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J. Phys. Chem. 1982, 86, 185-191
Mechanism of the Collision- Induced Dissociation of Polyatomic Ions Studied by Triple Quadrupole Mass Spectrometry D. J. Douglas Sclex Inc., Thornhill, Ontarlo, Cam& L3T 1P2 (Received: August 6, 1981; In Flnel Form: September 23, 1981)
The collision-induced dissociation (CID) of two ions of well-defined energetics,chlorobenzene and bromobenzene, has been studied by triple quadrupole mass spectrometry. The fragment yields as a function of target gas pressure, the variation of fragment yields with collision energy, and the fragment translational energy distributions are reported. The variation of fragment yields with target density can be understood in conventional kinetic terms for consecutive and competing reactions, each with a characteristic cross section. The variation of the cross section with collision energy and the fragment energies are interpreted within the framework of a two-step CID mechanism, an activation step followed by a separate unimolecular reaction. The fragment energies provide direct physical evidence for this mechanism. The variation of dissociation cross section with collision energy shows that a large fraction of the center of mass translational energy is converted to vibrational energy of the ion in the activation step. Comparison of the form of this cross section with the line-of-centers model and a statistical model developed here shows that, while the efficiency of the activation step is high, not all of the available energy is randomized in the organic ion through formation of a collision complex with the target.
Introduction During the last several years, there has been an increased use of tandem mass spectrometers for the direct analysis of complex mixtures (termed MS/MS for mass spectrometry/mass spectrometry). In these instruments a mass filter selects a parent ion of interest, the ion is fragmented by collision with a target gas, and the mass spectrum of the resulting daughter ions is scanned by a second mass filter. The development of MS/MS has prompted an increased interest in the mechanism of the collision-induced dissociation (CID) of polyatomic ions. Initial work was performed on magnetic and electric sector mass spectrometers where collisions take place at high energy (>1keV). The mechanism of such collisions has been deduced through work in several laboratories.’* At these high energies, the collision time is 10 times less than the period of a molecular vibration, and the process can be thought of as an electronic excitation and internal conversion followed by a separate unimolecular reaction. More recently, quadrupole mass analyzers have been applied to MS/MS.’*l9 In quadrupole instruments, ions (1) Boyd, R. K.; Beynon, J. H. In “Advances in Maas Spectrometry”; Daly, R., Ed.; Heyden: London, 1978; Vol. 7B, 1115-56. (2) Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81a-92a. (3) Durup, J. “Recent Developments in Mass Spectroscopy”, Ogata, K., Hayakawn, T., Ed.; University Park Press: Baltimore, 1970, pp 921-34. (4) Wachs, T.; McLafferty, F. W. Znt. J. Mass Spectrom. Zon Phys. 1977,23, 243-7. (5) Kim, M. S.; McLafferty, F. W. J. Am. Chem. SOC.1978, 100, 3279-82. (6) McLafferty, F. W.; Bente, P. F., m, Kornfeld, R.; Tsai, S. C.; Howe, I. J. Am. Chem. SOC.1973,95,2120-9. (7) Fedor, D. M.; Cooks, R. G. Anal. Chem. 1980,52,679-82. (8) Franchetti, V.; Freiser, B. 5.;Cooks,R. G. Org. Mass. Spectrom. 1978,13, 106-10. (9) Bowie, J. H.; Blumenthal, T. J. Am. Chem. SOC.1975,97,2959-62. (10) Yost, R. A.; Enke, C. G. J.Am. Chem. SOC.1978, 100, 2274-5. (11) Yost, R. A.; Enke, C. G. Anal. Chem. 1979,51,1251A-63A. (12) Yoet, R. A.; Enke, C. G.; McGilvery, D. C.; Smith, D.; Morrison, J. D. Znt. J. Mass. Spectrom. Zon Phys. 1979,30, 127-36. (13) Hunt, D. F.; Shabanowitz, J.; Giordani, A. B. Anal. Chem. 1980, 52. .?Fia-!X. --- --. (14) Siegel, M. W. Anal. Chem. 1980,52, 1790-2. (15) Zakett, D.; Cooks, R. G.; Fies, W. J. Anal. Chim.Acta 1980,129, 129-35. --I
