SIDDICK ICLI AND ROBERT W. KREILICK
3462
Nuclear Magnetic Resonance Studies of Phenoxy Radicals: Hyperfine Coupling Constants and Spin Densities of a Series of Partially Fluorinated Radicals by Siddick Icli and Robert W. KreiIick*l Department of Chemistry, University of Rochester, Rochester, New York 14627
(Received February 12, 1971)
Publication costs borne completely b y The Journal of Physical Chemistry
We have taken proton and fluorine nmr spectra of a series of partially fluorinated phenoxy radicals. The spectra of two unfluorinated parent radicals were also taken. Hyperfine coupling constants were determined from the shifts of the lines while fluorine spin densities were estimated through an analysis of the line widths. The fluorine spin densities were found to be a constant fraction of the carbon spin densities, and we were unable t o determine individual fluorine spin polarization parameters from our experimental data. The fluorine spin densities are found to be about 7.5y0of the carbon spin densities.
I. Introduction The hyperfine coupling constants of fluorine atoms in fluoro-substituted aromatic radicals depend on both the spin density at the adjacent carbon atom and the spin density at the fluorine. The coupling constants are related t o the spin densities by spin polarization parameters (Q's) for the 1s and 2s electrons. Three types of equations have been proposed to relate coupling constants (AF) to spin densities. These
To evaluate the spin polarization parameters one must be able to determine the sign and magnitude of the fluorine coupling constant, the carbon spin density ( p c ) , and the fluorine spin density (PF). I n the case of eq 3 one must also know the sign and magnitude of the carbon-fluorine bond spin density (PcF). The magnitude of the fluorine coupling and the sign and magnitude of the carbon spin density can normally be obtained from electron spin resonance spectra. Fluorine spin densities are usually not obtained by esr studies, and as a consequence most of the papers in which Q values have been reported rely on calculated values of fluorine spin densities. Nuclear magnetic resonance spectroscopy can be used to determine the sign and magnitude of electronnuclei hyperfine coupling constants in some instances. If a given radical is dissolved in a liquid radical solvent and spin exchange between solute and solvent rapidly averages the electron spin energy levels, one observes a single shifted nrnr line from each group of equivalent nucleiS7 The equation relating the shift of a given The Journal of Physical Chemistry, Val. 76, N o . 12, 1971
line ( A H ) to the hyperfine coupling constant ( A )is AH = -A(:)(%)
(4)
The width of the nmr lines can be used to estimate the spin density at the fluorine atom in fluorinated radica1s.6 The width of the nmr lines is dominated by the electron-nuclei dipole-dipole interaction and the Fermi contact interactionas I n the case of fluorine one must consider dipolar terms from the spin at carbon and from the spin a t fluorine. The equations for the line widths of signals from protons and from fluorine are given by6*s
(1) Alfred P. Sloan Foundation Fellow.
(2) (a) P. H. Fischer and J. P. Colpa, 2. Naturforsch. A , 24, 1980 (1969); (b) S. V. Kulkarni and C . Trapp, J . Amer. Chem. Soc., 92, 4801, 4809 (1970).
(3) A. Hudson and J. W.Lewis, MoZ. Phys., 19, 241 (1970). (4) J. Sinclair and D. Kivelson, J . Amer. Chem. Soc., 90, 5074 (1968). ( 5 ) W. Espersen and R. 1%'. Kreilick, Mol. Phys., 16, 577 (1969). (6) A . Hinchliffe and J. Murrell, ibid., 14, 147 (1968). (7) W. Espersen and R. W. Kreilick, J . Phys. Chem., 73, 3370 (1969); R. 1%'.Kreilick, J . Amer. Chem. Soc., 90, 2711, 5991 (1968). (8) I. Solomon, Phya. Rev., 99,559 (1955); N. Bloembergen, 1.Chem. Phys., 27, 572 (1957); G. W.Canters and E. DeBoer, MoZ. Phys., 13, 395 (1967); D. Stehlik and K. H. Hausser, 2. Naturforsch. A , 22, 914 (1967).
ESRSYMPOSIUM.NMRSTUDIES OF PHENOXY RADICALS
3463
0. Compound
RI
I11 IV
H H H H CHa CHa
R8
Ra
H F H
H H F F F H
CHa H
H
R3 Figure 1. Compounds investigated.
11. Experimental Section The phenolic precursors of radicals I-VI were prepared by the technique of E r s h ~ v . ~The overall yields, melting points, and analytical data for these compounds are listed in Table I. The radicals were
Table I: Analytical Data, Melting Points, and Yields of the Phenolsa
--TheoreticalPhenol
%C
I1 I11 IV V
79.96 79.96 80.22 80.22
Analytical------------Found-%H %C
8.33 8.33 8.65 8.65
80.04 79.98 80.14 79.85
3
phenoxy
I n these expressions t, is the correlation time for spin exchange, SCH is the separation between the electron in the carbon 2p orbit and the proton, r C F is the separation between the electron in the carbon 2p orbit and the fluorine, and r F is the separation between the electron in the fluorine 2p orbit and the fluorine. In an earlier paper we reported the nmr and esr spectra of three fluorinated phenoxy radicah6 We have extended this work to a series of four new fluorinated phenoxy radicals along with two unfluorinated parent radicals. The compounds which have been studied are shown in Figure 1.
