J . Phys. Chem. 1986, 90, 1564-1 57 1
1564
observe (Figure 5B-1,2) that higher-order radical cation aggregates are formed more readily in the less viscous solvent, n-hexane. A time-resolved study of C H D dimer cation formation (conditions were identical with that of the T M E experiment; [CHD] = 0.01M) yields spectral2 that are not as well resolved as those obtained for TME. However, the EPR spectral absorbances due to CHD+, disappear at a faster rate than those of TME'.. This can be rationalized in terms of the expected reduction in steric hinderance for the planar C H D molecule which lacks the methyl groups of TME.
Conclusions In this paper, we have observed the first complete EPR spectrum of an olefin radical cation dimer, (TME,'.), and higher aggregates in fluid solution. The estimated rate constant for dimer radical cation formation is comparable to that determined by other Dimer (and multimer) radical cation formation was found to be dependent on the temperature and viscosity of the
solvent. The dimer radical cation of T M E was also observed to form higher-order radical cation aggregates when the concentration was increased. At the highest concentration of TME and longest microwave pulse delay time, a new, previously unreported, species was formed. The new species is best described as an aggregate radical cation of T M E (TME,'.; n 4). Similar viscosity and concentration dependences are observed for the formation of aggregate cations of C H D . The results presented here demonstrate that time resolution is an essential tool of the FDMR technique if one is to fully delineate the structure and dynamics of transient monomer/multimer radical cations formed in radiolysis.
Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under Contract No. W-3 1-109-ENG-38. Registry No. CHD radical cation, 34504-5 1-3; tetramethylethylene radical cation, 34512-36-2.
Transannular Interactions in Dimer Cation Radicals of Naphthalene Derivatives. Conformation Anomaly and Stabilization Energy Atsushi Terahara, Hiroaki Ohya-Nishiguchi,* Noboru Hirota, Department of Chemistry, Faculty of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606, Japan
and Akira Oku Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku, Kyoto 606, Japan (Received: October 9, 1985) The dimer cation radicals (R2+.)of a number of naphthalene derivatives were studied by electrochemical ESR and lowtemperature cyclic voltammetry (LTCV) in CH2C12.Some of the ESR spectra could not be analyzed unless conformations with symmetries lower than a symmetrical eclipse structure were assumed. In order to determine the probable conformations for R2+.from the ESR results, Huckel-McLachlan-type calculations were carried out based on several possible dimer structures and taking the transannular effect in R2+'into consideration. From a comparison between the observed and calculated hfcc it is suggested that the most probable conformations for R2+'in a number of cases are rotated and/or translated structures. In the LTCV study, characteristic peaks corresponding to the reactions R2+'+ e 2R [11] and R2+'* R22++ e [III] were observed as the result of dimer formation following the reaction R + R" + e [I]. The stabilization energies of Rz+' can be obtained directly from the potential difference between peak I and peak 11. The decrease in the stabilization energies for derivatives (0.134.07 eV) with respect to that for naphthalene (0.16 eV) can be attributed to a reduction of the transannular interaction due to the steric hindrance of the substituting groups and the change in conformation of R2+'.
-
Introduction Transannular interaction existing in systems with two or more aromatic rings stacked together has attracted many investigators, because it plays important roles in numerous chemical and biological processes such as aggregation reactions and the transfer of energy and electrons in photosynthetic systems. The dimer cation radicals (R2+') produced by association of monomer cation radicals with neutral molecules form an attractive group of systems for examinng the transannular interaction in detail, for they have relatively simple structures and can be studied by a variety of techniques. Therefore, Rz+'s have been investigated by several spectroscopic techniques such as electron spin resonance (ESR),'" UV and mass spectro~copy.'~ I. C. Lewis and L. S. Singer, J . Chem. Phys., 43, 2712 (1965). 0. W. Howarth and G . K. Fraenkel, J . Chem. Phys., 52,6258 (1970). T. C. Chiang and A. H. Reddoch, J . Chem. Phys., 52, 137 1 (1 970). Y. Yoshimi and K. Kuwata, Mol. Phys., 23, 297 (1972). F. Gerson, G. Kaupp, and H . Ohya-Nishiguchi, Angew. Chem., Inr. Ed.'Engl., 16, 657 (1977). (6) H. Ohya-Nishiguchi, H. Ide, and N . Hirota, Chem. Phys. Lett., 66, 581 (1979). (7) 8. Badger and B. Brockelhurst, Nature (London), 219, 26 (1968). (8) B. Badger and B. Brockelhurst, Trans. Faraduy Sot., 65. 2576, 2582. 2588 (1969). (9) B. Badger, B. Brockelhurst, and R. D. Russel, Chem. Phys. Lett., 1, 122 (1972).
