Experimental determination of the heat of hydrogenation of phenylcyclobutadiene

Article abstract of DOI:10.1021/ja0525511

The heat of hydrogenation of phenylcyclobutadiene (ΔH° _hyd = 57.4 ± 4.9 kcal mol^-1) was determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-3-yl cation. This leads to an antiaromatic destabilization energy of 27 ± 5 kcal mol^-1, a difference of 9.6 ± 4.9 kcal mol^-1 for the first and second C-H bond dissociation energies of 1-phenylcyclobutene, and an estimate of 96 ± 5 kcal mol^-1 for the heat of formation of cyclobutadiene. These results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene.

Full text of DOI:10.1021/ja0525511

Published on Web 08/27/2005
Experimental Determination of the Heat of Hydrogenation of
Phenylcyclobutadiene
Alireza Fattahi, Lev Lis, and Steven R. Kass*
Contribution from the Department of Chemistry, UniVersity of Minnesota,
Minneapolis, Minnesota 55455
Received April 19, 2005; E-mail: kass@chem.umn.edu
Abstract: The heat of hydrogenation of phenylcyclobutadiene (∆H°hyd ) 57.4 ( 4.9 kcal mol-1) was
determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-
3-yl cation. This leads to an antiaromatic destabilization energy of 27 ( 5 kcal mol-1, a difference of 9.6 (
4.9 kcal mol-1 for the first and second C-H bond dissociation energies of 1-phenylcyclobutene, and an
estimate of 96 ( 5 kcal mol-1 for the heat of formation of cyclobutadiene. These results are compared to
G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy
of a monocyclic cyclobutadiene.
Introduction
A wide variety of computational methods have been em-
ployed to address the energetics of cyclobutadiene.16-36 Simple
Hu¨ckel calculations indicate that 1 has no π-electron delocal-
ization energy and is less stable than 1,3-butadiene, its acyclic
analogue. More sophisticated semiempirical, ab initio, and
density functional theory results lead to a heat of formation
spanning from 87 (BLYP/6-311G(2d,2p)) to 124 (HF/4-31G)
kcal mol-1. If only the most reliable ab initio methodologies
are considered (G2, CBS-Q, and CCSD(T)), then this range
narrows to 99-104 kcal mol-1. These latter predictions also
lead to an antiaromatic destabilization energy (ADE), as given
Cyclobutadiene (1) occupies an important place in chemistry
and has been the subject of extensive experimental efforts for
over 100 years.1-3 Numerous accomplishments relating to its
preparation, characterization, and antiaromatic nature have been
reported including matrix-isolated infrared spectra at 8 and 20
K,4-9 the synthesis and isolation of tetra-tert-butylcyclobutadiene
(and its corresponding tetrahedrane),10,11 and the formation of
a room-temperature stable molecule containing 1 encapsulated
in a polyether cage.12 As a result of all of this work, it is well-
known now that cyclobutadiene has a rectangular D2h geometry
and a singlet ground state, and is extremely reactive bimolecu-
larly. Quantitative data addressing the stability or lack thereof
of this 4π-electron annulene, however, is largely lacking and
has led to controversy regarding its heat of formation,13,14
relative vs absolute antiaromaticity,15 and the origin of its
instability (i.e. σ vs π framework).16-19
by the reaction in eq 1, of 31-36 kcal mol-1
.
In contrast, a recent photoacoustic calorimetry study was
reported which provided the heat of reaction for the photo-
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9
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J. AM. CHEM. SOC. 2005, 127, 13065-13069
13065

A R T I C L E S
Fattahi et al.
chemically induced conversion of the tetracyclic propellane 2
into cyclobutadiene, phthalene (3), and tricyclic tetraene 4 (eq
2).14
3-Phenyl-2-cyclobuten-1-ol (6). To a stirred solution of 0.31
g (2.15 mmol) of freshly sublimed 3-phenyl-2-cyclobuten-1-
one in 12 mL of methanol was added 0.51 g of CeCl3‚7H2O.
