Matrix and time-resolved infrared spectroscopy of chloro-p-nitrophenylcarbene and related species

Article abstract of DOI:10.1021/jp011841a

Chloro-p-nitrophenyldiazirine (2) was deposited in an argon matrix. Photolysis (350 nm) releases chloro-p-nitrophenylcarbene (1) as a persistent species. The IR and UV-vis spectra of carbene 1 were obtained, and the IR spectrum was adequately simulated by

Full text of DOI:10.1021/jp011841a

J. Phys. Chem. A 2001, 105, 8413-8416
8413
Matrix and Time-Resolved Infrared Spectroscopy of Chloro-p-nitrophenylcarbene and
Related Species
Meng-Lin Tsao, Zhendong Zhu, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210
ReceiVed: May 14, 2001; In Final Form: July 11, 2001
Chloro-p-nitrophenyldiazirine (2) was deposited in an argon matrix. Photolysis (350 nm) releases chloro-p-
nitrophenylcarbene (1) as a persistent species. The IR and UV-vis spectra of carbene 1 were obtained, and
the IR spectrum was adequately simulated by density functional theory (DFT) calculations. The carbene has
an intense IR vibration at 1206 cm-1 involving the carbene carbon and aromatic ring carbon. This band was
observed by TRIR spectroscopy upon laser flash photolysis (355 nm) of 2 in heptane at ambient temperature.
The presence of benzene did not influence the frequency of this vibration. The carbene lifetime as measured
by TRIR spectroscopy (≈400 ns) is consistent with previous LFP studies utilizing UV-vis detection. Upon
LFP of 2 in CCl4 or CF2ClCFCl2 the photogenerated carbene abstracts chlorine atom from solvent to form
R,R-dichloro-p-nitrobenzyl radical with a TRIR band at 1316 cm-1. Upon LFP of 2 in acetonitrile, acetone,
methyl acetate, pyridine, and tetrahydrofuran ylide species were produced. The ylides had similar TRIR spectra
with bands at 1584, 1504, and 1312 cm-1.
I. Introduction
(JASCO). The intensity change of the IR light induced by
photoexcitation is monitored as a function of time by an MCT
Carbenes are important intermediates in organic chemistry.1
As a consequence, they have been extensively studied as
transient species in the liquid phase by laser flash photolysis
methods with UV-vis detection.2 Carbenes have also been
studied as persistent species in solid argon at cryogenic
temperatures.3
There has been recent dramatic progress in the development
of time-resolved infrared (TRIR) spectroscopy. Prior to initiating
studies of new carbenes by the TRIR technique, we thought it
prudent to apply this methodology to a relatively well understood
system. We also hoped to probe carbene-solvent interactions
using IR spectroscopy, a potentially more sensitive probe than
transient UV-vis spectroscopy. Herein we are pleased to report
our TRIR investigation of chloro-p-nitrophenylcarbene (1) and
radical and ylide species derived from it.
photovoltaic IR detector (Kolmar Technologies, KMPV11-1-
J1), with a 50 ns rise time amplified with a low-noise NF
Electronic Instruments 5307 differential amplifier and digitized
with a Tektronix TDS784D oscilloscope. The TRIR spectrum
is analyzed by the IGOR PRO program (Wavematrics Inc.) in
the form of a difference spectrum, ∆At ) - log(1 + ∆It/I),
where ∆It is the intensity change induced by photoreaction at
time t, and I is the IR intensity with the sample without
photoexcitation. Thus, depletion of reactant and formation of
transient intermediates or products lead to negative and positive
signals, respectively.
Matrix Isolation Spectroscopy. The diazirine precursor 2
was placed in a glass U tube that was directly connected to a
closed cycle cryogenic system cooled by helium (Air Products).
Argon gas blew over the precursor and condensed on the surface
of a CsI window. The argon matrix formed was maintained at
14 K during the entire experiment. UV-vis spectra were
measured with a Lambda 6 UV-vis spectrophotometer, and
the IR spectrum was recorded with an FT-IR 2000 Perkin-Elmer
spectrometer with 2 cm-1 resolution. Ray-O-Net 350 nm lamps
were used to photolyze the sample and the resulting IR and
UV-vis spectra were recorded sequentially in each step of the
photolysis cycle.
II. Experimental Section
Chloro-p-nitrophenyldiazirine (2) was synthesized as de-
scribed in the literature.4
Density Functional Theory Calculations. Geometries of
singlet chloro-p-nitrophenylcarbene (1) and its radical (3) and
ylide (4-8) derivatives were fully optimized at the B3LYP level
of theory using the 6-31G* basis set.7 Vibrational frequencies
of these species were performed at the same level, and no
imaginary frequencies were found. All the calculations were
performed using the GAUSSIAN 98 program.8
Time-Resolved Infrared (TRIR) Studies. TRIR experiments
were conducted with a JASCO TRIR-1000 dispersive-type IR
spectrometer with 16 cm-1 resolution following the method
described in the literature.5,6 Briefly, a reservoir of sample
solution (10 mL of 8 mM of chloro-p-nitrophenyl diazirine with
or without 10-500 mM of acetonitrile, acetone, methyl acetate,
tetrahydrofuran, or pyridine) is continually circulated between
two calcium fluoride salt plates with 0.5 mm path length. The
sample was excited by 355 nm flash pulses of an Nd:YAG laser
(97 Hz repetition rate, 0.5-0.7 mJ/pulse power), which is
crossed with the broadband output of a MoSi2 IR source
III. Results and Discussion
Chloro-p-nitrophenyldiazirine (2) was deposited in an argon
matrix at 14 K. Photolysis (350 nm, 6 h) led to disappearance
10.1021/jp011841a CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/28/2001

