Theoretical mechanistic study on the radical–molecule reactions of cyanomethylidyne with PH3, H2S, and HCl

Abstract

The cyanomethylidyne (CCN) has been the long-standing subject of extensive theoretical and experimental studies on its structures and spectroscopies. However, there are few investigations on its reactivity. Our very recent theoretical work indicated that even with the simplest methane, the CCN reaction faces almost zero barriers following the carbyne mechanism as CH does. This was suggestive of the powerfulness of the nonatomic and nonhydrogenated CCN radical in depleting old molecules and synthesizing new cyanogen-containing molecules in either combustion or interstellar processes. In this paper, a detailed mechanistic study at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311g(d,p) and G2M(CC1)//B3LYP/6-311G(d,p) computational levels is reported for the reactions of CCN with a series of sigma-bonded molecules of the second row H(n)X (X,n) = (P,3), (S,2), and (Cl,1). The carbenoid insertion is confirmed as the most favored entrance channel, forming H(n) (-1)XC(H)CN. Subsequently, H(n) (-1)XC(H)CN will predominantly lead to product H(n) (-2)XC(H)CN+H via the H-extrusion processes (except X = Cl). Yet, the CCN+HX (X = Cl) reaction is the exception because XC(H)CN intrinsically has no H-atoms at X for extrusion or migration. At G2M(CC1)//B3LYP/6-311G(d,p) computational level, ClC(H)CN can only dissociate back to the reactant or be stabilized with its isomers upon sufficient collisions or radiation. The carbyne character confirmed in this paper provides a useful base for future experimental and theoretical study on the chemistry of this nonatomic and nonhydrogenated reactive radical. In addition, interestingly, the complexes H(n)X-CCN (X,n) = (P,3) and (S,2) formed in the reactions are found not to be the simple (loosely bound) donor-accepter complexes as those formed in the CCN insertions into other hydrides (NH(3), H(2)O, HF, HCl). On the basis of the comparison with the qualitative features of typical ylides, H(3)P--CCN and H(2)S--CCN are considered to be similar to the ylides in nature, being "ylide-like radicals." They might be observed in some experiments, since they are in deep potential wells on the energy surface.