J. Phys. Chem. A 2005, 109, 8209-8217
8209
Dibenzo-p-dioxin. An ab Initio CASSCF/CASPT2 Study of the π-π* and n-π* Valence Excited States Ivan Ljubic´ * and Aleksandar Sabljic´ † Department of Physical Chemistry, Ru]er BosˇkoVic´ Institute, P.O. Box 180, HR-10002, Zagreb, Republic of Croatia ReceiVed: April 11, 2005; In Final Form: July 8, 2005
The π-π* and n-π* valence excited states of dibenzo-p-dioxin (DD) were studied via the complete active space SCF and multiconfigurational second-order perturbation theory employing the cc-pVDZ basis set and the full π-electron active spaces of 16 electrons in 14 active orbitals. The geometry and harmonic vibrational wavenumbers of the ground state correlate well with the experimental and other theoretical data. In particular, significant improvements over previously reported theoretical results are observed for the excitation energies. All of the π-π* excited states exhibit planar D2h minima. Thus no evidence was found for a C2V butterflylike relaxation, although the wavenumbers of the b3u butterfly flapping mode proved exceedingly low in both the ground S0(1Ag) and the lowest dipole allowed excited S1(1B2u) state. The calculations of oscillator strengths established the 21B2u r 11Ag and 21B1u r 11Ag transitions as by far the most intense, whereas the only allowed of the n-π* transitions (1B3u) should possess only a modest intensity. Studies into dependence of the oscillator strengths on the extent of the butterfly-like folding showed that the electronic spectrum is more consistent with a folded equilibrium geometry assumed by DD in solution.
Introduction The polychlorinated derivatives of dibenzo-p-dioxin (PCDDs), besides their high acute toxicity, proved very hazardous also in longer terms, because numerous studies established them as tumor promoters and teratogens.1 Naturally occurring2 and generated as unwanted byproducts of many industrial processes, mainly iron ore sinter plants and nonferrous metal industry facilities,3 as well as during incineration of industrial, municipal, and hospital wastes,4 PCDDs are nowadays recognized as a widely encountered class of pollutants.5 Despite a considerable decrease in the PCDDs release to the environment throughout the past decade, semivolatility and high resistance to degradation made possible their transport over long distances, with many years old emissions still contributing to current exposure.5 A major environmental and health concern stems from their solubility in lipids and the consequent tendency toward accumulation in fatty, skin, and liver tissues, whereas the elimination from human body comes about only slowly.6 A design of efficient methods of detection and photodeactivation of PCDDs calls for further extensive research into their fundamental physical properties.7 Theoretical studies into the electronic structure of PCDDs, in particular their IR and electronic spectra, have a very important role here. Dibenzo-p-dioxin (DD) stirred a considerable number of experimental and theoretical studies as a nucleus compound and a natural starting point for investigation into PCDDs. The most recent efforts include sub-Doppler high-resolution excitation spectroscopy by Baba et al.,8 as well as thorough analyses of the condensed phase IR, Raman, and phosphorence spectra on the part of Gastilovich et al.9 The structure of DD is depicted in Figure 1. * To whom the correspondence should be addressed. E-mail: iljubic@ irb.hr. † E-mail:
[email protected].
Figure 1. Dibenzo-p-dioxin and labeling of the coordinate axes within the D2h point group symmetry.
Figure 2. Dibenzo-p-dioxin in the C2V butterfly conformation.
In labeling the coordinate axes in the D2h DD molecule, we adopted the recommendation by the IUPAC Commission on Molecular Structure and Spectroscopy,10 with the z-axis passing through the largest possible number of atoms and the x-axis perpendicular to the molecular plane (Figure 1). This convention differs from that adopted in the most recent spectroscopical investigation by Baba et al.8 The correspondence between the molecule fixed axes and the irreducible representations (irreps) of the DD molecular orbitals within the two labeling conventions is the following: this work ref 8
x y
y z
z x
ag ag
b1g b3g
b2g b1g
b3g b2g
au au
b1u b3u
b2u b1u
b3u b2u
In the so-called butterfly conformation, claimed by some authors to be the equilibrium structure of the ground and/or the lowest excited state (vide infra), the molecule assumes a nonplanar C2V structure, as the one depicted in Figure 2. The butterfly-like bending in DD preserves the σxy and σxz reflection planes, whereas only the x-axis remains 2-fold (C2). The middle dioxin ring may thus be twisted to a more favorable boat conformation,
