Experimental and theoretical investigation on the OH + CH₃C(O)CH₃ reaction at interstellar temperatures (T=11.7-64.4...

The rate coefficient, k(T), for the gas-phase reaction between OH radicals and acetone CH₃C(O)CH₃, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique (T = 11.7-64.4 K). 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. In agreement with previous studies we found that the reaction proceeds via initial formation of two pre-reactive complexes both leading to H₂O + CH₃C(O)CH₂ by H-abstraction tunneling. The experimental k(T) was found to increase as temperature was lowered. The measured values have been found to be several orders of magnitude higher than k(300 K). This trend is reproduced by calculations, with a special good agreement with experiments below 25 K. The effect of total gas density on k(T) has been explored. Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47×10¹⁷ cm^-3 is twice higher than the average values found at lower densities. The computed k(T) is also reported for 10³ cm^-3 of He (representative of the interstellar medium). 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. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8×10^-10 cm³ molecule^-1 s^-1 at 10 K and the reaction products are water and CH₃C(O)CH₂ radicals.