After cooling to 0 °C, 0.083 g (2.15 mmol) of sodium
borohydride was added in small portions over 5 min, and the
temperature was maintained for an additional 10 min. Removal
of the solvent under reduced pressure afforded a precipitate
which was suspended in CH2Cl2 and purified by flash chroma-
tography on a short (∼15 cm) silica gel column first using CH2-
Cl2 and then 50% more of 15% EtOAc in CH2Cl2. White
crystals of 3-phenyl-2-cyclobuten-1-ol were obtained (0.30 g,
96%) and can be stored in the dark at -20 °C for at least 3
If the heats of formation of 2, 3, and 4 were known, the heat of
formation of cyclobutadiene readily could have been obtained
from this result. Unfortunately, they have not been determined,
so they were estimated using computed molecular mechanics
(MM3) geometries and semiempirical AM1 energies. This led
Deniz, Peters, and Snyder to report a heat of formation for
cyclobutadiene of 114 ( 11 kcal mol-1 and an ADE of 46 kcal
mol-1; these values are 10-15 kcal mol-1 larger than the most
sophisticated computational predictions to date.
More recently, Broadus and Kass measured the heat of
formation of benzocyclobutadiene (5) in the gas phase using a
negative-ion thermodynamic cycle.13 By comparing it to 1 via
a homodesmotic reaction (eq 3),37,38
1
months without any noticeable decomposition. H NMR (300
MHz, CDCl3) δ 2.50 (br s, 1H), 2.63 (dt, 1H, J ) 12.9 and 1.2
Hz), 3.20 (ddd, 1H, J ) 12.9, 3.9, and 0.9 Hz), 4.80 (dt, 1H, J
) 3.9 and 0.9 Hz), 6.40 (q, 1H, J ) 0.9 Hz), 7.33-7.43 (m,
5H). This compares favorably to the literature spectrum recorded
at 60 MHz (i.e. 2.55 (dt, 1H, J ) 13.0 and 1.2 Hz), 2.66 (d,
OH, J ) 7.6 Hz), 3.15 (dq, 1H, J ) 13.0, 3.8, and 0.8 Hz),
4.84 (broad, 1H), 6.34 (dt, 1H), 7.33 (5H)).42 13C NMR (75.4
MHz, CDCl3) δ 40.5, 67.3, 125.5, 128.5, 128.8, 129.5, 134.0,
147.2.
Gas-Phase Experiments. A dual cell model 2001 Finnigan
Fourier transform mass spectrometer (FTMS) equipped with a
3 T superconducting magnet and controlled by an Ion Spec data
system running IonSpec99, ver. 7.0, was used for these studies.
3-Phenyl-2-cyclobuten-1-ol was added into the first (analyzer)
cell at a static pressure of ∼4 × 10-8 Torr and ionized with 50
eV electrons for 40 ms to afford positive ions at m/z 147 (M +
H, ∼5%), 146 (M, ∼15%), 145 (M - H, ∼55%), and 129 (M
+ H - H2O, ∼25%). The ion of interest (m/z 129 or 147) was
transferred to the second (source) cell where it was collisionally
cooled with up to three pulses of argon, each leading to pressures
of ∼10-5 Torr, and then it was re-isolated using a series of
chirp and stored waveform inverse Fourier transform (SWIFT)
excitations.44,45 Neutral reagents were introduced into the source
cell via slow leak valves, and the formation of product ions
were monitored as a function of time. In this work reported
reaction rates are estimated to have an uncertainty of ( 50%.
To probe the structure of the 1-phenylcyclobuten-3-yl cation
(7), it was reacted with a static pressure of H2O (∼4 × 10-8
Torr) in the analyzer cell. The resulting m/z 147 ion or the one
produced by chemical ionization of 3-phenyl-2-cyclobuten-1-
ol was transferred to the source cell and bracketed with standard
reference bases or fragmented via energetic collisions using
sustained off-resonance irradiation (SORI).
