Phenylcyclopropane Energetics and Characterization of Its Conjugate Base: Phenyl Substituent Effects and the C-H Bond Dissociation Energy of Cyclopropane

Article abstract of DOI:10.1021/acs.joc.6b01718

The α-C-H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally determined using Hess' law. An equilibrium acidity determination of 1 afforded ΔH°_acid = 389.1 ± 0.8 kcal mol^-1, and isotopic labeling established that the α-position of the three-membered ring is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor in correctly determining the proper ionization site (i.e., secondary amide ions are needed, and primary ones and OH^- lead to incorrect conclusions since they scramble the deuterium label). An experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ± 2.8 kcal mol^-1) was combined with the ionization energy of hydrogen (313.6 kcal mol^-1) to afford BDE = 93.0 ± 2.9 kcal mol^-1. This enabled the effect of the phenyl substituent to be evaluated and compared to other situations where it is attached to an sp³- or sp²-hybridized carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out, and a revised C-H BDE for cyclopropane of 108.9 ± 1.0 kcal mol^-1 is recommended.

Full text of DOI:10.1021/acs.joc.6b01718

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Article
Phenylcyclopropane Energetics and Characterization
of Its Conjugate Base: Phenyl Substituent Effects and
the C–H Bond Dissociation Energy of Cyclopropane
Alireza Fattahi, Lev Lis, and Steven Robert Kass
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01718 • Publication Date (Web): 06 Sep 2016
Downloaded from http://pubs.acs.org on September 13, 2016
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Phenylcyclopropane Energetics and Characterization of Its Conjugate Base:
Phenyl Substituent Effects and the C–H Bond Dissociation Energy of
Cyclopropane
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‡
‡‡
‡‡
Alireza Fattahi , Lev Lis , and Steven R. Kass *
‡
Department of Chemistry, Sharif University of Technology, Tehran, Iran
‡‡
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455,
E-mail: kass@umn.edu
Table of Content Graphic
ꢀ
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9
3.0 ± 2.9
+ H
[
kcal mol–1
]
389.1 ± 0.8
-313.6
1
7.5 ± 2.8
+ H+
+ e + H+
–
–
1
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Abstract
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The α-C–H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally
determined using Hess' law. An equilibrium acidity determination of 1 afforded ΔH˚acid = 389.1
–
1
±
0.8 kcal mol and isotopic labeling established that the α-position of the three-membered ring
is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor
in correctly determining the proper ionization site (i.e., secondary amide ions are needed,
–
primary ones and OH lead to incorrect conclusions since they scramble the deuterium label). An
experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ±
–
1
–1
2
.8 kcal mol ) was combined with the ionization energy of hydrogen (313.6 kcal mol ) to
–
1
afford BDE = 93.0 ± 2.9 kcal mol . This enabled the effect of the phenyl substituent to be
3
2
evaluated and compared to other situations where it is attached to an sp - or sp -hybridized
carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out and a
–
1
revised C–H BDE for cyclopropane of 108.9 ± 1.0 kcal mol is recommended.
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Introduction
Carbon-based radicals are commonly formed reactive intermediates that play an important role
1
-4
in organic synthesis as well as in atmospheric, biological, combustion and industrial processes.
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Their energetics are commonly analyzed using carbon-hydrogen bond dissociation energies
(BDEs) and are widely used to rationalize the chemical reactivity and selectivity of free
5
radicals. As a result, BDEs are of considerable importance and continued interest. One of the
most accurate and reliable methods for determining these quantities is to exploit Hess' law and
employ the thermodynamic cycle illustrated in Figure 1. In this approach, experimental
measurements of the gas-phase deprotonation enthalpy of HX (ΔH˚acid) and the electron affinity
.
of X (EA) are combined with the well-known ionization energy of hydrogen atom (IE = 313.6
–1
kcal mol ) to obtain the BDE of HX.
BDE(HX)
•
•
HX
H + X
ΔH˚acid(HX)
•
–
e–
EA(X )
•
•
– IE(H )
+
H
+ X
H + X
e–
+
•
•
BDE(HX) = ΔH˚acid(HX) – IE(H ) + EA(X )
Figure 1. Negative ion thermodynamic cycle for the determination of a bond dissociation
energy via the measurement of a deprotonation enthalpy and an electron affinity; the third term is
–1
the ionization energy of hydrogen atom, and a value of 313.6 kcal mol is well established.
