Laser flash photolysis study of phenylcarbene and pentafluorophenylcarbene

Article abstract of DOI:10.1021/jp963332s

Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of phenyldiazomethane and pentafluorophenyldiazomethane releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC), respectively. In acetonitrile solvent the carbenes react rapidly to form nitrile yl

Full text of DOI:10.1021/jp963332s

3832
J. Phys. Chem. A 1997, 101, 3832-3840
Laser Flash Photolysis Study of Phenylcarbene and Pentafluorophenylcarbene
Atnaf Admasu, Anna Do´ra Gudmundsdo´ttir, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 100 W. 18th AVenue, Columbus, Ohio 43210
ReceiVed: October 25, 1996; In Final Form: January 27, 1997X
Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of phenyldiazomethane and pentafluorophenyldiazomethane
releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC), respectively. In acetonitrile solvent the
carbenes react rapidly to form nitrile ylides which have convenient absorption maxima for optical detection
(λmax ) 350 nm). Phenylcarbene and pentafluorophenylcarbene each react with acetonitrile with an absolute
rate constant of 2.4 × 106 M-1 s-1 in CF2ClCFCl2 (Freon-113) at ambient temperature. The lifetimes of
spin-equilibrated PC and PFPC are 190 and 500 ns in Freon-113, respectively, and the lifetime of each carbene
is deduced to be 22 ns in neat acetonitrile. LFP of phenyl and pentafluorophenyldiazomethane in the presence
of pyridine leads to the expected pyridine ylide. The observed absolute rate constants of reaction of spin
equilibrated PC and PFPC with pyridine are 1.9 × 107 and 5.1 × 107 M-1 s-1, respectively, in Freon-113 at
ambient temperature. From this data, one can deduce singlet-triplet splittings (∆GST, 298 K) of 2.3 and 3.1
kcal/mol for PC and PFPC, respectively, which is the difference in energy between the triplet minimum and
the point of intersection of the singlet and triplet carbene plus pyridine surfaces. LFP of phenyldiazomethane
and pentafluorophenyldiazomethane in acetonitrile containing carbene scavengers (e.g., alcohols, alkenes,
and silanes) gives reduced yields of nitrile ylides. Analysis of the yield of ylide as a function of quencher
by the Stern-Volmer method gives kQτ values of carbene quenching. Low-temperature (77 K) photolysis of
pentafluorophenyldiazomethane generates the persistent EPR spectrum of triplet pentafluorophenylcarbene
which is the ground state of this carbene. However, photolysis of pentafluorophenyldiazomethane at ambient
temperature generates stable reaction products derived from capture of singlet pentafluorophenylcarbene. Thus
the properties of pentafluorophenylcarbene are remarkably similar to those of phenylcarbene. Each carbene
has a triplet ground state but reacts in solution at ambient temperature through a low-lying excited singlet
state. The lack of a fluorine substituent effect on the behavior of phenylcarbene is compared to that in
singlet phenylnitrene.
I. Introduction
good yields of products,9 the generation of phenylnitrene
invariably leads to polymeric tar, or interception of its ring-
expanded ketenimine isomer.10,11 The absolute kinetics of PC
and pentafluorophenylcarbene (PFPC) were determined in this
study in an attempt to discover fluorine effects in the carbenic
system and to place the fluorine effect on arylnitrene chemistry
into clearer focus. In addition to time-resolved studies, the
matrix EPR spectrum of PFPC has been obtained, and the stable
products of its chemical reactions at ambient temperature
identified in cyclohexane and in methanol. The data indicates
that, in contrast to phenylnitrene, the effect of fluorine substitu-
tion on the chemistry and dynamics of PC is negligible.8,11
In the past decade, numerous studies of the dynamics of
arylcarbenes have been reported.1 Laser flash photolysis (LFP)
studies of diphenylcarbene,2 fluorenylidene,3 and 1- and 2-naph-
thylcarbene,4,5 and halophenylcarbenes6 have been described.
The absorption spectra of these transient species and their
absolute rate constants of reaction were tabulated. Strangely
enough, the parent arylcarbene, phenylcarbene (PC), was not
II. Results
Chemical Analysis of Reaction Mixtures. PC has been well
studied by the many classical tools of carbene chemistry.9
Photolysis of phenyldiazomethane (PDM) and other precursors
at cryogenic temperatures generates PC as a persistent species
in rigid media. This has allowed its study by UV-Vis,12 fluo-
rescence,13 IR,12 and EPR14 spectroscopies. There is no question
that the triplet is the lowest energy state of phenylcarbene.