are dissociated in collisions at lower energies, typically 1-100 eV. Less discussion has appeared on the dynamics of collision-induced dissociation at these lower energies. The target gas pressures and ion energies reported in the triple quadrupole work differ widely, or have not been reported at all, and no systematic study of these important variables has appeared. An improved understanding of the basic CID process occurring in quadrupole instruments will be necessary for an intelligent choice of instrument parameters, for the comparison of spectra obtained in different labs, and for the setting up of standard conditions for library spectra. Following earlier work by Durup and co-workers,2°CID in the 1-100-eV energy range has been described as occurring through a “momentum transfer collision”12to distinguish it from the electronic excitation observed at higher energies. In some cases where there is a strong attraction between the parent ion and the neutral target, a long-lived collision complex can be formed, leading to efficient energy transfer or chemical reaction.21 If, however, the potential between the ion and the neutral is less attractive, the dynamics will differ. Some of the most elegant work on collision-induced dissociation and energy transfer has been performed in experiments using crossed beams, but this work has focused principally on diatomic, triatomic, or dimer molecule^.^-^^ Direct reaction to three (16) Glish, G. L.; Hemberger, R. G.; Cooks, R. G. Anal. Chim.Acta 1980,119, 137-44. (17) Glish, G. L.; Cooks, R. G. Anal. Chim.Acta 1980, 119, 145-8. (18)Zakett. D.: Hembereer. P. H.: Cooks. R. G . Anal. Chim.Acta 1980, i19, 144-52.‘ (19) Bwch, K. L.; Kruger, T. L.; Cooks, R. G. Anal. Chim. Acta 1980, 119, 153-6. (20) Yamaoka, H.; Dong, P.; Durup, J. J . Chem. Phys. 1969, 51, 3465-76. (21) Enke, C. G.; Chakel, J. A.; Darland, E. G., presented at the 27th Conference on Mass Spectrometry and Allied Topics, Seattle, WA, June 1979, Paper MPMOC4. (22) Parks, E. K.; Hansen, N. J.; Wexler, S. J . Chem. Phys. 1973,58, 5489-513. (23) Parks, E. K.; Kurhry, J. G.; Wexler, S. J. Chem. Phys. 1977,67, 3014-28. (24) Piper, L. G.; Hellemans, L.; Sloan, J.; Ross, J. J. Chem. Phys. 1972,57,4742-51. (25) Tully, F. P.; Lee, Y. T.; Berry, R. S. Chem. Phys. Lett. 1971, 9, 80-4. I
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particles is observed. Vibrational excitation is facile in this energy range. This efficient vibrational excitation, observed in simple molecules, is expected to carry over to larger polyatomic molecules, but no beam experiments of large molecules have been reported. The many degrees of freedom characteristic of polyatomic ions is expected to lead to different behavior for collision-induced dissociation. Since an activated polyatomic ion can survive many vibrations before dissociating, the CID of polyatomics is likely to proceed through separate activation and fragmentation steps rather than direct dissociation to three fragments, even though the collision time may be comparable to a vibrational period. In some (but not all) cases, CID spectra resemble electron impact (EI) spectra, suggesting a similar fragmentation mechanism in both cases, and this has been interpreted as evidence for such a two-step CID While this CID mechanism has been widely assumed, there has been little direct experimental evidence to support it. This paper reports a study of the CID of two model polyatomic ions, bromobenzene and chlorobenzene, performed on a triple quadrupole instrument. For low internal energies these ions fragment by loss of a halogen atom. The bond strengths and the rateenergy curves for these reactions have been well studied by the photoelectron-photoion coincidence (PEPICO) meth0d.3"~~Since these ions have well-defined energetics, a study of their fragmentation offers useful insights into the dynamics of CID. A complete study of the fragmentation of a complex organic ester by Dawson et al.= discusses the implications of the results of this basic study for the design and operation of triple quadrupole mass spectrometers. Here, the dependence of the fragment yields from C&Br+ on target gas pressure is used to demonstrate that the relative ion abundances observed in triple quadrupole mass spectrometry can be understood in conventional terms of consecutive and competing reactions, each with a characteristic cross section. The energy dependence of this cross section for loss of a halogen is shown for these two ions. The data are interpreted in terms of a two-step CID process. The fragment energy distributions provide direct evidence for this two-step process while the variation of cross section with colliiion energy shows that the activation step proceeds through vibrational excitation in which a large fraction of the center of mass translational energy is converted to internal energy of the parent ion. Comparison of the form of the CID cross section with a lineof-centers model and with a statistical model developed here shows that, while the efficiency of the activation step is high (greater than the line-of-centers model), not all of the translational energy is converted to ion internal energy through formation of a collision complex with the target. (26)Cheng, M. H.;Chiang, M. H.; Gialason,E. A; Mahon, B. H.; Two, C. W.; Werner, A. S. J. Chem. Phys. 1970,52,6150-6.
