2722
J. Phys. Chem. 1981, 85, 2722-2725
by the two approaches, we suggest four complexes for further detailed experimental and theoretical study: (CH3)2S-HNC0, (CH3)2S-S02,H3N-HNCO, and H3N-S02.
Acknowledgment. We thank the National Science Foundation (CHE-76-81718and 80-26560) for support of this research. Very helpful comments have been made by Professor R. Drago.
Competition between Isomerization and Fragmentation of Gaseous Ions. 1. Kinetic and Thermodynamic Control for C4H,+ Ions Tacheng Hsieh, Jerome P. Gilman, Morris J. Welss, and G. G. Meisels" Department of Chemistry, Universlfy of Nebraska, Lincoln, Nebraska 68588 (Recelved: March IO, 198 1; In Final Form: June 22, 198I )
Breakdown graphs of the six C4H8isomers have been determined by threshold photoelectron-coincident photoion (TPE-CPI) mass spectrometry. When internal energy is expressed relative to the most stable isomer structures, the breakdown graphs of the four olefins are identical, indicating that all ions rearrange to the thermodynamically most stable structure before they fragment. However, at higher internal energies, methylcyclopropane and cyclobutane show different behavior consistent with a scheme in which direct fragmentation from a structurally specific transition state begins to compete with isomerization followed by dissociation.
Introduction Structure elucidation of gaseous ions is one of the important tasks in mass spectrometry. It is complicated by the frequent incidence of rearrangement of molecular ions before fragmentation to a structure totally different from that of the neutral precursor. In general, the fragmentation thresholds of odd-electron ions are lower than those of even-electron ions. Therefore, in odd-electron hydrocarbon ions the isomerization barrier is usually below that for fragmentation, often leading to complete randomization between isomeric species.l The above argument, based on the stability of even- or odd-electron ions, should also be valid for unsaturated hydrocarbon ions; however, it is well-known that the double bond migrates readily. The situation is particularly complicated for small unsaturated hydrocarbons. The isomeric C4H8 molecules constitute a unique structurally representative system which can be used to probe the structural behavior of molecular olefin and cyclic ions before and during fragmentation. As precursor ions for intermediate states, the different possible fragmentation paths taken by each isomer can, in principle, be correlated with the structure of the intermediate. The low-energy electron impact spectra of the isotopically labeled C4H8isomers2 suggest that the open-shell C4H8+ions have completely equilibrated to a mixture of interconverting structures before fragmentation although it was recognized that some of the isomers showed dissimilarities in their mass spectra taken at 70 eV. Subsequent studies of C4H8+by metastable ion f~rmation?~ field ionization kinetics,6and charge-ex~hange~~' measurements (1) Karsten Levsen, "Fundamental Aspects of Organic Mass Spectrometry", Verlag Chemie, Weinhein, West Germany, 1978. (2) (a) G. G. Meisels, J. Y. Park, and B. G. Giessner, J. Am. Chem. SOC.,91, 1555 (1969), and references cited therein; (b) ibid., 92, 254 (1970); (c) M. S. H. Lin and A. G. Harrison, Can. J . Chem., 62, 1813 (1974). (3) G. A. Smith and D. H. Williams, J. Chem. SOC.B, 1529 (1970). (4) J. L. Holmes, G. M. Weese, A. S. Blair, and J. K. Terlouw, Org. Mass Spectrom., 12, 424 (1977). (5) R. P. Morgan and P. J. Derrick, Org. Mass Spectrorn., 10, 563 (1975).
