3370
WILLIAMESPERSEN AND ROBERT W. KREILICK
Magnetic Resonance Studies of a Series of Phenoxy Radicals. Substituent and Steric Effects on Spin Distributions by William Espersen and Robert W. Kreilick Department of Chemistry, University of Rochester, Rochester, New York 1.4627 (Received April 7, 1969)
We have investigated the nmr and esr spectra of a series of substituted phenoxy radicals to obtain information about electronic and steric effects on spin distributions. Relatively small electronic effects were observed. Groups which affectedthe geometry of theradicals were found to produce large changes in the coupling constants. Variable-temperature measurements (esr) were conducted on some of the radicals to obtain information about the temperature dependence of the coupling constants.
Introduction The spin distributions in negative and positive ion radicals have been reported to depend on the electrondonating or -withdrawing ability of substituent groups.'v2 Changes in geometry have been shown to affect the spin distribution in phenoxy radicals.a We have investigated the nmr and esr spectra of a series of phenoxy radicals in order to determine electronic and steric effects on spin distributions. The structures of the radicals which were investigated are shown in Figure 1.
0-
Compound
R
la-m
2 3 4
CO phenyl COzEt CN
Figure 1. Compounds investigated.
We were able to observe the nmr spectra of most of these radicals and determine electron-nuclei coupling constants from the shifts of the various lines. I n some cases the radicals were either too unstable or too insoluble for nmr measurements and the coupling constants were determined by analysis of esr spectra. The liquid radical di-t-butylnitroxide (DBNO) was used as a solvent for the nmr experiment^.^ Rapid spin exchange between solute and solvent radicals averaged the electron spin states and relatively sharp nmr lines were observed. The relation used to determine the hyperfine coupling constants (ai) from the shifts of the nmr lines ( A H ) is given by
The esr spectra of most of these radicals were very complicated and difficult to analyze. Computer simuThe Journal of Physical Chemistry
lations of some of the esr spectra were obtained. I n most cases, we had to make small changes in the coupling constants obtained from the nmr experiments in order to obtain good simulations. Small changes in coupling constants were found to produce very large variations in the appearance of the spectra. Radicals la-If were examined to determine electronic and steric effects on spin distributions. The compounds with substituents in the meta and para positions were investigated to determine electronic effects. When groups were substituted in the ortho position the geometry of the radicals was affected and a change in coupling constants was observed. The esr spectra of most of the radicals were temperature dependent. Detailed variable-temperature measurements were carried out on a few of the radicals whose spectra were not too complicated.
Experimental Section (1) Preparation of Compounds. The phenolic precursors of the radicals were made by the condensation of 3,5-di-t-butyl-4-hydroxybenzaldehyde with the appropriate nitrile in either piperidine or pyridine according to the procedure of M ~ l l e r . The ~ compounds were purified by elution through a column of neutral alumina with hexane-benzene and recrystallization from a mixture of hexane and benzene. The melting points, yields, and analyses of new compounds are given in Table I. The chemical shifts of the various phenols are given in Table 11. The radicals were made by oxidizing ethereal solutions of the phenols with aqueous alkaline KsFe(CN)o. Nitrogen was bubbled through the reaction mixture during (1) A. H. Maki and D. H. Geske, J . Amer. Chem. Soc., 83, 1852 (1961); E.T.Strom, ibid., 88,2065 (1966). (2) E.M. Latta and R. W. Taft, ibid., 89,5172 (1967). (3) R.Kreilick, ibid., 88,5284 (1966). (4) R. W. Kreilick, ibid,, 90, 2711, 5991 (1968); R. W. Kreilick, Mol. Phys., 14,495(1968). (5) E.Muller, R. Mayer, H. Spanagel, and K. Scheffler, Ann., 645, 53 (1961).
