J . Phys. Chem. 1987, 91, 5035-5037 torsion the Raman line at 206 cm-' for the d6 molecule has been used in the calculation but the Raman frequency is not known to better than two wavenumbers. Additionally, we used the shift factor of 1.364 for the analogous mode in the A" symmetry block since this methyl torsion was not observed in the spectrum of the d6 molecule in the gas phase. Thus, taking these factors into consideration, the agreement between the theoretical and observed ratios appears to be very satisfactory. For the shift ratio for the (CH3)2NC1and CH3(CD3)NC1molecules, the normal modes are all in the A symmetry block, so that the calculated value is 18.41 and the experimental value is 17.79, which gives an error of 3.4% which again is quite satisfactory. Therefore, the Teller-Redlich product rule supports the proposed vibrational assignment. The assignment of the NCI stretching mode is consistent with the recent work of McDonald et al.I5 on CINO. As expected, this stretching mode is far from a pure mode where mixing occurs with the CNCl bend, C-N symmetric stretch and N C 2 symmetric deformation. Remarkable consistency was found among the frequencies for the, normal vibrations associated with the methyl groups of dimethylamine and those of the corresponding modes of Nchloro-N-methylmethanamine. It is rather surprising that the heavy chlorine atom with its large electronegativity did not significantly affect the carbon-hydrogen motions of the methyl groups. However, it should be noted that the structural parameters for the common atoms in the (CH3)2NHand (CH3),NC1 molecules are very similar, including the nitrogenarbon distance^.'^^^' In (CH3),NH the barrier to internal rotation has been determined9 to be 1054 cm-' (3.01 kcal/mol); substitution of a chlorine atom for the hydrogen atom increases the barrier to 1658 cm-I (4.74 kcal/mol). This increase of 1.73 kcal/mol is in agreement with the observed increase in the torsional barrier from 679 cm-' (1.94 kcal/mol) in CH3NH222to 1323 cm-' (3.78 kcal/mol) in CH3NHCLZ3 One might expect this additivity to the barriers with substitution to continue with the addition of a second chlorine atom for a hydrogen atom in methylamine. Therefore, one would predict the barrier in CH3NCI2to be around 5.5 kcal/mol. Such substituent additivity has been found for the barriers in the corresponding halocarbon^.^^ The sample of CH3(CD3)NClcontained an impurity, which could not be removed, that exhibited interfering vibrations in the Raman spectrum of the liquid. Analysis of the mass spectrum (21) Wollrab, J . E.; Laurie, V. W. J . Chem. Phys. 1968, 48, 5058. (22) Takagi, K.; Kohma, T. J . Phys. SOC.Jpn. 1971, 30, 1145. (23) Caminati, W.; Cervellati, R.; Mirri, A. M. J . Mol. Spectrosc. 1974, 51, 288. (24) Durig, J. R.; Guirgis, G. A. Chem. Phys. 1979, 44, 309.
5035
of the CH3(CD3)NClsample indicates that the impurity may be CD3NC12. Many of the observed vibrational bands due to the impurity correlate with the expected vibrational frequencies of CH3NC1225upon deuteriation, although it is likely there is an additional impurity that cannot be identified. There have been few reported26force constants for the N-C1 bond. For the ClNO molecule this force constant has been reportedI5 to have a value of 1.243 mdyn/8,, which is only about half the value of 2.31 mdynJ8, that we found for this force constant for the (CH3)2NC1molecule. However, the N-C1 bond distance2' in ClNO is 1.973 A whereas the N-CI distanceI9 in (CH3),NC1 is 1.749 8,. The molecule CINO, is reportedZsto have an N-C1 bond distance of 1.83 8,with a force constant of 1.840 mdynJ8,. Thus, it appears that the short and presumably stronger N-CI bond in (CH3)2NC1is reflected by the larger force constant, and as the N-CI bond becomes longer, the force constant value decreases. The force constants for the remaining portion of the molecule appear reasonable compared to those reportedZ9from ab initio calculations for methylamine. In the vibrational studies of dimethylamine, the Urey-Bradley force field was utilized, so it is not possible to compare those force constants with the ones obtained in this study. Nevertheless, it appears that a reasonable set of force constants has been obtained for N-chloro-Nmethylmethanamine. In the Raman spectrum of solid (CH3),NCI there are at least nine observed lattice modes. Unfortunately, they were not nearly as pronounced in the Raman spectrum of either (CD,)2NC1 or CH3(CD3)NCI. Therefore, it is not possible to assign the individual transitions to translational or librational modes on the basis of the observed shifts with deuteriation. Nevertheless, the number of lattice modes indicates that there are at least two molecules per primitive cell. Acknowledgment. We gratefully acknowledge the financial support of this study by the National Science Foundation through Grant CHE-83-11279. Registry No. (CH3)2NC1, 1585-74-6; CH,(CD3)NC1, 109801-36-7; (CD,),NCI, 109838-65-5. (25) Haag, W. R. J . Znorg. Nucl. Chem. 1980, 42, 1123. (26) Hohne, V. K.; Jander, J.; Knuth, K.; Schlegel, D. Z . Anorg. Allg. Chem. 1971, 386, 316. (27) Cazzoli, G.; Degli Esposti, C.; Palmissi, P.; Simeone, S. J. Mol. Spectrosc. 1983, 97, 165. (28) Bernitt, D. L.; Miller, R. H.; Hisatsune, I. C. Specfrochim.Acta, Part A 1967, 23A, 237. (29) Hamada, Y.; Tanaka, N.; Sugawara, Y.; Hirakawa, A. Y.; Tsuboi, M.; Kato, S.; Morokuma, K. J . Mol. Spectrosc. 1982, 96, 313.
