Methyl-cation shuttles: Xe- and N2-catalyzed isomerization of CH3NO2- to CH3ONO-

Full text of DOI:10.1021/ja954162k

4500
J. Am. Chem. Soc. 1996, 118, 4500-4501
Methyl-Cation Shuttles: Xe- and N2-Catalyzed
+
Isomerization of CH3NO2 to CH3ONO+
Vladimir Baranov, Simon Petrie,1 and Diethard K. Bohme*
Department of Chemistry and Centre for Research in
Earth and Space Science, York UniVersity
North York, Ontario, Canada M3J 1P3
ReceiVed December 11, 1995
The isomerization of protonated molecules can be catalyzed
by molecules which transport or “shuttle” the proton from a
high-energy site to a lower-energy site on the protonated
molecule.2 Such catalysis recently has been characterized in
the gas phase for the isomerization of protonated CO,3 CN,4
N2O,5 and SiO6 in which the proton is transported by H2, CO
and CO2, and NO and CO as well as N2 and SO, respectively.
Proton-transport catalysis has also been invoked in the isomer-
ization of anions in the gas phase7 and is fundamental to
reactions in solution such as acid- and base-catalyzed pyrolysis
of amides and keto-enol isomerization of carbonyl compounds.8
Here we report what we believe to be the first observation of
an analogous catalysis involving the transport of a methyl cation,
Figure 1. Potential-energy diagram for the isomerization and frag-
mentation of the nitromethane and methylnitrite ions based on the results
of a threshold photoelectron-photoion coincident (TPEPICO) mass-
spectrometer study.9a Not shown are the dissociations of CH3ONO+
into CH3NO+ + O, NO+ + CH2OH, and CH2OH+ + NO which should
be kinetically unfavorable.9b,c
+
viz. the isomerization of CH3NO2 to CH3ONO+ which has a
barrier of 14.8 kcal mol-1 (see Figure 1) catalyzed by Xe and
by N2. The catalysis was achieved within the reaction region
of a selected-ion flow tube (SIFT) mass spectrometer,10 and the
two isomers were distinguished using multi-collision induced
dissociation (CID).11
+
CH3NO2 and CH3ONO+ ions were produced in separate
experiments upstream in flowing He carrier gas at 0.35 ( 0.01
Torr by electron transfer from nitromethane (IE ) 11.02 ( 0.04
eV)12 and methylnitrite (IE ) 10.38 ( 0.03 eV)12 to C2H3F+
according to reactions 1 and 2, respectively.
(C2H3F+)* + CH3NO2 f CH3NO2+ + C2H3F
C2H3F+ + CH3ONO f CH3ONO+ + C2H3F
(1)
(2)
C2H3F+ ions were produced by electron impact on vinyl
fluoride (IE ) 10.363 ( 0.015 eV)12 in a conventional ion
source at electron energies of 40 eV and 30 eV, respectively,
and mass-selected and injected upstream into the flow tube. The
upstream addition of nitromethane and methylnitrite was
minimized to avoid secondary association reactions. The CH3-
NO2+ and CH3ONO+ ions were allowed to thermalize with ca.
+
Figure 2. Multicollision CID spectra for CH3NO2 derived from
nitromethane (top), for the m/z 61 ion derived from the reaction of
+
CH3NO2 with ca. 1019 molecules s-1 of Xe (middle), and for CH3-
ONO+ derived from methylnitrite (bottom). The NO+ initially present
in the experiment with Xe (middle) can be attributed to the dissociation
of CH3ONO+ into NO+ + CH3O after isomerization since the shuttle
reaction can produce internally excited (CH3ONO+)* if it is fast
compared to collisional thermalization with He.
(1) Chemistry Department, University College, University of New South
Wales, A.D.F.A., Canberra, A.C.T. 2600, Australia.
(2) Bohme, D. K. Int. J. Mass Spectrom. Ion Processes 1992, 115, 95.
(3) (a) Wagner-Redecker, W.; Kemper, P. R.; Jarrold, M. F.; Bowers,
M. T. J. Chem. Phys. 1985, 83, 1121. (b) Freeman, C. G.; Knight, J. S.;
Love, J. G.; McEwan, M. J. Int. J. Mass Spectrom. Ion Processes 1987,
80, 255.
(4) Petrie, S.; Freeman, C. G.; Maut-Ner, M.; McEwan, M. F.; Ferguson,
E. E. J. Am. Chem. Soc. 1990, 112, 7121.
(5) Ferguson, E. E. Chem. Phys. Lett. 1989, 156, 319.
(6) Fox, A.; Bohme, D. K. Chem. Phys. Lett. 1991, 187, 541.
(7) See, for example: Stewart, J. A.; Shapiro, R. H.; DePuy, C. H.;
Bierbaum, V. M. J. Am. Chem. Soc. 1977, 99, 7650.
(8) See, for example: Streitweiser, A.; Heathcock, C. H. Introduction
to Organic Chemistry; Macmillan: New York, 1985.
