1. 1 HOONO ISOMERIZATION TO HONO 2 INVOLVING CONICAL INTERSECTIONS T. J. DHILIP KUMAR, and JOHN R. BARKER Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI JOHN F. STANTON Institute for Theoretical Chemistry, Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX OSU International Symposium on Molecular Spectroscopy meeting, June 22-26, Columbus, Ohio, USA.

2. 2 Contents  Introduction  Conclusions  Potential energy surfaces  Contour maps: Avoided crossings and Conical Intersections?

3. 3  The important atmospheric reactions HO 2 + NO and OH + NO 2 lead to formation and dissociation of the HOONO intermediate. Introduction  HOONO has cis- and trans- isomers. Generic reactions:  With the objective of studying the detailed dynamics of this reaction and its class of reactions, (RO 2 + NO), R = Alkyl, its chemistry is being studied. HO 2 + NO HOONO* OH + NO 2 HONO 2 * NO 2 + h  O + NO O+O 2 + MO 3 + M  Surface is computed in order to study the influence of low-lying excited electronic states on the ground state PES and on its reaction dynamics. L. P. Olsen, M. D. Bartberger and K. N. Houk, J. Am. Chem. Soc., 125, 3999 (2003).

4. 4  FONO isomerizes to FNO 2 through a tight transition state involving a two-state avoided curve crossing. (UCCSD(T)) (a) FO + NO reaction  HOONO, is isoelectronic with FONO and similar mechanism can be invoked. Contour map of the lowest energy model diabatic surface, plotted as a function of the cartesian coordinates of the F atom with NO 2 at its TS. G. B. Ellison, J. M. Herbert, A. B. McCoy, J. F. Stanton and P. G. Szalay, J. Phys. Chem. A 108, 7639 (2004). Potential Energy Surfaces

5. 5 (b) CH 3 OO + NO reaction  Multi-configurational CASSCF calculations located a conical intersection near where single-configurational DFT methods predict an intrinsic energy barrier.  The barrier in B3LYP suggested to be an artifact. CH 3 ONO 2 - CH 3 OONO reaction: (a) S 0 and S 1 surfaces with CAS(14,11) active space, (b) S 0 surface showing a saddle point between CH 3 ONO 2 and CH 3 OONO. J. F. Arenas, F. J. Avila, J. C. Otero, D. Pelaez and J. Soto, J. Phys. Chem. A, 112, 249 (2008). B3LYPCASSCF

6. 66  (a) UCCSD(T)/cc-pVTZ [ACES2] (b) UB3LYP/cc-pVTZ [G03]  PES is mapped as a function of: (1) distance between O and NO 2 and (2) angle between O, N and X while optimizing all other degrees of freedom.  In-plane PES is computed while H of OH is allowed to relax out-of plane. Coordinates Levels of theory: (c) CASSCF(12,11)/cc-pVTZ [MOLPRO]  “Rear end” and “front end” are merged. “Front end” “Rear end” N O O HO X N O O OH X with 2 roots (c) HOO + NO reaction

7. 7 cis-HOONOtrans-HOONO HONO 2 0.96 1.271 1.459 1.165 112.7 114.0 117.4 1.424 1.376 1.196 0.984 114.4 113.1 99.9 0.96 1.271 1.593 1.174 100.4 106.1 108.9 Dihedral (NOOH) = 100 o Minimum Energy Structures Molecules UCCSD(T) (kcal/mol) UB3LYP (kcal/mol) CASSCF(12,11) (kcal/mol)  H o (0 K) (kcal/mol) HO 2 + NO0.0 cis-HOONO-23.8-23.0-22.0-27.1 trans-HOONO-20.0-19.4-17.8-23.2 HONO 2 -53.2-52.7-53.2-55.1 HO + NO 2 -8.5-7.8-11.3-7.6 Comparison of minimum energies with zero point energy

8. 8 Contour maps of adiabatic surface UCCSD(T)/cc-pVTZ HONO 2 trans-HOONO cis-HOONO N O O

9. 9 CASSCF(12,11)//UCCSD(T)/cc-pVTZ Location of avoided crossing regions on S 0 surface.  Recently, CASPT2/cc-pVTZ show a small barrier resulting from an avoided crossing with an excited electronic state for cis-HOONO C. Chen, B. C. Shepler, B. J. Braams and J. M. Bowman, Phys. Chem. Chem. Phys., 11, 4722 (2009); J. Chem. Phys. 127, 104310 (2007) HONO 2 trans-HOONO cis-HOONO N O O 165 o

10. 10 HONO 2 trans-HOONO cis-HOONO UCCSD(T)/cc-pVTZ N O O UB3LYP/cc-pVTZ CASSCF(12,11)//UCCSD(T)/cc-pVTZ CI-1 CI-2 J. F. Arenas, F. J. Avila, J. C. Otero, D. Pelaez and J. Soto, J. Phys. Chem. A, 112, 249 (2008).

11. 11 Summary  Objective is to study the kinetics of HO 2 + NO reaction using the ground electronic state and including the coupling effects of low lying excited states.  Ab initio surface computed using CCSD(T) and CASSCF(12,11) methods.  CASSCF shows avoided crossing regions between S 0 and S 1 as in FO+NO system.  Computing non-adiabatic coupling matrix elements and mixing angles between the two states on the path of avoided crossings/conical intersections.

12. 12 Acknowledgments (Atmospheric Chemistry)

13. 13

14. 14 HONO 2 trans-HOONO cis-HOONO UB3LYP/cc-pVTZ N O O

15. 15 Energy difference of avoided crossing regions obtained using CASSCF(12,11)/cc-pVTZ.  This surface and others in the same class have been studied previously by others. L. P. Olsen, M. D. Bartberger and K. N. Houk, J. Am. Chem. Soc., 125, 3999 (2003).  Global potential energy surface (PES) is investigated by the UCCSD(T) and CASSCF methods.

16. 16 cis-HOONOtrans-HOONO HONO 2 0.96 1.271 1.459 1.165 112.7 114.0 117.4 1.424 1.376 1.196 0.984 114.4 113.1 99.9 0.96 1.271 1.593 1.174 100.4 106.1 108.9 D = 100.0 CI-1 CI-2