3461
J. Phys. Chem. 1995,99,3461-3464
Perturbation of Spin Density Distribution Due to Deuterium Substitution Han Zuilhof: Gerrit Ladder,*$? Robert P. van Mill: Patrick P. J. Mulder: David E. Kage? Richard C. Reiter,*?$and Cheryl D. Stevenson*y$ Gorlaeus Laboratories, Leiden Institute of Chemistry, P.O. Box 9502,2300 RA Leiden, The Netherlands, and the Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160 Received: July 15, 1994;In Final Form: September 12, 1994@
The EPR analysis of the anion radicals of 1,3,6$-tetradeuteriopyrene and 1,3,4,5,6,8,9,1O-octadeuteriopyrene in THF with K+ at 173 K shows that the observed coupling constant for the protons in the 2,7 positions is decreased in both anion radicals by 25 mG (AUH= -25 mG) from its value in the anion radical of pyrene itself. This implies that there is only a very small, if any, contribution to this reduction in the spin density at the 2,7 positions resulting from deuterium substitution in the 4,5,9,10 positions. Ab initio calculations (6-31G** and 6-311G** basis sets) carried out considering the C-D bond to be shorter than the C-H bond by 0.01 A predict the effects of octadeuteriation to be 21 and 18 mG, respectively, in good agreement with experiment. The calculations do, however, predict an approximately equal contribution from the two tetradeuterio substitution patterns, with the effect of 1,3,6,8-deuteriation being slightly larger than that of 4,5,9, IO-tetradeuteriation.
In 1993 it was observed that deuterium substitution of the ring hydrogens in the solid copper salt of 2,5-dimethyl-N,iV‘dicyanoquinonediimine increases the temperature for a phase transition by several degrees, alters the conductivity properties, and significantly alters the appearance of the EPR signals.’ These unprecedented large secondary deuterium isotope effects are attributed to an amplification of the zero-point energy and bond length changes by a collective of an “infinite number of layers of molecules”. Deuteriation also caused differences in the crystal structures revealed by X-ray analysis. This portion of the study was reminiscent of the X-ray structure determinations of tetracyanoanthraquinodimethanereported in the same year by Heimer and Matten.* They found that this compound (I), which is distorted from planarity into a bent “butterfly” shape
+ NC I CN
due to steric repulsion between the cyano groups and the encroaching pen hydrogens, is distorted to a smaller degree (the system is flatter) when the ring protons are replaced with deuteriums. These two nonplanar systems are designed in such a way as to magnify the effects of bond length changes upon chemical and physical properties. The causes of the effects are, however, comp1e:ely general. Due to the slight anharmonicity of the potential energy curves, the lower zero-point energy of the C-D bond relative to that of the C-H bond results in shorter mean and maximum bond lengths of the C-D bond. For naphthalene, X-ray crystallography reveals a reduction of the C-L (L = H or D) bond length from 1.085 (C-H) to 1.073 (C-D) A.3 Vibrational and/or geometrical changes significantly perturb electron di~tribution.~ Consequently, a direct measurement of Leiden University.
* Illinois State University.
‘Abstract published in Advance ACS Abstracts, February 15, 1995.
this property would represent a probe into such isotopic effects. Since it is possible to observe very small changes (milligauss) in EPR coupling constants ( U H ’ S ) when several lines exhibiting a particular aH can be identified, with the use of internal standard^,^ electron paramagnetic resonance provides a very sensitive tool with which changes in electron distribution can be made visible. This motivated us to investigate the existence of EPR observable perturbations in electron distribution as a result of deuteriation in a simple, planar, nondegenerate, hydrocarbon system. The system of interest must be nondegenerate to avoid isotope effects on the Jahn-Teller distortion?,6 To be EPR active, the hydrocarbon system would have to be in the form of the anion radical. Further, since the complex hyperfine pattern from such a system would probably preclude the isolation of individual components giving direct measure of UH perturbations, the general appearance of the overlapping EPR spectral lines must be very sensitive to changes in aH. The pyrene system possesses sufficient EPR complexity, solution electron affinity, and anion radical planarity to meet these criteria. Thus, as part of our studies of isotope effects occumng in the reduction of organic compounds, we have synthesized and reduced 1,3,6,8-tetradeuteriopyrene (1,3,6,8d4-PY), 4,5,9,1O-tetradeuteriopyrene(4,5,9,1O-d4-PY), and 1,3,4,5,6,8,9,10-octadeuteriopyrene (sym-ds-PY); see structures 11, 111, and IV. Furthermore, their EPR spectra were recorded
I1
I11
and simulated, and ab initio quantum chemical calculations were performed to mimic the effects of isotopic substitution.
