A Quantum Chemical Study of HZnCH 3 A Transition Metal Compound with 4s 2 Recoupled Pair Bonding David E. Woon & Thom H. Dunning, Jr. RI12

Motivation In late 2010, Flory et al. reported new experimental results for HZnCH 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, 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 ZnH & ZnH 2  Bonding, Structure, and Energetics of ZnCH 3 & HZnCH 3  Zn( 1 S, 3 P) Reactions with CH 4 Zn( 1 S) + CH 4  HZnCH 3 (recoupled pair bonding pathway) Zn( 3 P) + CH 4  ZnH + CH 3 & curve crossings (covalent bonding pathway)

Recoupled Pair Bonding: p 2 vs. s 2 RHF GVB Overlap GVB (MCSCF) 3s 2 3p x 3p y 3p z 2 S s 2 3p x 1 3p y 1 (3p z 2 - c 1 2 3d z 2 2 ) Be 2s 2 (2s 2 - c 2 2 2p z 2 ) Be

2s 2 (2s 2 - c 2 2 2p z 2 ) Zn 3d 10 4s 2 3d 10 (4s 2 – c 3 2 4p z 2 ) Zn Recoupled Pair Bonding: p 2 vs. s 2 RHF GVB Overlap GVB (MCSCF) 3s 2 3p x 3p y 3p z 2 S s 2 3p x 1 3p y 1 (3p z 2 - c 1 2 3d z 2 2 )

X2+X2+ Recoupled Pair Bonding in ZnH

X2+X2+

X2+X2+

X2+X2+

X2+X2+

Zn RCCSD(T)/AVQZ calcs H Zn + H  ZnH  E e = kcal/molR ZnH = Å

Recoupled Pair Bonding in ZnH & ZnH 2 Zn Zn + H  ZnH  E e = kcal/mol ZnH + H  ZnH 2  E e = kcal/mol R ZnH = Å R ZnH = Å Typical behavior observed in compounds with recoupled pair bonding: weak & long 1 st bond, stronger & shorter 2 nd bond. RCCSD(T)/AVQZ calcs

Recoupled Pair Bonding in ZnCH 3 Zn + CH 3  ZnCH 3  E e = kcal/molR ZnC = Å Zn RCCSD(T)/AVQZ calcs

Recoupled Pair Bonding in ZnCH 3 & HZnCH 3 Zn + CH 3  ZnCH 3  E e = kcal/molR ZnC = Å Zn RCCSD(T)/AVQZ calcs Zn ZnH + CH 3  HZnCH 3  E e = kcal/molR ZnC = Å R ZnH = Å

Recoupled Pair Bonding in ZnCH 3 & HZnCH 3 Zn + CH 3  ZnCH 3  E e = kcal/molR ZnC = Å Zn R ZnC = Å RCCSD(T)/AVQZ calcs R ZnH = Å R ZnC = (2) Å R ZnH = (1) Å Flory et al. (R 0 )

ZnH bond pairZnC bond pair Zn polarized toward Hpolarized toward CH 3 Recoupled Pair Bonding in HZnCH 3

H orbitalCH 3 singly occupied orbital Zn left lobe orbitalZn right lobe orbital Zn GVB overlap s=0.704 Recoupled Pair Bonding in HZnCH 3

Singlet Reaction: Zn( 1 S)+CH 4  HZnCH 3 Method: TS search at the B3LYP/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  HZnCH 3 IRC TS Barrier:B3LYP97.7 kcal/mol Zn+CH 4 HZnCH 3

Singlet Reaction: Zn( 1 S)+CH 4  HZnCH 3 TS IRC Zn+CH 4 HZnCH 3 Barrier:B3LYP97.7 kcal/mol CCSD(T)//B3LYP90.5 kcal/mol MP293.2 kcal/mol

Singlet Reaction: Zn( 1 S)+CH 4  HZnCH 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 HZnCH 3 ?

Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 Method:  TS search at the B3LYP/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

IRC Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 TS Zn+CH 4 ZnH+CH 3 Barrier:B3LYP 6.7 kcal/mol

IRC Zn+CH 4 ZnH+CH 3 Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 TS Barrier:B3LYP 6.7 kcal/mol CCSD(T)//B3LYP10.8 kcal/mol

IRC Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 TS Barrier:B3LYP 6.7 kcal/mol CCSD(T)//B3LYP10.8 kcal/mol

IRC crossing 1 crossing 2 Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 TS Barrier:B3LYP 6.7 kcal/mol CCSD(T)//B3LYP10.8 kcal/mol

 Zn( 1 S) + CH 4 non-reactive  HZnCH 3 reactive crossing 1 crossing 2 Triplet Reaction: Zn( 3 P)+CH 4  ZnH+CH 3 Relaxation of structures at crossing points:

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 Chem Phys 9, 331, 2007) found that reaction kinetics favor spin-allowed pathways at high temperatures.

Loose Ends  Calculate triplet to singlet surface hopping rates  Verify singlet and triplet transition states at full RCCSD(T) and/or MRCI levels of theory  Explore the Zn( 1 S) + CH 4  ZnH + 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 ZnH, ZnH 2, ZnCH 3, and HZnCH 3 all arise from recoupling the 4s 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 HZnCH 3 were found: while there is a large barrier on the singlet pathway, it may still be the dominant pathway if the surface hopping rate is very slow.

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