Reactivity and thermochemistry of quadricyclane in the gas phase

Article abstract of DOI:10.1021/ja9540278

The gas-phase acidity of quadricyclane C₇H₈ has been determined using a flowing after-glow selected ion flow tube at room temperature. Measurements of forward and reserve rate constants for the proton transfer reaction B^- + C₇H₈ ? C₇H₇^- + BH where BH = NH₃, CH₃NH₂ and C₆H₆ give Δ_acidG₃₀₀(C₇H₈) = 394.7±0.8 kcal/mol and Δ_acidH₃₀₀(C₇H₈) = 403.0±1.1 kcal/mol. Combining this value with the electron affinity, 0.868±0.006 eV, of the quadricyclyl radical, gives 109.4±1.3 kcal/mol for the C-H bond dissociation energy of quadricyclane at Cl. Reactions of quadricyclane ion with various reagents have been studied, and branching ratios of product channels have been determined. These results along with ab initio calculations and a companion photoelectron spectroscopy study, indicate that there are two isomers of quadricyclane ion which have similar energies. The gas-phase acidity of quadricyclane, C₇H₈, has been determined using a flowing afterglow-selected ion flow tube at room temperature. Measurements of forward and reverse rate constants for the proton transfer reactions B^- + C₇H₈ ? C₇H₇^- + BH where BH = NH₃, CH₃NH₂, and C₆H₆ give Δ(acid)G₃₀₀(C₇H₈) = 394.7 ± 0.8 kcal/mol and Δ(acid)H₃₀₀(C₇H₈) = 403.0 ± 1.1 kcal/mol. Combining this value with the electron affinity, 0.868 ± 0.006 eV, of the quadricyclyl radical, gives 109.4 ± 1.3 kcal/mol for the C-H bond dissociation energy of quadricyclane at C1. Reactions of quadricyclanide ion with various reagents have been studied, and branching ratios of product channels have been determined. These results, along with ab initio calculations and a companion photoelectron spectroscopy study, indicate that there are two isomers of quadricyclanide ion which have similar energies.

Full text of DOI:10.1021/ja9540278

5068
J. Am. Chem. Soc. 1996, 118, 5068-5073
Reactivity and Thermochemistry of Quadricyclane in the Gas
Phase
Hack Sung Lee, Charles H. DePuy,* and Veronica M. Bierbaum*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Colorado,
Boulder, Colorado 80309-0215
ReceiVed NoVember 30, 1995X
Abstract: The gas-phase acidity of quadricyclane, C7H8, has been determined using a flowing afterglow-selected
ion flow tube at room temperature. Measurements of forward and reverse rate constants for the proton transfer
-
reactions B- + C7H8 a C7H7 + BH where BH ) NH3, CH3NH2, and C6H6 give ∆acidG300(C7H8) ) 394.7 ( 0.8
kcal/mol and ∆acidH300(C7H8) ) 403.0 ( 1.1 kcal/mol. Combining this value with the electron affinity, 0.868 (
0.006 eV, of the quadricyclyl radical, gives 109.4 ( 1.3 kcal/mol for the C-H bond dissociation energy of
quadricyclane at C1. Reactions of quadricyclanide ion with various reagents have been studied, and branching
ratios of product channels have been determined. These results, along with ab initio calculations and a companion
photoelectron spectroscopy study, indicate that there are two isomers of quadricyclanide ion which have similar
energies.
Introduction
dependence of the vibrational spectra and the phase transition
of quadricyclane in the solid phase have also been studied.10
The accompanying paper (J. Am. Chem. Soc. 1996, 118, 5074)11
provides a good summary of recent literature on several isomers
of C7H8 and C7H7.
Quadricyclane (tetracyclo[3.2.0.0.2,70.4,6]heptane) is a highly
strained member (1) of the C7H8 family of hydrocarbon
compounds which include norbornadiene (2), toluene (3), and
cycloheptatriene (4) among others. The strain energy of
In contrast to norbornadiene, toluene, and cycloheptatriene,
very little is known about the gas-phase chemistry and ther-
mochemistry of quadricyclane and its anion. In this paper, we
report the gas-phase acidity of quadricyclane, and by use of
the electron affinity of the quadricyclyl radical reported in the
accompanying paper (J. Am. Chem. Soc. 1996, 118, 5074)11
we have determined the C-H bond dissociation energy. These
values are of interest because there is little information available
on the effect of strain on C-H bond dissociation energies, and
quadricyclane is a highly strained molecule. We have also
studied the reactivity of quadricyclanide ion with various
reagents, including CS2, COS, CO2, N2O, CH3Cl, and (C2H5)2O.
