J. Phys. Chem. 1980, 84, 2225-2233
pressure. This adds confidence to the establshed mechanism. It was shown that 2-methyl-1-butene is predominantly a secondary product, formed by reaction 8 and another source, probably reaction 13. The rate constants k5 and k7 have been measured individually, a task which has not, been possible by earlier techniques.
Acknowledgment. We thank the National Research Council of Canada for grants in support of this research. J.H.W. thanks the Dorothy J. Killam Trust for the award of a scholarship. Supplementary Material Available: An Appendix describing the derivation of eq 13 (2 pages). Ordering information is given on any current masthead page.
References and Notes P. D. Pacey and J. H. Wimabsena, Chem. phys. Lett.. 53, 593 (1978). P. M. Marquaire and G. M. Came, React. Kinet. Catal. Lett., 9, 165 (1978); 171 (1978). F. Baronnet, M. Dzierzynski, G. M. Came, R. Martin, and M. Niclause, Int. J. C h m . Kinet., 3, 197 (1971). M. P. Halstead, R. S. Konar, D. A. Leathard, R. M. Marshall, and J. H. Purnell, Proc. R . SOC.London, Ser. A, 310, 525 (1969). J. W. Wilt, “Free Radicals”, Vol. I, J. K. Kochl, Ed., Wiley, New York, 1973, p 340. See paragraph at end of paper regarding supplementary material. A. M. Held, K. C. Manthorne, P. D. Pacey, and H. P. Reinholdt, Can. J. Chem., 55, 4128 (1977). J. C. McCoubrey and N. M. Singh, Trans. Faraday Soc., 53, 877 (1957); J. 0.Hirschfeider, C. F. Curtiss, and R. 8. Bird, “Molecular Theory of Gases and Liquids”, Wiley, New York, 1954, p 533. “Selected Values of Properties of Hydrocarbons and Related Compounds”, Thermodynamics Research Centre, Texas A&M University, College Station, TX 1971, tables 23-24 1.200)-v, 23-2(5.1200)-t, 8-w, and 2-w.
2225
(10) S.W. Benson and H. E. O’Neal, ”Kinetic Data on Gas Phase UniMolecular Reactlons”, National Bureau of Standards, Washington, D.C., 1970, p 585. (1 1) J. N. Bradley and K. 0. West, J. Chem. Soc., Faradey Trans. 7, 78, 8 (1976). (12) J. Muller, F. Baronnet, G. Scacchi, M. Dzierzynski, and N. Niclause, Int. J. Chem. Kinet., 9, 425 (1977). (13) W. Tsang, Int. J . Chem. Kinet., 5, 929 (1973). (14) J. A. Clark and C. P. Quinn, J . Chem. Soc., Faraday Trans. 1 , 72, 706 (1976). (15) H. E. van den Bergh, Chem. Phys. Lett., 43, 201 (1976). (16) P. D. Pacey and J. H. Purnell, Int. J. Chem. Klnet., 4, 657 (1972). (17) P. D. Pacey, Can. J . Chem., 51, 2415 (1973). ( 18) R. M. Marshall, J. H. Purnell, and P. D. Storey, J. Chem. Soc., Faraday Trans. 7, 72, 85 (1976). (19) W. Tsang, J. Chem. Phys., 44, 4283 (1966). (20) A. Shepp, J. Chem. Phys., 24, 939 (1956). (21) R. E. March and J. C. Polanyi, Proc. R. SOC.London, Ser. A , 273, 360 (1963). (22) H. E. van den Bergh, A. 8. Callear, and R. J. Norstrom, Chem. my?. Lett., 4, 101 (1969). (23) N. Basco, D. 0. L. James, and R. D. Stuart, Int. J. Chem. Kdnet., 2, 215 (1970). (24) F. K. Truby and J. K. Rice, Int. J . Chem. Kinet., 5, 721 (1973). (25) D. A. Parkes, D. M. Paul, and C. P. Qulnn, J. Chem. Soc., Faraday Trans. 1 , 72, 1935 (1976). (26) F. C. James and J. P. Simons, Int. J . Chem. Kinet., 6, 887 (1874). (27) C. P. Hochanadel, J. A. Ghormley, J. W. Boyle, and P. J. Ogren, J. Phys. Chem., 81, 3 (1977). (28) K. Gianzer, M. Quack, and J. Troe, Symp. (Int.) Combust., [Proc.], 16, 949 (1976). (29) W. Braun, A. M. Bass, and M. Pilling, J. Chem. phys., 52, 5131 (1970). (30) K. Wnzer, M. Quack, and J. Troe, Chem. Phys. Lett., 39,304 (1976). (31) W. A. Chupka, J . Chem. Phys., 48, 2337 (1966). (32) R. J. CvetanovlC and R. S. Irwin, J. Chem. Phys., 46, 1694 (1967). (33) A. F. Trotman-Dickenson and E. W. R. Steacie, J . Chem. Phys., 19, 169 (1951). (34) J. A. Ken and M. J. Parsonage, “Evaluated Kinetic Data on Gas Phase Addition Reactions”, Butterworth, London, 1972, p 31; K. Yang, J. Am. Chem. Soc., 84, 3795 (1962).