(27)Cosby, P. C.; Moran, T. F. J. Chem. Phys. 1970,52,6157-65. (28)Cheung, J. T.;Dah, S. J. Chem. Phys. 1979,71,1814-8. (29)Armstrong, W. D.; Conley, R. J.; Creaser, R. P.; Greene, E. F.; Hall, R. B. J. Chem. Phys. 1975,63,3349-64. (30)Greene, E.F.;Hall, R. B.; Sondergaard, N. A. J. Chem. Phys. 1977,66,3171-80. (31)McGinnis, R. P.; Greene, E. F. J. Chem. Phys. 1978,69,4073-5. Todd, P. J.; McGilvery, D. C.; Baldwin, M. A.; (32)McLafferty, F.W.; Bockhoff. M. B.: Wendel. G. J.: Wixom, M. R.; Niemi. T. E. In 'Advances in Mass &ectrometry"i Quayle, B. A., Ed.; Heyden: London, 1980. (33) Baer, T.; Tsai, B. P.; Smith, D.; Murray, P. T. J. Chem. Phys. 1976,64,2460-5. (34)Rownstock, H.M.; Stockbauer, R.; Parr, A. C . J. Chem. Phys. 1979,71, 3708-14. (35)Rosenstock, H.M.; Stockbauer, R.; Parr, A. C . J. Chem. Phys. 1980,73,773-7. (36)D a m n , P. H.;French, J. B.; Buckley, J. A.; Douglas, D. J.; Simmons, D., submitted for publication in Org. Mass Spectrom.
Douglas
Experimental Section All experiments were performed on a prototype Sciex Inc. TAGA 6000 triple quadrupole MS/MS system.3638 An atmospheric pressure chemical ionization (APCI) source38was used to generate parent ions of bromo- or chlorobenzene. Typically, ions experience lo6 collisions in the ion source before entering the vacuum chamber housing the mass analyzers. Under these conditions, exceM internal energy of the ions is lost, and ions leaving the source are vibrationally cold (2' = 290 K). The vibrational temperature of the ions is further reduced in the supersonic expansion into the vacuum chamber (vibrational temperatures of approximately 10 K have been observed in such expansions). Lens elements on the vacuum side of the APCI interface are arranged to extract ions from the expanding gas and focus them into the first mass filter. If the voltage on these lens elements is set so that the ions are accelerated through the expanding gas, collisions can For the vibrationally excite or even dissociate present study concerning the energy dependence of the CID cross sections, these fields were made as small as possible to avoid excitation of the parents. As shown below, the threahold observed for the cross section confirms that this vibrational excitation was minimal ( X 0 . 5 eV). The gas target of the TAGA 6000 is formed from a free jet cross flow through the center of the second, rf only, quadrupole. Only the target thickness (number density times length) enters the solution to the rate equations describing the reactions occurring in the collision chamber. This thickness was calculated approximately by integrating the well-known density field of a free jet expansiona along the axis of the quadrupole. To facilitate comparison with work performed on other instruments, the target thickness has been converted to an equivalent target pressurg by dividing the integrated thickness by the length of the quadrupole (15 cm).This equivalent target pressure is the pressure which would give the same fragment yields if the collision chamber were uniformly filled with gas at this pressure. Ion collision energies were varied by changing the rod offset applied to the second quadrupole. Both fragment and parent ions move with aproximately the same speed (see below) and hence spend the same number of rf cycles in the quadrupole field. The transmissions of parents and fragments are thus similar over the range of kinetic energies explored despite the changing residence time in quadrupole 2. Since only the ratio of fragment to parent ion intensities enters the cross-section calculation, the energy dependence of the cross section is expected to be well reproduced. For both parent and fragment ions, kinetic energy distributions were measured by varying the rod offset on the third quadrupole to obtain a stopping curve. The first differences of these data then give the ion energy distributions. The instrument uses close coupled quadrupoles (rather than ion lenses), and the transmission between quadrupoles 2 and 3 is not expected to be strongly energy dependent.% Thus, the data approximate closely the true distributions. While the accuracy of this technique has not been fully assessed, it is believed to be correct to at least f 2 eV, sufficient for the discussion here. (37)Buckley, J. A.; Douglas, D. J.; Simmons, D.; Dawson, P. H.; French, J. B., presented at the 1981 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1981, paper 783. (38)Sakuma, T.; Davidson, W. R.; Thomson, B. A.; Danylewych, L. M.; Shushan, B.; Fulford, J. E.; Reid, N. M.; Rees, G. A. V.; Tosine, H., submitted for publication in Anal. Chem. (39)French, J. B. Can. Aeronaut. Space Inst. Trans. 1970,3,77-83.