MP2 and B3LYP calculations led to ∆H°f (1) ) 101 and 103
kcal mol-1, respectively. This is in accord with high-level
calculations, but to establish the heat of formation of cyclo-
butadiene and its ADE, experimental data are needed. We now
describe the culmination of an earlier study on the C3-H bond
dissociation energy (BDE) of 1-phenylcyclobutene39 and report
the gaseous heat of hydrogenation of phenylcyclobutadiene by
employing a positive-ion thermodynamic cycle. Our results are
compared to G3 theory predictions, and the effect of the phenyl
substituent on cyclobutadiene is estimated.
Experimental Section
General. 3-Phenyl-2-cyclobuten-1-one was prepared and
purified by vacuum sublimation as described in the literature.40-42
Reduction of the ketone to the desired alcohol (6) was carried
out using a procedure similar to that for the preparation of (4′-
carbomethoxymethyl)-3-phenyl-2-cyclobuten-1-ol rather than
the one reported for 6.43 Solvents were dried by standard
methods, and reagents were used as received. 1H and 13C NMR
spectra were recorded on a Varian VAC-300 spectrometer and
are reported in parts per million (δ).
Computations. Calculations were carried out using Gaussian
200346 on IBM and SGI workstations at the Minnesota Super-
computer Institute. Density functional theory optimizations and
vibrational frequencies were computed using the Becke three-
parameter hybrid exchange47 and Lee-Yang-Parr48 correlation
density functional (R(U)B3LYP) with the 6-31G(d) and 6-31+G-
(d) basis sets, and single-point energies were obtained with the
aug-cc-pvtz basis set.49 Since the 6-31+G(d) energies agree with
(33) Jursic, B. S. THEOCHEM 2000, 507, 185-192.
(34) Wiberg, K. B. Chem. ReV. 2001, 101, 1317-1331.
(35) Feng, Y.; Liu, L.; Wang, J.-T.; Zhao, S.-W.; Guo, Q.-X. J. Org. Chem.
2004, 69, 3129-3138.
(36) Kovacevic, B.; Baric, D.; Maksic, Z. B.; Mueller, T. J. Phys. Chem. A
2004, 108, 9126-9133.
(37) George, P.; Trachtman, M.; Bock, C. W.; Brett, A. M. Theor. Chim. Acta
1975, 38, 121-129.
(38) George, P.; Trachtman, M.; Bock, C. W.; Brett, A. M. Tetrahedron 1976,
32, 317-323.
(44) Wang, T. C. L.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1986, 58, 2935-
2938.
(39) Glasovac, Z.; Eckert-Maksic, M.; Dacres, J. E.; Kass, S. R. J. Chem. Soc.,
Perkin Trans. 2 2002, 410-415.
(45) Gauthier, J. W.; Trautman, T. R.; Jacobson, D. B. Anal. Chim. Acta 1991,
246, 211-225.
(40) Frimer, A. A.; Pizem, H. Tetrahedron 1999, 55, 12175-12186.
(41) Hassner, A.; Dillon, J. L., Jr. J. Org. Chem. 1983, 48, 3382-3386.
(42) Manatt, S. L.; Vogel, M.; Knutson, D.; Roberts, J. D. J. Am. Chem. Soc.
1964, 86, 2645-2653.
(46) Frisch, M. J. et al. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003.
(47) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(48) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(49) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007-1023.
(43) Qian, W.; Rubin, Y. J. Org. Chem. 2002, 67, 7683-7687.
9
13066 J. AM. CHEM. SOC. VOL. 127, NO. 37, 2005

Heat of Hydrogenation of Phenylcyclobutadiene
A R T I C L E S
those obtained using the larger aug-cc-pvtz basis set to within
1.0 kcal mol-1 on average, only the former data are given in
the text. G3 and G3(MP2) energies also are reported and were
obtained as previously described.50,51 All of the resulting
thermodynamic quantities were corrected to 298 K using
unscaled B3LYP or scaled (0.8929) HF frequencies.
