4182 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Niu et al.
CO2 (0m10, J) + hν (∼2270 cm-1) f CO2 (0m11, J ( 1)
abstraction step were performed for comparison with observed
trends in HCOOH production. The competition between the
Norrish Type I channel which results in CO2 formation and the
Norrish Type II elimination was also explored.
(6)
Carbon monoxide was also probed as shown below.
Most of the previous experimental and theoretical work on
Norrish Type II processes has been done on ketone or aldehyde
reactants. Choice of these reactants would present an additional
difficulty in the study of the reaction dynamics due to the enol-
keto isomerization step. Production of an acid from the
photolysis of esters eliminates this complication. Formate esters
represent the simplest system of this type and there is well-
documented11 HCOOH rovibrational spectroscopy, making this
the most attractive class of precursors for gas-phase photo-
dissociation dynamics studies.
CO(ν ) 0, J) + hν (∼2100 cm-1) f CO(ν ) 1, J ( 1) (7)
Experimental Section
Formic acid relative yield determinations were performed at 227.5
and 222 nm. For the 227.5-nm experiments, a Questek 2520 v-â XeCl
excimer laser, operated at 1 Hz, was used to pump a Lambda Physik
Fl3002 dye laser set at 445 nm. A dye solution of Coumarin 440 in
methanol with DABCO added for dye stability was used. The output
of the dye laser was directed through a BBO (B cut) crystal housed in
an InRad autotracker. The doubled output was separated from the
residual fundamental using a four-prism beam separator. Choice of
this wavelength was made to maximize the UV output from our
apparatus at a wavelength where the esters absorb. Upgrading the
excimer laser to model 2720 enabled operation at 222 nm (KrCl).
Absorption coefficients at 227.5 and 222 nm were determined for
each of the six esters using the experimental apparatus with a 185-cm
cell. Laser energies were measured with a Molectron J25 joulemeter
and were corrected for window absorption. Sample pressures were
varied from 200 to 800 mTorr. Within the limits of experimental
uncertainty, the absorption coefficient at a particular wavelength was
not dependent on the identity of the ester. The absorption coefficients
were found to be 4.5 ((0.4) × 10-3 and 5.8 ((0.5) × 10-3 cm-1 Torr-1
at 227.5 and 222 nm, respectively. These results are consistent with
the published ethyl formate spectrum which has absorption coefficients
of approximately 4.9 × 10-3 and 6.5 × 10-3 cm-1 Torr-1 at the
corresponding wavelengths.1
1
The onset of absorption for the (nπ*) transition of formate
esters is observed at 255 nm and the absorption peaks at
approximately 215 nm.1 Ethyl formate is the smallest ester
which can undergo the Norrish Type II elimination. Review
of the early photochemical studies on formate esters led to
postulation1 of four potentially important photodissociation
channels as shown below for ethyl formate.
CH3CH2OCHO + hν f CH3CH2O + HCO
f CH3CH2CO2 + H
(1)
(2)
(3)
(4)
f CH3CH2OH + CO
f CH2CH2 + HCOOH
The continuous wave infrared laser source was a Laser Photonics
liquid nitrogen cooled system. For the 227.5-nm experiments, the IR
beam was steered into the cell slightly off-axis with the UV beam so
that the beams would overlap over the 275 cm path length. Dichroic
beam splitters were used to overlap the UV and IR beams during the
222-nm experiments. Identical beam alignment was carefully main-
tained for all experiments at a given excitation wavelength. The infrared
output was directed through a PTI 1/4-m monochromator, which served
to filter the detector from stray light and unwanted second mode
emission from the IR source. Infrared intensities were monitored with
a Judson HgCdTe detector with matched preamplifier. The rise time
of the detection system was approximately 1 µs. Transient IR signals
were averaged over 1000 UV laser shots using a LeCroy 9410 digital
oscillosope and then transferred to a Northgate 386 PC for storage and
later analysis.
Channels 1 and 2 are the Norrish Type I cleavage reactions.
Facile thermal decarboxylation12 from the alkoxyl product of
channel 2 should result in CO2 production. Carbon monoxide
production has been reported for the photolysis of a variety of
formate esters.13,14 The Norrish Type II reaction is channel 4
above.
