1519
J. Phys. Chem. 1988, 92, 1519-1523 as dissolution or sublimation, by taking the relationship between the local imperfection in the material and the reaction mechanism into account. The relaxation process after each deletion was neglected in the present study. Instead, the local potential energy was recalculated after each deletion. Relaxation, together with the effect of lattice vibration, should be accounted for, in order to refine the simulation.
Further studies are needed to delete the obvious oversimplifications in the present study, e.g., in two dimensionality, the too small model size as less than 10 nm in diameter.
Acknowledgment. W e express our appreciation to Professor H. Kuno for valuable discussion and S. Hata for model construction studies.
Alkane Eliminations from Ionized Ketones in the Gas Phase: Dependence of Ion-Neutral-Complex-Mediated Decompositions on the Properties of the Ionic and Neutral Partners John C. Traeger, Chemistry Department, La Trobe University, Bundoora, Victoria 3083, Australia '
Charles E. Hudson, and David J. McAdoo* Marine Biomedical Institute and Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston, Texas 77550 (Received: July 7 , 1987; In Final Form: September 21, 1987)
Alkane eliminations and the associated alkyl losses from ionized ketones were characterized to explore influences of the nature of the partners on ion-neutral-complex-mediated dissociations. Photoionization ion efficiency curves, unimolecular and collision-induced dissociation patterns, and translational energy releases were determined. The energy range over which alkane eliminationswere significant increased substantially with the size of the alkane eliminated. This is attributed to increasing polarizability of the radicals in the intermediate ion-neutral complexes. The variation of R H formation with the size of R supports the reality of ion-neutral complexes in unimolecular decompositions. An anomalously weak methane elimination from ionized 2-butanone is attributed to an inability to form the intermediate complex due to the relatively low polarizability of the methyl radical and a decreasing attraction to ionic partners of increasing size. The dominance of methyl loss from ionized acetone at short times and methane loss in long-lived ions may reflect a larger centrifugal barrier in higher rotational states of the ion.
We examined the energy dependence of alkane eliminations, assumed to be ion-neutral-complex-mediated processes,' from a homologous series of ions for several reasons: first, to obtain additional evidence for the intermediacy of such complexes in unimolecular dissociations, even though a considerable body of information* already indicates their existence; second, because ion-neutral-complex-mediated decompositions are potential sources of information about the forces between ions and neutrals and their effects on the dynamics of unimolecular dissociations. Also, these reactions are probes of processes occurring in the energy range below the threshold for simple dissociation, a region not easily explored utilizing ion-molecule reactions. It has been suggested that complexes are formed predominantly between the thteshold for covalent bond breaking and the threshold for subsequently overcoming electrostatic attractions.' However, that conclusion was based largely on data from decompositions of long-lived, low-energy ions and was in need of further examination.' Finally, we wished to explore the influence of the properties (1) Hudson, C. E.; McAdoo, D. J. Inr. J. Mass Spectrom. Ion Processes 1984,59, 325-332. (2) (a) Bowen, R. D.; Stapleton, B. J.; Williams, D. H. J. Chem. Soc., Chem. Commun. 1978, 24-26. (b) Bowen, R. D.; Williams, D. H. J . Am. Chem. SOC.1980, 102, 2752-2756. (e) Wendleboe, J. F.; Bowen, R. D.; William, D. H. J. Am. Chem. Soc. 1981,103,2333-2339. (d) Morton, T. H. J. Am. Chem. Soc. 1980,102,1596-1602. (e) Morton, T. H. Tetrahedron 1982, 38, 3195-3243. ( f ) Longevialle, P.; Botter, R. J. Chem. SOC.,Chem. Commun. 1980, 823-825. (8) Longevialle, P.; Botter, R. Inr. J . Mass Specrrom. Ion Phys. 1983, 47, 179-182. (h) Tumas, W.; Foster, R. F.; Pellerite, M.J.; Brauman, J. I. J . Am. Chem.Soc. 1983, 105, 7464-7465. (i) Tumas, W.; Foster, R. F.; Brauman, J. I. J . Am. Chem. SOC.1984, 106, 4053-4054.
(3) McAdoo, D. J.; Traeger, J. C.; Hudson, C. E.; Griffin, L. L., accompanying paper in this issue.
