Thermodynamic stability of trichlorocyclopropenyl cation. An experimental (FTICR) and computational [G2(MP2)] study

Article abstract of DOI:10.1021/jo981369y

The standard Gibbs energy change for the halide transfer between tert- butyl chloride and trichlorocyclopropenyl cation has been determined by means of Fourier transform ion cyclotron resonance mass spectrometry. Trichlorocyclopropenyl cation is found to be substantially more stable than tert-butyl and 1-adamantyl cations. This reaction and cognate processes were also studied at the ab initio G2(MP2) level. The agreement between experimental and calculated changes of thermodynamic state functions is remarkably good. On the basis of these results, isodesmic and homodesmotic reactions were constructed. They led to the delocalization energy in 1 and to the quantitative assessment of substituent effects on the stability of the aromatic 2π-electron system.

Full text of DOI:10.1021/jo981369y

J . Org. Chem. 1998, 63, 8995-8997
8995
Th er m od yn a m ic Sta bility of Tr ich lor ocyclop r op en yl Ca tion . An
Exp er im en ta l (F TICR) a n d Com p u ta tion a l [G2(MP 2)] Stu d y†
J .-L. M. Abboud,*,‡ O. Castan˜o,§ M. Herreros,‡ I. Leito,‡ R. Notario,*,‡ and K. Sak‡
Instituto de Quı´mica Fı´sica “Rocasolano”, C. S. I. C., c/ Serrano 119. E-28006, Madrid, Spain, and
Departamento de Quı´mica Fı´sica, Universidad de Alcala´. E-28871, Alcala´ de Henares, Madrid, Spain
Received J uly 14, 1998
The standard Gibbs energy change for the halide transfer between tert-butyl chloride and
trichlorocyclopropenyl cation has been determined by means of Fourier transform ion cyclotron
resonance mass spectrometry. Trichlorocyclopropenyl cation is found to be substantially more stable
than tert-butyl and 1-adamantyl cations. This reaction and cognate processes were also studied at
the ab initio G2(MP2) level. The agreement between experimental and calculated changes of
thermodynamic state functions is remarkably good. On the basis of these results, isodesmic and
homodesmotic reactions were constructed. They led to the delocalization energy in 1 and to the
quantitative assessment of substituent effects on the stability of the aromatic 2π-electron system.
In tr od u ction
Cyclopropenium ion, (CH)3+, is the simplest Hu¨ckel 4n
+ 2 π-electron monocyclic system. This ion and cognate
species have recently received considerable attention.1 Its
standard enthalpy of formation has been determined
experimentally (1075 kJ mol-1)2 and calculated at the G2
defining the onset of reactions 2a and 2b under the same
experimental conditions:
level (1074 kJ mol-1).1b Here we report the results of a
quantitative experimental study of the thermodynamic
stability of trichlorocyclopropenyl ion, (CCl)3+,1 in the gas
phase. To our knowledge, this is the first study of this
kind on substituted cyclopropenium ions.1 Data thus
obtained are compared to quantum-mechanical calcula-
tions at the G2(MP2) level.3
∆G°(1), the standard Gibbs energy change for the
chloride exchange between tert-butyl chloride (2) and 1
in the gas phase [reaction 1], was determined by means
of Fourier transform ion cyclotron resonance spectrom-
etry (FT ICR).4
2(g) + B1H+(g) f 4(g) + HCl(g) + B1(g) (2a)
3(g) + B2H+(g) f 1(g) + HCl(g) + B2(g) (2b)
Exp er im en ta l Section
A. Th e F T ICR Sp ectr om eter . The study was carried
out on a modified Bruker CMS 47 FT ICR mass spectrometer4c
used in previous studies.5,7 A detailed description is given in
refs 5a and 7. Some modifications have been introduced with
respect to the standard instrument. They are described in
these references. The substancial field strength of the supra-
conducting magnet, 4.7 T, allows the monitoring of ion-
molecule reactions for relatively long periods of time, and also,
the use of relatively high pressures (of the order of 5 × 10-4
mbar) during a few seconds.
