The photodissociation dynamics of the ethyl radical, C2H5, investigated by velocity map imaging

Abstract

The photodissociation dynamics of the ethyl radical C(2)H(5) has been investigated by velocity map imaging. Ethyl was produced by flash pyrolysis from n-propyl nitrite and excited to the à (2)A(') (3s) Rydberg state around 250 nm. The energetically most favorable reaction channel in this wavelength region is dissociation to C(2)H(4) (ethene) + H. The H-atom dissociation products were ionized in a [1+1(')] process via the 1s-2p transition. The observed translational energy distribution is bimodal: A contribution of slow H-atoms with an isotropic angular distribution peaks at low translational energies. An expectation value for the fraction of excess energy released into translation of <f(T)> = 0.19 is derived from the data, typical for statistical dissociation reactions. In addition, a fast H-atom channel is observed, peaking around 1.8 eV. The latter shows an anisotropic distribution with β = 0.45. It originates from a direct dissociation process within less than a rotational period. Time-delay scans with varying extraction voltages indicate the presence of two rates for the formation of H-atoms. One rate with a sub-nanosecond time constant is associated with H-atoms with large translational energy; a second one with a time constant on the order of 100 ns is associated with H-atoms formed with low translational energy. The data confirm and extend those from previous experiments and remove some inconsistencies. Possible mechanisms for the dissociation are discussed in light of the new results as well as previous ones.

The photodissociation dynamics of alkyl radicals

“…Amaral et al investigated the photodissociation of ethyl by H-atom high-n Rydberg time-of-flight (TOF) spectroscopy, 21 while our group applied VMI. 1 A bimodal translational energy distribution was found for the H-atoms in both studies, with an isotropic contribution that peaks at low translational energies and a fast H-atom channel that shows an anisotropic angular distribution. Interestingly, the experimental data obtained from partially deuterated samples yielded a large degree of site-selectivity for ethyl 1,[21][22][23] and i-propyl, 24 and little isotopic scrambling takes place.…”

“…Note that the P(E T ) of ethyl was taken from our previous work. 1 For comparison with ipropyl, we recorded a H-atom image without pyrolysis, given in the supplementary material. 44 It corresponds to H-atom loss from the precursor i-butyl nitrite and does not resemble the distribution depicted in Figure 2…”

Ultraviolet photodissociation dynamics of the n-propyl and i-propyl radicals

“…35 In the photodissociation of ethyl via its 3s Rydberg state, 32-34 bimodal product translational energy release and energy-dependent angular distribution of the Hloss products were observed and attributed to two dissociation pathways. 33,34 The excited 3s Rydberg state decays via a conical intersection in a nonclassical H-bridged geometry, producing a fast and anisotropic channel with direct H-atom scission via the nonclassical H-bridged transition state and a slow and isotropic channel from unimolecular dissociation of the hot radical after internal conversion. 33,36,37 Site-selective loss of the β hydrogen atom is confirmed by using the partially deuterated CH 3 CD 2 radical.…”

“…37,46 The direct dissociation from the excited state to the H + C 2 H 4 in the repulsive C 2v pathway leads to the fast channel with an anisotropic angular distribution and repulsive translational energy release. 33,34,37 The excited 3s state also decays via the conical intersection to the ground electronic state (i.e., internal conversion) in the classical structure, which then undergoes unimolecular dissociation of a hot radical, leading to the slow and isotropic product channel. 33,34,37 The n-propyl radical can be viewed as an ethyl radical with a β H atom substituted with a methyl group.…”

“…33,34,37 The excited 3s state also decays via the conical intersection to the ground electronic state (i.e., internal conversion) in the classical structure, which then undergoes unimolecular dissociation of a hot radical, leading to the slow and isotropic product channel. 33,34,37 The n-propyl radical can be viewed as an ethyl radical with a β H atom substituted with a methyl group. The ground state n-propyl has two lowest energy conformers separated by a small energy difference of <0.05 kcal/mol, one with the radical carbon 2p z orbital eclipsed to the β-CC bond and another to a β-CH bond.…”