A Quantum Chemical Study of HZn CH 3

A Quantum Chemical Study of HZn. CH 3 A Transition Metal Compound with 4 s 2 Recoupled Pair Bonding David E. Woon & Thom H. Dunning, Jr. RI 12

Motivation In late 2010, Flory et al. reported new experimental results for HZn. CH 3 (methyl zinc hydride), which is thought to be formed by direct insertion of Zn into a methane CH bond. M. A. Flory, A. J. Apponi, L. N. Zack, and L. M. Ziurys, J Am Chem Soc 132, 17186, 2010. featured in the 6 December 2010 issue of Chemical & Engineering News

Motivation Our two reasons to study this species: (1) Its formation pathway is not known: Zn(1 S) or Zn(3 P)? (2) It looks like a case of s 2 recoupled pair bonding analogous to the behavior of elements like Be, B, and C.

Motivation Our two reasons to study this species: (1) Its formation pathway is not known: Zn(1 S) or Zn(3 P)? (2) It looks like a case of s 2 recoupled pair bonding analogous to the behavior of elements like Be, B, and C.

Outline Recoupled Pair Bonding: p 2 and s 2 cases S vs. Be & Zn Zn. H & Zn. H 2 Bonding, Structure, and Energetics of Zn. CH 3 & HZn. CH 3 Zn(1 S, 3 P) Reactions with CH 4 Zn(1 S) + CH 4 HZn. CH 3 (recoupled pair bonding pathway) Zn(3 P) + CH 4 Zn. H + CH 3 & curve crossings (covalent bonding pathway)

Recoupled Pair Bonding: p 2 vs. s 2 RHF S 3 s 23 px 3 py 3 pz 2 GVB Overlap GVB (MCSCF) 3 s 23 px 13 py 1(3 pz 2 - c 123 dz 22) 0. 855 Be Be 2 s 2 (2 s 2 - c 222 pz 2) 0. 681

Recoupled Pair Bonding: p 2 vs. s 2 RHF S 3 s 23 px 3 py 3 pz 2 GVB (MCSCF) GVB Overlap 3 s 23 px 13 py 1(3 pz 2 - c 123 dz 22) 0. 855 Zn Be Zn 2 s 2 (2 s 2 - c 222 pz 2) 0. 681 3 d 104 s 2 3 d 10(4 s 2 – c 324 pz 2) 0. 713

Recoupled Pair Bonding in Zn. H X 2 S+

Recoupled Pair Bonding in Zn. H X 2 S+

Recoupled Pair Bonding in Zn. H X 2 S+

Recoupled Pair Bonding in Zn. H X 2 S+

Recoupled Pair Bonding in Zn. H X 2 S+

Recoupled Pair Bonding in Zn. H RCCSD(T)/AVQZ calcs H Zn Zn + H Zn. H DEe = -23. 1 kcal/mol RZn. H = 1. 5991 Å

Recoupled Pair Bonding in Zn. H & Zn. H 2 RCCSD(T)/AVQZ calcs Zn Zn Zn + H Zn. H DEe = -23. 1 kcal/mol RZn. H = 1. 5991 Å Zn. H + H Zn. H 2 DEe = -81. 7 kcal/mol RZn. H = 1. 5365 Å Typical behavior observed in compounds with recoupled pair bonding: weak & long 1 st bond, stronger & shorter 2 nd bond.

Recoupled Pair Bonding in Zn. CH 3 RCCSD(T)/AVQZ calcs Zn Zn + CH 3 Zn. CH 3 DEe = -17. 5 kcal/mol RZn. C = 2. 0092 Å

Recoupled Pair Bonding in Zn. CH 3 & HZn. CH 3 RCCSD(T)/AVQZ calcs Zn Zn + CH 3 Zn. CH 3 DEe = -17. 5 kcal/mol RZn. C = 2. 0092 Å Zn Zn. H + CH 3 HZn. CH 3 DEe = -81. 5 kcal/mol RZn. C = 1. 9428 Å RZn. H = 1. 5365 Å

Recoupled Pair Bonding in Zn. CH 3 & HZn. CH 3 RCCSD(T)/AVQZ calcs Zn Zn + CH 3 Zn. CH 3 DEe = -17. 5 kcal/mol RZn. C = 2. 0092 Å Zn Flory et al. (R 0) RZn. C = 1. 9281(2) Å RZn. H = 1. 5209(1) Å RZn. C = 1. 9428 Å RZn. H = 1. 5365 Å