0022-3654/88/2092-1519$01.50/0
SCHEME I 0
.+
TABLE I: Metastable and Collision-Induced Decomposition Patterns of Ionized Ketones
ketone (RC(=O) R') CH,C(=O)CH, CH3C(=O)CH2CHp CH2CH2C(=O)CH2CH3 CH,C(=O)CH(CH,), (CH3)2CHC(=O)CH$H, CH,CH2C(=O)CH(CH3)2 (CH,),CHC(=O)CH(CH,)2 CH,CH2C(=O)C(CH,)3 (CHj),CC(=O)CH2CH3
ion 1 2 3
4 5 5 6
7 7
metastable spectrum -R" -RH" 96
100 17b
100
36 46 4b 100 3e
CAD spectrum -R -RH
100 6
100
1oc
100
100
19
100
72
1
14d 100
100 0.5 0.5
'R is always R, not R', from RC(=O)R'. bLargely collision-induced, data from a Kratos MS 50TA mass spectrometer. cMay be formed totally or in part by processes other than loss of CH4. d-C2H6 = 100. eMay be C4H9+. of the ionic and neutral partners on decompositions involving ion-neutral complexes. The stability of a complex will be determined by the polarity and polarizability of the neutral: the distance between the neutral 0 1988 American Chemical Society
Traeger et al.
1520 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 TABLE II: Trsasktiod Energy Releases in Alkane Losses from Metastable Ketone Ions" ketone RH lost T,, kJ mol-'
CH3C(=O)CH3b CH&(=O)CH, CH$(=O)CHZCH3 CHzCHZC(=O)CHzCH3 (CHo)&!HC(=O)CHZCHI (CH3)2CHC(=O)CH2CH3 (CH3)2CHC(=O)CH(CH3),
CH4 CH4 CH4 C2H.5 C2Hs C3Hs C3Hs
3-PENTANONE
* U
,
57 56
m/z
z
-
U
0.81 f 0.04 1.06 i 0.1 13.5 i .05 3.6 i 0.2 4.3 i 0.2 1.5 i 0.2
'
A
. . *.
I
u
LL LL W
*.
Z
...
0 I
a
4.2 f 0.1
a.
N I
Values are average energy releases and are isd. Ion source temperature = 320 K; all other data obtained at 530 K.
2-PROPANONE P U
m/z
z
W
43 42
.... . .. .
A
I
u
I
LL LL
W
z
..
0 I
b-
a
..
..
.:
z
2 0
k-
0
z
.
......... ,,:::.......aA
11 ...........r.*bdtt
9.E
9.6
10.2
10.0
10.8
10.6
10.4
PHOTON ENERGY /eV
Figure 2. Photoionization ion efficiency curves for the losses of ethane ( m / z 56) and ethyl ( m / z 57) from ionized 3-pentanone.
2-METHYL-3-PENTANONE
N I
z
0
..
M
0 I-
O
m/z 71 *
.*
70
A
L 11
.**. 10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
... . ... .. .....
10.9
PHOTON ENERGY /eV
*.
Figure 1. Photoionizationion efficiency curves for the losses of methyl ( m / z 43) and methane (m/z 42) from ionized acetone.
and center of charge in the ion, and the charge distribution in the ion. Alkyl free radicals should have small permanent dipole moments, but their polarizabilities should increase with size. Thus study of RH elimination with increasing size of R provides a means of exploring the role of polarizability in ion-neutral complex formation. Simple ketone ions were studied because they eliminate alkanes at low e n e r g i e ~probably ,~ through ion-neutral complexes (Scheme 1).lv6 Their alkane eliminations are replaced by simple dissociations with increasing internal energy.'
Results Decomposition patterns of metastable ketone ions are given in Table I. Alkane eliminations were usually dominant, but alkyl losses produced the major peaks in several spectra. In two of the latter, eliminations of the alkane corresponding to the alkyl lost were prevented by the absence of a needed a-hydrogen. The intensity of the metastable elimination of methane from ionized acetone (1) was dramatically temperature sensitive, increasing 6-fold in intensity between 320 and 530 K (the temperature at which most of the reported data were taken), and to (4) (a) Gioumousis, G.; Stevenson, D. P. J. Chem. Phys. 1958, 29, 294-299. (b) Su, T.; Bowers, M. T. Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic: New York, 1979; pp 83-117. (5) (a) Cooks,R. G.; Yeo, A. N. H.; Williams,D. H. Org. MarsSpectrom. 1%9,2,985-995. (b) Litton, J. F.; Kruger, T. L.; Cooks, R.G. J. Am. Chem. Soc. 1976, 98, 2011-2013. (c) Hammerum, S.; Donchi, K. F.; Derrick, P. J. In?. J. Mass Spectrom. Ion Phys. 1983, 47, 347-350. (6) Traeger, J. C.; Hudson, C. E.; McAdoo, D. J. Int. J . Mars Spectrom.