Styrene (PhCH=CH
2
) recently was investigated and its α-C–H BDE was found to be 100.1 ±
–
1 6
–1
3
.4 kcal mol . This value is 10.6 ± 3.5 kcal mol smaller than for ethylene and is a direct
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measure of the stabilizing effect of a phenyl group at an sp -hybridized carbon center; the high-
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–1
3
level computationally predicted G4 energy difference is 6.9 kcal mol . At an sp carbon a
phenyl ring has a larger impact. That is, BDE(CH
3
–H) – BDE(PhCH
2
–H) = 15.2 ± 0.6 kcal
0
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9
0
–1 5
–1
mol , and this difference compares favorably to a G4 theory prediction of 14.7 kcal mol .
–
1
These results reveal that there is a 4.6 ± 3.6 kcal mol experimentally determined difference and
–
1
a corresponding G4 value of 7.8 kcal mol for the differential stabilization of a phenyl
2
3
substituent at sp - and sp -hybridized carbon atoms.
Cyclopropane is comprised of three tetravalent carbon atoms each of which is bound to four
3
atoms. In an introductory organic chemistry course this commonly would lead to a sp
assignment for the carbon centers. It is well-known, however, that the carbon-carbon bonds in a
three-membered ring are bent and employ more p-character than in other such situations.
Bonding models developed by Coulson and Moffit as well as that by Walsh indicate that the
2
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hybridization of the carbon atoms in the C–H bonds of cyclopropane are sp . This assignment
1
3
is in accord with the enhanced acidity of cyclopropane, its strong C–H BDE, a large C–H
heteronuclear coupling constant of 161 Hz and the well-known enhanced reactivity of
1
0,11
cyclopropanes with electrophiles.
Consequently, phenylcyclopropane (1) offers an
2
opportunity to determine the effect of a phenyl substituent at a different type of sp -hybridized
carbon center. Experimental and computational results are reported herein and in the process the
–
1
acidity of 1 is resolved and the C–H BDE of cyclopropane is revised upwards by 3 kcal mol . It
is also shown how incorrect conclusions can be drawn regarding the favored ionization site of a
compound by mass spectrometry.
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Results and Discussion
–
–
–
Phenylcyclopropane was deprotonated by several different bases (NH
2
, Me
2
N , and HO with
–
1
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proton affinities of 403.4 ± 0.1, 396.4 ± 0.9 and 390.27 ± 0.02 kcal mol , respectively) and the
resulting conjugate base was found to abstract a proton from H
2
O. These results indicate that
–
1
ΔH˚acid(1) ~390 kcal mol and this value was refined by measuring the equilibrium acidity as
illustrated in equation 1. The forward (k
1
1
) and reverse (k-1) rate constants were found to be k =
k1
–
–
1-c-C H Ph + HO
1-c-C H Ph + H O (1)
3 4 2
3
5
k
–
1
K = k /k
1
–1
–
9
–11
3
–1 –1
2
.03 ± 0.05 x 10 and k-1 = 1.93 ± 0.31 x 10 cm molecule s , where the given uncertainties
are the standard deviations of the data. This leads to an equilibrium constant of 105 ± 17, but a
more conservative uncertainty of ± 100% was adopted for the subsequent derivation of the
acidity of 1. Given that ΔG˚ = -RTlnK and ΔG˚ = ΔG˚acid(1) - ΔG˚acid(H
2
O) for eq. 1, this leads to
–
1
12a
2
ΔΔG˚acid = -2.8 ± 0.6 kcal mol and can be combined with ΔG˚acid(H O) = 383.68 ± 0.02 kcal
–1
–1
mol to afford ΔG˚acid(1) = 380.9 ± 0.6 kcal mol . Computed entropies for phenylcyclopropane
(85.6 e.u.) and its conjugate base (86.9 e.u.) from G4 calculations lead to TΔS˚acid(1) = 8.2 kcal
–1
–1
mol . If one assumes an error of ± 2 e.u. for ΔS˚acid(1), then ΔH˚acid(1) = 389.1 ± 0.8 kcal mol
is obtained.