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somewhat. This introduces a small mass-dependent correction to the ion intensities of the fragments if they are to be considered as coming from the same parent intensity in each case.% This correction has been applied to the data of Figure 1. The fragments at masses 51 and 77 correspond to loss of Br and further loss of C2H2and are simply accounted for by the following scheme:
of IOd torr) 16
I6
C&$r+ m / e 156 I
-% C&+ + Br m / e 77
N2
C4H3+ 4- CzH2 m / e 51
The C4H3+ion is formed in sequential reaction requiring two collisions (at higher energies it can be formed in a single collision from C&$r+). The solutions to the rate equations describing this scheme are
L
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m e 1. Ion intensities vs. target thickness for CeH,Br+ (150) incklent on a N, target: (0)ma88 156; ( 0 )mass 77; (A)mass 51. The solid lines are the solutions to the rate equations describlng the fragmentation.
Results and Discussion Pressure Dependence of Fragment Yields. A comprehensive discussion of the pressure dependence of fragment ion yields and the use of this for structure elucidation can be found in ref 36. Here, it is demonstrated that, despite the complex ion trajectories in the quadrupole field, daughter ion yields can be understood in terms of conventional rate equations governing an ion beam moving through a uniform target. There is a characteristic cross section for each reaction. The cross sections depend on ion internal energy, collision energy, and target gas. As well, the observed yields depend somewhat on the rf voltage applied to the second quadrupole, due to the different confinement efficiencies of parents and daughters.% For a given set of instrument parameters, however, rate equations can fully describe the production of fragment ions. As an example of this, the fragmentation yields from bromobenzene ions, C6H$r+, incident on a nitrogen target were measured for a range of target densities. The results are shown in Figure 1. The collision energy was 25 eV (laboratory (LAB)). For these measurements, the RF voltage on quadrupole 2 was set to be 0.50 that of quadrupole 3. As the third quadrupole scans, the rf voltage on the second quadrupole tracks at this lower value and correspondingly the transmission of the parent changes
where I,,,@) is the ion current of mass m at the exit of the collision chamber, S is the target thickness, u1 is the cross section for mass 156 to undergo a reaction to mass 77, and a2 is the cross section for reaction from 77 to 51. These solutions are shown by the solid lines in F' e 1. Reaction cross sections al = 13.8 A2 and a2 = 2.3 were used. It is not necessary to include scattering losses in the kinetics. The total ion signal (sum of parents and fragments) is nearly constant over the range of target densities explored, despite the variation in the relative contributions of parents and daughters. Since fragment ions have masses and energies different from those of parents (see below), it follows that the ion optics and the detection system have minimal energy or mass discrimination over the mass and energy range encountered here. (Further evidence for and a theoretical discussion of the low scattering losses in rfonly quadrupoles can be found in ref 41.) The rate equations can account for the ion intensities over the full range of target pressures. Mass 51 is formed entirely by sequential reaction in this case. (In fact, a plot vs. SZ is linear at low target densities as expected for of 151 the product of a reaction requiring two collisions.) Cross sections are appreciable, indicating fairly efficient dissociation of the parent ions. Sequential reactions are readily observed for a target thickness of 2 X 1014cm-2 or greater. Variation of the Dissociation Cross Section with Collision Energy. Dissociation cross sections were studied for both bromobenzene and chlorobenzene ions. The yield of C6H6+from C6H5Br+incident on a nitrogen target was measured for laboratory collision energies up to 65 eV. The torr equivtarget thickness was 1.6 X 1013cm-2 (3 X alent pressure) well within the single collision regime (cf. Figure 1). At energies below 50 eV (LAB), only loss of a halogen atom was observed. At higher energies, a second channel to produce C4H3+becomes accessible. Relative cross sections were calculated from u -In [l - If/(Z + If)]where If is the total fragment ion current (I7, + &), Ipis the parent ion current, and a is the cross section to fragment to C6H5+or to fragment through the higher energy channel to C4H3+. These cross sections and the contribution to the total cross section from the second channel are shown by the points in Figure 2. The shape
(40) Potter, W. E.;Mauemburger, K. Rev. Sei. Instrum. 1972, 43, 1327-30.
(41) Dawson, P.H.;Fulford, J., submitted for publication in Int. J. Mass Spectrom. Ion Phys.
