Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

Abstract

By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v = 1, 2) and CO(v = 1-3) are detected in one-photon dissociation of acetyl cyanide (CH(3)COCN) at 308 nm. The S(1)(A(")), (1)(n(O), π(∗) (CO)) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 μs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1-7) × 10(-12) cm(3) molecule(-1) s(-1). The measurements of O(2) dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJ∕mol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN + CH(2)CO, in which the CH(2)CO moiety may further undergo secondary dissociation to release CO. The production of CO(2) in the reaction with O(2) confirms existence of CH(2) and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH(3) fragments that dominate the dissociation products at 193 nm are not detected.

Note: Photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy: Is CO a primary or secondary product?

“…The corresponding energy state of this channel is higher by 31 kJ/mol than the excitation energy to make the CH 2 CO decomposition much less favored. 1 Second, if CH 2 CO decomposes to CO and CH 2 , the obtained CO 2 intensity should be as comparable as the CO intensity observed. However, it is difficult to explain an unmatched CO 2 spectral intensity that brings about a very small yield.…”

“…1 The HCN fragment was excited up to v 3 = 2 with ro-vibrational energy disposal of 81 ± 7 kJ/mol. Further, a marked CO spectrum was analyzed to yield a Boltzmann rotational temperature of 520 ± 30, 410 ± 20, and 400 ± 40 K for v = 1, 2, and 3, respectively, and a Boltzmann vibrational temperature of 3600 ± 300 K. Thus, the CO population ratio for v = 0, 1, 2, and 3 corresponds to 0.62:0.25:0.09:0.05, yielding ro-vibrational energy disposal of 32 ± 3 kJ/mol.…”

“…The consequence evidences CO as a secondary reaction product. The following primary isomerization/dissociation channels 1 were also checked with their individual activation barriers, E act . Before recasting the isomerization transition state (TS) barrier that turns out to be a new global minimum energy path for the scheme (3), all these barriers are well above the excitation energy of 387.9 kJ/mol, equivalent to 308 nm, such that the probabilities for the primary production of CH 2 and CO were excluded in the previous study.…”

“…(3)) found previously that has a tight TS structure with a CCN angle constrained to ∼81 o . 1 These two pathways should lead to a distinct dynamical signature in the products.…”

Photodissociation of gaseous CH3COSH at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Observation of three dissociation channels

“…6,[27][28][29] In brief, it contained a step-scan FTIR emission spectrometer for spectral detection of the fragments and a six-way stainless steel cube as reaction chamber. The photolysis laser propagated through the sample beam ejected from an effusive nozzle, and the fragment emission signals were guided along a perpendicular axis to focus onto the entrance of the FTIR spectrometer.…”