COMMUNICATION TO THE EDITOR
768
Vol. 66
COMMUNICATION TO THE EDITOR VARIATIOXS I N T H E COUPLIKG COXSTANT BETWEEN 13C AND DIRECTLYBONDED FLUORINE Sir: A few measurements of the coupling constant J l q - F in saturated systems have been made,1.2 but only with carbon, fluorine, and hydrogen atoms directly bonded to the I3C atom. We have evaluated this coupling constant in the presence of other directly-bonded substituents (Table I). The values exhibit correlations with the position of these substituents in the periodic table. Increase in the atomic number of a substituent atom within a given group of the periodic table is accompanied by an increase in JlaC-F. Examples of this regularity are seen in the pairs of molecules CF4 and CC18F, (CF& and (CF3Se)2r(CF8S)2Hgand (CF3Se)2Hg, (CF2C1)2 and (CF2Br)2, and (CFa)2NN02 and (CF&PCl. The effect of the atoms directly bonded to the 13Catom appears to be greater than that of remote substituents. This is not always the case for the analogous proton coupling constant, JIPC-H (where both the absolute magnitude and the amount of variation are smaller), since its value in methyl groups directly bonded to carbon or nitrogen are similar in m a g n i t ~ d e . ~As the atomic number of a directly-bonded substituent increases within a given row of the periodic table, J1aC-F decreases, as can be seen for molecules of the type CF3X. The lowest value found is that of 259.2 c./s. for CF4 itself. The value for (CF3)2NKO~ is 273.6 c./s., while for the two CFaC compounds listed, the coupling constants are 282.5 c./s. and 283.2 c./s.1 Similarly in the second row of the periodic table the value for (CF&PCl is greater than any of the three values given for C F B compounds. These variations are of lesser magnitude than those within a group of the periodic table, noted above, and are opposite in sign to differences observed for J1aC-H.' It is of interest to note from Table I that the value of JlSC-F for CFsH is greater than that for CF4,z being roughly the same as that for (CF&.NKO2. MalinowskiShas attempted to show an additivity relation for JlSC-H for compounds of the type CHXYZ using contributions according to the equation Jlac-~I = bx by {z, and he has obtained good agreement between predlcted and observed values. A similar procedure for (13C,F) coupling constants gives reasonably consistent
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(1) G. V. D. Tiers, J . P h w . SOC.Japan, 16, 354 (1960). ( 2 ) S. G. Frankiss, t o be published. (3) E. R. Malinowski, J . AmqChem. SOC., 88, 4479 (1961).
TABLE I VALUESOF THE ( W , F ) COUPLING CONSTANT FOR VARIOUS SUBSTITUENTS, AND THE ASSOCIATED ZETA VALUES The compounds are arranged in the order of the atomic number of the element for which zeta is quoted. hIolecule
Jiac-F
Atom directly bonded t o 1aC
(a) Trifluoromethyl 259.22 274.32 282.51 283.21 273.6 320.2 313.8 308.3 309 331.3 337.1 332.5
r
compounds F 86.4' H 101.5 C 109.7 C 110.4 N 100.8 1' 147.4 141.O S S 135.5 136. S 158.5 Se 164.3 Se 159.7 Se
(b) Other compounds 234. 82 H 74.2 157.42 H 52.5 298.0 C 105.8 336.5 112.2 c1 104.4b 299.0 c1 115.7b 311.6 Br a This value is used in the derivation of all the others. Assuming CC = 108. c The 1% satellite bands of this compound have broadened triplet structure (with 1: 1: 1 relative intensities), the splitting being 3.0 c./s. Details of the fine structure of the other compounds will be published later.
values for the contributioiis of directly-bonded substituents (Table I), with the notable exception of hydrogen ({H varies markedly in the series CFaH, CE'2H2, CFH3).2 This procedure emphasizes the variation of J1aC-F with the atomic numbers of the substituents, since we obtain {P > { S > (a; bse > l B r ; and {B~ > {GI > {F. These empirical correlations may be of use in confirming proposed structures. For example, the value of JlaC-F for CF3SKC04 strongly supports the evidence that the perfluoromethyl group is attached directly to the sulfur atom. The author is grateful to the Inorganic Chemistry Department, Cambridge University, for the samples of the sulfur- and selenium-containing compounds; and to the Department of Scientific and Industrial Research for a research studentship. (4) H. J. Emelbus and A. A. Haas, private communlcation.
THEUNIVERSITY CHEMICAL LABORATORY LENSFIELD ROAD R. K. HARRIS CAMBRIDGE, ENGLAND RECEIVED JANUARY 29, 1962
