A R T I C L E S
Peon et al.
In methanol and ethanol, the reaction of Ph2CH+ with a
neutral alcohol molecule appears to dominate ion pair collapse.
Bartl et al. measured the rates of reaction between Ph2CH+ and
various nucleophilic quenchers, including neutral alcohol mol-
ecules, by nanosecond photoheterolysis of Ph2CHCl in aceto-
nitrile.25,26 From these rates and the molarities of the neat
solvents, time constants of 34, 69, and 246 ps are predicted for
methanol, ethanol, and 2-propanol, respectively. Our measured
decay times in methanol and ethanol (τ2 in Table 2) agree very
well with these estimates, and we conclude that solvolysis by a
neutral molecule is faster than addition of the geminate alkoxide
ion to Ph2CH+. In the case of 2-propanol, however, the value
of τ2 in Table 2 is nearly a factor of 2 smaller than predicted
from the bimolecular rate constant measured in acetonitrile by
Bartl et al.
Of course, caution must be used when extrapolating bimo-
lecular rate constants measured in acetonitrile to the neat alcohol
solvents. Kirmse et al. reported pronounced solvent effects on
the reaction rates of Ph2CH+ with a given nucleophile.56 These
authors found that reaction rates were 40-100 times lower in
2,2,2-trifluoroethanol (TFE) than in acetonitrile, while still lower
rates were observed in 3,3,3,3′,3′,3′-hexafluoro-2-propanol
(HFIP). They argued that hydrogen bonding to the nucleophile
hinders its reaction with the carbocation.56 However, lower rates
of reaction with increasing hydrogen bonding would predict
slower rates in the neat solvents, in contrast to our observation
in 2-propanol. In other systems, scavenging rates have been
reported to increase faster than expected with quencher con-
centration. For example, a plot of the decay rate of Ph2CH+
vs alcohol concentration shows upward curvature in 1,2-
dichloroethane57 and in HFIP.56 Kirmse et al. rationalized this
behavior by the ability of a second alcohol molecule to act as
a base, preventing dissociation of an initial alcohol-carbocation
adduct.56 In this case, it is unclear why this mechanism would
accelerate quenching in 2-propanol, but not in ethanol and
methanol. In the latter two solvents the decay rates agree with
those inferred from measurements of the bimolecular rate con-
stants at low alcohol concentration. Furthermore, the quenching
rates for alcohols in acetonitrile were reported to be independent
of alcohol concentration.58
with solvent separated ion pairs are familiar from electron trans-
fer in polar solvents.60 This could retard the geminate reaction,
offsetting the Coulombic attraction of the two oppositely charged
ions. Further study is required to fully understand the competi-
tion between geminate ion pair collapse and reaction with a
neutral alcohol molecule. A similar competition occurs in
excited-state proton-transfer reactions to solvent at low pH.
Complex kinetics can be observed in these systems due to
the diffusion-influenced reaction of geminate partners. These
effects can be modeled with the time-dependent Smoluchowski
equation.61
To end this section, we note that the rate of decay of the
diphenylmethyl cation is just 10% slower in CH3OD than in
CH3OH. Yoshihara and co-workers determined by dynamic
Stokes shift measurements that solvation of photoexcited
coumarin 102 occurs approximately 10% more slowly in deu-
terated methanol.62 This small isotope effect is thus consistent
with rate-limiting solvation and is not believed to indicate proton
transfer.
(4) Intermolecular Proton Transfer from Alcohols to
1Ph2C. The τ1 values in Table 2 show that proton transfer from
oxygen to carbon can take place on an ultrafast time scale.