–
1
Benzene is 10.3 kcal mol more acidic than cyclopropane (ΔH˚acid = 401.2 vs 411.5 kcal
–1 12a,13
mol )
and consequently it is not immediately clear which of the six different sites in 1
corresponds to the most acidic position. To address this issue experimentally, 1-deuteriophenyl-
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–
–
cyclopropane (1D) was prepared as previously described and reacted with H
2
N , Me
2
N and
–
+ –
HO . Both amide and hydroxide anions abstracted a proton or a deuterium to afford (M – H )
+
–
and (M – D ) ions. The obvious indication is that there are two or more acidic positions in 1 but
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this ignores the beautiful mechanistic work carried out by Bierbaum and DePuy et al on
1
4
hydrogen-deuterium exchange. In particular, they showed that multiple proton transfers can
occur in a single collision between an anion and an acid, and that less acidic sites sometimes
undergo isotopic exchange. This takes place when the acid or the conjugate acid of the base
possesses two or more exchangeable hydrogens in a process that was termed extraordinary
–
–
exchange. The "obvious" inference from the labeling results with H
2
N and HO , consequently
–
could be incorrect as illustrated in Scheme 1. It is for this reason that Me
2
N was also used
because the conjugate acid of this secondary amide only has one exchangeable hydrogen and can
–
2
not undergo extraordinary exchange. That is, it can not lead to isotopic scrambling. When Me N
Ph
–
M – D+)–
(
–HOD
HOD
Ph
Ph
Ph
+
OH–
OH–
–
D
D
Ph
OH–
OD–
HOD
–
D
H
H
H
H O
2
–
–H O
–
2
D
D
M – H+)–
(
Scheme 1. Isotopic label scrambling mechanism for the deprotonation of 1-deuteriophenylcyclo-
propane.
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+ –
was reacted with 1D only the (M – D ) ion was observed revealing that the α-position of
phenylcyclopropane is the most acidic site in this compound. This result is in keeping with the
importance of charge delocalization in the gas phase and previous assumptions about the
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1
5,16
structure of 1a.
1
5
Several reports on the acidity of 1 have appeared previously. A bracketing result of 390 ± 3
–1
–1
kcal mol is in accord with a value of ~392 kcal mol based upon hydrogen-deuterium
exchange studies and both of these findings are in agreement with our more accurate
–
1
–1
determination of 389.1 ± 0.8 kcal mol . All three values differ by ~10 kcal mol , however, from
–1
16
an equilibrium measurement of 380.5 kcal mol by Peerboom et al. A plausible explanation for
the latter discrepancy is the presence of a small impurity of 1-phenyl-1-propene as this could
account for the extremely slow deprotonation observed with methoxide ion. To address this
further, M06-2X and G4 computations were carried out. Predicted values of 386.1 (M06-2X/aug-
–
1
cc-pVTZ//M06-2X/aug-cc-pVDZ) and 389.2 (G4) kcal mol were obtained and are consistent
with the higher values (i.e., less acidic determinations); isodesmic reactions using methane,
ethylene, and cyclopropane as the reference give M06-2X deprotonation enthalpies of 387.2,
–
1
388.4 and 385.8 kcal mol . G4 theory has been shown to be especially accurate and reproduces
–1 7
a large well-established energetic data set with an average error of 0.8 kcal mol . In this
–
1
instance our experimental determination is reproduced within 0.1 kcal mol . The relative
stabilities of the different conjugates bases of 1 were also calculated as indicated in Figure 2 and
(
10.7)
(11.2)
(
6.3)
H (10.7)
H (11.4)
(0.0) H
–
1
Figure 2. Computed G4 relative deprotonation enthalpies in kcal mol for the six different
acidic sites in phenylcyclopropane.
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the α-carbanion is found to be the most stable structure as one would expect. This is in accord
with our deuterium-labeling study and that of DePuy et al. but inconsistent with suggestions by
–
Peerboom et al. that deprotonation with OH takes place at the ortho position of the aromatic
0
1
2
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0
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9
0
1
2
3
4
5
6
7
8
9
0
1
5,16
–1
ring.
That is, this pathway is predicted by G4 theory to be endothermic by 5.2 kcal mol and
thus energetically unfavorable.