PDM and other precursors have been photolyzed in several
solvents, and the stable mixture of products formed have been
analyzed.9 Generation of PC in the presence of alkenes (A)
produces the expected cyclopropane with almost perfect ste-
reospecificity.9,15
studied by LFP techniques until very recently,7 a fact which
provided partial motivation for this study. A stronger impetus
driving this work was the finding of very dramatic effects of
fluorine substitution on the properties of phenylnitrene.8 Whereas
PC is easily intercepted by a variety of trapping agents to give
* Author to whom correspondence should be addressed: phone, 614-
292-0401; fax, 614-292-1685; e-mail, platz.1@osu.edu.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
S1089-5639(96)03332-4 CCC: $14.00 © 1997 American Chemical Society

Laser Flash Photolysis Study of Phenylcarbenes
J. Phys. Chem. A, Vol. 101, No. 20, 1997 3833
SCHEME 1
SCHEME 2
Photolysis of PFPDM in cyclohexane cleanly produces adduct
4 (>98% of the volatile reaction products) at ambient temper-
ature.
This result does not change upon dilution of the alkene with
less reactive cosolvents.9b This result and the work of Baer
and Gutsche16 indicate that spin equilibration of PC is rapid in
solution relative to the rate of its bimolecular reactions.
Typically, interception of the higher energy, more reactive
singlet state predominates (Scheme 1), as shown by Moss and
Significantly neither radical dimer 5 nor 3 was formed in
this reaction. Apparently, only the singlet state of pentafluo-
rophenylcarbene (1PFPC) reacts with cyclohexane at ambient
temperature.
Photolysis of PFPDM in methanol cleanly yields the product
of OH insertion, a reaction that is characteristic of singlet
carbenes.2
3
Tomioka who have chemically intercepted PC, but at -196°,
where the equilibrium greatly favors the ground triplet state.9b,17
We have obtained results consistent with expectations, and
with Scheme 1, upon photolysis of PDM in cyclohexane. The
product isolated is formed rather cleanly and is that of singlet
carbene CH insertion (1). Only trace amounts of diphenylethane
PDM
No products other than 6 were formed in methanol in more
than trace quantities. Photolysis of PFPDM in the presence of
a triplet sensitizer, under conditions where all of the light is
absorbed by the triplet sensitizer, again leads cleanly to the
formation of 6. This demonstrates that the low-lying singlet
state of PFPC is easily accessed from the ground triplet state
(Scheme 2).
2 or bicyclohexyl 3 were detected, which indicated that there
is very little chemistry of triplet phenylcarbene in this solvent
at ambient temperature.
The triplet nature of the ground state of pentafluorophenyl-
carbene (3PFPC) was confirmed by low-temperature spectros-
copy. A solution of PFPDM in perfluorohexane was cooled to
-196° and irradiated with 350 nm light. Photolysis generated
a new set of EPR absorption bands characteristic of a ground
state triplet carbene. The zero-field parameters |D/hc| ) 0.521
cm-1 and |E/hc| ) 0.0259 cm-1 are very similar to those of
PC which have |D/hc| ) 0.515 cm-1 and |E/hc| ) 0.0251
The isotope effect (C6H12 vs C6D12) to singlet insertion CH
in cyclohexane is approximately 2. This was determined by
photolyzing PDM in a 1:1 (v:v) mixture of cyclohexane and
cyclohexane-d12 followed by GC-MS of the product mixture.
Adducts 1 and 1-d12 were easily separated and their relative
intensities measured to determine the isotope effect. The isotope
effect of 2 realized with PC is very similar to that obtained
with 1-naphthylcarbene4 and is characteristic of singlet
carbenes.1,9c Substantially larger isotope effects are observed
in the reaction of triplet carbenes (e.g., diphenylcarbene) with
C-H bonds.2c
cm-1 14
The photogenerated resonance absorption bands disap-
.
pear upon thawing the matrix. It is clear that the triplet is either
the ground state of PFPC or is within a few cal/mol of the
ground state.
The product studies and low-temperature EPR studies reveal
that, although PFPC has a triplet ground state, the singlet state
is readily accessible. The singlet state of PFPC must be much
more reactive than the triplet state as its chemistry predominates
at ambient temperature. The chemistry of PC and PFPC follows
the same pattern. Thus it seems safe to assume that, as per

3834 J. Phys. Chem. A, Vol. 101, No. 20, 1997
Admasu et al.
Figure 2. The formation of ylide 9 following LFP of PDM in Freon-
113 containing 1.12 M acetonitrile-d6.
Figure 1. The transient absorption spectrum of ylide 10 produced by
LFP of PFPDM in acetonitrile.
