Electronic spectra of H and OH adducts to benzene

16 June 1995

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 239 (1995) 258-262

Electronic spectra of H and OH adducts to benzene M. Krauss a, R. Osman b a Center for Advanced Research in Biotechnology, National Institute of Standards and Technology, 9600 Gudelsky Drive, Rockville, MD 20850, USA b Department of Physiology and Biophysics, Mount Sinai School of Medicine, City University of New York, New York, NY 10029, USA

Received 9 January 1995; in final form 29 March 1995

Abstract Hydrogen and hydroxyl radical adducts to benzene are intermediates in its oxidative degradation. Their presence can be detected by a UV absorption near 310 nm. However, ab initio calculations of the 1,4-cyclohexadienyl structure for the ground state in both adducts suggests the likelihood of an additional absorption in the visible. Calculations indicate that the observed absorption is to the second excited state. Absorption to the first excited state is calculated to have a much smaller oscillator strength than the observed second excited state. The predicted spectra of the H and OH adducts are similar.

1. Introduction The hydroxyl radical and the hydrogen atom add readily to benzene. Formation of the adducts is the initial step in oxidation of benzene when oxygen is present in the solution. A recent study of these reactions followed a broad absorption band with a peak at 310 nm attributed to the hydroxyl adduct [1]. Two mesomeric forms of the radical adducts, M1 and M2 in Fig. 1, are assumed to exist. Their relative contribution to the final electronic structure would determine the carbon site for the addition of oxygen. However, theoretical calculations suggest that the 1,4-cyclohexadienyl structure, M1, in which the benzene resonance is lost is lower in energy [2], and dominates the ground state electronic behavior. In the ground state the double bond is primarily localized on C 1 - C 2 and C 4 - C 5 as described in Fig. 1. The localized double bonds in the 1,4-cyclohexadienyl radical suggest that the diagnostic absorption at 310 nm may not be the lowest electronic transition

available. Strong coupling is possible between the radical open shell orbital and the excited orbitals of the nearest neighbor 'rr or conjugated orbitals [3]. The resulting excited state would be substantially lower in energy than normally expected and a visible transition is predicted for this radical adduct. Thus, the observed transition can be assigned to the second excited state. It is evident that a catalogue of the states of the radical adducts is necessary for interpretation and assignment of the spectra. This note will analyze the electronic structure of the ground and excited states of both the hydrogen and hydroxyl adducts. Although, the electronic structure of the ground states have already been investigated [2,4], there have been no studies of the excited states.

2. Method Optimized structures are generated using analytical gradients for both the H and OH adducts in the

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M. Krauss, R. Osman / Chemical Physics Letters 239 (1995) 258-262

M1

M2a

M2b

Fig. 1. Schematic of mesomeric valence bonding structures and atom numbering.

ground and first excited state at both the restricted open-shell Hartree-Fock (ROHF) and complete active space multi-configuration self-consistent-field (CAS-MCSCF) levels using the GAMESS [5] suite of codes. The standard defaults in GAMESS were used in the solution of the ROHF and MCSCF equatons. To reduce the computational effort the excited state solution of the OH adduct was converged to an RMS gradient of 0.0006 which is an order of magnitude larger than the ground state. The excited state geometries are provided with one less significant figure than the ground state. The H and OH adducts have C2v and C s symmetry, respectively, with the symmetry plane perpendicular to the ring. However, the electronic structure is discussed with more clarity if the orbitals are referred to the plane of the ring atoms. In the H adduct the ring is perfectly planar while it is slightly outof-plane in the OH adduct. The dominant excitations within the active orbital space of the H adduct include orbitals that are anti-symmetric with respect to reflections in the ring plane. A similar behavior is observed for the OH adduct with respect to the plane that best approximates the ring atoms. All of the discussion therefore denotes the orbital symmetry with respect to the approximate symmetry plane of the ring. Orbitals are designated as cr when they are not changed by reflection in the plane and as ,rr when they change sign. The active space includes the essential five "rr orbitals suggested by Chipman [2]. The two valence 'rr orbitals are very similar to those required to study the valence excited states of benzene [6,7]. Eleven additional o- orbitals are included for the OH adduct and nine for the H adduct. Twenty three electrons are distributed among 14 orbitals for the H adduct and twenty seven electrons among 16 orbitals for the OH adduct. Including the cr orbitals does not alter the

259

dominant excitation among the ~ orbitals. The inclusion of the cr orbitals in the active space was done to insure that excitations within this sub-space was not important. Adding a cr valence orbital to the active space did not alter the excitation energies significantly and was not pursued further. The natural orbitals from a first-order configuration interaction (FOCI) were used as the initial orbitals in the MCSCF solution. Five of the orbitals which energetically correspond to the most tightly bound were kept as core orbitals. The excitation energies are obtained from average density CAS-MCSCF calculations with the ground state weighted 0.5 and each excited state 0.25. The core orbitals are replaced by compact effective potentials (CEP) used with the concomitant CEP-31G basis [8]. Polarization functions were not added because it rendered the CAS-MCSCF calculation prohibitively large at this time. Reasonably accurate valence excitation energies for benzene were obtained with the same basis set [6]. The excitation energies to the 1Blu, 3Blu , and 3Elu states of benzene are calculated within 1000 c m - 1 of experiment. The dynamical correlation tends to cancel between states where orbital excitations are among valence orbitals of the same size. It is possible that more diffuse functions are needed for the description of the second excited state, but the accuracy obtained in this work is also about 1000 cm -1 from experiment and is sufficient for assignment of this state to the observed absorption.