Ion Processes, in press. (7) (a) McLafferty, F. W.; McAdoo, D. J.; Smith, J. S.;Kornfeld, R.J. Am. Chem. Soc. 1971,93,3720-3730. (b) McAdoo, D. J.; Witiak, D. N. J. Chem.Sor., Perkin Trans. 2 1981,770-773. (c) Lifshitz, C.; Tzidony, E. Int. J. Mars Spectrom. Ion Phys. 1981,39,181-195. (d) Lifshitz,C. Int. J. Mass Spectrom. Ion Phys. 1982,43, 179-193. (e) Lifshitz, C. J. Phys. Chem. 1983, 87, 2304-2313. (f) Burgers, P. C.; Holmes, J. L.; Szulejko, J. E.; Mommcrs, A. A.; Terlouw, J. K. Org. Mass Spectrom. 1983,18. 254-262. (g) Turecck, F.; Hanus, V.Org. MussSpectrom. 1984,19,631-638. (b) McAdoo, D. J.; Hudson, C. E . In?. J. Mass Spectrom. Ion Processes 1984, 59, 77-83. (i) McAdoo, D. J.; Farr, W.; Hudson, C. E. J. A h . Chem. Soc. 1980, 102, 5265-5169. McAdoo, D. J.; Hudson, C. E. Org. Mass Spectrom. 1983, 18, 159-161. (k) Zwinselman, J. J.; Harrison, A. G. Org. MassSpectrom.
u)
1984, 19, 573-576.
.".' ..-: "..... a ..,
9.4
9.6
9.8
10.2
10.0
l
10.6
10.4
~
l
10.8
11.0
~
l
11.2
11.4
PHOTON ENERGY /eV
Figure 3. Photoionization ion efficiency curves for the losses of ethane ( m / z 70) and ethyl ( m / z 71) from ionized 2-methyl-3-pentanone.
2-METHYL-3-PENTANONE
*
57
m/z
U
z * w
56
1
!LLA LL W
..........
L
e
~~~~~~~~*:~~~*A'*,,.,'A.,.'AA'A
*..P
/ 1
9.4
9.6
9.8
,
I
10.0
*"C.*'
.M I
10.2
I
I
1
10.4
10.6
10.8
'
1
1
11.0
11.2
'
1 11.4
PHOTON ENERGY /sV
Figure 4. Photoionization ion efficiency curves for the losses of propane ( m / z 5 6 ) and propyl ( m / z 5 7 ) from ionized 2-methyl-3-pentanone.
11-fold at 600 K. This temperature dependence was missing even at very low energies in the ion source, as at 320 K and nominally 10.5 eV electron energy m l z 42 was 0.4% as intense and m1.z 43 13% as intense as the molecular ion, identical with values at 530 K of 0.4% and 13%. Although it is possible that a small portion of the methyl loss from metastable 1 was unimolecular, the major fraction was collision-induced. Most translational energy releases (Table 11) were in the range 1-5 kJ mol-', similar to those reported for other alkane elimi-
~
l
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
Alkane Eliminations from Ionized Ketones
1521
TABLE III: Thermochemical Data at 298 K (kJ mol-')
ketone (RC(=O)R') CH,C(=O)CH, CH,C(=O)CH,CH, CH,C(=O)CH(CH,), CHjCH2C(=O)CH2CH, CH,CH,C(=O)CH(CH,), (CH3)2CHC(=O)CH2CH3 (CH,)2CHC(=O)CH(CH3),
AHdproducts)b -R -RH
AHdRC(=O)R'Y -217.2 -240.8 -262.4 -258.4 -286.1 -286.1 -311.3
800.9 735.1 712.4 107.5 684.8 667.3 644.6
805.0 703.1 610.1 693.6 600.6 673.1 580.1
-R
-RH
predicted threshold -R -RH
15.9 18.7 21.5 20.8 23.5 24.3 27.1
15.6 17.3 19.0 19.0 20.7 21.7 23.4
1002.2 957.2 953.3 945.1 947.4 929.1 928.8
AHcorb
obsd thresholdC -R -RH
1006.6 926.6 853.5 933.0 866.0 937.5 868.0
1001.5d 955.2e 953.3 946.5' 947.5 931.1 930.1
1001.5 933.0' 927.2 930.1 895.4
OReference 17. bCalculated from data in Table VI. cCalculated by using 1.00 eV = 96.487 kJ mol-I. dReference 18. Reference 19. ~~
~
~~~~~
~~
TABLE IV: Energies Required To Reach the -R/-RH Crossover Points and Plateaus in the -RH Curves
2,4-DIMETHYL-3-PENTRNONE
* U
m/z
z
2 U
71 70
-
ketone (RC(=O) R') CH3C(=O)CHS CH,C(=O)CH,CH3 CH$(=O)CH(CH3)2 CH,CH$(=O)CH2CH, CH,CH,C(=O)CH(CH,), (CH,)2CHC(=O)CH,CH, (CH,)2CHC(=O)CH(CH3),
A
3
LL L
w
.....