Experimental Section
1,3,6,8-Tetradeuteriopyrene(1,3,6,844-PY).4,5,9,10-Tetrahydropyrene was allowed to undergo acidic exchange in perdeuteriated sulfuric acid producing 1,2,3,6,7,8-hexadeuterio-
0022-365419512099-3461$09.00/00 1995 American Chemical Society
3462 J. Phys. Chem., Vol. 99, No. 11, 1995 4,5,9,1O-tetrahydropyrene.This material was brominated at the 2 and 7 positions with bromine and FeC13 in water. The resulting 2,7-dibromo-l,3,6,8-tetradeuterio-4,5,9, lo-tetrahydropyrene was treated with dichlorodicyanoquinone (DDQ)to give the desired 1,3,6,8-d4-PY. This product proved to be 80% 1,3,6,8-d4-PY and 20% 1,3,6-d3-PY. 4,5,9,10-Tetradeuteriopyrene(4,5,9,10&PY). The symdg-PYwas brominated at the 1, 3, 6,and 8 positions with Brz in CC4, and the bromines were replaced with hydrogens using TiC&.LiAl&. This product is 75% 4,5,9,1O-d4-PY with 14% 2,4,5,9,1O-d5-PY, 9% 4,5,9-d3-PY, and 2% 1,4,5,9,1O-d5-PY. 1,3,4,5,6,8,9,l&octaaeutenopyrene (symdn-PY). 1,2,3,6,7,8Hexahydropyrene was allowed to undergo acidic exchange in perdeuteriated sulfuric acid producing 4,5,9, lo-tetradeuterio1,2,3,6,7&hexahydropyrene. This material was treated with NaH in DMSO-d6repetitively to yield 1,1,3,3,4,5,6,6,8,8,9,10dodecadeuterio-l,2,3,6,7,8-hexahydropyrene,which was converted to sym-dg-PYwith DDQ. This product proved to consist of 62% desired product and 18% 1,2,3,4,5,6,8,9,lO-d9-PY, 5% 1,3,4,5,6,9,1O-d7-PY, and 15% 1,3,4,5,6,8,9-d7-PY. Full synthetic details of these and some related deuterio pyrenes will be presented in a forthcoming paper. NMR,mass spectral, and EPR analyses were utilized to determine the isotopic purity of each of these products. The EPR work, which was carried out on the potassium-reduced substrates in tetrahydrofuran, is discussed in detail in the next section. EPR spectra were recorded with low modulation amplitude (0.05 G) as previously described,5a and simulated utilizing standard procedures.5b
Results and Discussion EPR Data. The reduction of pyrene with potassium metal in THF under high vacuum conditions produces a light red solution that yields a very well resolved spectrum upon EPR analysis at -100 OC. This spectrum is best simulated utilizing coupling constants of 1.012G (2H's, aH(2,7)), 2.140G (4H's), and 4.830G (4H's). These couplings are consistent with those previously r e p ~ r t e d . ~ The reduction of our sample of 1,3,6,8-d4-PY under identical conditions also yields a very well resolved spectrum upon EPR analysis at -100 "C (Figure 1). This spectrum is well simulated utilizing coupling constants of 0.9875 G (2H's, aH(2,7)),2.140 G (4 H's), and 0.7425 G (4 D's). The agreement between simulation and real spectrum is significantly improved by considering the spectrum to be a result of 80% 1,3,6,8-d4-PY'and 20% 1,3,6-trideuterio-pyreneanion radical, Figure 1. This impurity (1,3,6-trideuteriopyrene)anion radical was simulated with the above set of coupling constants except that a single deuterium splitting was replaced with a proton splitting of 4.830 G. The appearance of the simulation is distorted in several places if the ~ ~ ( 2 . in 7 ) 1,3,6,8-d4-PY'is not reduced by about 25 & 3 mG from its usual value of 1.012G (see Figure 1). The error is estimated from the fact that if 22 or 28 mG changes are used in the simulations they are cosmetically poorer. Since this anion radical sample does not include a sensitizing internal standard, the following experiment was carried out to confirm this observed isotopic distortion of spin distribution upon tetradeuteriation. The 1,3,6,8-d4-PY sample was mixed with pyrene and reduced to produce a ratio of anion radicals of [1,3,6,8-d4-PY'-]:[1,3,6-d3-PY'-]:[PY'-] = 70:18:12.The resulting EPR spectrum and simulations are shown in Figure 2. Once again, if the protons in the 2 and 7 positions of 1,3,6,8= 1.012G d4-PY'-are considered to have a splitting of a~(2,7) (identical to that in pyrene), clear discrepancies between the simulation and the real spectrum are evident (shown in Figure