all led to similar conclusions. The role of the C4H8+ions as intermediates in unimolecular fragmentation has also been investigated in photoionization: y i r r a d i a t i ~ n , ~ photodissociation,1° and ion-molecule rea~tions?J~-'~ All of the various experiments mentioned above suffer from a drawback in that the precursor ions do not possess a well-specified amount of internal energy but have an energy distribution determined by the ionization process. This is thought to be largely responsible for the differences in the 70-eV mass spectra of C4Hs. A more elegant method, which enables one to observe fragmentation as a function of energy deposition, is the technique of threshold photoelectron-coincident photoion (TPE-CPI) mass spectrometry. Since photoions are detected in coincidence with threshold photoelectrons, the internal energy of the ion is uniquely determined by the photon energy. Although Baer et applied this technique to C4H8+ions, they only addressed the threshold for the dissociation process to C3H6+and CH3 and the unimolecular dissociation rate leading to the formation of C3H6+fragment ion. They find that the butenes and methylcyclopropane are not distinguishable; cyclobutane was not included in their investigation. (6) (a) T. 0. Tiernan and L. P. Hills, presented at the 19th Annual Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 2-7, 1971; (b) C. Lifshitz and T. 0. Tiernan, J. Chem. Phys., 66,3555 (1971). (7) J. Sunner, Int. J.Mass Spectrom. Ion Phys., 32,285 (1980). (8) (a) L. W. Sieck, S. K. Searles, and P. Ausloos, J. Am. Chern. SOC., 91,7627 (1969); (b) L. W. Sieck, S. G. Lias, L. Hellner, and P. Ausloos, J. Res. Natl. Bur. Stand., Sect. A, 76, 115 (1972). (9) S. G. Lias and P. Ausloos, J. Res. Natl. Bur. Stand., Sect. A, 75, 591 (1971). (10) (a) J. M. Kramer and R. C. Dunbar, J. Chem. Phys., 59, 3092 (1973); (b) M. Riggin, R. Orth, and R. C. Dunbar, ibid., 66,3365 (1976). (11) (a) 2.Herman, A. Lee, and R. Wolfgang, J.Chem. Phys., 51, 462 (1969); (b) A. Lee, R. L. Leroy, Z. Herman, R. Wolfgang, and J. C. Tully, Chem. Phys. Lett., 12, 569 (1972). (12) T. Huntress, Jr., J. Chern. Phys., 56, 5111 (1972). (13) P. R. LeBreton, A. D. Williamson, and J. L. Beauchamp, J.Chern. Phys., 62, 1623 (1975). (14) W. J. Chesnavlch and M. T. Bowers, J. Am. Chem. SOC.,98,8301 (1976). (15) T. Baer, D. Smith, B. P. Tsai, and A. S.Werner, Ado. Mass Spectrom., A7, 56 (1978). Professor Baer has reinterpreted their data with a single lifetime distribution.
0022-3654/81/2085-2722$01.25/00 1981 American Chemical Society
Isomerization and Fragmentation of Gaseous Ions
The Journal of Physical Chemistry, Vol. 85, No. 19, 1981
2723
m/e 41 w
45
CIS-2-BUTENE 11.804 eV
8
0.8
0
-
z 41
93
0.6 -
m
30[
e q
m/e 5 5 , 5 6
4: W
0.4 -
2
+ 5W
15
0.2 -
p: ,
, , :I. '
20
I
.
,
, '.
. .
.(,
. . . .... . ... ..
...
25
30 FLIGHT TIME
,
0.0
.... . . . . . . . .., ...,.: , . :. .. . . . . . .... .,,..
I
2.2
35 USEC
Flgure 1. Time-of-flight Coincidence spectrum of cis-2-butene taken at 11.804 eV.
Present evidence indicates that isomerization of the six C4H8molecular ions to a common structure is followed by fragmentation from a series of transition states which appear to be described best by a model which requires both tight and orbital transition states for each channel.16 We fit this model by evaluating breakdown graphs for all C4H8 isomers and extend the internal-energy range to a point where higher energy dissociation paths should compete with isomerization.