We have carried out ab initio calculations to assess the relative
stability of two quadricyclanide ion structures, their electron
binding energies and their entropies of acidity. These studies
provide a comprehensive overview of the chemistry and
thermochemistry of quadricyclane in the gas phase.
quadricyclane is evident in a comparison of the heats of
formation of these compounds which are 80 ( 1, 57.0 ( 1.0,
12.0 ( 0.1, and 43.7 ( 0.2 kcal/mol, respectively.1,2 Quadri-
cyclane is formed by rapid ring-closure of norbornadiene when
irradiated by ultraviolet light; it is relatively stable at room
temperature but reverts to norbornadiene when heated to 140°.3,4
Due to these properties, quadricyclane has been investigated as
a candidate for solar-to-thermal energy conversion systems.5,6
Quadricyclane radical cation has been studied by time-resolved
EPR spectroscopy in solution7 and by low-temperature EPR
spectroscopy in matrices.8,9 The temperature and pressure
Experimental Section
Reactions were studied at room temperature with our tandem flowing
afterglow-selected ion flow tube (FA-SIFT), which has been described
in detail elsewhere.12 Negative ions are created by electron impact
ionization and ion-molecule reactions in the source flow tube at helium
pressures of 0.2 Torr. Ions are selected by a quadrupole mass filter
and injected into the reaction flow tube at 0.5 Torr helium pressure.
The gases and liquids used in this experiment are as follows:
quadricyclane (Aldrich, 99%), ammonia (Matheson, Anhydrous 99.99%),
monomethylamine (Matheson, 99.5%), benzene (J. T. Baker, 99.9%),
and ammonia-d3 (Cambridge Isotope Laboratories, 99% d). Proton
NMR spectra of the quadricyclane sample indicates that there is a minor
X Abstract published in AdVance ACS Abstracts, May 1, 1996.
(1) Rogers, D. W.; Choi, L. S.; Girellini, R. S.; Holmes, T. J.; Allinger,
N. L. J. Phys. Chem. 1980, 84, 1810.
(2) NIST Standard Reference Database 19B: Negative Ion Energetics
Database Version 3.01 1993, program written and data compiled by
Bartmess, J. E. This compilation is based on original data from Gas-Phase
Ion and Neutral Thermochemistry; 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. No. 1.
(3) Hammond, G. S.; Turro, N. J.; Fischer, A. J. Am. Chem. Soc. 1961,
83, 4674.
(4) Dauben, W. G.; Cargill, R. L. Tetrahedron 1961, 15, 197.
(5) Canas, L. R.; Greenberg, D. B. Solar Energy 1985, 34, 93.
(6) Arai, T.; Oguchi, T.; Wakabayashi, T.; Tsuchiya, M.; Nishimura,
Y.; Oishi, S.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1987, 60,
2937.
(7) Ishiguro, K.; Khudyakov, I. V.; McGarry, P. F.; Turro, N. J. J. Am.
Chem. Soc. 1994, 116, 6933.
(9) Cromack, K. R.; Werst, D. W.; Barnabas, M. V.; Trifunac, A. D.
Chem. Phys. Lett. 1994, 218, 485.
(10) Kawai, N. T.; Gilson, D. F. R.; Butler, I. S. J. Phys. Chem. 1992,
96, 8556.
(11) Gunion, R. F.; Karney, W.; Wenthold, P. G.; Borden, W. T.;
Lineberger, W. C. J. Am. Chem. Soc. 1996, 118, 5074.
(12) Van Doren, J. M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V. M.
Int. J. Mass Spectrom. Ion Proc. 1987, 81, 85.
(8) Barnabas, M. V.; Werst, D. W.; Trifunac, A. D. Chem. Phys. Lett.
1993, 206, 21.
S0002-7863(95)04027-3 CCC: $12.00 © 1996 American Chemical Society

Quadricyclane in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 21, 1996 5069
Table 1. Rate Constants, Equilibrium Constants and Gas-Phase Acidity Values for Quadricyclane Relative to Three Bases at 300 K
kf
B- + C7H8 {
\
} C7H7- + BH
kr
BH
NH3
∆
acidG(BH) (kcal/mol)
k (cm3/s)
Keq
∆
acidG(C7H8) (kcal/mol)
395.0 ( 0.5
396.4 ( 0.4a
kf ) 8.1 ( 0.2 (-10)b
kr ) 8.0 ( 1.0 (-11)
kf ) 7.2 ( 0.8 (-10)
kr ) 9.7 ( 0.6 (-11)
kf ) 2.2 ( 0.8 (-12)
kr ) 3.2 ( 0.4 (-11)
10.1 ( 1.3
CH3NH2
C6H6
395.8 ( 0.5c
392.9 ( 0.4d
7.4 ( 0.9
394.6 ( 0.6
394.5 ( 0.6
0.067 ( 0.027
a References 28 and 29. b 8.1 (-10) represents 8.1 × 10-10
.
c This work. d Reference 14.
norbornadiene impurity (<1%) but no detectable toluene impurity
(<0.1%). Amide ions, NH2-, are created in the source by electron
impact on ammonia, then mass selected by a quadrupole mass filter
and allowed to react with quadricyclane in the reaction flow tube. For
the reverse reaction, quadricyclanide ion is created in the source by
reaction of amide ion and quadricyclane and then mass selected and
injected into the reaction flow tube and allowed to react with ammonia.
