5/21/2023 0 Comments Spacechem a brief introduction![]() The reaction products (CH 3C(O)CH 2 and CH 3 radicals) have been detected by several groups 17, 18, 21– 25 with contradictory conclusions, and also studied by theoretical methods. Regarding the products of reaction (1), there are two possible exothermic channels: H-atom abstraction leading to water and acetonyl (CH 3C(O)CH 2) radical ( Reaction 1) and OH-addition to the carbonyl C-atom followed by methyl elimination leading to acetic acid, CH 3C(O)OH ( Reaction 2). 15 reported experimental and theoretical rate coefficients down to 79 K and 10 K, respectively. The kinetics of the gas-phase reaction of CH 3C(O)CH 3 with OH radicals has been extensively investigated as a function of temperature both experimentally 6– 15 and theoretically, 10, 12, 15– 20 but mainly at T>200 K. The depletion of CH 3C(O)CH 3 in the gas-phase involves reactions initiated by the hydroxyl (OH) radicals in the three environments (ISM, Earth’s atmosphere and in high-temperature combustion processes). In diesel-biodiesel blends, acetone can be used as oxygenator 4 and it can also be an additive in gasoline fuel blends. In the terrestrial atmosphere, CH 3C(O)CH 3 is one of the most important trace organic molecules which are emitted mainly from anthropogenic sources. In much hotter environments, such as the Earth’s atmosphere and internal combustion and injection diesel engines, CH 3C(O)CH 3 is also of great importance. 2 suggested that this large saturated COM is not easily formed in the gas phase at 10 K and the current mechanisms proposed to explain the gas phase abundance of interstellar acetone are based on grain mantle chemistry 3. 1 proposed that its formation in this cold environment starts with the radiative association reaction of CH 3 + and acetaldehyde (CH 3CHO) forming CH 3C(O)CH 4 + followed by the dissociative recombination of CH 3C(O)CH 4 +. In particular, acetone (CH 3C(O)CH 3) has been detected in 1987 in Sagittarius B2 molecular cloud and Combes et al. The mechanism of formation of many complex organic molecules (COMs) detected in star-forming regions of the interstellar medium (ISM), such as cold dense molecular clouds ( T~10-100 K), is not fully known. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8×10 -10 cm 3 molecule -1 s -1 at 10 K and the reaction products are water and CH 3C(O)CH 2 radicals. ![]() ![]() The predicted rate coefficients at 10 K surround the experimental value which appears to be very close to the low pressure regime prevailing in the interstellar medium. ![]() The computed k( T) is also reported for 10 3 cm -3 of He (representative of the interstellar medium). Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47×10 17 cm -3 is twice higher than the average values found at lower densities. The effect of total gas density on k( T) has been explored. This trend is reproduced by calculations, with a special good agreement with experiments below 25 K. The measured values have been found to be several orders of magnitude higher than k(300 K). The experimental k( T) was found to increase as temperature was lowered. In agreement with previous studies we found that the reaction proceeds via initial formation of two pre-reactive complexes both leading to H 2O + CH 3C(O)CH 2 by H-abstraction tunneling. The temperature dependence of k( T = 10-300 K) has also been computed using a RRKM-Master equation analysis after partial revision of the potential energy surface. The rate coefficient, k( T), for the gas-phase reaction between OH radicals and acetone CH 3C(O)CH 3, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique ( T = 11.7-64.4 K).
0 Comments
Leave a Reply. |