Experimental Section The threshold photoelectron-coincident photoion mass spectrometer has been described previo~sly'~ and is reviewed here only briefly. The light source is a quartz capillary discharge tube similar to that of Huffman et al.18 The monochromator was operated with an optical resolution of 0.083 nm; in combination with the threshold photoelectron detector, this led to an energy resolution of -21 meV (fwhm) at a photon energy of 14 eV. Both the helium Hopfield continuum and hydrogen many-line spectra were used as light sources. The spectrum is obtained by using the threshold photoelectron and ion signal as start and stop inputs to a time-to-pulse-height converter. The output of the converter is fed to and stored in a multichannel pulse-height analyzer. A typical result for the time-of-flight coincidence spectrum of cis-2-butene is shown in Figure 1 at an energy of 11.804 eV. The two major peaks shown in the figure result from the ions of unresolved masses 56 (c4H8+)and 55 (C4H7+),and from ions of m/e 41 (C3H5+).Separation of masses 55 and 56 is achieved in a separate experiment by operating the quadrupole mass filter in the mass analyzer mode. The threshold photoelectron detector used in this study transmits small but significant numbers of electrons at energies up to 2 orders of magnitude higher than the energy resolution of this apparatus.l9 Transmission of these nonzero-energy electrons causes false coincidence and thus may lead to a false interpretation of breakdown graphs. A method to correct experimental data such as breakdown graphs for the transmission of energetic electrons has been developed; 2o it consists of convoluting the transmission (16) W. J. Chesnavich, L. Bass, T. Su, and M. T. Bowers, J. Chem. Phys., 74, 2228 (1981). (17) (a) C. F. Batten, J. A. Taylor, B. P. Tsai, and G. G. Meisels, J. Chem. Phys., 69, 2547 (1978); (b) M. J. Weiss, T. Hsieh, and G. G. Meisels, ibid., 71, 567 (1979). (18) R. E. Huffman, Y. Tanaka, and J. C. Larrabee, Appl. Opt., 2,617
2.6
INTERNAL
3.0 3.4 ENERGY ( eV )
3.8
Figure 2. Breakdown graph of four olefinic C,H, compounds. Internal-energy scale is photon energy minus 9.13 eV for cis- and trans-Bbutene, 9.03 eV for 1-butene, and 9.23 eV for methylpropane. I
D-
I
W
y
I
0.8.
a
0
z 3
m
0.6.
a
INTERNAL ENERGY ( eV ) Flgure 3. Breakdown graph of methylcyclopropane. Internal-energy scale: hv = 8.74 eV.
Z
3
m
a
INTERNAL ENERGY
( eV )
Flgure 4. Breakdown graph of cyclobutane. Internal-energy scale: hv = 8.75 eV.
function of the TPE detector with the photoelectron spectrum and postulated breakdown curves until the experimental one is matched, and was applied to all results obtained in this study. Mass-analyzed ion kinetic energy spectra (MIKES) are obtained by triple-sector mass spectrometry (TSMS; Kratos MS-50-TA). Results and Discussion The breakdown graphs, after correction for nonzeroenergy electrons transmitted through the TPE detector, for the four olefinic C4H8isomers, methylcyclopropane, and
(10fiR)
(19)T . Hsieh, J. P. Gilman, M. J. Weiss, G. G. Meisels, and P. M. Hierl, Int. J. Mass Spectrum. Ion Phys., 36, 317 (1980).
(20) J. P. Gilman, T. Hsieh, and G. G. Meisels, to be submitted for publication.
2724
The Journal of Physical Chemjstty, Vol. 85, No. 19, 1981
Hsieh et al.