TABLE 1: Absorption Maxima of Arylcarbene Ylides
PC, diphenylcarbene, fluorenylidene, and 1-naphthylcarbene,1-5,9
singlet and triplet pentafluorophenylcarbene are in rapid equi-
librium in solvents of relatively low reactivity at ambient
temperature.
ylid former
PC (nm)
PFPC (nm)
C5H5N
CH3CN
CD3CN
tBuCN
PhCN
474
345
340
345
343
486
351
351
356
392
343
Laser Flash Photolysis Studies of PC and PFPC. 1-Naph-
thylcarbene (NC) was among the first aryl carbenes to be studied
by the LFP technique.4 It is difficult to detect the ground triplet
C6F5CN
PC. Equation 1 is predicted by Scheme 1,
kobs ) 1/τ + kCH CN[CH3CN]
(1)
3
where kCH CN is the rate constant of the spin-equilibrated mixture
3
of singlet and triplet carbene in Freon-113 with acetonitrile, and
τ is the lifetime of spin-equilibrated carbene in this solvent in
the absence of acetonitrile. As expected, a plot of kobs vs [CH3-
CN] is linear (Figure 3). Similar results were obtained with
pentafluorophenylcarbene. The results are collected in Table
2.
It was also possible to trap PC and PFPC with pyridine upon
LFP of the appropriate precursor (see Table 1 and Figure 4).
The formation of these ylides was resolved as a function of
pyridine concentration, and rate constants of reaction were
deduced as per acetonitrile (Table 2). According to Scheme 1,
ground state triplets 3PC and 3PFPC serve as reservoirs for the
more reactive singlet states.9 If Scheme 1 is correct then the
rate constants of the spin-equilibrated carbenes with pyridine
are related to those of the singlet carbenes by eq 2.
state (3NC) directly by optical methods because its UV
absorption maximum falls underneath that of 1-naphthyldiaz-
omethane. Thus we were not surprised when LFP of PDM and
PFPDM in pentane did not lead to significant transient absorp-
tion between 300 and 700 nm. 1-Naphthylcarbene (and later
2-naphthylcarbene5) were most easily studied by capture with
acetonitrile solvent to form ylides 7 and 8 which were easily
monitored because of a favorable absorption maximum (λ g
386 nm), intense molar absorptivity, and very long lifetime (τ
> 100 µs) relative to the carbene.
1kpyrK ) kpyr
(2)
If we assume that 1kpyr ) 1 × 109 M-1 s-1, a common value
for singlet carbenes,18 we deduce that
Thus we were not surprised when LFP of PDM and PFPDM
in various nitrile solvents gave intense transient absorption
spectra attributed to 9 and 10, respectively (see Figure 1 and
Table 1).
KPC ) 0.019 and ∆G298ST(PC) ) 2.3 kcal/mol
KPFPC ) 0.005 and ∆G298ST(PFPC) ) 3.1 kcal/mol
The singlet-triplet gaps of PC and PFPC have not been
determined by experiment. However, recently high-level cal-
culations on PC have been reported which predict ∆EST ) 4
kcal/mol.19 Thus, our data, when interpreted within the context
of Scheme 1, gives a somewhat smaller singlet-triplet splitting
than the theoretical estimate. As the calculations are for gas
phase species, one is tempted to attribute this discrepancy to a
solvent effect although other interpretations are possible (Vide
infra). The magnitude of kpyr varies somewhat in pentane,
cyclohexane-d12, Freon-113, and acetonitrile, which indicates
that the role of solvent on K and 1kpyr is small but not zero. As
It was possible to resolve the growth of ylide 9 in Freon-113
(CF2ClCFCl2) containing dilute acetonitrile. The growth of 9
was a simple exponential process (Figure 2), which could be
fit to yield observed rate constant kobs (eq 1) for spin-equilibrated

Laser Flash Photolysis Study of Phenylcarbenes
J. Phys. Chem. A, Vol. 101, No. 20, 1997 3835
Figure 3. A plot of kobs vs [acetonitrile]; see text.
and probably are unity within experimental error, which
indicates that CH insertion is a rather minor reaction of PC in
acetonitrile and even less important for PFPC where an inverse
isotope effect is actually observed. The latter is likely a result
of experimental error.
One can shorten the lifetime of the carbene and reduce the
yields (φy) of ylides 9 and 10 by adding a competitive quencher
(Q) to neat acetonitrile (Scheme 1). A Stern-Volmer analysis
of this mechanistic scenario predicts eq 3 and 4 where τ is the
φyo
φy
kQ[Q]
) 1 +
(3)
(4)
kCH CN[CH3CN]
3
φyo
Figure 4. The transient absorption spectrum of the ylide formed upon
reaction of pentafluorophenylcarbene with pyridine. The spectrum was
obtained by LFP of PFPDM in 1:1 pentane:pyridine (v:v).
) 1 + kQ[Q]τ
φy
carbene lifetime in neat acetonitrile, φoy is the yield of ylide in
neat acetonitrile when [Q] ) 0, and Q is a typical carbene
quencher, such as an alkene (Q ) A, Scheme 1) or an alcohol,
and kQ is the absolute rate constant of spin-equilibrated carbene
with Q.