MAGNETIC RESONANCE STUDIES OF
3371
SERIESOF PHENOXY RADICALS
A
Table I: Melting Points, Yields, and Analytical Data %
Compd-------
X
NO.
lb IC
Id le If 1g lh li 1j lk 11 lm 2
MP, C o
yield
90 175 105 145 137 144 114 186 121 103 185 168 143
2-Fluor0 2,6-Dichloro 2-Methyl 4-Me thy1 4-Methoxy 4-Chloro 4-Fluor0 4-Nitro 3-Chloro 3-Fluor0 3,4,5-Trimethoxy 3-Pyridyl
11 9 15 32 34 23 40 41 33 57 38 60 41
__-_ Theoretical----
__--Found----
%C
%H
% C
% H
78.60 68.66 82.95 82.95 79.30 75.08 78.60 72.99 75.08 78.60 73.73 79.01 79.74
7.46 6.21 8.41 8.41 8.04 7.12 7.46 6.92 7.12 7.46 7.85 7.84 7.53
78.74 68.82 82.87 82.98 79.31 75.27 78.66 73.08 75.32 78.72 73.46 79.14 79.61
7.43 6.49 8.53 8.52 8.06 7.29 7.44 6.98 7.14 7.38 7.84 7.73 7.53
Table 11: Chemical Shifts of Phenols' 7 -
Compd
7
NO.
X
&Butyl
OH
CH
la lb
H 2-Fluor0 2,6-Dichloro %-Methyl 4-Methyl 4-Methoxy 4-Chloro 4-Fluor0 4-Nitro 3 Ch 1oro 3-Fluor0 3,4,5-Trimethoxy
1.43 1.48 1.50 1.50 1.49 1.50 1.51 1.49 1.50 1.50 1.48 1.53
5.58 5.65 5.70 5.60 5.58 5.60 5.68 5.62 5.70 5.70 5.66 5.67
7.41 7.49 7.23 7.04 7.42 7.37 7.45 7.39 7.56 7.48 7.42 7.41
7.77 7.82 7.83 7.80 7.80 7.80 7.92 7.80 7.83 7.84 7.80 7.83
7.46 7.23 7.15 7.24 7.36 7.28 7.48 7.35 8.05 7.44 7.24 6.89
3-Pyridyl
1.53 1.48 1.48 1.47
5.78 5.98 5.94 6.11
7.52 8.05 8.18 7.68
7.85 7.98 7.92 7.84
8.36 7.72
IC
Id le If 1g lh li 1j lk 11 lm 2 3 4
-
Shifts are in ppm from TMS in CDC13.
m-Phenoxy
Aromaticb
Other
CHz-4.36 CHs-1.39
' Generally the center of a group of lines.
the reaction. The ether layer was washed with a solution of KzC03, dried, and evaporated. The resulting radicals were dissolved in DBNO and the nmr spectra were taken as quickly as possible. All of the radicals except compound 11 were green. This radical was a deep red. Samples for esr experiments were made by oxidation with PbOz in toluene. (2) Instruments. The nmr spectra were taken on a JEOLCO 4H-100 100-MHz nmr spectrometer equipped with a broad-line unit employing 35-Hz field modulation. Esr spectra were taken on a JEOLCO 3BSX esr spectrometer employing 100-KHz field modulation. Spectra simulations were carried out with a JEOLCO RA-1 digital computer.
Results and Discussion The radicals with substituted phenyl rings (la-lm) were investigated to obtain information about steric and
electronic effects on the over-all spin distribution. The phenyl ring in radicals lb-ld contained an ortho substituent while radicals le-li contained para substituents and radicals lj-lk contained meta substituents. Variations in the coupling constants of the radicals with meta and para substituents appear to be due to electronic effects. The changes in the couplings of the compounds with ortho substituents appear to be due to variations in relative geometries of these molecules. The nmr spectra of the radicals showed peaks from each of the protons. Representative spectra are shown in Figure 2. The lines from the m-phenoxy protons, the methylene proton and the m-phenyl protons were shifted to low field. The low-field shifts show that the coupling constants are positive and indicate negative spin densities a t the adjacent carbon atoms. The lines from the o- and p-phenyl protons were shifted to high fields showing that these couplings are negative Volume 75,Number 10 October 1960
3372
WILLIAMESPERSEN AND ROBERT W. KREILICK
A ~
phenoxy
C- H
~
lie 3 2 ~
o rpt haor a
lr lOkHr
..
