Rearrangement Reactions in the Electron Impact Fragmentation of Isobutenel

Transcription

1 Rearrangement Reactions in the Electron Impact Fragmentation of Isobutenel MARGARET S.-H. LIN AND ALEX. G. HARRISON Depcrrttnrtlt ofc11rtnistry. Utliversity of Toror~ro, Tororlro, Otltnrio M5S IAl Received November 19, 1973 The detailed mass spectrum of isobutene has been examined using both D and 13C labelling. It is shown that at low average internal energies of the molecular ion complete randomization of hydrogens and of carbons occurs prior to fragmentation to form C3H,+. As the average internal energy of the molecular ion increases (by increasing the ionizing electron energy) the extent of both carbon and hydrogen randomization decreases. Carbon scrambling is complete in the molecular ion prior to fragmentation to form C2 ions under all conditions studied. The results are consistent with a skeletal isomerization of the isobutene molecular ion by a mechanism involving a series of 1,3 ring closures to form methylcyclopropane type ions. On a examint en dttail le spectre de masse de l'isobutylkne utilisant les techniques de marquage au deuttrium et au "'C. On a montrc qu'h des niveaux d'tnergies internes relativement basse pour l'ion rnoltculaire il y a distribution au hasard des hydrogtnes et des atomes de carbone avant la fragmentation conduisant 2. I'ion CtHi+. A mesure que l'tnergie interne moyenne de l'ion moltculaire augrnente (en augmentant l'tnergie d'ionisation) le taux de distribution au hasard du carbone et de l'hydrogtne diminue. La redistribution des carbones est compltte dans l'ion moltculaire avant la fragmentation pour conduire h des ions contenant deux atornes de carbone dans toutes les conditions Ctudites. Les rtsultats obtenus sont en accord avec l'hypothtse qu'il y a une isomtrisation au nivau du squelette de l'ion rnoltculaire de l'isobutylkne par un micanisme impliquant une strie de fermetures de cycles 1,3 pour donner des ions du type mtthylcyclopropane. [Traduit par le journal] Can. J. Chem., (1974) Introduction It is well known (1-4), from deuterium labelling studies, that considerable hydrogen rearrangement occurs in unsaturated hydrocarbon molecular ions prior to fragmentation. The deuterium retention in the c,(h,d),+ ion formed from 1-butene-4-d3 near the appearance potential threshold (1, 4) can be explained only on the basis of randomization of all HID and is not consistent with a mechanism involving 1,3 hydrogen shifts only. Similar results have been obtained (3) in a study of labelled pentenes and it has been suggested (3) that the hydrogen rearrangements occur predominantly by 1,2 shifts. However, Meisels et al. (4) have shown that the 13C retention in the C3H5+ ion produced near the appearance potential threshold from I-butene-4-13C is 75% demonstrating loss of carbon identity and rearrangement of the carbon skeleton.' Such skeletal rearrangements 'The work reported here will constitute part of the Ph.D. Thesis to be submitted by M. S.-H. Lin to the University of Toronto. 'By contrast, in the 75 ev mass spectrum of propene- 3-'" (5) the C2H3+ and CH3+ fragment ions show -50Z ','C retention. This is not consistent with loss of carbon identity but has been rationalized in terms of 1,3 hydrogen shifts. will also result in hydrogen rearrangement and implicate a mechanism(s) other than simple 1,2 or 1,3 hydrogen shifts. Meisels et al. (4) have proposed a sequence of 1,3 ring closures to methylcyclopropane ions to explain the skeletal isomerization in the I-butene molecular ion. Both the deuterium labelling studies (1, 4) and the I3C labelling study (4) present some evidence that the amount of hydrogen rearrangement and skeletal rearrangement decrease with increasing internal energy indicating that fragmentation becomes competitive with isomerization at higher internal energies. In connection with other studies we had available deuterium and I3C labelled isobutenes and we have carried out a detailed study of the electron impact fragmentation of isobutene. The results show that for low energy C,H,+ ions fragmenting to form C3H5' both carbon and hydrogen randomization are complete, whereas the extent of hydrogen and skeletal rearrangement clearly decreases for ions of higher excitation energy. In contrast carbon skeletal rearrangement is complete prior to fragmentation to C,H,+ at all internal energies. The results are not consistent with hydrogen rearrangement by simple 1,2 or 1,3 shifts only but, in agreement