cyclobutane are shown in Figures 2-4 for the processes
--
C4H8+
C4H,+ + H
(2)
C3H5++ CH3
(3)
C2H4+ + C2H4
(4)
Instead of plotting breakdown graphs as a function of photon energy, we analyze these in terms of excess internal energy E* above the most stable ionic isomer. This quantity is calculated from the relationship E* = hv + - IP,, where the enthalpy term refers to the energy released when the ground-state ion X+ rearranges to the most stable ionic structure B+ (either 2-butene or 2-methylpropene),and IP is the ionization potential of the particular isomer. It is apparent from Figure 2 that breakdown graphs of the four olefinic C4H8+ions are indistinguishable from each other, indicating that these ions have completely equilibrated before fragmentation and are presumably a mixture of structures interconverting rapidly. Therefore, breakdown graphs of the olefinic isomers only reflect the excess internal energy which they possess above the most stable ionic structure. On the other hand, the breakdown graphs of methylcyclopropane and cyclobutane are identical with that of the olefinic isomers only in the very narrow energy region from onset of C3H5+at ca. 2.25-2.6 eV. A t higher internal energies, the breakdown behaviors of methylcyclopropane and cyclobutane are different. Metastable ion spectra of C4H8+ions generated from the six C4H8 isomers are essentially identical; reactions 2 and 3 have intense metastable peaks while reaction 4 shows only very weak metastable peaks. This is in good agreement with published report^.^^^ Lifshitz and Tiernansb studied the fragmentation of cyclobutane by charge exchange in a tandem mass spectrometer and suggested that the high yield of CzH4+ion observed in their experiment resulted from a strained intermediate ion having the tetramethylene structure formed by rupture of a carbon-carbon bond. They proposed that this strained tetramethylene ion dissociates via a second carbon-carbon bond rupture to give C2H4+ and C2H4 at higher internal energies and that this process competes with isomerization. Our results support the interpretation of Lifshitz and Tiernan. The basic mechanisms which govern the retention of ion structure in competition with isomerization to the thermodynamically most stable structure depend on the relative threshold energies Ei and Edfor isomerization and dissociation, respectively, and on the nature of the transition state. This can be characterized by the rate constant vs. internal energy curve &(E)curves). The shape of k(E) curves is predominantly determined by the geometry of the transition state and the threshold energy of the particular process. The transition state of a rearrangement process such as isomerization is usually much tighter than that of a direct bond cleavage. Loose transition states have larger numbers of low-lying energy states than tight ones. Therefore, the rate constants for loose transition states rise more rapidly with energy at low internal energies than tight ones and level off at higher values at higher energies. This behavior is also reflected in the metastable observations which show that the metastable ion for processes in which rearrangement is rate determining is more abundant than that for direct fragmentation. At a given internal energy, the relative intensities of fragment ions are proportional to the ratio of their rate constants. Thus, the relative ion
2.2
2.6 3.0 3.4 INTERNAL ENERGY, eV
Figure 5. Hypothetical k ( € ) curves. (A) Fragmentation to C3Hs+: (solid portion) Bowers et al. (ref 16); (experimental points) Baer et al. (ref 15); dashed portion is extended arbitrarily. (B) Isomerization process among olefinic ion structures. (C) CpH4+ formatlon by direct fragmentation from methylcyclopropane. (D) C2H4+formatlon by direct fragmentation from cyclobutane.
intensities reflect the threshold energy at low internal energies, but the transition-state geometry at higher internal energies. We note that these considerations ignore contributions from very loose orbiting transition states which are available for only very narrow energy bands in each reaction channel.16 On the basis of the breakdown graphs and the metastable intensities observed in this study, we have constructed hypothetical k(E)curves as illustrated in Figure 5 for the dissociation processes of ions derived from the C4H8isomers from fragmentation onset at ca. 2.2-3.4-eV excess internal energy. In Figure 5, the solid portion of curve A is taken from the values calculated by Bowers and co-workers16 using their transition-state switching model. The experimental points are those of Baer and co-workers15for the apparent unimolecular dissociation rate constant for C4H8+ions from four butenes and methylcyclopropane, obtained in a coincidence approach similar to ours. We have arbitrarily extended curve A (dashed portion) to the energy range covered in this discussion. Such extrapolations are a good approximation over a limited energy range. Caution must be exercised in relating such extrapolations to observed ion intensities when alternate channels leading to the same product ion may become available at higher energies, or when the internal energy carried forward in the fragment ion may begin to exceed the threshold for its dissociation. Since the four butenes have identical breakdown graphs (when plotted on a common internal-energy scale), the isomerization must always be rapid in comparison to dissociation and Ei must be less than E d . Therefore, the rate constant for olefin ion isomerization 1-butene+ F! &-%butene+ e trans-Z-butene+ e isobutene+ (K5) is probably at least 1order of magnitude higher than that for dissociation; this is indicated in curve B. It is drawn arbitrarily through a point at least 1order of magnitude larger than that for curve A at 3.6 eV. Its shape assumes that the transition state has a lower onset than dissociation2 and is somewhat tighter. The transition states for both isomerization and dissociation must be relatively tight; this is supported by the intense metastable peaks