According to the ESR results obtained so far for Rz+', the following two relations are known to hold: (a) the total value of hyperfine coupling constants (hfcc) in R2+'is equal to that in R", while the number of equivalent protons in R2+.is twice that in R+', and (b) the hfcc values in R2+.are close to one-half of the corresponding hfcc values in R+'. These relations are a consequence of the high spatial symmetry of R2+'and the equal distribution of the unpaired electron on the two aromatic moieties. On the basis of these results it has been proposed that the conformation of Rzf' is a symmetrical eclipse structure such as shown in Figure 1A. This proposition is also supported by the results of ESR studies on radical cations of cyclophanes with symmetrical eclipse structure^,^^^^^'^ which satisfy the above two relations. There are, however, some cases in which the conformations of R2+' are not a symmetrical eclipse. In the case of 1,4-dimethylnaphthalene studied by Yoshimi and K ~ w a t arelation ,~ (b) is not satisfied, which was explained in terms of a 180' rotated (10) S. Arai, A. Kira, and M. Imamura, J . Chem. Phys., 54,4890 (1971). (1 1) A. Kira, S. Arai, and M. Imamura, J . Phys. Chem., 76, 11 19 (1972). (12) M. A. J. Rodgers, J . Chem. SOC.,Faraday Trans. 1 , 6 8 , 1278 (1972). (13) M. Meot-Ner (Mautner), J . Phys. Chem., 84, 2724 (1980). (14) H. Ohya-Nishiguchi, A. Terahara, N. Hirota, Y. Sakata, and S. Misumi, Bull. Chem. SOC.Jpn., 55, 1782 (1982). (15) A. Terahara, H. Ohya-Nishiguchi, N. Hirota, Y. Sakata, S. Misumi, and K. Ishizu, Bull. Chem. SOC.Jpn., 55, 3896 (1982).
OO22-3654/86/2090-1564$01.50/0 0 1986 American Chemical Societv
Transannular Interactions in Naphthalene Derivatives n
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1565
B-
A-
U
B
( B)
(A)
Figure 1. Symmetrical eclipse structure (A) for 12+'and 180' rotated structure (B) for 4*+'.
V
U
cii3 cli3
Figure 2. Molecular structures of the naphthalene derivatives investigated.
structure shown in Figure 1b. Furthermore, the conformations of R2+. in the dimer cation salts R2+'PF6- of naphthalene and fluoranthene are not symmetrical eclipses, but 90' and 180' rotated structures, re~pectively.'~." These results indicate that the conformation of R2+'is not necessarily a symmetrical eclipse, but varies depending on the molecular structure and the environment. Thus it is worthwhile to examine the conformation of R2+'in various systems to understand the factors influencing them. In the present work we have investigated R2+'and R+' of a series of naphthalene derivatives shown in Figure 2 by electrochemical ESR and cyclic voltammetry (CV) in the hope of determining the conformations and stabilities of R2+.. First, we have attempted to clarify the conformations of R2+'from an analysis of the ESR spectra. The hfcc of a number of naphthalene derivatives fail to satisfy the above-mentioned relations (a) and (b), indicating that the conformations are not likely to be symmetrical eclipses. Rotated or translated conformations are proposed as probable dimer structures on the basis of a careful comparison between the observed hfcc and those calculated by the Huckel-McLachlan method. The relationship between steric hindrance and conformation is discussed in view of the obtained results. Second, we have made CV measurements under experimental conditions similar to those of the ESR measurements. The cyclic voltammograms confirm the dimerization processes to form R2+'. The stabilization energies of R2+'were studied in detail and the steric effect was examined in a systematic way,. To our knowledge the present paper seems to be the first report which discuss the results of ESR and CV measurements of R2+' at the same time.
Experimental Section Apparatus. The ceil for electrochemical ESR reported previously'* was modified for the sake of convenience as shown in Figure 3. ESR spectra were measured by using a JEOL-FE3X spectrometer equipped with a temperature controller. The electrochemical cell for low-temperature CV was similar to that in Figure 3 except for the following changes: use of a working (16) H. P. Fritz, H. Gebauer, P. Friedrich, and U. Schubert, Angew. Chem., Int. Ed. Engl., 17, 215 (1978). (17) V. Enkelman, B. S. Morra, C. Krohnke, G. Wegner, and J. Heinze, Chem. Phys., 66, 303 (1982). (1 8) H. Ohya-Nishiguchi, Bull. Chem. SOC.Jpn., 52, 2064 (1979).
Figure 3. A schematic view of the electrochemical ESR cell. A, working electrode (0.5 mm gold wire); B, counterelectrode (0.5 mm gold wire); C, tefron tube; D, E, and F, Pyrex cell; G, sample tube (5-mm-0.d. quartz); H, temperature control dewar; and 1, cylindrical ESR cavity.
electrode (A) in the shape of a 5-mm-0.d. loop instead of a helix, addition of a reference electrode made of 0.3-mm-0.d. silver wire coated with Teflon, and use of a Pyrex sample tube (G) of 8 mm 0.d. Cyclic voltammograms were measured with a PAR 173 potentio/galvanostat equipped with an I/V converter, PAR 176, in conjunction with a H P 3310A function generator. Preparation of Sample Solution. Dichloromethane (DCM) containing lo-] M tetrabutylammonium tetrafluoroborate (TBABF4) was used as the solvent for both experiments. In the case of ESR, about 5% trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA) were added to the sample solution. M ferrocene (Fc) was added to the solution In the case of CV, in order to calibrate the potential with a Fc/Fc+ couple (0.400 V vs. NHE).I9 The solution was degassed by the freezepump-thaw method and the air in the cell was replaced by dry nitrogen gas with a purity of 99.995%. Material. Spectroscopic grade DCM was distilled over CaH,. TFA, TFAA, and Fc of special grade and TBABF, for polarographic use were used without further purification. Commercially obtained compounds 1 , 7 , 8 , and 9 were recrystallized once before use. 2, 3, and 4 were also commercially obtained and were used after vacuum distillation. 5,6, 10, 11, 12, and 13 were synthesized according to the literatures.20,2' The ring protons in 7, 8, and 9 were partly exchanged by deuterons in a solution of TFAdl/CC1,22 for analysis of the ESR spectra. The ratio of H / D was determined as about 1/5 by N M R .
