An experimental thermochemical and theoretical study of triquinacene: Definitive disproof of its neutral homoaromaticity

Article abstract of DOI:10.1021/ja981680x

The enthalpy of formation (ΔH(f)/°(g) = 57.51 ± 0.70 kcal/mol) of triquinacene (1), newly determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that previously reported and corresponds to ab initio and density functional theory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be -29.4 ± 0.3 kcal/mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC measurements (28.4 ± 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6311+G**+ZPE.

Full text of DOI:10.1021/ja981680x

11130
J. Am. Chem. Soc. 1998, 120, 11130-11135
An Experimental Thermochemical and Theoretical Study of
Triquinacene: Definitive Disproof of Its Neutral Homoaromaticity
Sergey P. Verevkin,† Hans-Dieter Beckhaus,† Christoph Ru1chardt,*,† Rainer Haag,‡
Sergei I. Kozhushkov,‡ Tosja Zywietz,‡ Armin de Meijere,*,‡ Haijun Jiao,§ and
Paul von Rague´ Schleyer*,§
Contribution from the Institut fu¨r Organische Chemie und Biochemie der Albert-Ludwigs-UniVersita¨t
Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, Institut fu¨r Organische Chemie der
Georg-August-UniVersita¨t Go¨ttingen, Tammannstrasse 2, D-37077 Go¨ttingen, Germany, and Computer
Chemistry Center of the Institut fu¨r Organische Chemie der Friedrich-Alexander-UniVersita¨t
Erlangen-Nu¨rnberg, Henkestrasse 42, D-91054 Erlangen, Germany
ReceiVed May 15, 1998. ReVised Manuscript ReceiVed September 3, 1998
Abstract: The enthalpy of formation (∆H°(g) ) 57.51 ( 0.70 kcal/mol) of triquinacene (1), newly
f
determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that
previously reported and corresponds to ab initio and density functional theory computational results. As a
consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from
enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported
by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state
from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization
of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be -29.4 ( 0.3 kcal/
mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC
measurements (28.4 ( 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6-
311+G**+ZPE.
Introduction
failed to find evidence of substantial homoaromatic stabilization
in 1. In particular, the computed stepwise heats of hydrogena-
tion of 1, 2, and 3 were nearly the same with various molecular
mechanics and semiempirical methods and at various ab initio
levels (Table 1). This contradiction between theory and
experiment has been interpreted in various ways. For example,
Dewar and Holder5 ascribed the unusually small differences in
In 1964, Woodward, Fukunaga, and Kelly1 conceived the
tantalizing possibility that the three double bonds of triquinacene
(1), although separated, might still interact in benzene-like
fashion. Indeed, a 1986 experimental determination by Liebman
et al.2 found a significantly lower (4.5 kcal/mol) enthalpy of
hydrogenation of 1 to dihydrotriquinacene (2) than that of the
subsequent steps, 2 to 3 and 3 to 4. However, an extensive ab
enthalpy of formation between 1 and 2 [∆∆H°(1)] to a
f
decreased stability of 2 rather than an increased stability of 1,
and the discrepancy between theory and experiment to com-
putational errors.
Holder9 applied atoms in molecules (AIM) theory to analyze
the bonding pattern in 1 but was unable to locate any bond
critical points among the supposedly homoaromatically bonded
atoms. This finding was declared as “another (and hopefully
the final) nail to the coffin of neutral homoaromaticity” of 1.
The present study was undertaken to obtain new experimental
information.
initio study by Schulman et al.3 both questioned the experimental
heats of hydrogenation and estimated the enthalpy of formation
of 1 to be 56.6-57.0 kcal/mol rather than the experimental 53.5
( 1 kcal/mol. Several other theoretical investigations4-9 also
* To whom correspondence should be addressed. E-mail: ameijer1@uni-
goettingen.de; ruechardt@oca.chemie.uni-freiburg.de; pvrs@organik.uni-
erlangen.de.
Results and Discussion
† Albert-Ludwigs-Universita¨t Freiburg.
While homoaromaticity clearly is present in ionic systems,
such stabilization, if at all detectable, is often considered to be
very small in neutral hydrocarbons.2,6 Spectroscopic (IR,10
‡ Georg-August-Universita¨t Go¨ttingen.
§ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
(1) Woodward, R. B.; Fukunaga, T.; Kelly, R. C. J. Am. Chem. Soc.
1964, 86, 3162-3164.
(2) Liebman, J. F.; Paquette, L. A.; Peterson, J. R.; Rogers, D. W. J.
Am. Chem. Soc. 1986, 108, 8267-8268.
(3) Miller, M. A.; Schulman, J. M.; Disch, R. L. J. Am. Chem. Soc. 1988,
110, 7681-7684.
(4) Spanget-Larsen, J.; Gleiter, R. Angew. Chem. 1978, 90, 471-472;
Angew. Chem., Int. Ed. Engl. 1978, 17, 441-442.
(5) Dewar, M. J. S.; Holder, A. J. J. Am. Chem. Soc. 1989, 111, 5384-
5387.
(6) Storer, J. W.; Houk, K. N. J. Am. Chem. Soc. 1992, 114, 1165-
1168.
(7) Schulman, J. M.; Miller, M. A.; Disch, R. L. J. Mol. Struct.
(THEOCHEM) 1988, 169, 563-568.