At high target gas pressures, multiple collisions can relax the translational energy distributions. To check that this did not influence the results reported here, we measured parent and fragment ion energy distributions for chlorobenzene ions (C6H6C1+)incident on an argon target for a range of target thickness up to 6.2 X 1014 cm-2. For a thickness of 3 X 1014cm-2 or less, the shape of the energy distributions was constant. The results reported here were obtained at a target thickness of 1.6 X 1013cm-2 and were not distorted by multiple collisions. Saturated pulse counting was used for ion detection. Mass discrimination has been shown to be modest (20 am^).^
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is the target gas temperature, and E C M is the nominal center of mass collision energy. For 27-eV bromobenzene ions incident on nitrogen at 290 K, this is calculated to be W = 0.96 eV. The complete width, from both contribuor 1.06 tions, is given approximately by [(0.45)2+ (0.96)2]1/2 eV in this case. Equation 4 describes a thermal target rather than a free jet and does not rigorously apply to these experiments. A free jet gives target molecules with a near uniform distribution of speeds, but a large angular spread. The distribution in collision energies derives mainly from the distribution in angle between the velocity vectors of the ion and the neutral,42and thus eq 4 gives a fair approximation to the energy spread here. The cross section of Figure 2, then, is a convolution of the true cross section over a distribution of energies with 1-eV width (center of mass). No detailed deconvolution has been attempted. Nevertheless, the general form of the cross section of Figure 2 reveals some features of interest and has sufficient information to permit comparison with two simple theoretical models which help clarify the CID process. The threshold in the empirical cross section is approximately at the bond strengths reported in ref 33 and 35 (marked with arrows a and b, respectively). A kinetic shift in this threshold might be expected if ions fragment by unimolecular reaction following a separate activation step. Unfortunately, any kinetic shift is largely masked by the spread of collision energies in this experiment. Parent ions of 25-eV energy (LAB) spend 13.6 ps traveling from the center of the second quadrupole (where collisions are most probable) to the third quadrupole. To produce daughters they must then fragment in less than this time. The PEPICO experiments indicate that the internal energy must then exceed 3.4 eV to produce fragments, and this threshold is indicated by arrow c in Figure 2. The experimental cross section begins to rise at 19 f 3 eV (LAB). Because of the distribution in collision energies, the experimental cross section rises from zero somewhat before the true threshold by -0.5 eV in center of mass coordinates or -3 eV LAB (cf. Figure 6 of ref 42). Deconvolution over the spread of collision energies would then be expected to shift the threshold to 22 f 3 eV (3.4 eV center of mass) in accord with the kinetic shift expected from the rate-energy curves of ref 33 and 35. Observation of a kinetic shift would provide direct evidence for a two-step CID process. The data of Figure 2 are consistent with this but do not clearly demonstrate a kinetic shift. The threshold of Figure 2 is within 0.5 eV (center of mass) of those predicted by the PEPICO experiments, and most of this uncertainty can be attributed to the spread of collision energies. It follows that the sum of ion internal energy and any kinetic energy acquired from the rf field is less than 0.5 eV, since both of these effects would shift the threshold to lower energies. Thus, (i) the ion source produces ions of less than 0.5-eV internal energy (probably much less) and (ii) parent ions receive less than 3-eV (LAB) kinetic energy from the rf field. The quadrupole focuses the ions but does little to change the ion kinetic energy, as suggested by Yost et al.12 and predicted by computer models of the ion t r a j e ~ t o r i e s . l ~ * ~ ~ * ~ ~ From threshold, the cross section rises to a maximum over -4 eV of center of mass energy. This major feature of the form of the cross section is not strongly affected by the spread in ion collision energies since the dominant effect of this spread is to blur the threshold behavior; Le., the true cross section is expected to rise from 10% to 90% of its maximum over this energy range (a few eV). Fur= 0.5) thermore, the position of this rise (taken at u/ummaX is scarcely shifted at all by the spread of collision energies.42
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Flguro 2. Cross section for fragmentation of C6H,Br+ by N, vs. translational energy: (0)experiment 1; (A)experiment 2; (0)contribution of mass 51 to the cross section. The Sdkl lines I and I1 are the results of the lineofcenters and statistical models, respectively. Arrows a and b indicate the t h r e expected from published bond strengths. h o w c Is the threshold expected allowing for a kinetic shift.
