A New View of the Norrish Type II Elimination:  Product Rotational State Distributions

Abstract

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...

Formate Ester Norrish Type II Elimination: Diode Laser Probing of Gas-Phase Yields

“…Comparison of gas-phase HCOOH production from different precursors using a quantum-state-specific probe technique, such as time-resolved infrared diode laser absorption spectroscopy, requires consideration of potential differences in product state distributions. At lower sample pressures and short times, nonstatistical rotational state distributions are observed as discussed in an accompanying communication …”

“…At lower sample pressures and short times, nonstatistical rotational state distributions are observed as discussed in an accompanying communication. 31 Since the nascent internal energy partitioning and translational energy release into HCOOH may not be the same for each of the esters, quantum yield measurements must be performed on equilibrated samples. Working with moderate pressure samples serves to provide adequate collisional equilibration since no clear pressure dependence of the yields is observed.…”