(8) McEwen, A. B.; R. Schleyer, P. v. R. J. Org. Chem. 1986, 51, 4357-
4368.
(9) Holder, A. J. Comput. Chem. 1993, 14, 251.
(10) Childs, R. F. Acc. Chem. Res. 1984, 17, 347-352.
10.1021/ja981680x CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/20/1998

Disproof of Neutral Homoaromaticity of Triquinacene
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11131
Table 1. Calculated and Experimentally Determined Enthalpies of Formation for Triquinacene (1) and Heats of Hydrogenation for
Hydrocarbons 1-3 (All Values in kcal/mol)
∆∆H°(g)
f
∆∆Hf°(g)
method
MINDO/3
MM2
MM3
MMP2
1
2-1
3-2
4-3
ref
78.7
59.8
58.6
61.5, 60.2
56.3
4
5-7
6
8
5
3
3, 7
3
-27.7
-26.8
-27.4
-27.0
-25.7
-27.4
AM1
-32.6
-32.0
-31.7
HF/STO-3G
HF/6-31G*
HF/6-31+G*
MP2/6-31G*
B3LYP/6-31G* a
B3LYP/6-311+G** a
expt
-26.0
-26.3
-26.4
53.0, 57.0b
-27.0
-26.4
-26.3
-27.0
-26.6
-26.4
56.6b
57.2
57.2
-27.2
-25.8
-26.3
3
-27.7
-27.3
-26.7
-27.6
-27.3
-26.8
53.5 ( 1,c 56.4d
57.51 ( 0.70
-23.0 ( 0.9
-27.5 ( 0.7
-27.5 ( 0.3
2, 6
expta
a This work. b Derived from calculated enthalpies of hydrogenation and experimentally determined ∆H°(g) of perhydrotriquinacene (4, -24.47
f
( 0.86 kcal/mol17). c Derived from experimentally determined enthalpies of hydrogenation. d Derived from calculated ∆H°(g) of perhydrotri-
f
quinacene and experimentally determined enthalpies of hydrogenation.
Table 2. Calculated Zero-Point Energies (ZPE, kcal/mol),
UV,10 CD,11 PES12) and structural13 data, as well as the
Thermal Energies (Eth, kcal/mol) at 298 K, Number of Imaginary
Frequencies (NImag), and Magnetic Susceptibilities (øtot, ppm cgs)
calculated overlap integral of nonbonded interaction,14 offer no
evidence of homoaromatic character in 1. We determined
ZPE (NImag)a
Eth
øtot
b
c
∆H°(g) for triquinacene (1) for the first time by measuring its
f
1 (C10H10, C3V)
2 (C10H12, C1)
3 (C10H14, C1)
4 (C10H16, C3)
C5H10 (C2)
106.9 (0)
122.1 (0)
137.2 (0)
152.5 (0)
88.8 (0)
111.1
126.5
141.9
157.3
92.0
-66.2
energy of combustion.
The hydrocarbon 1 was prepared by using the established
procedure of Deslongchamps et al.15 and purified thoroughly
(see Experimental Section). The energy of combustion mea-
-83.6
-44.5
-38.5
-83.7
-101.5
surements, performed as described previously,16 gave ∆H°(g)
C5H8 (Cs)
73.6 (0)
76.5
f
5 (C10H10, C3V)
6 (C10H10, C3V)
adamantane
107.7 (0)
105.2 (1)
154.1 (0)
112.2
109.0
158.3
) 57.51 ( 0.70 kcal/mol for triquinacene (1). This value agrees
well with the computed estimate of Schulman et al.3 at the
RMP2/6-31G* level of theory and with AM1 semiempirical
calculations (also see Table 1).
a B3LYP/6-31G*. b B3LYP/6-311+G**. c CSGT-B3LYP/6-31G*//
B3LYP/6-311+G**.
In contrast, the previously reported ∆H°(g) ) 53.5 ( 1.0
f
kcal/mol for 1, derived from the experimental enthalpy of
corrections, and number of imaginary frequencies are given in
Table 2. Magnetic susceptibilities (ø) were calculated at the
B3LYP/6-31G*//B3LYP/6-311+G** level with the CSGT19
method, and nucleus-independent chemical shift (NICS) at
B3LYP/6-31G*/6-31G*//B3LYP/6-311+G** with the GIAO20
method as implemented in Gaussian 94.
hydrogenation of 1 to 4, -78.0 ( 0.5 kcal/mol, and ∆H°(g) of
f
4 (-24.47 ( 0.86 kcal/mol17), is 4.0 kcal/mol lower. This
difference is practically the same as the 4.5 kcal/mol previously
claimed to be the homoaromatic stabilization for 1. We do not
presume to explain the observed anomalously low value for the
∆H°(g) of 1 to 2, but the new experimental enthalpy of
f
The optimized structural parameters of 1 (C3V), e.g., the CdC
double bond length (1.333 Å, the same as in cyclopentene) and
the nonbonded distance (2.549 Å), agree with the X-ray data
(1.319, questionably too short, and 2.533 Å13). Thus, there are
no geometric changes which might be caused by homoconju-
gation in 1. On the other hand, 2 and 3 have C1 symmetry,
and the ethene unit of the five-membered ring in 2 is twisted
by 36.2°, 38.2°, and 31.9° in 3 as compared with the value for
cyclopentene (41.1°). Perhydrotriquinacene (4) has C3 sym-
metry; the three ethene units are twisted by 37.9°. All these
results agree well with Schulman et al.’s calculations.3
We have employed Schulman et al.’s3 method to derive the
formation is, at least, in line with the most reliable computational
results. The only logical conclusion is that 1 is not homoaro-
matic.