8414 J. Phys. Chem. A, Vol. 105, No. 37, 2001
Tsao et al.
Figure 1. TRIR spectrum produced upon LFP (355 nm) of 2 from 0
to 200 ns in heptane at ambient temperature.
Figure 2. TRIR spectrum produced upon LFP (355 nm) of 2 from 0
to 500 ns (solid line) and 1500 to 2000 ns (dashed line) in CCl4 at
ambient temperature.
of the IR and UV-vis of the diazirine and the appearance of
new spectral features summarized in the Supporting Information.
These new features will be assigned to carbene 1 on the basis
of calculations and by analogy to related compounds.
The IR spectrum of the product of photolysis of 2, given in
Supporting Information Figure S1(A), is in agreement with the
previous study of Wierlacher et al.9 The vibrational spectrum
of carbene 1 calculated using density functional theory (DFT)
at the B3LYP level of theory using the 6-31G* basis set is
shown in Figure S1(B) after application of a scaling factor of
0.96. The agreement is satisfactory. The carbene has an intense
absorption at 1206 cm-1, in a spectral region where the diazirine
absorbs weakly. This vibration largely (∼95%) consists of a
C-C stretch involving the carbene carbon and aromatic ring
(see Table S2, Supplementary Information). Unsubstituted
chlorophenylcarbene has a similar absorption band at 1225
Figure 3. Decay of transient absorption at 1206 cm-1 (A) and the
growth of transient absorption at 1316 cm-1 (B) following LFP (355
nm) of 2 in CCl4.
The frequency of the carbene vibration at 1206 cm-1 is not
influenced by the presence of 0.1-1 M benzene. Thus, there is
no evidence of complexation of this carbene with benzene from
vibrational spectroscopy. This is in accord with our kinetic
studies of this system.13
LFP of 2 in either Freon-113 (CF2ClCFCl2) or carbon
tetrachloride (Figure 2) leads to the presence of a new transient
species absorbing at 1316 cm-1. This species grows and decays
with comparable time constants. If k1 and k2 are the rate
constants for the formation and decomposition of this intermedi-
ate, we find that their apparent values are k1 ) 1.5 × 106 s-1
and k2 ) 4.8 × 105 s-1. These values are only approximate
because of the complex nature of the signal (Figure 3).
The species absorbing at 1316 cm-1 is associated with radical
3, produced by chlorine atom abstraction reaction of the carbene
with solvent, because the lifetime of the carrier of the 1316
cm-1 10 The large band of carbene 1 at 830 cm-1 is the mixing
.
of the bending of the nitro group and the asymmetric stretching
of C-C-Cl.
The UV-vis spectrum of the product of photolysis of 2 has
a strong absorption at 320 nm, in reasonable agreement with
the reported absorption maximum of carbene 1 observed in laser
flash photolysis experiments.4,11 In addition to the strong
absorption in the UV region there is a weak broad band in the
visible above 700 nm. This is a characteristic band of closed
shell singlet carbenes. Chlorophenylcarbene, for example,
absorbs at 700 nm. This band is essentially a HOMO-LUMO
transition in which an electron is promoted from the in-plane
hybrid orbital to the out-of-plane p orbital.12
Time-Resolved Infrared Spectroscopy. The IR band of 1
at 1206 cm-1 is attractive for TRIR spectroscopy because it is
an intense band unique to the carbene system. Laser flash
photolysis (LFP) of 2 in heptane produces the TRIR spectrum
of Figure 1, which clearly shows the presence of the 1206 cm-1
vibration. The carrier of this signal decays exponentially with
a time constant of 400 ns at ambient temperature, a result
consistent with LFP studies of carbene 1 with UV-vis detec-
tion.4,11 The lifetime of 1, measured at 1210 cm-1 in CCl4, is
slightly sensitive to the presence of oxygen. The observed rate
constant of disappearance of the carbene increases (k ) 1.5 ×
106 and 1.8 × 106 s-1) when bubbling the solution with argon
and oxygen, respectively, as expected from published re-
ports.2,11c
cm-1 vibrational band is sensitive to the presence of oxygen
and because DFT calculations predict that 3 will have an intense
absorption at 1328 cm-1 (Supporting Information). Chlorine
atom abstraction reactions of arylhalocarbenes have been studied
previously by laser flash photolysis methods with UV-vis
detection.14
Because of absorption by the solvent, carbene 1 cannot be
monitored in Freon-113. Both the carbene and radical can be
observed in CCl4 (Figure 2). The formation of the radical
appears to be faster than the decay of the carbene (Figure 3).
We suspect this is an artifact of the poor separation of the growth
and decay dynamics of the radical.