Results and Discussion E S R Results. a . General Feature. The voltages at which the ESR signals appeared agreed well with the oxidation potentials ( E , , 2 [ I ] determined ) by CV. The favorable conditions to observe t h e ESR spectra of R," were a high initial concentration of t h e order of lo-, M, a low temperature below 200 K, and an applied voltage as low as possible. For the observation of R+' the following conditions are required: a low initial concentration below (19) R. R. Gagne, C. A. Coval, and G. C. Lisensky, Inorg. Chem., 19,2854 (1980). (20) A. Oku, Y . Ohnishi, and F. Mashio, J. Org. Chem., 37, 4264 (1972). (21) B. M. Trost, G. M. Bright, C. Frihart, and D. Britteri, J . A m . Chem. SOC.,93, 737 (1971). (22) G.Dallinga, P. J. Smit, and E. L. Mackor, Mol. Phys.. 3, 130 ( I 960).
Terahara et al.
1566 T h e Journal of Physical Chemistry. Vol. 90,No.8,1986 TABLE I: Observed and Calculated hfcc" for 1 and 10 10
1
2
1
2
1
monomer 1.313 (8 H) 0.203 (4 H) 1.465 -0.178
obsd
calcd
-0.525
dimer obsd
-0.130
0.276 (8 H)
(n) calcd'.' (n)
-0.270 (1.94) calcdd.' -0.261 (n) (2.01)
0.103 (8 H)
-0.058 (2.24) -0.066 (1.97)
"in mT. *Estimated by averaged hfcc. structure. ,/3' = 0.lP.
0.562 (8 H) 0.115 (8 H) 0.548 (8 H) (2.37)' (1.77) 0.787 -0.085 (1 36) (2.10) 0.65 1 -0.097 (2.25) (1.83) [O,O] structure.
[90Z,O]
TABLE 11: Observed and Calculated hfcc" for 2 and 3 2>+. 3,+'
calcdb*d calcd'-d
obsd 1
2 3 4 5 6 7 8 a
0.589 0.093 0.031 0.476 0.186
(6 (2 (2 (2 (2
H) H) H) H) H)
0.124 (2 H)
0.420 -0.078 -0.052 -0.298 -0.269 -0.053 -0.063 -0.260
0.513 -0.106 -0.070 -0.356 -0.221 -0.047 -0.055 -0.214
obsd 0.580 0.106 0.023 0.342 0.212 0.083 0.023 0.212
(6 H) ( 6 H) (2 H) (2 H) (2 H) (2 H) (2 H) (2 H)
calcdc,d 0.522 0.216 -0.040 -0.317 -0.188 -0.052 -0.038 -0.199
'
in mT. [ 180X,O] structure. [ l802,+ 1 x] structure. dR' = 0.18.
M, a high ratio of TFA and TFAA to DCM (ca. lo%), and electrolysisat about 210 K with a high voltage (about 0.3 V higher than E1,2[1]) followed by lowering the temperature to below 180 K. However, R" of 1, 2, 3, 8, and 9 could not be detected under any conditions. On the other hand, the ESR spectra of R2+.of 11, 12, and 13 were not observed. No significant temperature dependence of the ESR spectra was found except for 4*+'. Assignment of the hfcc was made from a comparison with the computer-simulated spectra by taking into consideration the number of equivalent protons and the results of the MO calculations described in following section. In the following we discuss the main features of the individual systems. b. Naphthalene ( l ) ,1-Methylnaphthalene(2),and 1,2-Dimethylnaphthalene (3). The observed ESR spectrum of 12+' agreed with that and was analyzed easily (Table I). An additional single sharp signal appeared at the center of the spectrum when the applied voltage was increased. This signal was identified as that due to the solid salt of 12+'BF4-grown on the working electrode. The intense ESR spectra of 22+'and 3*+. were observed and reconstructed well with the hfcc listed in Table 11. Figure 4 shows the observed and simulated ESR spectra of 2*+'as an example. Taking the molecular symmetries of 1, 2, and 3 into account, we find that relation (a) holds. Whether or not (b) holds could not be examined, because the ESR spectra of R" were not observed. c. 1,4-Dimethylnaphthalene ( 4 ) , 1,2,3,4-Tetramethyl(6).The naphthalene ( 5 ) ,and 1,4,6,7-Tetramethylnaphthalene ESR spectra of both R+' and Rz+'were observed for these three compounds. The ESR spectrum of is shown in Figure SA as an example. The ESR spectra were all analyzed (Figure SB) well by using four hfcc listed in Table 111. The result for 4*+' agreed with that r e p ~ r t e d ,but ~ the values of hfcc for 4" are different. It is considered that the poor resolution due to the undesired overlap of 42+.and 4" led to the erroneous analysis of the 4" spectrum in the previous report. When the hfcc of 4*+' were compared with those of 4+', it is seen that relation (a) holds but (b) does not as was pointed out previ~usly.~ Similarly, the values of hfcc in 5*+.and 62". deviate largely from one-half of the corresponding hfcc in .'5 and 6+', especially the hfcc assigned to the methyl protons in the dimers being close to one-third those in the monomers.
Figure 4. The observed (A, 190 K) and simulated (B) ESR spectra of 22+..
Figure 5. The observed (A, 170 K) and simulated (B) ESR spectra of 5*+'.
d. 1,5-Dimethylnaphthalene( 7 ) , 1,8-Dimethylnaphthalene ( 8 ) ,and Acenaphthalene (9).The ESR spectra for these compounds observed under conditions favorable for dimer formation were anomalous, because they could not be analyzed by four groups of hfcc expected from the conformations shown in Figure 1. However, the CV results described later confirm that the observed spectra were genuinely due to R2+'. The ESR spectrum of 7 can be analyzed by using hfcc listed in Table IV. Strangely.