Care must be taken in the creation of quadricyclanide ion in the source,
since isomers of lower energy might be formed, as indicated by the
slight upward curvature in the plot of logarithm of the signal Vs the
reaction distance. The effect of more slowly reacting isomers can be
minimized, and accurate reaction rate coefficients can be obtained by
kinetic analysis at early reaction times and small reagent flow rates.
The acidity of quadricyclane was also measured relative to mono-
methylamine and benzene. We have re-examined the gas-phase acidity
of monomethylamine with our SIFT instrument; this value was
previously determined in flowing afterglow studies.13 We have
measured the proton transfer rate coefficients for the forward reaction
of amide ion with monomethylamine and reverse reaction of mono-
methylamide ion with ammonia. Amide ion was created as described
above, and CH3NH- ion was created in the source by the reaction of
amide ion with monomethylamine (direct electron impact on CH3NH2
was not very efficient in producing CH3NH-). The measured rate
coefficients are kf ) 1.1 (( 0.1) × 10-9 cm3/s and kr ) 4.0 (( 0.3) ×
10-10 cm3/s, resulting in Keq ) 2.8 ( 0.3 and ∆rxnG ) -0.6 ( 0.1
kcal/mol. For the reactions with quadricyclane, the same method was
used to create CH3NH- ion in the source.
experiments demonstrate that quadricyclanide ion retains its structure
in the source and does not readily isomerize to norbornadienide anion
or benzyl anion.
The number of acidic sites in quadricyclane was probed by H/D
exchange reactions with ND3. Electron transfer reactions of quadri-
cyclanide ion with SO2, CS2, and O2 were studied to estimate the
electron affinity (EA) of the C7H7 radical. Reactions with CS2, COS,
CO2, and N2O were examined and found to give numerous products
including adducts. Nucleophilic substitution (SN2) reactions with CH3-
Cl and elimination (E2) reactions with (C2H5)2O were also studied and
rate coefficients were measured.
Results and Discussion
The gas-phase acidity of quadricyclane has been obtained
from the equilibrium constant, Keq, by measuring forward (kf)
and reverse (kr) reaction rate coefficients with compounds of
known acidity (eq 1), i.e., Keq ) kf/kr.
kf
B- + C7H8 {
\
} C7H7- + BH
(1)
kr
The values for the acidity of quadricyclane, ∆acidG, along with
forward/reverse reaction rate coefficients determined using three
bases are shown in Table 1. Note that the acidities of
monomethylamine and benzene were determined using ammonia
as an anchor base. From the known ∆acidG (B) of these three
bases, ∆acidG (QC) of quadricyclane can be determined (eq 2).
Since the gas-phase acidity of benzene was re-measured recently,14
we employ this value in our study. For the reactions with quadricy-
clane, phenide ion, C6H5-, was created in the source from reaction of
amide ion with benzene. For all the reactions studied, small amounts
of HO- ion were formed in the downstream flow tube by reaction with
water impurity, since the quadricyclanide ion and NH2-, CH3NH-, and
∆
acidG (QC) ) ∆acidG (B) - RT ln Keq
(2)
The three values of ∆acidG fall within experimental uncertainties;
an average value ∆acidG ) 394.7 ( 0.8 kcal/mol is reported.
We have studied H/D exchange reactions of quadricyclanide
ion with ND3 to determine the number of acidic sites in
quadricyclane. Up to five H/D exchanges were readily ob-
served. These results suggest that there are six acidic sites and
that the four equivalent hydrogens at the four-membered ring
[sites C1, C5, C6, and C7 in 1], and the two equivalent
hydrogens at the three-membered rings [sites C2 and C4 in 1]
have similar acidities. Thus, structures 5 and 6 are predicted
to have similar stability; there is no evidence for the formation
of structure 7. An alternative explanation for the H/D exchange
results is that the anion rapidly rearranges to scramble the 1-
and 2-positions. However, skeletal rearrangements of anions
are not common.
-
C6H5 ions are more basic than hyroxide ion.
