J . Phys. Chem. 1990, 94, 5809-5812
5809
probabilities per collision which are 1-2 orders of magnitude smaller than those measured in the gas phase. at room temperature. The temperature dependence of the relaxation probabilities is consistent with predictions of the SSH, repulsive collision, energy-transfer model. O2 is found to be a factor of 4-5 more efficient than Ar in relaxing vibrationally excited OCS in the cryogenic liquid solutions.
excited vibrational levels at cryogenic temperatures and a consequent reduction in the thermal photodissociation cross section. The isotope enrichment factors achieved in cryogenic solution in this study are limited mainly by vibrational relaxation of the IR laser pumped, 2u2 vibrational level of OCS by Ar(l). It is possible that larger isotope enrichment factors may be achieved by two-step photolysis with laser pulse widths and IR/UV laser delay times on the order of IO ns. The rates measured in this study for relaxation of the v I and v2 vibrational levels of OCS by liquid Ar at 84.4 K correspond in the isolated binary collision approximation to relaxation
-
Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy.
Effective Ion Temperatures in a Quadrupole Ion Trap B. D.Nourse and H. I. Kenttamaa* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: June 22, 1989; I n Final Form: January 2, 1990)
The reaction of 02*+ with methane is used as a “chemical thermometer” to obtain information concerning effective ion temperatures in a Paul-type quadrupole ion trap using He buffer gas. The rate constant as well as the product distribution indicates an effective temperature of 600-700 K for 0;+ions trapped for long times; Le., a similar rate constant and a similar ions with a Maxwell-Boltzmann energy distribution. Other results product distribution is obtained at 600-700 K for 02*+ suggest that this temperature may also adequately describe the behavior of some polyatomic ions in the ion trap. The effective ion temperature can be increased (up to approximately 1300 K for the O;+/methane system) by using appropriate radio-frequency voltages to kinetically excite the ions.
Introduction Mass spectrometers designed to trap gas-phase ions have attracted much interest recently.14 Experimental characterization of the temperature of ions in a static magnetic ion trap, the ion cyclotron resonance instrument, has been of concern for a long time but still remains pr~blematic.~At very low trapping potentials, the experimentally determined rate constants have been found to be similar to those obtained under thermal conditions6~’ where the average translational energy of the reactant ions is less than 0.05 eV and takes a Maxwell-Boltzmann di~tribution.~,~ Paul-type quadrupole ion traps represent another kind of an ion trapping mass spectrometer.I0 These mechanically simple devices consist of a chamber formed between three electrodes with hyperbolic inner surfaces. Radio-frequency (rf) voltages, sometimes in combination with direct-current fields, are used to trap, isolate, excite, and eject ions. Recently, Paul-type ion traps became commercially available. These instruments are gaining recognition as simple and relatively inexpensive mass spectrometers that are well suited, for example, for chemical ionization experiments, ion-molecule reaction studies, and tandem mass spectrometry ( I ) March, R. E.; Hughes, R. J.; Todd, J. F. J . Quadrupole Mass Spectrometry; Chemical Analysis Series, Vol. 102; Wiley: New York, 1989. (2) Johlman, C. L.; White, R. L.; Wilkins, C. L. Muss Spectrom. Reo. 1983, 2, 389. (3) Brodbelt, J. S.; Cooks, R. G.Spectra 1988, 11, 30. (4) Nourse, B. D.; Cooks, R. G.Anal. Chim. Acra 1990, 228, 1 . ( 5 ) See, for example: (a) Bursey. M. M.; Lehman, T. A. In Ion Cyclotron Resonance Spectrometry; Bursey, M. M., Lehman, T. A,, Eds.; Wiley-Interscience: New York, 1976. (b) Huang, F A ; Dunbar, R. C. J . Am. Chem. SOC.1989, I I I, 6497. (6) Bloom, M.; Riggin, M. Can. J . Phys. 1974, 52, 436. (7) Meisels, G. G. I n Interaction between Ions and Molecules; Ausloos, P., Ed.; Plenum Press: New York, 1974; pp 611-618. (8) Levson, K. In Fundamental Aspects of Organic Mass Spectrometry; Weinheim: New York, 1978; Chapter 1 . (9) Henchman, M. In Ion-Molecule Reucfions; Franklin, J. L., Ed.; Plenum Press: New York, 1972; Vol. I, Chapter 5. (IO) Todd, J . F. J.; Lawson, G.; Bonner, R. G. In Quadrupole Mass Spectrometry; Dawson, P. H., Ed.; Elsevier Scientific: New York, 1976; Chapter 8.