Transannular Interactions in Naphthalene Derivatives
T h e Journal of Physical Chemistry, Vol. 90,NO.8, 1986
1567
TABLE 111: Observed and Calculated hfcc" for 4, 5, and 6 4+' obsd calcd 4*+' obsd
(4 calcdbf
(n) calcd'f
(n) calcddf
(n) calcd'f
(n) 5+' obsd calcd 52" obsd
(n) calcdef
(n)
1
5
2
6
0.961 (6 H ) 0.858
0.395 (2 H) -0.458
0.217 (2 H) -0.149
0.132 (2 H) -0.120
0.328 (12 H) (3.16) 0.451 (1.90) 0.441 (1.95) 0.489 (1.75) 0.365 (2.35)
0.212 (4 H) (1.86) -0.235 (1.95) -0.241 (1.90) -0.206 (2.22) -0.283 (1.62)
0.119 (4 H ) (1.82) -0.067 (2.22) -0.066 (2.25) -0.079 (1.89) -0.056 (2.04)
0.090 (4 H ) (1.47) -0.053 (2.26) -0.087 (1.38) -0.05 1 (2.35) -0.073 (2.14)
0.995 (6 H ) 0.889
0.337 (2 H) -0.447
0.236 (6 H) 0.227
0.141 (2H) -0.1 18
0.349 (12 H) (2.71) 0.381 (2.33)
0.222 (4 H) (1.70) -0.274 (1.61)
0.137 (12 H) (1.72) 0.087 (2.60)
0.092 (4 H) (1.53) -0.073 (1.62)
0.850 (6 H ) 0.829
0.439 (2 H) -0.483
0.205 (2 H) -0.147
0.205 (6 H) 0.184
0.314 (12 H) (2.71) 0.351 (2.36)
0.243 (4 H) (1.81) -0.291 (1.66)
0.128 (4 H) (1.60) -0.057 (2.57)
0.120 (12 H) (1.71) 0.139 (1.33)
6" obsd calcd 62" obsd
(n) calcdef
(4 'In mT. [O,O] structure.
[ 18OZ,O] structure. d[180Z,+1X] structure.
the hfcc consist of two nonequivalent hfcc due to the methyl protons and a smaller number of total protons than that in 72+.. The ESR spectrum for 8 shown in Figure 6A, on the other hand, consists of a superposition of a highly resolved spectrum and a broad background. The former was simulated well by the hfcc given in Table IV as shown in Figure 6B. The latter could be due to aggregates. The well-resolved ESR spectrum was also obtained for 9 and analyzed as listed in Table IV. These two groups of hfcc for 82'' and 92+.consist of a larger number of hfcc than the four expected for typical dimers, though the total number of protons is equal to those expected for 82'' and 92+'. In order to ensure the analysis of the spectra and the assignment of the hfcc due to the methyl and methylene protons, partly deuterated compounds of 7,8, and 9 were investigated under the same conditions as above. As an example, the ESR spectrum of deuterated 8 is shown in Figure 6C, which was well reconstructed (Figure 6D) by using the two methyl hfcc equal to those given for 82" in Table IV. Similarly, two methyl hfcc were deduced from the observed spectrum for deuterated 7. For 9, the three hfcc were determined to be due to the methylene protons, as assigned in Table IV. Smaller hfcc due to deuterons and residual protons were not resolved. The ESR spectrum of 7" was well resolved and analyzed by four hfcc, which agreed with those r e p ~ r t e d .This ~ radical was unstable and disappeared within a few minutes. Since 8+' and 9" could not be detected by our method, the hfcc given in Table IV are taken from thq l i t e r a t ~ r e . ~ . ~ ~ The sums of the two methyl hfcc, 0.693 and 0.870 m T for 7*+' and 82+',are close to the methyl hfcc in the monomers, 0.707 and 0.825 mT, respectively. This fact supports the assignments of the observed spectra to the dimers. The methylene hfcc in 9 are more complicated because the two protons bonded to a methylene carbon are no longer equivalent in 92+..Such nonequivalencyof methylene protons is shown more clearly for lo2+'described below. Neither relations (a) nor (b) is satisfied in 72+', 8*+',and 92+'. Therefore, some distorted conformations with symmetries lower than the (23) A. C . Buchanan, 111, R. Livingston, A. S . Dworkin, and G. P. Smith, J . Phys. Chern., 84, 423 ( 1 9 8 0 ) .
[180Z,-lX] structure. 'p' = O.l@.
TABLE IV: Observed and Calculated hfcc' for 7, 8, and 9 1
4
2
3
7+. obsd calcd 72'' obsd calcdbqd
0.707 (6 H) 0.534 (2 H ) 0.169 (2 H) 0.169 (2 H) 0.772 -0.517 -0.153 -0.116 0.405 (6 H) 0.199 (2 H) 0.270 (6 H) 0.109 (2 H ) 0.476 -0.308 -0.089 0.322 -0.209 -0.056
-0.068 -0.044
8" obsd' calcd 8,+' obsd
0.825 (6 H) 0.573 (2 H) 0.245 (2 H) 0.1 16 (2 H) 0.762 -0.525 -0.164 -0.105
0.586 (6 H ) 0.364 (2 H) 0.222 (2 H) 0.080 (2 H ) 0.284 (6 H) 0.284 (2 H) 0.061 (2 H) 0.019 (2 H) calcdCsd 0.538 -0.362 -0.1 13 -0.071 -0.182 -0.041 0.251 -0.016 -0.352 -0.113 calcde 0.525 -0.07 1 -0.176 -0.049 0.253 -0.024
9+'
obsdg calcd 92+. obsd
1.318 (4 1.253
H) 0.659 (2 H) -0.540
0.313 (2 H) 0.059 (2 H ) -0.300 -0.009
0.962 (2 H) 0.567 (2 H) 0.080 (2 H ) 0.882 (4 H) 0.080 (2 H ) 0.064 (2 H ) 0.400 (2 H) calcdc*d 0.692 -0.329 -0.185 0.578 -0.237 -0.129
0.064 (2 H) 0.016 (2 H) 0.001 0.004
"In mT. * [ 0 , + 1 4 structure. c[180Z,+lx] structure. d@' = 0.lP. eIon-pair model based on [18OZ,O] structure (see text). /Reference 5. SReference 23.