z
.*
0 3
+ a
N 3
z
0 3
0
c
0
I
U'
L
g" 9.2
9.4
9.6
9.6
........e.
.......10.0
10.2
crossover point,' kJ mol-'
1 2
4 3
5 5 6
plateau onset: kJ mol-'
0 0 0
15
60 25 120 110
50 50 70 70
RTheenergy at which the two curves cross, minus the measured threshold for R'CO+ + R formation. bThe energy of the onset of the plateau in the -RH curve, minus the measured threshold for R'CO+ + R formation.
....
10.4
ion
10.6
10.8
11.0
I
PHOTON ENERGY /eV
TABLE V Metastable Decompositions of Deuterium-Labeled 3-Pentanone Ions
Figure 5. Photoionization ion efficiency curves for the losses of propane ( m / z 70) and propyl ( m / z 71) from 2,4-dimethyl-3-pentanone.
nations.' The energy releases correlated approximately with differences between the thresholds for the a'ssociated alkyl and alkane losses (Table 111). Ionized 2-butanone released 3-6 times more energy than other ions upon alkane elimination. Photoionization ion efficiency (PI) curves for alkyl and alkane losses are given in Figures 1-5. Alkane eliminations from ionized 2-butanone (2) were too weak to give satisfactory PI curves. The alkane losses were usually favored near threshold, but soon reached plateaus, indicating that they ceased to be important at higher internal energies.8 In contrast, curves for the alkyl losses rose steadily over the energy range examined, consistent with results of a previous photoionization study of ionized ketones.8 The PI curves were too noisy to obtain breakdown plots by taking first derivatives, a procedure that would give directly the dependence of a given fragmentation on internal energy. The thresholds for the alkane losses are from 0 kJ mol-' (acetone) to 35 kJ mol-' below the thresholds for alkyl losses (Table 111). As would be expected, the differences between the thresholds were smallest when the two types of products were quite close in energy, and largest when the alkane loss products were much more stable than the products of alkyl loss. Table IV provides indicators of the competition between alkane eliminations and alkyl losses in the form of the energies required to reach the crossover points of the PI curves for the two types of reactions and the onsets of the plateaus in the curves for alkane losses. The alkane eliminations become more competitive with the alkyl losses as the size of R increases. (Compare the m / z 56 and m l z 57 curves for 3-pentanone (3) to those for 2-methyl-3pentanone ( 5 ) and the m f z 70 and m f z 71 curves for 5 and 2,4-dimethyl-3-pentanone(6).) Increasing the size of the ion associated with a given radical diminished the energy needed to reach the crossover point but did not affect the energy needed to reach the plateau in the -RH curves. Our thresholds for the decompositions of 1, 10.38 and 10.38 eV, are slightly higher than the previously reported 10.33 eV for loss of CH, and ca. 10.30 h0.02 eV for loss of CH4.9 We cannot (8) Murad, E.; Inghram, M. G. J . Chem. Phys. 1984, 40, 3263-3275.