3. Results and discussion The optimized geometries are given in Table 1 for both ROHF and MCSCF calculations with the same notation as presented by Chipman [2] to ease comparison. Somewhat surprisingly the critical C1-C2 bond distance in the ROHF geometry of the ground state is in better agreement with the geometry reported by Chipman [2] who also did not use any polarized functions in calculating the geometry. The differences are probably due to the use of the CEP and CEP basis. The C - H bond distance is larger than expected which is attributed to the lack of polarization functions on the hydrogen atom. Calculation of the excitation energies at the two geome-

26O

M. Krauss, R. Osman / Chemical Physics Letters 239 (1995) 258-262

Table 1 Geometry of C6H 7 and C6H6OH

tries f o r t h e O H a d d u c t , o p t i m i z e d at the M C S C F ROHF

C6H 7

C6H6OH

B 2 [1]

bond distance C1-C6 C1-C2 C2-C3 C6-H C1-H C2-H C3-H C6-O O-H angles (deg) C5C6C1 C6C1C2 C1C2C3 C2C3C4 C2C1H C1C2H C2C3H C6OH

A 2 [2]

,~ [1]

g e o m e t r i e s h a v e little e f f e c t o n t h e e x c i t a t i o n e n e r g y

A" [21

ROHF

MC

MC

ROHF

MC

MC

(,~) 1.522 1.366 1.457 1.104 1.093 1.092 1.090

1.524 1.395 1.449 1.103 1.093 1.092 1.091

1.52 1.46 1.41 1.10 1.09 1.09 1.09

1.515 1.364 1.455 1.102 1.091 1.091 1.090 1.462 0.966

1.516 1.394 1.447 1.102 1.091 1.092 1.090 1.461 0.966

1.52 1.47 1.43 1.10 1.09 1.09 1.09 1.47 0.97

113.0 122.9 121.2 118.9 120.0 120.0 120.6

113.1 122.6 121.2 119.4 120.0 119.6 120.3

114 121 121 122 120 119 119

113.1 122.9 121.1 119.0 119.0 120.0 120.5 112.2

113.2 122.6 121.2 119.4 119.4 120.0 120.3 112.3

112 122 121 119 119 118 120 112

0 0 180 180 180

0 0 180 180 180

0.7 - 0.4 182.2 178.6 178.6

-0.6 0.4 177.3 181.0 181.4

-6 0 191 180 179

torsions (deg) C4C2C1C6 0 C5C1C2C3 0 C4C2C1H 180 C5C1C2H 180 C4C2C3H 180

Complete geometries are available upon request. All states are effectively P~' with respect to reflections in the plane of the carbon ring; the state symmetries heading the columns are given for the overall molecular symmetry.

Table 2 H and OH benzene adduct excitation energies and dipole moments H

Total energy

Excitation energy

B 2 [1]

A 2 [2]

B 2 [3]

(23, 14)

- 36.89250

479.1

299.9

OH a

Total energy [11

(27, 16) I (27, 16) II

- 52.54715 - 52.54284 2.14

or

l e v e l ( s e e T a b l e 2), s h o w s that the d i f f e r e n t

Table 3 Dominant configurations in MCSCF for the ground and excited states of H and OH adducts of benzene State

A~ [31

483.2 444.5 2.13

302.7 295.7 2.18 D

Energy in the first column is in au and the excited state transition energy is in nm. a Geometry: I MCSCF, II ROHF. (27, 16) MCSCF for 27 electrons in 16 orbital active space.

Coefficient

la"

2a"

3a"

4a"

5a"

2 2 1 2 1

2 1 2 0 1

1 1 1 1 1

0 1 0 2 1

0 0 1 0 1

0.922 0.213 -0.162 -0.142 0.139

2A 2

2 2 2 2 0 2 2

2 1 1 0 2 2 1

0 2 1 2 2 2 0

1 0 0 1 1 2 2

0 0 1 0 0 0 0

0.641 0.604 0.234 -0.125 -0.108 -0.108 0.107

3B 2

1 2 2 1 2 1 1

2 2 1 1 1 1 2

2 0 1 2 0 1 0

0 0 1 1 1 1 2

0 1 0 0 1 1 0

0.623 -0.501 - 0.442 -0.169 -0.123 -0.118 0.104

State

Orbital occupancies a

C6H7 1B 2

Coefficient

la

2a

3a

4a

5a

2 2 1 1 2

2 1 2 1 0

1 1 1 1 1

0 1 0 1 2

0 0 1 1 0

0.926 0.197 -0.163 0.139 -0.134

2,V

2 2 2 1 2 0

1 2 1 2 0 2

2 0 1 1 2 2

0 1 0 1 1 1

0 0 1 0 0 0

0.682 0.551 0.253 - 0.224 -0.112 -0.104

3~

1 2 2 1 2 1 0

2 2 1 1 1 1 2

2 0 1 2 0 1 2

0 0 1 1 1 1 0

0 1 0 0 1 1 1

0.649 - 0.483 - 0.442 -0.159 -0.120 -0.106 0.103

C6H6OH

Excitation energy ~ ' [21

Orbital occupancies a

a All orbital symmetries with respect to reflections in the plane of the ring.

