1142
The Journal of Physical Chemistry, Vol. 83,
No. 9, 1979
et al.,i5~'6log k,(L/mol s) = 9.4- (8.7/8).Hence at -25 OC, [RO,], N M while [RO,H] E 0.84 M. k-, is not expected to represent a diffusion-controlled process since a > (1 - a )and can be estimated at about loa2(L/mol s). Log k , (L/mol s) = 8.2-2.218is obtained from ref 16 which also gives a = 0.88at 30 OC in CCI, solvent. This allows us to estimate a 0.7at -30 OC so that we have (1 ia) = 1.7in CH,CI, solvent. (18)J. Czarnowski and J. H. Schumacher, Int. J. Chem. Kinet., 10, 1 1 1
-
(1 978). (19) J. Czarnowski and H. J. Schumacher, Int. J. Chem. Kinet., in press. (20) L. Batt and R. D. McCulloch, Int. J . Chem. Kinet., 8,491 (1976). (21) J. L. Arnau and P. A. Gigfiere, Can J . Chem., 53, 2490 (1975). (22) The A factor for the rate constant k,' is estimated from the values reported for the reactions of HO, with different substrates and are summariyd in Natl. Bur. Stand., Spec. Pubi., No. 513 (May 1978), while E , E 4 kcal is taken from a similar reaction, RO, tetralin hydroperoxae TOOH itetralyl peroxy radical, reported by Howard
+
-+
L. L. Griffin and D. J. McAdoo et al., Can. J . Chem., 53, 623 (1975). (23) S.W. Benson, "Thermochemical Kinetics", 2nd ed, Wiley, New York, 1976. (24) D. D. Wagman, Natl. Bur. Stand., Spec. Pub/., No. 513 (1978), Appendix 1.
(25)L. R. Mahoney and M. A. DaRooge, J. Am. Chem. SOC., 92, 4063 (1970),and references cited therein. (26)L. R. Mahoney, F. C. Ferris, and M. A. DaRooge, J. Am. Chem. Soc., 91,3883 (1969). (27)A. J. Colussi, F. Zabel, and S . W. Benson, Int. J . Chem. Kinet., 9, 161 (1977). (28) J. H. B. Chenier, E. Furimsky, and J. A. Howard, Can. J . Chem., 52, 3682 (1974). (29) J. H. B. Chenler and J. A. Howard, Can. J. Chem., 53, 823 (1975). (30) R. R. Baker, R. R. Baldwin, C. J. Everett, and R. W. Walker, Combust. Flame, 25, 285 (1975);27, 147 (1976),and earlier references cited therein.
Decompositions of the Metastable [CH,C(OH),]+* Ion. A Test of the Applicability of the Energy Randomization Hypothesis to Unimolecular Fragmentations Lawrence L. Griffin Moody College, Texas A&M University System, Galveston, Texas 77553
and David J. McAdoo" The Marine Biomedical Institute and Department of Human Biological Chemistry and Genetics, University of Texas Medlcai Branch, Galveston, Texas 77550 (Received December 18, 1978) Publication costs assisted by Moody College
The energy randomization postulate of the quasi-equilibriumtheory of mass spectrometry is tested by comparing the decomposition patterns of metastable [CH,C(OH)(OD)]+.ions generated so that the origins of OH and OD were interchanged. The [CH,C(OH),]+. ion either loses water directly, or it rearranges to the acetic acid ion and loses OH or CH3. Metastable [CH,C(OH)(OD)]+-ions gave the same decomposition patterns, independently of whether the OH or the OD was formed by rearrangement. This is consistent with randomization of the internal energy of the decomposing ions being complete in the 10-6-10-5 s that preceded the studied decompositions.
Introduction The quasi-equilibrium theory (QET)l has been widely applied to the fragmentation of ions in the mass spectrometer. The fundamental postulate of the QET and related theories2 is that the internal energy of a reacting species is randomized rapidly relative to its rate of reaction. Although the QET has been widely applied to mass spectral fragmentations, there continue to be questions about its unqualified applicability to unimolecular decomposition~.~ Thus additional direct experimental tests of the energy randomization assumption would be useful. In "chemical activation" experiment^,^ reactive forms of the same chemical species are generated by two different processes, and the subsequent reaction patterns compared. Any nonrandomization of internal energy would result in differences between the reaction patterns of the differently prepared species. Such differences have been observed in chemically activated molecules undergoing very fast reaction~.~ Two types of chemical activation experiments have been performed in the mass spectrometer. In one, the relative abundances of metastable peaks for the same ions generated by differing mechanisms were compared.6 Identical ratios of the peak intensities were observed at low ionizing 0022-3654/79/2083-1142$01 .OO/O
Scheme I
'OH7 0 It I1 , C H , @ C C H ~ -+ ~ ' CH,WCH,Q . H?