conformations shown in Figure 1 should be considered for these dimers. e. Pyracene ( l o ) , 5,6-Dimethylacenaphthalene (11), 1,4,5,8-Tetramethylnaphthaiene(IZ), and Octamethylnaphthalene (13).The intense ESR spectra due to R'. of these compounds were observed even at room temperature. The spectra
1568 The Journal of Physical Chemistrv, Vol. 90, No. 8. 1986
Terahara et al.
Figure 7. The ESR spectra of 10: (A) central part of 10" observed at 270 K, (B) observed at 170 K due to lo2+'and lo'', and (C) simulation for lo2+'i n (B)
are close to one-half of the hfcc due to the ring and methylene hfcc in lo+',respectively. Therefore, relations (a) and (b) basically hold in lo2+'. The dimer cation radicals of 11, 12, and 13 could not be observed probably because of strong steric hindrance acting between two moieties with many substituents. Probable Dimer Conformation. a. MO Calculation. In order to assign the hfcc obtained and to determine the probable conformation of R2+',we calculated the unpaired spin distribution by the modified Hiickel-McLachlan method described below. For the methyl and methylene groups hyperconjugation models were adopted according to the l i t e r a t ~ r e . ~ ~Comparison .~' for R" reveals a good correlation between the observed and the calculated hfcc as listed in Tables I-IV. Therefore, it is easy to assign the hfcc observed for the monomer cation radicals. The most probable conformations for R2+.were proposed from a comparison between the observed hfcc and those calculated on the basis of several possible conformations for R2+'. In comparing the experimental with the calculated hfcc, we employ the hyperfine index n, defined by2*
n, = a I M / a l D
Figure 6. The ESR spectra of S2+.: (A) observed at 180 K, (B) simulation for (A), (C) observed for deuterated 8 at 180 K, and (D) simulation for (C).
were easily analyzed and the hfcc of lo+',12+.,and 13" agreed with those reported.'^^^,^^ The hfcc of 11" are as follows in mT; 1.289 (4 H), 0.859 (6 H), 0.251 (2 H), 0.114 (2 H). In Figure 7B is shown the ESR spectrum for 10 obtained under conditions favorable for observing R2+', which consists of the spectrum of 10" and lo2+'.Fortunately the hf lines due to 10" (Figure 7A) were simple enough to be separated from the spectrum due to lo2+'.The simulated spectrum for lo2+.is shown in Figure 7C. The hfcc of lo2+'listed in Table I consists of three kinds of hfcc. Only two kinds of hfcc are expected based on relation (a). This disagreement is understood by considering that the methylene protons in 10," are divided into two groups as with 92c.. The smallest hfcc and the average value of the larger two hfcc in lo2+' (24) E. de Boer and E. L. Mackor, Mol. Phys., 5, 493 (1962). (25) K. D. Root and M. T. Rodgers. J . Mugn. Reson., 1, 568 (1969).
where alu and aiDare the hfcc due to the ith proton in R+. and Rz". It was found in this work that the calculated n, is sensitive to the conformation of R2+'. We further define the conformation representation [Ai,Bj] which describes the relative configuration of the second moiety with respect to the first placed at the origin. The first element in the square bracket indicates rotation around the i axis by an angle A and the second represents the translation along the j axis by a distance B, where i a n d j denote the molecular axes shown in Figure 2 and B is taken in units of one benzene ring. Since the translation along the Z axis changes the transannular interaction p' described below, such a translation is not considered here. [O,O] and [ 18OZ,O], for example, represent the symmetrical eclipse and the 180' rotated structures shown in Figure 1, respectively. In calculating the M O of R2+.,an additional resonance integral p' between the carbon atoms in one moiety and those in the other was introduced as the transannular interaction according to the (26) G. Gerson, High Resolution E S R Spectroscopy, Wiley, New York, 1970, pp 50. (27) M. Iwaizumi, M. Suzuki. T. Isobe, and H . Azumi, Bull. Chem. SOC. Jpn., 40, 2754 (1967). (28) n, was called the "aggregation number" for the chlorophyl a dimer: J. R. Norris, H. Scheer, M. E. Druyan, and J. J . Katz, Proc. Null. Acad. Sci. U . S . A . ,71, 4897 (1974).