Energy Transfer Reactions of N2(A32,+). 5. Quenching by Hydrogen Halides, Methyl Halides, and Other Molecules W. G. Clark and D. W. Setser” Department of Chemistry, Kansas State University,Manhattan, Kansas 66508 (Received: February 25, 1980)
The 300 K quenching rate constants for Nz(A3Z,+,u’=0,1)have been measured with 20 small molecules, mostly of the hydrogen halide or methyl halide variety. The metastable Nz(A)molecules were prepared by the reaction of Ar(3P02)with Nz in a discharge-flow system. Rate constants were measured by observing the variation of the N2(ABZ,+-X1Zg+) emission intensity as a function of added reagent concentration. The magnitude of the rate constants increase in both the HX and CH3Xseries as X changes from F to I. The temperature dependence of the NO, Oz,CzHz,and C2H4reactions were qualitatively investigated with the discharge flow technique by doing experiments at -120 K. On the basis of the limited data, the rate constants are either virtually invariant with temperature or decrease mildly with decreasing temperature. In contrast the Nz(A)bimolecular, energy pooling, N2(C)formation rate constant appears to increase with decreasing temperature. The results of the present study are compared to previous investigationsfrom this laboratory and to other studies of Nz(A3Z1,+).
Introduction During recent years, several methods have been developed which faciliate direct study of the chemistry of Nz(A3&+), the first electronically excited state of molecular nitrogen, in the absence of other reactive intermediates.l* The method developed in our laboratory utilizes a discharge-fJow apparatus in which the reaction of Ar(3P,,2) with Nzis used to convert the metastable argon atoms into metastable Nz(A38,+) molecules, via cascade from higher lying triplet states of Nz.7 This method provides a continuous flow of NJA) molecules, in the u’ = 0 and 1levels, at a concentration of -2 X 1O1O molecules ~ m - ~A.new use8 of this Nz(A) source is the investigation of the chem0022-3654/80/2084-2225$0 1.OO/O
istry of Nz(B3n,), which can be produced by laser excitation from Nz(A). A similar discharge flow source has been developed for the study of CO(a311).g Since the N,(A) discharge-flow source gives a mixture of u’ = 0 and 1levels (1.00.77 for the operating conditions of the present study), the rate constant measurements must be evaluated carefully to determine the role, if any, of vibrational excitation. Using pulse radiolysis, Dreyer and co-workers5 have studied the kinetics of Nz(A) in even higher vibrational levels. In our earlier work we studied the quenching reactions of N,(A) with some common inorganic and unsaturated organic molecules,lb*cobserved excitation transfer for some reactions,lc-e and compared the reactivity of 0 1980 American Chemical Society
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Clark and Setser
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
CO(a) and N2(AhgbIn the present work we have extended the rate constant measurements to include a series of HX and CH3X (X = halogen, OH, or SH) and related molecules. Attempts, only partially successful, were made to study the temperature coefficients of the 02,C2H2,C2H4, NO, and energy-pooling (giving N2(C) N2(X))reactions by doing experiments at temperatures below 300 K. These studies of the temperature dependence of the rate constants are less complete than the work of Slanger et al.4 A compilation of N2(A)quenching rate constants is presented; the agreement between the measurements using various methods for producing N2(A)1-6generally is satisfactory. Since this is the final paper in this series, experimental details are provided, including description of some of the less-successful experiments. Our earlier rate constant measurements were done with the fixed observation point method with either direct monitoring of the strongest bands of the