0022-3654/90/2094-5809$02.50/0
involving photoactivated or collision-activated dissociation of mass-selected ion^.^,^ These devices can be operated in the mass-selective instability mode in which a wide range of selected ions are simultaneously stored in the ion trap and then massanalyzed by sequential mass-selective ejection.” Typically 1 mTorr of helium buffer gas is used in the ion trap. This is believed to dampen the motion of the ions and thereby to improve ion trapping efficiency.” Paul-type ion traps utilize high rf amplitudes (often thousands of volts) which might be expected to result in very high ion kinetic energies. This, however, has proven not to be the case. The dynamics of ion motion in Paul-type quadrupole ion traps has been studied extensively.12-’8 Theoretical considerations suggest that ions migrate toward the center of the trap while their kinetic energy decreases. The majority of the ions studied have been found to have an average kinetic energy of 1 0 . 2 eV after about 15 collisions.’2*18These results suggest that the internal energies of ions trapped for long times may be relatively low. Indeed, rate constants measured in Paul-type traps for some chemical systems agree with literature values obtained under thermal or nearthermal conditions.19 On the basis of miscellaneous observations, (11) (a) Stafford, G. C.; Kelley, P. E.; Syka, J . E. P.; Reynolds, W. E.; Todd, J . F. J. Int. J . Muss Spectrom. Ion Phys. 1984,60, 85. (b) Louris, J. N.; Cooks, R. G.; Syka, J . E.; Kelley, P. E.; Stafford, G. C.; Todd, J. F. J. Anal. Chem. 1987, 59, 1677. (12) (a) Bonner, R. F.; March, R. E.; Durup, J. Int. J . Mass Spectrom. Ion Phys. 1976, 22, 17. (b) Doran, M. C.; Fulford, J . E.; Hughes, R. J.; Morita, Y.; March, R. E. Int. J . Mass Spectrom. Ion Phys. 1980, 33, 139. (1 3) Lawson, G.; Bonner, R. F.; Mather, R. E.; Todd, J. R. J.; March, R. E. J . Chem. SOC.,Furaday Trans. I 1976, 72, 545. (14) Dehmelt, H. G. Ado. At. Mol. Phys. 1967, 3, 53. (15) Dehmelt, H. G.; Major, R. G. Phys. Reo. 1968, 170, 91. (16) Dawson, P. H. Int. J . Mass Spectrom. Ion Phys. 1976, 20, 237, (17) Lawson, G.;Todd, J . F. J.; Bonetti, R. F. Dynamics Mass Spectrometry; Price, D., Todd, J. F. J., Eds.; Heyden: London, 1970; Vol. 4, Chapter 4. (18) Siemers, I.; Blatt, R.; Sauter, Th.; Neuhauser, W. Phys. Reu. A 1988, 38, 5121. (19) Armitage, M . A.; Higgins, M. J.; Lewars, E. G.; March, R. E. J . Am. Chem. SOC.1980, 102, 5064, and references therein.
0 1990 American Chemical Society
5810
The Journal of Physical Chemistry. Vol. 94, No. 15, 1990 $ t t
Reaction
Nourse and Kenttamaa
Analysis
I
Ionization
RF Voltage Electron
Gale
AC Vonage
2
DCVoltaQe
t
I
Figure 1. Sequence of rf, ac, and dc pulses used to study ion-molecule reactions and collision-activated dissociation in the quadrupole ion trap.