Zuilhof et al.
Figure 1. Upper: A low-field 8 G portion (beginning at the second peak and ending at the spectral center) of the EPR spectrum at 173 K of 1,3,6,8-&-PY'-. Center: A computer simulation of 1,3,6,8-d~PY'generated using coupling constants of 0.9875 G (2 Hs), 2.14 G (4 H's), and 0.7425 G (4 D's),and a peak-to-peak line width of 0.07 G. Also included is a 20% contribution from a 1,3,6-&PY'- impurity, simulated by replacement of a 0.7425 G (1 D) with a 4.83 G (1 H) coupling constant. Lower: A computer simulation generated using parameters as in the center simulation, except that at the 2 and 7 positions, the 1.012 G coupling constant observed in ordinary pyrene was used for simulation of all contributions from deuterated species. Note the significantly poorer fit to the upper (real) spectrum; for example see the point indicated by a vertical arrow.
2). However, this disagreement between the simulated and real spectrum is obviated when ~ ~ ( 2 . is 7 )reduced to 0.9875G in the simulation of 1,3,6,8-d4-PY'-.A 25 mG reduction in the splitting constant for the protons in the 2 and 7 positions (A~H(z,~) = -25 mG) is absolutely necessary for accurate simulation of this mixture of anion radicals. Can this AuH(z,~) be further augmented via deuteriation of the remaining four positions? The EPR spectrum of the anion radicals generated from the reduction of our sym-dg-PY sample is best simulated by including four anion radical species in the simulation. These are sym-dg-PY'-,1,3,4,5,6,8,9-heptadeuteriopyrene*-, 1,2,3,4,5,6, 8,9,1O-nonadeuteriopyrene'-, and 1,3,4,5,6,9,10-heptadeuteriopyrene'-. They are present in a ratio of 62:15.5:17.5:5, respectively (Figure 3). Again, it is necessary to include a 25 mG reduction in ~ ~ ( 2 .to 7 )obtain the best simulation. However, the spectral pattern is not sufficiently sensitive to this alteration in aH(2,7)to be conclusive in the absence of an internal standard. When the EPR of this anion radical mixture is recorded in the presence of the anion radical of pyrene, it is clear that aH(2.7)in sym-dg-PY'- must be reduced by 25 mG from its value in the pyrene anion radical (Figure 3). It is important to note that the very small reduction in ~ ~ ( 2 . resulting 7) from deuteriation of the 1,3,4,5,6,8,9,10 positions is only observable because the presence of the pyrene anion radical yields a complex pattern of overlapped lines with a morphology that is extremely sensitive
Perturbation of Spin Density Distribution
Figure 2. Upper: A low-field 10 G portion (beginning about 2.9 G upfield from the first pyrene peak) of the EPR spectrum at 173 K of a
mixture of 1,3,6,8-d4-PY'- and PY-. Contributions from 1,3,6,8-d4PY'- begin near the center of the displayed portion. Center: A computer simulation generated using the coupling constants listed in Figure 1 (center) for 1,3,6,8-d4-PY'- and d3 contributions, and for pyrene, 1.012 G (2 H's), 2.14 G (4 H's), and 4.83 G (4 H's). The peak-to-peak line width is 0.095 G. The d4, d3, and ordinary pyrene anion radicals are assumed to be present in the ratio 70:18:12, respectively. Lower: A computer simulation with parameters as in the center simulation, except that at the 2 and 7 positions, the 1.012 G coupling constant observed in ordinary pyrene was also used for simulation of contributions from both deuterated species. Note the significantly poorer fit to the upper (real) spectrum; for example, see the point indicated by a vertical arrow. to these UH'S. Consequently, tiny alterations in this UH yield significant obvious appearance changes in the EPR pattern. The negative 25 mG A u H ( ~for , ~ )the tetradeuterio system compared with the same value in the octadeuterio system