soecies ~-~ lost ~-~ - r -
ion 3-1,1 ,I-d, 3-2,2-d2
C2H3D3
C2H4D2
C2H5D
100
5"
C2H6
62
100 5
Corrected for the collision-induced loss of CH3CD2utilizing the abundance of the associated C2H5loss and the -CH3CD2/-C2H5 ratio in the CAD spectrum of ionized 3-pentanone-2,2-d2. pinpoint the source of the discrepancy due to a lack of information regarding the earlier measurements. However, it could be due to our corrections for hot band structure. The two sets of measurements agree that the thresholds are very similar, that the curve for CH4 elimination levels off at about 10.5 eV, and that the curve for CH, loss rises monotonically over the entire energy range of the measurements. The observed threshold for CH2=C=O'+ CH4 formation from 1 is 5 W mol-' below the thermochemically predicted one. This is probably within the combined uncertainties in IE(CH2=C=O), AE(CH2=C=O'+) and AHHr(CH2=€=0). C2Hs formaA similar problem exists for (CH3)2C=C=O'+ tion. Very large isotope effects often accompany alkane eliminat i o n and ~ many ~ ~ other ~ ~complex-mediated ~ ~ ~ ~ ~metastable ~ decompositions.' Isotope effects were evident in the metastable spectra of deuteriated 3-pentanone ions (Table V). The 2,2-d2 ion lost CH3CHD2about 20 times as often as it lost CH3CH2D, demonstrating the presence of a substantial primary isotope effect, paralleling previous observations on 1? A secondary isotope effect due to a-deuteriation could have been obscured by the strong opposing primary isotope effect. CD,CH3 was lost from 3-1,1,1-d3 62% as frequently as CH3CH3,indicating the presence of a slight secondary isotope effect.
+
+
Discussion
Influence of the Polarizability of the Neutral on Complex Formation. The overall PI curves and the data on Table IV indicate that, when the size of the radical that would be associated (9) Baer, T., personal communication reported in ref 7d. (10) (a) Mead, P. T.; Donchi, K. F.; Traeger, J. C.; Christie, J. R.; Derrick, P. J. J . Am. Chem. SOC.1980, 102, 3364-3369. (b) Holmes, J. L.; Burgers, P. C.; Mollah, Y . A. Org. Muss Spectrom. 1982, 17, 127-130.
Traeger et al.
1522 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 TABLE VI: Supplementary Thermochemical Data (kJ mol-') species mf,298 Hf.298 - mf,o
CH3 CH4 C2H5 C2H6
(CHS),CH CIH8 CH,=CO'+ CH3CO+ CH3CH=CO'+ C2H5CO+ (CH3)2C=CO'+ (CH3)2CHCO+
143.9" -74.56
10.4" 10.0"
116.3'
12.4' 1 1.7d
-84.0' 76. l e -104.5'
16.0" 14.4d
879.5' 657.0'
11.89 1 1.7'
777.6h
591.2h
13Sh 14.5h
684.6' 568.5"
15.2j 17.3'
"Traeger, J. C.; McLoughlin, R. G. J . Am. Chem. SOC.1981, 103, 3647-3652. 'Reference 17. CReference18. dCalculated by using vibrational frequencies from Shimanouchi, T. Natl. Stand. Ref. Data Ser. (US.Natl. Bur. Stand.) 1972. 39. 'McMilIen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493-532. 'Calculated by using IE(ketene) = 9.61 eV (Levin, R. D.; Lias, S. G. Natl. Stand. Ref Data Ser. (US., Natl. Bur. Stand.) 1982, 71) and AHdketene) = 147.7 kJ mol-l.16 gCalculated by using vibrational frequencies from Moore, C. B.; Pimentel. G. C. J . Chem. Phys. 1963, 38, 2816-2829. hReference 19. 'Calculated by using IE(dimethy1ketene) = 8.38 eV (Levin, R. D.; Lias, S. G. Natl. Stand. Re$ Data Ser. ( U S . Natl. Bur. Stand.) 1982, 71 and AHddimethylketene) = -124 kJ mol-', based on AHf(methylketene) = -86 kJ mol-I (ref 19) and -38 kJ mol-' for replacing -H with -CH3. 'Estimated from CH,CHCO'+. kCalculated from the m / z 71 AE for 3-methyl-2-butanone (this work, Table 111). 'Estimated from C2H5CO+. with a given ion in a complex is increased, alkane elimination becomes much more competitive with alkyl loss. For example, the crossover point for the CH3CH2CO+and CH3CH=C=O'+ curves shifts upward by 60 kJ mol-' and the plateau onset by 20 kJ mol-' when the accompanying radical is changed from ethyl to isopropyl. Similar results are observed for formations of (CH3)2CHCO+and (CH3)2C=C=O'+. Furthermore, C3H8 elimination from 6 was much more competitive than was C2H6 elimination from the same precursor. The apparent exception to this trend, the absence of C4Hloelimination from the 2,2-dimethyl-3-pentanone ion (7), is probably due to preferred bond scission on the opposite side of the carbonyl group, preventing formation of the necessary complex. Competitiveness of the alkane loss does not increase with greater stability of the products of alkane elimination, as ethane elimination from 5 competes relatively weakly with ethyl loss, even though its products are 84 kJ mol-' more stable than the accompanying ethyl loss products. Elimination of propane from the same ion competes very well with propyl loss, although, based on the measured appearance energies, the products