N,(A-X) tran&ongb or by monitoring the Hg(3Pl-1So) emission intensitylb at 253.7 nm from the N2(A) Hg excitation transfer reaction. In the present study the total N2(A) concentration was monitored via observation of the N2(A-X) emission intensity with a solar blind photomultiplier tube. The decay of the [N2(A)]was monitored as a function of both distance and reagent concentration to obtain rate constants. This method was selected in anticipation of the possible advantages for the temperature variation studies and to avoid any problems arising from surface effects that might vary in the presence of reagents. Unfortunately, the potential advantages of the direct observation of N2(A)was largely offset by the low intensity of the N2(A-X) etnission and by scattered background radiation from the discharge and from the Ar N2 mixing zone that was detected by the solar blind photomultiplier tube. For some reactions the fixed observation point was used to monitor the strongest bands of the N2(A-X) emission with a monochromator in order to distinguish between the rates of reaction for the u' = 0 and 1 levels. In addition to the decay studies, steady-state intensity measurements of N2(A) and product emissions were made to characterize the NO excitation transfer reaction and the bimolecular N,(A) energy-pooling reaction. With the benefit of hindsight, we would recommend the fixed observation point method with direct monitoring of the 0-6 and 1-9 band intensities for study of N2(A,u=O) and N2(A,u=1)reactions. For reagents that do not react with Hg or NO, the NO(A-X) or Hg(3Pl-1So)emission intensities are recommended for monitoring the NZ(A,u= 0,l) concentration. The latter method permits the quenching to be followed over more than an order of magnitude change in N2(A) concentration.
+
+
+
Experimental Section The generation of N2(A) by the reaction of Ar(3Po,2) atoms with N2 has been thoroughly described.lSl0 The metastable argon atoms are generated by flowing purified argon through a low-power, hollow cathode discharge; the N2 is subsequently added and N2(A3Z,+)is formed from radiative and collisional cascade from the N2(C311,) and N2(B3n,) states,laY7It is important to remove impurities from tank gases; we used liquid nitrogen cooled molecular sieve traps (at low pressure) for this purpose. A rather large flow (1/2-1/3 of the Ar flow) of nitrogen was used to maximize [N2(A)]. High flows are needed to quench the Ar(3Po,2)atoms, to increase the pressure (which reduces the loss by diffusion to the wall), and to provide for rapid vibrational relaxation of the NJA) molecules into u = 1 and 0. Systematic studies of the decay of N2(A)in various N2/Ar mixtures were not done and other factors also may
influence the need for large N2 flows. The total flow rate was 2 mmol s-l which gave a pressure of 4 torr for the pump (1000 L m i d ) used in this work. For unthrottled conditions the flow velocity was -20 m s-l in a 25-mm diameter flow tube. The maximum Ar(3Po,2)and N2(A) concentrations are obtained for operation of the hollow cathode discharge at minimum power, -230 V and 4 mA, for sustaining the discharge. Although the N2(A)decay rate is decreased at higher pressure, the efficiency of Ar(3P0,2) production declines above -2 torr and the best operating pressure was -4 torr for this apparatus. The discharge section and the Ar*/N2 mixing zone of the apparatus was similar to that shown in Figure 1 of ref lb, except that the N2was mixed coaxially with the argon flow. Wood's horn light traps separated the discharge from the Ar*/N2 mixing zone and the Ar*/N2 mixing zone from the reagent mixing zone. These protect the downstream portion of the flow apparatus from scattered light from the discharge and from the intense N2(C-X) and N2(B-A) emission from the Ar(3Po,2)+ N2 reaction. The apparatus must run for -30 min in order to obtain stable