and for a rich flame in Figure 4, with and without the addition of 1% sulfur. In the lean-flame series, the radicals OH,H, and 0 and the trace species H2 exhibit larger nonequilibrium overshoots and subsequent rates of decay toward equilibrium as temperature is
Schofield and Steinberg
718 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
r
0
t
I
I
I
I
I
I 2 3 4 DOWNSTREAM TIME, (ms)
'
l
"
I
o
I
5
i
ci L
Id2
0
I
2
3 TIME, (ms)
4
5
Figure 3. Trace species profiles in a lean HZ/O2/N2= 1.4/ 1 / S flame in the presence of 1% SO2(solid lines) or in the absence of sulfur (broken
10'~
H,/O,/N, = 1.4/1/5 T- l680K
0
I 2 3 4 5 DOWNSTREAM TIME, (ms) Figure 2. Measured (Na) (top) and (OH) (bottom) downstream profiles in a lean H2/02/N2flame with varying amounts of SOz.
decreased. As noted in Figure 3, the only significant sulfur species in the lean flames is SO2. SO and SO3concentrations are generally less than 1% of those of the SO2 levels. In the rich flames, the radicals OH, H, and 0 and the trace species O2exhibit a more enhanced rate of decay with the addition of sulfur than is exhibited in the lean flames. Figures 3 and 4 can be compared in this regard. Decreasing temperature, as in the lean flames, exhibits increased radical decay. The sulfur chemistry in the rich flames is more complex than in the lean flames due to the presence of significant concentrations of numerous sulfur species. SO2, SO, and S profiles are rather flat and not very sensitive to temperature. With increasing H 2 / 0 2 ratios, the differences between their concentrations are decreased, but the relative ordering is maintained. The reduced-sulfur species H2S, SH, and S2exhibit similarly shaped profiles and have approximately equivalent concentrations in any given rich flame as seen in Figure 4. With increasing H2/02,their concentrations increase but always are less than the SO levels. SO3 is negligible in rich flames. The broad data base, exhibiting H2/02ratios from 0.6/ 1 to 4/ 1 and temperatures from 1650 to 2400 K, spans a wide range of differing conditions and proved to be invaluable for the identification and characterizationof the dominant sodium/sulfur chemistry. Initially, checks were made on the equilibration of reactions 1-3 suggested by Fenimore6and Durie et al.' Assuming that the
lines).
enhanced sodium depletion with the addition of sulfur formed NaS02, NaS, or NaSO,, as indicated by these reactions, the corresponding equilibrium constants could be evaluated. However, the log K vs 1/T graphs exhibited no trends approximating establishment of any such equilibria. A list of about 200 possible reactions can be written for the production and consumption of NaS, NaSH, NaS2, NaOS, NaS02, and NaS03, which appear to be the most likely sodium/sulfur compounds to exist in rich or lean H2/02/N2flames. At the low sodium levels (l2 Recently, new methods to investigate fragmentation mechanisms for consecutive reactions using the collisionally activated dissociation have been developed by Cooks and co-workers13and Maas and Nibbering.14 Determination of the rate constant for ion fragmentation is very important to understand the dissociation kinetics of polyatomic ions. The absolute rate constant for single-step dissociation can be determined using various methods such as photoelectronphotoion coincidence spectrometry:*15 field ionization kinetics,16 multiphoton ionization mass ~pectrometry,'~~'~ and ion cyclotron resonance technique.19 Recently, a new method based on mass-analyzed ion kinetic energy spectrometry (MIKES) has been developed to determine the rate constant for a photodissociation on a nanosecond time scale.20,21In the present work, the kinetics and mechanism of consecutive dissociation of energy-selected n-heptane molecular ion investigated using this technique are reported.
The experimental apparatus and techniques employed have been described in detail previouslyzoand will be reviewed only briefly here. The experimental apparatus shown in Figure 1 consists of a double-focusing mass spectrometer with reverse geometry (VG Analytical Model ZAB-E) modified for photodissociation study and an argon ion laser (Spectra Physics Model 164-09). Ions generated in the source and accelerated upon exiting the source
*To whom correspondence should be addressed.
(1) Levsen, K. Fundamental Aspects of Organic Mass Spectrometry; Verlag Chemie: Weinheim, 1978. (2) Cooks, R.G.;Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Zons; Elsevier: Amsterdam, 1973. (3) Wendelboe, J . F.; Bowen, R. D.; Williams, D. H. J . Am. Chem. Soc. 1981, 103, 2333. (4) Cooks, R.G.,Ed.Collision Spectrtmopy; Plenum: New York, 1978. (5) Levsen, K. Org.Mass Spectrom. 1975, 10, 43. (6) (a) Harris, F. M.; Beynon, J. H. In Gas Phase Zon Chemistry; Academic: Orlando, 1984; Vol. 3, Chapter 19. (b) Dunbar, R.C. In Gas Phase Zon Chemistry; Academic: Orlando, 1984; Vol. 3, Chapter 20. (7) Chen, J. H.;Dunbar, R. C. Znt. J. Mass Spectrom. Zon Processes 1986, 72, 115. (8) Beckey, H. D.Z . Naturforsch., A 1971, 26, 1243. (9) Oliveira, M. C.; Baer, T.; Olesik, S.; Almoster Ferreira, M. A. Znt. J. Mass Spectrom. Zon Processes 1988, 82, 299. (10) McLafferty, F.W., Ed. Tandem Mass Spectrometry; Wiley: New York, 1983. ( 1 1 ) Proctor, C. J.; Kralj, B.; Brenton, A. G.; Beynon, J. H.Org. Mass Spectrom. 1980, 15, 619. (12) Boyd, R.K.;Shushan, B. Znt. J . Mass Spectrom. Zon Phys. 1981, 37, 355. (13) Louris, J. N.; Wright, L.G.; Cooks, R. G.; Schoen, A. E. Anal. Chem. 1985, 57, 2918. (14) Maas, W. P.M.; Nibbering, N . M. M. Znt. J . Mass Spectrom. Zon Processes 1989, 95, 171. (1 5) Baer, T.Adv. Chem. Phys. 1986,64, 1 1 1. (16) Brand, W. A,; Levsen, K. Znt. J . Mass Spectrom. Zon Phys. 1983,51, 135. (17) Kiihlewind, H.; Kiermeier, A.; Neusser, H. J. J . Chem. Phys. 1986, 85,4427. (1 8) Stanley, R.J.; Cook, M.; Castleman, Jr., A. W. J . Phys. Chem. 1990, 94, 3668. (19) Dunbar, R. C. J . Phys. Chem. 1990.94, 3283. (20) Choe, J. C.; Kim, M. S.J . Phys. Chem. 1991, 95, 50. (21) Choe, J. C.; Kim, M. S. Znt. J . Mass Spectrom. Zon Processes 1991, 107, 103.
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