2 1814 CAN. J. CHEM. VOL with the results for 1-butene (4), are consistent with a sequence of 1,3 ring closures to form methylcyclopropane ions. Experimental Low resolution spectra were obtained using a 90" 6 in. radius-of-curvature magnetic deflection mass spectrometer described previously (6). Samples were introduced through a room temperature inlet system to the source at -100 "C. The ionizing electron current was -10 FA with a repeller voltage of 3 V (1.1 ev ion exit energy) and the electron energy variable between 11 ev (nominal) and 75 ev. For the 13C labelling studies mass spectra were obtained using an AEI MS-902 double focussing mass spectrometer at a resolution ( ) sufficient to resolve I3C-CH do~lblets in the mass region of interest. The ratios '3CC,H,/C,+,H,+ were obtained from repeated measurements of the intensities of the relevant peaks. Samples were introduced through a room temperature inlet system with a source temperature of "C. The ionizing electron current was 100 FA with the electron energy being varied between 20 ev (nominal) and 70 ev. Metastable peak intensities corresponding to fragmentation reactions occurring in the first drift region of the MS-902 were measured by the defocussing technique described earlier (7). Unlabelled isobutene (Matheson research grade) was used without further purification. Isobutene-d2 (2-methylpropene-1,l-d2) and isobutene-dc, (2-methyl-d3-propene- 3,3,3-d3) were obtained from Merck, Sharp and Dohme, Montreal and were used without further purification. Low energy analysis in the molecular ion region showed the isobutene-rl, to be 96.8% d2, 0.9% dl, and 2.3% do, while the isobutene-dc, was 93.1% dr, 6.5% d5, 0.3% d4, and 0.1% do. Corrections for these isotopic impurities have been made only in the molecular ion region. Isobutene-I3C (2-methylpropene-2-'T) was obtained from Merck, Sharp, and Dohme, Montreal. Repeated measurements at low electron energy and low resolution yielded the ratio 13CC3H8+/C4H8+ = ; this ratio was confirmed by high resolution measurements at 70 ev. Assuming normal I3C isotopic abundance in the methyl and methylene positions this ratio yields the following % abundances. (CH3)zC = l3ch2 0.74% The mass spectrum (75 ev) of isobutene is shown in Fig. la. The major fragment ions are C,H,+ (tnle 41), C3H3+ (mle 39), and C2H4+ (mle 28) with other significant fragment ions being C4H7+ (mle 55), C3H4+ (mle 40), C2HSf (mle 29), and C2H3+ (tnle 27). Metastable peaks (first drift region) were observed for formation of C3H,+ and C3H4+ from the molecular ion. No metastable peaks were observed for forma- FIG. 1. Mass Spectra of (a) (CH3)C=CH2, (CH3)2C=CD2, and (c) (CD3),C=CH2. (b) tion of C2H4+ and it is probable that this ion also originates by direct fragmentation of the molecular ion. Consequently the major emphasis in the present work was directed toward a study of the label distribution in the C3HSf and C2H4+ ions since this gives the clearest indication of the extent of rearrangement in the molecular ion prior to fragmentation. 'The results concerning formation of the C4H7+ ion were inconclusive because of the isotopic impurities present and the possibility of significant primary hydrogen-deuterium isotope effects. Mass Spectra of Isobutene-d2 and Isobutene-d, The 75 ev mass spectra of the two deuterium labelled isobutenes are shown in Figs. lb and c.~ The mass spectra of isobutene and the labelled isobutenes in the C, region are presented in Table 1 as a function of electron energy. The spectra at each electron energy for the three compounds were determined immediately fol- 3The complete spectra, in tabular form, are available, at a nominal charge from the Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa, Canada KIA 0S2.