suggests that there is minimal, if any, effect (Au~(2,7) FZ 0) due to deuteriation of the 4,5,9,10 positions. The EPR pattern from the 4,5,9,10-&PY anion radical is well simulated using coupling constants of 1.012 G (2 H's), 4.82 G (4H's), and 0.34 G (4 D's). These same parameters can be used in simulations of mixtures of 4,5,9,10-d4-PY'- and PY'- to reproduce the spectrum. Thus, these results seem to substantiate the fact that deuteriation in the 4,5,9,10 positions does not alter aH(2,7). Unfortunately, however, the lowering of UH(2,7) by 25 mG in the simulation does not drastically alter the general appearance of the hyperfine pattern. Hence, the general pattern of EPR lines in this system does not yield an overlap situation that is sufficiently sensitive to ~ ~ ( 2 , to 7 ) confirm or preclude a possible finite h a ~ ( 2 , 7 ) . However, based upon the results from the 1,3,6,8 substitution and the octadeuteriation experiments, the perturbation from the 4,5,9,10 deuteriation must be small. It should be pointed out that the 0.9875 G splitting for aH(2,7) is not the smallest splitting in the deuteriated systems, as the UD'S are smaller. This and the fact that there are impurities with varying deuteriation present complicates the use of the
J. Phys. Chem., Vol. 99, No. 11, I995 3463
Figure 3. Upper: A low-field 10 G portion (beginning about 7.5 G upfield from the first pyrene peak) of the EPR spectrum at 173 K of a and PY-. Contributions from mixture of 1,3,4,5,6,8,9,10-d8-PY'-
1,3,4,5,6,8,9,lO-d8-PY-begin near the center of the displayed portion. Center: A computer simulation generated by assuming a 22% contribution from pyrene, and contributions from a dg and two d7 impurities as described in text. Coupling constants used are as follows: for pyrene, 1.012 G (2 H's), 2.14 G (4 H's), and 4.83 G (4 H's); for dg-pY'-, 0.9875 G (2Hs), 0.33 G (4 D's),and 0.7425 G (4D's);for dg-PY'-, 0.155 G (1 D)replaced a 0.9875 G (1 H) value; for one d7 impurity 2.14 G (1 H) replaced a 0.33 G (1 D)value; for the other d7 impurity 4.83 G (1H) replaced a 0.7425 G (1 D)value. The peak to peak width is 0.085 G. Lower: A computer simulation with parameters as in the center simulation, except that at the 2 and 7 positions, the 1.012 G value observed in ordinary pyrene was used for simulation of all contributions from deuterated species. Note the significantly poorer fit to the upper (real) spectrum; for example, see the point indicated by a vertical arrow. outermost (lowest S I N ) lines for the measurement of tiny changes in ~ ~ ( 2 , 7 )The . changes described above in the EPR spectra due to deuteriation are quite small, unlike the rather dramatic alterations in the absorption spectra observed by Nilsson and Lund produced by the deuteriation of the radicals formed from the y-irradiation of C(CHzOH)4.* Ab Initio Calculations, Ab initio calculations were performed to investigate the hypothesis that the C-H bond length reduction which occurs upon deuteriation is (partially) causing the observed reduction in spin density. To simulate these very small deuterium isotope effects, basis sets which include polarization functions on both the carbon and hydrogen atoms are required. Therefore, UHF full optimization calculations were performed with both the 6-31G**9 and 6-311G**1° basis sets, using the Spartan program.ll The isotope effect was simulated by using a fixed reduction of the relevant C-D bond lengths of 0.010 8, with respect to the optimized C-H bond length. The structure of the radical anion was subsequently reoptimized with this restriction. The proportionality between the coupling constant ( U H ) of the odd electron to a hydrogen nucleus attached to the ring, and the unpaired spin density at the carbon to which the hydrogen is bonded, as described by
3464 J. Phys. Chem., Vol. 99, No. 11, 1995
Zuilhof et al.
