Two roaming pathways in the photolysis of CH 3 CHO between 328 and 308 nm

Photodissociation of isobutyraldehyde (C3H7CHO) at 248 nm is investigated using time-resolved Fourier-transform infrared emission spectroscopy to demonstrate the growing importance of the roaming pathway with increasing molecular size of aliphatic aldehydes. Each acquired CO rotational distribution from v = 1 to 4 is well characterized by a single Boltzmann rotational temperature from 637 to 750 K, corresponding to an average rotational energy of 5.9 ± 0.6 kJ mol(-1). The roaming signature that shows a small fraction of CO rotational energy disposal accompanied by a vibrationally hot C3H8 co-fragment is supported by theoretical prediction. The energy difference between the tight transition state (TS) and the roaming saddle point (SP) is found to be -27, 4, 15, 22, and 30 kJ mol(-1) for formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, and 2,2-dimethyl propanal, respectively. The roaming SP is stabilized by a larger alkyl moiety. It is suggested that the roaming photodissociation rate of aldehydes increasingly exceeds those via the tight TS, resulting in the dominance of the CO + alkane products, as the size of aldehydes becomes larger. Along with formaldehyde, acetaldehyde, and propionaldehyde, in this work isobutyraldehyde is further demonstrated that this aldehyde family with special functional group is the first case in the organic compound to follow predominantly a roaming dissociation pathway, as the molecular size becomes larger.

Section: Methodsmentioning

confidence: 99%

Roaming as the dominant mechanism for molecular products in the photodissociation of large aliphatic aldehydes

Tsai

Kasai

et al. 2015

“…That is, the ionization efficiency is independent of the rovibrational state of the products, which readily compensates for the apparent limitation on the detection sensitivity. 55 A significant problem for measurements of CH3CHO photolysis is formation of clusters in the supersonic expansion, as previously noted by Lee et al 24 Peaks corresponding to (CH3CHO)n clusters up to n = 5 were readily observed in the mass spectrum following VUV ionization. Dissociation of CH3CHO molecules in clusters produces a translationally slow component that dominates the center of the CH3 ion images.…”

Section: Resultsmentioning

confidence: 86%

“…17,22,23 More recent ion imaging experiments, however, have identified translationally slow low-J CO products, with an onset wavelength consistent with the threshold for channel IIb, leading to the suggestion that the radical products CH3CO + H substantially contribute to roaming. 24 Of the two radical product channels, IIa is most important and dominates the long wavelength (λ > 266 nm) photochemistry. 1 The radical products CH3 + HCO can be formed via unimolecular dissociation on S0 following internal conversion or, if sufficient energy is available, dissociation over a barrier on the T1 surface following intersystem crossing.…”

Section: Introductionmentioning

confidence: 99%

Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy

Toulson

Fishman

Murray

2018

Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm. Ion images recorded probing CH3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH3 speed distributions become bimodal at λ < 292 nm, in addition to the translationally fast T1 products a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH3CHO obtained probing CH3 and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S0, accessed via a conical intersection between the S1 and S0 surfaces at extended C-C distances. Time-resolved ion images of CH3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T1 and S0 via the conical intersection occurs with time constants of 240 and 560 ps, respectively.3

“…A well-known problem encountered when working with acetaldehyde expansions is cluster formation. 36,46 Acetaldehyde dimers, trimers, and larger clusters are also observable in the mass spectrum following ionization at 118.2 nm. Ion imaging measurements of CH3 fragments produced by photolysis of the neutral molecule provide a more discriminating diagnostic; under cluster-free conditions, the image at 308 nm appears as a clean, isotropic ring at speeds of ~1700 m s -1 , while photolysis of clusters results in higher ion counts near the center.…”

Section: Resultsmentioning

confidence: 99%

UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization

Kapnas

McCaslin

Murray

2019