Jong Ho Choi

A theoretical study of the reaction of O(3P) with an allyl radical C3H5

Ab initio calculations of the reaction of ground-state atomic oxygen [O(3P)] with an allyl radical (C3H5) have been carried out using the density functional method and the complete basis set model. On the calculated lowest doublet potential energy surface, the barrierless association of O(3P) to C3H5 forms three energy-rich addition intermediates, which are predicted to undergo subsequent isomerization and decomposition steps leading to various products: C3H4O+H, CH2O+C2H3, C2H4+CHO, C2H2O+CH3, C2H5+CO, C3H4+OH, and C2H4O+CH. The respective reaction mechanisms through the three addition intermediates are presented, and it has been found that the barrier height, reaction enthalpy, and the number of intermediates involved along the reaction coordinate are of extreme importance in understanding such reactive scattering processes. With the aid of Rice–Ramsperger–Kassel–Marcus calculations, the major reaction pathway is predicted to be the formation of acrolein (C3H4O)+H, which is consistent with the previous ...

Crossed beam investigations of the reaction dynamics of O(3P) with allyl radical, C3H5

The reaction of ground-state atomic oxygen (O(3P)) with allyl radical (C3H5) was investigated in the crossed beam configuration. O(3P) and C3H5 were generated by the photodissociation of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. The nascent internal distributions of the OH(X2Π : v″=0,1) reaction product from the newly observed channel of O(3P)+C3H5→C3H4+OH were probed by laser induced fluorescence (LIF) spectroscopy. The distributions showed significant excitations with an unusual bimodal feature: the low and high rotational components without spin-orbit and Λ-doublet propensities in the ground and first excited vibrational states. On the basis of population analysis and comparison with the ab initio and statistical calculations, the experimental distributions are estimated to be totally non-statistical and suggest that the dynamics of the reaction might be described by two competing mechanisms: a major direct abstraction process and an indirect short-lived addition-complex for...

Exploring the dynamics of hydrogen atom release from the radical–radical reaction of O(3P) with C3H5

The gas-phase radical-radical reaction dynamics of O(3P) + C3H5 --> H(2S) + C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-alpha region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H + C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H + C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition.

Crossed-beam radical-radical reaction dynamics of O(P3)+C3H3→H(S2)+C3H2O

The radical-radical oxidation reaction, O(3P)+C3H3 (propargyl)-->H(2S)+C3H2O (propynal), was investigated using vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed-beam configuration, together with ab initio and statistical calculations. The barrierless addition of O(3P) to C3H3 is calculated to form energy-rich addition complexes on the lowest doublet potential energy surface, which subsequently undergo direct decomposition steps leading to the major reaction products, H+C3H(2)O (propynal). According to the nascent H-atom Doppler-profile analysis, the average translational energy of the products and the fraction of the average transitional energy to the total available energy were determined to be 5.09+/-0.36 kcal/mol and 0.077, respectively. On the basis of a comparison with statistical prior calculations, the reaction mechanism and the significant internal excitation of the polyatomic propynal product can be rationalized in terms of the formation of highly activated, short-lived addition-complex intermediates and the adiabaticity of the excess available energy along the reaction coordinate.

Radical–radical reaction dynamics: The OH formation in the reaction of O(3P) with propargyl radical, C3H3

The radical–radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated by applying laser induced fluorescence spectroscopy in a crossed beam configuration, together with ab initio calculations. A new exothermic channel of O(3P)+C3H3→C3H2+OH was identified and the nascent distributions of OH reaction product in the ground vibrational state (X 2Π:υ″=0) showed substantial rotational excitations with an unusual bimodal feature composed of the low- and high-N″ components. No spin–orbit propensities were observed, whereas the averaged ratios of Π(A′)/Π(A″) were determined to be 0.60±0.28. With the aid of the CBS-QB3 level of ab initio theory it is predicted that on the lowest doublet potential energy surface the reaction proceeds through the addition complexes formed through the barrierless addition of O(3P) to C3H3, and that the counterpart of C3H2 of the probed OH product is cyclopropenylidene (1c-C3H2). On the basis of the comparison with statis...

Ab initio investigations of the radical-radical reaction of O(P3)+C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O((3)P)] with a propargyl (C(3)H(3)) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O((3)P) to C(3)H(3) on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C(2)H(3)+CO, C(3)H(2)O+H, C(3)H(2)+OH, C(2)H(2)+CHO, C(2)H(2)O+CH, C(2)HO+CH(2), and CH(2)O+C(2)H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C(3)H(2)+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.

The nature of rotational barriers of the C–N bond in thioamides and the origin of the nonplanarity for thiourea

The most important donor/acceptor interactions about the C(O)–N bond rotation in formamide are the barrier-forming N(lp)/C–O(π)* interaction and the antibarrier-forming N(lp)/C–O(σ)* interaction. There is a large difference between the rotational barriers of formamide and urea. The most significant causes which lead to the low rotational barrier of urea are the reduction of the barrier-forming N(lp)/C–O(π)* interaction energy and the existence of the antibarrier-forming N′(lp)/C–O(π)* interaction. The nonplanarity of the ground state for urea is caused by the competition of the two former interactions. They come into conflict with each other to a greater extent in the planar structure than in the nonplanar structure and this excessive conflict results in the instability of the planar structure. To relax this strain, the geometry is transformed into a stable nonplanar one. We found that the same interpretation as for formamide is applied to the thioamide system. The amide π-orbital system was investigated to understand the change in the C–N and C–O(S) bond length during the rotation process.