An Analysis of the Electronic States of Acephenanthrylene B. F. Plummer Department of Chemistry, Trinity University, San Antonio, Texas 78284 (Received: January 7 , 1987; In Final Form: April 24, 1987)
The absorption and emission spectrum of acephenanthrylene is examined experimentally and theoretically by the use of simple perturbational molecular orbital theory and by use of a semiempirical PPP SCF CI calculation. The long-wavelengthtransition near 4 4 0 nm is assigned to a new state called a K-transition that is not present in the precursor analogue phenanthrene. The excited states are found to contain extensive configuration interaction. A weak fluorescence near 540 nm with a quantum yield of 0.0035 is identified as a normal Stokes shifted emission.
Benzo-fused derivatives of acenaphthylene (A) continue to attract study because of their potential carcenogenicity',2 and their 0022-3654/87/2091-5035$01.50/0
possible occurrence in the e n ~ i r o n m e n t . ~We are interested in learning more about these substances because of their anticipated 0 1987 American Chemical Society
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5036 T h e Journal of Physical Chemistry, Vol. 91, No. 19, 1987
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Plummer acephenanthrylene
phenanthrene
p /
)
20x
Figure 2. Perturbational molecular orbital diagram for the correlation of states of acephenanthrylene. The circles a r e drawn with their diameters proportional to the size of the coefficients of the atomic orbitals at the atom indicated in the molecular orbital. T h e energy levels are computed from first-order Hiickel molecular orbital theory and plotted in units of 0. 400
500
wavelength (rim)
Figure 1. (Top) Absorption (-) spectrum of acephenanthrylene in cyclohexane. Excitation and emission spectra (- - -) are in arbitrary units. (Bottom) Calculated transitions based upon PPP SCF CI calculations. Bar thickness corresponds to approximate oscillator strength. A circle represents a very weak band. Flags at the top of each bar are transition moment directions calculated with respect to the structure positioned as shown.
novel photophysical and photochemical The availability of acephenanthrylene (AP) through improved syntheses9-" encouraged us to examine it in more detail. Previous molecular orbital calculation^^^^'^ reported few details about the expected spectroscopic behavior of AP. We also are concerned about the reported" fluorescence characteristics of AP because the emission and excitation data seemed incompatible with our understanding of the excited-state properties to be expected from the yellow hydrocarbon. We report here the experimentally determined absorption and emission spectra and analyze the spectroscopic transitions by qualitative M O theory and through the use of the semiempirical PPP S C F CI calculation.
Results and Discussion The absorption, emission, and excitation spectra and calculated electronic transitions for AP are reproduced in Figure 1. The (1) Sangaiah, R.; Gold, A,; Toney, G. E. In Polynuclear Aromatic Hydrocarbons: Formation, Metabolism, and Measurement; Cooke, M., Dennis, A. J., Eds.; Butterworth: Woburn, MA, 1983; pp 1067-1076. (2) Kohan, M. J.; Sangaiah, R.; Ball, L. M.; Gold, A. Mutat. Res. 1985, 155, 95. (3) Fu, P. P.; Beland, F. A,; Yang, S. K. Carcinogenesis (London) 1980, I , 725. (4) Plummer, B. F.; AI-Saigh, 2. Y. J . Phys. Chem. 1983, 87, 1579. (5) Plummer, B. F.; AI-Saigh, Z. Y.; Arfan, M . J . Org. Chem. 1984, 49, 2069. (6) Plummer, B. F.; AI-Saigh, Z. Y.; Arfan, M. Chem. Phys. Lett. 1984, 104, 389. (7) Plummer, B. F.; Hopkinson, M. J.; Zoeller, J. H. J . Am. Chem. SOC. 1979, l01, 6779. (8) Ferree, W. I., Jr.; Plummer, B. F.; Schloman, W. W. Jr., J . Am. Chem. SOC.1974, 96, 7741 and references cited therein. (9) Scott, L. S.; Reinhardt, G.; Roelofs, N . H . J . Org. Chem. 1985, 50, 5886. ( I O ) Neumann, G.; Mullen, K. Chimia 1985, 39, 275. (1 1) Amin, S.;Balanikas, G.; Huie, K.; Hussain, N.; Geddie, J. E.; Hecht. S. S. J . Org. Chem. 1985. 50, 4642. (12) DasGupta, A,; DasGupta, N. K. Can. J . Chem. 1976, 54, 3227. (13) Titz, M.; Hochmann, P. Collect. Czech. Chem. Commun. 1967, 32, 2343.