TABLE 2. Lifetimes of Spin-Equilibrated PC and PFPC in
Various Solvents
carbene
PC
solvent
quencher kobsQ (×106 M-1 s-1
)
τ (ns)
CF2ClCFCl2 CD3CN
CH3CN
2.1
2.4
260
190
205
pyridinea
19
Plots of φo/φy vs [Q] are predicted and are found to be
y
PC
PC
PFPC
C6H12
C5H12
pyridinea
9.3
8.5
5.1
2.4
2.8
74
85
500
214
236
linear. A typical plot is given in Figure 5, values of kQτ are
pyridinea
collected in Table 3.
CF2ClCFCl2 pyridinea
CH3CN
A plot of φo/φy vs [CF3CH2OH] shows two linear regions
y
CD3CN
with distinctly different slopes (Figure 6). This type of behavior
has been previously observed in the reaction of carbenes and
alcohols.6,20 The data obtained at low concentrations of alcohol
is attributed to reaction of the carbene with monomeric alcohol.
The data at concentrations of alcohol >0.2 M is attributed to
the reaction of carbene with dimeric or oligomeric alcohol.
Water as quencher yields a linear plot at all concentrations
employed. The data seems more consistent with water as an
oligomeric rather than as a monomeric species.
a Reference 7.
is typical, the rate constants of the bimolecular reactions of
singlet carbenes are slower in the more polar solvent.6
The molarity of neat acetonitrile is 19.2 M. If we assume
that kCH CN is the same in Freon-113 and in acetonitrile, we can
3
calculate the pseudo-first-order rate of formation of ylides 9
1
and 10 as kCH CN[CH3CN] ) /τ and deduce lifetimes of spin-
3
equilibrated PC and PFPC in neat acetonitrile as 22 ns in each
case.
III. Discussion
On the basis of this analysis, it seems reasonable that the
lifetime of PC and PFPC in neat acetonitrile is limited solely
by reaction of the singlet carbene with solvent. However, ylide
formation is not the only possible mode of reaction of PC and
PFPC with acetonitrile, because CH insertion is possible as well.
However, the isotope effects (Table 2) are much smaller than
those observed (2) for other singlet carbenes1,4 in cyclohexane
Mechanism of the Reaction of 1PC and 1PFPC with
Alcohols. Singlet carbenes can, in general, react with the OH
group of alcohols by one of three mechanisms. The reaction
can proceed by (a) proton transfer to give an ion pair which
subsequently collapses to product,21 by (b) formation of an
oxonium ion type ylide, or by (c) concerted C-H insertion.

3836 J. Phys. Chem. A, Vol. 101, No. 20, 1997
Admasu et al.
TABLE 3: Stern-Volmer kQτa Values of PC and PFPC with Various Carbene Traps in CD3CN at Ambient Temperature
PC
PFPC
<0.20 M
>0.20 M
<0.20 M
>0.20 M
(CH3)2CHOH
CH3CH2OH
CH3OH
2.68
7.80
11.91
4.31
1.30
4.39
5.71
2.65
1.05
1.99
3.58
1.29
0.15
0.13
8.32
0.57
CH3OD
H2O
D2O
3.09
2.41
1.45
0.46
CF3CH2OH
triethylsilane
trans-1,2-dichloroethylene
cyclohexene
benzene
toluene
mesitylene
27.68
14.78
4.04
0.80
4.71
22.22
7.99
1.05
1.02
6.00
3.44
11.55
4.30
0.98
1.06
1.25
a τ(PC,CD3CN) ) 26 ns; τ(PFPC,CD3CN) ) 20 ns.
Figure 6. A plot of φo/φy vs [CF3CH2OH] obtained by LFP of PDM
in neat acetonitrile containing CF2CH2OH; see text.
y
Figure 5. A plot of φo/φy vs [cyclohexene], obtained by LFP of PDM
y
in neat acetonitrile containing cyclohexene; see text.
methanol.21,22 Laser flash photolysis data with carboethoxy-
carbene 1123 and tetrachlorocyclopentadienylidene 1224 has been
The exact choice of mechanism will depend, of course, on the
nature of the carbene and the alcohol.22
interpreted in favor of pathway b.
The data of Table 3 indicates that the reactivity of both PC
and PFPC with alcohols increases with the acidity of the alcohol.
This immediately excludes oxonium ylide mechanism b for both
carbenes. This result was surprising for PFPC as oxonium ylide
13 appears to have a favorable structure.
The existence of isotope effects in the reaction of PC and
PFPC with the OH bond is further evidence against the oxonium
ion mechanism.