of these two reactions are about equal in stability. The obvious source of the effect of the size of R is the increased polarizability of the propyl radical relative to the ethyl radical. Polarizabilities of methyl (2.7 A')" and methane (2.6 A3)I2are quite similar, so polarizabilities of ethyl and propyl radicals should be similar to those for ethane and propane, 4.44 and 6.29 A3, respectively.12 The attractive force between an ion and a polarizable neutral some distance away is given4 by the expression E = ctq2/2# (ais the polarizability of the ion, q the charge on the ion, and r the distance between the ion and the neutral.) The increase in polarizability with the size of the radical would substantially increase the attraction between it and an ion. This would in turn increase the lifetime of the ion-neutral complex and thereby the competitiveness of hydrogen transfers taking place through such complexes. No apparent feature of the radicals other than their polarizabilities would enhance the alkane losses as the radical size increases. We can think of no reason why concerted elimination of the alkanes would become more competitive with increasing ( 1 1) Klots, C. E. J . Chem. Phys. 1976, 64, 4269-4275. (12) Miller, K. J.: Savchik, J. A. J . Am. Chem. SOC.1979, 101, 7206-7213.
size of the species lost. Thus present results provide further evidence that alkane eliminations take place through intermediate ion-neutral complexes formed by scission of a C-C bond followed by hydrogen transfer to the nascent radical. The differences between the observed thresholds for alkane eliminations and alkyl losses are 0-35 kJ mol-', reaching the maximal differences that have been observed for such reactions.Ioa The smallest differences occur when the heats of formation of the products of alkane elimination are close to those of the products of simple cleavage. Strong primary isotope effects on the alkane eliminations from 17cand 3 (present results) indicate' that in those ions the hydrogen transfer takes place above the actual threshold for bond scission to form the complex. When the products of alkane losses are low enough in energy to permit it, those processes take place 20-35 kJ mol-l below the threshold for separation of the products of simple cleavage. In the latter cases, we equate the thresholds for alkane elimination with the thresholds for complex formation.lq3 These observations correlate well with predictions that in ionic dissociations the incipient fragments will begin to rotate freely in all dimensions about 20 kJ mol-' below the threshold for simple diss~ciation.'~Thus the threshold for alkane elimination is set either by the heat of formation of the products or by the threshold for complex formation, whichever is higher. The increase in the difference between the onsets of alkyl and alkane losses from 20 kJ mol-' when the complex is [ ( C H 3 ) 2 C H C O + - C H 2 C H 3 ] to 35 kJ mol-' when [(CH3)2CHCO+-CH(CH3)2]is formed demonstrates that the difference between onset of complex formation and simple dissociation increases with the polarizability of the neutral partner. This difference becomes even larger when the neutral is polar, as recent calculations indicate that the complex between the H 2 0 molecule and the ethylene ion is formed about 70 kJ mol-' below the threshald for its diss~ciation.'~ The major portion of each alkane elimination in the ion source takes place above the onset of the corresponding alkyl losses. Rather than a sudden shift from alkane to alkyl loss as the threshold for the latter process is surpassed, competition between the processes takes place over a range of energies that expands with increasing size of the radical in the complex. Acetone ions formed by isomerization over a high barrier from their enol isomer lose only methyl, indicating complete dominance of simple cleavage about 134 kJ mol-' above the threshold for methyl loss from the acetone well above the onset of the plateau in the -CH4 curve for acetone. Metastable 3-pentanone ions formed by isomerization from ionized 1-penten-3-01lose C2H62-4% as often as ethyl over a narrow range of energies starting 71 kJ mol-' above AH(CH3CH2CO+ CH2CH3)."J,I5 Therefore a small amount of ethane is eliminated even above the onset of the plateau in the photoionization efficiency curve for C3H40'+ formation from 3-pentanone. The Influence of the Ionic Partner in the Complex on Alkane Elimination. The weak loss of CH4 and the large associated energy release from the metastable 2-butanone ion is an anomaly in the series of ions studied. Loss of methane from the 2-butanone ion is thermochemically favored over loss of methyl (Table 111). The decompositions of ionized acetone and 3-pentanone imply that there should be no difficulty in forming either methane or CH$H=C=O'+ from ionized 2-butanone. This has previously been noted, but not e~p1ained.I~
