Unimolecular Dissociation of the CH₃OCO Radical:  An Intermediate in the CH₃O + CO Reaction

Abstract

This work investigates the unimolecular dissociation of the methoxycarbonyl, CH 3 OCO, radical. Photolysis of methyl chloroformate at 193 nm produces nascent CH 3 OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the CH 3 O + CO and CH 3 + CO 2 product channels. Both electronic ground-and excited-state radicals undergo competitive dissociation to both product channels. The experimental product branching to CH 3 + CO 2 from the ground-state radical, about 70%, is orders of magnitude larger than Rice-Ramsperger-KasselMarcus (RRKM)-predicted branching, suggesting that previously calculated barriers to the CH 3 OCO f CH 3 + CO 2 reaction are dramatically in error. Our electronic structure calculations reveal that the cis conformer of the transition state leading to the CH 3 + CO 2 product channel has a much lower barrier than the trans transition state. RRKM calculations using this cis transition state give product branching in agreement with the experimental branching. The data also suggest that our experiments produce a low-lying excited state of the CH 3 OCO radical and give an upper limit to its adiabatic excitation energy of 55 kcal/mol.

Characterization of the Methoxy Carbonyl Radical Formed via Photolysis of Methyl Chloroformate at 193.3 nm

“…Photofragment Ion Imaging Data. Imaging experiments incorporating state-selective UV photoionization found translational energy distributions roughly consistent with those presented by McCunn et al 15 The raw images for m/e ) 35 are shown in Figure 6. Because both the 193.3 nm photodissociation light and the 235 nm photoionization light can dissociate methyl chloroformate, the background signal was measured using only 235 nm and it was subtracted from the images obtained using both the 193.3 and 235 nm light.…”

“…The calculations clearly indicate a correlation between the stability of the alkyl radical leaving group and the barrier height. The barrier of 16.8 kcal/mol for CO 2 production from the methoxy carbonyl radical is slightly higher than the barrier presented by McCunn et al 15 but still lower than the predicted barrier to dissociate to CH 3 O + CO. Note also that recent calculations at the B3LYP/6-311++G(d,p) level of theory on sulfur analogues found that the CH 3 OSO dissociation barrier to CH 3 + SO 2 , 37.3 kcal/mol, 20 is lower in energy than the barrier to form CH 3 O + SO.…”

“…Except for the different orientation of the methyl group relative to the terminal carbonyl group, in fact, the cis-TS and the trans-TS have very similar geometries at the B3LYP/6-311G(2df,p) and CCSD(T)/ 6-311G(2df,p) levels. 15 One may postulate that the lower energy profile for dissociation via the cis-TS could be attributed to a bonding feature exclusively present on the cis pathway. From the NBO analysis along the pathway between the cis-methoxy carbonyl and the cis-TS, we found that there is a strong stabilization effect due to the donation of electron density from the nonbonding C orbital into the σ* orbital of the methoxy C-O bond (n C f σ* CO ).…”

“…This establishes the qualitative applicability of the n C f σ* CO hyperconjugative picture over the relevant portion of the IRC. Thus, the NBO analysis gives important qualitative insight into the relative energetic difference between the cis-TS and the trans-TS (the cis-TS is more stable than the trans-TS by ∼20 kcal/mol at the CCSD(T) level 15 ) explaining the substantial branching to the CH 3 + CO 2 product channel observed by McCunn et al…”

DFT and ab initio theoretical study for the CF3S+CO reaction

“…It is very important to compare the PES feature of the CF 3 S + CO reaction with that of analogous reactions CH 3 O + CO, CH 3 S + CO and CF 3 S + CO, and the three reactions were studied experimentally and theoretically already [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Although the methods used are different, the comparisons will give some interesting similarities and differences and imply the properties of the same family elements.…”