TABLE 1: Sites of Deuterium Substitution on the Pyrene Anion Radical and the Resulting Perturbation in the Spin Density in mG), Including Both the Empirical (EPR)and the and Proton Coupling Constant at the 2,7 Positions (A@and AuH(~,T) Calculated (from the Two Different Basis Sets) Results EPR sites
1,3,6,8-d4 4,5,9,10-d4 1,3,4,5,6,8,9,10-d~
6-31G**
-& x 103 0.75
-Aa~t2.71
-& x 103
-
25
0.75
25
0.431 0.344 0.689
-
the McConnell relationship (UH = allows the calculation of isotope effects. The precise value of Q depends on the coupling mechanism in a planar n-radical and has been suggested to be 30 G.12d The isotope effect on the coupling constant for the hydrogens in the 2,7 positions was calculated as the difference in the pt spin densities (A@)of the perprotiated and partially deuterated systems, (AUH= QAe). This yields the data presented in Table 1. The calculations are in excellent agreement with the experimental data for the octadeuteriated system. This strongly suggests that a change in the C-H(D) bond lengths at the 1,3,6,8 and the 4,5,9,10 positions due to deuteriation does indeed effect the spin densities at the 2,7 positions. It is presently not clear that this is the only factor influencing this spin density, since agreement with the experimental data is less favorable for the tetradeuteriated systems. An effect due to the 4,5,9,10tetradeuteriation is theoretically implied but not observed. The 1,3,6&tetradeuteriated system, however, shows an experimental isotope effect about double that calculated and essentially the same size as that measured for the octadeuteriopyrene system. The experimentally implied insensitivity of the spin densities at the 2,7 positions to deuteriation at the 4,5,9,10 positions is not corroborated by the ab initio calculations. The calculations suggest only a minor reduction of the isotope effect with increasing distance to the atom of interest. Intuitively, the experimental outcome is easier to comprehend. To check whether basis set effects might play a role, calculations using both the 6-31G** and 6-311G** basis sets were performed. The data in Table 1 show that there are no substantial differences between the results of the 6-31G** and the 6-311G** calculations. This shows that basis set effects do not play an important role. The actual value for the 6-311G** calculated eZin the 2,7 position in P Y - is -0.0433. An arbitrary aspect of the calculation is the size of the reduction of the C-D bond length with respect to the C-H bond length. The value of 0.010 A was chosen, since it is slightly smaller than that observed experimentally in the naphthalene ~ y s t e r n and , ~ therefore it is presumably not overestimating the effects of this bond length reduction. Though the quantitative outcome of the calculations varies, of course, with the bond length reduction used, qualitatively this does not influence the picture. This was shown by two 6-311G** calculations on the octadeuteriated radical anions, in which the C-D bond length was shortened by 0.005 and 0.015 A, respectively. The calculations indicate that there is no change in aH(1,3,6,8) or aH(4,5,9,10) due to deuteriation of the 2,7 positions. This is in agreement with observation. We were also unable to observe
6-311G**
-Aa~tz.n 13.1 10.3 20.7
-& x 103
- A a ~ c z71
0.332 0.286 0.607
10.0 8.6 18.2
any Aa~(4,5,9,10) due to deuteriation of the 1,3,6,8 positions or vice versa. Calculations (6-31 lG**) suggest that d4 substitution at either the 1,3,6,8 or 4,5,9,10 positions should result in a percent change in ~ ~ ( 2 , that 7 ) is a factor of 4 larger than that of the unsubstituted (4 hydrogen) positions. So, the calculations as well as the experiment show the 2,7 positions to be especially sensitive to deuteriation in the 1,3,6,8 or 1,3,4,5,6,8,9,10 positions. Finally, we realize that calculation improvements by inclusion of Mdler-Plessett corrections or diffuse functions might be helpful. However, we do observe the correct trends without the improvements, and we do not have the computer hardware needed for these calculations.