To support the new experimental finding, we carried out high-
level density functional theory (DFT) computations using the
Gaussian 94 program.18 In contrast to the earlier calculations,
the electron-correlated DFT method (B3LYP) and a larger basis
set (6-311+G**) were used. All the structures are energy
minima, as shown by the B3LYP/6-31G* frequency calculations.
The computed total energies, zero-point energies, thermal
(11) Paquette, L. A.; Kearney, F. R.; Drake, A. F.; Mason, S. F. J. Am.
Chem. Soc. 1981, 103, 5064-5069.
(12) (a) Bischof, P.; Bosse, D.; Gleiter, R.; Kukla, M. J.; de Meijere,
A.; Paquette, L. A. Chem. Ber. 1975, 108, 1218-1223. (b) Christoph, G.
G.; Muthard, J. L.; Paquette, L. A.; Bo¨hm, M. C.; Gleiter, R. J. Am. Chem.
Soc. 1978, 100, 7782-7784.
(13) Stevens, E. D.; Kramer, J. D.; Paquette, L. A. J. Org. Chem. 1976,
41, 2266-2269.
(14) Paquette, L. A.; Snow, R. A.; Muthard, J. L.; Cynkowski, T. J.
Am. Chem. Soc. 1979, 101, 6991-6996.
(15) Deslongchamps, P.; Cheriyan, U. O.; Lambert, Y.; Mercier, J.-C.;
Ruest, L.; Russo, R.; Soucy, P. Can. J. Chem. 1978, 56, 1687-1704.
(16) Beckhaus, H.-D.; Ru¨chardt, C.; Kozhushkov, S. I.; Belov, V. N.;
Verevkin, S. P.; de Meijere, A. J. Am. Chem. Soc. 1995, 117, 11854-
11860.
enthalpies of hydrogenation, the ∆H°(g) of 1-3, and the
f
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, 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.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, 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.; Gonzalez, C.; Pople, J. A. Gaussian 94; Gaussian Inc.:
Pittsburgh, PA, 1995.
(19) Keith, T. A.; Bader, R. F. W. Chem. Phys. Lett. 1993, 210, 223-
231.
(17) Clark, T.; Knox, T. M.; McKervey, M. A.; Mackle, H.; Rooney, J.
J. J. Am. Chem. Soc. 1979, 101, 2404-2410.
(20) (a) Dichtfield, R. Mol. Phys. 1974, 27, 789-807. (b) Wolinski, K.;
Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251-8260.

11132 J. Am. Chem. Soc., Vol. 120, No. 43, 1998
VereVkin et al.
Table 3. Estimated Enthalpies of Formation (kcal/mol) for 1-4
1
2
3
4
expta
53.5 ( 1.0
(57.45 ( 0.68)c
57.0
56.6
57.2
30.5 ( 1.0
3.0 ( 1.0
-24.47 ( 0.86d
HF/6-31G* b
30.0
29.4
29.5
29.6
3.6
3.6
2.2
2.3
-22.7
MP2/6-31G* b
B3LYP/6-31G* c
B3LYP/6-311+G** c
-23.8
-24.7
57.2
a Reference 2. b Reference 3. c This work. d Reference 17.
stabilization energy of 1. Thus, the enthalpies of hydrogenation
of 1, 2, and 3 are obtained from the homodesmotic eqs 1-3, in
which cyclopentane (C5H10) and cyclopentene (C5H8) are used
as reference molecules.
In addition to this energetic evidence, we have also computed
the magnetic properties to characterize the homoaromaticity of
1. If 1 were homoaromatic, it should have significant diamag-
netic susceptibility exaltation23 and NICS values.24 Homodes-
motic equations are appropriate not only to estimate aromatic
stabilization energies but also to evaluate magnetic susceptibility
exaltations.25
The calculated (via eq 6) magnetic susceptibility exaltation
of 1, -0.6 ppm cgs, is negligible for a system with six
π-electrons and a large ring perimeter.25 The benzene exaltation
is -16.7.26 In addition, the calculated NICS value at the
geometrical center of the three double bonds, -1.6 ppm, may
be comparable with those of the nonaromatic perhydro-
triquinacene 4 (-4.6), cyclopentane (-4.2), cyclopentene (-2.3)
as well as cyclohexane (-2.0). Hence, 1 is also not homoaro-
matic magnetically.
1 + C5H10 ) 2 + C5H8
2 + C5H10 ) 3 + C5H8
3 + C5H10 ) 4 + C5H8
∆Hr(1) ) -0.6 kcal/mol (1)
∆Hr(2) ) -0.4 kcal/mol (2)
∆Hr(3) ) +0.2 kcal/mol (3)
Using the experimental hydrogenation enthalpy of cyclopen-
tene to cyclopentane (which corresponds to the difference in
the enthalpies of formation, -26.94 ( 0.13 kcal/mol)2 and the
enthalpies of reaction, the calculated stepwise enthalpies of
hydrogenation (in kcal/mol) are -27.6 for 1, -27.3 for 2, and
-26.8 for 3 (Table 1). In agreement with other theoretical
values, the B3LYP results indicate that all three double bonds
in 1 are essentially independent and do not interact. The
enthalpy of hydrogenation for the first CdC double bond, -27.6
kcal/mol, is 4.6 kcal/mol larger than the experimental value
reported by Liebman et al.2
Can Neutral Trishomoaromatic Systems Exist?