The findings above enabled us to determine the substituent effect of a phenyl group on the
acidity of phenylcyclopropane at the α- and β-positions as well as the impact of the three-
membered ring on benzene. The results are summarized in Table 1 and the G4 values indicate
Table 1. Experimental and Computed Phenylcyclopropane Acidities and Substituent
a
Effects.
cmpd
ΔH˚acid
substituent effect
b
b
expt
calc
expt
calc
c
C
6
H
6
401.2 ± 0.2 399.4 (400.6)
d
c-C
3
H
6
411.5 ± 2.0 411.8 (413.2)
c
PhCH
3
382.3 ± 0.5 379.6 (382.8)
3 5
1-Ph-c-C H
α
389.1 ± 0.8 386.1 (389.2) 22.4 ± 2.2
398.5 (399.9)
25.7 (24.0)
13.3 (13.3)
12.5 (12.6)
4.5 (5.1)
β
β
cis
399.3 (400.6)
trans
o-
m-
p-
394.9 (395.5)
399.1 (399.9)
0.3 (0.7)
399.7 (400.4)
-0.3 (0.2)
a
–1 b
All values in kcal mol . These results correspond to M06-2X/aug-cc-pVTZ//M06-2X/aug-cc-
c
d
pVDZ and G4 (in parentheses) energies. Ref. 12. Ref. 13.
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–1
that a cyclopropyl group can enhance the acidity of benzene by up to 5.1 kcal mol whereas a
–
1
phenyl ring increases the acidity of cyclopropane by 12.6 to 24.0 kcal mol . The much larger
effect in the latter case (eq. 2) is significantly less than in the acyclic example given in eq. 3 and
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+
+
–
(2)
–
ΔH˚ = 22.4 ± 2.2 kcal mol–1
2
5.7 (M06-2X), 24.0 (G4)
–
–
PhCH2 + CH4
PhCH3 + CH3 (3)
ΔH˚ = 34.4 ± 0.9 kcal mol–1
3
6.1 (M06-2X), 36.5 (G4)
can be attributed to differential electron delocalization. To assess the importance of resonance
stabilization, the phenyl rotation transition structures for 1a and benzyl anion were
computationally located. In the former case, barriers of 13.7 (M06-2X/aug-cc-pVTZ//M06-
–
1
2
X/aug-cc-pVDZ) and 10.8 (G4) kcal mol were obtained and account for only half of the
stabilization due to the phenyl group. That is, additional factors such as polarization and the
inductive effect when taken together appear to be as important as the resonance stabilization of
1
a; conjugation between the three-membered ring and the phenyl group also plays a role in the
transition structure but it is worth noting that the rotation barrier for 1 is 1.3 (M06-2X/aug-cc-
–
1
pVTZ//M06-2X/aug-cc-pVDZ) and 0.9 (G4) kcal mol . For benzyl anion the rotation barrier is
significantly larger (25.1 M06-2X/aug-cc-pVTZ//M06-2X/aug-cc-pVDZ and 23.2 (G4) kcal
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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3
3
3
3
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
–1
mol ) and the apparent contribution due to resonance delocalization is roughly two thirds of the
substituent effect.
The electron affinity of 1-phenylcyclopropyl radical (1r) was measured by reacting its
corresponding anion with reference reagents of known electron affinity and monitoring the
occurrence or nonoccurrence of electron transfer. Sulfur dioxide and chloropentafluorobenzene
both were found to abstract an electron from 1a whereas o-trifluoromethylcyanobenzene and
weaker reagents did not (Table 2). These results bracket the electron affinity of 1r between the
values for chloropentafluorobenzene and o-trifluoromethylcyanobenzene, so EA(1r) = 0.76 ±
0
1
2
3
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–1
17
0
.12 eV (17.5 ± 2.8 kcal mol ) is assigned. Direct M06-2X/aug-cc-pVTZ//M06-2X/aug-cc-
–
1
pVDZ and G4 computations of this quantity give 23.6 and 22.9 kcal mol , respectively.
Alternatively, if one calculates the electron affinity difference between phenylcyclopropyl and
benzyl radicals and uses the experimental value for the latter compound (21.03 ± 0.14 kcal
–
1
18
–1
mol ), then predictions of 21.4 (M06-2X) and 22.2 (G4) kcal mol are obtained. These results
imply that the electron affinity of 1r is at the high end of the experimental range.
Table 2. Electron Affinity Bracketing Results for 1-Phenylcyclopropyl Radical.
a
ref. cmpd.