-
When hydroxide ion, HO-, is used as a precursor to create C7H7
ion in the source, quite different reactivity was observed. With this
mode of preparation, C7H7- ion is primarily benzyl anion, as confirmed
by studies of its chemical reactivity and by parallel photoelectron
studies.11 For example, the reaction rate with water is 3 orders of
magnitude slower than the collision rate. Reaction of this ion with
D2O results in only 2 H/D exchanges, confirming that this ion is indeed
benzyl anion. The benzyl anion is presumably formed in the source
by an extremely slow proton transfer-isomerization reaction via a long-
-
lived collision complex: HO- + (1) f C6H5CH2 + H2O. This
reaction is exothermic by 78 kcal/mol. Collision induced dissociation
(CID) studies of quadricyclanide, norbornadienide, and benzyl anion
were carried out by raising the SIFT injector plate to 75 V relative to
the ion source. CID of the quadricyclanide ion, generated from the
reaction of amide and quadricyclane, forms acetylide ion (74%) as the
major product, with smaller amounts of C5H5- (18%) and C3H3- (8%).
CID of the norbornadienide ion was distinctly different, forming
acetylide ion (98%) almost exclusively with only a trace of C5H5- (2%);
there was no evidence for production of C3H3-. Injection of the benzyl
anion under these conditions did not produce fragment ions. These
(13) Mackay, G. I.; Hemsworth, R. S.; Bohme, D. K. Can. J. Chem.
1976, 54, 1624.
(14) Davico, G. E.; Bierbaum, V. M.; DePuy, C. H.; Ellison, G. B.;
Squires, R. R. J. Am. Chem. Soc. 1995, 117, 2590.
The existence of six sites in quadricyclane of similar acidity
is corroborated by our studies of the reaction of amide ion with

5070 J. Am. Chem. Soc., Vol. 118, No. 21, 1996
Lee et al.
dideuterated quadricyclane (8). This reaction proceeds by 63%/
25.0 cal/mol K ) 2.5 cal/mol K, we obtain ∆rxnH ) -0.9 kcal/
mol for reaction 7. ∆fH(C7H7-) is determined to be 117.0 kcal/
mol using ∆fH(NH2-) ) 26.9 kcal/mol and ∆fH(NH3) ) -11.0
kcal/mol. In these estimations, ∆fH(C7H8) is assumed to be 80
kcal/mol; however, this value is not needed in determining the
upper limit of the EA, since it cancels out in the calculation.
Hence EA(C7H7) is less than 21.6 kcal/mol (0.937 eV). Another
way to estimate the EA is from bracketing reactions (Table 4b);
electron transfer from quadricyclanide ion to SO2 (EA ) 1.107
eV)17,18 is fast; however, no electron transfer was observed to
O2 (EA ) 0.451 eV).19 These reactions indicate that the EA
of quadricyclyl radical is greater than 0.451 eV and less than
1.107 eV. Electron transfer to CS2 (EA ) 0.51 eV)20 was not
observed but the presence of other rapid reaction channels
precludes any firm conclusions from this system. These
estimates were helpful in the analysis of spectra in a concurrent
photoelectron spectroscopy (PES) study. From the PES study,11
two different EAs for quadricyclane are deduced, 0.868(6) and
0.962(6) eV. These two values probably correspond to two
different isomers of structure 5 and 6.
To estimate the relative energies of the quadricyclanide ions
(5) and (6), we have performed ab initio calculations: MP3
(frozen core)/6-31++G** single point energy with fully
optimized geometries and vibrational frequencies at the RHF/
6-31++G** level (Table 2). The HF energy, MP3 energy, and
zero point energy of quadricyclanide ion are -268.96181,
-269.97073, and 0.12109 Hartrees for 5 and -268.96156,
-269.97084, and 0.12074 Hartrees for 6, respectively. The
energy of 5 is greater than that of 6 by less than 0.3 kcal/mol
including zero point energies; thus the stability of these two
isomers is almost identical. The calculations show that structure
7 is a transition state.
For the electron affinities, calculations of the two quadricyclyl
radical structures have been performed: MP3 (frozen core)/6-
31++G** single point energy with fully optimized geometries
and vibrational frequencies at the UHF/6-31++G** level. The
HF energy, MP3 energy, and zero point energy of quadricyclyl
radicals are -268.98626, -269.95620, and 0.12422 Hartrees
for 5 and -268.98624, -269.95390, and 0.12412 Hartrees for
6, respectively. The calculated electron binding energies (eBE)
are eBE(5) ) 0.40 eV, eBE(6) ) 0.46 eV without zero point
energies, and eBE(5) ) 0.48 eV, eBE(6) ) 0.54 eV with 90%
zero point energy corrections. The 0.06 eV energy difference
is almost entirely due to higher correlation energy of the
quadricyclyl radical 5 compared to that of the radical 6. These
calculated eBEs are much lower than the experimental values.