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similarity of the absorption spectrum to that of phenanthrene14 (P) is noteworthy. However, an important difference is that associated with the very weak transition ( t 300) that has its 00band near 445 nm. The yellow of AP is undoubtedly the result of this weak transition with a diffuse vibrational progression that rises on the edge of the transition that occurs near 368 nm. This weak transition bears a strong similarity to that of the K-band occurring at long wavelengths in the spectrum of A.'5$'6 The remainder of the spectrum of AP progressing from 368 nm down to 200 nm is similar in its pattern to the fine structure detail identifiable in the spectrum of P. We have simulated the spectrum of AP by a simple M O perturbational model (Figure 2) based upon the elegant theoretical analysis of the spectrum of the nonalternant hydrocarbon A proposed by Michl and T h u l ~ t r u p . ' ~We have analyzed the spectroscopic behavior of AP by use of the semiempirical Pariser-Pople-Parr S C F CI c a l c ~ l a t i o n . ~ ~ - ~ ~ The first transition in AP is characterized by a low-energy HOMO LUMO energy difference that is not found in P. In the linear combination of molecular orbitals, the LUMO of ethylene has the correct orbital phase relationship to interact strongly with the LUMO of P to produce a new LUMO (labeled -1) of AP whose energy is lower than that of either contributor. The new HOMO of AP (labeled +1) results from the out-of-phase combination of the bonding MO of ethylene with an appropriate lower energy orbital of P. This interaction creates a new orbital (+1) of higher energy than the remaining P molecular orbitals, whose energies are slightly lowered by the perturbation. This 1 -1 transition, which bears similar characteristics to that found in A, we suggest should also be labeled a K-transition. It has a significant degree of charge transfer from the ethylene bridge of the polycyclic ring similar to that found for A . If there is significant photochemistry to be found for this system, we would anticipate that the excited-state processes will have considerable electrophilic character. Orbitals numbered +2 and +3 of AP appear, on the basis of their calculated symmetry, to be derived from the perturbed orbitals of P that originally were its HOMO and second highest occupied orbitals (the solid correlation lines).
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(14) Jaffe, H . H.; Orchin, M. Theory and Applications of Ultracioler Spectroscopy; Wiley: New York, 1962; pp 323-326. (15) Thulstrup, E. W.; Michl, J. J . Am. Chem. Soc. 1976, 98, 4533. (16) Michl, J . J . Am. Chem. Sot. 1976, 98, 4546. ( 1 7) Michl, J.; Warnick, S. M. J . Am. Chem. Sot. 1974, 96, 6280. (18) Michl, J. J . Am. Chem. SOC.1978, 100, 6801, 6812, 6819. (19) Pariser, R.; Parr, R. G . J . Chem. Phys. 1953, 21, 466. (20) Pople, J . A. Trans. Faraday SOC.1953, 49, 1375.
Electronic States of Acephenanthrylene
The Journal of Physical Chemistry, Vol. 91, No. 19, 1987 5037
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LUMO
E
"OMO
*
Figure 3. HOMO and LUMO orbital electron densities of AP calculated from PPP SCF CI approximation. (See caption to Figure 2.)