/
CH 3 ac=o 4- , CH 2a' H y
\ CH,"
+
HyC-O+
+
.CH,'
Scheme I1
+
RCH=CH2
electron energies, supporting the validity of the QET for slow fragmentations. (Metastable decompositions occur 10-6-10-5 s after the formation of the decomposing ion.) In a second experiment, the rates of loss of methyl from opposite sides of deuterated forms of the ionized acetone ion generated by rearrangement of the corresponding enolic ion (Scheme I) were compared.7 The results indicated that the unlabeled ion loses CHzaHY about 1.5 times as often as CH3", contrary to the expectation that 0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
Decomposition of Metastable [CH2C(OH),]+.
the two methyls would be lost at equal rates if energy were fully randomized prior to the decomposition of the acetone ion formed by rearrangement. Appearance potential measurements indicate that the acetone ion formed from its enol isomer has about 20 kcal more energy than it needs to decompose: so it would decompose very rapidly following its formation. In light of the proposal that energy is not completely randomized in the reaction depicted in Scheme I, studies of the reactions of other symmetrical ions are needed. Symmetrical ions are useful systems to examine for evidence of nonrandomization of energy in that identical reactions occurring in parts of the ion differing only in their origins can be compared. We here compare the rates of loss of the two hydroxyl groups from the enol form of the acetic acid ion generated by the McLafferty rearrangement of aliphatic acids (Scheme II).9 A previous study'' has shown that the ratios of the metastable peak intensities for the decompositions of this ion vary slightly as a function of the source of the ion. This variation is probably due to small differences in the amounts of internal energy in the decomposing ions.1°
Results and Discussion Decompositions of the C2(H,D)402+Ions. The relative intensities of the peaks resulting from the metastable decompositions of the enol isomer of the acetic acid ion, the acetic acid ion, and deuterated forms of both ions are given in Table I. Both ions lost water and hydroxyl and methyl radicals, but at different relative rates. The metastable spectrum of the enol is similar to that previously reported.'O Mechanisms of Decomposition of the C2H402+*Ions. Water molecules lost from CH2C(0H),+ (1)contained both hydroxyl hydrogens, as at least 98% of the water lost from l-0,0-d2was D20, and more than 99% of the water lost from 1-2,2-d2was Hz10 (Table I). Therefore, 1 loses water directly to give the ketene ion (Scheme 111). The loss of hydroxyl from 1could occur either by simple cleavage or following rearrangement to the acetic acid ion (4). By analolgy to enolic C3H60+.(Scheme I), 1 might be expected to lose hydroxyl by the pathway 1 4 5. The rearrangement step 1 4 would be subject to a primary isotope effect, but the direct loss of hydroxyl (1 2) would not be. The isotope effect would slow the transfer of a hydroxyl deuterium relative to the transfer of a hydroxyl hydrogen in 1-0-d, 4, resulting in a preference for the loss of OD over the loss of OH from 1-0-dl. OD was lost 1.8 times as often as OH. This value is close to the primary isotope effect of about 2.1 observed on the rearrangement of the enolic hydrogen in C3H60+.,7and it is not likely that a secondary isotope effect would be large enough to give this OD/OH ratio. The isotope effect demonstrates that 1 loses hydroxyl by the pathway 1 4 5. The probable loss of OH from 1-0,0-d2may indicate that some exchange between the hydroxyls and the methylene (in up to 20% of the decomposing ions) precedes the loss of hydroxyl. This contrasts with the specific loss of the water molecule (see above), and is therefore difficult to explain. If 1 loses OH by II 4 5 , then the loss of methyl would be expected to occur by 1 4 6 (Scheme 111). The similar losses of CH, and CH2D from l-O-dl and the absence of CIHzD loss from 1-2,2-d2confirm this expectation. The acetic (acidion loses methyl and hydroxyl directly, as the acetic acid-0-dl ion loses CH, and OD with little scrambling (Scheme IV). Loss of water from ionized acetic acid involves Loss of the hydroxyl and a methyl hydrogen, as acetic acid-0-d, and acetic-2,2,2-d3acid both lost HDO.