Pathway a has been demonstrated for diphenylcarbene and

Laser Flash Photolysis Study of Phenylcarbenes
J. Phys. Chem. A, Vol. 101, No. 20, 1997 3837
SCHEME 3
Thus the data supports either proton transfer to form ions
which are not detected (pathway a) or a highly polarized,
concerted insertion reaction.
The other data of Table 3 is unremarkable. PC and PFPC
react rapidly with alkenes, aromatics, and a silane. As τ of PC
and PFPC in neat acetonitrile-d3 (assuming that kCH CN is solvent
3
independent) can be deduced, the kQτ values of Table 3 can be
converted to absolute rate constants (τ(PC) ) 26 ns, τ(PFPC)
) 20 ns).
reflect the energy separation from the triplet minimum to the
point of intersection of the singlet and triplet carbene plus
pyridine surfaces.
Comparison of Phenylcarbene and Phenylnitrene. Singlet
phenylnitrene (1PN) undergoes ring expansion so rapidly
(∼0.1-1.0 ns) in solution at ambient temperature that it is nearly
impossible to trap this species with external reagents.11,27
Singlet and Triplet Surface Crossings. Considerable
3
literature suggests that PC serves as a reservoir for the more
reactive 1PC.9 3PC reacts with alkanes, alkenes, aromatics, and
alcohols9 to give products consistent with the trapping of 1PC.
This is classically interpreted9 with the aid of Scheme 1, which
predicts that kQ ) 1kQK. If we assume that 1kQ is 1 × 109 M-1
s-1 with typical traps, then values of K and ∆GST can be
deduced. This analysis predicts that ∆GST for PC (and
presumably PFPC) is smaller than the value predicted by
theory.19
This situation has been encountered before, and an interpreta-
tion has been put forward by Griller, Nazran, and Scaiano
(GNS). In this mechanism there is a surface crossing below
the isolated carbene minimum of the singlet surface.25
It is easy to demonstrate that the thermodynamics of a GNS
type of mechanism are in place for the reaction of PC with
hydrocarbons (Scheme 3). Theory and experiment indicate that
the insertion of singlet carbenes into simple C-H bonds
surmounts little or no barrier.26 This is clearly the case with
phenylchlorocarbene.26
The corresponding reaction of PC is known only in the gas
phase, and it requires several hundred degrees.28 Recent
calculations of Borden and Karney indicate that ketenimine
formation must be a stepwise process involving an azirine
intermediate (14).29
1
Insertion reactions of PC are rapid because it has a closed
The barrier for the reaction of 3PC with hydrocarbons to form
radical pairs is not known and cannot be determined at ordinary
temperatures as this reaction is slower than the singlet pathway.
However, hydrogen atom abstraction reactions of triplet diphen-
ylcarbene are known2c and traverse barriers greater than 5 kcal/
shell configuration which allows the empty p orbital to complex
with and react quickly with pairs of electrons.30
3
mol. Thus the activation energy for the reaction of PC with
R-H exceeds the sum of ∆HST (4 kcal/mol) and ∆Hq (of the
singlet insertion reaction) predicated by theory. As the reaction
1
of PC with RH is highly exothermic and surmounts little or
no barrier,26 the triplet hydrogen atom abstraction surface must
cross the surface of the singlet insertion pathway. If there is
any coupling of these surfaces, the triplet will cross over to the
singlet surface at a point that must be below the singlet carbene
minimum. Perhaps the relative reactivity of 3PC with alkanes,
alkenes, and aromatics (to give singlet products) merely reflects
the strength with which the singlet and triplet surfaces can
couple in a particular bimolecular process. The same arguments
can be applied to the reaction of arylcarbenes with pyridine.
Thus, the singlet-triplet splittings deduced earlier actually
Singlet phenylnitrene has an open shell configuration which
means that it cannot undergo concerted insertion and addition
reactions as rapidly as 1PC.31 However, as 1PN can be thought
of as an iminyl biradical31 (15), it is easy to see how it can
readily cyclize to form azirine 14.29

3838 J. Phys. Chem. A, Vol. 101, No. 20, 1997
Admasu et al.
As the corresponding open shell structure of 1PC is substan-
tially higher in energy than the lower energy closed shell
singlet19 configuration, cyclization of 1PC is much slower than
Series 1400 with a thermal conductivity detector using 5% SE-
30 GC, and W-HP 80/100 and 5% OV-17, and W-HP 80/100
columns.
1
that of PN.
Tetrahydrofuran, benzene, hexafluorobenzene, and toluene
were purified by distillation from sodium benzophenone.
Acetonitrile and methylene chloride were purified by shaking
over 4 Å molecular sieves and distilling from calcium hydride.
Methanol and ethanol were distilled from sodium metal.