such as the temperature measured for the neutral gas in a Paul-type trap,19 rotational temperature of nitrogen ions,20and dissociation product distributions obtained for proton-bound pyridine dimers,2' low temperatures (around 300 K) have been suggested for ions in Paul-type quadrupole ion trap^.^^^^^ It is usually not possible directly to measure the actual temperature of ions at low pressure in a mass s p e c t r ~ m e t e r . ~In~ - ~ ~ some instances, the ions may have significantly higher temperature than the device itself, for example, from being kinetically heated by an applied electric field. In order to characterize experimental conditions prevailing in a specific device, one may choose to determine an 'effective temperature." In the present work, the effective temperature is defined as the actual temperature that would result in a specified rate (or product distribution) for a well-characterized, temperature-dependent reaction of ions with a Maxwell-Boltzmann energy distribution. In order to examine effective ion temperatures in the Paul-type ion trap using helium buffer gas, a number of "chemical thermometers" were employed; Le., effective ion temperatures were investigated indirectly by examining temperature-dependent properties of chemical react i o n ~ The . ~ ~reaction of 02'+ with methane is one of the systems selected as a chemical thermometer. This reaction has been the focus of a considerable research effort and is one of the best understood bimolecular gas-phase reaction^.^^-^* Both the rate constant and the product distribution for this reaction are known to be temperature dependent. Experimental Section
Experiments were carried out with a prototype quadrupole ion trap mass spectrometer described earlier.4v1'b A modified version of the software was used to implement the necessary radio-frequency (rf) and direct-current (dc) voltage sequences. Figure 1 illustrates the sequence of voltage pulses necessary for ion-molecule reaction studies and collision-activated dissociation experiments. ion generation and single ion selection is followed by a variable reaction time. Single ion selection can be obtained by applying a dc voltage (typically 30 V for 7 ms in the case of 0:+,but both values are mass dependent) to the ring electrode during the ion isolation interval. During the reaction time, a supplementary ac voltage, "tickle voltage" (0.1-5.0 V (p-p) amplitude), applied across the end caps at the fundamental frequency of the selected ion permits acceleration of the ion, which can lead to dissociation upon collisions with the helium buffer gas.IIb Mass analysis is (20) Muntz, E. F. UTlA Report No. 71, 1961; Phys. Fluids 1%2,5,80. (21) Brodbelt-Lustig, J. S.;Cooks, R. G. Talanta 1989, 36, 255. (22) Henchman, M. In Interaction between Ions and Molecules; Ausloos, P., Ed.; Plenum Press: New York, 1974; pp 21-24. (23) Bartmess, J. E. In Structure/Reactivity and Thermochemisiry of Ions; Ausloos, P., Lias, S.G., Eds.; D. Reidel: 1986; pp 367-380. (24) DurupFerguson, M.; BBhringer, H.; Fahey, D. W.; Fehsenfeld, F. C.; Ferguson, E. E. J . Chem. Phys. 1984, 81, 2657. (25) Rowe, B. R.;Dupeyrat, G.; Marquette, J. B.; Smith, D.; Adams, N . G.;Ferguson, E. E. J . Chem. Phys. 1984.80, 241. (26) Adams, N . G.;Smith, D.; Ferguson. E. E. Int. J . Mass Spectrom. Ion Processes 1985, 67, 67. (27) Dotan, I.; Fehsenfeld, F. C.; Albritton, D. L. J . Chem. Phys. 1978, 68, 5665. (28) Barlow, S. E.; Van Doren, J . M.; DePuy, C. H.; Bierbaum, V. M.; Dotan. I.; Ferguson, E. E.; Adams, N. G.; Smith, D.; Rowe, B. R.; Maquette. J. B.: Dupeyrat, G.; Durup-Ferguson, M. J . Chem. Phys. 1986, 85, 3851.
z
OO
800
400
1200
1600
Time (msec) Figure 2. Relative abundance of 02'+ as a function of time for the reaction with CHI. The helium pressure was 5 X IO-' Torr.
