A theoretical study of the reactions of CH3+ with second-row atoms in
their ground states, which are interesting in general gas-phase chemis
try but particularly in interstellar chemistry, has been carried out.
For this purpose ab initio molecular orbital calculations of the (XH(n
)C)(+) systems, X being Si, P, S, and Cl (n = 1-3), have been made. Th
e lowest-lying stable species of each system have been characterized,
and we have searched for all the relevant saddle points. We have found
three exothermic channels for the reaction of CH3+ With Si: the elect
ron-transfer process, production of SiCH2+(B-2(1)) + H(S-2), and produ
ction of SiCH+((3) Sigma(+)) + H-2((1) Sigma(g)(+)). We have found tha
t quenching into Si+(P-2) + CH3((2)A(1)) should be a likely process wh
en the system follows the lowest (3)A'' potential energy surface and t
hat there are two barrier-free paths for the channel giving SiCH2+(B-2
(1)) + H(S-2). Production of SiCH+((3) Sigma(+)) + H-2(1 Sigma(g)(+))
is only slightly exothermic, and one of the transition states involved
lies below the reactants by a very small quantity, so it is hard to m
ake a definitive statement on whether the process might take place. Th
e reaction of CH3+ With P has no exothermic reaction channels within t
he quartet state. However, there are indications that spin flipping in
to the doublet state is a likely process, opening a possibility for th
e production of PCH2+((1)A(1)) + H(S-2). For the reactions of CH3+ wit
h S and Cl there are two exothermic channels in each case, but only th
ose giving SCH2+(B-2(2)) + H(S-2) or ClCH2+((1)A(1)) + H(S-2), respect
ively, are not subject to energy barriers. The electron-transfer react
ion is endothermic in both cases, whereas production of SCH+((3)A(1))
+ H-2((1) Sigma(g)(+)) or ClCH+((2)A') + H-2((1) Sigma(g)(+)) is preve
nted by small barriers.