Pyridine was distilled from barium oxide and stored over
potassium hydroxide pellets. Triethylamine and diethylamine
were purified by distillation over barium oxide and stored over
potassium carbonate. Olefinic quenchers were purified by
distillation and stored over 4 Å sieves. All solvent distillations
were performed under a dry nitrogen atmosphere. Deuterated
and other solvents were purchased from Aldrich and used as
received.
It is clear that fluorine substitution retards the rate of
rearrangement of pentafluorophenylnitrene.8 We have argued
that the fluorine substituent makes the p-π2 configuration of
the nitrene less accessible because of differential stabilization
of the p-σ2 configuration, in part by π backbonding of fluorine,
and by delocalization of the filled, in plane, orbital through the
σ system onto the electronegative fluorine atom.32
Perhaps this effect brings a closed shell singlet configuration
below that of the open shell configuration and consequently
reduces the rate of rearrangement.32 This view has received
some support by calculations of Cramer.33 However, unpub-
lished calculations of Borden and Karney suggest another
interpretation.34
ESR Measurements. ESR measurements were made with
a Varian E-112 X-band ESR spectrometer. Carbene spectra
were obtained at 10 mW of microwave power. All samples
were prepared in 4 mm Suprasil quartz sample tubes. Samples
of 15 mmol of diazo precursor were sealed under vacuum after
three freeze-thaw cycles to remove traces of oxygen and
photolyzed for 3-5 min using 300 or 350 nm lamps. Samples
were stored in liquid nitrogen between measurements.
Laser Flash Photolysis Studies35. The laser flash photolysis
apparatus for transient absorption and kinetic studies consists
of a Lumonics TE-861-4 excimer laser (351 or 249 nm) or a
Lambda Physik LPX-100 excimer laser (308 nm). Stock
solutions of the photolabile carbene precursors were prepared
in the solvents of interest. Hexafluorobenzene, acetonitrile,
acetonitrile-d3, and pentane were commonly used as solvents.
Pyridine and various nitrile solvents were used as ylide-forming
carbene traps. The concentration of each solution of carbene
precursor was adjusted to an optical density of 0.6-1.0 at either
351, 308, or 249 nm, depending upon which laser pulse was
employed.
Solutions of samples to be studied by laser flash photolysis
were contained in quartz cells for excitation at either 308 or
249 nm. The cells were fitted with rubber septa, and the samples
were degassed by bubbling with dry, oxygen free nitrogen for
three minutes. The sample cells were then irradiated at ambient
temperature with excimer laser pulses that impinged on the
sample at a right angle to a Xe arc lamp monitoring beam. The
monitoring beam was focused on a monochromatomater that
was selected for the wavelength of interest. Signals were
obtained with a photomultiplier tube and converted to a digitized
signal by a Tektronix 7912 transient digitizer. These signals
were analyzed by a Macintosh computer.
1
The abundant intermolecular chemistry of PFPN is due to
its much lower rate of rearrangement than that of singlet
phenylnitrene. As singlet PC is closed shell and does not ring
expand in solution,28 fluorine substitution, which will stabilize
closed shell configurations, has only a negligible effect on the
properties of the aryl carbene.
IV. Conclusions
The absolute kinetics of phenylcarbene (PC) and pentafluo-
rophenylcarbene (PFPC) were determined by laser flash pho-
tolysis methods. The absolute rate constant of both spin
equilibrated PC and PFPC with acetonitrile is 2.4 × 106 M-1
s-1 in CF2ClCFCl2 (Freon-113) at ambient temperature. In this
solvent, the rate constants of PC and PFPC with pyridine are
1.9 × 107 and 5.1 × 107 M-1 s-1 at ambient temperatures. From
this data, one can deduce singlet-triplet splittings (∆GST, 298
K) of 2.3 and 3.1 kcal/mol of PC and PFPC, respectively. These
values are smaller than those predicated by theory,19 which
indicates that there is a Griller-Nazran-Scaiano25 type of
surface crossing mechanism. Absolute rate constants of PC and
PFPC with various quenchers are reported.
V. Experimental Section
General. Melting points were taken with an electrothermal
capillary melting point apparatus and are reported uncorrected.
1H NMR spectra were recorded with a Bruker AM-200 (200
MHz) or Varian AM-250 (250 MHz) instrument. 13C NMR
and 19F NMR spectra were recorded in units of ppm relative to
tetramethylsilane (0 ppm). Infrared spectra were recorded on
a Perkin-Elmer model 1710 infrared fourier transform spec-
trometer and are given in cm-1. UV-vis spectra were taken
using a Perkin-Elmer Lambda 3B UV-Vis spectrophotometer
and recorded on the Perkin-Elmer 3700 data station or a Hewlett
Packard 8452A diode array spectrophotometer interfaced with
an IBM portable personal computer. Gas chromatograph-
fourier transform infrared-mass spectra were taken on a
Hewlett-Packard 5890 gas chromatograph with infrared and
mass detection. Gas chromatography was performed on a
Perkin-Elmer 8500 gas chromatograph with a flame ionization
detector using a 30 m × 0.254 mm column (DB-1). Preparatory
gas chromatography was performed on a Varian Aerograph
Transient absorption spectra were obtained using EG and G
Princeton Allied Research Model 1460 optical multichannel
analyzer (OMA). In a typical optical multichannel analyzer
experiment, a cell containing a solution of carbene precursor
and/or any additional reagent (usually ylide trap) was placed in
the path of the laser pulse and xenon arc beam and four
background (no laser pulse) and three active laser pulse signals
in an alternating manner were collected with the OMA. The
average spectrum was recorded, and the most favorable transient
absorption obtained from this spectrum was used as a monitoring
wavelength in the subsequent kinetic studies.