carried out by mass-selective instability induced by an rf voltage ramp. A typical electron ionization time period was 0.5 ms. The ion-molecule reaction time was varied from 0 to 1000 ms. The variable rf voltage (1-1 5 OOO V (p-p)) applied to the ring electrode was approximately 310 V (p-p) (q = 0.39) at 1.1 MHz during the reaction time for the 02'+/CH4 study. These conditions are referred to as the normal trapping conditions in the Results and Discussion sections. For the activation experiments, the 02*+ ions were accelerated for 7 ms with an ac pulse of a frequency of approximately 146000 Hz (1 V (p-p)). The samples were introduced into the ion trap through a Granville-Phillips valve at a nominal pressure of 5 X IO" Torr. The helium damping gas was similarly introduced, yielding a total pressure of 5 X Torr, as measured in the vacuum manifold using an ionization gauge. The samples were obtained commercially. Triethylsilyl cation and ionized dimethyl phosphonate were generated by 70-eV electron ionization. Ionized tetraethylsilane was generated by 70-eV electron ionization or by Ar" charge exchange. The C6H110+isomers were generated by chemical ionization, using different reagent ions (tert-butyl cation, protonated cyclohexadiene, methylcyclopentyl cation). Some experiments were carried out with a Finnigan ion trap detector described earlier.29 The ionization and reaction times stated above, along with the helium and reagent gas pressures, are representative of the conditions used for the ion trap detector, as well. In this device, single ion isolation can be carried out only for the ion with the highest m / z value by raising the rf level in order to eject all ions of lower m / z values. Collision-activated dissociation experiments cannot be carried out using this device. The ion-molecule reaction of ionized methane with neutral methane (CH4'+ + CH4 CH5+ CH3') is a temperature-independent reaction,30 and it was used to calibrate the pressure measurements. This calibration was carried out each day by generating a pressure calibration curve from a plot of the measured pressure (read from the ion gauge and corrected for ion gauge sensitivity for methane) versus the calculated pressure, assuming k = 1.1 X cm3 molecule-' s - ' . ~ O The rate constants for the reaction of 0:+ with methane and for the reaction of C2H2'+ with neutral acetylene were determined from the known pressures of the neutral reagents and the slope. of the natural logarithm of the relative abundance of reactant ion versus time. A typical plot is shown in Figure 2 for the reaction of 02*+ with methane.
-
+
Results To test the performance of the quadrupole ion trap for reaction rate measurements, the rate constant for the reaction of the (29) Brodbelt, J. S.; Louris, J. N.; Cooks, R. G.Anal. Chem. 1987, 59, 1278. (30) Ikezoe, Y.; Matsuoka, S.; Takebe, M.; Viggiano, A. In Gas Phase Ion-Molecule Reaction Rare Consianrs rhru 1986; Maruzen: Tokyo, 1987.
Effective Ion Temperatures in a Quadrupole Ion Trap
The Journal of Physical Chemistry, Vol. 94, No. IS, 1990 5811
SCHEME I: Lowest Energy Dissociation Reactions for Some of the Systems Studied ( E , is the Activation Energy)4**u*4s
(CH3CH2)4Si'+
(CH3CH2)3Si+
€0
-
0.5 eV
Eo- 1.5eV
(CH3O)2P(O)H0+
-
c
(CH,CH&SP
+ CH3CH;
(CH3CH2)2SH++ CH,=CH,
Eo 0.7 eV
CH30P(OH)H'+ + CH2O
-
molecular ion of acetylene with neutral acetylene (C2H2'+ + C2Hz C4HZ+, C4H3+)was measured. According to earlier research, this reaction is temperature independenta30 The rate constant for the reaction was determined, with and without helium damping and (8.1 f 1.6) X gas, to be (1.6 f 0.3) X cm3 molecule-' s-l, respectively. These rate constants compare with literature values obtained by using flowing afterglow instruments ((9.0-14.0) X 1O-Io cm3 molecule-I s-1).30,31 The reason for the somewhat different rates obtained with and without helium buffer gas can be explained on the basis of different rates of relaxation of ions to the center of the cell.12b The rate constant for the ion-molecule reaction of 02*+ with methane was determined in the presence and absence of helium damping gas to be 8.2 X and 6.6 X cm3 molecule-I s-I, respectively. The estimated precision of these measurements is k20%. The estimated reaction efficiency is 0.007. The ratio of exothermic to endothermic products (m/z 47:(m/z 15 + m/z 16)) is 1.00:0.33 under these conditions. (When excess methane is present, m/z 15 and