Results/Discussion
Structural Authentication. Electron ionization of a static
pressure of 3-phenyl-2-cyclobuten-1-ol (6) leads to 1-phenyl-
cyclobuten-3-yl cation (7, m/z 129, ∼25%) among other ions
presumably via fragmentation of the M and M + H ions (eq
4).
calculations were carried out, and phenylcyclobutadiene is found
to be significantly more stable than the other isomers. Conse-
quently, the acidity of 1-phenylcyclobuten-3-yl cation was
measured by reacting it with standard reference bases and
observing the occurrence or nonoccurrence of proton transfer
(Table 1). Strong bases such as pyrrolidine, diisopropylamine,
The identity of this substituted 1-phenylallyl cation was not in
doubt since it is stabilized by the phenyl group and has no
apparent low-energy rearrangement pathway available and our
thermodynamic measurements are reproduced by high-level
computations; the acidity also is structurally diagnostic (see
below). Nevertheless, we probed this species further by reacting
it with water and examining the resulting adduct in comparison
to the independently prepared ion formed by self-chemical
ionization of 6. In both cases, the resulting C10H11O+ ion (m/z
147) protonates N,N-diethylhydroxylamine (Et2NOH, PA )
218.6 ( 2.0 kcal mol-1) and aniline (PhNH2, PA ) 210.9 (
2.0 kcal mol-1) but not pyrrole (PA ) 209.2 ( 2.0 kcal mol-1),
ammonia-d3 (PA ) 204.0 ( 2.0 kcal mol-1), and weaker
bases.52 One H-D exchange is observed with ND3 and the rate
constant for the disappearance of the d0 species is (6.6 ( 1.0)
× 10-10 cm3 molecule-1 s-1. Likewise, sustained off-resonance
irradiation (SORI) collision-induced dissociation (CID) leads
to ions at m/z 129 (M - H2O, ∼45%), 128 (M - H3O, ∼10%),
91 (M - C3H4O, ∼10%) and 69 (M - C6H6, ∼35%) regardless
of how the m/z 147 ion is produced.53 These results indicate
that both 6 and 7 lead to the same species.
Table 1. Proton Affinity Measurement Results for
Phenylcyclobutadiene (8)
∆H°
acid
(kcal mol-
1
cmpd
)
proton transfer
ammonia-d3
pyrrolidine
diisopropylamine
1,3-propanediamine
1,4-butanediamine
204.0 ( 2.0a
226.6 ( 2.0
232.3 ( 2.0
235.9 ( 2.0
240.3 ( 2.0
245.0 ( 2.0
no
no
no
no
nob
yesc
3-(dimethylamino)propylamine
a This value is for NH3. b An adduct - NH3 at m/z 200 was observed.
c k ) 3.5 × 10-10 cm3 molecule-1 s-1
.
and 1,4-butanediamine do not abstract a proton from 7, whereas
the more basic 3-(dimethylamino)propylamine does with a rate
constant of 3.5 × 10-10 cm3 molecule-1 s-1. These observations
enable us to assign PA(8) ) 242.7 ( 2.7 kcal mol-1, which is
a much larger value than that measured for any other hydro-
carbon and is in excellent accord with computed results of 242.6
(G3), 241.5 (G3(MP2)), and 245.7 (B3LYP) kcal mol-1 (eq 6).56
Bracketing Experiments. Deprotonation of 1-phenylcy-
clobuten-3-yl cation affords phenylcyclobutadiene (8) because
a π bond is formed and the energetic benefit (∼60 kcal mol-1
)
outweighs the antiaromatic destabilization of the system (∼30
kcal mol-1, see below). The latter interaction, however,
decreases the acidity of 7 so that a large numerical value for
the proton affinity of 8 is expected. Additional deprotonation
products can be envisioned (eq 5), but the corresponding carbene
(9, -HB), allene (10, -HC), and benzocyclopropene (11, -HD)
would seem to be much less stable; 9 and 10, as previously
noted, are best described as 3-phenylbicyclo[1.1.0]but-1(2)-ene
and 1-phenyl-2-bicyclo[1.1.0]butylidene and are illustrated as
such.54,55 To test this expectation, G3(MP2) and B3LYP
The ionization potential of 1-phenylcyclobuten-3-yl radical
(7r) also was measured by bracketing (Table 2). In particular,
tetraethylhydrazine (Et2NNEt2) and species with higher ioniza-
tion potentials do not undergo electron transfer with 7r, whereas
(50) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J.