--
-
0 0 r(
co
o o m
22"
ddcD
mmm
0 0 rl
m
moor-0
v
fv rl
E
0
.3
z
Y
bl
.3
M a E
.-0
-+
--
-- --
E
0
.e
+
cow
m
v
mm
1143
1144
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
L. L. Griffin and D. J. McAdoo
Scheme I11
+
CH~=C=OH+
SOH
-
S
C
H
~
2
1
Ot
Ot
I1
I1
CH~YH O c OHY
c H 2 a HYCOHO
4b
4a
CH 2
a Y ~C
so+
5a
. C H ~ " H ~
Scheme IV
CH,~CO+ + .OHO
C O , i o ++ s C H , ~ 6
It is interesting to note that the same ions (3,5, and 6) are formed by the losses of the corresponding neutral species from both 1 and 4. Energy Randomization in .CH&(OH),+. If the internal energy of 1 is completely randomized prior to its metastable decomposition, then the decomposition pattern of l-O-dl would be independent of the ion's origin. However, if energy randomization were incomplete in 1, then the OH/OD ratios might vary between the ions generated from 1-butanoic acid-0-dl and l-butanoic-4,4,4-d3acid, since the origins of OH and OD are inverted between the two sources. The same arguments apply to the other decompositions. Within experimental error, the metastable spectrum of l-O-dl was independent of its origin, so we conclude that complete randomization of internal energy precedes the decompositions of 1. The differences in energy randomization between 1 and enolic C3H60+.probably result from the different times between the formation and further reactions of the symmetrical ions in the two pathways. Since loss of water competes with rearrangement of metastable 1 to 4, 1 4 probably occurs after 10-6-10-5 s. Decomposition by 4 5 or 4 6 then occurs in a much shorter time. The long period between formation and further reaction of 1 permits complete internal energy randomization. On the other hand, the acetone ion formed by rearrangement of enolic C3H60+probably decomposes very rapidly following its formation (see above), so that complete randomization of
- -
*OH
COzHof
6b
+ *CHz'HY
internal energy does not have time to occur. Other symmetrical ions which isomerize on the metastable time scale and then rapidly decompose need to be examined in further tests for incomplete energy randomization.
\
o+
+
+
*OHo
H HOC ~
5b
6a
+
CH;COHO 4
C
C02Ho+
-
Experimental Section All spectra were obtained on a DuPont 21-491 mass spectrometer at 70 eV at an ion source temperature of 250 "C. The metastable peak heights were determined in the first field free region of the mass spectrometer by adjusting the electrostatic analyzer potential.ll The preparation of the deuterated butanoic acids is described elsewhere.12 All of the butanoic acids were purified by gas chromatography. Acetic acid-0-dl was prepared from acetic acid by exchange with D 2 0 in the mass spectrometer inlet. Acknowledgment. We thank the Robert A. Welch Foundation for generous financial support (Grant H-609).
References and Notes (1) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftlg, and H. Eyring, Proc. Natl. Acad. Sci. U . S . A . , 38, 667 (1952). (2) P. J. Robinson and K. Holbrook, "Unimolecular Reactions", WileyInterscience, New York, 1972. (3) J. L. Franklin, Science, 193, 725 (1976). (4) J. N. Butler and G. B. Kistiakowsky, J . Am. Chem. SOC.,82, 759 (1960). (5) (a) J. D. Rynbrandt and 6. S. Rabinovitch, J . Phys. Chem., 75, 2164 (1971); (b) J. Meagher, K. J. Chao, J. R. Barker, and B. S. Rablnovitch, ibid., 78, 2535 (1974). (6) H. M. Rosenstock, V. H. Dibeler, and F. N. Harllee, J . Chem. Phys., 40, 591 (1964). (7) F. W. McLafferty, D. J. McAdoo, J. S. Smith, and R. Kornfeld, J . Am. Chem. Soc., 93, 3720 (1971). (8) D. J. McAdoo, Doctoral Dissertation, Cornell University, 1971. (9) F. W. McLafferty, Anal. Chem., 31, 82 (1959). (10) J. L. Occolowitz, J . Am. Chem. Soc., 91, 5202 (1969). (11) F. W. McLafferty, J. Okamoto, H. Tsuyama, Y. Nakajima, T. Noda, and H. W. Major, Org. Mass Spectrom., 2, 751 (1969). (12) D. J. McAdoo, 0. N. Wltiak, and F. W. McLafferty, J . Am. Chem. Soc., 99, 7265 (1977).