10.1021/jp051867s CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005
8210 J. Phys. Chem. A, Vol. 109, No. 36, 2005 however, at the expense of less pronounced stabilization via the π-electron delocalization. The angle closed by the two benzene rings commonly serves as a convenient measure of the degree of molecular distortion. From the character tables of the D2h and C2V point groups, on employing the molecule fixed coordinate system (Figure 1) the irreps of the C2V subgroup are correlated with the D2h irreps (in parentheses) as follows: A1 (Ag + B3u), A2 (B3g + Au), B1 (B1u + B2g), and B2 (B2u + B1g). The equilibrium structure of DD in the ground state (S0) has long raised controversies, as to whether the nuclear configuration has a D2h planar form or is bent from the plane as in a butterfly form. The issue is not only of an academic interest, for the reason that certain studies correlate higher ecotoxic properties of some dioxin isomers with their planarity.11 Thus Colonna et al.12 measured the nonzero (0.55 D) dipole moment of DD in the benzene solution and showed it to be consistent with a folded conformation, in which the two benzene rings close the angle of 164°. An analogous conclusion was reached on the grounds of dipole polarization studies, albeit with a somewhat larger dipole moment (0.64 D) determined.13 A semiempirical extended Hu¨ckel theory (EHT) for an isolated molecule yielded the planar minimum, although the conformational energy was found to be rather insensitive to changes in the angle between the two benzene rings.12 It was concluded that the DD structure must be markedly nonrigid in solution, owing to a very low barrier to the butterfly flapping motion. The analogous structural peculiarities, planar ground state minima coupled with very low barriers to the butterfly-like folding, were observed at the B3LYP/6-31G(d) level for the most toxic of dioxin congeners, tetrachlorinated DDs.14 The electronic transitions and oscillator strengths calculated at the semiempirical SCF-PPP-CIS level12,15 were in a fair agreement with the absorption spectrum of DD in heptane solution due to Lamotte and Berthier.16 Zimmermann et al.17 determined the first ionization energy of DD (7.598 ( 0.002 eV) using the resonance-enhanced twocolor two-photon ionization technique. These authors also recorded a clearly structured intermediate state spectrum of the first excited singlet state S1. Apart from the most intense vibronic line at ν˜ (0-0) + 22 cm-1, attributed to the out-of-plane butterfly flapping mode of the S1 state, the spectrum exhibited an unusual number of low-frequency modes (6.2 eV). With the exception of 1B2g and 3B2g, the n-π* states should occur below the first ionization threshold of DD (7.598 ( 0.002 eV).17 Within the D2h symmetry, the only dipole allowed n-π* transition, albeit with an almost negligible intensity (Table 4), is the x-polarized 1B3u r 1Ag occurring ∼0.6 eV below the ionization threshold. A relaxation to the butterfly C2V symmetry is expected to enhance this absorption, possibly significantly, owing to a larger overlap of the pz oxygen lone pairs with the π-orbitals of the benzene rings. The 11B1g state is completely analogous to the 2Ag state in that it is vibronic allowed via coupling with the butterfly mode, i.e., dipole allowed in the butterfly C2V symmetry, where it would match the third lowest B2 state. However, because the corresponding transition is y-polarized, it is expected to experience a much larger intensity gain than the 2Ag r 1Ag transition. Conclusions The properties of the singlet and triplet π-π* and n-π* valence excited states of dibenzo-p-dioxin (DD) were investigated via the CASSCF/CASPT2 approach22 with the g2 modification of the zeroth-order Hamiltonian.25 The use of the relatively small cc-pVDZ basis set was imposed by the size of the system, as well as the large active spaces employed, which comprised the full π-orbital space. Resorting to the real27 and imaginary28 level shift techniques proved necessary in calculating the n-π* excitation energies to eliminate the singularities due to intruder states. The calculated excitation energies for the 11B2u r 11Ag and 13B3g r 11Ag transitions, the two most extensively studied by the experiment,8,9 are in a very good agreement with the measured data. The CASSCF harmonic vibrational wavenumbers of the ground state exhibit a good correlation with the wavenumbers derived from the IR, Raman, and phosphorescence spectra.9 The geometry optimizations of all the excited states end up in a planar D2h minimum. Thus no evidence was found for a C2V butterfly-like relaxation, although the wavenumbers pertaining to the b3u butterfly flapping mode proved exceedingly low in both the ground and the lowest dipole allowed excited state (11B2u). The remarkably flat potential for the butterfly motion was verified by the calculations on the folded DD geometries. According to the CASSCF oscillator strengths, the 21B2u r 11Ag and 21B1u r 11Ag transitions are by far the most intense, whereas the only allowed n-π* transition (1B3u) should possess only a modest intensity. The vibronic allowed 21Ag r 11Ag transition gains only a small intensity on the butterfly-like
