Time-Resolved IR Study of Benzoyl Radicals
A R T I C L E S
concentrations ranging from 15 to 50 mM for the TRIR experiments.
Sample purity was confirmed by gas chromatography/mass spectrometry
(GC-MS) (>95%). Acetonitrile (Aldrich, spectrophotometric grade) was
used as received.
kinetic analysis of the transient to be problematic. It has been
demonstrated that time-resolved infrared (TRIR) spectroscopy
is a powerful technique for the direct detection of benzoyl and
aliphatic acyl radicals.19-22 This is due to the fact that these
radicals generally have strong νCO absorptions, in the range
1780-1880 cm-1, with high extinction coefficients; e.g., for
the unsubstituted benzoyl radical with νCO ) 1828 cm-1 in
Diode laser-based TRIR experiments, at Nottingham23 and Colum-
bia,6 employed pulses from a Nd:YAG laser (Spectra Physics GCR-
150-30 or GCR-12) as an excitation source. The IR probe consisted of
a diode laser (Mu¨tek MDS 1150/2) fitted with a monochromator (Mu¨tek
MDS 1400S) and sample chamber with a fast mercury cadmium
telluride (MCT) detector (Kolmar Technologies KMPV11-1-J1). The
signal from the detector was amplified by a low-noise preamplifier
(Stanford Research Systems SR 560) and captured with a digital storage
oscilloscope (Tektronix TDS 360). Argon-saturated acetonitrile solutions
were flowed continuously through a 1 mm path length gastight IR cell
fitted with calcium fluoride windows.
n-hexane, ꢀ ≈ 1300 M-1 cm-1 22
.
Time-resolved step-scan Fourier transform infrared (s2-FTIR) experi-
ments were conducted with a combination of a Nicolet Magna 860
interferometer and a Nd:YAG laser (Spectra Physics GCR-12).24
Synchronization of the laser with data collection was achieved by use
of a pulse generator (Stanford DG535). The interferometer is equipped
with both an internal 100 kHz 16-bit digitizer and an external 100 MHz
12-bit digitizer (Gage 8012A). In these experiments a 1 mm photo-
voltaic MCT detector was used with a 20 MHz preamplifier. This
detector has AC and DC outputs and both outputs are digitized
simultaneously to ensure proper phase matching. The AC signal was
amplified by an external preamplifier (Stanford SR 560) to use the
full dynamic range of the digitizer. Single-sided interferograms were
obtained from one laser pulse at each mirror position. We have used
an external optical bench (Nicolet-TOM) for locating the sample cell
and MCT detector. This allows easy manipulation of the UV/visible
laser beam through the cell. The s2-FTIR spectra in this paper were
recorded at 8 cm-1 resolution with four or eight scans of the
interferometer.
The ps-TRIR facility at the Rutherford Appleton Laboratory has been
described in detail elsewhere.25,26 Briefly, part of the output from a 1
kHz, 800 nm, 150 fs, 2 mJ Ti:sapphire oscillator/regenerative amplifier
was used to pump a white light continuum seeded BBO OPA. The
signal and idler produced by this OPA were difference frequency mixed
in a type I AgGaS2 crystal to generate tunable midinfrared pulses (ca.
150 cm-1 fwhm, 1 µJ). Second harmonic generation of the residual
800 nm light provided 400 nm pulses, which were used to excite the
sample. Changes in infrared absorption were recorded by normalizing
the outputs from a pair of 64-element MCT linear array detectors on a
shot-by-shot basis.
DFT calculations were performed with Q-Chem software.27 The
EDF1 functional28 was used with the 6-31+G* basis set to calculate
the geometrical structure and harmonic vibrational frequencies and
intensities of radicals 1b-6b. EDF1 is an empirical exchange-
correlation density function that was constructed with a set of accurate
experimental data for calibration.28 The justification for using this
functional with the 6-31+G* basis set will not be discussed in this
In this paper we report a systematic TRIR study of the
reactivity of benzoyl radicals 1b-6b toward the acrylic
monomer n-butylacrylate and toward oxygen addition. We have
also measured their reactivity toward hydrogen and bromine
atom donors. The benzoyl radicals were generated by laser
photolysis of photoinitators 1a-6a. The photophysics and
photochemistry of 5a have been studied in detail by both
nanosecond and picosecond TRIR. In addition, density func-
tional theory (DFT) has been used in order to help elucidate
some of the experimental observations.
Experimental Section
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The photoinitiators, 1-hydroxycyclohexyl phenyl ketone (1a), 2-hy-
droxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (2a), 2-meth-
yl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone (3a), 2-benzyl-
2-(dimethylamino)-1-(4-morpholinophenyl)-1-butanone (4a), (2,4,6-
trimethylbenzoyl)diphenylphosphine oxide (5a), and bis(2,6-di-
methoxybenzoyl)(2,4,4-trimethylpentyl)phosphine oxide (6a), (all from
Ciba Specialty Chemicals) were used as received and made up to
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M. W.; Grills, D. C. Central Laser Facility, Rutherford Appleton Laboratory
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