“Van der Waals” Wells are Important in Chemical Reactions

1. “Van der Waals” Wells are Important in Chemical Reactions University of Florida, QTP Nov. 6, 2002 Acknowledgments: Dunyou Wang (now at NASA/Ames), Tiao Xie (Emory), David Manolopoulos (Oxford), $$ from US Dept. of Energy

2. Cl + HD D+HCl, H+DCl reaction • Importance of this reaction • It plays a central role in fundamental chemical kinetics, and has served as a critical test case for bimolecular reaction rate theory, especially transition-state and kinetic isotope effect. And, the theory of isotope effects was derived from it. • This reaction is also a prototype for a host of Cl reactions that are in atmospheric chemistry and photochemical air pollution. • This reaction is the rate determining step in the mechanism of the Cl2 + H2 2HCl chain reaction.

3. Studies of the Cl + H2 reaction • Experimental studies: • Rate constants for Cl + H2 and D2 reactions over the temperature range 296-3000 K. • Branching ratio of Cl + HD reaction has been studied in crossed molecular beam experiment. • Theoretical studies: • Many potential energy surfaces have been constructed for this reaction, among which, the G3 surface most successful one. • VTST have been used to calculate rate constants on these surfaces, and compared with experimental data. Truhlar and co. • Quantum reactive scattering on G3 and a new pes Manolopous, Werner and co-workers

4. The “G3” potential energy surface • G3 surface was constructed by Truhlar et al. in 1996. • It’s based on the so-called GQQ surface, which has been shown to give good agreement with experiment on Cl + H2 and D2 reactions. • G3 surface improves on the GQQ surface in the region of Cl-H-H bending potential. • Linear saddle point geometry: RHCl (Å) = 1.4011 RHH’ (Å) = 0.9896 RH’Cl (Å) = 2.3907 V (kcal/mol) = 7.88

5. G3 Success Cl + D2 Cl + H2

6. Failure of the G3 surface Branching ratio determined in cross-beam experiment as a function of collision energy for HD(j=0). K. Liu (1999) Collision energy (kcal/mol)

7.  r R Cl H H Contour Plot of G3 Surface Jacobi Coordinates

8. G3 surface and Bian-Werner surface • BW and G3 surface are broadly similar • Barrier height: (kcal/mol) 7.88 (G3) 7.61 (BW) • Saddle point frequencies (cm-1) bending: 581 (G3) 540 (BW) stretching: 1358 (G3) 1360 (BW) • Difference • Imaginary frequency (cm-1) 1520i (G3) 1294i (BW) This indicates that G3 surface has a thinner barrier. • BW has a Van der Waals well with a depth of 0.5 kcal/mol at a T-shape equilibrium geometry.

9. G3 surface and Bian-Werner surfaces

10. Theory and ExperimentManolopoulos Science (1999)

11. Cl H D Cl D H G3 and BW surfaces Prob to form HCl reduced On BW relative to G3

12. Conclusion Van der Waals well (very shallow) in Cl+HD has a significant effect on branching ratio for Cl + HD(j=0) but not on rate constant

13. The O(3P)+HCl Reaction A challenging reaction, non-linear saddle point, ‘heavy-light-heavy’ system. Barrier height of KSG adjusted down by KSG to get agreement with exp on k(T). Those calculations were not converged so later calcs showed disagreement with Experiment - barrier height too small. New surface ‘S4’ by Ramuchandran, barrier height is higher than KSG, but ...

14. RATE CONSTANT FOR O(3P)+ HCl ON S4 S.Z.B.A.T.L.R.G.L JPC (2001)

15. The exact expression for k(T) N(E) is the Cumulative Reaction Probability

16. (Variational) Transition State Theory

17. TST Derivation

18. POTENTIALS FOR O(3P)+ HCl REACTION

19. The O(3P)+HCl Reaction

20. Cl 9.8 kcal -1.6 -5.2 Cl O H O O H H Cl The O(3P)+HCl Reaction

21. The O(3P)+HCl Reaction S. Skokov, T. Tsuchida, S. Nanbu, J. M. Bowman, and S. K. Gray, J Chem. Phys(2000). K. Nobusada, H. Nakamura, Y. Lin, B. Ramachandran, J. Chem. Phys. (2000) CRP(J=0) =

22. The O(3P)+HCl ReactionXie, Wang, Bowman, Manolopoulos (2002)

23. The O(3P)+HCl Reaction

24. The O(3P)+HCl Reaction Quasi-bound states Bound states

25. Resonances and density of states Eth Resonances are therefore like bound states in some respects, or bound states are resonances with zero widths.

26. Resonances and lifetimes The more conventional relationship is given as follows: This is unimolecular decay of an (isolated) resonance, with a decay rate equal to

27. The (quasi) bound state approach Resonances are quasibound eigenstates with complex energy eigenvalues, Er,n-iG n/2 HC = H - ilU(R)

28. Quasibound State calculations A primitive basis of twenty Legendre functions, Eight vibrational functions of HCl for O+HCl (range: 1.6 a0 to 3.3a0) and 8 OH vibrational functions for Cl + OH (range 1.2a0 to 3.6a0) and 100 sine functions in R for each arrangement Ranges of R are [3.4a0 ,10.2a0] for the O+HCl channel and [3.2a0 , 8.0a0] for the C+lOH channel. Length of the absorbing potential: 2.0a0 A contraction scheme was used to reduce the direct product basis from to 16,000 to 4770. 400 of the real wavefunctions used to construct complex H-matrix. The range of l was 0.001 to 0.5 h, in steps of 0.01 h.

29. Quasibound State calculations

30. Comparison of resonance energies and quasibound State energies of VdW wells (eV)

31. Comparison of resonance energies and quasibound state energies of VdW wells (eV) Overlap = quasibound density in the saddle point region

32. Assignment of resonances

33. Quasibound state wavefunctions O-HCl state at 0.2496 eV

34. Quasibound state wavefunctions Cl-HO state at 0.2414 eV

35. CONCLUSIONS Resonances in the tunneling region due to Van der Waals minima. Important effect on k(T) - increasing, why? a) Resonances “prepare complexes” b) Non-adiabaticity? Recall Question bend zpe. Do wells destroy bending Adiabaticity?

36. Other examples

37. OH+HNO3 Negative T-dependence indicates fairly complex and positive T-dependence indicates a barrier, as usual.