Another approach for evaluating eBEs is Koopmans' theorem
which states that the electron binding energy of a closed-shell
molecule is equal to the negative of the highest occupied orbital
energy. Using this theorem, we obtain 1.61 and 1.74 eV for
the eBEs of anions 5 and 6, respectively. In this case, the eBEs
are considerably higher than the experimental values, but the
order is the same as from the previous estimation using the total
energy. These estimates suggest that radical 6 has a higher EA
than radical 5.
37% loss of H+/D+; these values are close to the statistical ratios
(67%/33% loss of H+/D+) in a system containing four hydrogens
and two deuteriums of comparable acidity. It should be
mentioned that, for norbornadienide ion reacting with ND3, three
H/D exchanges have been observed.15
To estimate the entropy of acidity, we employed ab initio
calculations. The geometries and energies of quadricyclane (1)
and quadricyclanide ion (5, 6) were calculated using Gaussian-
9216 with MP3 (frozen core)/6-31++G**//RHF/6-31++G**
basis sets. The rotational constants and vibrational frequencies
(uncorrected) were used to calculate the entropy change of
deprotonation. The entropies of 1, 5, and 6 are 67.9, 69.4, and
69.3 cal/mol K, respectively, at 300 K (Table 2). Using the
entropy of the proton, 26.0 cal/mol K, we obtain ∆acidS (5) )
27.5 cal/mol K and ∆acidS (6) ) 27.4 cal/mol K (eq 3).
-
∆
acidS ) S(C7H7 ) + S(H+) - S(C7H8)
(3)
The enthalpy change at 300 K was then obtained using eq 4.
∆
acidH ) ∆acidG + T∆acidS
(4)
The acidity at 0 K (∆acidH0) was calculated using eq 5.
T
-
∆
acidH0 ) ∆acidH -
{C (C H ) + C (H+) -
∫
T
p
7
7
p
0
Cp(C7H8)} dT (5)
where Cp is the heat capacity. The heat capacities can be
expressed as a function of temperature using rotational constants
and vibrational frequencies obtained from ab initio calcula-
tions.14 The integral term is 1.6 kcal/mol for the quadricyclanide
ion 5. The C-H bond dissociation energy (BDE) can be
calculated using eq 6, once EA(A) is known
BDE(A-H) ) ∆acidH(AH) - IP(H) + EA(A)
(6)
where IP is the ionization energy of hydrogen atom (313.6 kcal/
mol).
To estimate the upper limit of the EA of the C7H7 radical,
the energetics of several reactions were compared. When O-
ion (heat of formation ∆fH ) 25.8 kcal/mol) reacts with
quadricyclane, the C7H6- ion as well as HO- ion (∆fH ) -32.8
kcal/mol) are formed. Exothermic formation of the hydroxide
ion and quadricyclyl radical requires that ∆fH(C7H7) be less
than 138.6 kcal/mol (Table 4a). To estimate the heat of
formation of C7H7- ion, consider the enthalpy change of reaction
7.
Since sites C1 and C2 in quadricyclane have similar acidity,
but the electron binding energies of the resulting anions 5 and
6 differ by almost 0.1 eV, the C-H bond dissociation energies
at C1 and C2 must differ by this amount. Table 3 summarizes
the acidities, ∆acidG, ∆acidS, and ∆acidH, and C-H bond
NH2- + C7H8 a C7H7- + NH3
(7)
From the free energy change ∆rxnG ) -1.7 kcal/mol and the
entropy change ∆rxnS ) ∆acidS (C7H8) - ∆acidS (NH3) ) 27.5-
(15) Lee, R. E.; Squires, R. R. J. Am. Chem. Soc. 1986, 108, 5078.
(16) Frisch, M. J.; Tucker, G. W.; Head-Gordon, M.; Gill, P. M. W.;
Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M.
A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley,
J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.;
Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92, Revision C; Gaussian Inc.:
Pittsburgh, PA, 1992.
(17) Nimlos, M. R.; Ellison, G. B. J. Phys. Chem. 1986, 90, 2574.
(18) Grabowski, J. J.; Van Doren, J. M.; DePuy, C. H.; Bierbaum, V.
M. J. Chem. Phys. 1984, 80, 575.
(19) Travers, M. J.; Cowles, D.; Ellison, G. B. Chem. Phys. Lett. 1989,
164, 449.
(20) Kebarle, P.; Chowdhury, S. Chem. ReV. 1987, 87, 513.