Transannular Interactions in Naphthalene Derivatives HQ
4
Figure 8. Proposed conformation of R2+': (A) [9OZ,O] for IO2+', (B) [18OZ,-lX] for 4*+., (C) [18OZ,+lX] for 22+.,and (D) [180Z,+lX] for 8,+'.
relative c o n f ~ r m a t i o n . ~ ,For ' ~ simplicity the same p' between substituents was also assumed. For the conformation without translation ( B = 0), no remarkable change was seen in spin distribution even if p' was changed. The results with p' = O.l@ are given in most cases. Such calculations were previously performed on the radical cations of cyclophanes with satisfactory
result^.^^^^^ b. [9OZ,O]f o r 12+' and lo2+'. Since 1 and 10 have a high symmetry of D2h,[lSOZ,O] is identical with [O,O]. Taking into account the relations which hold for these two compounds shows that two conformations of [O,O] and [9OZ,O] are possible. We calculated the hfcc of 12+.and lo2+'based on these two conformations and compared them with the values observed. Because the calculated hfcc are both close to the observed values, as seen in Table I, it is difficult to choose a probable dimer structure on the basis of these hfcc data only. However, comparison by using nj for IO2+' reveals a clear difference. The calculated n, for the methylene protons is smaller than 2.0 for [O,O], but larger for [902,0], while the opposite trend is found for n2. The observed trend for nj is obviously in favor of the conformation [90Z,O] (Table I). Therefore, it is concluded that the probable conformation for lo2+'is [9OZ,O] as shown in Figure 8A. A similar trend was also found for the calculated ni of 12+, but the conformation of 12+' cannot be decided because of the absence of the data for l+.. c. [180Z,-lx] f o r 42+',5,+', and 62". The hfcc of these compounds do not satisfy relation (b), though they satisfy (a). The observed n , for the methyl protons are much larger than 2.0, but smaller for the ring protons. The origin of such anomalous spin distributions must be attributed to their dimer structures. We discuss the conformation for 42+'as a representative case. First we calculated the hfcc based on the two conformations [O,O] and [ 180Z,O]. The agreement between the observed and calculated hfcc is not satisfactory as shown in Table 111, because the calculated hfcc of 42+'are all close to one-half of the corresponding hfcc in 4'' ( n j N 2 for all i), contrary to the observation. Consequently, we calculated the translated structures of [ 1 8 0 Z , f l x ] whose sxmmetries satisfy relation (a). It is noted in Table 111 that the trend of n,for [1802,-1x] rather than that for [ 1802,+ 1x1 is closer to that observed, namely, n, is larger than 2.0 but n5 is smaller than 2.0. Therefore, the translated structure of [ 18OZ,-lx] shown in Figure 8B is considered as the probable conformation for 42+.. Here n2 and n6 were not taken into account, because small changes in the hfcc cause large changes in n2 and n6. Similar arguments hold for s2+'and 62+'. d. [180Z,+Ix] f o r 22f' and 32+'. In spite of the C, symmetry of 2 and 3, the observed hfcc indicate symmetries higher than C2 for R2+. Within a number of conformations possible for 22+'and 32+',four conformations, [O,O],[ISOZ,O],[18OY,O], [18OX,O],with large overlaps between the two aromatic moieties may be regarded
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1569 as probable. Therefore, calculations for these conformations were carried out first. However, the calculated hfcc are similar in all four cases and the agreement between the observed and calculated hfcc was too poor to choose one of these conformations. The result of [ 180X,O] for 22f' is given in Table I1 as an example. Consequently, we made calculations based on the translated structures. Considerable improvements were made by adopting [18OZ,+lx], as listed in Table 11. [18OY,+lx] gave a result similar to that of [18OZ,+lx], but it should be discarded because of its large steric hindrance. From this we conclude that the probable conformations for 22+'and 32f'are [1802,+1x] as shown in Figure 8C. e. Conformations f o r 72+',8*+',and 92+'. Relations (a) and (b) are not satisfied in these compounds. Lowering the symmetry of conformation of R2+' is required from the hfcc observed and [O,O] or [ 180Z,O] type conformations cannot be accepted. Two possible causes for the symmetry lowering can be considered in this case. One is a conformation with a lower symmetry as in the other cases and another is "ion-pair" formation between R2+ and the counterion of BF4-. First we consider the conformation of 82". Possible conformations with lower symmetries can be achieved by translating along the X or Y axis or rotating about the 2 axis by 0' < 19 < 180'. The translated structure along the Yaxis and the rotated structure, however, did not reproduce the hfcc observed. In Table IV we give the calculated hfcc based on [ 180Z,+ 1x1, which agree well with the observed hfcc. [O,+lx] also gave a similar result but we eliminated it because of the steric effect of the methyl groups. Secondly, a calculation based on the ion-pair model was attempted. Electrostatic interaction between R2+'and the counterion was introduced by the McClelland method29 which was successfully applied to a number of ion pairs between aromatic anion radicals and metal cations.30 The ratio of the spin density on one moiety close to the anion against that on the other increases when the anion approaches to R2+'from infinity. The ratio of 2:l observed for the two methyl hfcc was reproduced with a distance of 15 A (p' = 0.05@).31 The calculated hfcc with this model are also given in Table IV. It is difficult to choose conclusively the suitable conformation for 82+.from these two models. But the translated [ 18OZ,+lx] conformation shown in Figure 8D is preferred to the ion-pair model on the basis of the following experimental facts: temperature dependence of the ESR spectra was not observed, and no change in the spectra was observed when other counteranions were used (PF6- and C104-). Similar translated conformations may be considered to be probable for 7*+'and 92+.for the same reasons. The results of the calculations based on [O,+lYl and [ 18OZ,+1 yl for 72+.and 92+.,respectively, are listed in Table IV. However, the agreement between the observed and calculated hfcc is not satisfactory. At present, we cannot find a conformation which satisfactorily reproduces the observed spin distribution. Cyclic Voltammetry. a. Cyclic Voltammetry of R2+'.In 1972, van Duyne and R e i l l e ~observed ~~ the characteristic peak due to the dimer cation radical of 9,lO-dimethylanthracene in butyronitrile by low temperature cyclic voltammetry (LTCV). Recently, Enkelmann et al. reported" LTCV for fluoranthene in DCM in reference to the electrocrystallization of a R2+'PF6-salt. These two reports indicated the utility of the LTCV technique in studying R2+'. We investigated LTCV for naphthalene derivatives for the purpose of obtaining stabilization energies of R2+'. In Figure 9, cyclic voltammograms for 8 are shown as typical examples. Three peaks denoted I, 11, and 111 can be clearly identified in the voltammogram recorded at 200 K (Figure 9a), but peaks I1 and 111 disappeared at room temperature (Figure (29) B. J. McClelland, Trans. Faraday Soc., 57. 1458 (1961). (30) N . Hirota, MTP Znf. Reu. Sci., Phys. Chem. Ser. I I , 4, 139 (1975). (31) The distance at which the calculated hfcc fit to observed hfcc decreases as p' increases. Therefore, there is no strict meaning for the distance used in the calculation. (32) R. P. van Duyne and C. N. Reilley, Anal. Chem., 44, 158 (1972).