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This analysis supports our observation that the spectrum of A P is similar to that of P. The transitions 2 -1 and 3 -1 are representative of a perturbed phenanthrene transition with respect to their symmetry properties. The results of the PPP S C F CI calculation are fortuitously close to the observed absorption spectrum of AP. The calculated oscillator strength of the first transition is weak, suggesting a symmetry-forbidden transition that occurs through vibrational coupling with higher energy allowed states. Substantial configuration interaction can be identified from the computation. The first transition is calculated to be transversely polarized and to contain 30% 1 -1 in AP, with an additional admixture of 45% of the -1. In Figure 3, we higher energy transitions 2 -1 and 3 illustrate the computed MOs for the H O M O and LUMO. The wave-function symmetry derived from the correlation of states for the H O M O and LUMO is surprisingly similar to those from the PPP S C F CI calculations. The next transition near 368 nm is also calculated to be transversely polarized, and we tentatively 'A transition. There is also extensive assign it as the 'Lb configuration interaction in this transition. About 30% is admixture of 1 -1, and another 35% is admixture of the third transition and some mixture of a higher electronic state above the LUMO. The third transition near 336 nm is calculated to be mostly of longitudinal polarization, and we assume that it rep'A transition. This transition also shows exresents the 'La tensive configuration interaction with the 1 -1 state and with a higher electronic level above the LUMO (specifically, the next energy level above LUMO). The remaining transitions are in a congested region of the spectrum, and their assignments are even more speculative until further data can be obtained. On the basis of the computed energies, we tentatively propose that the transition near 266 nm is analgous to a Bb 'A transition. The band near 'A. The transition near 229 247 nm appears analogous to B, 'A while one near 219 nm is likely c, nm is possibly a Cb 'A. These assignments are based primarily on the calculated polarizations and energies and by comparison to the assignment of states in the analysis of the spectrum of phenanthrene.2',22 The fluorescence of A P has a broad and diffuse maximum located near 540 nm and a quantum yield of 3.5 X This
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(21) Vasak, M.; Whipple, M. R.; Michl, J. J . Am. Chem. SOC.1978, 100, 6867. (22) Thulstrup, E.; Michl, J.; Eggers, J. H.1.Phys. Chem. 1970, 74, 3868.
contrasts markedly with a previously reported emission supposed to occur in the 360-380-nm region. Given the weak K-band with its onset near 440 nm, we would expect a relatively diffuse emission with a large Stokes shift into the visible portion of the spectrum. This long-wavelength emission is also similar in character to that of A.7323 The excitation spectrum is a further corroboration of the origin of the 540-nm emission from AP. We have also excited AP near 260 nm and produced an emission near 350-370 nm which has similar characteristics to that initially reported]' for AP. However, when we attempted to verify its origin with a corrected excitation spectrum, we could not find a similarity between the excitation spectrum and the absorption spectrum. We conclude that the emission occurring near 360 nm when using 260-nm excitation is a spurious one that may be related to the presence of a contaminant that is difficult to remove. It should be noted that the molar absorptivity values reported for the compound producing the unusual fluorescence are similar to those reported by Scott9 with one exception: that being a peak at 270 nm with an e = 172 000 that we do not see in pure AP. This may represent a contaminant whose emission occurs at the wavelength of emission purported to be that of AP. In our experience, these annelated derivatives of acenaphthylene often exhibit unusual fluorescence behavior such as anomalous fluorescence but with very low quantum yield^.^*^^^ However, AP does not have a sufficient difference in its energy between the first and second excited states. If anomalous fluorescence were to be expected,24 then the energy gap between S,and S2typically should be greater than 9000 cm-I. In AP, the gap is less than 5000 cm-'. The absence of significant quantum yield of fluorescence from these annelated acenaphthylenes is characteristic of their behavior. They all seem to have high radiationless rates of internal conversion that compete with emission. The presence of the K-band with its extensive configuration interaction and attendant vibrational coupling seems to be the apparent source of the radiationless deactivation of the excited states because the precursor aromatics are all significantly fluorescentZ5when not coupled to the ethylene bridge through a peri set of carbon bonds.
Experimental Section Acephenanthrylene was synthesized and purified by vacuum sublimation, and its properties were confirmed to be identical with a sample furnished by Professor Scott.9 Absorption spectra were run on a Cary 118C spectrophotometer in spectroscopic-grade cyclohexane (Aldrich). Fluorescence and excitation spectra were measured with a Perkin-Elmer M P F 44B using a DCSU-2 corrected spectra microprocessor and slit widths of 5 nm (excitation) and 10 nm (emission). Molecular orbital calculations were carried out with the PPP S C F C I program as previously described.17J8 All of the modifications of the formalism described in ref 17 and 18 have been retained. The 65 lowest singly excited configurations below 100 eV were used in the calculation. Acknowledgment. We gratefully recognize the support of the Petroleum Research Fund, administered by the American Chemical Society, and the support of the National Science Foundation. We thank Professor L. S. Scott for a sample of acephenanthrylene and Dr. J. Downing and Professor J. Michl for computational support. Registry No. Acephenanthrylene, 201-06-9. (23) Heilbronner, E.; Weber, J.-P.;Michl, J.; Zahradnik, R. Theor. Chim. Acta 1966, 6, 141. (24) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (25) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970.