of the cross section, i.e., u/uma, is not affected by the correction for mass-dependent transmission of quadrupole 2, and hence this correction was unnecessary here. The results of two experiments are shown. In experiment 1, the second fragment was not recorded and so only data below 50 eV are shown. The fraction of the laboratory translational energy (ELAB) that is available for inelastic scattering is the energy of the collision in a coordinate system moving with the For an ion center of mass of the collision partners (ECM). of mass m, incident on a target of mass m2,this is given The center of mass colby E C M = [mz/(ml+ m2)]ELAB. lision energy is shown by the top scale of Figure 2. The cross section rises from a threshold to a maximum over a range of 30 eV (LAB). In order to compare this cross section with the predictions of two simple models, the maximum cross section, u-, has been normalized to 1; i.e., Figure 2 records g/u-. For the case of bromobenzene ions, this maximum cross section was found to be 250 A2,approaching gas kinetic. A cross section of this magnitude indicates that translational energy is efficiently used to promote reaction. At a collision energy of 8 eV (center of mass), for example, more than 40% of the translational energy (3.5 eV) is transferred to internal energy in essentially every collision. Both the spread of energies in the parent ion beam and the thermal motion of the target contribute to broadening the distribution of collision energies about the nominal value, and this introduces a distortion in the experimental cross-section curve. Under the conditions of this experiment, the greatest contribution to the energy spread comes from the thermal motion of the target. The data of Figure 2 were recorded with an ion beam having an energy spread of -3 V fwhm in the LAB frame or 0.45 eV in center of masa coordinates. As shown by Chantry,&a monoenergetic beam incident on a thermal target will have a breadth in center of mass collision energies (W) given by where y = ml/(ml (42) Chantry,
+ m2),k is Boltzmann's
constant, To
P.J. J. Chem. Phys. 1971,55, 2746-59.
Collision-Induced Dissociation of Polyatomlc Ions
This rise in the cross section offers clues to the mechanism of the activation step and is compared here with the predictions of two models. The solid line I in Figure 2 is the cross section predicted by a line-of-centers model for the activation step drawn with a threshold predicted from the PEPICO experimenta (3.4 eV CM). This model is widely used for modeling endothermic processesa and has had some success in describing the CID of d i a t ~ m i c s . ~ ~ The model predicts u/u- = (1- E0/EcM)where Eois the threshold for reaction (3.4eV). The line-of-centers cross section increases from 10% to 90% of its maximum over a center of mass energy range of 30 eV. Clearly this model underestimates the rate of increase of the cross section with
The Journal of Physical Chemlstty, Vol. 86, No. 2, 1982 189
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The line-of-centers model gives efficient energy transfer for direct, head-on collisions and describes, approximately, diatomic CID.@ That the experimental cross section rises more rapidly for polyatomics suggests that the many degrees of freedom of the organic ions may be accepting vibrational energy in an efficient manner, and this in turn suggests that a statistical model may be suitable to describe the activation step. Statistical theories have been applied with success to the description of energy tramfer and CID for cases where "sticky" collisions are e ~ p e c t e d . ~ ~In M particular, Eastes and Toennies have shown that, as the complexity of the collision partners increases (from diatomic through triatomic to polyatomic), energy transfer in ion-neutral collisions assumes a more statistical nature.* (It should be emphasized that the statistical model discussed here refers to the activation step, Le., the breakup of a parent ion-neutral complex, and not to the subsequent unimolecular reaction of the excited C6H5Br+. The unimolecular decay of excited CeH&b+ (also statistical) is well described in ref 33-35). A simple statistical model for the activation step can be formulated from the phase space theory of Light.& If a collision complex is formed in which energy is randomized, the complex will decay statistically populating each accessible quantum state of the products with equal probability. The probability of forming an ion with internal energy Eint,then, is given by P(Eint) p(E,t)P(EbM), where p(EbJ is the density of internal states of the ion at energy Ebtand p(EbM) is the density of states for product translational energy EbM. A simple analytical approximation to p(Ebt)is p(Eht) E,,", with s taken to be the number of "effective" oscillators (usually about '/3-'/2 the number of vibrational degrees of freedom). The density of translational states is p(EbM) E'&2. For the case where the ion has no internal energy before the collision, the final translational energy is E ~ =MECM- Eht, and
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Flgure 3. Cross sections for fragmentation of CaH5CI+vs. translational energy in (A) LAB coordinates and (B) center of mass coordinates: (0) N, target: (A)argon target.
The fragmentation cross section is readily obtained by recognizing that the cross section is proportional to the probability of transferring an energy greater than the threshold energy Eo (3.4eV in this case). Thus
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The integral ensures normalization. A similar expression was derived by Safron et al.4' by considering the RRKM theory of complex breakup and was found to be in reasonable agreement with experiments for chemical reaction and inelastic scattering occurring through a complex.48 (43) Bemetein, R. B.; Levine, R. D. "Molecular Reaction Dynamics"; Clarendon Press: Oxford, 1974. (44) Donohue, T.; Chou, M. S.; Fisk, G. A. Chem. Phys. 1973, 2, 271-82. (46) Eastes,W.; Toennies, J. P. J. Chem. Phys. 1979, 70, 1644-61. (46) Light, J. C. Discuss.Faraday SOC.1967,44, 14-29. (47) Safron, S. A.; Weinstein, N. D.; Herschbach, D. R. Chem. Phys. Lett. 1972, 12, 664-8.