m/z 16 are rapidly converted to m/z 17 and m/z 29.) The most abundant product ion observed for the reaction of 02'+with methane is an ion of m/z 47. In order to verify that this ion was indeed the desired reaction product, Le., H2COOH+, it was allowed to undergo previously examined ion-molecule reactions.32 As expected, the ion of m/z 47 was found to react with acetone to give protonated acetone and acetylium cation and with water to give protonated water only. In another experiment, the 02'+ ion was isolated and a supplementary ac voltage (1 V (p-p)) was employed for about 7 ms to increase its kinetic energy. These conditions resulted in an increase in the production of the endothermic products relative to m/z 47. The ratio of the exothermic to endothermic products varied with the ac voltage amplitude and was approximately 1:5 under the most energetic conditions. The signal-to-noise ratio was poor in these experiments, apparently because some of the kinetically excited 02'+ions were lost from the trap. No rate constant measurements were carried out under these conditions. Other chemical systems used to investigate effective ion temperatures in the ion trap are shown in Scheme I. These ions were stored in the trap for up to 3000 ms. Decomposition caused by collisions with the helium buffer gas was observed for tetraethylsilane (Eo= 0.5 eV) and protonated cyclohexene oxide (Eo = 0.5 eV) but not for the other ions. With a helium pressure of approximately 5 mTorr, the half-lives (corrected for ion loss) of protonated cyclohexene oxide and (Et),Si'+ were 600 and 300 ms, respectively. The rate constants measured for these dissociation and 8.8 X cm3 molecule-' s-I, reactions are 1.7 X respectively. The efficiency of decomposition (kdissoc/kADO) was 0.0003 for (Et),Si'+ and 0.0002 for protonated cyclohexene oxide upon collisions with helium. The dissociation rates were not sensitive to variation of the q value (between 0.1 and 0.5 during the reaction time), the method used to ionize the samples (for molecular ions or their fragments, electron ionization or Ar" charge exchange; for protonated molecules, chemical ionization using different protonating reagent ions), length of the cooling (31) Munson, M. S. B. J . Phys. Chem. 1965, 69, 572. (32) Van Doren, J . M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V. M.; Dotan, 1.; Ferguson, E. E. J. Phys. Chem. 1986, 90, 2772.
Temperature (K)
'
(b)
1wO
1C
00
Temperature (K) Figure 3. Rate constant (a) and the product distribution (b) for the reaction of CH4 + 02*+ as a function of effective temperature. The data were obtained from refs 24 and 26.
time prior to the reaction period, or the rf level during ion isolation. For the other ions investigated, mass-independent ion loss of approximately 20% was observed during the 3000-ms storage time, but no decomposition products were observed. Discussion The reaction of ionized oxygen with methane has been the focus of a number of research efforts, and it is one of the best characterized bimolecular gas-phase reaction^.^^,^^.^^ Previous studies have demonstrated that the rate constant as well as the product distribution is temperature and kinetic energy d e ~ e n d e n t . ~ , * * ~ , ~ ~ There is only one exothermic channel (1) available for the ion-
-
CH4 + 02'+ H2COOH+ (m/z 47)
+ H' + 1.0 eV
(1)
molecule complex. At higher energies, two endothermic reactions, (2) and (3), can dominate the product d i s t r i b ~ t i o n . ~ ~The *~~v~~
+
02*+ CH4
-
02*+ + CH4
CH3+(m/z 15) + H02' - 0.24 eV (2) (m/z 16) + O2 - 0.60 eV
CH,"
(3)
products of these reactions, CH3+ and CH4*+,react rapidly with CH,, as shown in (4) and (5) (the rate constant is 1.1 X lo", cm3 molecule-l s-I for both reaction^).^, Thus, the endothermic channels are monitored by formation of CHS+ and C2Hs+.24 Figure 3 shows the previously determined26dependence of the overall rate constant and the branching ratio on the effective temperature in a flow tube. CHI'+
+ CHI
CH3+ + CHI
-
+ CH,' C2H5+(m/z 29) + H2
CHS+ (m/z 17)
-
(4) (5)
Examination of the reactions of O +; is complicated by the fact that electron ionization of oxygen is known to result in a large proportion of electronically and vibrationally excited 02*+.34935
(33) Ferguson, E. In Advances in Atomic and Molecular Physics; Bates, D., Bederson, B., Eds.; Academic Press: London, 1988; Vol. 25, p 61. (34) Tanaka, K.; Kato, T.; Koyano, 1. J . Phys. Chem. 1980, 84, 750.