N,(A) concentrations. The discharge and Ar*/N2 mixing section of the apparatus were constructed of Pyrex and terminated (after the second light trap) in a 29/42 standard taper (outer) joint. This joint fitted to a quartz inner counterpart that was blown to the quartz flow tube. The reagent inlet, 3-mm tubing terminating in a 5-mm ball with numerous small holes, extended through the second Wood's horn light trap and terminated about 2 cm beyond the end of the (outer) standard taper joint. The experiments described in the first part of this paper, termed method I, were done with a 25-mm i.d. quartz flow tube of 60 cm length. Since the first-order decay constant in the Ar/N2 mixture at 4 torr was 120 s-l, the N2(A-X) emission intensity had declined to an unusable low level after 40 cm and only the first 27 cm length of the tube was used for quenching studies. The zero time position was taken as 3 cm beyond the mixing jet in order to allow adequate time for good mixing (confirmed by inspection of the chemiluminescence from reactions between added reagents and the metastable argon atoms). In the present work, observations were made at five positions along the flow tube separated by 6 cm (or a time of 2.6 ms). One attempt was made to reduce the loss from quenching at the wall by coating the tube with Fluorolube grease, a saturated halocarbon (-CF2CFC1-),, but this had no effect upon the decay rate of N2(A). No other coatings were investigated, but we are not optimistic that coatings would be useful. A significant improvement could be achieved by increasing the pumping speed by a factor of -5, which can be obtained by adding a small Roots blower to an apparatus such as the present one. This would greatly reduce the loss of N,(A) from diffusion to and quenching by the wall. Reagents were diluted with argon and stored in 5-L reservoirs. The flow rates were measured with calibrated capillary flow meters. The pressure drop across the capillary was measured with silicone oil. Some difficulties were encountered in reliable metering for the reagent gases that readily dissolved in the silicone oil. The N2(A-X) emission spectrum, recorded with a monochromator at a position corresponding approximately to the second observation point, is shown in Figure 1. For the quenching studies, a solar blind Hammamatsu 1R166 photomultiplier tube was used. This tube has a response that peaks at 220 nm and declines smoothly to an absolute cutoff at 320 nm; the short wavelength cutoff is determined by the quartz window. Figure 1 shows that the 1R166
-
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980 2227
Energy Transfer Reactions of N2(A32,+)
N2
EMISSION SPECTRA v'= 0
vL 1
r
I 7
V'.5
I 8
I
9
I
10
11
5 260
280
300
320
340
360
380
NM
Flgure 1. The N2(A3Z>-X1Zg+) emission spectrum from the discharge flow source as recorded by a spectrometer fitted with a grating blazed at 250 nm and an EM1 9558Q photomultiplier tube. Most scattered light from the Ar*/N, mixing zone has been eliminated and the majority of the N2(C3n,-B311g)emission arises from the 2N,(A) energy-pooiing reaction. The weak NO(A-X) emission is from traces of NO impurity.
photomultiplier tube observed the strong 0-5,6, 7 bands and the 1-7, 8 and I-3,4 bands (not shown). The photomultiplier (IPM) tube was placed 6 cm from the flow tube and viewed a 3-cm length of flow tube, i.e., 1.5 cm on either side of the center position. The output of the photomultiplier tube was measured with a Keithley electrometer and recorded on a strip-chart recorder. Without added reagent,, the typical signal level at 4 torr pressure was 1 X A at the first observation point and -0.3 X A 24-cm downstream from that point. After quenching of all of the [N,(A)], the typical residual "dark" signal was 1X A. The use of a 10-nm half-width band pass interference filter centered at 280 nm, which would pass only the 0-6 and 0-7 bands, was