Time-resolved infrared absorption spectroscopy was utilized to
monitor the production of HCOOH, CO2,
and CO following ultraviolet laser excitation of gas-phase formate
esters. Excitation of ethyl formate at 227.5 nm
resulted in formation of HCOOH and CO2. The
CO2 quantum yield was estimated to be 0.5 ± 0.1. No
evidence
for CO formation was obtained at this wavelength. Relative quantum
yields for the Norrish Type II elimination of
HCOOH from ethyl, n-propyl, n-butyl, isopropyl,
isobutyl, and tert-butyl formate were obtained at 227.5 and
222
nm. Normalization of the observed HCOOH yields with respect to the
number of γ-hydrogen atoms resulted in
reactivity trends at 227.5 nm of 1:3:9 for the abstraction of primary,
secondary, and tertiary hydrogen atoms,
respectively. At 222 nm, a similar reactivity trend was observed
with yields per available γ-hydrogen of 1:3:7 for
abstraction of primary, secondary, and tertiary hydrogen atoms.
Yields were found to be independent of ester pressure
over the range 100−550 mTorr. Semiempirical and ab
initio calculations of the excited state hydrogen
abstraction
step were performed and enthalpies of activation of 8−12 kcal/mol
were obtained using AM1 with configuration
interaction.
Investigation of the patterns of reaction product internal energy release is an important tool in developing a fundamental understanding of the molecular-level mechanisms of reactions and, in particular, revealing insight into the details of the relevent potential energy surfaces (PES). Photochemical reactions which produce simple products have been the focus of a great deal of attention. 1 Our goal is to investigate the dynamics of reactions which produce polyatomic species, such as the well-known Norrish Type II elimination. 2 Reported here is the first study of Norrish Type II photodissociation dynamics. Relative populations were determined for selected rotational states of HCOOH produced in the 222-nm photolysis of isobutyl formate as shown in reaction 1.Formic acid can be treated as a planar, nearly-prolate rotor. 3 Rotational states can be identified where the total angular momentum is projected on either the a-axis or the c-axis. Projection on the a-axis corresponds approximately to rotation about the CdO bond axis and is designated with the rotational quantum state J J,0 . The c-axis projection, J 0,J , corresponds to rotation in the plane of the molecule. 4 The experimental method involves monitoring the populations of rotational states, as shown below, which correspond to the two limiting cases for angular momentum projections described above.The general approach is similar to recently reported experiments on the Norrish Type I dissociation of acetaldehyde where no K-level dependence was observed for the HCO fragment. 5 Here, we demonstrate that a nonstatistical rotational state distribution with an apparent propensity for rotation about the a-axis can be observed even in room temperature bulb gas experiments.The experimental apparatus is described in a paper on quantum yield measurements. 6 All of the dynamics experiments were performed with 222-nm photolysis of 100-mTorr samples of isobutyl formate. Transient IR signals were obtained for seven rotational states with J e 10 using R-branch transitions of the carbonyl stretching vibrational band. In order to normalize for infrared transition intensities, formic acid (Lancaster, 97%) was carefully degassed via multiple freeze-pumpthaw cycles in liquid nitrogen. Under the experimental conditions used here, the vapor was significantly enhanced in HCOOH 7 and no correction was made for the presence of water vapor. Pressures were corrected for the HCOOH monomerdimer equilibrium. 8 IR absorption coefficient measurements were made using diode laser current modulation settings which were identical to those used in the transient IR measurements. Line strengths were normalized with respect to the fraction of HCOOH molecules in a given rotational state at room temperature. The standard asymmetric rotor expansions were used to calculate rotational energies. 9 Normalized rotational state populations were obtained by dividing the IR ∆I/I 0 by the (2J + 1) rotational state degeneracy and by the infrared line strength. In addition, the relative populations were nor...
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