Determination of the Barrier Height for Acetyl Radical Dissociation from Acetyl Chloride Photodissociation at 235 nm Using Velocity Map Imaging

Abstract

This work uses velocity map imaging to determine the barrier height for acetyl radical, CH 3 CO, dissociation to CH 3 + CO. Photodissociation of acetyl chloride at 235 nm generates acetyl radicals with an internal energy distribution spanning this barrier. We determine the velocity and internal energy distribution of all nascent acetyl radicals, stable and unstable, by measuring the velocities of the Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) cofragments. These Cl cofragments are detected with 2 + 1 resonance-enhanced multiphoton ionization (REMPI) in a spin-orbit branching ratio Cl( 2 P 3/2 ):Cl( 2 P 1/2 ) of 3.3 ( 0.2. Using 157 nm photoionization, we then detect the recoil velocities of the energetically stable acetyl radicals. The radicals and momentum matched Cl atoms evidence parallel angular distributions. Comparison of the total recoil translational energy distribution P(E T ) for all radicals to that obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 25.0 ( 0.4 kcal/mol. From this onset we can calculate the barrier height for CH 3 CO f CH 3 + CO, but this relies on prior determinations of the C-Cl bond energy of acetyl chloride. Using an experimental bond dissociation energy of 83.4 ( 0.2 kcal/mol yields a dissociation barrier of 14.2 ( 0.5 kcal/mol. Our data evidence that a portion of the acetyl radicals formed with total internal energy above the barrier are stable due to the partitioning of energy into rotation during the C-Cl bond fission of the precursor. Thus, the internal energy onset for dissociation is not as sharp as was assumed in prior determinations of the barrier. The experimentally determined onset is compared with that predicted from electronic structure calculations at the G3//B3LYP and CCSD(T) levels of theory.

The photodissociation dynamics of alkyl radicals

“…It has been shown that VMI is applicable to reactive intermediates that are only available in small number densities. [35][36][37][38] Excited-state computations of the larger alkyl radicals are on the other hand still scarce and only vertical excitation energies have been calculated for a number of alkyl radicals. 39 Complete active space self-consistent field (CASSCF) computations on t-butyl suggested that in addition to the C-H coordinate, the C-C coordinate might be relevant in the photodissociation of alkyl radicals.…”

Assessing an Impulsive Model for Rotational Energy Partitioning to Acetyl Radicals from the Photodissociation of Acetyl Chloride at 235 nm

“…D 0 (C-Cl) is the bond dissociation energy of the C-Cl bond, experimentally determined as 83.62 ( 0.15 kcal/mol, 24,25 which is quite close to the theoretical value calculated at the UCCSD(T)/CBS//UCCSD(T)/aug-cc-pVTZ level of theory as 83.4 kcal/mol. 13 It is slightly higher than the G4//B3LYP/6-311++G(3df,2p) value of 82.57 kcal/mol. 26 Figure 1.…”

“…The details of this apparatus and the experimental results are described elsewhere. 1,13 In short, a supersonic molecular beam of acetyl chloride molecules was photodissociated at 235 nm, and the Cl and CH 3 CO cofragments were photoionized and detected with velocity map imaging to measure the distributions of velocities, P Cl (υ) and P acetyl (υ) respectively. The acetyl radicals were produced in a range of velocities corresponding to internal energies that span the barrier to further dissociation.…”

Acetone cation chemistry under varying electron density conditions: from unimolecular decomposition to dissociative recombination processes

“…Many works have been published on this process, the products of which are associated with tautomerization to the keto isomer followed by rapid preferential cleavage of the newly formed C–C bond to yield methyl radicals and the acetyl cation. The acetyl cation would be expected to yield the highly unstable acetyl radical on neutralization and subsequently to decompose very rapidly to methyl radicals and CO in our apparatus. As these are the same products as our primary ‘keto cation’ decomposition, it is not expected that we would observe any new species that grow in at the expense of secondary ionization of 1‐propen‐2‐ol with ionization current increases.…”