3 LIN AND HARRlSON : MASS SPECTRAL REARRANGEMENTS 1815 lowing each other without alteration of any of the instrumental parameters. Also shown in Table 1 are the spectra observed for fragmentation reactions occurring in the first drift region of the MS-902. The experimental results for the labelled isobutenes are compared in Table 1 with the ion intensities calculated assuming complete HID randomization in the C,(H,D),+ molecular ion prior to fragmentation. To carry out these calculations it was assumed that the total intensity for each C,(H,D),,+ species was the same as the intensity of C,H,,+ in the spectrum of the unlabelled isobutene at the same electron energy. As can be seen the metastable peak intensity data and the electron impact mass spectral data at and 11.5 ev (nominal) for the labelled isobutenes are in good agreement with the dis- tribution calc~~lated assuming complete HID scrambling in the molecular ion prior to fragmentation to form C,(H,D),+ and C,(H,D),+." AS the electron energy is increased the experimental distribution begins to deviate consider- ably from the distribution calculated ass~~rning complete scrambling. This is particularly noticeable for formation of the C3(H,D),+ ion where for isobutene-cl, the C3H3D2+ (m/e 43) and C,H,+ (tnle 41) intensities are considerably greater than the calculated intensities whereas the C3H4D+ (tnle 42) intensity is considerably less. Although part of the tnle 41 increase could be due to an abnormally high intensity for C3HD2+ (i. nonrandom distribution in forming C,(H,D),+) the mle 43 signal can only be C3H3D2+. The low intensity of C3H4+ (and thus C,(H,D),+) indicates that 'the decreased intensity at mle 42 relative to the calculated intensity cannct be entirely accounted for by nonrandom distribution in C,(H,D),+ (i.e. abnormally low intensity of C3H2D2+). The results indicate that, compared to the complete randomization observed at low electron energies, isobutene-2, preferentially loses CH, or CHD, (to form C,H,D,+ and C,H,+, respectively) at higher electron energies (14 ev and higher). This conclusion is confirmed by the results for the cl, isobutene. Of the C,(H,D),+ ions only 4The fragmentation to form C,(H,D),+ could not be examined in the absence of C,(H,D),+ ions because the appearance potentials of the two species were found to be practically identical. C3H2D3+ (mle 44) suffers from interference (C,D,+) and since the total C,(H,D),+ intensity is relatively small this interference is minor. Comparing the experimental distributions in the region mle 44 to mle 46 at 14 ev or higher with those calculated ass~~ming randomization we observe that the C,H,D,+ (rille 44) and C,D,+ (tn/e 46) intensities are higher than the calculated intensities while the C,HD,+ intensity is lower. Thus loss of CD, or CH2D is favored over loss of CDH,. Thus for both compounds the preferred fragmentation at 14 ev or higher involves loss of the three methyl hydrogens or of the two ethylenic hydrogens along with one methyl hydrogen. Loss of two methyl hydrogens with one methylene hydrogen is of considerably lower probability. The latter loss can originate only by considerable isotopic exchange between the two positions. This aspect will be discussed fi~rther below in light of the 13C labelling results. Table 2 summarizes the mass spectral data for the C, ions as a filnction of electron energy for the unlabelled and deuterium labelled iso- butenes. The intensities are expressed as a of the total C, ion intensity. For each of the labelled compounds the experimental res~llts are compared with the relative intensities calc~llated ass~~ming complete HID scralnbling prior to formation of the C2(H,D),+ (x = 2 to 5) ions. For these calculations the total intensities for each C,(H,D),+ species were assumed to be identical to that observed for the corresponding C2H,+ ion at the same electron energy. At the lowest electron energies only C2- (H,D),+ is observed and for the labelled conipounds it is observed that the experimental intensities are close to those calculated ass~~ming complete HID scrambling. As the electroll energy is increased and more C2 ions are observed the experimental distribution begins to differ from that calculated ass~~lning complete scrambling. In light of the observation (see below) that at 70eV tlie carbon retention is -50% for all C, ions (C2H3+ to C,H,+) indicating complete scrambling of the carbons the significance of the deviations from scrambling in the deuterium labelling results is not clear. Of the C, ions only C,H,+ appears to originate directly from the molec~llar ion with C,H,+ and + C2H5 originating by further fragmentation of the (M - 1)' species. It is possible that the deviations from scrambling reflect isotope effects in formation of the (M - H)+ and (M - D)+

4 TABLE 1. Mass spectra of isobutenes in C, region as function of energy (intensities Y, total C3 ions) -- Metastable spectra 11.0 ev 11.5 ev 14.0 ev 18.0 ev 30.0 ev 75.0 ev - nz/c Exp. Calcd. Exp. Ca!cd. Exp. Calcd. Exp. Calcd. Exp. Calcd. Exp. Calcd. Exp. Calcd.