Isomerization and Fragmentation of Gaseous Ions
which we observe. Since the transition states are similar and the dissociation process has the higher threshold, fragmentation is always preceded by isomerization. This mechanism is often responsible for hydrogen scrambling and limits the utility of deuterium labeling as a probe of fragmentation mechanisms for olefins and a number of other molecules. For methylcyclopropane and cyclobutane, the breakdown graphs differ from that of the butenes at internal energies in excess of 2.6 eV, notably through the production of relatively larger amounts of ethylene ions as shown in Figures 3 and 4. This reflects the ability of ions derived from the cyclic molecules to dissociate directly from their original configurations through transition states which are not accessible to the olefinic structures. Both methylcyclopropane and cyclobutane have intense metastable peaks for the formation of C3H5+ions indicating that the production of this ion is preceded by an isomerization process. If one assumes that all C3H5+ions are produced via isomerization, one can use the relative abundance of CzH4+ to estimate the contribution of CzH4+ ions resulting from the isomerization process. We assume the following scheme, illustrated for cyclobutane ion, cB+, dissociating directly or isomerizing to olefinic ions, OB':
The Journal of Physical Chemistty, Vol. 85,No. 19, 1981 2725
Curve C for methylcyclopropane and curve D for cyclobutane (Figure 5) are then drawn relative to curve B on the basis of such calculations using the C2H4+ data in Figures 2-4. This approach assumes for clarity of presentation that the transition state for ring opening of both ions is the same as that for isomerization among the olefinic ion structure. The calculation actually provides only the ratio of the rate constant for ring opening to that for direct dissociation of the cyclic structures. Using the above analyses and the data reported by Tiernan et ala6from the charge-exchange study for the breakdown graphs of C4Hs isomers a t internal energies higher than 3.4 eV suggests that curves B and D should cross over at an internal energy of -4.8 eV; Le., formation of C2H4+ ions by direct fragmentation should become the predominant process for cyclobutane at energies above 4.8 eV. On the other hand, the direct fragmentation process for methylcyclopropane does not become predominant at any energy. We conclude that curve D must result from a looser transition state than curve C.
- 0.15) = 0.12.
kik4 + (ki + kd)(kz + k, + k4) Substitution of k4/(k2 + k3 + k4) = FoB(C2H4+)and rear-
Conclusion Breakdown graphs for the four olefinic C4Ha isomers, in the energy range from 0 to 3.4 eV above the thermodynamically most stable ionic structure, are indistinguishable, suggesting that they have completely equilibrated to a mixture of rapidly interconverting structures before fragmentation. For those molecular ions, the threshold energy for isomerization (Ei)is always smaller than that for fragmentation (Ed). Results are consistent with a tight transition state for both isomerization and fragmentation. The breakdown graphs of methylcyclopropane and cyclobutane are similar to those of the olefinic butenes only in a very narrow energy region; at higher internal energies, direct dissociation to C2H4+ proceeds through a different transition state which is not accessible to the olefinic structures. The breakdown graphs of methylcyclopropane and cyclobutane imply that for methylcyclopropane both isomerization and dissociation take place via tight transition states but that for cyclobutane the transition state for isomerization is tighter than that for direct fragmentation. The direct fragmentation from cyclobutane becomes predominant at internal energies above 4.8 eV.
- Fo~(C2H4+) _ -- FCB(C~H~+) ki 1 - FCB(C&+) For example, at 3.2 eV in cyclobutane, the ratio of direct dissociation to isomerization processes is (0.15 - 0.05)/(1
Acknowledgment. We thank the Department of Energy (contract DEAS0276-ER02567), the National Science Foundation (Regional Facility in Mass Spectrometry at the University of Nebraska) and the Research Council of the University of Nebraska for support. We also thank Professor M. T. Bowers for a preprint of ref 16.
k,
cB
cB+ + e
cB+ 4 C2H4+ + CzH4 kd
cB+
ki
OB+
2C4H7++ H
OB+
ka
C3H5++ CH3
k4
C2H4+ + C2H4
+
The fraction of ethylene ion FcB(C2H4+)observed in the breakdown graph of cyclobutane is given by FcB(c2H4+)
=
kd
rangement yields kd
N