I - %
B
OCH,
C-H
phenoxy meta
meta
ortho
+ 10 k H r A H
, b
Figure 2. Nmr spectra: (A) radical Id; (B) radical If. Different regions of the spectra are recorded at different gains: (1)DBNO; (2) diamagnetic t-butyl; (3) paramagnetic t-butyl.
with positive spin densities a t the adjacent carbons. The shifts and coupling constants are given in Table 111. The data for the fluorinated radicals have previously been reported6but are included to give a complete tabulation. Compounds IC,lm, and l i were very insoluble in DBNO and we were unable to observe all of the lines from these radicals. Radicals 2, 3, and 4 were rather unstable and we were unable to observe their nmr spectra. The coupling to the methyl and methoxy protons in radicals l e and If can be used to calculate proportionality constants for these substituents. The over-all spin distribution is not changed very much by these substituents and one can use the coupling of the para proton in the unsubstituted compound to calculate the spin density a t the para carbon ( p p a T a = 0.0635 with Q = -22.5). I f the couplings of the methyl and methoxy protons are related to the spin density by ai = Qppam (6) W. Espersen and R. Kreiliok, Mol. Phys., 16, 577 (1969).
The Journal of Physical Chemistry
(2)
MAGNETIC
RESONANCE ~ R T D I E sOF A SERIES OF PHENOXY RADICALS
*
*
one obtains values of 26.6 3 and 4.3 0.5 G for the respective proportionality constants. The substituent groups are not hindered in these molecules and these numbers should be the values for freely rotating groups. An analogous procedure can be used to calculate proportionality constants for the methyl group in radical Id and the p-methoxy group in radical 11. The spin density a t the ortho position can he calculated from the coupling to the ortho aromatic proton in radical Id. (porma = 0.026 with Q = -22.5). The methyl and methoxy proportionality constants determined for these compounds are +16 1 and +0.93 0.5 G , respectively. The substituents are sterically hindered in these radicals and the smaller values of the proportionality constants may reflect hindered rotation.' Introduction of substituents a t the para or meta position of the phenyl ring produced small changes in most of the coupling constants. I n earlier papers concerning substituent effects on spin distributions, linear plots of coupling constants us. Hammett u values have been We were unable to find a linear relation between any of the couplings in this series of phenoxy radicals with a variety of u values. The coupling constants of the m-phenoxy protons and the meta aromatic protons were decreased on introduction of para substituents while the coupling of the ortho protons was increased. ortho substituents were found to produce much larger changes in the coupling constants. The splittings from protons on the phenyl ring were much smaller than those of the unsuhstituted radical. These couplings appeared to decrease as the steric hulk of the ortho suhstituents increased. This behavior can be explained by an increase in the value of twist angle 0% (Figure 1). The coupling constants of the m-phenoxy protons increased showing a greater localization of spin in the phenoxy ring. The coupling of the methylene hydrogen was increased in the case of radical l h while it decreased in the cases of ICand Id. This behavior may he due to differencesin values of twist angle el. Radicals 2, 3, and 4 were rather unstable in concentrated solutions and we were unable to obtain their nmr spectra. The coupling constants of these radicals were obtained by analysis of their esr spectra. Representative spectra are shown in Figure 3. The value of the coupling to the methylene proton was lower than that found for radicals la-lm while the coupling to the mphenoxy protons was slightly greater. The methylene protons on the ester group of radical 3 produced a small splitting. The two nitrogens coupling from radical 4 were identical. The temperature dependence of the esr spectra of radicals la, lb, IC, lh, li, lk, 11,2,3, and 4 were investigated. The spectra of all of the radicals except 11,2,3, and 4 were very complicated and we were unable to determine which coupling constants were temperature dependent. I n some cases we observed alternations in
n
3373
,
*
B
Figure 3. Esr spectrs: (A) radical 3 in toluene at 294O; (B) radical 4 in toluene at 297".
-80
-40
0
40
80
T e m p e r o t u r s . 'C
Figure 4. Plot of methylene coupling constants 0%. tempersture: 0 , radical 2; 0, radical 4.
line widths as the temperature was lowered. The spectra of 11, 2, and 4 were analyzed at a series of temperatures. The methylene proton's coupling constant was the only splitting which showed a measurable temperature dependence. A plot of this splitting us. temperature is shown in Figure 4. The temperature dependence of this coupling can he explained by changes in twist angle 01. The variation in this angle must he relai tively small or one would expect to see changes in the coupling of the nitrogen and the protons on the R group. Acknowledgment. This work was supported in part by National Science Foundation Grant GP-9339. (7) E. W. Stone and A. H. Maki. J . Chem. PAYS.,37,1326 (1962)
Volume 73, Number io Octobpr 1989