The solid line marked I1 in Figure 2 shows this cross section calculated with s = 7. (The number of vibrational degrees of freedom of C6H5Br+is 30, and taking s = 7 corresponds to a relatively low number of effective oscillators.) This simple statistical theory which works well in other ~ o n t e x t s ~ ' *overestimates ~*~' the slope of the rise in the cross section, reaching 90% of the maximum within 1 eV of the threshold. For large s, the theory predicts a near step function rise, since the many degrees of freedom of the polyatomic ion lead to a high density of states which accommodates nearly all of the available energy. Any statistical theory which allows all of the degrees of freedom of the organic ion to accept energy without restriction will give a cross section which rises very rapidly, regardless of the detailed model used for the density of states. Somewhat better agreement between this theory and the experiment can be obtained by taking s = 3, an unrealistically small value for calculating the density of states of C6H&h+. Thus, agreement with the model can only be forced if the number of degrees of freedom accepting vibrational energy is severely restricted. This strongly suggests that, if a collision complex is formed, translational energy is not completely randomized in all of the degrees of freedom of the ion during the lifetime of this complex. (48) Lee,k; Leroy, R. L.; Herman, Z.; Wolfgang,R.; Tully, J. C.Chem. Phys. Lett. 1972, 12, 669-73.
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22
E ~ A (Be v ) Flgure 4. Energy distribution of CeH,+ from CeH,Br+ IncMent on N2 at 25 eV (LAB). (---) Parent energy distribution shifted to lower energies by a factor of m,/m, (771156 in this case). The arrows indicate the expected energies for scattering angles of O', 90°, and 180' in the activation step.
The above model does not allow for vibrational and rotational excitation of the target (N2). If this is allowed, it can be shown that P(E,,) E h < ( E c ~- EhJ5I2. This distribution produces somewhat better agreement with the experiment, suggesting that monatomic and diatomic targets might display significantly different cross-section curves. Thus, the CID cross sections for chlorobenzene ions on N2 and Ar targets were measured. These cross sections are shown in Figure 3 plotted against laboratory (Figure 3A) and center of mass energies (Figure 3B). (For these experiments, the energy distribution of the parent ion beam was quite wide (12 eV fwhm (LAB) corresponding to 2.5 eV (CM) with an argon target), giving poorly defined thresholds.) When plotted against center of mass energy, there is little difference in the cross sections for Ar and N2 targets.49 If energy is fully randomized in a collision complex, the cross section for an argon target will rise more steeply than that for nitrogen since vibrational and rotational excitation of nitrogen can remove energy which would otherwise be available for excitation of the parent ion. The maximum cross sections at EcM = 11 eV were equal for N2 and Ar targets (56 A2). The "equivalence" of N2 and Ar targets provides further evidence that energy is not fully randomized in these ionneutral collisions. Evidently, the actual energy transfer process is intermediate between the direct line-of-centers and the fully statistical models. The statistical model could be modified by using a more realistic density of states function (Le., as in ref 33-35), but any realistic density of states will give a very fast rise in the cross section, which does not match the experimental data. Most likely a model which allows partial randomization of the collision energy will be successful. Such a model has been advanced by Eastes and Toennies,* to describe inelastic collisions between lithium ions and triatomic molecules. The extension and application of this theory to CID would be of interest but is beyond the scope of this paper. Translational Energy Distributions of the Fragments. The variation of the cross section with collision energy was discussed within the context of a two-step CID mechanism. Direct evidence that this mechanism applies here comes
E ,t
(eVi
Figure 5. Energy distribution of CeHS+from CsH5Br+incident on N, at 56 eV (LAB). Notation as in Figure 4.
N
(49) It ha^ been reported (ref 10 and 12) that heavier target gases give a greater fragment yield for a given LAB collision energy, and Figure 3A illuetmtaa this for N2 and Ar targets. To a large extent this simply reflects the fact that heavier targets give a greater center of mass energy for a given LAB energy. It would be of interest to further compare different targets at the same center of m a s energy, as is done in Figure 3B.
