Reaction of H + ketene to formyl methyl and acetyl radicals and reverse dissociations

Thermochemical properties for reactants, intermediates, products, and transition states important in the ketene (CH 2 CO) + H reaction system and unimolecular reactions of the stabilized formyl methyl (C·H 2 CHO) and the acetyl radicals (CH 3 C·O) were analyzed with density functional and ab initio calculations. Enthalpies of formation (Δ H f ° 298 ) were determined using isodesmic reaction analysis at the CBS‐QCI/APNO and the CBSQ levels. Entropies ( S ° 298 ) and heat capacities ( C p °( T )) were determined using geometric parameters and vibrational frequencies obtained at the HF/6‐311G(d,p) level of theory. Internal rotor contributions were included in the S and C p ( T ) values. A hydrogen atom can add to the CH 2 ‐group of the ketene to form the acetyl radical, CH 3 C·O ( E a = 2.49 in CBS‐QCI/APNO, units: kcal/mol). The acetyl radical can undergo β‐scission back to reactants, CH 2 CO + H ( E a = 45.97), isomerize via hydrogen shift ( E a = 46.35) to form the slight higher energy, formyl methyl radical, C·H 2 CHO, or decompose to CH 3 + CO ( E a = 17.33). The hydrogen atom also can add to the carbonyl group to form C·H 2 CHO ( E a = 6.72). This formyl methyl radical can undergo β scission back to reactants, CH 2 CO + H ( E a = 43.85), or isomerize via hydrogen shift ( E a = 40.00) to form the acetyl radical isomer, CH 3 C·O, which can decompose to CH 3 + CO. Rate constants are estimated as function of pressure and temperature, using quantum Rice–Ramsperger–Kassel analysis for k (E) and the master equation for falloff. Important reaction products are CH 3 + CO via decomposition at both high and low temperatures. A transition state for direct abstraction of hydrogen atom on CH 2 CO by H to form, ketenyl radical plus H 2 is identified with a barrier of 12.27, at the CBS‐QCI/APNO level. Δ H f ° 298 values are estimated for the following compounds at the CBS‐QCI/APNO level: CH 3 C·O (−3.27), C·H 2 CHO (3.08), CH 2 CO (−11.89), HC·CO (41.98) (kcal/mol). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 20–44, 2003