Triquinacene (1) is not homoaromatic, but can neutral
trishomoaromatic systems exist?6,23a This question was dis-
cussed recently. Semibullvalenes, if annelated appropriately,
can result in homoaromatic ground states.27 What would happen
if the three CdC double bonds in 1 were to approach more
closely? During the isomerization of diademane (5)28 into 1,
the three cyclopropyl σ bonds in 5 are converted into three
separated double bonds as in 1 simultaneously. Such isomer-
1 + 4 ) 2 + 3
1 + 3 ) 2×2
∆Hr ) -0.8 kcal/mol
∆Hr ) -0.3 kcal/mol
(4)
(5)
1 + 3C5H10 ) 4 + 3C5H8
∆Hr ) -0.8 kcal/mol (6)
Although 1 is a normal hydrocarbon as deduced from eq 1,
we can calculate the “stabilization energy” for 1 using the
homodesmotic eqs 4 and 5, in which not only the number of
double bonds but also the strain effects can be balanced to a
large extent. Clearly, the quite small reaction enthalpies of eqs
4 (-0.8 kcal/mol) and 5 (-0.3 kcal/mol) also show that there
is no homoaromatic interaction among the three double bonds
in 1. Thus, 1 is not homoaromatic energetically.
ization can be induced thermally28,29 as well as carried out under
(23) (a) Schleyer, P. v. R.; Jiao, H. Pure Appl. Chem. 1996, 68, 209-
218. (b) Schleyer, P. v. R.; Freemann, P. K.; Jiao, H.; Goldfuss, B. Angew.
Chem. 1995, 107, 332-335; Angew. Chem., Int. Ed. Engl. 1995, 34, 337-
340. (c) Dauben, H. J., Jr.; Wilson, J. D.; Laity, J. L. In Nonbenzenoid
Aromatics; Snyder, J. P., Ed.; Academic Press: New York, 1971; Vol. II,
pp 167-206. (d) Zywietz, T. K.; Jiao, H.; Schleyer, P. v. R.; de Meijere,
A. J. Org. Chem. 1998, 63, 3417-3422.
(24) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van E.
Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317-6318.
(25) (a) Jiao, H.; van E. Hommes, N. J. R.; Schleyer, P. v. R.; de Meijere,
A. J. Org. Chem. 1996, 61, 2826-2828. (b) Jiao, H.; Schleyer, P. v. R.
Angew. Chem. 1996, 108, 2548-2551; Angew. Chem., Int. Ed. Engl. 1996,
35, 2383-2386.
(26) Jiao, H.; Schleyer, P. v. R.; Beno, B. R.; Houk, K. N.; Warmuth,
R. Angew. Chem. 1997, 109, 2929-2933; Angew. Chem., Int. Ed. Engl.
1997, 36, 2761-2764.
(27) Jiao, H.; Nagelkerke, R.; Kurtz, H. A.; Williams, R. V.; Borden,
W. T.; Schleyer, P. v. R. J. Am. Chem. Soc. 1997, 119, 5921-5929.
(28) IUPAC name: hexacyclo[4.4.0.0.2,100.3,50.4,807,9]decane. Prepara-
tion: (a) de Meijere, A.; Kaufmann, D.; Schallner, O. Angew. Chem. 1971,
83, 404-405; Angew. Chem., Int. Ed. Engl. 1971, 10, 417-418. (b)
Kaufmann, D.; Fick, H.-H.; Schallner, O.; Spielmann, W.; Meyer, L.-U.;
Go¨litz, P.; de Meijere, A. Chem. Ber. 1983, 116, 587-609.
(29) (a) de Meijere, A.; Kaufmann, D.; Schallner, O. Tetrahedron Lett.
1973, 553-556. (b) Spielmann, W.; Fick, H.-H.; Meyer, L.-U.; de Meijere,
A. Tetrahedron Lett. 1976, 4057-4060. (c) Meyer, L.-U.; de Meijere, A.
Chem. Ber. 1977, 110, 2545-2560.
Using the calculated enthalpy of reaction (eq 6) and the
enthalpies of formation of cyclopentane,21 cyclopentene,21 and
4,17 the enthalpy of formation of 1 is estimated to be 57.2 kcal/
mol. This is larger than the value reported by Liebman et al.2
but close to the MP2/6-31G* result and, in particular, to the
new experimental value (57.51 ( 0.70 kcal/mol). The same
procedure gives the enthalpies of formation of 2 and 3 as 29.6
and 2.3 kcal/mol, respectively, close to the other theoretical and
experimental results (Table 3). Using the enthalpy of isomer-
ization from 4 to adamantane, corrected to 298.15 K, and the
experimental ∆H°(g) of adamantane (32.2 ( 0.55 kcal/mol21),
f
the ∆H°(g) of 4 is computed to be 24.7 kcal/mol,22 in
f
agreement with the experimental value (24.47 ( 0.86 kcal/mol).
(21) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of
Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986.
