Wh. Breckenridge, ACTIVATION OF H-H, SI-H, AND C-H BONDS BY NSNP EXCITED-STATES OF METAL ATOMS, Journal of physical chemistry, 100(36), 1996, pp. 14840-14855
We present a comprehensive overview of our current knowledge of the in
teractions of valence M(nsnp P-3) and M(nsnp P-1(1)) excited states wi
th H-H, Si-H, and C-H bonds, where M = Mg, Zn, Cd, and Hg. It is propo
sed that the high reactivity of M(nsnp P-3(1)) states with H-H and Si-
H bonds compared to C-H bonds is due to the lack of steric hindrance i
n the localized, side-on, M(np pi)-XH(sigma) donor-acceptor molecular
orbital interactions, since the Si-H bond length in SiH4 is similar t
o 1.5 Angstrom compared to C-H bond lengths of similar to 1.1 Angstrom
. It is also concluded that Mg(3s3p P-1(1)) and Zn(4s4p P-1(1)) effici
ently activate C-H bonds as well as H-H and Si-H bonds not just becaus
e of their higher energy but because of better M(np pi)-XH(sigma) ene
rgy matches and overlap, which overcomes M(ns)-XH(sigma) repulsion and
the steric hindrance. It is further proposed that the striking differ
ences in the microscopic mechanisms of attack of C-H bonds by Mg(P-1(1
)) versus Zn(P-1(1)) may be due to the fact that the Zn(4s) ''core'' i
s substantially (similar to 0.2 Angstrom) smaller than the Mg(3s) ''co
re'' allowing true insertion of the Zn(P-1(1)) state (but not the Mg(P
-1(1)) state) into C-H bonds to form (by surface hopping) long-lived g
round-state zinc alkyl hydrides which decompose in a non-RRKM fashion
to yield the observed ZnH product. Finally, the experimental results t
o dale (as well as ab initio calculations) indicate that direct, end-o
n ''abstractive'' attack of M(nsnp P-1(1)) states [as well as O(D-1(2)
)] of H-H, SI-H, and C-H bonds probably does not occur.