Quadricyclane in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 21, 1996 5071
Table 2. Parameters of Quadricyclane 1 and Quadricyclanide Ion 5 and 6 as Determined from ab Initio Calculations using Gaussian-92b
units
C7H8 (1) (C2V)
C7H7- (5) (C1)
C7H7- (6) (C1)
HF energy
MP3 energy
zero point energy
Hartrees
Hartrees
Hartrees
-269.63632
-270.63748
0.13751
-268.96181
-268.96156
-269.97073
-269.97084
0.12109
0.12074
rotational constants GHz
symmetry number
4.466, 4.407, 3.285
2
4.559, 4.517, 3.334
1
69.4
412.6, 564.7, 690.3, 743.3, 768.2, 417.8, 574.3, 719.7, 756.5, 788.6,
784.6, 809.2, 855.4, 888.7, 934.5, 790.2, 818.3, 851.6, 864.6, 936.5,
4.563, 4.463, 3.370
1
69.3
entropy
cal/mol K 67.9
vibrational
cm-1
(A1)
797.2, 878.1, 983.3, 1037.6, 1095.0,
frequenciesa
1197.3, 1414.1, 1502.1, 1621.4, 3188.2, 973.5, 997.3, 1009.6, 1028.4,
939.7, 994.6, 1003.0, 1005.7,
1055.0, 1067.4, 1088.4, 1126.6,
1145.7, 1156.3, 1231.4, 1267.6
1302.9, 1320.6, 1350.6, 1361.2,
1388.4, 1493.8, 1599.6, 3113.5,
3131.8, 3244.3, 3250.5, 3262.5,
3278.9, 3299.3
3340.0, 3377.6
1039.1, 1086.0, 1126.8, 1130.9,
1143.8, 1174.9, 1182.7, 1270.8,
1332.0, 1353.3, 1385.8, 1398.6,
1462.1, 1490.2, 1609.9, 3100.0,
3132.8, 3221.1, 3245.8, 3259.7,
3261.0, 3286.2
(A2)
581.9, 805.8, 911.3, 1151.1, 1164.3,
1295.6, 1332.7, 3352.4
(B1)
423.9, 839.3, 969.0, 1000.0, 1130.7,
1196.5, 1403.8, 3226.8, 3363.7
(B2)
748.0, 777.2, 1018.5, 1077.7, 1131.0,
1367.1, 1427.7, 1529.3, 3336.9, 3365.3
a The vibrational frequencies are not corrected. b Geometry and vibrational frequencies (RHF/6-31++G**), single point energy (MP3/6-31++G**).
Table 3. Gas-Phase Acidities and Bond Dissociation Energies (BDE) of Some C7H8 Isomers and Electron Affinities (EA) of the
Corresponding C7H7 Radicals, at 300 K
C7H8 isomers
∆
acidG (kcal/mol)
394.7 ( 0.8a
∆
acidS (cal/mol K)
∆
acidH (kcal/mol)
EA(C7H7) (eV)
BDE(C-H) (kcal/mol)
quadricyclane (5)
(6)
norbornadiene
toluene
27.5 ( 0.1a
27.4 ( 0.1a
27.4 ( 3.0g
23.8 ( 1.4g
20.1 ( 2.6g
403.0 ( 1.1a,b
402.9 ( 1.1a
399.5 ( 1.7g
380.8 ( 2.1g
375.2 ( 2.1g
0.868 ( 0.006c
0.962 ( 0.006c
1.286 ( 0.006c
0.912 ( 0.006f
0.49 ( 0.13g
109.4 ( 1.3a,d
111.5 ( 1.3a,d
115.6 ( 1.9a
88.0 ( 1.0g
391.3 ( 1.5e
373.7 ( 2.0g
369.2 ( 2.0g
cycloheptatriene
73.0 ( 2.0g
b
a This work.
∆
acidH(0 K) ) 401.4 ( 1.1 kcal/mol calculated using eq 5. c The accompanying paper, ref 11 (J. Am. Chem. Soc. 1996, 118,
5074). d Two values of BDE(C-H) correspond to isomers of quadricyclyl radical corresponding to isomers of quadricyclanide ion (structures 5 and
6). e Reference 15. f Reference 30. g References 2 and 31.
Table 4
a. Estimating the Upper Limit of the Electron Affinity of
Quadricyclyl Radical (C7H7) from Exothermic Reactions
(Hydrogen Abstraction and Proton Transfer Reactions)
O- + C7H8 f HO- + C7H7
∆Hrxn e 0
∆Hrxn ) -0.9 kcal/mol
NH2- + C7H8 f C7H7- + NH3
b. Bracketing of the Electron Affinity of Quadricyclyl Radical
(C7H7) from Electron Transfer Reactions
C7H7- + SO2 f SO2- + C7H7
C7H7- + CS2 fe CS2- + C7H7
C7H7- + O2 fe O2- + C7H7
EA(SO2) ) 1.107 eV
EA(CS2) ) 0.51 eV
EA(O2) ) 0.451 eV
radicals formed by C-H bond dissociation will be far from
planarity, and, presumably, this accounts for the strength of these
bonds.
