J. Am. Chem. Soc. 1996, 118, 3527-3528
The Lifetime of Formylcarbene Determined by
3527
Transient Absorption and Transient Grating
Spectroscopy
John P. Toscano and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity
120 West 18th AVenue, Columbus, Ohio 43210
Valerij Nikolaev
Department of Chemistry
St. Petersburg State UniVersity
UniVersity Prospekt 2, St. Petersburg, Russia
Figure 1. Double reciprocal treatment of the yield of ylide 1 produced
by LFP (308 nm) of FDM in methylene chloride at ambient temperature.
Yanni Cao and Matthew B. Zimmt*
Department of Chemistry, Brown UniVersity
ProVidence, Rhode Island 02912
Scheme 1
ReceiVed NoVember 13, 1995
Aromatic carbenes have been thoroughly studied by transient
absorption spectroscopy.1,2 In condensed phases simple aryl-
carbenes do not undergo intramolecular rearrangements which
might otherwise limit their lifetimes. The aromatic component
also provides a convenient chromophore for UV-vis detection.
Recently much attention has been directed toward the study
of alkylcarbenes3 and acylcarbenes.4 These species do undergo
facile rearrangements and thus are much shorter lived than
typical arylcarbenes. They also lack convenient chromophores.
We have reported lifetimes of various alkyl- and acylcarbenes
using the pyridine ylide-nanosecond laser flash photolysis
technique.5 This technique utilizes a competition between all
first-and pseudo-first-order carbene reactions with a pyridine-
dependent second-order reaction which produces a useful UV-
vis active probe (ylide 1, Scheme 1).
Laser flash photolysis (LFP) of formyldiazomethane (FDM)
produces singlet formylcarbene (FC) which can isomerize,6 or
react with solvent or pyridine (Scheme 1).4 It is not possible
to resolve the rate of formation of ylide 1 in methylene chloride
with the spectrometer available in Columbus (20 ns time
resolution). However, standard double reciprocal analysis
(Figure 1) of the optical yield of ylide with pyridine concentra-
tion yields the ratio kPYR/k0 (or kPYRτ) equal to 0.73 M-1 where
k0 is the sum of all first-order and pseudo-first-order rate
constants of all processes which consume the carbene in the
absence of pyridine and τ (1/k0) is the carbene lifetime as defined
in Scheme 1.3 The value of kPYR is typically taken as 1-5 ×
109 M-1 s-1 which indicates that the lifetime of singlet formyl
carbene lies in the range of 0.15-0.73 ns in methylene chloride.
This is consistent with our previous estimate of τ as 0.5-2.3
ns in Freon-113 (CF2ClCFCl2).4c,d The ratio kCH OH/kPYR ) 4.6,
in Freon-113, was determined by Stern-Volmer3analysis of the
quenching of the yield of 1 as a function of methanol
concentration at a constant concentration (3.1 M) of pyridine
(Figure 2) (Scheme 1). At this concentration of pyridine in
Freon-113, all FC produced in the laser pulse is captured by
pryridine. Thus, in Freon-113 we can deduce that k
τ )
CH3OH
3.4 M-1. This analysis assumes that all of the chemistry of FC
in solution proceeds through the singlet state of the carbene.
To confirm this analysis and its implicit assumptions, a
technique with superior time resolution, transient grating
spectroscopy, was utilized. Transient grating based calorimetric
techniques provide sufficient time resolution to determine
transient lifetimes of 1 ns or less.7 Gratings were generated in
1.5-2.0 × 10-3 M solutions of FDM in CH2Cl2 by crossing
two 279-nm pulses (45 ps fwhm, <1 µJ, 400 µm) at a 37° angle
in a 1 mm suprasil flow cell. The time dependence of the
resulting thermal phase grating was probed using a 633-nm dye
laser pulse (60 ps fwhm, <0.5 µJ, 250 µm) incident on the
sample at the Bragg angle. The photochemically induced
diffraction waveform was adequately fit (Figure 3) using a
model containing only two heating terms: a kinetically un-
resolvable (fast) heat release and one resolvable (slow) expo-
nential heat release.7d No dispersive or absorptive terms in the
model were required to fit the data.7b The rate constant of
(1) Platz, M. S.; Maloney, V. M. Kinetics and Spectroscopy of Carbenes
and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p 239.
(2) Moss, R. A.; Turro, N. J. Kinetics and Spectroscopy of Carbenes
and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p 213.
(3) Platz, M. S.; Modarelli, D. A.; Morgan, S.; White, W. R.; Mullins,
M.; Celebi, S.; Toscano, J. P. Prog. React. Kinet. 1994, 19, 93.
(4) (a) Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popic, V. J. Am. Chem.
Soc. 1994, 116, 8146. (b) Wang, J. L.; Toscano, J. P.; Platz, M. S.; Nikolaev,
V.; Popik, V. J. Am. Chem. Soc. 1995, 117, 5477. (c) Toscano, J. P.; Platz,
M. S.; Nikolaev, V. J. Am. Chem. Soc. 1995, 117, 4712. (d) It was not
possible to saturate the yield of ylide in methylene chloride, thus, Stern-
Volmer experiments were performed in Freon-113.
(5) Jackson, J. E.; Platz, M. S. AdVances in Carbene Chemistry; Vol. 1,
Brinker, U., Ed.; JAI Press: Greenwich, CT, 1994; Vol 1, p 89.
(6) (a) Baron, W. J.; DeCamp, M. R.; Hendrick, M. E.; Jones, M., Jr.;
Levin, R. H.; Sohn, M. B. In Carbenes; Jones, M., Jr., Moss, R. A., Eds.;
Krieger: Malabar, FL, 1983; Vol. I. (b) Moss, R. A.; Jones, M., Jr. In
ReactiVe Intermediates; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New York,
1981, p 61. (c) Moss, R. A.; Jones, M., Jr. In ReactiVe Intermediates; Jones,
M., Jr., Moss, R. A., Eds.; Wiley: New York, 1985; p 95. (d) Marchand,
A. P.; Brockway, N. M. Chem. ReV. 1974, 74, 431. (e) Kirsme, W. Carbene
Chemistry, 2nd ed.; Academic Press: New York, 1978. (f) Meier, H.; Zeller,
K.-P. Angew. Chem., Int. Ed. Engl. 1975, 14, 32. (g) Torres, M.; Lown, E.
M.; Gunning, H. E.; Strausz, O. P. Pure Appl. Chem. 1980, 52, 1623. (h)
Zeller, K.-P. Tetrahedron Lett. 1977, 707. (i) Zeller, K.-P.; Meier, H.;
Kolshor, H.; Muller, E. Chem. Ber. 1972, 105, 1875. (j) Timm, U.; Zeller,
K.-P.; Meier, H. Tetrahedron 1977, 33, 453.
(7) (a) Miller, R. J. D. In AdVances in Spectroscopy; Clark, R. J., Hester,
R. E. Eds.; John Wiley and Sons: New York, 1989; Vol. 18, p 1. (b) Morais,
J.; Zimmt, M. B. J. Phys. Chem. 1995, 99, 8863. (c) Ma, J.; Dutt, B.;
Waldeck, D. H.; Zimmt, M. B. J. Am. Chem. Soc. 1994, 116, 10619. (d)
74% of the detected expansion is prompt, the remaining 26% occurs with
a time constant of 900 ps.
0002-7863/96/1518-3527$12.00/0 © 1996 American Chemical Society