5812
The Journal of Physical Chemistry, Vol. 94,No. 15, 1990
TABLE I: Increase in the Average Internal Energy upon a Raise in Temperature from 298 to 600 K for Different Systems with Maxwell-Boltzmann Energy Distribution ((Ha- H2,) - '/$T)" and the Experimental Dissociation Threshold for Each of These Systems (E0)42~43*45
( H p - H29&12kT, eV O O H '
(6H3CH2)4Si'+ (CH30)2P(O)H'+ (C H 3CH 2)3Si+
0.53 >0.92 -0.34 >0.69
0.53
E,, eV
reaction observed
0.5
yes
0.5 0.7
Yes
1 ,s
no no
1.6
no
Vibrationally excited 0 2 ' + ( u 1 2 ) rapidly reacts with methane to give the products shown in (2) and (3).36 It has been well established, however, that 02*+(u21)is rapidly quenched (to u = 0) by collisions with CHI (rate 6 X cm3 molecule-' s-')~~ and that O;+(u=O) is not vibrationally excited in He buffer gas in a drift In order to lower the internal energy of any excited 0;+ as much as possible under the experimental conditions prevailing in the Paul-type trap, a relatively long cooling time (up to 200 ms) was used prior to examination of the ion-molecule reaction of 02'+ with methane. A cooling time of 200 ms corresponds to 18 000 collisions with the helium buffer gas and 50 collisions with methane. This number of collisions is more than enough3638 to allow quenching of the higher electronic and vibrational states of 02'+ generated during electron ionization, the final states of the system depending on the conditions prevailing in the ion trap. Indeed, we observed a single rate constant and a time-independent product distribution for the reaction of 02'+ with methane. On the basis of the measured overall rate constant and the data shown in Figure 3a,24*26 the estimated effective temperature of the 02'+/CH4 system in the ion trap using He buffer gas is 590 K (either 200 or 590 K is indicated; 200 K can be ruled out on the basis of the suprathermal kinetic energies of the ions12.18and the other results obtained in the present study). The ratio of exothermic to endothermic products was found to be 1.00:0.33. This ratio corresponds to a temperature of 700 K (see Figure 3b). If the ion internal and kinetic energies were in an equilibrium and approximately in a Maxwell-Boltzmann distribution, the 02*+ ions would have an average internal energy of 0.12 eV and a kinetic energy of 0.08 eV under these conditions (at 600-700 K). We examined the stability of a number of ions (Table I) in the trap by storing them for long times (up to 3000 ms). The excess internal energy that each of these ions would gain upon a temperature increase from 298 to 600 K, assuming Maxwell-Boltzmann energy distribution, is indicated in Table I, together with the dissociation threshold reported in the literature for each ion. Note that the amount of energy indicated for tetraethylsilane and protonated cyclohexene oxide is larger than the corresponding dissociation thresholds. Thus, these two ions are expected to decompose in the trap if the effective temperature were 600 K (assuming the energy distribution is not too different from Maxwell-Boltzmann). Rapid decomposition was indeed observed for tetraethylsilane and protonated cyclohexene oxide but not for the other ions studied (Scheme I). These decomposition reactions (35) Rowe, B. R.; Dupeyrat, G.; Marquette, J. B.; Smith, D.; Adams, N.
G.;Ferguson, E . E . J. Chem. Phys. 1984, 80, 241. (36) Bohringer, H.; DurupFcrguson, M.;Fahey, D. W.; Fehsenfeld, F. C.;
Ferguson, E . J. Chem. Phys. 1983, 79, 4201. (37) Durup-Ferguson, M.;Bohringer, H.; Fahey, D. W.; Ferguson, E. E. J . Chem. Phys. 1983. 79, 265. (38) Bohringer. H.; DurupFerguson, M.; Ferguson, E. E.;Fahey, D. W. Planet Space Sci. 1983, 31, 483.
