Spectra of tert-Butoxy and 2-Butoxy Radicals
J. Phys. Chem. A, Vol. 103, No. 41, 1999 8211
Mode assignments cannot be made for most of the progres-
sions noted here, and numerous additional bands are evident in
the spectra for which progressions cannot be reliably described.
Further progress is likely to require analysis of jet-cooled
spectra, and the complete analysis of these radicals poses a
significant challenge to spectroscopy. It is hoped that this work
will aid future investigations of the chemistry and kinetics of
alkoxy radicals as well as increasing interest in the spectroscopy
and photodissociation dynamics of alkoxy radicals containing
more than one or two carbon atoms.
Acknowledgment. Our interest in this topic originated in
conversations with R. Atkinson; we are in debt to him for his
encouragement and for many helpful discussions. This work
was funded by the National Science Foundation and by the
donors of the Petroleum Research Fund, administered by the
American Chemical Society. We are indebted to J. O. Clevenger
for designating us as the recipient of the R. W. Field group’s
surplus Nd:YAG laser. We further thank K. J. Guerin for
repeatedly going beyond the call of duty, Dr. J. Chaiken for
his advice and encouragement, and D. R. Katz for assistance
with some experiments.
Figure 5. Pressure dependence fluorescence lifetimes of 2-butoxy at
three different excitation wavelengths. Delay time: 10 µs. 5% 2-butyl
nitrite in N2 at a total pressure of 5.6 Torr. As mentioned in the text,
error bars on each data point were less than or equal to 3 ns.
TABLE 4: Zero-Pressure Fluorescence Lifetimes (ns) of
Different Vibrational Levels of B˜ State
References and Notes
radical
label
band origin (nm)
τ0 ((ions)
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tert-butoxy
(4-0)
(3-0)
(2-0)
a5
357.5
364.3
371.4
347.3
353.4
367.0
358.6
365.8
361.0
172
135
138
124
167
131
70
a4
a2
2-butoxy
(3-0)
(2-0)
a2
99
79
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(for CH3O, at least) a rather more dramatic effect on fluores-
cence lifetimes. The fluorescence lifetime of jet-cooled CH3O
exhibits some mode dependence while decreasing by as much
as 40% upon excitation of ∼3000 cm-1 of vibration in the A˜
state (well before the onset of predissociation).12,13,34 The extent
of the LIF excitation spectrum of 2-butoxy (4 C-O stretch
bands) is narrower than that of ethoxy and 2-propoxy (5 C-O
stretch bands),4,8,28 which is in turn narrower than that of
methoxy (6 or 7 C-O stretch bands). Predissociation of excited
methoxy radical begins at ν′C-O ) 6 and predissociation
essentially eliminates fluorescence from ν′C-O ) 8.11,12,34
(16) Reisler, H.; Noble, M.; Wittig, C. In Molecular Photodissociation
Dynamics; Ashfold, M. N. R., Baggott, J. E., Eds.; Royal Society of
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transitions of both tert-butoxy and 2- butoxy as B˜-X˜ .
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IV. Conclusions
Laser-induced fluorescence excitation spectra of tert-butoxy
and 2-butoxy (B˜-X˜ ) are observed following laser photolysis
of the corresponding butyl nitrites at 355 nm. For tert-butoxy,
the spectral range studied was extended down to the apparent
origin at 25 866 cm-1 (386.6 nm), far to the red of the region
examined in the initial study by Blitz and co-workers (330-
360 nm).6 This enables us to identify two progressions consisting
of 16 vibronic bands. Our results for 2-butoxy represent the
first observations of electronic spectra for this species. The
transition origin of 2-butoxy is tentatively assigned at 26 185
cm-1 (381.9 nm), and 15 vibronic bands in four progressions
are labeled. In each radical the dominant progression is assigned
to a C-O stretching vibration, with frequencies for tert-butoxy
and 2-butoxy radicals of ν′C-O ) 521 ( 10 cm-1 and ν′C-O
)
567 ( 10 cm-1, respectively. The other progressions labeled
here in the spectrum of 2-butoxy do not originate from C-O
stretching mode excitation. Overall fluorescence lifetimes for
tert-butoxy and 2-butoxy are ca. 150 and ca. 85 ns, respectively.
(28) Inoue, G.; Okuda, M.;Akimoto, H. J. Chem. Phys. 1981, 75, 2060.