Acknowledgment. C.D.S. thanks the National Science Foundation (Grant CHE-9011801) for support of this work. We also thank Dr. Jean Standard of Illinois State University for helpful discussion. References and Notes (1) Sinzger, K.;Hunig, S.; Martina, J.; Bauer, D.; Bietsch, W.; Ulirich von Schutz, J.; Wolf, H. C.; Kremer, R. K.; Metzenthin, T.; Bau, R.; Kahn, S. I.: Lindbaum. A.:. Leneauer. . C. L.: Tillmanns. E. J. Am. Chem. SOC.
199% 115,7696: (2) Heimer. N. E.;Matten, D. L. J . Am. Chem. SOC. 1993,115.2217. (3) (a) Berger, S.; Kunzer, H. Tetrahedron 1983,39, 1327. (b) For another case of isotopic bond length reduction see: Bartell, L. S.; Roth, E. A.; Hollowell, C. D. J . Chem. Phys. 1965,42, 2683. Melander, L.; Saunders, W. H. Reaction Rates of Isotopic Molecules; Wiley: New York, 1980;pp 189-197. (4) A single deuterium will split the degeneracy of the antibonding MO's of benzene, and this is in part due to vibronic interactions; see: (a) Reddoch, A. H.; Dodson, C. L.; Paskovich, D. H. J. Chem. Phys. 1970,52, 2318. (b) Lawler, R. G.; Bolton, J. R.; Fraenkel, G. K.; Brown, T. H. J. Am. Chem. SOC. 1964,86,520. (c) Alper, J. S.; Silby, R.; J. Chem. Phys. 1970,52,569. (d) Lawler, R. G.; Fraenkel, G.K. J. Chem. Phys. 1968, 49, 1126. (5) (a) Stevenson, G. R.; Ballard, M. K.; Reiter, R. C. J . Org. Chem. 1991, 56, 4070. (b) Morse, P. D. EPRWare User Manual; Scientific Software Services: Bloomington, Il, 1990. (c) Morse, P.D., Reiter, R. C. EWSim User Manual; Scientific Software Services: Bloomington, Il, 1990. (6) (a) Matsuura, K.; Nunome, K.; Okazaki, M.; Toriyama, L;Iwasaki, M. J . Phys. Chem. 1989,93,6642.(b) Eriksson, L. A.; Lunell, S.J. Am. Chem. SOC. 1992,114,4532. (7) Hoijtink, G.K.;Townsend, J.; Weissman, S . I. J. Chem. Phys. 1961, 34,507. (8) Nilsson, G.;Lund, A. J. Phys. Chem. 1984,88, 3292. (9) Hariharan, P. C.; Pople, J. A.; Theor. Chim. Acta 1973,28,213. (10) Krishnan, R.; Binkley, J. S.;Seeger, R.; Pople, J. A. J. Chem. Phys. 1980,72,650. (11)Spartan program, version 3.1;Wavefunction Inc., M e , CA (1994). (12) (a) McConnell, H. M. J . Chem. Phys. 1956, 24, 632. (b) McConnell, H. M. J . Chem. Phys. 1956,24,764.(c) McConnell, H. M. J. Chem. Phys. 1958,28,107. (d) Lowry, T.H.; Richardson, K. S . Mechanism and Theory in Organic Chemistry; Harper & Row: New York, 1987;p 740. JP941817F