Mp,
Yield,
%H
OC
%
8.58 8.48 8.76 8.60
99 107 105 78
55 73 41 27
Compounds I and VI have previously been reported; ref 9 and 10.
made by oxidizing an ethereal solution of the parent phenols with aqueous alkaline K3FeCNs. After the ether layer was dried and evaporated, the radicals were dissolved in the liquid radical di-tert-butyl nitroxide (DBNO) for the nmr experiments.' The nmr spectra were taken on a JEOLCO 4H-100 100-MHz nmr spectrometer. A Princeton Applied Research Model HR-8 lock-in amplifier was used with the high-resolution spectrometer for the broad line studies. A Digital Equipment PDP-12 digital computer was used for the various calculations. Programs were written in FOCAL.
Radical
B
ortho para
phenyl
IV
2:
meta
5 Khz
I
ortho phenyl
--3H
Figure 2. Nmr spectra of radicals I and IV: 1, diamagnetic aromatic peak; 2, diamagnetic tert-butyl peak; 3, DBNO peak.
111. Results and Discussion The proton nmr spectra of the radicals showed peaks from each group of equivalent protons. Typical spectra are shown in Figure 2. Shifts and coupling constants are listed in Table 11. The coupling constants from compound I are similar to those determined in an earlier esr study.'" The signs of the coupling constants from the ortho' and para protons of the phenyl ring are negative while the signs of the coupling from the meta protons of the phenyl and phenoxy rings are positive. The fluorine couplings had opposite signs from the corresponding proton couplings in each case. The magnitudes of the fluorine spin densities were estimated from an analysis of the proton and fluorine line widths. A plot of the fluorine and proton line widths vs. the square of the coupling constants is shown in Figure 3. The plot also includes points from the three radicals reported in our earlier paper." The experiments were carried out in a liquid radical solvent in which the correlation time for spin exchange is short (9) V. V. Ershov, G. N. Bogdanov, and A . A. Volodkin, 1.m. A k a d . N a u k , 1, 167 (1963); G. N. Bogdanov and V. V. Ershov, ibid., 6, 1084 (1963). (10) A. Rieker and K. Scheffler, Ann. Chem., 689, 78 (1965). (11) We re-examined the spectra of these radicals and with better instrumentation found the fluorine line widths to be slightly smaller than the values previously reported. The Journal of Physical Chemistry, Vol. 76, N o . 08, 1971
SIDDICK ICLIAND ROBERT W. KREILICK
3464 Table 11: Shifts and Coupling Constant for the Radicals Radical
I I1 I11 IV V VI
--
Phenoxy ring ,--metaAHa Ab
,---AH
A
-126.6 -128.8 -130.2 -127 -133.2 -129
f131.0 $131.5 $135.8 f134.9 +79.8 f81.8
-1.75 -1.75 -1.81 -1.80 -1.06 -1.11
$1.69 $1.72 f1.74 $1.69 $1.78 $1.74
--Ortho--?
Phenyl ring --Meta-AH A
-51.4 -49.1 -49.3 -47.2 -47.9 -48
f0.68 f0.66 3-0.66
-----Para---
-Methyl
AH
A
+143.6 +143.2
-1.92 -1.91
AH
f0.63
$0.64 $0.63
+81.8
-1.11
grouA
$43.8 -64.2 -59.5
---Fluorine-
-0.58 4-0.86 $0.81
AH
A
$106.2 -333.5 -312.3 -194
-1.42 $4.51 f4.22 +2.62
* The coupling
a The shifts are in ppm from the corresponding diamagnetic peaks. Positive signs indicate a shift to high field. constants are in gauss.
Table I11 : Carbon and Fluorine Spin Densities Radical
Position
Meta Para Para Para Ortho Meta Para
I1 I11 IV V Aa
B C
PC
x
lo*
PF
-2.45 +7.11 +7.11 f4.1 +3.64 -1.77 +5.31
x
PB/PO
x
10’
-1.7 f5.28 $4.94 f3.05 f2.23 -1.68 f4.61
io1
7.0 7.4 7.0 7.10 6.1 9.5 8.6
a Compounds A, B, and C are the radicals studied in our previous paper; ref 5.