SO
EA (eV)
electron transfer
2
1.107 ± 0.008 yes
C
6
F
5
Cl
o-CF
CS
0.82 ± 0.11
0.70 ± 0.10
0.51 ± 0.10
0.46 ± 0.20
0.22 ± 0.10
yes
no
no
no
no
3
6 4
C H
2
COS
NO
a
All values come from ref. 12b.
1
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Hess' law as illustrated in Figure 1 along with the acidity of 1 and the electron affinity of 1r
determined above can be used to derive the C–H BDE of phenylcyclopropane at the α-position.
–1
A value of 93.0 ± 2.9 kcal mol is obtained which is in reasonable accord with direct predictions
10
11
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–
1
of 97.0 (M06-2X/aug-cc-pVTZ//M06-2X//M06-2X-aug-cc-pVDZ) and 97.4 (G4) kcal mol and
–
1
isodesmic comparisons of 96.7 (M06-2x) and 97.4 (G4) kcal mol (eq. 4). To assess the impact
of the phenyl group on this quantity, it can be compared to the BDE of cyclopropane (106.3 ±
–
1 11
–1
0
.3 kcal mol ) and a difference of 13.3 ± 2.9 kcal mol is obtained. This compares favorably
+
PhCH3
+ PhCH2
(4)
ΔH˚ = BDE (PhCH –H) – BDE (1)
2
to M06-2Xaug-cc-pVTZ//M06-2X//M06-2X-aug-cc-pVDZ) and G4 predictions of 10.3 and 10.8
–
1
kcal mol , respectively. In this case this difference appears to be largely due to electron
delocalization in that the rotation barrier of the phenyl group in 1r is 9.2 (M06-2Xaug-cc-
–
1
pVTZ//M06-2X//M06-2X-aug-cc-pVDZ) and 7.8 (G4) kcal mol . Similar effects were noted for
–
1
toluene and styrene, but the impact of the aromatic ring is 4 kcal mol larger in the former case
–
1
6
and 3 kcal mol smaller in the latter one (Table 3). These results are surprising in that phenyl
substitution has the largest impact on methane and the smallest effect on ethylene even though
4 3 6 2
the C–H BDEs follow the order, CH < c-C H < CH=CH . That is, greater stabilization occurs
for the more stable radicals (as indicated by their BDEs). This trend can be rationalized by
resonance stabilization of the corresponding radicals in that it correlates to their computed C–Ph
rotation barriers.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
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4
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5
5
5
5
5
5
5
5
6
–1
Table 3. Bond Energies and C–Ph Rotational Barriers in Kcal Mol .
RCH
3
1-R-c-C
3
H
5
RCH=CH
2
a
b
a
c
BDE (R = H)
BDE (R = Ph)
ΔBDE
104.99 ± 0.03 106.3 ± 0.3 110.7 ± 0.6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
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2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
89.8 ± 0.6
93.0 ± 2.9
13.3 ± 2.9
10.3, 10.8
100.1 ± 3.4
10.6 ± 3.5
7.5, 6.9
15.2 ± 0.6
ΔBDE (M06-2X, G4) 13.9, 14.7
rotation barrier
(M06-2X, G4)
11.1, 10.9
9.2, 7.8
5.1, 3.2
a
b
c
Ref. 5. Ref. 11. Ref. 6.
Previous G3 and W1 calculations indicate that the experimental C–H BDE of cyclopropane is
–1
19
3
kcal mol too small. To address this further, four different computational approaches were
employed (i.e., M06-2X/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ, G4
and W1BD) and the BDE was obtained directly and by the isodesmic reactions illustrated in eqs.
5
and 6. These results along with new G3 and W1 determinations via eqs. 5 and 6 are
summarized in Table 4 and lead us to recommend a C–H BDE for cyclopropane of 108.9 ± 1.0
–
1
–1
kcal mol which corresponds to a 2.6 kcal mol upward revision of the currently accepted
17
value.
+
CH3
+ CH4
(5)
ΔH˚ = BDE (c–C H ) – BDE (CH )
3
6
4
+
CH =CH
+ CH =CH
2
(6)
2
2
ΔH˚ = BDE (c–C H ) – BDE (C H )
3
6
2 4
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Table 4. Computed C–H BDEs for Cyclopropane Using Different Theoretical Methods and
Chemical Reactions.