0
I
o
l
4l
I
1
12
'
I
16
l
l
20
(ev)
Figure 8. Energy distribution of CeH5+from CeH,CI+ incident on Ar at 25 eV (LAB). Notation as in Figure 4.
from the translational energy distribution of the fragments. Figures 4 and 5 show the energy distributions of C6H5+ fragments from bromobenzene ions incident on a nitrogen target at two LAB energies, 25 and 56 eV, respectively. fragments Figure 6 shows the energy distribution of C6H6+ from chlorobenzene ions incident on argon with 25-eV energy (LAB). In each case these fragment energies are just those to be expected from unimolecular decay of a parent ion which has lost some energy in an inelastic collision. For a parent ion of mass m l , undergoing an inelastic collision with a target of mass m2,the ratio of the parent ion LAB energy after the collision (E'LU) to that before the collision is given by EiAB -- Em-
(7)
where OCM is the scattering angle for the inelastic collision
Collision-Induced Dissociation of Polyatomic Ions
in center of mass coordinates, EM is the energy transferred to the ion, and M = ml m2.% If the parent ion then fragments to a daughter of maw m3 with no kinetic energy release, the fragments will move at the same speed as the parent and have an energy (Efrw)given by &rag = ( ~ ~ / ~ I ) E ’ L A B (8)
+
Fragment energies calculated from eq 7 and 8 are shown by the arrows in Figures 4-6 for center of mass scattering angles of Oo, 90°, and M O O . The values of Eht used were 3.4 and 4.0 eV for bromobenzene and chlorobenzene, res p e c t i ~ e l y . ~ In ~ ”every ~ case the predicted fragment energies agree well with the experiment. It is seen that ions acquire a spread in energy from the range of scattering angles occurring during the activation step. A wide range of angles can contribute to the observed fragments. These large scattering angles suggest that it is vibrational (not electronic) excitation which leads to dissociation. Fragment ions have lower energies than the parents because they are lighter (hence, less energy for the same speed) and because recoil of the neutral in the activation step removes some additional LAB energy from the parents. For the case of a heavy ion incident on a light target, EtLAB Em and the fragment kinetic energy is approximately given by = (m3/m1)ELAB (9)
-
The kinetic energy distributions predicted by this formula are shown by the dashed lines in Figures 4-6. These are the parent distributions, measured in the same experiment, shifted by the mass ratio of eq 9. The parent distributions are normalized to the same height to conveniently display the median energy predicted by eq 9. Equation 9 is useful as a rule of thumb for the expected energy.% It is most suitable for heavy ions colliding with light targets, essentially because all parents and fragments move at approximately the same speed, that of the center of mass of the ion-neutral pair. This will commonly be the case found in many analytid applications. For lighter ions or heavier targets (e.g., C6HSC1+on Ar, Figure 6) the more exact considerations of eq 7 and 8 are necessary. If kinetic energy is released in the fragmentation, there will be an additional spread of fragment energies convo(50) Appel, J. In “Collision Spectroscopy”;Cooks,R. G., Ed.;Plenum Preee: New York, 1978; pp 252-5.
The Journal of Physical Chemistty, Vol. 86, No. 2, 1982 191
luted over the spread of fragment energies arising from the activation step, given by AEfr, =
4m31/2(ml- m3)1/2(EiABr)1/2
(10) ml where T is the kinetic energy release and m3 is the mass of the fragment ion. In favorable cases, this could be observed by triple quadrupole mass spectrometry but was not a necessary correction to describe the main features of the energy distributions here. For the simple bond cleavage of these reactions, minimal kinetic energy release is expected. Kinetic energy release may, however, contribute a few electron volts to the breadth of these fragment energy distributions, but this is a minor contribution compared to the width arising from the range of scattering angles involved in the activation step. Conclusions The fragment energy distributions reported here provide direct physical evidence for a two-step CID mechanism, an activation step followed by mimolecular fragmentation. The efficiency of the energy transfer in the activation step is high, intermediate between the line-of-centers and statistical models. The large scattering angles involved in the activation step suggest that it is vibrational rather than electronic excitation which obtains in this energy regime. Further study of collision-induced dissociation for a wide range of systems, is likely to reveal an equally wide range of behavior, of which the mechanism discussed here is but one case. This mechanism, however, is likely to apply to many cases of interest to analysts. Triple quadrupole mass spectrometers are well suited to the study of ion-neutral collisions. While the development of these instruments has been spurred by their many analytical applications, these instruments also facilitate studies such as the investigation of ion-molecule reaction schemes, the measurement of reaction cross sections, and the determination of product energies. Thus, it is likely that triple quadrupoles will be usefully applied to future studies of the dynamics of ion-molecule collisions. Acknowledgment. I thank P. H. Dawson, J. B. French, and the scientific staff of Sciex Inc. for many helpful discussions. I thank D. Lane for programming assistance. This work was carried out under contract to the National Research Council of Canada.