Use was made of a bracketting technique known as
the “dissociative proton attachment method” (DPA).5 In
this method, ∆G°(1) is given by the difference of the gas-
phase basicities,6 GB, of the two bases, B1 and B2,
B. DP A Exp er im en ts. The basic concepts of the method
have been developed in ref 5. The experimental technique is
quite similar to that used in these studies. Some minor
changes are introduced in order to avoid the unwanted
deprotonation of R+(g) by B(g) (this possibility exists in the
case of t-Bu+(g)). In general, the reference base B is introduced
into the high-vacuum section of the instrument and subject
to electron ionization (using nominal energies in the range of
10-13 eV). Nominal pressures of B are ca. 2 × 10-7 mbar.
Charged fragments from B act as primary proton sources. In
general, after 1-2 s, the main ion present is BH+(g). After
† Dedicated to Professor Erwin Buncel.
‡ Instituto de Qu´ıica F´ısica “Rocasolano”.
§ Universidad de Alcala´.
(1) See, e.g., the following and references therein: (a) Glukhotsev,
M. N. J . Chem. Educ. 1997, 74, 132-136. (b) Glukhotsev, M. N.; Laiter,
S.; Pross, A. J . Phys. Chem. 1996, 100, 17801-17806. (c) Burk, P.;
Abboud, J .-L. M.; Koppel, I. A. J . Phys. Chem. 1996, 100, 6992-6997.
(d) J emmis, E. D.; Srinivas, G. N.; Leszczynski, J .; Kapp, J .; Korkin,
A. A.; Schleyer, P. v. R. J . Am. Chem. Soc. 1995, 117, 11361-11362.
(e) Budzelaar, P. H. M.; Schleyer, P. v. R. J . Am. Chem. Soc. 1986,
108, 3967-3970. (f) An important experimental study of C3Cl3+AlCl4
(X-ray structure) and HF/6-311G(d,p) calculations on C3Cl3 have
recently been published: Clark, G. R.; Taylor, M. J .; Steele, D. J . Chem.
Soc., Faraday Trans. 1993, 89, 3597-3601.
-
+
(2) Lias, S. G.; Bartmess, J . E.; Liebman, J . F.; Holmes, J . L.; Levin,
R. D.; Mallard, W. G. J . Phys. Chem. Ref. Data 1988, 17, Suppl. 1.
(3) Curtiss, L. A.; Raghavachari, K.; Pople, J . A. J . Chem. Phys.
1993, 98, 1293-1298.
(4) (a) Bowers, M. T.; Aue, D. H.; Webb, H. M.; McIver, R. T. J . Am.
Chem. Soc. 1971, 93, 4314-4315. (b) Freiser, B. In Techniques for the
Study of Ion-Molecule Reactions; Farrar, J . M., Saunders, W., J r., Eds.;
Wiley: NewYork, 1988; Chapter 2. (c) Laukien, F. H.; Allemann, M.;
Bischofberger, P.; Grossmann, P.; Kellerhals, H. P. Kopfel, P. In Fourier
Transform Mass Spectrometry. Evolution, lnnovations and Applica-
tions; Buchanan, M. V., Ed.; ACS Symposium Series 359; American
Chemical Society: Washington, DC, 1987; Chapter 5.
(5) (a) Abboud, J .-L. M.; Castan˜o, O.; Herreros, M.; Elguero, J .;
J agerovic, N.; Notario, R.; Sak, K. lnt. J . Mass Spectrom., Ion Proc.
1998, 175, 35-40. (b) Abboud, J .-L. M.; Castan˜o, O.; Della, E. W.;
Herreros, M.; Mu¨ller, P.; Notario, R.; Rossier, J .-C. J . Am. Chem. Soc.
1997, 119, 2262-2266. (c) Abboud, J .-L. M.; Notario, R.; Ballesteros,
E.; Herreros, M.; Mo´, O.; Ya´n˜ez, M.; Elguero, J .; Boyer, G.; Claramunt,
R. J . Am. Chem. Soc. 1994, 116, 2486-2492.
(6) The gas-phase basicity GB(B) of base B is the standard Gibbs
energy change pertaining to the reaction: BH+(g) f B(g) + H+(g). See,
e.g., ref 2.
(7) Abboud, J .-L. M.; Herreros, M.; Notario, R.; Esseffar, M.; Mo´,
O.; Ya´n˜ez, M. J . Am. Chem. Soc. 1996, 118, 1126-1130.