–1
method
BDE(c-C
direct
3
H
5
–H) (kcal mol )
10
11
12
13
14
15
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21
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a
eq. 5 eq. 6 avg.
M06-2X/aug-cc-pVTZ
107.2
108.3 109.1 108.2
109.5 108.8 108.9
110.0 109.6 109.6
108.7 109.0 108.6
109.7 109.3 109.3
109.0 108.8 108.9
109.2 109.1 108.9
b
CCSD(T)/aug-cc-pVQZ 108.5
G3
109.2
108.2
109.0
109.0
108.5
G4
W1
W1BD
avg.
a
b
5•
•
Direct = c–C
3
H
6
→ c–C
3
H
+ H . CCSD(T)/aug-cc-pVTZ optimized structures and unscaled
M06-2X/aug-cc-pVTZ vibrational frequencies for zero-point energies and thermal corrections to
298 K were used.
Conclusion
Phenylcyclopropane is slightly more acidic than water in the gas phase and as in solution the
α-position is most labile. The resulting benzylic anion is stabilized by electron delocalization
over the aromatic ring but this accounts for only half of the phenyl substituent effect. By
measuring the electron affinity of 1-phenylcyclopropyl radical and applying Hess' law, the C–H
–
1
BDE of 1 was found to be 93.0 ± 2.9 kcal mol . Resonance stabilization plays a larger role in 1r
and accounts for the observed phenyl substituent effect order: PhCH > c-C Ph > PhCH=CH
High level calculations also indicate that the experimental C–H BDE of cyclopropane is too
3
3
H
5
2
.
–
1
–1
small by 2.6 kcal mol and we recommend a value of 108.9 ± 1.0 kcal mol for this quantity
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based upon very accurate computational methodologies. An experimental redetermination of this
bond energy, however, clearly is warranted.
Experimental
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All reagents were obtained from commercial sources and used as supplied except for 1-
deuteriophenylcyclopropane. This isotopically labeled compound was prepared by treating
phenylcyclopropane with sodium hydride in DMSO-d at 35 ˚C for 5 days as previously
6
2
0
described.
Negative ions were generated by electron ionization in a Fourier transform mass spectrometer
(FTMS) equipped with a 3 T magnet and subsequently were allowed to react with 1 or 1D. The
+
–
+ –
–5
resulting (M – H ) or (M – D ) ions were cooled with a pulse of argon to a pressure of ~10
Torr and transferred to the second cell of the dual cell instrument. Another argon pulse up to
–
5
~
10 Torr was employed and the ion of interest was mass isolated using a combination of a
2
1
22
stored−waveform inverse Fourier transform (SWIFT) and/or chirp excitation pulses.
Reactions were monitored using constant amounts of the reagents of interest as a function of
time. For the forward direction of the equilibrium acidity determination of 1, the anionic base
–
(
HO ) was formed from water and transferred to the second reaction region where it was allowed
to react with phenylcyclopropane.
Computations were carried out with Gaussian 09 at the Minnesota Supercomputer Institute
2
3
24
7
for Advanced Computational Research. Both M06-2X and G4 calculations were performed on
1
, all six of its conjugate bases (i.e., the 1-, 2- (cis and trans), and o-, m- and p-anions) and 1-
phenylcyclopropyl radical (1r). Geometry optimizations and vibrational frequencies with the
former method and the aug-cc-pVDZ basis set were carried out, and single point energies were
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obtained with the larger aug-cc-pVTZ basis set. For methane, ethylene, cyclopropane, methyl,
vinyl and cyclopropyl radicals, M06-2X and CCSD(T) structures were also computed with the
aug-cc-pVTZ basis set, and in the former cases vibrational frequencies were computed as well.
Coupled cluster single point energies (i.e., CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ)
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11
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2
5
and the G4 and W1BD methods were employed too. All of the resulting energetic quantities are
given at 298 K.
ACKNOWLEDGMENT. Generous support from the National Science Foundation (CHE-
1
361766) and the Minnesota Supercomputer Institute for Advanced Computational Research are
gratefully acknowledged.
ꢀ
Supporting Information Available: Computed structures and energies are provided as is the
complete citation to reference 23 (12 pages). This material is available free of charge via the
internet at http://pubs.acs.org.
Competing financial interests: The authors declare no competing financial interests.
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(
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(
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(
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(
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