519
Substituent Effects on Nitroaromatic Radical Anions
Substituent Effects on Nitroaromatic Radical Anions in Aqueous Solution' P. Neta and Dan MelseP Radiation Research Laboratories and Department of Chemistry, Meiion Institute of Science, Carnegie-MelionUniversity, Pittsburgh, Pennsylvania 152 13 (Received October 6, 1975) Publication costs assisted by Carnegie-Mellon University and the U S . Energy Research and Development Administration
The effect of substituents on the ESR parameters of nitroaromatic radical anions in aqueous solutions has been studied by observing the spectra of some 20 such radicals using the in situ radiolysis steady-state ESR technique. The results focus on the effect of substitution and disubstitution at positions ortho to the nitro group. Higher nitrogen splittings were found to result in lower hyperfine constants for both para and ortho protons (or methyl groups), while meta hydrogens are affected to a much smaller extent. An inverse linear correlation is in fact found between a j o 2 and a&,. When ayOz increases to values greater than -20 G the meta proton splittings become higher than those for the para and ortho protons. The effect of an ortho OH group is relatively small but the effect of its basic form (0-) is comparable to that of a methyl group. A strong synergistic effect of the second substituent in ortho disubstituted nitroaromatics is observed. It is also found that the rate of electon-transfer reaction from (CH3)&OH to the parent compounds can be correlated with the spin density on the ring of the resultant radical anion as indicated by the inverse correlation with ajo2.
The effect of substituents on the geometry and the electronic structure of nitroaromatic molecules and their radical anions have been studied in detail. Several techniques, including uv and NMR spectroscopy and dipole moment measurements, have been utilized in elucidating these effects in the parent molecules.2 The same effects in the radical anions have been studied using ESR spectro~copy,~-' while p o l a r ~ g r a p h i c ~and - ~ kinetic studies2 yield information concerning the effects of substituents on the energetics of the transformation from the molecule to its radical anion and the activated complex involved. Recently we have been able to correlate the one electron reduction potentials of several nitro aromatic compounds in aqueous solutions for the radiwith the nitrogen hyperfine constants (~$0,) cals. l o The effect of electron-withdrawing groups in decreasing u;02 and of electron-donating groups in increasing ago2was correlated with Hammett's constant^.^ The relevance of such a correlation was intensified by the comparison of the effect of the same substituents in different conjugated syst e m ~ These .~ correlations were, however, confined mainly to substitution a t positions para and meta to the nitro group. On the other hand, the most striking effect of substitution or disubstitution occurs a t the ortho position. The work of Geske et aL7 clearly establishes that the effect of such a substitution exceeds by far any mesomeric effect and has to be attributed to spacial twisting of the nitro group out of the plane of the benzene ring. Recent INDO MO calculations corroborates these conclusions.ll The calculated twist angles in the radicals resemble those estimated for the molecule^.^ The only substituents studied thus far from this point of view are the bulky methyl and tert-butyl groups in nitrobenzenes and nitroanilines in organic solvents. We have undertaken the present study with the hope of gaining further insight into the effect of polar charged and uncharged ortho substituents in nitrobenzene radical anions in aqueous solutions. Some specific effects due to the water are to be expected and a{on is usually found to be higher in this solvent than in aprotic solvents.12 Indeed, it was found that
hydrogen bonding between the nitro and the hydroxyl group in o-nitrophenol radical anion results in a pronounced increase in the pK, of this proton.13 Two techniques were used in this study. One was the in situ radiolysis steady-state ESR technique and the other was kinetic spectrophotometric pulse radiolysis which enables us to measure the absolute rate constants for electron transfer from the 2-propanol radical to the nitro compound.