In the area of time resolved kinetic studies, two types of
measurements were conducted. These were direct measurement
of a rate constant and Stern-Volmer quenching. In a typical
direct measurement, an aliquot of fixed volume of a stock
solution of the carbene precursor was syringed into a given
number of laser cells. The required ylide-forming reagent was

Laser Flash Photolysis Study of Phenylcarbenes
J. Phys. Chem. A, Vol. 101, No. 20, 1997 3839
then added in successively larger increments to each cell. Each
cell was capped with a septum, degassed, and pulsed with laser
radiation, and the data so produced was recorded.
were placed in 4 mm pyrex tubing which had been thoroughly
washed with ammonium hydroxide and dried prior to use.
Solvents used in these studies were purchased from Aldrich
Chemical Company (99+%), and were used without further
purification. Deuterated solvents were 99.5% atom percent
purity. The samples were sealed under vacuum after three freeze
pump-thaw cycles and photolyzed for 30-120 min using four
Rayonet RPR-3500 lamps (3500A) in a home-built photochemi-
cal reactor. After photolysis was complete, the sealed tube was
opened and 0.10 mL acrylonitrile was added to remove residual
diazo precursor. The samples were allowed to stand in the dark
for 2 h prior to analysis to allow for complete reaction with
acrylonitrile.
Analysis of product mixtures by gas chromatography was
accomplished using a Perkin-Elmer 8500 gas chromatograph
equipped with a flame ionization detector and a 30 m × 0.254
nm column (DB-1). Typical chromatography times were 60-
120 min/injection over a temperature range of 70-225 °C.
Products of the photolysis mixture were isolated in their pure
state from some of the reaction mixtures by using either a Varian
Series 1400 preparatory gas chromatograph or by preparatory
thin layer chromatograph. In both product isolation methods
the reaction mixtures were concentrated by pumping off most
of the solvent prior to chromatography.
In a typical Stern-Volmer quenching experiment, the samples
were prepared in a number of cells by adding a fixed amount
of stock solution and a constant volume of ylide-forming
reagent. To these solutions was added a known amount of the
quencher of interest in successively larger increments to each
cell. The quencher concentration in each sample was calculated
on the basis of the amount present in the total volume of the
solution. The capped, degassed cell was pulsed with laser
irradiation, and the data so produced recorded.
Synthesis. The synthesis of phenyldiazomethane has been
described previously.7,9
2,3,4,5,6-Pentafluorobenzaldehyde Tosylhydrazone.36 p-Tol-
uenesulfonhydrazide (9.30 g, 0.050 mol) was added to a solution
of 9.90 g (0.050 mole) of 2,3,4,5,6-pentafluorobenzaldehyde
in 90 mL of absolute ethanol. The suspension was stirred under
nitrogen for 48 h at room temperature. The solution was then
cooled in an ice bath. The white crystalline solid was filtered
and washed with cold absolute ethanol. The filtrate was further
concentrated to 90 mL by pumping off the solvent under
vacuum. The concentrated solution was cooled in ice bath, and
then the crystalline solid was filtered and washed with cold
ethanol. The mother liquid and the washing solution were
combined to give a third crystallization. The total yield of the
colorless crystal was 15.80 g (87%). The melting point of the
crystal was 162-163 °C.
Product assignments were made on the basis of mass spectra
data obtained with a Hewlett Packard 5890 GC/IR/MS and
supported by NMR spectroscopy.
