7296
Concerning the Relative Importance of T Inductive Effects and Polar Field Effects on 1'9FChemical Shifts in Aromatic Derivatives William F. Reynolds*Ia and Gordon K. HamerIb Contribution from the Department of Chemistry, University of Toronto, Toronto, Ontario, Canada MSS 1A l . Receiced March 2, 1976
Abstract: I3C chemical shifts for ammonia derivatives of aromatic compounds, molecular orbital calculations for the same compounds, and correlations of I3C chemical shifts for 4-substituted biphenyls with 61 and UR all indicate that T polarization is an important mechanism of transmission of polar substituent effects and appears to be the dominant polar effect on I9F chemical shifts in aryl fluorides. The constant in the Buckingham electric field equation A = 27 - 34 X IO-'? for C - F bonds. This value is used to show that direct field effects are unimportant in most aryl fluorides. Molecular orbital calculations for B&difluorostyrenes show that the H component of C(sp2)-F bonds is relatively insensitive to direct field effects, thus accounting for the relative unimportance of these effects upon I9F chemical shifts in aryl fluorides.
There is a continuing controversy concerning the importance of a-inductive and polar electric field effects on I9F chemical shifts in aromatic derivatives.'~~ Taft and co-workers have interpreted the observed 01 dependence of I9Fchemical shifts for a series of systems 1 in terms of r-inductive effects:?
groups.2-5 The importance of direct field effects is also evaluated.
Results and Discussion A. Evidence for *-Polarization Effects. Exocyclic polar or charged groups in phenylalkanes induce a-electron density changes which can be represented schematically by:'
1 z
Similarly, the enhanced (TI dependence of I9Fchemical shifts in 10-substituted 9-fluoroanthracenes (2) relative to 4-substituted fluorobenzenes (3) has been attributed to a-inductive On the other hand, Adcock and co-workers have argued that polar field contributions are more important than a-inductive effects, based on I9Fchemical shifts for the systems 4-tkd
10
h/
This seemed inconsistent with the claim that field/inductive effects are unimportant in 4-substituted biphenyl derivatives5 To test the latter conclusion, 13Cchemical shifts were measured for 4-ammoniobiphenyl in CF3C02H. These shifts, expressed relative to the isoelectronic 4-methylbiphenyl in the same solvent12 (see Table I), indicate a-electron density c changes in the unsubstituted phenyl group which are identical with 10. Therefore, we correlated the I3Cchemical shifts for the other 4-substituted biphenyls5 with Taft's 01 and q0 constants.13,14 Results of these correlations are summarized 4 5 N in Table I. The pattern of pi values is amazingly similar to the actual chemical shifts for 4-ammoniobiphenyl and to the chemical shift and a-electron density patterns in phenylalkane derivative^.^ Since I3C chemical shifts in similar systems appear to reflect r-electron density these results indicate that polar substituents successively polarize the two phenyl groups. Thus, a polarization appears to be an important Schulman and co-workers5 have analyzed I3C chemical shifts mechanism for transmission of polar effects in the biphenyl system. The failure of Schulman et al. to detect this effect can for 4-substituted biphenyls (9) using the F M M F approach to substituent effects6 I n contrast with both other groups, they be attributed to a weakness in the F M M F treatment; it assumes that the only polar effects are direct field effects.h concluded that the I3C chemical shifts are insensitive to field/inductive effects and that only mesomeric effects are Molecular orbital calculations for ammonio derivatives of important. Thus using similar aromatic derivatives, different 1 have shown that phenyl a polarization leads to proportionate groups have either'concluded that a-inductive effects conchanges in the Zp(r)-electron density of the attached f l ~ o r i n e . ~ tribute to I9F chemical that polar electric field effects Results for further derivatives (Table 11) show a close parallel are more i m p ~ r t a n t ,or ~ .that ~ neither effect is i m p ~ r t a n t . ~ between carbon and fluorine a-electron density changes and p~ values for these derivatives, confirming that the U I depenBased on I3C chemical shifts and molecular orbital calculations for ~ h e n y l a l k a n e styrene,8 ,~ and phenyla~etylene~ dedence of 1 and the enhanced (TI dependence for 2 relative to 3 rivatives, we concluded that field-induced polarization of the reflect a-polarization effects. This explanation can also account for the observation that p~ decreases more rapidly than pi as phenyl a-electron system is an important mechanism of transmission of polar substituent effects. Further evidence for phenyl group conjugation decreases in 1' since a polarization a polarization has been reported r e ~ e n t l y . W ~ ~e ~now ~ , ~ ~ involves both a conjugative component and a through-space component which is present even when conjugation is depresent evidence demonstrating significant a polarization in s t r ~ y e d .Thus ~ , ~ the major features of the field/inductive de1-9, thus reconciling the contradictory conclusions of other d
Journal of the American Chemical Society
/ 98:23 /
November 10, 1976
1297
Table I. (a) I3C Chemical Shifts for 4-Ammoniobiphenyl (in CF3COOH) Relative to 4-Methylbiphenyl and (b) Correlations of I3C Chemical Shifts for Other 4-Substituted Biphenyls with
(TI
and
URO
Correlations without halogens
Correlations with halogens Carbon 4 3 2 1' I' 2' 3' 4'
6
1
3
~
-9.64 -5.82 +1.92 +5.97 -2.33 +0.19 +0.28 +1.45
Plb
~
PR
r"
SDd
PI
PR
r
SD
-1.21 f 0.53 f 1 5 . 0 5 f 0.67 -1.2550.27 +1.02f0.13 +0.32 f 0.07 +2.13 f 0.04
0.799 0.993 0.956 0.949 0.905 0.999
0.54 0.69 0.27 0.14 0.07f 0.04
f 1 . 9 0 f 0.23 +6.26 f 1.08 -3.17f0.16 +0.41f0.22 +0.49 f 0.11 +2.00 f 0.06
-0.84 f 0.15 f 1 4 . 9 6 f 0.70 -1.14fO.lO +1.11fO.14 +0.26 f 0.07 +2.09 f 0.04
0.968 0.996 0.997 0.973 0.960 1,000
0.14 0.66 0.09 0.13 0.06 0.04
e
e +2.87 +5.77 -2.86 +0.53 +0.37 +1.94
& 0.78
f 0.99 f 0.39 f 0.19 i 0.11 f 0.06
(' I3C chemical shifts in ppm relative to 4-methylbiphenyl. Low field shifts are positive. Correlations were also performed using the same sign convention. Weighting factors in correlation: 6 = 60 pIuI ~ R U R " Experimental . data from ref 5. Multiple correlation coefficient. Standard deviation between calculated and experimental values. No correlations were performed for C(4) and C(3) since these carbon shifts may not reflect changes in ground-state electron density.' /Standard deviation comparable with experimental error.
+
+
Table 11. Comparison of Carbon and Fluorine 7-Charge
NH;
Densities (X103) for Ammonio Derivatives of 1, 2, and 3 and p1 Values from I9F Chemical Shift Correlations for the S a m e Derivatives in CCIq
NH; -53
(A)
+6
NH;
16
*20
Compd
G
1 1 1 1 1 3
-CH2-N=CH-C H=C H -N=l\i-
qC*'
+199 +329 +311 +329 +349 +564 +I155
-