5 pp - LIN AND HARRISON : MASS SPECTRAL REARRANGEMENTS 1817 TABLE 2. Mass spectra of isobutenes in C2 region (intensities CC, total ions) 14.0 ev 18.0 ev 25.0 ev 75.0 ev mle Exp. Calcd. Exp. Calcd. Exp. Calcd. Exp. Calcd Can. J. Chem. Downloaded from by on 05/04/18 ions in the labelled isobutenes. Such isotope effects will correspondingly influence the intensity distributions in fragment ions originating by further decomposition of (M - H)+ and (M - D)'. Isobutet~e-I C (2- Methyl ~ro~et~e-2-i C) Table 3 presents the intensity ratios (13CC,,- H,+/C,H,+) measured for various ions at 70 ev using the MS-902 at a resolution sufficient to resolve 13C-CH doublets. The sample contained 30.2% I3C in the 2-position. From the intensity ratios measured the % C2 retention in each fragment ion has been calculated and is shown in the final column of Table 3. The details of the method of calculation are given in the Appendix. Assuming loss of only a methyl or methylene carbon in formation of the C, ions one would expect 100% C2 retention, whereas assumption of complete scrambling of the carbons would lead to a prediction of 75% C2 retention. The C,H,+ and C,H,+ ions both show -86% C2 retention indicating extensive skeletal rearrangement but not sufficient to result in complete carbon equivalence. On the other hand the C, ions all show - 50z retention of C2, which is the value predicted assuming complete carbon scrambling. Surprisingly the C3H3+ ion shows almost complete retention of C, (96z). Metastable peaks were observed for the following reactions forming C,H,+. Since there are so many routes leading to formation of C,H,+ the mechanistic conclusions to be drawn from the high retention result are not clear. Table 4 presents the 13C retention data for the C3H5+ ion as a function of electron energy. The 13CC2H5+/C3H5' ratios from 70 to 20 ev (nominal) electron energy were obtained from the mass-resolved peaks. The ratio in the energy range 11.5 to 13.5 ev (nominal) was obtained from the low resolution spectra after correction of the tn/e 41 intensity C,H,+ for l3cc2h4+. This correction amounted as a maximum to

6 1818 CAN. J. CHEM. VOL. 52, 1974 TABLE 3. 13C retention data for ions in 141 I3CC3Hs+ --> 13CC2H5+ + CH3 Can. J. Chem. Downloaded from by on 05/04/ Intensity % C2 Ion species ratio retention the metastable peak intensity ratio nz,*/n~,* was found to be for fragmentation I3CC3H,+ reactions occurring in the first drift region of the double focussing mass spectrometer. This leads to C4H8 a calculated C2 retention of 74.2 i 5% in agree- 13CC2H, ment with the 75% retention calculated assuming C3H5 complete carbon equivalence in C4H,+ prior to 13CC2H fragmentation. The results clearly show that the C3H4+ "?,,- retention of C2 decreases as the electron 13CC2H3+ energy, and hence, the average excitation energy C3H3+ of CdH8+, decreases. I3CCH5+ Over the nominal electron energy range CzH ev the C2H4+ ion is the only significant 13CCH4+ fragment ion in the C, region. The ratio '3CCH,+/C2H4+ was found to be CzH, from which one obtains C2 13CCH ~ retention. This compares favorably with the C2H3+ result at 75 ev and both are in good agreement with the 50% retention calculated assuming TABLE 4. 13C retention in C3H5+ as function of electron energy complete carbon randomization. Discussion Electron energy (ev, nominal) % c.2 retentron 0.5% of the total n~/e 41 intensity and was calculated from the measured C3H4+ intensity assuming complete scrambling of the carbons in formation of C3H4+.5 Over the electron energy range 11.5 to 13.5 ev the observed ratio remained constant within experimental error. The Y, C2 retention in C3H,+ decreases from -86x at 75 ev to -- 78% at low electron energy. For the two fragmentation reactions 5The alternative of assuming 100% retention of C2 in the C3H4+ ion changes the measured I3CC2H5 + + /C3H5 ratio to from the reported value of Since the retention in C3H4+ is -86% at 70 ev and appears to decrease with decreasing electron energy the assumption of complete scrambling appears more reasonable. The I3C labelling results clearly show that for the lowest energy C4H8+ ions fragmenting to form C3H5+ carbon skeletal isomerization has proceeded sufficiently prior to fragmentation to make all carbon atoms equivalent. Hence the % retention of C2 is that predicted assuming complete randomization of the carbons. As the average internal energy of the C,H8+ ions is increased by increasing the electron energy the extent of skeletal rearrangement prior to fragmentation decreases as shown by the increased C2 retention in the C3H5+ fragment ion. In contrast, over the entire electron energy range studied, skeletal rearrangement has led to complete randomization of carbons prior to fragmentation to form C2H4+. The deuterium labelling results parallel the 13C labelling results. At the lowest average internal energies resulting in fragmentation to form C3(H,D),+ complete H/D randomization has occurred prior to fragmentation. As the average internal energy of the fragmenting ion increases the extent of HID randomization decreases. The results clearly show that the hydrogen and carbon randomization reactions are rate processes and that fragmentation of the molecular ion becomes kinetically competitive with the randomization reactions as the internal energy