A. J. Chem. Phys. 1998, 109, 7764-7776.
(51) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J.
A. J. Chem. Phys. 1999, 110, 4703-4709.
(52) Bartmess, J. E. In NIST Chemistry WebBook; NIST Standard Reference
Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; National
Institute of Standards and Technology: Gaithersburg, MD 20899 (http://
webbook.nist.gov).
(55) Wiberg, K. B.; Marquez, M. J. Am. Chem. Soc. 1998, 120, 2932-2938.
(56) B3LYP computations for the proton affinity of cyclobutadiene decrease
by 4.9 kcal mol-1 when diffuse functions are included in the 6-31G(d)
basis set, but the value is still 5.7 kcal mol-1 larger than that predicted
using G3 theory. Using the larger aug-cc-pvtz basis set further reduces
this difference but by only 1.2 kcal mol-1, so we have empirically corrected
(53) Exact mass measurements were made to verify the elemental composition
of the fragment ions.
(54) Kollmar, H.; Carrion, F.; Dewar, M. J. S.; Bingham, R. C. J. Am. Chem.
Soc. 1981, 103, 5292-5303.
the B3LYP/6-31+G(d) energy by 5.7 kcal mol-1
.
9
J. AM. CHEM. SOC. VOL. 127, NO. 37, 2005 13067

A R T I C L E S
Fattahi et al.
Table 3. Experimental and Computed Heats of Formation
Table 2. Ionization Potential Bracketing Results for
1-Phenylcyclobutenyl Radical (7r)
1
∆H°
f (kcal mol-
)
cmpd
IP (eV)
electron transfer
Benson’s
methoda
noa
noa
noa
yesb
cmpd
G3(MP2)b
G3b
expt
N,N-diethylaniline
6.98 ( 0.05
6.80 ( 0.02
6.50 ( 0.05
6.24 ( 0.07
N-phenylpyrrolidine
tetraethylhydrazine
dicyclopentadienylnickel
cyclobutane
cyclobutene
cyclobutadiene
6.8
37.3
7.1
39.2
102.2
29.5
7.1
39.9
103.9
31.1
6.78 ( 0.14c
37.45 ( 0.37c
phenylcyclobutane
1-phenylcyclobutene
3-phenylcyclobutene
phenylcyclobutadiene
32.8
59.6
63.1
a Adduct formation was observed. b k ) 1.9 × 10-9 cm3 molecule-1
56.8
61.8
118.4
58.9
64.1
121.5
s-1
.
116 ( 6d
dicyclopentadienylnickel (Cp2Ni) donates an electron efficiently
(k ) 1.9 × 10-9 cm3 molecule-1 s-1). This leads to IP(7r) )
6.37 ( 0.14 eV or 146.9 ( 3.2 kcal mol-1, which is in
reasonable accord with computed values of 6.57 (G3), 6.50 (G3-
(MP2)), and 6.44 (B3LYP) eV (eq 7).
a See ref 53. b Computed via atomization energies. c See ref 54. d Based
upon the measured heat of hydrogenation and an estimated heat of formation
for 1-phenylcyclobutene of 59 ( 3 kcal mol-1, where the given uncertainty
represents a conservative estimate.