Figure 1. Gas-phase acidity scale of C7H8 isomers.
We examined the reactions of the mixture of quadricyclanide
ions 5 and 6 with CS2, COS, CO2, and N2O (Table 5).23 In
previous studies, we have shown that these reagents react in
characteristic ways with carbanions of different structural types.
For example, ions as strongly basic as 5 and 6 (e.g., the phenide
ion, C6H5-) react with CS2 almost exclusively by sulfur atom
abstraction,24 and indeed that is the predominant pathway (77%)
observed. A host of other ions are formed in small amounts;
we shall discuss their origin below.
dissociation energies of some isomers of C7H8 (quadricyclane,
norbornadiene, toluene, and cycloheptatriene), and the electron
affinities of the corresponding radicals. Figure 1 shows the
∆
acidH300 scale for these compounds. Both C-H bonds are
extremely strong for saturated hydrocarbons, 109.4 ( 1.3 and
111.5 ( 1.3 kcal/mol for 5 and 6, respectively, and are much
more similar to those in ethylene (111.2 ( 0.8)21 than to those
in cyclohexane (95.5 ( 1).22 However, the C-H bonds in
cyclopropane have a BDE of 106.3 ( 0.3 kcal/mol,22 indicating
that internal strain strengthens the bond. In general, alkyl
radicals prefer to be planar or near-planar. The quadricyclyl
Carbonyl sulfide (SdCdO) also transfers a sulfur atom in
many of its gas-phase reactions with carbanions. This process
is more exothermic than the corresponding reaction of CS2, since
the neutral product is CO rather than CS. It seems reasonable
to propose that reactions of (5) and (6) with COS begin by sulfur
(21) Ervin, K. M.; Gronert S.; Barlow, S. E.; Gilles, M. K.; Harrison,
A. G.; Bierbaum, V. M.; DePuy, C. H.; Lineberger, W. C.; Ellison, G. B.
J. Am. Chem. Soc. 1990, 112, 5750.
(22) McMillen, D F.; Golden, D. M Ann. ReV. Phys. Chem. 1982, 33,
493.
(23) Kass, S. R.; Filley, J.; Van Doren, J. M.; DePuy, C. H. J. Am. Chem.
Soc. 1986, 108, 2849.
(24) DePuy, C. H. Org. Mass Spectrom. 1985, 20, 556.

5072 J. Am. Chem. Soc., Vol. 118, No. 21, 1996
Lee et al.
Table 5. Branching Ratios of the Products of the Reactions of Quadricyclanide Ion with CS2, COS, CO2, and N2Oa
C7H7- + CS2
C7H7S- + CS
adduct
C7H7- + COS
C7H7S- + CO
77%8
6%8
5%8
5%8
4%8
2%8
1%8
33%8
25%8
14%8
12%8
6%8
HC2S- + [C5H6 + CO]
C5H5- + HC2COSH
HS- + [C7H6CO]
HC2COS- + C5H6
adduct
HS- + [C7H6CS]
C5H5- + HC2CS2H
HC2S- + [C5H6CS]
HC2- + [C5H5CS2H]
HC2CS2- + C5H6
6%8
HC2- + [C5H5COSH]
4%8
k(CS2) ) 7.4 × 10-10 cm3/s
k(COS) ) 5.2 × 10-10 cm3/s
C7H7- + CO2
C5H5- + HC2CO2H
C5H5CO2- + C2H2
adduct
HC2CO2- + C5H6
HC2- + [C5H5CO2H]
C7H7- + N2O
HC2O- + [C5H6N2]
OH- + [C7H6N2]
adduct
HC2N2O- + C5H6
C7H5- + H2O + N2
C5H3O- + [C2H4 + N2]
C5H5- + HC2N2OH
C7H7O- + N2
35%8
24%8
16%8
15%8
10%8
26%8
20%8
15%8
12%8
10%8
7%8
5%8
5%8
a Total rate coefficients are shown for CS2 and COS. Some neutral products, which may not be bound, are denoted by brackets. Experimental
uncertainty for branching ratios of 10% or more is (5%.
abstraction, and, indeed, C7H7S- accounts for 33% of the
product ions. Small amounts of adduct are also observed (eqs
8 and 9).
m/z 25) are obviously formed by fragmentation of 11 and 12,
most of which probably remains undecomposed.
If we now return to the product ions from reaction with CS2
-
we can see that these same ions are formed (with HC2CS2
replacing HC2COS-) (eq 11). Fragmentation of (9) again
Because the reactions with COS are more exothermic than
those with CS2, the product ions will be generated with more
internal energy, and they can undergo further reactions. For
example, the ions of m/z 57 and m/z 65 (formulated as 13 and
14, respectively) can arise naturally from 9 by reverse cycload-
dition reactions (eq 10). A similar mechanism was invoked by
MacMillan et al.25 in the reactions of metal cations with
quadricyclane.