Ljubic´ and Sabljic´ folding of the DD molecule. Studies into dependence of the oscillator strengths on the extent of the folding show that the experimental electronic spectrum in solution35 is more consistent with a butterfly-folded equilibrium geometry of DD. An excellent reproduction of the electronic spectrum can be obtained provided minor adjustments are made to the vertical band centers calculated in a mildly folded geometry. At the same time, however, the 21Ag r 11Ag oscillator strength has to be increased considerably (75 times), to reproduce faithfully the shape of the 265-310 nm region. This large discrepancy remains unresolved, knowing that the remaining oscillator strengths exhibit a very good agreement with those derived from the experimental spectrum. Acknowledgment. This work was supported by the Ministry of Science and Technology of the Republic of Croatia under project number 0098033. Supporting Information Available: A detailed discussion on the intruder states emerging in the CASPT2 calculations on the π* transitions, graphs depicting dependence of the CASPT2 excitation energy upon various values of real and imaginary level shifts, pictorial representation of the (16,14) active space, and corresponding active occupancies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sax’s Dangerous Properties of Industrial Materials, 10th ed.; Lewis, R. J., Sr., Ed.; John Wiley & Sons: 2000; Vols. 1-3. (b) Shepard, B. M.; Young, A. L. In Human and EnVironmental Risks of Chlorinated Dioxins and Related Compounds; Tucker, R. E., Young, A. L., Gray, A. P., Eds.; Plenum Press: New York, 1983. (c) Kociba, R. J.; Cabey, O. Chemosphere 1985, 14, 649. (d) Aylward, L. L.; Hays, S. M.; Karch, N. J.; Paustenbach, D. J. EnViron. Sci. Technol. 1996, 30, 3534. (e) Grassman, J. A.; Masten, S. A.; Walker, N. J.; Lucier, G. W. EnViron Health Perspect. 1998, 106, 761. (f) Hassoun, E. A.; Stohs, S. J. Comp. Biochem. Physiol. 1996, 113C, 393. (g) Mackie, D.; Liu, J.; Loh, Y.-S.; Thomas, V. EnViron. Health Perspect. 2003, 111, 1145. (h) Williamson, M. A.; Gasiewicz, T. A.; Opanashuk, L. A. Toxicol. Sci. 2005, 83, 340. (2) Rappe, C.; O ¨ berg, L. G.; Andersson, R. Organohalogen Compd. 1999, 43, 249. (3) Rappe, C. Pure Appl. Chem. 1996, 68, 1781. (4) Yoneda, K.; Ikeguchi, T.; Yagi, Y.; Tamade, Y.; Omori, K. Chemosphere 2002, 46, 1309. (5) (a) Atkinson, R. Atmospheric chemistry of PCBs, PCDDs, and PCDFs. In Chlorinated Organic Micropollutants: Issues in EnVironmental Science and Technology; Hester, R. E., Harrison, R. M., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1996. (b) Baker, J. I.; Hites, R. A. EnViron. Sci. Technol. 2000, 34, 2879. (6) (a) Shiu, W. Y.; Doucette, W.; Gobas, F. A. P. C.; Andren, A.; Mackay, D. EnViron. Sci. Technol. 1988, 22, 651. (b) Sabljic´, A. Chemosphere 2001, 43, 363. (c) Fuster, G.; Schumacher, M.; Domingo, J. L. EnViron. Sci. Pollut. R. 2002, 9, 241. (d) Ogura, I. Organohalogen Compd. 2004, 66, 3376. (7) (a) Khasawneh, I. M.; Winefordner, J. D. Talanta 1988, 35, 267. (b) Funk, D. J.; Oldenborg, R. C.; Dayton, D.-P.; Lacosse, J. P.; Draves, J. A.; Logan, T. J. Appl. Spectrosc. 1995, 49, 105. (c) Klimenko, V. G.; Nurmukhametov, R. N. J. Fluoresc. 1998, 8, 129. (d) Harada, H.; Tanaka, M.; Murakami, M.; Shimizu, S.; Yatsuhashi, T.; Nakashima, N.; Sakabe, S.; Izawa, Y.; Tojo, S.; Majima, T. J. Phys. Chem. A 2003, 107, 6580. (8) Baba, M.; Doi, A.; Tatamitani, Y.; Kasahara, S.; Katoˆ, H. J. Phys. Chem. A. 2004, 108, 1388. (9) (a) Klimenko, V. G.; Nurmukhametov, R. N.; Gastilovich, E. A. Opt. Spectrosc. 1997, 83, 84. (b) Gastilovich, E. A.; Klimenko, V. G.; Korol’kova, N. V.; Rauhut, G. Chem. Phys. 2001, 270, 41. (c) Gastilovich, E. A.; Klimenko, V. G.; Korol’kova, N. V.; Nurmukhametov, R. N. Chem. Phys. 2002, 282, 265. (10) (a) Schutte, C. J. H.; Bertie, J. E.; Bunker, P. R.; Hougen, J. T.; Mills, I. M.; Watson, J. K. G.; Winnewisser, B. P. Pure Appl. Chem. 1997, 69, 1633. (b) Schutte, C. J. H.; Bertie, J. E.; Bunker, P. R.; Hougen, J. T.; Mills, I. M.; Watson, J. K. G.; Winnewisser, B. P. Pure Appl. Chem. 1997, 69, 1641. (11) Grainger, J.; Reddy, V. V.; Patterson, D. G., Jr. Appl. Spectrosc. 1988, 42, 643.
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