Preliminary survey work was performed to look for stable
molecular productions from ethyl formate channels 2 through
4. Relative HCOOH yields were determined at 227.5 and 222
nm from methyl, ethyl, n-propyl, n-butyl, isopropyl, isobutyl,
and tert-butyl formate. The basic experimental scheme is shown
below for detection of HCOOH, CO2, and CO using infrared
diode laser absorption spectroscopy. HCOOH was monitored
via the ν3 carbonyl stretching mode. The rotational states of
The 1986 AFGL HITRAN database16 was used for identification of
CO2 and CO transitions and for calculations of quantum yields of these
species. Calibration of the IR laser wavelength in the HCOOH probe
region was performed using water vapor as a reference gas, with
assignments taken from the HITRAN database. Formic acid transitions
were then identified from literature assignments11 of ν3 lines. During
transient IR measurements, the diode laser was tuned to the peak of a
transition for the product molecule of interest. No transient signals
were observed with the laser detuned slightly from the probe line. All
of the quantum yield experiments were performed using the HCOOH
this asymmetric rotor are designated by JKa,Kc
.
HCOOH(ν3 ) 0, J′′Ka′′,Kc′′) +
hν(∼1770 cm-1) f HCOOH(ν3 ) 1, J′Ka′,Kc′) (5)
Use of the diode laser technique to monitor CO2 and CO has
been well-documented.15 CO2 was monitored via the antisym-
metric stretching transition. The bending state, ν2, quanta are
designated as ml and, in the work reported here, the probe
transitions always involved lower states with no quanta of ν1
symmetric stretch or ν3 antisymmetric stretch excitation.
transition (ν3 ) 0, J ) 90,9) f (ν3 ) 1, J ) 100,10) at 1784.023 cm-1
.
Reference samples of formic acid (Lancaster, 97%), CO2 (Airco,
99.9%), and CO (Airco, C. P. Grade) were prepared by degassing prior
to use with several freeze-pump-thaw cycles in liquid nitrogen.
Commercial samples of ethyl formate (Aldrich, 97%), n-propyl formate
(Kodak, 99%), isopropyl formate (Pfaltz & Bauer), n-butyl formate
(Sigma, 97%), isobutyl formate (Aldrich, 97%), and tert-butyl formate
(Aldrich, 99%) were carefully purified17 before use to eliminate any
residual HCOOH which would interfere with experimental measure-
(11) Weber, W. H.; Maker, P. D. J. Mol. Spectrosc. 1987, 121, 243.
(12) (a) Gray, P.; Thynne, J. C. J. Nature 1961, 191, 1357. (b) Thynne,
J. C. J. Trans. Faraday Soc. 1962, 58, 676.
(13) Ausloos, P. Can. J. Chem. 1958, 36, 383.
(14) Yee Quee, M. J.; Thynne, J. C. J. Trans. Faraday Soc. 1967, 63,
1656.
(15) (a) Kreutz, T. G.; O’Neill, J. A.; Flynn, G. W. J. Chem. Phys. 1987,
87, 4598. (b) Holland, J. P.; Rosenfeld, R. N. J. Chem. Phys. 1988, 89,
7217. (c) Weiner, B. R.; Pasternack, L.; Nelson, H. H.; Prather, K. A.;
Rosenfeld, R. N. J. Phys. Chem. 1990, 94, 4138. (d) Hall, G.; Vanden Bout,
D.; Sears, T. J. J. Chem. Phys. 1991, 94, 4182. (e) Suzuki, T.; Kanamori,
H.; Hirota, E. J. Chem. Phys. 1991, 94, 6607. (f) Alvarez, R. A.; Moore,
C. B. J. Phys. Chem. 1994, 98, 174.
(16) Rothman, L. S.; Gamache, R. R.; Goldman, A.; Brown, L. R.; Toth,
R. A.; Pickett, H. M.; Poynter, R. L.; Flaud, J.-M.; Camy-Peyret, C.; Barbe,
A.; Husson, N.; Rinslaud, C. P.; Smith, M. A. H. Appl. Opt. 1987, 26,
4058.
(17) Riddick, J. A.; Bunger, W. B. Organic SolVents, Physical Properties
and Methods of Purification; Wiley-Interscience: New York, 1970; Vol.
2, pp 748-55.