Photolysis of (Pentafluorophenyl)diazomethane. Pen-
tafluorophenyldiazomethane was photolyzed in various solvents
(15 mM) with a standard Rayonet photoreaction apparatus
equipped with 350 nm source bulbs. All photolysis reactions
were done at room temperature. After photolysis the solvents
were pumped off under vacuum and the major products were
isolated by preparative TLC or preparative GC. In these studies
the photolysis were done in the following solvents. (a)
Cyclohexane. Only one product was formed and isolated by
preparative TLC, and this product was analyzed by GC-MS,
and 1H NMR, 13C NMR, and 19F NMR spectroscopies. 1NMR
(CDCl3): δ 0.83-1.7 (m, 11H, C6H11), 2.57 (dt, J ) 1 and 7
Hz, 2H, CH2). 13C NMR (CDCl3): δ 26.47 (s), 26.78 (s), 30.32
(s), 32.32 (s), 39.47 (s), 113.7 (m), 135.3-138.0 (m), 139.5-
142.5 (m), 143.24-147.12 (m). 19F NMR (CDCl3): δ -144.1
(dd, J ) 7 and 22 Hz, 2F, 2ArF), -159.56 (t, J ) 22 Hz, 1F,
ArF), -164.5 (m, 2F, 2ArF). MS: m/e 264 (M+), 181 (loss of
C6H11).
1H NMR (DMSO-d6): δ 2.35 (s, 3H, CH3); 7.39 (d, 8 Hz,
2H, ArH), 7.56 (d, J ) 8 Hz, 2H, ArH); 7.81 (s, 1H, N)CH),
11.96 (s, 1H, NH). 13C NMR (DMSO-d6): 20.9 (s), 109.1 (dt,
J ) 4 and 13 Hz), 127.16 (s), 129.66 (s), 134.32 (q, J ) 3 Hz),
135.76 (s), 139.0-142.8 (m), 143.78 (s), 142.4-146.4 (m), 19
F
NMR (DMSO-d6): 139.84 (dd, J ) 7 and 22 Hz, 2F, 2ArF), δ
-150.5 (t, J ) 22 Hz, 1F, ArF), δ -159.94 (m, 2F, 2ArF).
Sodium Salt of 2,3,4,5,6-Pentafluorobenzaldehyde Tosyl-
hydrazone. To an ice-cooled solution of 2.00 g (5.50 mmol)
of 2,3,4,5,6-pentafluorobenz-aldehyde tosylhydrazone in 60 mL
freshly distilled ether was added 0.132 g (5.50 mmol) of NaH
(60% in mineral oil) upon stirring. The solution was then stirred
at room temperature for 24 h under N2. The large solid
suspension formed was filtered and washed with dry ether. The
solid was finally dried under high vacuum at room temperature.
The yield of the dry colorless sodium salt was 1.68 g (79%)
(2,3,4,5,6-Pentafluorophenyl)diazomethane. In a two-
necked round bottom flask was placed a magnetic stirrer. One
of the necks of the round bottom flask was connected to a
modified side arm addition flask containing 0.50 g (1.30 mmol)
sodium salt of 2,3,4,5,6-pentafluorobenzaldehyde tosylhydrazone
while a short distillation path was connected to the second. A
receiving flask attached to the distillation path was cooled in
dry ice-isopropyl alcohol bath. After the round bottom flask
was heated under vacuum (ca. 0.05 mmHg) in a 100 °C oil
bath in the dark, the salt was slowly added in small portions
upon stirring and by tapping the addition flask in 2 min intervals.
The orange-colored gas produced upon decomposition of the
sodium salt was collected as an orange solid in the receiving
flask. The flasks were finally flashed with nitrogen, and the
product was warmed to room temperature to give 0.22 g (82%)
of an orange-colored oil. 1H NMR (CDCl3): δ 4.90 (s, 1H,
HC-N2). 19F NMR (CDCl3): δ -147.92 (dd, J ) 7 and 22
Hz, 2F, 2ArF), -163.3 (t, J ) 22 Hz, 1F, ArF), -164.0 (m,
2F, 2ArF).
(b) Methanol. Only one product was formed and isolated
by preparative-GC, and this product was analyzed by GC-MS
1
and NMR, 13C NMR, and 19F NMR spectroscopies. 1NMR
(CDCl3): δ 3.39 (s, 3H, CH3), 4.55 (t, J ) 1 Hz, 2H, CH2).
13C NMR (CDCl3): δ 58.41 (s), 61.11 (s), 111.2 (m), 135.2-
139.7 (m), 143.3-147.8 (m). 19F NMR (CDCl3): δ -143.6
(dd, J ) 7 and 22 Hz, 2F, 2ArF), -154.85 (t, J ) 22 Hz, 1F,
ArF), -163.0 (m, 2F, 2ArF). MS: m/e 212 (M+), 181 (loss of
OCH3).
Acknowledgment. Support of this work by the National
Science Foundation (NSF-CHE Grant 8814950) is gratefully
acknowledged. One of us (A.D.G.) gratefully acknowledges a
NATO Fellowship. The authors are indebted to Drs. Borden
and Karney for a preprint of their manuscript.
References and Notes
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Carbenes and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p
239.
Product Analyses. Product studies were performed as
follows: Solutions of photolabile carbene precursors (0.015M)

3840 J. Phys. Chem. A, Vol. 101, No. 20, 1997
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