14840
J. Phys. Chem. 1996, 100, 14840-14855
FEATURE ARTICLE Activation of H-H, Si-H, and C-H Bonds by nsnp Excited States of Metal Atoms W. H. Breckenridge Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: March 6, 1996; In Final Form: May 30, 1996X
We present a comprehensive overview of our current knowledge of the interactions of valence M(nsnp 3P) and M(nsnp 1P1) excited states with H-H, Si-H, and C-H bonds, where M ) Mg, Zn, Cd, and Hg. It is proposed that the high reactivity of M(nsnp 3P1) states with H-H and Si-H bonds compared to C-H bonds is due to the lack of steric hindrance in the localized, side-on, M(npπ)-XH(σ*) donor-acceptor molecular orbital interactions, since the Si-H bond length in SiH4 is ∼1.5 Å compared to C-H bond lengths of ∼1.1 Å. It is also concluded that Mg(3s3p 1P1) and Zn(4s4p 1P1) efficiently activate C-H bonds as well as H-H and Si-H bonds not just because of their higher energy but because of better M(npπ)-XH(σ*) energy matches and overlap, which overcomes M(ns)-XH(σ) repulsion and the steric hindrance. It is further proposed that the striking differences in the microscopic mechanisms of attack of C-H bonds by Mg(1P1) versus Zn(1P1) may be due to the fact that the Zn(4s) “core” is substantially (∼0.2 Å) smaller than the Mg(3s) “core”, allowing true insertion of the Zn(1P1) state (but not the Mg(1P1) state) into C-H bonds to form (by surface hopping) long-lived ground-state zinc alkyl hydrides which decompose in a non-RRKM fashion to yield the observed ZnH product. Finally, the experimental results to date (as well as ab initio calculations) indicate that direct, end-on “abstractive” attack of M(nsnp 1P1) states [as well as O(1D2)] of H-H, Si-H, and C-H bonds probably does not occur.
Introduction M*(nsnp) +
It has been known for decades that nsnp excited states of group II metal atoms, such as Hg, can activate C-H bonds or the H-H bond.1,2 In fact, the first photosensitization reaction studied, in 1923, was the decomposition of H2 into H atoms by reaction with the Hg(6s6p 3P1) excited state.3 It was shown in later years that M(nsnp) excited states (where M ) Mg, Zn, Cd, Hg) can activate a variety of X-H bonds (where X ) H, C, Si, Ge, N, P).4-59 There has been, of course, wide interest in understanding such fundamental chemical interactions. For example, it has been suggested1-5 that such excited states attack X-H bonds “side-on” rather than by the “end-on” abstractive mechanisms common for, say, halogen atom attack of X-H bonds. Furthermore, “half-collision” experiments have been developed in the past decade30,35-45 whereby either the M(nsnp) state is photolytically excited within a jet-cooled M‚XH van der Waals complex or the M(nsnp) state is excited (off-resonance) while it is colliding with an XH molecule. This type of experiment is providing, and will continue to provide, valuable insights into fundamental chemical and energy-transfer dynamics. Recent discoveries of practical processes which depend on Hg(3P1) activation of C-H bonds (unique vapor phase organic synthetic methods60) and Si-H bonds (chemical vapor deposition of semiconductor films61) have also given new impetus to studies of the mechanisms of X-H bond activation by excited metal atoms. The rates of reaction and the products of bond activation depend on the particular M*(nsnp) excited state and on the particular X-H bond (as well as on the molecule containing it):4,5 X
Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)00700-9 CCC: $12.00
X H
•M
H+
M(nsns) + H• +
H M
X
(1a)
X•
X•
(1b)
(1c)
For several cases, studied in our laboratories21,24-26,28,29,32,33,37 as well as in others,30,44,45,47,48,53,54 accurate quantum-stateresolved distributions of the vibrational and rotational energy of the MH(V,N) products in exit channel 1a have also been determined using laser pump-probe techniques. A variety of mechanisms, at various levels of detail, have been proposed over the years to explain both the dominant products observed (1a, 1b, or 1c) and the distributions of MH(V,N) energies when process 1a occurs. Ab initio calculations18,22,62-69 of M(nsnp) + mX-H potential surfaces have also been reported which have often been helpful mechanistically. We have recently reported70 that both Cd(5s5p 3P1) and Zn(4s4p 3P1) readily activate Si-H bonds in SiH4, forming the metal hydride molecules and SiH3 in the gas phase. In contrast, these reagents1,2,4,5,13,15,49-51 [as well as the analogous Hg(6s6p 3P0,1) states1,2,4,5] are very inefficient in activating the C-H bonds in CH4. However, the singlet Zn(4s4p 1P1) state, as well as the analogous Mg(3s3p 1P1) state, reacts at essentially every collision with CH4 to produce the metal hydride molecules and CH3.4,5,11,12,14,15,19,20,47,48,56,57 In this Feature Article, a comprehensive overview is presented of our current knowledge of the interactions of nsnp excited states of Mg, Zn, Cd, and Hg with H-H, Si-H, and C-H bonds. Mechanistic explanations are presented for the detailed “state-to-state” dynamical results © 1996 American Chemical Society
Feature Article
J. Phys. Chem., Vol. 100, No. 36, 1996 14841 TABLE 1: ∆H°298 Values (kcal/mol) for the Reactions M* + XH f MH + X71,72 and Bond Energies (kcal/mol) of MH Molecules71
a
M* 3
Cd( P1) Zn(3P1) Hg(3P1) Mg(3P1)
b
XH
∆H°298
H2 CH4 SiH4 H2 CH4 SIH4 H2 CH4 SiH4 H2 CH4 SiH4
+0.1 +0.7 -13.8 -9.2 -8.6 -23.1 -18.0 -17.4 -31.9 +11.5 +12.1 -2.4
M* 1
Mg( P1)
Zn(1P1)
XH
∆H°298
H2 CH4 C(CH3)4 SiH4 GeH4 H2 CH4 C(CH3)4 SiH4
-26.2 -25.6 -30.4 -40.0 -47.3 -49.9 -49.3 -54.1 -63.7
MH
∆H°298(MHfM+H)
MH
∆H°298(MHfM+H)
HgH CdH
9.5 16.5
ZnH MgH
20.5 30.2
TABLE 2: Comparison of Experimental versus “Prior” Vibrational Population Distributions70 reaction Zn(3P1) + SiH4 f ZnH(V) + SiH3 Figure 1. (a) Nascent population distribution P(N) of ZnH(V)0;N) in the reaction of Zn(4s4p 3P1) with SiH4.70 (b) Nascent population distribution P(N) of CdH(V)0;N) in the reaction of Cd(5s5p 3P1) with SiH4.70
which have been obtained in this area over the past several years in our laboratories as well as in others. Discussion 1. Activation of Si-H Bonds by Zn(4s4p 3P1) and Cd(5s5p 3P1). A. Zn(3P1) + SiH4. The nascent probability distribution of ZnH(V)0;N) product rotational quantum states for the reaction
Zn( P1) + SiH4 f ZnH(V;N) + SiH3 3
Cd(3P1) + SiH4 f CdH(V) + SiH3
product state V)0 V)1 V)2 V)0 V)1 V)2 V)3
PV(exptl)
PV(prior)a
1.00