Nourse and Kenttamaa were not due to excess energy obtained by the ions when generated or isolated since the rates were not sensitive to change of the ionization method, variation of cooling time before isolation of the reactant ions, or the rf voltage level during ion isolation. Measurement of the rate constant of a simple endothermic reaction may yield semiquantitative information about effective ion temperature if a functional form for the temperature dependence of the rate constant is By applying the empirical expression k( 7') = k , e ~ p ( - E , / R 7 ' ) , * ~where - ~ ~ ~E, is the activation energy, to the &sociation of (C2H5)$i'+ upon collisions with helium (kdi-/kADO = 0.0003), an approximate temperature of 720 K is obtained. In a similar manner, the dissociation of cyclohexene oxide, upon collisions with helium, would give a temperature of 530 K. These estimates, while very crude in nature, give a temperature range 530-720 K, which is in agreement with the temperature range proposed above (590-700 K). The reaction of 02*+ with methane was also examined under more energetic conditions wherein the 02'+ ions were accelerated by using a supplementary ac voltage. The absolute and relative abundances of the endothermic products (CHI'+ and CH3+) increased with the ac voltage amplitude, being approximately 1:5 under the most energetic conditions. This suggests that the effective temperature can be as high as 1300 K for this system (Figure 3b), corresponding to 0.29-eV internal energy for 02'+ ions with Maxwell-Boltzmann energy distribution. Note that polyatomic ions could have significantly higher internal energies at this temperature. Indeed, collisional activation experiments carried out for dimethyl phosphonate and dimethyl phosphite ions as a function of the amplitude of the supplementary ac voltage indicatez5that internal energies up to 5.8 eV can be deposited in these polyatomic ions. Further, studies on collision-activated dissociation of the molecular ion of n-butylbenzene, a chemical system that has often been used as an internal energy g a ~ g e , ' l ~ * ~ , ~ ' indicate that approximately 2-eV internal energy can be deposited in these ions upon acceleration in the ion trap.IIb In all these studies, an increase in ion internal energy was observed upon increasing the amplitude of the excitation pulse. Conclusion
Our results indicate an effective temperature of 600-700 K for the 02'+/CH4 system and for some polyatomic organic ions in a quadrupole ion trap. The effective temperature can be as high as 1300 K (for the 02'+/methane system) when the ion kinetic energy is increased by a supplementary ac voltage. The dependence of the effective temperature on the pressure and nature of the buffer gas, the mass and abundance of the ion, and the voltages applied to the trap remain to be examined in detail. Acknowledgment. Financial support provided by the National Science Foundation (Grant CHE-87 17380) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is greatly appreciated. R. G. Cooks is acknowledged for the use of the quadrupole ion traps and for various useful discussions, and R. E. March for helpful comments. (39) (a) Smith, D.; Adams, N. G.; Lindinger, W. J. Chem. Phys. 1981, 7 5 , 3365. (b) Bass, L. M.; Cates, R. D.; Jarrold, M. F.; Kirchner, N. J.; Bowers, M. T. J. Am. Chem. Soc. 1983, 105, 7024. (40) McLuckey, S. A.; Sallans, L.; Cody, R. B.; Burnier, R. C.; Verma, S.; Freiser, B. S.; Cooks,R. G. Int. J. Mass Spectrom. Ion Phys. 1982, 44, 215.
(41) Chen, J. H.; Hayes, J. D.; Dunbar, R. C. J. Phys. Chem. 1984,88, 4759. (42) Rosenstock, H. M.; Draxl, K.; Steiner, B.; Herron, J. T. J. Phys. Chem. ReJ Data 1977,6 (Suppl. 1). (43) Kenttamaa, H. I.; Pachuta, R. R.; Rothwell, A. P.: Cooks, R. G. J . Am. Chem. Soc. 1989, 111, 1654. (44) Selected Values of Physical and Thermodynamic Properties of Hy-
drocarbons and Related Compounds; Rossini, F. D., Pitzer, K. S., Braun, R. M.; Pimental, G. C., Eds.; Carnegie Press: Pittsburgh, PA, 1953. (45) de Ridder, J. J.; Dijkstra, G. R e d . Trau. Chim. 1967, 86. 737.