Intermolecular proton transfer reactions are challenging to study
by time-resolved methods because of the difficulty of initiating
reaction at the same instant of time for an ensemble of donors
and acceptors. Photoacids, molecules that undergo proton-
transfer reactions in an electronically excited state, overcome
this problem and have been extensively studied in recent
years.63-65 The fastest known photoacids transfer a proton to a
water molecule in slightly less than 10 ps. Excited-state proton
transfer to H2O occurs in 8 ( 1 ps at 25 °C for 5-cyano-1-
naphthol63 and in 7.1 ps for 7-hydroxy-4-methylflavylium.66
Proton transfer from excited 8-hydroxypyrene-1,3,6-trisulfonate
to acetate ion, which is present at high concentration in aqueous
solution, occurs in 3 ps.64 1Ph2C, which accepts a proton in
9 ps in neat methanol, belongs to this same class of ultra-
fast intermolecular proton-transfer reactions. Interestingly,
proton transfer to methanol by 5-cyano-1-naphthol is nearly
2 orders of magnitude slower than in water, requiring 390 ps
in CH3OH.63
The anomalous kinetics in 2-propanol are best explained in
our view by reaction between Ph2CH+ and the alkoxide ion
formed by protonation of the carbene. The reactive decay of
this ion pair is apparently faster than nucleophilic attack by
neutral 2-propanol, which is a weaker nucleophile than either
methanol or ethanol. A crude estimate is that Ph2CH+ and the
2-propoxide ion react in 100 ps. The geminate reaction rate is
likely limited by the energetic cost of desolvating two singly
charged ions in order for reaction to take place. If the rate of
protonation of 1Ph2C is controlled by the solvent reorganization
necessary to solvate the incipient alkoxide ion (see next section),
then solvent separated alkoxide/Ph2CH+ ion pairs could be
formed quickly.59 The large reorganization energies associated
The high basicity of singlet carbenes follows from the high
pKa values of their corresponding carbocations. For example,
the stable carbene imidazole-2-ylidene, which deprotonates
weakly acidic hydrocarbons such as indene, is estimated to have
a pKa of 24, or a pKb of -10.67 By comparison, the powerful
photoacid 5-cyano-1-naphthol has an excited-state pKa of -2.8.63
Ab initio calculations predict a lower proton affinity for
imidazol-2-ylidene68 than for diphenylcarbene,9 suggesting that
the latter may be even more basic.
The rate constant for the disappearance of singlet carbenes
longer lived than 1Ph2C have been measured in the presence of
(60) Gould, I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522.
(61) Agmon, N.; Szabo, A. J. Chem. Phys. 1990, 92, 5270.
(62) Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K. J. Phys. Chem. 1996,
100, 14575.
(63) Pines, E.; Pines, D.; Barak, T.; Magnes, B. Z.; Tolbert, L. M.; Haubrich,
J. E. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 511.
(64) Genosar, L.; Cohen, B.; Huppert, D. J. Phys. Chem. A 2000, 104, 6689.
(65) Cohen, B.; Huppert, D. J. Phys. Chem. A 2001, 105, 2980.
(66) Lima, J. C.; Abreu, I.; Brouillard, R.; Macanita, A. L. Chem. Phys. Lett.
1998, 298, 189.
(56) Kirmse, W.; Guth, M.; Steenken, S. J. Am. Chem. Soc. 1996, 118, 10838.
(57) Sujdak, R. J.; Jones, R. L.; Dorfman, L. M. J. Am. Chem. Soc. 1976, 98,
4875.
(58) As reported in ref 26. The authors did not specify the range of quencher
concentrations used in acetonitrile, but the maximum concentration is
presumably considerably below 1 M.
(59) Alkoxide ions have high mobilities due to the proton relay mechanism
(Grotthus mechanism). Slower escape by the bulkier alkoxide ion derived
from 2-propanol could be another reason that ion pair recombination is
more important in this solvent than in the shorter alcohols.
(67) Alder, R. W.; Allen, P. R.; Williams, S. J. J. Chem. Soc., Chem. Commun.
1995, 1267.
(68) Dixon, D. A.; Arduengo, A. J. J. Phys. Chem. 1991, 95, 4180.
9
6436 J. AM. CHEM. SOC. VOL. 124, NO. 22, 2002