IR Spectroscopy of Chloro-p-nitrophenylcarbene
J. Phys. Chem. A, Vol. 105, No. 37, 2001 8415
The TRIR spectra of ylides 5-8 are given in the Supporting
Information. The spectra are adequately simulated by DFT
calculations. The TRIR spectra of ylides 4-8 are all rather
similar.
IV. Conclusions
The IR and UV-vis spectra of chloro-p-nitrophenylcarbene
(1) as a persistent species have been obtained in argon at 14 K.
The spectra are adequately simulated by DFT calculations.
Carbene 1 has a prominent vibration at 1206 cm-1 that involves
a C-C stretch between the carbene carbon and adjacent aromatic
ring carbon. This band of 1 is readily detected upon LFP of
diazirine 2 in heptane because the precursor and stable reaction
products have little absorption in this region. The frequency of
this vibration is not influenced by the presence of benzene. There
is no TRIR evidence of complexation of this carbene with
benzene. Upon LFP of 2 in CCl4 or CF2ClCFCl2 the TRIR
spectrum of the R,R-dichloro-p-nitrobenzyl radical is detected
at 1316 cm-1. Upon LFP of 2 in acetonitrile, acetone, methyl
acetate, pyridine, or tetrahydrofuran, the TRIR spectra of ylides
are observed. The TRIR spectra of the radical and ylides derived
from chloro-p-nitrophenyl carbene are well simulated by DFT
calculations. The spectra of the ylides are all quite similar and
have prominent bands of 1584, 1504, and 1312 cm-1. These
vibrations are all associated with the p-nitrophenyl ring. It is
demonstrated that TRIR spectroscopy, in parallel with DFT
calculations, is a useful way of studying carbenes and related
reactive intermediates.
Figure 4. TRIR spectrum produced upon LFP of 2 from 500 to 1000
ns in CD3CN at ambient temperature (A). IR spectrum of ylide 4
calculated by DFT with the B3LYP/6-31G* basis set after scaling by
0.96 (B).
Figure 5. TRIR spectrum produced upon LFP of 2 from 500 to 1000
ns in CH3CN at ambient temperature.
Acknowledgment. We are indebted to Professor Gustafson
for training on the TRIR spectrometer. Financial support of the
National Science Foundation (NSF CHE-9613861) is gratefully
acknowledged.
Laser flash photolysis of 2 in CD3CN and CH3CN produces
the TRIR spectrum of Figures 4 and 5. This spectrum is
attributed to ylide 4, previously observed by UV-vis spectros-
Supporting Information Available: Figures of IR and UV-
vis spectra of 1 and 2 and TRIR spectra of 5-8. Tables of
optimized geometries and vibrational frequencies of 1 and its
radical and ylide derivatives. This information is available free
of charge via the Internet at http://pubs.acs.org.
copy.4,11,13 The ylide is formed with observed rate constant 5.8
× 106 s-1 and has a lifetime of several microseconds at ambient
temperature.
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