M. Krauss, R. Osman / Chemical Physics Letters 239 (1995) 258-262

rations, described in Table 3, show the transfer of the electron open-shell hole among the "rr orbitals upon excitation from the ground state. The order of the excited states is determined by the number of effective node changes involved in the excitations from the ground state. The first excited state is dominated by two configurations. In one, the initially singly occupied at3 is excited to "rr4 and in the other there is a single excitation from 'rr2 to 'rr3. In the second excited state the doubly occupied "rr1 orbital is singly excited to "rr3 while the configuration with singly occupied rr3 is excited to "rr5. The dipole moments in Table 2 of the ground and excited states are very similar because all the -rr orbitals are delocalized around the ring and the charge centroid is not altered. Thus, no meaningful solvent shift of these excitations is expected. The dipole and the velocity oscillator strengths are both calculated to be small at 0.0005 and 0.0017, respectively, while for the second excited state they are 0.0041 and 0.0066. This explains the difficulty in observing the first excited state. Although the electronic structure of the H and OH adducts are very similar near the equilibrium geometry of the ground state, the asymptotic correlation behavior of these systems are very different suggesting that the energy surfaces of the excited states have to eventually behave very differently. For example, the first excited state of the OH adduct will adiabatically correlate with the benzene and OH (2H) ground state asymptote while the excited state of the H adduct correlates with the lowest triplet excited state of benzene and H(2S). However, there is no evidence for any substantial difference between the optimized geometries of the H and OH adducts first excited state. The adiabatic behavior apparently does not manifest itself until the molecule is distorted far from the equilibrium geometry of either the ground or the excited state.

of the second excited state but a substantial one on the first excited state. Some insight into the behavior of the excited state electronic structure can be obtained from optimizing the geometry of the first excited state and comparing the geometry and electronic structure to the ground state. The geometry of the first excited state for both the H and OH adduct (see Table 1) was optimized at the MCSCF level. Optimization of the geometry at the MCSCF level affects the shorter double bonds primarily because the rr orbital excitations determine the dominant configurations in the MCSCF. The electronic structure is described in Table 3. In the first excited state, the dominant configurations involve an excitation from a non-bonding to an antibonding orbital or a bonding orbital to a non-bonding orbital. The ground state double bonds at C1-C2 and C4-C5 are substantially weakened but some of the resonance interaction is retained between bonds C2-C3 and C3-C4. There is also a concomitant transfer of charge from the carbon para to the adduct site to the carbon atoms in the ortho position. It is evident from Table 1 that the H and OH adducts have similar ring geometries in the ground as well as in the first excited state. Replacing H by OH does not affect the overall geometry or electronic structure of these adducts. In the ground state the open shell is localized at position C3, para to the addition site generating a higher probability of binding of an open shell molecule like 0 2. This conclusion is in agreement with the experimental analysis [1]. The in vacuo excitation energies are given in Table 2 at the geometries of all optimized ground state structures. The dominant configurations in the ground and first two excited states all involve excitations within the "rr space. The nodal properties of the -rr orbitals are described in Fig. 2. The dominant ground state configuration has two electrons in both rr I and "rr2 with one electron in ar 3. These configu-

r~1

~2

261

~3

~4

re5

Fig. 2. Schematicof "~' orbitals whose occupanciesare given in Table 3 as orbitals la-5a.

262

M. Krauss, R. Osman / Chemical Physics Letters 239 (1995) 258-262

In conclusion, the calculated energy of the second excited state of the OH adduct is found to be in reasonable agreement with the experimental absorption energy [1]. The in vacuo energies are expected to be accurate for these valence states. The solution spectra are not expected to shift since the dipole moments of the ground and both excited states are essentially the same. As expected from the implications of the ground state geometry and electronic structure, there is also an allowed lower energy transition predicted to the first excited state. This visible transition may be useful as another signature for these molecules in environments where the UV transition could become obscured.

References [1] X.-M. Pan and C. von Sonntag, Z. Naturforsch. 45b (1990) 1337. [2] D. Chipman,J. Phys. Chem. 96 (1992) 3294. [3] M. Krauss and R. Osman, J. Phys. Chem. 97 (1993) 13515. [4] S. Olivellaand A. Sole, J. Am. Chem. Soc. 113 (1991) 8628. [5] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis and J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [6] J.M.O. Matos, B.O. Roos and P. Malmquist, J. Chem. Phys. 86 (1987) 1458. [7] M. Krauss and D.R. Garmer, J. Phys. Chem. 97 (1993) 831. [8] W.J. Stevens, H. Basch and M. Krauss, J. Chem. Phys. 81 (1984) 6026.