Methylcarbyne radical [CH3C((X)over-tilde(2)A '';(a)over-tilde(4)A(2))] and the chemiionization reaction: CH3C+O -> CH3CO++e(-)

The heat of formation of the methylcarbyne radical (CH3C) is calculated usi ng various ab initio approaches. The most accurate value, at the CCSD(T)/6- 311G(3df,3pd)//MP2/6-311G(3df,3pd) level of theory, is derived as Delta H-f (298)[CH3C((X) over tilde(2)A ")] = 122 +/- 1 kcal mol(-1); in addition Del ta H-f(298)[CH3C((a) over tilde(4)A(2))] is calculated as 152 +/- 2 kcal mo l(-1). The (a) over tilde(4)A(2)-(X) over tilde(2)A " excitation energy is derived as 1.3 +/- 0.1 eV [29 +/- 2 kcal mol(-1)]. For both electronic stat es, the equilibrium geometry and harmonic vibrational frequencies are calcu lated. The use of the Gaussian-2 (G2) theoretical model to calculate the ma ximum electron kinetic energy from chemiionization reactions is explored. I n particular, the CH3C + O --> CH3CO+ + e(-) and CH + O --> HCO+ + e(-) che miionization reactions at 298 K are considered for the doublet and quartet states of the hydrocarbon radical. These calculations lead to a maximum ele ctron kinetic energy of 1.04 and 2.46 eV for the former reaction, and 0.22 and 1.05 eV for the latter reaction, where the first number in each case re fers to the ground doublet state of the reacting radical and the second ref ers to the lowest quartet. It is concluded that the G2 method is adequate f or determining the thermodynamics of chemiionization reactions involving sp ecies in their ground electronic states; however, a higher level of theory is required for excited states.