Terahara et al.
1570 The Journal of Physical Chemistry, Vol. 90, No. 8. 1986
a) 200K
L-.2
-.2
0
o
-.2
0
E-E,/z[Il / V Figure 10. Digital simulation for CV (see text) TABLE V: Observed Oxidation Potentials and Stabilization Energies (AE) for Naphthalene Derivatives compd El,,iIIR E1,,[II1" E,,2[II11G AEb 1.42 0.16 1 1.58 1.44 1.55 1.71 0.1 1 2 1.48 1.67 0.13 1.35 3 1.40 1.49 1.56 0.09 4
b) RT
I
I
l
,
.
0.5
l
l
l
l
l
l
,
/
,
,
1.5
1.0
E [ V , v s NHEI Figure 9. The observed cyclic voltammograms for 8 at (a) 200 K and (b) room temperature.
9b). It is obvious that peak I is related to the one-electron oxidation of R given as R
R+'
+e
E,;,[I]
(1 1
Peak I1 appeared only at the cathodic sweep and its peak height with respect to that of I depended on the concentration of R.33 Thus peak I1 was assigned to the reduction of R2+' written by
-
2R
R2+' + e
E1/2[II]
(2)
On the other hand, the peak current of I11 is close to one-half of that of I, which indicates, according to Enkelmann et a1.,I7that peak 111 is attributable to the redox couple of
* R22++ e
R2+.
El/z[III]
(3)
Therefore, it is conclusively shown that R2+.was produced by the dimerization reaction R+'
+ R == Rz+'
(4) following the electrochemical process of (1). The appearance of peaks I1 and 111 provides evidence for the formation of R2+'. In order to confirm the peak assignment, we further carried out digital simulation of the CV curve.34 The calculated CV curve including only a reversible couple of (1) is shown in Figure loa. When the dimerization reaction 4 is taken into consideration, a new peak related to peak I1 appears, as seen in Figure lob. The peak height of I1 relative to I depended on the bulk concentration of R as observed experimentally. Further addition of the redox couple of R2+./R22+produces the curve shown in Figure lOc, which reproduces Figure 9a satisfactorily. The ratio of the peak height of 111 relative to I becomes 1/2 as pointed out in the previous report.I7 Therefore, the results of the digital simulation support the validity of the peak assignment. The experimental LTCV results for 2 and 3 were quite similar to those for 8. The peak separation between I and I11 was, however, poor for 4, 8, 9, and 10. On the other hand, for 1, 5, and 6 the shape of peak I1 was deformed due to the formation (33) Unpublished results. (34) S. Feldberg, Digiral Simulation: A General Merhod f o r Solving Electrochemical Diffusion Kinetic Problems, in Electroanalytical Chemistry, Vol. 3, A. J. Bard, Ed., Marcel Dekker. New York, 1969, pp 199-296.
5 6 7 8 9 10 11 12 13
1.33 1.36 1.48 1.41 1.32
1.20 1.19
1.23 1.29 1.38 1.33
1.22 1.13
1.40
0.10 0.07 0.10 0.08 0.10
1.28
0.07
1.61 I .56
1.26 0.90
Volts vs. "E.
*In eV
of the salts R2+'BF,- adsorbed on the electrode, and peak 111 was not observed clearly. Furthermore, additional peaks possibly attributable to reaction products appeared at a cathodic sweep near 0 V for 1,4,5, and 6. Neither peak I1 nor 111 was observed for 11, 12, and 13, which coincided with the results of the ESR experiments described above. b. Stabilization Energies of R2+'.In Table V are summarized the peak potentials of El12[I],EIj2[II],and EIj2[III]estimated in reference to the Fc/Fc+ couple. The order of Elj2[I]corresponds well to that predicted from the orbital energies of the highest occupied molecular orbitals (HOMO) calculated by the Hiickel method described in the previous section.35 El12[II], however, cannot be compared with the orbital energies, because they change with the magnitude of the transannular interaction introduced as a parameter. The stabilization energies (AE) of R2+'defined by the energy difference between R+' and R2+.can be obtained directly as the potential difference between E,,,[I] and EIl2[II] in eV. A E for R2+'of the naphthalene derivatives investigated are listed in the last column in Table V. For 12+', a AH' of 2.7 kcal/mol has been reported'' for the dimerization reaction 4 of 1 in acetone, which is close to A E for 1,0.16 eV (3.7 kcal/mol). Therefore, the values of AE obtained here are regarded as reasonable. On the other hand, Meot-Ner13 reported the dissociation energies, AH,', for 12+' and 92+'to be 17.8 and 17.0 kcal/mol, respectively, in the gas phase. H e also obtained the contribution of the resonance interaction AH,,,' to AH,' from the difference between AHD'(R2'') and MD'(R2H+) for the protonated dimers as 3.7 and 2.7 kcal/mol for 12+' and 9*+.,respectively. AE obtained here for 12+.and 92+',3.7 and 2.3 kcal/mol, respectively, are in excellent agreement with AH,,,' rather than hHDo(R2+').This can be explained as follows. AHD'(R2'') consists of AH,,,' and AH,,' due to electrostatic interactions in R2+.. On the other hand, A E can be written as AE = AHo'(R2")
+ Esol(RZ+')- E,,,(R+')
where E,,, are the solvation energies. The relation AE AH,,,' is obtained when E,,,(R+') - Esoi(R2+') N AH,,' is satisfied. This
Transannular Interactions in Naphthalene Derivatives
0
9
0
6
rotation
0
3
0
0
1
2
3
4
translation (XI /A
Figure 11. The dependences of stabilization energy due to the transannular interaction on the rotation about Z axis and translation along X axis for 12+.
