Photofragmentation of Acetyl Cyanide at 193 nm

Abstract

The photodissociation of gaseous acetyl cyanide has been examined following excitation at 193 nm. CN X2Σ+ photofragments were probed via laser fluorescence excitation to determine their rotational, vibrational, and translational energy distributions. CN was produced in v‘‘ = 0 and 1 with mean rotational energy (13.5 ± 2) kJ mol-1, and v‘‘ = 2 with mean rotational energy (10 ± 4) kJ mol-1. Mean translational energies of the CN fragments were (32 ± 10) kJ mol-1. Ab initio electronic structure theory has been used to characterize the heat of formation for acetyl cyanide along with its geometries and vibrational frequencies. The acetyl cyanide heat of formation, ΔH 0 f,0, is predicted to be (−0.4 ± 8) kJ mol-1 using Gaussian-2 theory (G2). The theoretical results are used to compute bond dissociation energies of acetyl cyanide for further interpretation of the experimental photodissociation data. Evidence is presented that the majority of CN fragments are produced via dissociation of the parent acetyl cyanide to CH3CO + CN, with subsequent decomposition of the acetyl fragment. The alternate possible primary α-cleavage pathway to CH3 + OCCN is proposed as a possible source for the OCCN radical.

Quantum chemical study of mechanisms of dissociation and isomerization reactions in some molecules and radicals of astrophysical significance: Cyanides and related molecules

“…Further, the calculated value of the dissociation energy for COCN • → CO + CN • is found to be 35.8 kcal/mol (DFT) (table 2) which is close to the estimated experimental value of 35.9 kcal/mol [1]. Horwitz et al [8], however, reported a much lower value of 30.1 kcal/mol from RHF ab initio calculations using G2 theory.…”

“…Present calculations further tend to rationalize some of the findings of Furlan et al [1] on photodecomposition in CO(CN) 2 and Horwitz et al [8] in CH 3 COCN by 193 nm radiation. Since the photon energy of 193 nm radiation is 148.2 kcal/mol, the available energy in photodecomposition experiment of Furlan et al [1] in a sequential reaction is 51.0 kcal/mol.…”

“…carbonyl cyanide CO(CN) 2 by photofragment translational spectroscopy, Furlan et al [1] detected the presence of OCCN radical which further decomposed into CO and CN • . However, in a related study on acetyl cyanide [8], no OCCN radical could be detected. The prevalence of carbonyl and cyano compounds in the interstellar medium [9] also suggests the likelihood of molecules containing these groups to be of astrophysical importance.…”

“…Lower basis MP2/6-31G(d) calculations, however, predict a much higher value of 126.0 kcal/mol. After taking into account exit barrier for α-cleavage of about 13 kcal/mol [8], the dissociation energies should be about 110 kcal/mol. (DFT) and 139 kcal/mol (MP2).…”

Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride

“…[128][129][130][131] In particular, one usually expects that most of the potential energy released along the reaction coordinate after passing the barrier is converted to relative translational energy of the photofragments, and that the translational energy distribution is rather insensitive to the total available energy. 108,[132][133][134][135] In contrast, reactions without an exit channel barrier typically have an asymmetric bell-shaped translational energy distribution with its maximum somewhat away from zero translational energy. 128,130 It can be seen from Also, from the 6S-MC-QDPT energies in Fig.…”