10.1021/jo981369y CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/30/1998

8996 J . Org. Chem., Vol. 63, No. 24, 1998
Ta ble 1. Exp er im en ta l Deter m in a tion of DP A On setsa for Sp ecies 2 a n d 3
Abboud et al.
compd
ref,b (GB)c transfer?
ref,d (GB)c transfer?
DPA onsete
2
t-C4H9CN (186.2), yes
t-C4H9SH (187.6), yes
(n-C3H7)2S (199.1), yes
(CH3CO)2CH2 (199.9), yes
n-C4H9OCH3 (189.2), no
CH3COOCH3 (189.3), no
2-cyanopyridine (201.0), no
(n-C4H9)2S (201.4), no
188.4
200.5
3
a
b
All values in kcal mol-1
.
Strongest base able to lead to DPA. c Gas-phase basicities of the reference bases, taken from ref 9. See text.
Weakest base not leading to DPA. e Average of the two closest GB values from columns 2 and 3.
d
Ta ble 2. Com p u ta tion a l Resu lts a t th e G2(MP 2) Levela ,b
value, 50.6 ( 11.8 kJ mol-1 (from data in Table 1).
Combining this datum with the entropy change computed
at the MP2/6-31G(d) level for reaction 1, ∆S°(1) ) 33.0 J
mol-1 K-1, we obtain a value of ∆H°(1) of 60.5 ( 11.8 kJ
mol-1. The G2(MP2) value is 61.3 kJ mol-1. Thus, from
the standpoint of the chloride-ion affinity, 1(g) appears
as substantially more stable than tert-butyl and, even,
1-adamantyl cation.12
species
enthalpy
Gibbs energy
(CCl)3+, 1
t-C4H9Cl, 2
C3Cl4, 3
-1492.916633
-617.226718
-1952.950661
-157.169332
-2412.335136
-2872.452549
-115.492839
-575.525577
-1492.955329
-617.264073
-1952.993256
-157.206640
-2412.381853
-2872.499932
-115.518867
-575.557431
t-C4H9+, 4
+
C3Cl5
C3Cl6 +
(CH)3
The experimental value of the standard enthalpy of
formation of 3(g), ∆fH°m(3,g), is not yet available. It was
thus computed at the G2(MP2) level using the data from
Table 2, including the spin-orbit correction.11 A value
of 128.9 kJ mol-1 was obtained. Combining this datum
with the experimental values of ∆H°(1) (this work),
∆fH°m(2,g)9 and ∆fH°m(4,g),9 we get ∆fH°m(3,g) ) 964.6
C3H3Cl
a
b
All values in hartrees. See text.
reaction times of ca. 5 s, all ions, with the exception of
BH+(g), are ejected off of the ICR cell8 by means of radio
frequency ejection “chirps” (broad band). Great care is taken
in order to prevent the excitation of this ion, and so, use is
made of an “ejection safety belt” (a feature of the Bruker
software that strictly prevents the irradiation of a preselected
frequency range around the resonance frequency of BH+ in
order to avoid its accidental excitation). At this point, a “burst”
of R-X(g) is allowed to enter the high-vacuum section of the
spectrometer by means of a pulsed piezoelectric valve.
These pulses last some 0.1 s, and the total pressure reaches
values of ca. 5 × 10-4 mbar. The pressure goes down to ca. 1
× 10-6 mbar in about 3 s. During this period of time, the main
reaction observed is the formation of R+(g). The system is
routinely monitored over a period of about 20 s after the
injection of R-X(g). The same experimental protocol was
applied to both 2 and 3. Given the very low pressures
prevailing in these experiments, reactions 2a and 2b are
essentially irreversible (the partial pressure of XH is extremely
small), and so, while a true equilibrium is not reached, the
onset of the DPA process can be clearly observed.
kJ mol-1
.
The delocalization energy, ∆Edeloc, defined as the
standard energy change (equal to the standard enthalpy
change) for the homodesmotic13 reaction 3 is of great
conceptual importance1a,14 in the case of the Hu¨ckel
systems such as (CH)3+ and 1. The best value for ∆H°(3)
presently available is -247.3 kJ mol-1 (from a G2
calculation).1b This is a very large value, suggestive of
the “archetypal aromatic character”1b of (CH)3+. For 1(g),
we can similarly construct the homodesmotic reaction 4:
C. Sa m p les. Compounds 2 and 3 were Aldrich products.
Their purity was assessed by standard methods.