7 LIN AND HARRISON : MASS SPECTRAL REARRANGEMENTS Can. J. Chem. Downloaded from by on 05/04/18 of the C,H,+ ions is increased. Although the hydrogen randomization can be rationalized in terms of a series of 1,2 or 1,3 hydrogen shifts, this cannot be the only isomerization reaction occurring since such hydrogen shifts will not lead to carbon randomization. In addition, appearance potential measurements (8) indicate that at the threshold the C3H,+ ion formed from isobutene has the allyl structure; such a structure cannot be achieved by simple 1,2 or 1,3 hydrogen shifts. A plausible mechanism for skeletal isomerization leading to carbon randomization and to formation of allyl ions on fragmentation involves a sequence of 1,3 ring closures to form methylcyclopropane ions as illustrated schematically in Scheme This mechanism has been suggested by Wagner (9) to explain the radiation-induced rearrangement of solid pentenes and hexenes at 77 OK and by Meisels et al. (4) to explain the carbon skeletal rearrangement in the 1-butene molecular ion at low ionizing energies. It is obvious that a number of ring closures and openings are necessary before complete ran- 6For simplicity only the carbon skeleton is shown in Scheme 1 and the positive charge is omitted. domization of the carbons is attained with formation of allyl ion. Consequently it is not surprising that C,H,+ ions of high internal energy, with a consequent short lifetime with respect to fragmentation, do not achieve complete randomization. The sequence of ring closures and openings proposed in Scheme 1 also leads to hydrogen randomization and it is not necessary to post~~late 1,2 or 1,3 hydrogen shifts to explain the results, although such shifts are not precluded. The authors are indebted to the National Research Council of Canada for financial support. Appendix To calculate the ratio of '3CC2Hx+/C3Hx+ we consider the fragmentation of the unlabelled species (CH3),C=CH2 (67.57%) and the singly labelled species CH3('3CH3)C=CH2 (1.4973, (CH,),C=='~CH, (0.74%), and (CH3)2'3C=CH2 (30.20%). In addition molecular ions containing two 13C labels also can contribute to formation of 13CC2H,+. The only significant 13C2C2H, species will contain a label at C2 and may be represented as 13CH3(CH3)'3C=CH2 with an

8 1820 CAN. J. CHEM. VOL. 52, 1974 abundance of -3.3x of the (ch,),'~c=-ch, CH3 species. In the following derivation ci represents the fraction of CIHvf.,. ions containing C2. We / further assume that for those C, ions containing CH 3 C2 the remaining two carbons are selected at From these relative amounts one calculates the random from the remaining three carbons. The relation relation between the measured intensity ratio and ci may be derived as follows: '3CC,H,f ~ C,Hxf ~ 13CH3 '3CC2H.x+((1 - a) + 3~1) \ 7 Similar reasoning leads for the C, ions to the C=CH2 / I CH3 I3CC2H,+ (ja + (1 - a)) \ 7 C=I3CH, / I CH3 C3H,+ (+a) CH3 13CC2H,+ (a) \ 7 13C=CH, 13CH3 13CC2H,+ (:a + (1 - a)) \ /1 ' %=CH 2 / I CH3 CaH,+ 0 expression I. W. A. BRYCE~~~ P. KEBARLE. Can. J. Chem. 34,1249 ( 1956). 2. W. H. MCFADDEN. J. Phys. Chem. 67, 1074(1963). 3. B. J. MILLARD and D. F. SHAW. J. Chem. Soc. (B), 664 ( 1965). 4. G. G. MEISELS, J. Y. PARK, and B. G. GIESSNER. J. Am. Chem. Soc. 91, 1555 (1969). 5. H. H. VOGE, C. D. WAGNER, and D. P. STEVENSON. J. Catal. 2,58 (1963). 6. B. G. K ~y~sand A. G. HARRISON. J. Am. Chem.Soc. 90,5671 (1968). 7. C. W. TSANG and A. G. HARRISON. Org. Mass Spectrom. 3,647 (1970). 8. G. G. MEISELS, J. Y. PARK, and B. G. GIESSNER. J. Am. Chem. Soc. 92,254 (1970). 9. C. D. WAGNER. J. Phys. Chem. 71,3445 (1967).