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Triple ion formation by dinitrobenzophenone radical anion. Effects on the internal hindered rings rotation
Sprcrrochm,rca Acra. Vol. 39A. No. 2. pp 191-192.
0584-8539~83~020191~2%03.000
1983
J: 1983 Pergamon Press Ltd.
Prmred I” Great Bntam.
RESEARCH
TRIPLE
NOTE
ION FORMATION BY DINITROBENZOPHENONE THE INTERNAL HINDERED RINGS
RADICAL ROTATION
ANION.
EFFECTS
ON
(Received 13 dune 1982)
The formation of ion pairs and triple ions by reduction of aromatic nitro derivatives by alkali metals in ethereal solvents is widely studied in literature ([l&6] and refs. therein). In particular it is well known that the formation of triple ions is greatly favoured by addition of salts with a common cation to the ion pair solution. In most cases when two equivalent sites are present with high spin density in the organic frame, triple ions are the intermediates of intermolecular cation exchange between the radical pair and the added salt, and a strong linewidth alternation can be observed in the ESR spectra. However, in a preceding work [5] on the triple ion formation by dinitrodiphenyletilene radical anion we have reported that no line-width alternation was observed on the ESR time scale. In this note the similar case of the formation of triple ion by dinitrobenzophenone (DNBF) radical anion with sodium cations in tetrahydrofuran is given and a thermodynamical model for the triple ion-ion pair equilibrium based on the detection of the hyperfine splitting (h.f.s.) constant of the nitrogen at different temperatures is proposed. The ion pairs DNBF-‘, Na+ have been widely studied in a preceding work [6]. The experimental spectra (see Fig. la) are all composed by a triplet due to the h.f.s. constant of the nitrogen, by the splitting due to the protons in orrho position and in meta position with respect to the nitro group and by the
quartet due to the cation. An alternation line-width effect is also observed due to the p-nitrophenyl rings rotation. An activation enthalpy of 4.9 f 0.6 kcal mol _ ’ for this process has been reported. When a second Na+ cation is supplied to the ion pair by adding sodium tetraphenylborate (NaBPh,) (0.4 Mj the following significant changes occur in the ESR spectra (see Fig. lb): (a) the sodium quartet disappears; (b) the meta protons h.f.s. constant disappears; (c) the large coupling constant of the nitrogen decreases (see Fig. 2); (d) the linewidth alternation is no more observable.
a)
Fig. 2. Hyperfine splitting constants of the ion pair DNBF -., Na+ at different temperatures. The h.f.s. constant of the nitrogen is also shown when NaBPh, is added to the solution (0).
The reported fact (a) is typical when triple ions are formed[ld]. As the nitrogen h.f.s. constant decreases by formation of triple ions, the second cation could be localized or near the C=O group or near the nitro group which is uncomplexed in the ion pair, and subtract spin density from the other nitro group. However, the latter position is the only one which agrees with the above reported observations (b) and (d). In fact in thiscase the bond order C,&, (between the carbonyl and the rotating rings) decreases as well as the spin densities on the meta protons to the nitrogroup and the two rings can rotate freely. These considerations also agree with similar observations already reported [6] about the effects of the ion pairing on the internal hindered rotation barrier. The second cation is very looser than the first and it easily dissociates when temperature is lowered. In this case the observed nitrogen h.f.s. constant li is given by the relation
b)
Fig. I. Experimental spectra of DNBF-‘, furan (a) before and (b) after the addition at room temperature.
Na + in tetrahydroof NaBPh, (0.4 Mj where at each temperature 191
alp is the h.f.s. constant
of the
Research
192
nitrogen in the ion pair and a,, is the h.f.s. constant of the nitrogen in the triple ion. The last constant can be evaluated by linear extrapolation of the trend of zi at higher temperatures when Kdlss = 0. By this way it is also possible to draw a Van’t Hoff’s plot from which the following thermodynamic parameters have been evaluated for the triple ion dissociation process: AS dlss= -38.72calK~‘mol-’ AH
dlss=
-
note reported [9] for the pure solvent, and is lower than values reported elsewhere [2, 31 for other systems. The particularly low value for E, suggests that in this case also in presence of NaBPh, the solute-solvent interactions are negligible. Isriruro di Chimica Fisica, Univcrsitb Via Go& 19 20133 Milano, Ital)
7.67 kcal molt I.