investigated. However, the peak transmittance was rather low for this filter and the total signal was reduced so severely that the filter was not used. The photomultiplier tube housing was designed so that the flow tube could be immersed in a transparent coolant bath and the N2(A-X) emission intensities could still be recorded. Our attempt to use this apparatus at low temperature is described in the Appendix. For some experiments the intensities of the strongest N2(A,u=O)and N,(A,u=l) bands were observed with a monochromator at a fixed observation point downstream of where the reagent was added. These experiments, which are identified as method 11, were done with the apparatus described in ref 9b. A few experiments also were done with this apparatus in which Ha, CHI, or C2F6were added to give vibrational relaxation of u = 1 and then the decay of N2(A,u=O)was measured in the presence of added reagent (see also the Appendix of ref 9b). A procedure, identified as method 111, also was used to simultaneously observe the Nz(A) and the product emission intensities with a monochromator. From steady-state relationships, rate constants for the product excitation can be ascertained. The discharge and Ar + Nz mixing zone section were the same as described for method I, but the long tubular reactor was replaced by a 25-mm i.d. 15-cm long quartz flow tube. Standard taper joints blown to the top of the tube at each end served to connect the tube to
-
the entrance and exit lines. Each end was approximately flat and served as a window for viewing the emission down the 15 cm length tube, which was backed with a mirror. The decay of the N,(A) from diffusion and quenching at the walls during transversal of the tube was about 30% and the [N2(A)]was treated as being effectively constant. Cooling and heating of the reactor were accomplished by placing a tightly fitted aluminum block around the flow tube and entrance line. An extension of the aluminum block was immersed in a cooling or heating bath to control the temperature of the block. The temperature was measured at the exit side of the flow tube with a thermocouple. Replacement of the reagent entrance port with a thermocouple well showed that the temperature gradient along the reactor tube was about 20 "C. The flow pat,tern in this short quartz tube was rather poor because the gas flow cut across the inside of the two turns and only the center portion was evenly filled with the flowing gas.
Experimental Results The decay of N2(A)in the absence of added reagent, can be summarized by eq la-e. The first process represents N2(A)
-+ 7 ~ 2 1 ~ ~ '
hv
N2(X)
(la)
N2(X)
Nz(A) + N2(A) N2(A) + Ar
kN2"
Nz(X) + Nz*
Nz(X) + Ar
(IC)
(14
Nz(A) + Nz(X) -.!% 2Nz(X) (le) spontaneous radiative decay (the effective lifetime is 1.94 sll) which is a negligible loss process for our flow speeds. Since Nz(A)is quenched with virtually unit efficiency upon diffusion to the wall, kw = p-' and (lb) is the dominant loss process in the absence of added reagent.lb The third process is the bimolecular loss of N2(A) and has been termed energy pooling because of the formation of N2*
2228
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
1';
I
'
2
I
O2 o r CH3CI
4I
'
6I
'
I
I
10 I
I
'
CONCENTRATdN ,IO-"MOLES/CM3
Flgure 2. Decay plots of the N,(A-X) concentration vs. added reagent, O2 and CH,CI, concentration at different observation points (corresponding to different reaction times). The zero time position for CH,CI was omitted for convenience of presentation. The solid lines (squares) identify O2 data and dotted lines (circles) identify CH,Cl data.
Clark and Setser
Figure 3. Plots of decay constants, slopes from plots such as shown in Figure 2, vs, reaction time for 02, CH,CI, and CH3CN. The O2and CH3CIdecay constants were determined from the data in Figure 2. Note that molecule cm-, and ms units are used in this plot.