The heat of formation of phenylcyclobutadiene can be derived
from our data if the heat of formation of 1-phenylcyclobutene
is available. Unfortunately, this quantity has not been measured,
so we estimated it using Benson’s group equivalents method57
and computed G3 and G3(MP2) atomization energies (Table
3). If the average of the first two approaches is used (59 ( 3
kcal mol-1), then ∆H°f (8) ) 116 ( 6 kcal mol-1 is obtained.
This can be equated to the sum of the Benson strain-free energy
components (51.2 kcal mol-1), the strain energy, and the
antiaromatic destabilization energy. A value of 32.8 kcal mol-1
has been used for the SE of cyclobutadiene (this corresponds
to the strain for cyclobutane and an additional 1.5 kcal mol-1
per sp2-hybridized carbon)14 and leads to an ADE of 32 kcal
mol-1, which is in reasonable accord with the other predictions
of this quantity.
Thermochemistry. By combining our measurements for the
proton affinity of phenylcyclobutadiene and the ionization
potential of 1-phenylcyclobuten-3-yl radical with the recently
determined allylic C3-H bond dissociation energy of 1-phen-
ylcyclobutene (85.6 ( 2.6 kcal mol-1 39
and the well-known
)
values for the ionization energy of the hydrogen atom (313.6
kcal mol-1) and the bond energy of molecular hydrogen (104.2
kcal mol-1), one can obtain an experimentally determined heat
of hydrogenation for 8 of 57.4 ( 4.9 kcal mol-1 (eq 8).
Our PA and IE measurements (eqs 6 and 7) enable us to
derive the C-H bond strength of 1-phenylcyclobuten-3-yl
radical (eq 11, BDE2 ) 76.0 ( 4.2 kcal mol-1) and compare
it to the allylic C3-H BDE of 1-phenylcyclobutene (eq 12,
BDE1 ) 85.6 ( 2.6 kcal mol-1).59
This compares favorably with predicted values of 62.1 (G3),
60.5 (G3(MP2)), and 59.0 (B3LYP) kcal mol-1 56
,
but is more
than double the value for 1-phenylcyclobutene (∆H°hyd ) 26.8
kcal mol-1 57
and almost twice the value for cyclobutene
)
(∆H°hyd ) 30.7 ( 0.4 kcal mol-1).58 These large differences
are a reflection of the antiaromatic destabilization energy of
phenylcyclobutadiene, which is 26.7 ( 4.9 kcal mol-1 as given
by eq 9.
The difference is a mere 9.6 ( 4.9 kcal mol-1 as compared to
66 ( 1 kcal mol-1 for ethane and 61 ( 1 kcal mol-1 for
cyclobutane. This disparity is due in large part to the reduced
value for BDE1 because of the additional stability of the
resulting allylic radical. If one attempts to correct for this by
using model compounds such as ethylbenzene and cyclopentene
Alternatively, one can use the reaction illustrated in eq 10, but
the same result is obtained because the phenyl substituent on
cyclobutane and at the 3-position of cyclobutene effectively
cancel each other.
(BDE1 - BDE2 ) 38.4 ( 2.1 and 36.7 ( 1.4 kcal mol-1
,
respectively), then the gap is reduced, and the remaining
difference of ∼28 kcal mol-1 can be attributed to the ADE. In
(57) This quantity was obtained using Benson’s group equivalents approach.
Cohen, N.; Benson, S. Chem. ReV. 1993, 93, 2419-2438.
(58) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic
Compounds; Academic Press: New York, 1970; pp 1-643.
(59) The B3LYP value for BDE2 has been corrected for the apparent error in
the computed energy of phenycyclobutadiene as indicated in ref 56.