Compound
A
B C
”
R
odluoro m-Fluoro p-Fluoro
&R
Figure 3. Proton and fluorine line widths vs. coupling constants squared. Data from our earlier study are included.
compared to the rotational correlation time. In this case t, is the appropriate correlation time for both the dipolar and scalar interactions. One may determine the value of t, from eq 5 if one estimates rCH. We assumed rcH to be the carbon-hydrogen bond length and find t, to be 3.6 X 10-l2 see. To determine p~ from eq 6 one must estimate TCF and TF. We used the carbon-fluorine bond length as an ~ estimation of ?“CF and calculated the value of ~ / Y Ffrom the 2p radical distribution function.l 2 Slater screening constants were used to determine the effective nu2lear charge. This calculation gives a value of 0.294 A for ?“F. We were unable to determine the sign of PF as the dipoIar term contains pp2. FJ7e assumed that PF had the same sign as the spin density at the adjacent carbon in each case. Table 111 lists the fluorine spin densities obtained by this technique as well as the spin densities at the adjacent carbon atoms. We assumed that the substitution of fluorine for The Journal of Physical Chemistry, Vol. 76, N o . 3.3,1971
hydrogen did not disturb the overall spin distribution and calculated carbon spin densities from the appropriate proton coupling constants. A QCHvalue of -27 G was used. This assumption appears to be justified as the proton couplings did not change very much upon introduction of a fluorine. The spin densities in the phenyl rings of the two compounds with o-methyl groups (radicals V and VI) were smaller than those from the other radicals. The phenyl ring in these compounds is probably twisted with respect to the phenoxy ring because of an interaction between the bulky methyl groups and the phenoxy ring. In our earlier paper we estimated the Q values of eq 2 by plotting AF/pc 21s. p ~ / p c . For this technique to be valid the spin density at fluorine cannot be a constant fraction of the spin density a t the adjacent carbon. In cases in which PF = Cpc eq 2 can be written as
AF = (12)
[QCC
I. Waller, Z . Phys., 38, 635
+
C&FF]PC
(1926).
(7)
ESRSYMPOSIUM. NMRSTUDIESOF PHENOXY RADICALS
3465
where C = constant. When this is the case, one' cannot determine individual values for Qcc and QFF. The sum of the terms in eq 7 gives an effective Q relating fluorine couplings to carbon spin densities. Figure 4 shows a plot of the spin density a t fluorine vs. the spin density at the adjacent carbon. This plot indicates that the spin density at fluorine is about 7.5% of the spin density a t the adjacent carbon for this series of radicals. This result is similar to the result obtained through a study of the anisotropic coupling of the fluoroacetamide radi~a1.I~I n this case the fluorine spin density was found to be about 14% of the carbon spin density. AF
.G
Figure 5. Carbon spin density us. fluorine coupling constants. Data from our earlier study are included.
Table IV : Calculated and Experimental Fluorine Coupling Constants
Radioal
I1 -
2
0
2
4
I
6
pFx I03 Figure 4. Carbon spin density vs. fluorine spin density. Data from our earlier study are included.
A plot of the carbon spin densities vs. fluorine coupling constants is given in Figure 5 . The points from the radicals reported in this paper fall very close to the line while the points for the o- and p-fluoro radicals of our previous study fall slightly off the line. The slope CQFF. of this line gives a value of +62 G for QCC I n our earlier paper we attempted to estimate values of QCC and QFF from a limited set of experimental data. Our current work indicates that the ratio of PF/PC is nearly constant for this set of phenoxy radicals. One cannot determine individual values for polarization parameters when this is the case. We carried out calculations of fluorine coupling constants using our spin densities along with the polarization parameter determined by Kulkarni and TrappZb and by Fischer and Colpa.2a Table IV contain$ experimental fluorine coupling constants along wit$ the coupling constants calculated with this techqique. The results obtained with Kulkarni and Trapp's polarization parameters are completely different froih the experimental results while Fischer and Colpa's values gave good results in most instances. The agreement with Fischer and Colpa's results can be explained by introducing their Q values into eq 7 along with our
+
I11 IV V Ac B C
Experimental
Kulkami and Trapp"
Position
AF
AF
Fischer and Colpab AF
Meta Para Para Para Ortho Meta Para
-1.42 $4.45 $4.17 $2.59 +1.66 -1.11 +3.6
$0. 31 -0.53 -0.88 -0.47 -0.77 -0.24 + O . 30
-1.43 $4.18 $4.13 $2.51 $2.07 -1.09 f3.23
Calculated with QCC = -85 G , QFF = 1043 G; ref 2b. Calculated with &cc = 48.1 G, QFF = 146 G ; ref 2a. c Radicals from previous paper; see Table 111.
value for C. This calculation leads to a value of 59.1 G for QCC3- CQFF.
Acknowledgment. This work was supported in part by National Science Foundation Grant No. GP-9339.
Discussion ROGERV. LLOYD.Why is the ratio of AF/AH nearly constant in your radicals whereas in the simple fluorophenoxy radicals it differs for o-fluoro and p-fluoro?
R. W. KREILICK.Only one o-fluoro compound was included in our study. The ratio of AF/AH was different for this radical from any of the meta- or para-substituted radicals.
JAMESR. BOLTON.Trapp, et al., have recently shown that A F / A Hvaries considerably in a series of fluoro-substituted triphenylmethyl radicals. Do you feel you can determine the Q's for this series? R. W. KREILICK.We should be able to determine the fluorine couplings and spin densities for the fluoro-triphenylmethyl radicals if they are soluble in DBNO and are not dimerized. (13) R. J. Cook, J. R. Rowlands, and D. H. Whiffen, Mol. Phys., 7 , 31 (1968).
The Journal of Physical Chemistry, Vol. 76, No. 22, 1971