+
(13) (a) Bowers, M. T.; Jarrold, M. F.: Wagner-Redeker, W.; Kemper, P. R.; Bass, L . M. Faraday Discuss. Chem. SOC.1983, 75, 57-76. (b) Dodd, J . A.; Golden, D. M.; Brauman, J. 1. J . Chem. Phys. 1984, 80, 1894-1899. (14) Postma, R.; Ruttink, P. J. A.; van Baar, B.; Terlouw, J. K.; Holmes, J. L.; Burgers, P. C. Chem. Phys. Lett. 1986, 123, 409-415. (15) Traeger, J. C.; McAdoo, D. J. Int. J. Mass Spectrom. Ion Processes 1986, 68, 35-48. (16) McAdoo, D. J.; Barbalas, M. P. Int. J . Mass Specrrom. Ion Phys. 1980, 36, 281-284. (1 7) Pedley, J. B.; Rylance, J. Sussex-NPL Computer-Analyzed Thermochemical Data: Organic and Organometallic Compounds; University of Sussex, Brighton, U.K., 1977. (18) Traeger, J. C.; McLoughlin, R. G.; Nicholson, A. J. C. J . Am. Chem. Sot. 1982, 104, 5318-5322. (19) Traeger, J. C. Org. Mass Spectrom. 1985, 20, 223-227.
Alkane Eliminations from Ionized Ketones We propose an explanation based on the ionization efficiency curves obtained in this study. The differences in energies between the decomposition thresholds and the crossover points of the alkyl and alkane curves (Table IV) demonstrate that the alkane eliminations become less competitive in decreasing degrees with each addition of a methyl group to the ion. We attributed this diminution in the attractive forces with increasing size of the ion to the charge being less concentrated in the larger ions and/or an increase in the distance between the radical and the charge center in the ion. Recent computationsZohave indicated decreases in binding energies of 18% and 34% respectively on going from tetramethylammonium ions to the larger tetraethylammonium ions when the associated neutrals are water and acetone. This was attributed to the greater dispersion of charge in the larger ions. Since methyl and methane loss from ionized acetone have identical thresholds, any decrease in the attraction for methyl from that of the acyl ion would greatly reduce the frequency of methane loss. If the difference in activation energies for the two processes were equal, the lower energy process should become more favored with increasing size of the precursor due to the distribution of available energy over a larger number of degrees of freedom.2f,2’ The decreased attraction with increasing ion size must be sufficient to more than offset this “degrees of freedom” effect, at least in smaller ions. Contrasts in Metastable and Photoionization Decomposition Behaviors of Ionized Acetone. The losses of methyl and methane from ionized acetone have the same threshold according to our measurements, and the PI curve for methyl loss is above that for methane loss over the entire energy range examined. A similar pattern is implied in the electron impact ion source, as the methyl loss was 32 times as intense as the methane elimination at the lowest electron energy at which the latter intensity could be accurately measured. It is therefore surprising that methane elimination strongly dominates the metastable spectrum of ionized acetone. The losses of methyl and methane from ionized isopropyl methyl ether behave ~imilarly.~ It could be that the threshold for methane elimination is slightly below that for methyl loss, and that ions containing enough energy to lose methyl disappear before reaching the field-free region where the metastable decompositions take place. Although such a difference was not detected for the acetone ion, this explanation is consistent with the determinants of the relative rates of the decompositions of the other ketone ions. We speculate that simple dissociation may dominate even at threshold in the initially formed acetone ions but that, as time elapses, ions in low rotational states disappear by simple cleavage while ions in higher rotational states are restrained from so doing by the increased centrifugal barrier.’3a Higher rotational states would become more populated with increasing temperature, enhancing a reaction favored in high rotational states at longer times. This is consistent with the marked effect of temperature on the long-lived ions and the absence of such an effect at short times. Increasing the temperature may simply increase the number of acetone ions with enough energy to lose CH4, as the decomposition threshold is at the start of a valley in the ion population versus energy content curve for ionized acetone.22 This explanation (20) Meot-Ner (Mautner), M.; Deakyne, C. A. J . Am. Chem. SOC.1985,
107, 469-474.