k-
I
I
I
I
I
I
I
E30m
states lying above N2(A)in energy.12 Although this process E ,"25has a large rate constant (- 1O1O cm3 molecule-' SKI), loss by (IC)is insignificant for concentrations of -Wo molecules ~ m - Reactions ~. I d and l e are negligible under our conditions, as shown by the fact that the observed firstl hw + h A r + h N z , varied order rate constant, k = 7 N z ( ~ ) - + inversely with the total pressure.lb This was explicitly examined again and plots very similar to Figures 6 and 7 of ref l b were obtained. There was no significant improvement in the accuracy of the data and further discussion of kw is not warranted. These experiments did 8.0 10.0 12.0 demonstrate that the impurity level is sufficiently low that T I M E , MSEC impurities play no role in the decay of N,(A). Recent Flgure 4. Plots of decay constants, slopes from plots such as shown studies in other laboratories13have shown that hArand kN2 in F' ure 2, vs. reaction time for HBr, CpH,, and C&lH,. Note that molecule are ( cm3 molecule-' s-' and kNO = 9.0 X lo-'' cm3 molecule-' s-l. The three Fzo2values at 300 and -120 K give an Arrhenius rate constant in good agreement with Slanger et aL4
References and Notes (1) (a) Setser, D. W.; Stedman, D. H.; Coxon, J. A. J. Chem. Phys. 1970, 53, 1004. (b) Meyer, J. A.; Setser, D. W.; Klosterboer, D. H. Ibid. 1971, 55,2084. (c) Meyer, J. A.; Setser, D. W.; Stedman, D. H. J . Phys. Chem. 1970, 74,2238. (d) Stedman, D. H.; Meyer, J. A.; Setser, D. W. J. Mol. Spectrosc. 1972, 44,206. (e) Stedman, D. H.; Meyer, J. A.; Setser, D. W. J. Am. Chem. SOC.1968, 90,6856. (2) (a) Young, R. A,; St. John, G. A. J . Chem. Phys. 1968, 48, 898, 2572. (b) Young, R. A.; Black, G.; Slanger, T. G. Ibid. 1969, 50, 303. (3) (a) Callear, A. B.; Wood, P. M.Prans. Faraday Soc. 1971, 67, 272. (b) Callear, A. E.; Wood, P. M. Ibid. 1971, 67, 548. (4) Slanger, T. G.;Wood, B. J.; Black, G. J. Photochem. 1973, 2,63. (5) (a) Dreyer, J. W.; Perner, D. J. Chem. Phys. 1973, 58, 1195. (b) Dreyer, J. W.; Perner, D.; Roy, C. R. Ibid. 1974, 67, 3164. (c) Roy, C. R.; Dreyer, J. W.; Perner, D. Ibid. 1975, 63, 2031. (6) Madel, A,; Ewing, J. J. J. Chem. Phys. 1971, 57, 3490. (7) Kolts, J. A,; Brashears, H. C.; Setser, D. W. J. Chem. Phys. 1977, 67,2931. This work, which is a reinvestigation of the Ar($P,,) 4N, reaction, shows that the N,(C)/N,(B) ratio is about 1:l rather 'than 1:6 as reported in ref la. The exact value of this ratio if still controversal; see Touzeau, M.; Pagnon, D. Chem. Phys. Lett. 1978,
53,355. (8) (a) Heidner IV, R. F.; Sutton, D. G.; Suchard, S. H. Chem. Phys. Lett. 1976, 37, 2431. (b) Cook, J. M; Miller, T. A.; Bondybtty, \I. E. J. Chem. Phys. 1978, 69,2562. (9) (a) Taylor, G.W.; Setser, D. W. J. Am. Chem. Soc.1971, 93,4930. (b) Taylor, G. W.; Setser, D. W. J, Chem. Phys. 1973, 58,4840. (c) Clark, W. G.; Setser, D. W. Chem. Phys. Lett. 1975, 32, 71. (10) Setser, D. W. "Reactive Intermediates: Generation and Monitoring";
Academic Press: New York, 1979, (11) (a) Shemansky, D. W.; Carleton, N. P. J. Chem. Phys. 1969, 51, 682. (b) Shemansky, D. E. Ibid. 1969, 57, 689. (12) (a) Stedman, D. H.; Setser, D. W. J . Chem. Phys. 1969, 50. 2256. (b) Hays, G. H.;Oskam, H. J. Ibid. 1973, 59, 1507,6088.