(22) Recent MM4 calculations also gave an enthalpy of formation for
perhydrotriquinacene (4), ∆H°(g) ) 24.01 kcal/mol: Allinger, N. L.;
f
Chen, K.; Liu, J.-H. J. Comput. Chem. 1996, 17, 642-668.

Disproof of Neutral Homoaromaticity of Triquinacene
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11133
Table 4. Calculated and Experimentally Determined ∆H°(g) for
f
Diademane (5) (All Values in kcal/mol)
method MINDO/3
MM2
86.3
107.9
this work this work
MM3
70.7
92.3
6-31G*
exptlb
∆H°(g)
106.3
127.9
4
89
107
7
86.90 ( 0.33
108.6
this work
f
SEa
ref
a Calculated using an increment of -2.16 kcal/mol per CH group
for an unstrained molecule.40 b Derived from experimentally determined
∆Hf°(g) for triquinacene and experimentally determined ∆H° for
r
diademane f triquinacene isomerization.
Table 5. Results from the Measurements of the Enthalpy of
Reaction ∆H° and Eyring Parameters ∆Hq and ∆Sq for the
r
Isomerization Diademane (5) f Triquinacene (1) at T ) 368.2 K in
the Liquid Phase by DSC38
mass of
sample,
mg
Figure 1. Relative energies (RHF/6-31G*) and magnetic susceptibili-
ties as well as NICS values (RHF-GSGT/6-31G*//RHF/6-31G*) for
discrete points along the RHF/6-31G* intrinsic reaction coordinate
(IRC) with IRC ) 0 for the transition state 6, IRC ) 2 for triquinacene
(1), and IRC ) -2 for diademane (5).
- ∆H°,
∆Hq,
r
medium
tetradecane
tetradecane
tetradecane
tetradecane
tetradecane
tetradecane
kcal/mol
kcal/mol
∆Sq, eu
3.0
1.44 29.5
4.12 28.8
1.85 29.0
3.95 30.1
2.25 28.0
1.95 28.7
28.8
28.1
28.5
28.7
28.7
28.8
28.1
27.5
1.6
2.4
2.7
2.6
3.1
1.9
0.4
ment of magnetic susceptibility exaltation (-17.5 ppm cgs) and
NICS (-9.4 ppm) values over those of nonaromatic 1 is due to
contributions from these three-membered rings.27,32 For ex-
ample, cis,cis-1,2,3-trimethylcyclopropane has a small magnetic
susceptibility exaltation of -5.7 ppm cgs, the NICS value at a
distance of 1.5 Å (nearly the same distance as the central point
of the six active carbons to the three-membered ring in
diademane) over the ring center is -3.6 ppm, and the sum of
these values is close to that of diademane. Thus, 5 is not
aromatic.
However, we cannot reproduce the experimental activation
barrier of the isomerization from 5 to 1 at B3LYP/6-
311+G**+ZPE (B3LYP/6-31G*). The calculated barrier of
25.9 kcal/mol is 2.5 kcal/mol lower than the experimental value
of 28.4 ( 0.2 kcal/mol, determined here by differential scanning
calorimetry (DSC), which in turn is in good agreement with
the value of 28.3 ( 1.0 kcal/mol reported before for the
isomerization in solution.29a
di-n-butylphthalate 1.75 30.2
N-methylacetamide 2.60 30.8
mean values
29.39 ( 0.33 28.40 ( 0.16 2.21 ( 0.32
transition metal or acid catalysis.30 This reaction is allowed
according to the rules of orbital symmetry conservation and has
a relatively low Arrhenius activation energy in solution (Ea )
31.7 ( 0.7 kcal/mol)28b as well as in the gas phase (Ea ) 28.3
( 1.0 kcal/mol).29a Calculated as well as experimentally deter-
mined values of ∆H°(g) and strain energy (SE) for 5 are pre-
f
sented in Table 4. Experimental values for the enthalpy of
isomerization 5 f 1 and the enthalpy of activation for this
process as derived from DSC measurements are compiled in
Table 5.
The concerted and synchronous pathway of this isomerization
5 f 1 is also indicated by the calculated energy surface along
the RHF/6-31G* intrinsic reaction coordinate (IRC) descending
from both sides of the transition state.31 Moreover, the change
of magnetic susceptibility (∆ø) and the change in NICS
(∆NICS) at each of the discrete points along the IRC can be
plotted as shown in Figure 1. The calculated relative magnetic
susceptibility (∆ø), the relative NICS values (∆NICS), and the
relative energy (Erel) have maxima in magnitude at the transition-
state geometry (IRC ) 0, and the delocalized C-C bond
separations of 1.924 and 1.376 Å at RHF/6-31G*). Thus, this
transition state is to be aromatic.31
At B3LYP/6-311+G**, the transition state 6 C-C bond
separations of 1.867 and 1.414 Å are highly delocalized as
compared with the corresponding distances in 5 (1.523 vs 1.508
Å) and in 1 (2.549 vs 1.333 Å). On this basis, this transition
state is aromatic geometrically. This conclusion is supported
by the computed magnetic susceptibility exaltation of -35.3
ppm cgs (difference between the transition state and 1) and the
calculated NICS value (-24.7 ppm) at the geometric center of
the six active carbons and is in agreement with our preliminary
Hartree-Fock computations.31
Conclusion
The experimental enthalpy of formation of triquinacene (1)
has been revised by measuring its energy of combustion in a
microcalorimeter and by high-level density functional theory
computations. The new experimental value (∆H°(g) ) 57.51
f
( 0.70 kcal/mol) agrees with various ab initio and density
functional theory calculations but is about 4 kcal/mol higher
than the previously reported value derived from an enthalpy of
hydrogenation measurement. This revised enthalpy refutes the
claimed stabilization due to homoaromatic interaction. What-
ever the reasons for the discrepancy between the conclusion
from the heat of hydrogenation (∆HH) measurements and our
current results may be, this is not the first case in which ∆HH
measurements pointed toward homoconjugative stabilizations
which could not be confirmed by computational or other
experimental methods.25a That 1 is not homoaromatic also is
shown by the negligible calculated stabilization energy (-0.8
kcal/mol), diamagnetic susceptibility exaltation (-0.6 ppm cgs),
and NICS (-1.6 ppm), as well as the quite normal CdC bond
lengths. This situation changes when the three double bonds,
too far apart to interact in 1, are brought together. The concerted
and synchronous transition state of the isomerization from
diademane (5) to 1 is highly aromatic on the basis of the same
As discussed above, triquinacene (1) is not aromatic, but
diademane (5) has relatively more negative magnetic suscep-
tibility than 1 (Table 2). Does 5 have some aromatic character?