9
13068 J. AM. CHEM. SOC. VOL. 127, NO. 37, 2005

Heat of Hydrogenation of Phenylcyclobutadiene
A R T I C L E S
Table 4. Predicted Heat of Formation of Cyclobutadiene
any case, the BDE1 - BDE2 difference indicates that phenyl-
cyclobutadiene is an excellent radical trap from a thermody-
namic perspective and does not provide a good measure of the
strength of its second π bond; a better estimate for the π bond
strength is 34 kcal mol-1 (the π bond strength for cyclobutene
+ ∆H°rxn (eq 9)).
A natural question to ask at this point is what effect does the
phenyl group have on the stability of cyclobutadiene? This
question can be answered by comparing the heats of hydrogena-
tion of 1 and 8 as illustrated in eq 13.
1
∆H
°
f (kcal mol-
)
rxn
B3LYP
G3(MP2)
G3
atomization
hydrogenation
equation 1
102.2
99.4
99.0
103.9
101.0
99.3
105.0
103.8
avg (( std dev)
104.4 ( 0.8
100.2 ( 1.7
101.4 ( 2.3
formation is ∼100 kcal mol-1. This finding is consistent with
our previous estimate of 102 kcal mol-1 but is much smaller
than the value of 114 kcal mol-1 reported by Deniz, Peters,
and Snyder, in part because of the computational approach they
used.14 More sophisticated MP2 or DFT calculations were found
to lower the heat of formation by ∼4 kcal mol-1 61
.
Conclusions
The gas-phase heat of hydrogenation for phenylcyclobuta-
diene has been experimentally determined via a thermodynamic
cycle by making measurements on 1-phenylcyclobuten-3-yl
cation. This enables a wealth of thermochemical data to be
derived, including the antiaromatic destabilization energy, a
comparison of the first and second C-H bond energies of
1-phenylcyclobutene, and an estimate of the heat of formation
of phenylcyclobutadiene based upon the prediction of the heat
of formation of 1-phenylcyclobutene. Interestingly, phenylcy-
clobutadiene is found to be a remarkably good radical trap from
an energetic perspective because the antiaromaticity of the
ground state is relieved and the 1-phenylallyl radical produced
is stabilized by delocalization. Our results also suggest that the
parent compound is significantly more stable than the only
“experimental” determination reported to date.
Homodesmotic reactions such as this one should be reliably
reproduced at a variety of computational levels since electron
correlation effects largely cancel each other, and indeed, diverse
methods such as HF, MP2, MP3, MP4, QCISD(T), G3(MP2),
and G3 all predict that this transformation is slightly endothermic
(i.e. the values range from 1.1 to 2.0 kcal mol-1). In other words,
the heat of hydrogenation of cyclobutadiene is ∼2 kcal mol-1
larger than that for phenylcyclobutadiene, presumably because
of the additional conjugation in the latter species. If we combine
the G3 prediction with our experimental value for ∆H°hyd (8),
then ∆H°hyd (1) ) 59 ( 5 kcal mol-1 is obtained60 and ∆H°f
(1) ) 96 ( 5 kcal mol-1 can be derived on the basis of the
experimental heat of formation of cyclobutene (37.45 ( 0.37
kcal mol-1); ∆H°f (1) ) 99 kcal mol-1 is obtained if the G3
heat of formation for cyclobutene obtained via its atomization
energy is used. This result is on the low side of recent
predictions; therefore, we also computed the heat of formation
of cyclobutadiene via its atomization energy, heat of hydrogena-
tion, and homodesmotic reaction with cyclobutane (i.e., eq 1).
These results are summarized in Table 4 and lead us to predict
that 96 e ∆H°f (1) e 104 kcal mol-1 or that the heat of
Acknowledgment. Support from the National Science Foun-
dation, the donors of the Petroleum Research Foundation, as
administered by the American Chemical Society, and the
Minnesota Supercomputer Institute are gratefully acknowledged.
Supporting Information Available: Computed structures and
energies; complete ref 46. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0525511
(60) We assumed that the G3 reaction energy has an uncertainty of (2 kcal
mol-1
.
(61) Glasovac, Z.; Eckert-Maksic, M.; Kass, S. R. Unpublished results.
9
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