Adiabatic and Nonadiabatic Reaction Pathways of the O(³P) with Propyne

Abstract

For the reaction of O((3)P) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton-Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene (3)CH(3)CCHO or (3)CH(3)COCH which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH(3)CCO + H (CH(2)CCHO + H), CH(3) + HCCO, CH(2)CH + HCO, CH(2)CO + CH(2), CH(3)CH + CO, and the nonadiabatic channels of C(2)H(4) + CO, C(2)H(2) + H(2) + CO, H(2) + H(2)CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C(2)H(4) + CO channel, but not with the triplet CH(3)CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene ((1)CH(3)CHCO) dissociation following the intersystem crossing of the triplet ketocarbene adduct ((3)CH(3)CCHO). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of O((3)P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.

Multiplexed Photoionization Mass Spectrometry Investigation of the O(3P) + Propyne Reaction

“…Thus, we conclude that the only C 2 H 4 species we observe is ethene in its ground electronic state. Although we cannot definitely rule out formation of 3 C 2 H 4 or 3 CH 3 CH followed by rapid deactivation/isomerization to 1 C 2 H 4 , pathways leading to these products are disfavored based on calculated stationary points (discussed in section 4; see ref 21). However, there is evidence that at our experimental conditions (298 K and 4 Torr) vibrationally hot ethene formed by channel R1d decomposes to acetylene (C 2 H 2 ) and other products, which will be discussed in more detail in section 4.1.…”

“…AIEs for C 3 H 3 O isomers that are above 8.2 eV (see Table 2 The Journal of Physical Chemistry A Article isomerizes prior to being ionized. The lowest-lying triplet state of ethene is ∼2.7 eV above the singlet ground state, 21 and a lack of m/z = 28 signal at 10.313 eV also suggests that 3 C 2 H 4 is not directly observed. Thus, we conclude that the only C 2 H 4 species we observe is ethene in its ground electronic state.…”

“…55 However, in the same way that there are many pathways in R1 that can form CO, there are also multiple H 2 -generating pathways, making observation of C 3 H 2 O a more definitive candidate for characterization of R1h. Calculations by Zhao et al 21 and Chin et al 22 suggested that both the propadienal (CH 2 CCO) and 2-propynal (CHCCHO) isomers of C 3 H 2 O are thermodynamically allowed products of R1h, with propadienal being energetically favored.…”

“…Reported energetics are at 0 K based on calculations by Zhao et al21 (except that for R1e′ which comes from calculations in ref 22). In the experiments by Kanofsky et al, partially deuterated propyne precursors were used, and the isotopologue yields for products of R1c support that methylketenyl (CH 3 CCO) is the major C 3 H 3 O isomer formed.…”

Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

“…The last four channels can only originate from the singlet PES. A VUV LIF study by Xing et al [138] provided information on H, H 2 and CO channels by determining the translational energy distribution of the H product and the internal energy of H 2 and CO. More recently, electronic structure calculations have been reported for this reaction, together with an FTIR determination of the vibrational distribution of the CO product (from channels 5f, 5g, 5l and 5m) [139]. Despite a considerable amount of previous experimental (mostly kinetics) work, a detailed characterisation of the product BRs and of the role of ISC was still lacking.…”