REFERENCES
The negative sign of AS,,,, is in agreement with the hypothesis that the second cation is more solvated when it dissociates. The reported results could be affected by imprecision due to the small number of different temperatures at which Kdlsscan be evaluated. However, it is worth noting that they have the same order of magnitude of analogous reported [2-4] parameters. According to the rotational diffusion model for the electron spin resonance in liquids [7], the activation energy of the viscosity 9 of the studied paramagnetic system can be obtained from the slope of the plots of In (B$) and of In (C$) vs l/T, where B, and C, are obtainable from the contribution to the line-width [7,8] Wrelax,;.= A + &.,M, + C,M
:
where i labels the quantum spin states of nitrogen nucleus whose eigenstates are MA. In this case one obtains from the experimental data: E, = 1.2 f 0.1 kcal mol _ ‘. This value is close to the one reported [6] for the ion pair DNBF- , Na+,and also to the value
[‘I R. F. ADAMS and N. M. ATHERTON, Trans Faraday Sot.
64, 7 (1968). M. BARZAGHI, P. CREMASCHI,A. GAMBA,G. MOROSI,~. OLIVA and M. SIMONETTA, J. Am. Chcm. Sot. 100,3132 (1978). PI M. BARZAGHI, C. OLIVA and M. SIMONETTA, J. Phys. Chem. 84, 1717 (1980). i-41M. BARZAGHI, C. OUVA and M. SIMONETTA, J. Phys. Chem. 85, 1799 (1981). [51 C. OLIVA, Istituto Lombard0 (Rend. SC.) A (in press). 161M. BARZAGHI, S. MIERTLJS, C. OLIVA, E. ORTOLEVA and M. SIMONETTA, J. Phys. Chem., in press. r71 L. T. Muss, P. W. ATKINS, Eds. Electron Spin Relaxation in Liquids. Plenum Press, New York (1972). PI J. H. FREEDOMS G. K. FRAENKEL, J. Chem. Phys. 39,326 (1963). r91 C. CARVAJAL, K. J. TOLLE, J. SMID and M. SZWARK, J. Am. Chem. Sot. 87, 5548 (1965); D. NICHOLLS, C. SUTPHEN and M. SZWARK, J. Phys. Chtrm. 72, 1021 (1968).
PI
0584-8539~83~020191~2%03.000
1983
J: 1983 Pergamon Press Ltd.
Prmred I” Great Bntam.
RESEARCH
TRIPLE
NOTE
ION FORMATION BY DINITROBENZOPHENONE THE INTERNAL HINDERED RINGS
RADICAL ROTATION
ANION.
EFFECTS
ON
(Received 13 dune 1982)
The formation of ion pairs and triple ions by reduction of aromatic nitro derivatives by alkali metals in ethereal solvents is widely studied in literature ([l&6] and refs. therein). In particular it is well known that the formation of triple ions is greatly favoured by addition of salts with a common cation to the ion pair solution. In most cases when two equivalent sites are present with high spin density in the organic frame, triple ions are the intermediates of intermolecular cation exchange between the radical pair and the added salt, and a strong linewidth alternation can be observed in the ESR spectra. However, in a preceding work [5] on the triple ion formation by dinitrodiphenyletilene radical anion we have reported that no line-width alternation was observed on the ESR time scale. In this note the similar case of the formation of triple ion by dinitrobenzophenone (DNBF) radical anion with sodium cations in tetrahydrofuran is given and a thermodynamical model for the triple ion-ion pair equilibrium based on the detection of the hyperfine splitting (h.f.s.) constant of the nitrogen at different temperatures is proposed. The ion pairs DNBF-‘, Na+ have been widely studied in a preceding work [6]. The experimental spectra (see Fig. la) are all composed by a triplet due to the h.f.s. constant of the nitrogen, by the splitting due to the protons in orrho position and in meta position with respect to the nitro group and by the
quartet due to the cation. An alternation line-width effect is also observed due to the p-nitrophenyl rings rotation. An activation enthalpy of 4.9 f 0.6 kcal mol _ ’ for this process has been reported. When a second Na+ cation is supplied to the ion pair by adding sodium tetraphenylborate (NaBPh,) (0.4 Mj the following significant changes occur in the ESR spectra (see Fig. lb): (a) the sodium quartet disappears; (b) the meta protons h.f.s. constant disappears; (c) the large coupling constant of the nitrogen decreases (see Fig. 2); (d) the linewidth alternation is no more observable.
a)
Fig. 2. Hyperfine splitting constants of the ion pair DNBF -., Na+ at different temperatures. The h.f.s. constant of the nitrogen is also shown when NaBPh, is added to the solution (0).