Note that 5 has three three-membered rings, and the enhance-
(30) de Meijere, A. Tetrahedron Lett. 1974, 1845-1848.
(31) Jiao, H.; Schleyer, P. v. R. J. Phys. Org. Chem., in press.
(32) Sauers, R. R. Tetrahedron 1998, 54, 337-348.

11134 J. Am. Chem. Soc., Vol. 120, No. 43, 1998
VereVkin et al.
Table 6. Results from Typical Combustion Experiments of
Triquinacene (1) at T ) 298.15 Ka
Table 7. Densities F (293 K), Specific Heat Capacities cp (298.15
K), Enthalpy of Fusion ∆H°fus, Temperature of Fusion Tfus, and
Expansion Coefficients of the Materials
run
F,a
Cp,b cal
∆H° ,b
Tfus
K
,
10-6(δV/δT)p,c
b
fus
1
2
3
4
g cm-3 K-1 g-1 kcal/mol
dm-3 K-1
m(substance)/gb
m′(Mylar)/gb
m′′(cotton)/gb
∆Tc/Kc
0.042 532 0.034 211 0.028 466 0.030 375
0.014 696 0.014 604 0.015 072 0.014 901
0.000 644 0.000 705 0.000 543 0.000 642
triquinacene (1) 1.06
diademane (5)
0.390 3.54 ( 0.05 289.0
0.359 365.2
1.0
cottond
Mylare
1.500 0.399
1.380 0.315
0.1
0.1
1.467 91
-515.69
-0.96
0.29
1.227 61
-431.27
-0.77
0.24
1.067 17
-374.91
-0.68
0.21
1.117 83
-392.70
-0.71
0.22
-ꢀcalor∆Tc/cal
-ꢀcont∆Tc/cal
∆Ecorr/cald
a Measured with a pycnometer. b From DSC measurements. c Esti-
mated. d Cotton ∆u°(CH1.774O0.887) ) -4050.0 ( 1.0 cal g-1 e Mylar:
.
c
∆uc°(C10H8O4) ) -5458.6 ( 1.1 cal g-1, m(Mylar) ) mmoist [1 - (4.64 ×
-m′∆u°′/cal
80.22
79.72
82.37
81.34
c
10-5) (relative moisture of air in %)].
-m′′∆u°′′/cal
2.61
2.86
2.20
2.60
∆u°(subc)/cal g-1 -10 184.4 -10 197.2 -10 206.4 -10 184.2
c
Table 8. Enthalpies of Combustion ∆H° at T ) 298.15 K from
a For the definition of the symbols, see ref 35; Th ) 298.15 K; Vbomb
) 0.0460 dm3; pgi as ) 3.04 MPa; mwi ater ) 0.23 g; Eignition ) 0.35 cal;
mplatin ) 2.883 g. b Masses obtained from apparent masses. c ∆Tc )
Tf - Ti+ ∆Tcorr; ꢀcalor ) 351.31 ( 0.05 cal K-1; -ꢀcont∆Tc ) ꢀicont(Ti
c
All Combustion Experiments and Their Mean Value
-∆Hc° in kcal/mol for Triquinacene (1)
∆H°
-∆H°
c
c
- 298.15 K) + ꢀfcont(298.15 K - Tf + ∆Tcorr). d ∆Ecorr is the sum of
items 81-85, 87-90, 93, and 94 in ref 35.