Experimental Section Deoxygenated aqueous solutions containing
to
M of the nitroaromatic compound, 0.1 M isopropyl alcohol, and 5 X 10-3 M phosphate buffer a t pH 7 (unless otherwise stated) were irradiated by 2.8-MeV electrons from a Van de Graaff accelerator. Radiation produced eaq- reacts with ArN02 a t diffusion-controlled rates, while H and OH react with the alcohol to produce (CH&COH which also reduces ArNOz efficiently. The nitroaromatic radical anions thus produced were studied by the steady-state in situ radiolysis ESR technique.13 Rate constants for the reduction of ArNOz by (CH3)zCOH were determined by kinetic spectrophotometric pulse radiolysis using NzO saturated solutions. Experimental conditions were adjusted such that eaq- is quantitatively converted into OH by reaction with NzO and then all OH and H react with isopropyl alcohol. The absolute rate constant for the reaction of (CH&COH with ArNO2 was determined by following the kinetics of formation of the ArN02- optical absorption, usually a t 300-350 nm. The total radical concentration produced by the pulse was kept a t 1-3 pM to minimize second-order decay. Details of the computer-controlled pulse radiolysis system have been given previ0us1y.l~
Results The ESR spectra recorded with irradiated aqueous solutions of the nitroaromatic compounds were usually of high line intensities because of the relatively long lifetime of the radical anions observed. In a few cases, the spectra were quite complex and despite the reasonable line intensities The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
P.Neta and Dan Meisel
520
TABLE I: ESR Parameters of Nitroaromatic Radical Anionsa and Rate of Reduction of Their Parent Compounds by (CH,)zCOH aH a(others) k,,M-'s-' Radical PH k? ago, a," a: P NO,-
4
I
2.004706
11.57
3.24(2)
1.08(2)
I
2.00448b
13.66
3.38(2)
1.12
aFH, = 0.43
3.8 x 1 0 9 ~
COCH3 NOz-
COCH3
*2wozNo,-
3
1.17(2)
3.36
1.5 X 10'
3.38(2)
1.15(2)
3.65
1.6 x 109e
14.20
3.30(2)
1.10
2.00442
14.45
3.39(2)
1.13(2)
3.82(CH3)
7
2.00458
14.64
1.01; 0.91
3.45
7
2.00468
14.68
f
7
2.00461
14.82
f
7,14
2.00451d
14.85
3.39
1.04; 0.91
I
2.00441
14.90
3.35(2)
1.05(2)
7,14
2.00459
13.68
6,12
2.00448d
14.20
I
2.00451
I
p-
2"
NO; 1
a&
= 0.28
3.3 X
lo*
NOzI'
NOz-
I
= 1.05 =
7.2 x 10'
1.05
2"
No2I
2.00448
14.94
3.62; 3.18
1.08
= 0.43
a E H Z= 0.43
Bo;. 13
2.00457
The Journal of Physical Chemistry, Vol. 80, No. 5. 1976
15.22
f
9.2 x 10'
521
Substituent Effects on Nitroaromatic Radical Anions TABLE I (Continued) g
ana,
7
2.00463
15.60
7
2.00474
16.69
7
2.00477
7
Radical 0
PH
a,"
"PH
:0
a(others)
k , , M-'s-*
0
:\:/: H N H
3.41
2.73; 2.22(CH3)
1.23; 0.95
2.13
4.8
X
lo8
17.37
1.94(CH3)
0.92(2)
2.48
2.9
X
lo*
2.00474
17.67
2.71; 2.02(CH3)
1.14; 0.88
-*
2.00501
-02cxv
19.18
0.98(CH3)
0.98(2)
1.43
1.9 x
lo8
1 M K O H 2.00489
19.64
0.89; 0.13
1.24
7
2.00502
21.65
0.93; 0.75
0.41
1 M KOH 4MKOH
2.00499
21.68 21.84
0.82(2) 0.82( 2)
0.37 0.30
7
2.00501
21.958
0.30(2CH3) 0.89(2)
1 M KOH
2.00497
22.12
0.24(CH3)
2.0 04 9 5h
2 5.96
H
a g H = 0.34(2) 6.8 X
lo8
0.99( 2)
N
b" NO'-
17
OH NO;
7
NO,.
cop-
l9
NO,-
2o
21
0.32(CH3)
2.3 X 10'
CH30*
-O@$0;
22
""-b"'
0.30(CH,)
I
CHJ
$0;
23
24
0.92(2)