The reported fact (a) is typical when triple ions are formed[ld]. As the nitrogen h.f.s. constant decreases by formation of triple ions, the second cation could be localized or near the C=O group or near the nitro group which is uncomplexed in the ion pair, and subtract spin density from the other nitro group. However, the latter position is the only one which agrees with the above reported observations (b) and (d). In fact in thiscase the bond order C,&, (between the carbonyl and the rotating rings) decreases as well as the spin densities on the meta protons to the nitrogroup and the two rings can rotate freely. These considerations also agree with similar observations already reported [6] about the effects of the ion pairing on the internal hindered rotation barrier. The second cation is very looser than the first and it easily dissociates when temperature is lowered. In this case the observed nitrogen h.f.s. constant li is given by the relation
b)
Fig. I. Experimental spectra of DNBF-‘, furan (a) before and (b) after the addition at room temperature.
Na + in tetrahydroof NaBPh, (0.4 Mj where at each temperature 191
alp is the h.f.s. constant
of the
Research
192
nitrogen in the ion pair and a,, is the h.f.s. constant of the nitrogen in the triple ion. The last constant can be evaluated by linear extrapolation of the trend of zi at higher temperatures when Kdlss = 0. By this way it is also possible to draw a Van’t Hoff’s plot from which the following thermodynamic parameters have been evaluated for the triple ion dissociation process: AS dlss= -38.72calK~‘mol-’ AH
dlss=
-
note reported [9] for the pure solvent, and is lower than values reported elsewhere [2, 31 for other systems. The particularly low value for E, suggests that in this case also in presence of NaBPh, the solute-solvent interactions are negligible. Isriruro di Chimica Fisica, Univcrsitb Via Go& 19 20133 Milano, Ital)
7.67 kcal molt I.
REFERENCES
The negative sign of AS,,,, is in agreement with the hypothesis that the second cation is more solvated when it dissociates. The reported results could be affected by imprecision due to the small number of different temperatures at which Kdlsscan be evaluated. However, it is worth noting that they have the same order of magnitude of analogous reported [2-4] parameters. According to the rotational diffusion model for the electron spin resonance in liquids [7], the activation energy of the viscosity 9 of the studied paramagnetic system can be obtained from the slope of the plots of In (B$) and of In (C$) vs l/T, where B, and C, are obtainable from the contribution to the line-width [7,8] Wrelax,;.= A + &.,M, + C,M
:
where i labels the quantum spin states of nitrogen nucleus whose eigenstates are MA. In this case one obtains from the experimental data: E, = 1.2 f 0.1 kcal mol _ ‘. This value is close to the one reported [6] for the ion pair DNBF- , Na+,and also to the value
[‘I R. F. ADAMS and N. M. ATHERTON, Trans Faraday Sot.
64, 7 (1968). M. BARZAGHI, P. CREMASCHI,A. GAMBA,G. MOROSI,~. OLIVA and M. SIMONETTA, J. Am. Chcm. Sot. 100,3132 (1978). PI M. BARZAGHI, C. OLIVA and M. SIMONETTA, J. Phys. Chem. 84, 1717 (1980). i-41M. BARZAGHI, C. OUVA and M. SIMONETTA, J. Phys. Chem. 85, 1799 (1981). [51 C. OLIVA, Istituto Lombard0 (Rend. SC.) A (in press). 161M. BARZAGHI, S. MIERTLJS, C. OLIVA, E. ORTOLEVA and M. SIMONETTA, J. Phys. Chem., in press. r71 L. T. Muss, P. W. ATKINS, Eds. Electron Spin Relaxation in Liquids. Plenum Press, New York (1972). PI J. H. FREEDOMS G. K. FRAENKEL, J. Chem. Phys. 39,326 (1963). r91 C. CARVAJAL, K. J. TOLLE, J. SMID and M. SZWARK, J. Am. Chem. Sot. 87, 5548 (1965); D. NICHOLLS, C. SUTPHEN and M. SZWARK, J. Phys. Chtrm. 72, 1021 (1968).
PI