(21) Bente, P. F.; McLafferty, F. W.; McAdoo, D. J.; Lifshitz, C. J . Phys. Chem. 1975, 79, 7 13-721. (22) Based on the assumption that P(E) for the initial population of ions is closelv aooroximated bv the ohotoelectron soectrum. A ohotoelectron spectrum oia’cetone is given in Powis, I.; Danby, C.J. In?. J . Miss Spectrom. Ion Phys. 1979, 32, 27-33.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1523 predicts that methane loss should also increase substantially in the ion source with increasing temperature, but an appreciable change was not observed. Methane elimination displays kinetic energy releases up to 4 kJ mol-’, so either some metastable methane elimination does occur above the threshold for methyl loss, or the methane loss products are more stable than those produced by methyl loss. Our results may fulfill the suggestion that effects of rotational state on reaction rates should be sought in observations made very near threshold.23 The lack of a temperature effect on methane loss at short times suggests that rotational effects have little influence on the rapid eliminations of methane. Summary
Three significant conclusions have been drawn from the energy dependences of ion-neutral-complex-mediated reactions of a homologous series of ions: (1) Covalent bonds in ions can be broken to form complexes between ions and nonpolar neutrals up to 20-35 kJ mol-’ below the threshold for the dissociation of the resulting fragments which are then held in association by electrostatic attractions. This difference corresponds reasonably to prediction^'^ that incipient fragments in decomposing ions will begin to rotate freely about 20 kJ mol-’ below the threshold for complete dissociation of the partners. (2) Formation of ion-neutral complexes is strongly enhanced with increasing polarizability of the neutral fragment. (3) Stabilities of ion-neutral complexes appear to diminish with increasing size of the ionic partners. In addition, it was speculated that the increased centrifugal barrier in higher rotational states may also enhances ion-neutral complex formation. Further studies of the energy dependence of ion-neutral-complex-mediated decompositions involving a variety of ionic and neutral partners should contribute significantly to our understanding of the dynamics of the dissociations of ions in the gas phase. Experimental Section
Procedures for acquiring the PI curves and metastable spectra and for determining translational energy releases are given in the companion paper.3 3-Pentanone-1 ,I,l-d3 and 3-pentanone-2,Z-d2 were prepared by adding CD3CH2MgBr and CH3CD2MgBr, respectively, to propanal followed by oxidation with Cr03/H2S0,. The Grignard agents were prepared from the ethyl bromides, which were prepared by reducing acetic acid-d4 with LiA1H4 and acetic anhydride with LiAlD4, followed by the conversion of the alcohols to the bromides with HBr/H2S04. Acknowledgment. We thank D. Pavlu for preparing the manuscript, Professor M. L. Gross and the Midwest Center for Mass Spectrometry (NSF Grant C H E 78-18572) for use of the MS 50TA mass spectrometer, and the Australian Research Grants Scheme and the Robert A. Welch Foundation (Grant H-609) for financial support. Registry No. CH3C(0)CH3,67-64-1; CH3C(0)CH2CH3,78-93-3; CH3CH2C(O)CH2CH3, 96-22-0; CH,C(O)CH(CH3)2, 563-80-4; (CH,),CHC(O)CH2CH3, 565-69-5; (CH3)2CHC(O)CH(CH3)2, 565-80-0; CH$H2C(O)C(CH,)3, 564-04-5;CHpCH2C(O)CH2CD,, 1 12896-22-7; CHjCH,C(O)CD2CH3, 1 12896-23-8. (23) Bowers, M. T. Faraday Discuss. Chem. SOC.1983, 75, 96-91.
