Week 6. Some Tricks of Doing PES Calculations

Week 6. Some Tricks of Doing PES Calculations PES = Potential Energy Surface

F2 + CH3SCH3 (DMS) Reaction Reaction between two Closed Shell molecules J. Chem. Phys. 127, 101101 (2007) Usually a closed-shell molecule does not like to react with another closed-shell one. M-M Reaction

Closed Shell means that the configuration of doubly occupied molecular orbitals predominate. Usually a molecule with closed shell electronic structure is more stable and easier to be calculated.

H H H C H N H H H H CH3 NH2 OH F H N H O H F H C H H H H H Cl Cl H H H H Cl + H2 HCl + H H + Cl2 HCl + Cl Cl Cl Atom, Molecule & Radical Atom H C N O F Ne 原子 H C N O F Ne Molecule H2 CH4 NH3 H2O HF 2 or 8 electrons (Octet) H 分子 H O H F Radical 1 or 7 electrons (1 unpaired) テジカル Chemical Reaction + + 化学反应

F2 + CH3SCH3 CH3-S-CH2F + HF F  CH3-S=CH2 + HF m80 m20 Distinguish Isomers F F CH3SFCH3 + F m81 m19

8.7 eV 8.7 eV 7.8 eV

3.4 kcal/mol + F F2+ 0.0 TS1 ≲ -2.5 TS2 VDW INT (-12) + HF -80.1 Geometry: CAS-PT2 & QCISD(T) /cc-pVDZ (C,H); cc-pV(T+d)Z (S); aug-cc-pVTZ (F) Energy (kcal/mol): CCSD(T) & CAS-PT2 / Complete Basis Set Extrapolation for S, F.

Effect of Basis Sets F + F2 + No aug-basis on F

Similar (CH3)2S structures Smaller basis sets for C and H are OK. The error will be canceled in the relative energy. S-C=1.806 ∠CSC=98o 1.42 F2 DMS 1.78 1.92 1.45 2.76 ∠CSF=87o S-C=1.807 ∠CSC=98o ∠SFF=170o ∠SFF=171o ∠CSF=91o S-C=1.794 ∠CSC=100o INT VDW 1.49 2.52 2.00 ∠CSF=89o S-C=1.788 ∠CSC=101o ∠CSF=90o S-C=1.807 ∠CSC=99o ∠SFF=173o (CH3)2SF TS1

2-D PES with fixed (CH3)2S geometry F-FS(CH3)2 distance (Å) FF-S(CH3)2 distance (Å)

“safe” ☻ ☹? PE profile on the reaction coordinate (CH3)2S + F2 INT (CH3)2SF + F

e e n (CH3)2S: s* e e s F-F three-center four-electron bonding s* e e n e e s INT

3.4 kcal/mol + F F2+ 0.0 TS1 ≲ -2.5 TS2 VDW INT (-12) + HF -80.1

TS2 is Very Special! Free HF bond distance = 0.92Å for transition states: typical H-F distance = 1.1~1.4 Å 1.90 1.83 H-F: 1.95Å

Charge on Fout TS2 INT Dipole(z) (atomic unit) F-H distance (Å)

Auto Charge Separation (Harpoon) mechanism F + CH3SFCH3 HF + CH2SFCH3 Vertical IE of CH3SFCH3 = 7.8 eV Electron Affinity of F = 3.5 eV CH3SFCH3+ + F- -1/R 4.3 eV CH3SFCH3 + F 3.4Å

Covalent or Ionic? ~ F- TS2 INT F-S=1.792 S-CH3=1.793 CH2-S=1.621 ∠CSC=103o ∠FS(CH3)=88o ∠FS(CH2)=117o CH2SFCH3

最穩定的分子不一定會先形成 F2 + DMS CH3 + 0.0 -14.8 -29.0 CH3 + Rel. Energy (kcal/mol) -100.6 SF2(CH3)2

One more thing about M-M Reaction

F2 + CH3-S-S-CH3 CH3SF + CH3SF, Barrier << 1 kcal/mol  F + CH3-SF-S-CH3 ,EC > 4kcal/mol CH3SF+ CH3SS(F)CH3+ from CH3S2FCH3

H3C S F F S H3C H3C S F F S H3C Proposed mechanism

Acknowledgement Exp: Yu-Ju Lu (呂育儒), Lance Lee (李久林), Dr. Chu Shuan Chang (張志鉉), H-J Shao (邵華潔), Chi-Wei Liang (梁記偉), 方靜雯, 陳學穎 Cal: James Pan (潘俊維), Dr. Tingxian Xie (解廷獻), Prof. Henryk Witek Prof. Yuan T. Lee, Prof. Yuan-Pern Lee NSRRC for beam-time and help from Dr. Shih-Huang Lee’s group $$$: Academia Sinica, NSC

Thank You for Your Attention Product Distribution Potential Energy Surface

Week 6. Some Tricks of Doing PES Calculations