1330.27
1329.07
1327.41
1327.38
1326.57
1328.14 ( 0.67
criteria. While neutral trishomoaromaticity does not exist in
triquinacene, other more promising systems have been pro-
posed.8,23a
using the Clausius-Clapeyron equation. About 0.2 g of the sample
was mixed with glass beads and placed in a thermostated U-tube of
length 20 cm and diameter 0.5 cm. At constant temperature ((0.1
K), a nitrogen stream was passed through the U-tube, and the
transported amount of material was collected in a cooled trap. The
flow rate, 0.28-0.56 cm3 s-1, of the nitrogen stream was optimized in
order to reach the equilibrium of saturation of transporting gas at each
temperature of the investigation. The amount of condensed substance
was determined by GC analysis using an internal standard. The vapor
pressure p at each saturation temperature was calculated from the
amount of product collected within a definite time period with the help
of the ideal gas equation p ) mRT/V(N2)M, in which R ) 8.31451 J
K-1 mol-1, m is the mass of transported compound, V(N2) is the volume
of transporting gas, M is the molar mass of compound, and T is the
saturation temperature. The vapor pressure p was corrected for the
residual vapor pressure at the condensation temperature. The latter
was calculated from a linear correlation between ln p and T-1 obtained
by iteration. The molar enthalpy of vaporization was calculated from
Experimental Section
The hydrocarbons 115 and 528b were prepared following the previously
published procedures. Triquinacene (1) was purified by a sequence of
column chromatography on silica gel, then repeated (two times)
preparative gas chromatography (20% SE 30 on Chromosorb W-AW-
DMCS, 1000 mm × 8.2 mm column, 90 °C), and finally distillation
over 4-Å molecular sieves. Analysis of 1 by gas chromatography failed
to show any impurity peaks (greater than 0.01%). For this compound,
a purity of 99.99% (mp 288.9 K) was additionally established by DSC
measurements of the melting process.33 Diademane (5) was purified
by working up the mixture with snoutene through oxidation with
dimethyldioxirane, separating it from snoutene epoxide by column
chromatography on silica gel, then repeatedly (two times) subliming
under vacuum, 0.01 Torr at 303 K. 5 is not stable enough thermally
to test its purity by gas chromatography.
Thermochemical Measurements. (i) Combustion Calorimetry.
For measurements of the energy of combustion of 1, an isoperibolic
aneroid microcalorimeter with stirred water bath was used. The
substance was placed in Mylar bags, which were burned in oxygen at
a pressure of 3.04 MPa with a mass of 0.23 g of water added to the
bomb. The detailed procedure has been described previously.34 The
energy equivalent of the calorimeter, ꢀcalor (see Table 6), was determined
with a standard reference sample of benzoic acid (sample SRM 39i,
U.S. NIST). The summary of auxiliary quantities for the combustion
experiments and information necessary for reducing apparent mass
measured in air to mass, converting the energy of the actual bomb
process to that of the isothermal process, and reducing to standard
states35 is given in Table 7. Some typical combustion experiments for
triquinacene are given in Table 6. The individual values of the
the slope of the linear Clausius-Clapeyron correlation: ∆H°
)
vap
-R(d ln p/dT). The observed enthalpy of vaporization, ∆H° (T), at
vap
the temperature T obtained by this procedure is given in Table 9.
Because of the deviations from T ) 298.15 K of the average temperature
(T ) 308.9 K) of measurement by the transpiration method, the
observed value of the enthalpy of vaporization of 1 (Table 9) was
extrapolated (see Table 10) to this reference temperature using the
“Sidgwick correction”:37
{∆H° (T) - ∆H° (298.15 K)}/kcal/mol )
vap
vap
-0.014(T - 298.15)/K
(iii) DSC Measurements.38 The first experiments on 5 were carried
out with pure 5 without addition of any solvent. In these experiments,
two signals were observed simultaneously (the summarized enthalpy
of transformations was -26.0 kcal/mol). It was not possible to separate
the signals from the melting process and the process of interconversion
of the diademane (even in a wide range of variations of the heating
rates). Therefore, further experiments were carried out as follows: 1-4
mg of diademane together with a solvent (see Table 5) was placed in
small screwable high-pressure steel pans. The sample and the reference
pan with pure solvent (10 µL) were heated in a differential scanning
calorimeter (DSC-2, Perkin-Elmer, calibrated with indium) at a constant
enthalpies of combustion ∆H° together with the mean value and its
f
standard deviation, are given in Table 8. The given standard deviations
of the mean include the uncertainties from calibration and the
combustion energies of the auxiliary materials.
(ii) Transpiration Method. The enthalpy of vaporization of 1 was
determined with the method of transference in a saturated N2 stream36
(33) Hemminger, W. F.; Cammenga, H. K. Methoden der Thermischen
Analyse; Springer: Berlin, 1989; pp 268-277.
(34) Beckhaus, H.-D.; Ru¨chardt, C.; Smisek, M. Thermochim. Acta 1984,
79, 149-159.
(35) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental
Thermochemistry; Rossini, F. D., Ed.; Interscience: New York, 1956; pp
75-128.
(37) Chickos, J. S.; Hesse, D. G.; Panshin, S. Y.; Rogers, D. W.;
(36) (a) Beckhaus, H.-D.; Kratt, G.; Lay, K.; Geiselmann, J.; Ru¨chardt,
C.; Kitschke, B.; Lindner, H. J. Chem. Ber. 1980, 113, 3441-3455. (b)
Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic
Compounds; Academic Press: New York; 1970.
Saunders, M.; Uffer, P. M.; Liebman, J. F. J. Org. Chem. 1992, 57, 1897-
1899.
(38) Peyman, A.; Hickl, E.; Beckhaus, H.-D. Chem. Ber. 1987, 120, 713-
725.