5 × 105 He collisions at 294 ( 3 K before being subjected to
multi-collision CID downstream at the sampling orifice. The
potential of the sampling nose cone (Unc) was varied from 0 to
-80 V. Figure 2 shows that NO+ is the predominant product
ion (>99%) in the dissociation of both isomers over the voltage
range employed, but that the measured onsets for the two
dissociation reactions 3 and 4 are distinctly different,
CH3NO2+ + He f NO+ + CH3O + He
CH3ONO+ + He f NO+ + CH3O + He
(3)
(4)
(9) (a) Gilman, J. P.; Hsieh, T.; Meisels, G. G. J. Chem. Phys. 1983, 78,
1174. (b) Leyh-Nihant, B.; Lorquet, J. C. J. Chem. Phys. 1988, 88, 5606.
(c) Schro¨der; Su¨lzle, D.; Dutuit, O.; Baer, T.; Schwarz, H. J. Am. Chem.
Soc. 1994, 116, 6395.
(10) (a) Mackay, G. I.; Vlachos, G. D.; Bohme, D. K.; Schiff, H. I. Int.
J. Mass Spectrom. Ion Phys. 1980, 36, 259. (b) Raksit, A. B.; Bohme, D.
K. Int. J. Mass Spectrom. Ion Processes 1983/84, 55, 69.
(11) (a) Wang, J.; Baranov, V.; Bohme, D. K. J. Am. Soc. Mass Spectrom.
1996, 7, 261. (b) Baranov, V.; Bohme, D. K. Int. J. Mass Spectrom. Ion
Processes, in press.
viz. 51 and 29 V, respectively. The measured ratio of onset
voltages, 1.8, is comparable to the ratio of the energies required
for dissociation, 14.8 kcal mol-1/10.4 kcal mol-1 ) 1.4.
Figure 2 also shows that the multi-collision CID spectrum
(12) Lias, S. G.; Bartmess, J. E.; Liebman, J. J.; Holmes, J. L.; Levin,
R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data, Suppl. 1988, 17 (1).
+
of the m/z 61 ion produced when Xe is added to CH3NO2
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Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 18, 1996 4501
generated upstream by reaction 1 is identical in onset to that
for the CH3ONO+ ion produced by reaction 2, viz. the onset
for NO+ production is shifted to 29 V. In other words, the
addition of Xe has resulted in the following isomerization:
(MCA) of X is intermediate between MCA(NO2) and MCA-
(ONO), then both of the CH3+ transfer steps 6b and 6d will be
exothermic, ostensibly permitting an efficient overall reaction.
The methyl-cation affinities MCA(NO2) ) 33 kcal mol-1 and
MCA(ONO) ) 46 kcal mol-1 can be obtained from existing
thermochemical data;12 thus the requirement for an efficient
methyl-cation shuttle reaction (6) is that 33 kcal mol-1 < MCA-
(X) < 46 kcal mol-1. This criterion is met by N2 (MCA ) 44
CH3NO2+ + Xe f CH3ONO+ + Xe
(5)
A similar result was obtained with N2, but CO had no
measurable effect.
kcal mol-1 13
) and, within experimental uncertainty, by Xe (MCA
) 46.5 kcal mol-1 13
) but is clearly not met by CO (MCA ) 79
The mechanism which we propose for the observed isomer-
kcal mol-1).12 It follows, therefore, that Xe and N2 are capable
+
ization of CH3NO2 to CH3ONO+, namely
+
of effecting the isomerization of CH3NO2 to CH3ONO+ as
+
CH3NO2+ + X f [O2NCH3 ...X]
(6a)
(6b)
(6c)
(6d)
(6e)
observed, while the “back-transfer” step (6d) is very endothermic
for CO which cannot act as a shuttle reagent in this system.
Our observation of the isomerization reaction (6) involving
Xe and N2 suggests that this mode of catalysis, which has
hitherto been reported in the gas phase only for isomerization
of protonated species, may be rather general. Attempts have
begun to compute details of the potential-energy surface for
reaction 6, and we shall extend our experimental study of such
reactions to include other possible shuttle atoms and molecules.
+
[O2NCH3 ...X] f [O2N...CH3X+]
[O2N...CH3X+] f [ONO...CH3X+]
+
[ONO...CH3X+] f [ONOCH3 ...X]
Acknowledgment. The authors thank Alwin Cunje for technical
assistance, and D.K.B. thanks the Natural Sciences and Engineering
Research Council of Canada for the financial support of this research.
+
[ONOCH3 ...X] f CH3ONO+ + X
is analogous to that which has been invoked previously for the
observed “proton shuttle” isomerization of protonated mol-
ecules.2 In the above mechanism, the species X acts as a
“methyl-cation shuttle” by allowing the migration of CH3+ from
the N atom to the adjacent O atom. If the methyl-cation affinity
JA954162K
(13) (a) McMahon, T. B.; Heinis, T.; Nicol, G.; Hovey, J. K.; Kebarle,
P. J. Am. Chem. Soc. 1988, 110, 7591. (b) Glukhovtsev, M. N.; Szulejko,
J. E.; McMahon, T. B.; Gauld, J. W.; Scott, A. P.; Smith, B. J.; Pross, A.;
Radom, L. J. Phys. Chem. 1994, 98, 13099.