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HOONO ISOMERIZATION TO HONO2 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.

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Contents

Introduction

Conclusions

Potential energy surfaces

Contour maps: Avoided crossings and Conical Intersections?

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The important atmospheric reactions HO2 + NO and OH + NO2 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, (RO2 + NO), R = Alkyl, its chemistry is being studied.

HO2 + NO HOONO*

OH + NO2

HONO2*

NO2 + h O + NO

O+O2 + M O3 + 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).

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FONO isomerizes to FNO2 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 NO2 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(b) CH3OO + 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.

CH3ONO2 - CH3OONO reaction:(a) S0 and S1 surfaces with CAS(14,11) active space, (b) S0 surface showing a saddle point between CH3ONO2 and CH3OONO.

J. F. Arenas, F. J. Avila, J. C. Otero, D. Pelaez and J. Soto, J. Phys. Chem. A, 112, 249 (2008).

B3LYPCASSCF

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(a) UCCSD(T)/cc-pVTZ [ACES2] (b) UB3LYP/cc-pVTZ [G03]

PES is mapped as a function of: (1) distance between O and NO2 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

OHX

with 2 roots

(c) HOO + NO reaction

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cis-HOONO trans-HOONO HONO2

0.96

1.27

1

1.459

1.165

112.

7

114.0 117.4

1.424

1.37

61.

196

0.98

4

114.

4113.1 99

.9

0.96

1.27

1

1.593

1.17

4

100.

4

106.

1

108.

9

Dihedral (NOOH) = 100o

Minimum Energy Structures

MoleculesUCCSD(T)(kcal/mol)

UB3LYP(kcal/mol)

CASSCF(12,11)(kcal/mol)

Ho (0 K) (kcal/mol)

HO2 + NO 0.0 0.0 0.0 0.0

cis-HOONO -23.8 -23.0 -22.0 -27.1

trans-HOONO -20.0 -19.4 -17.8 -23.2

HONO2 -53.2 -52.7 -53.2 -55.1

HO + NO2 -8.5 -7.8 -11.3 -7.6

Comparison of minimum energies with zero point energy

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Contour maps of adiabatic surface

UCCSD(T)/cc-pVTZ

HONO2trans-HOONO

cis-HOONO

N

O

O

9CASSCF(12,11)//UCCSD(T)/cc-pVTZ

Location of avoided crossing regions on S0 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)

HONO2

trans-HOONO

cis-HOONO

N

O

O

• 165o

•

• • •

• •

• •

• •

•

•

•

•

•

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HONO2

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).

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Summary

Objective is to study the kinetics of HO2 + 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 S0 and S1 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.

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Acknowledgments

(Atmospheric Chemistry)

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14

HONO2trans-HOONO

cis-HOONO

UB3LYP/cc-pVTZ

N

O

O

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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.

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cis-HOONO trans-HOONO HONO2

0.96

1.27

1

1.459

1.165

112.

7

114.0 117.4

1.424

1.37

61.

196

0.98

4

114.

4113.1 99

.9

0.96

1.27

1

1.593

1.17

4

100.

4

106.

1

108.

9

D = 100.0

CI-1 CI-2