indicates that the difference between the solvation energies of R+' and R2+. is very large and nearly equal to the electrostatic interaction in Rz+'. In this case we may consider that AE represent the resonance (transannular) interactions in R2+'. It is seen in Table V that AE for the derivatives are smaller than that for 12+, which is considered to be due to the change in the transannular interactions in R2+'. Transannular Interaction and Steric Hindrance. The transannular interactions depend on the intermolecular distance as well as the relative configuration of the two aromatic moieties in R2+'. It is considered that the steric hindrance of the substituting groups must have large effects on the transannular interaction through both the intermolecular distance and the relative configuration. It is of interest to examine the dependence of the transannular interaction on the relative configuration. Figure 11 shows the change of the stabilization energy due to the transannular interaction (ET,)for 12+' against the rotation around the Z axis and the translation along the X axis.36 ETIwas estimated from the energy difference between the HOMO of R+'and R2+'. In applying the results shown in Figure 11 to the actual system, further changes of ET[ must be taken into consideration corresponding to the steric hindrance in a given conformation and to the change in spin distribution caused by substitution. It is noted that the magnitude of ET, at the rotation angle 0 = 90' is almost equal to that at 8 = 0' (Figure l l ) , which is understood by taking into account the large overlap between the p~ orbitals of the a-carbons in the 90' rotated structure. On the other hand, the magnitude of ETIin the translated structure (35) E. S. Pysh and N. C. Yang, J . Am. Chem. SOC.,85, 2124 (1963). (36) The transannular interaction p ' s were calculated from p' = 4.0Sp in order to take account of the variation on changing conformations. The overlap integrals between the carbon atoms in one moiety and those in the other, S, were calculated on the basis of the proper molecular structure with an inter-ring distance of 3.3 A (see ref 15).
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1571 becomes roughly one-half of that in the symmetrical eclipse structure because of the decrease of the overlap between the two naphthalene rings. It is important to note that there are two additional minima of the energy surface at the 90' rotated structure and the translated structure along the X axis. Therefore, R2'. may take one of these two conformations when steric hindrance makes the symmetrical eclipse structures unstable. For 11, 12, and 13,Rz+' do not exist as stable species because of the large steric hindrance between the substituents. On the other hand, the smaller steric hindrance of the methylene groups than the methyl groups and the reduction of repulsion in the 90' rotated structure allows 10 to form R2+'. Since ET, in the 90' rotated structure is identical with that in [O,O], the difference in AE between 12+ and lo2+',0.09 eV, can be attributed to steric hindrance of the methylene groups. The steric repulsion between the substituents must be large in the [O,O] structures of 2 to 9. Such repulsion may be negligible in the 180' rotated structure, and we may expect such a structure. However, the hyperfine data suggest that R2+'of these compounds are likely to take translated structures despite the decreased transannular interaction and questions arise as to the causes which make these structures stable. Here we consider two factors in favor of the translated structure. The first is repulsive interaction between the substituting groups and the corresponding protons. The steric hindrance is reduced by taking the translated structures. The second is the difference in the solvation energy between the 180' rotated and translated structures. In the last section we have noted that the difference in the solvation energy between R2+'and R" is very large. Since more space is available for solvation in the translated structure, a considerable gain is obtained in the solvation energy by taking a translated structure. However, the actual conformation of the dimer would be determined by a subtle balance of all existing interactions including electrostatic interaction, transannular interaction, steric hindrance, and solvation.
Summary and Concluding Remarks Anomalous conformations of the rotated and translated structures are suggested for R," in this paper. Rotated structures are familiar in the salts of R2+', but this is the first report which suggests such structures in solution. Further striking results are the hfcc data in favor of the translated structure, which resembles the structure of the chlorophyll a dimer. The present results indicate that the dimer conformation is changeable depending on internal and external perturbations. It is shown that LTCV provides a convenient and direct method to obtain the stabilization energy of R2+.. Registry No. 1, 91-20-3; lt', 34512-27-1; 2,90-12-0; 2", 34475-76-8; 3,573-98-8; 3+',74219-36-6; 4,571-58-4; 4+', 34475-77-9; 5,3031-15-0; 5", 38479-71-9; 6, 13764-18-6; 6", 100516-01-6; 7, 571-61-9; 7". 36652-47-8; 8,569-41-5; 8", 36652-48-9; 9,208-96-8; 9", 42299-22-9; 10,567-79-3; lo', 100679-97-8; 11, 56138-04-6; lit', 100516-02-7; 12, 2717-39-7; 12", 100678-81-7; 13, 18623-61-5; 13", 34531-77-6.