The ion of m/z 85 can be written as HC≡C-COS-, and can
be formed analogously by fragmentation of the COS adduct
exceeds that of (11), although the total amount of fragmentation
ion 10. Thus taken together, 45% of the product ions appear
is reduced because of the lesser amount of internal energy in
to arise by fragmentation of 9 and 10 while only 4% (HCC-,
(9) and (11).
This same pattern of reaction and fragmentation is also seen
with CO2, although with this reagent the initial step is addition
(25) MacMillan, D. K.; Hayes, R. N.; Peake, D. A.; Gross, M. L. J. Am.
Chem. Soc. 1992, 114, 7801.

Quadricyclane in the Gas Phase
J. Am. Chem. Soc., Vol. 118, No. 21, 1996 5073
Table 6. SN2 and E2 Reactions of Quadricyclanide Ion with
as the major pathway and proton abstraction as the minor
channel. This reactivity pattern is expected for highly basic
anions. The elimination reaction of quadricyclanide ion with
diethyl ether is very slow; the endothermic proton abstraction
which initiates this reaction is known to be slow for carbanions.
This is in sharp contrast to the rapid elimination reactions that
Methyl Chloride and Diethyl Ethera
C7H7- + CH3Cl
Cl- + C7H7CH3
CH2Cl- + C7H8
C2H5O- + C2H4 + C7H8 k e 1.2 × 10-11 cm3/s
k ) 5.0 × 10-10 cm3/s
93%8
7% 8
C7H7- + (C2H5)2O
8
are often observed for basic anions of heteroatoms, such as HO-
a Due to H2O impurity in diethyl ether, only the upper limit is given.
-
and NH2
.
rather than atom transfer. With no neutral product to carry away
internal energy, the adduct fragments to a still greater degree,
and we can see that 34% of the products must derive from the
reaction of 6 (eq 12), while 50% of the products derive from
reaction of 5.
Conclusions
The gas-phase acidity of quadricyclane has been measured
to be ∆acidG300(C7H8) ) 394.7 ( 0.8 kcal/mol and is the highest
among the C7H8 isomers measured to date. H/D exchange
reactions with ND3 indicate that the hydrogen atoms at the three-
and four-membered rings have similar acidity, giving rise to
two isomeric quadricyclanide ions of similar stability. These
-
conclusions are supported by reactions of C7H7 with various
reagents, by ab initio calculations and by examination of
dideuterated quadricyclane. Bracketing of the electron affinity
of quadricyclyl radical by electron transfer reactions proved
helpful to parallel photoelectron spectroscopy experiments.
Combination of the gas phase acidity of quadricyclane with the
electron affinity of the quadricyclyl radical provides the C-H
bond dissociation energy of quadricyclane.
Acknowledgment. The authors would like to thank Professor
W. C. Lineberger and Dr. R. F. Gunion for providing their
experimental data on the electron affinity of quadricyclanide
ion, for many stimulating discussions, and for sharing a sample
of dideuterated quadricyclane which was kindly provided by
Professor Wes Borden. This work is supported by the National
Science Foundation under Grants No. CHE-9421747 and
CHE-9421756.
This same general pattern of addition or atom abstraction
followed by ring-opening and fragmentation can be seen for
the N2O reactions, giving rise, for example, to HCCO-,
JA9540278
(26) DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1981, 103, 5034.
(27) DePuy, C. H.; Gronert, S.; Mullin, A.; Bierbaum, V. M. J. Am.
Chem. Soc. 1990, 112, 8650.
(28) Gibson, S. T.; Greene, J. P.; Berkowitz, J. J. Chem. Phys. 1985,
83, 4319.
(29) Wickham-Jones, C. T.; Ervin, K. M.; Ellison, G. B.; Lineberger,
W. C. J. Chem. Phys. 1989, 91, 2762.
(30) Gunion, R. F.; Gilles, M. K.; Polak, M. L.; Lineberger, W. C. Int.
J. Mass Spectrom. Ion Proc. 1992, 117, 601.
-
HCCN2O-, and C5H5 from 5 and most likely C5H3O- from
6.
Thus the chemical reactions of the M - 1 ions from
quadricyclane are consistent with the formation of a mixture of
5 and 6, with 5 probably predominating.
Table 6 summarizes the reactions of quadricyclanide ion with
methyl chloride and with diethyl ether.26,27 Reaction with
methyl chloride proceeds rapidly with nucleophilic substitution
(31) Bartmess, J. E.; Scott, J. A.; McIver, R. T., Jr. J. Am. Chem. Soc.
1979, 101, 6046.