Disproof of Neutral Homoaromaticity of Triquinacene
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11135
Table 9. Results from Measurements of the Vapor Pressure p of
Triquinacene (1)
decane). This fact was a reason to admit the very similar change of
enthalpy of interconversion of diademane into triquinacene in the
gaseous phase, too. Therefore, the average value of enthalpy of
reaction, measured in solution, was suggested to be equal for the gaseous
state. The temperature dependence of the enthalpy of reaction was
expected to be negligible within the limits of the experimental
uncertainties. Therefore, the enthalpy of reaction (-29.39 ( 0.33 kcal/
mol), measured at 368.2 K, was accepted without extrapolation to the
reference temperature, 298.15 K.
T, Ka
m, mgb
V(N2), dm3 c
p, Pad
291.3
296.4
301.5
306.5
311.9
316.5
321.5
326.5
4.09
3.48
3.31
3.39
3.77
3.96
3.79
5.29
0.747
0.390
0.259
0.195
0.162
0.129
0.098
0.098
119.2
172.6
246.0
333.4
445.4
586.7
738.3
1030.0
Estimation of the ∆H° of Diademane (5). Due to the thermal
vap
lability of 5, the transpiration method failed to provide reliable results.
A good approximation for the magnitude of ∆H° of hydrocarbons
∆Hv°ap,T ) 11.31 ( 0.16 kcal/mol
vap
can be obtained from correlation with the solvent-accessible surface
a Temperature of saturation, N2 gas stream 0.26-0.52 cm3 s-1 b Mass
.
area F:39
of transferred sample condensed at T ) 243 K. c Volume of nitrogen
used to transfer mass m of sample. d Vapor pressure at temperature T,
calculated from m and the residual vapor pressure at T ) 243 K.
∆H° (298.15 K)/kcal/mol )
vap
(0.0793 ( 0.0029)(F/Å2) + (1.25 ( 1.60)
Table 10. Thermochemical Results at T ) 298.15 K in kcal/mol
for Triquinacene (1) and Diademane (5)
The value of ∆H° ) 11.85 ( 0.38 kcal/mol calculated by this
vap
equation for 1 (F ) 133.5 Å2) is in good agreement with the experi-
∆H°(g) ∆H°(g)
f
(calfcd)f
HS
mental value (Table 10). Therefore, this equation could also be used
g
compound
∆H°(liq)
∆H°
vap
(expt)
f
to calculate the ∆H° ) 11.34 ( 0.37 kcal/mol for 5 (see Table 10).
vap
triquinacene (1)
46.05a
11.46b
57.51
43.0
14.5
((0.68) ((0.16) ((0.70)
75.56e 11.34c 86.90d -21.7 108.6
((0.85) ((0.37) ((0.77)
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft (Projects ME 405/15-3 (A.M.), Ru 60/
29-4 (C.R.), and Schl 190/23-1 (P.S.)) and the Fonds der
Chemischen Industrie. We thank the companies BASF, Bayer,
Hoechst, Degussa, and Hu¨ls AG for generous gifts of chemicals.
The authors are indebted to Dr. B. Knieriem for his careful
reading of the manuscript and one of the reviewers for helpful
comments.
diademane (5)
a Calculated from the specific energies of combustion. b From the
measurements of vapor pressures at different temperatures from Table
9 using the Clausius-Clapeyron equation. c Calculated from the
equation for the solvent-accessible areas (see text). d From the measure-
ments of enthalpy of isomerization from Table 5 and ∆H°(liq) of
f
triquinacene. e Calculated from the values for diademane from this
table: ∆H°(liq) ) ∆H°(g) - ∆H° . f Calculated as the sum of incre-
f
ap
ments for a strain-free fmodel:40,41 [CH] ) -2.16; [HCd] ) 8.6 kcal/
Supporting Information Available: Cartesian coordinates
and total energies computed at the B3LYP/6-311+G** level
for compounds 1-6, cyclopentane, cyclopentene, and adaman-
tane (5 pages, print/PDF; available in ASCII format on the
Web). See any current masthead page for ordering information
and Web access instruction.
mol. g Strain enthalpy: HS ) ∆H°(g,expt) - ∆H°(g,calcd).
f
f
heating rate of 5 K min-1 from 308 to 473 K. All experiments showed
an exothermic peak between 348 and 423 K as a result of intercon-
version of 5 into 1. For details about the calculation of the area of the
signal which was interpreted as enthalpy of reaction, see ref 38. The
results of the DSC measurements are given in Table 5. The activation
parameters of the thermal interconversion of diademane were obtained
by simulation of the signals.38 The acceptable deviation of the activation
enthalpy ∆Hq from the measured enthalpy of reaction is evidence that
the studied reaction proceeds according to a rate law of first-order
reactions. No significant difference was found for the value of the
enthalpy of reaction investigated in rather polar solvents (di-n-butyl
phthalate and N-methylacetamide) and in a nonpolar solvent (tetra-
JA981680X
(39) Chickos, J. S.; Hesse, D. G.; Hosseini, S.; Liebman, J. F.;
Mendenhall, G. D.; Verevkin, S. P.; Rakus, K.; Beckhaus, H.-D.; Ru¨chardt,
C. J. Chem. Thermodyn. 1995, 27, 693-705.
(40) Schleyer, P. v. R.; Williams, J. E.; Blanchard, K. R. J. Am. Chem.
Soc. 1970, 92, 2377-2386.
(41) Benson, S. W. Thermochemical Kinetics; Wiley-Interscience: New
York, 1976; p 272.