The experimental results are summarized in Table 1.
Com p u ta tion a l Deta ils
Calculations were performed on a Silicon Graphics “Chal-
lenge” computer using the Gaussian 94 package of programs.10
Calculations at the G2(MP2) level3 were performed on the
species 1-4. The results are reported in Table 2.
Discu ssion
The computed ∆G°(1) value amounts to 51.2 kJ mol-1
in remarkably good agreement with the experimental
,
Using the data given in Table 2, we obtain the
corresponding standard enthalpy change, ∆H°(4) ) -218.9
kJ mol-1
.
Again, this is a very substantial value,
(8) Soo, O. G.; Buchanan, M. V.; Comisarow, M. R. In Fourier
Transform Mass Spectrometry. Evolution, lnnovations and Applica-
tions; Buchanan, M. V., Ed.; ACS Symposium Series 359; American
Chemical Society: Washington, DC, 1987; Chapter 1.
(9) NIST Chemistry WebBook. NIST Reference Database; No. 69.
August 1997 release. http://webbook.nist.gov/chemistry/. Proton Af-
finity Data: compiled by E. P. Hunter, S. G. Lias. Neutral Thermo-
chemical Data: compiled by H. Y. Afeefy, J . F. Liebman, and S. E.
Stein.
(10) Gaussian 94, Revision B.3. Frisch, M. J .; Trucks, C. W.;
Schlegel, H. B.; Gill, D. M. W.; J ohnson, B. G.; Robb, M. A.; Cheeseman,
J . R.; Keith, T.; Petersson, G. A.; Montgomery, J . A.; Raghavachari,
K.-, Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.;
Peng C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andre´s, J . L.;
Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J ., Binkley, J . S.;
Defrees, D. J .; Baker, J .; Stewart, J . P.; Head-Gordon, M.; Gonza´lez,
C., Pople, J . A. Gaussian lnc.: Pittsburgh, PA, 1995.
indicating that substitution has not significantly affected
the delocalization energy. The existence of multiple Cl-
Cl interactions prevents a more detailed comparison of
∆H°(3) and ∆H°(4).
(11) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J . A. J .
Chem. Phys. 1997, 106, 1063-1079.
(12) Sharma, R. B.; Sen Sharma, D. K.; Hiraoka, K.; Kebarle, P. J .
Am. Chem. Soc. 1985, 107, 3747-3757.
(13) (a) Cremer, D. Tetrahedron 1988, 44, 7427-7454. (b) George,
P.; Tratchman, M.; Bock, C. W.; Brett, A. M. Tetrahedron 1976, 32,
317-323.
(14) Minkin, V. I.; Glukhotsev, M. N.; Simkin, B. Ya. Aromaticity
and Antiaromaticity. Electronical and Structural Aspects; J ohn
Wiley: New York, 1994.

Stability of Trichlorocyclopropenyl Cation
J . Org. Chem., Vol. 63, No. 24, 1998 8997
Furthermore, reaction 5 can be constructed. It directly
links the stabilities of both ions.
substituents in 1 is offset by their stabilizing resonance
and polarizability contributions.15
Using data from Table 2, we obtain for ∆H°(5) and
∆G°(5) 3.4 and -1.7 kJ mol-1, respectively. This result
strongly supports the conclusion based on reaction 4.
Ack n ow led gm en t. This work was supported by
grant PB96-0927-C02-01 of the Spanish D.G.E.S. Valu-
able assistance by lng. J . M. Castro, Servicios lnfor-
ma´ticos, Universidad de Alcala´ is most appreciated.
Work by I.L. and K.S. was supported in part by EC
TEMPUS grants.
We last compare the effect of chlorine substitution on
the standard enthalpies of formation of (CH)3+(g) and
benzene. To this end, we use the isodesmic reaction 6
and compute ∆H°(6) using experimental data for all the
J O981369Y
species. We obtain ∆H°(6) ) 19.7 kJ mol-1
.
The fact that reaction 6 is slightly endothermal clearly
shows that the potentially destabilizing field effect of the
(15) Taft, R. W.; Topsom, R. D. Prog. Phys. Org. Chem. 1987, 16,
1-87.