Recoupled Pair Bonding in HZn. CH 3 Zn. H bond pair Zn. C bond pair Zn Zn polarized toward H polarized toward CH 3

Recoupled Pair Bonding in HZn. CH 3 H orbital CH 3 singly occupied orbital Zn Zn Zn left lobe orbital Zn right lobe orbital Zn Zn GVB overlap s=0. 704

Singlet Reaction: Zn(1 S)+CH 4 HZn. CH 3 Method: • TS search at the B 3 LYP/AVTZ-PP level • IRC calculations to characterize reaction path connectivity • Single point calculations at the CCSD(T)/AVTZ level

Singlet Reaction: Zn(1 S)+CH 4 HZn. CH 3 TS HZn. CH 3 Zn+CH 4 IRC Barrier: B 3 LYP 97. 7 kcal/mol

Singlet Reaction: Zn(1 S)+CH 4 HZn. CH 3 TS HZn. CH 3 Zn+CH 4 IRC Barrier: B 3 LYP 97. 7 kcal/mol CCSD(T)//B 3 LYP 90. 5 kcal/mol MP 2 93. 2 kcal/mol

Singlet Reaction: Zn(1 S)+CH 4 HZn. CH 3 The barrier for direct insertion on the singlet surface is very large. What happens on the triplet surface? Does it lead to a more efficient pathway to forming HZn. CH 3?

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 Method: TS search at the B 3 LYP/AVTZ-PP level IRC calculations to characterize reaction path connectivity Single point calculations at the CCSD(T)/AVTZ level · Perform singlet calculations at triplet IRC geometries for both methods to explore the existence of surface crossings and the possibility of surface hopping

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 TS Zn+CH 4 Zn. H+CH 3 IRC Barrier: B 3 LYP 6. 7 kcal/mol

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 TS Zn+CH 4 Zn. H+CH 3 IRC Barrier: B 3 LYP 6. 7 kcal/mol CCSD(T)//B 3 LYP 10. 8 kcal/mol

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 TS IRC Barrier: B 3 LYP 6. 7 kcal/mol CCSD(T)//B 3 LYP 10. 8 kcal/mol

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 crossing 1 crossing 2 TS IRC Barrier: B 3 LYP 6. 7 kcal/mol CCSD(T)//B 3 LYP 10. 8 kcal/mol

Triplet Reaction: Zn(3 P)+CH 4 Zn. H+CH 3 Relaxation of structures at crossing points: Zn(1 S) + CH 4 non-reactive HZn. CH 3 reactive crossing 1 crossing 2

Singlet vs. Triplet Reactions The singlet surface reaction leads to direct insertion, but the barrier is very high. The triplet surface reaction leads to insertion if hopping occurs to the singlet surface late in the IRC. There is still a modest barrier to overcome, but surface hopping may or may not be very efficient. Harvey (Phys Chem Phys 9, 331, 2007) found that reaction kinetics favor spin-allowed pathways at high temperatures.

Loose Ends Verify singlet and triplet transition states at full RCCSD(T) and/or MRCI levels of theory Calculate triplet to singlet surface hopping rates Explore the Zn(1 S) + CH 4 Zn. H + CH 3 abstraction reaction: if it occurs, is it competitive with the insertion reaction? Compare to prior theory for both spin surfaces.

Conclusions § Zinc species such as Zn. H, Zn. H 2, Zn. CH 3, and HZn. CH 3 all arise from recoupling the 4 s 2 pair in a manner similar to recoupling s 2 pairs in early p-block elements (Be-C, Mg-Si, etc. ) and p 2 pairs in late p-block elements (S-Cl, etc. ). § Singlet and triplet pathways to HZn. CH 3 were found: while there is a large barrier on the singlet pathway, may still be the dominant pathway if the surface hopping rate is very slow. it

Acknowledgement Funding: The Distinguished Chair for Research Excellence in Chemistry at the University of Illinois at Urbana-Champaign L-R: Lina Chen, Jeff Leiding, Beth Lindquist, Thom Dunning, Lu Xu, David Woon, Tyler Takeshita