J, Phys. Chem. 1980, 84, 2233-2237 (13) Levron, D.; Phelps, A. V. J . Chem. Phys. 1978, 6 9 , 2260. (14) Koits, J. H.; Setser, D. W. J . Chem. Phys. 1978, 68, 2023. (15) Stedman, D. H.; Setser, D. W. Chem. Phys. Lett. 1968, 2 , 542. (16) Ottinger, Ch.; Simonis, J.; Setser, D. W. 5er. Bunsenges. phys. Chem. 1978, 82, 655. (17) Meyer, J. A.; Setser, D. W.; Clark, W. G. J. Phys. Chem. 1972, 76, 1. (18) Thrush, B. A.; Wlld, A. H. J . Chem. Soc., Faraday Trans. 2 1972, 68, 2023. (19) Garner, E. M.;Thrush, B. A. R o c . R. SOC.London, Ser A 1975, 346, 121. (20) Hill, R. M.; Gutcheck, R. A.; Huestis, D. L.; Mukherjee, D.; Lorents, D. C. Stanford Research Institute Technical Report to Advanced
2233
Research Projects Agency, Menlo Park, CA. July, 1974. (21) (a) Nadler, I.; Setser, D. W.; Rosenwaks, J. Chem. Phys. Lett. 1980 in press. (b) One possible complication that could affect our results is selective quenching by the spin states, which would interact with the selection of the proper 7N4A. (22) Deperasinska, 1.; Beswick, J. A., framer, A. J. Chem. Phys., 1979, 7 1 , 2477. (23) Lofthous, A.; Krupenie, P. H. J . Phys. Chem. Ref. Data. 1977, 256. (24) Nadler, I.; Rosenwaks, S. Chem. Phys. Lett. 1980, 6 9 , 286. (25) Velazco, J. E.; Koks, J. H.; Setser, D. W. J . Chem. Phys. 1978, 69, 4357. (26) The triplet product emission observed from C2H21dand CSZ1' tend to confirm this mechanism.
A Novel Bimolecular Reaction Sequence Yielding H(OH2)2+ at Low Pressures. Ion Cyclotron Resonance Studies of the Reactions of Doubly Solvated Protonst D. Wayne Berman and J. L. Beauchamp" Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 9 7 125 (Received: November 19, 1979; In Final Form: May 6, 1980)
A sequence of bimolecular ion-molecule reactions is identified which generates H(OH&+ in mixtures of H20 and 1,l.-dihaloethanesat lo4 torr. The possibility is discussed of using such a sequence as a source of doubly solvated protons to study H30+transfer in the absence of competing termolecular processes. As examples, thermochemical data derived from equilibrium H30+transfer between H 2 0 and HCN, and HzO and CHzO, are presented.
Introduction Numerous e~perimenta1l-l~ and t h e ~ r e t i c a Pstudies ~~ involving proton-bound dimers are providing a growing tabulation of hydrogen bond energies HBdefined by eq 1,
+
BlH+ Bz (BlHBz)' AH = -HB (1) where all quantities refer to the gas phase. In eq 1, the acidic species BIH+ associates with a Lewis base Bz forming the hydrogen-bonded complex (B1HB2)+. Though the species B might include Lewis bases such as H2,17JEC0,19 and even the rare gases,20~21 it is not surprising that the majority of studies have considered common solvents such as HzO, NH,, and CH3CN.1-13These complexes are usually generated through reaction 2 at pressures above -+
-
BIH+ .t Bz + [BiHB2+]*
[MI
BIHB2'
(2) torr.1-3~22~23 More highly solvated species have also been generated in this manner, providing sequential association energies of several molecules in a proton-bound complex. 1-4,8-10,17-25 Reaction 2 represenis a termolecular process where BIH+ and Bzform an intermediate complex [B1HB2+]* which is stabilized by collision with a third body M. In the absence of collisional stabilization, the internal energy of this complex is sufficient to permit dissociation into the original reactants. Thus, termolecular processes are only important at pressures where intervals between collisions are on the same order as lifetimes of the intermediate complexes. However, if a system is observed for a sufficiently long period of time, and there are no interfering bimolecular reactions, the products of termolecular processes can be detected at lower pressures. If B = H 2 0 for example, H(OHJ2+ should be observable in ICR trapped ion experiments from the direct condensation reaction 3 Contribution No. 6110. 0022-3654/80/2084-2233$01 .OO/O
[MI
H30+ + Hz0 H(OH2)Z' (3) at pressures greater than 6 X lo" torr.26v27It is interesting to note that, at thermodynamic equilibrium, the abundance of H(OHz)z+will be greater than H30+in reaction 3 whenever the H 2 0 pressure is in excess of 3.5 X One of the appealing features of ICR studies of equilibrium proton transfer, reaction 4, is that processes such B1H+ + B2 + BzH' + B1 (4) as reaction 2 can usually be avoided in low-pressure (
