NOTES
Dec., 1958 SOLVENT EFFECTS I N THE NUCLEAR MAGNETIC RESONANCE SPECTRA OF FLUORO-ORGANICS BY R.E. GLICKAND S.J. EHRENSON Contribution jrom the Department of Chemistry, Whitmore Laboratory, The Pennsuluania State Universitv, University Park, Pennsyluania. Received June 1968
The effect of solvent on the position of a fluorine high resolution nuclear magnetic resonance line has been reported2 recently to deviate substantially from that predicted by the magnetostatic model.a In this connection we have examined the behavior of the nuclear magnetic resonance frequency of four fluorine containing materials each extrapolated to infinite dilution in approximately forty solvents. The measurements were made on a Varian High Resolution nuclear magnetic resonance spectrometer a t 40 mc. with spectral shifts obtained by the usual frequency measuring techniques. *J The results for one of these materials, 1,2-dibromotetrafluoroethane (Fl), for a typical series of solvents are recorded in Table I, column 1. Column 2 contains a diamagnetic susceptibility correction as determined from the behavior of a proton containing material in the same solvent. (The latter, therefore, contains the experimental defect in Column 3 is the sum of columns 1 and 2 and is thus the deviation of F1 from equation 1. Another compound examined was benzotrifluoride in order to compare the solvent behavior when both hydrogen and fluorine were present in the same molecule. In this compound the hydrogen resonance frequency varied, essentially, according to equation 1 , 6 while the fluorine shifts paralleled thoseQf F1. The deviations, AH", listed in column 3 are found to be proportional to the molecular polarization ( P ) of the solvent molecule,6 as listed in column 4, according to equation 2 A H + AH' = AH" = -3.8P + 62 (2) Thus, the experimental value of AH for Fl in the gas phase as compared to that in infinite dilution in carbon tetrachloride is found to be 167 f 5 cycles while the value determined from equation 2 is 176 f 8 cycles. A similar extrapolation through the (1) This work was supported in part by the Office of Naval Research, Project NR055-328. Reproduction in whole or in part is permitted for any purpose of the United States Government. (2) D. F. Evans, Proc. Chem. SOC.,115 (1958). (3) The magnetostatic model is given by equation 1.
H i
=
siHo(1
- LYK)
(1)
For ( l ) , Ha is a reference resonant field (for a proton, Ho is the resonant field for the bare nucleus i n vacuo), si is a specific shielding factor and is a function of the electron environment of the ith nucleus, (I is a shape factor, x is the magnetic susceptibility of the medium, and H i is the observed field for the ith nucleus. Using the Lorena-Lorenta approximation for the molecular cavity and a cylindrical sample cell oriented transversely to the magnetic field, (I has the theoretical value of - 2 ~ / 3 (-2.09). The variations of proton resonance frequency with solvent have been found to follow equation 1 if a i s given the value of -2.60.' (4) A. A. Bothner-By and R. E. Glick, J . Chem. Phys., 26, 1647 (1957). (5) A. A. Bothner-By and R. E. Glick, ibid., 1651 (1957). (8) Y . K. Syrkin and M. E. Dyatkina, "Structure of Molecules and the Chemical Bond," Interscience Publishers, Inc., New York, N. Y., 1950, p. 201.
1599
three tetrahalo-methanes listed in Table I was made for carbon tetrafluoride. In this instance AH for the gas phase and that for infinite dilution in carbon tetrachloride had a theoretical value of 369 f 15 cycles while that determined experimentally by Evans2was 369 f 3 cycles and in this study was 370 f 3 cycles. TABLE I F1 ext. to m dilution in
1
2
3
AHa,b cycles
AH',C cycles
AH"-d cycles
CHzClz CHC13
+ +
+
4 Polariaability of solvent (P) cc.6
0 2.1 2.1 16.4 -25.0 4.2 -20.8 21.3 cc4 -32.0 - 1.2 -33.2 26.1 CHzCIBr -27.0 +12.3 -14.7 19.3 CHzBrz -58.0 +22.3 -35.7 22.1 CHClzBr -45.0 +12.9 -32.1 24.1 CHClBrz -67.0 +21.3 -45.7 27.0 CHBrs -77.0 +27.1 -49.9 29.8 -48.0 6.2 CC13Br -41.8 28.9 CClzBrz -68.0 $12.9 -55.6 31.8 A H values are given in frequency equivalents referred to a fixed fre uency of 40 mc. b A H is the frequency shift from that oypure F1. AH'is the frequency shift for a pure proton containing material of the same diamagnetic susceptibility as F1 to infinite dilution in the given solvent. AH"is the deviation of F1 from equation 1. Calculated from ref. 6.
+
Since solvent polarization (including self-polarization in the pure liquid) contributes substantially to the fluorine resonant frequency, intermolecular influences may be evaluated exactly by an extrapolation to zero polarization. This procedure contrasts markedly with that for standard hydrogen chemical shift assignments where measurements in magnetically isotropic media such as carbon tetrachloride are ~ u i t a b l e . ~Nevertheless, proton resonance, as evidenced by the deviation noted in a, equation 1, may be influenced by solvent polarization. These results are presently being extended, both theoretically and experimentally, to include intramolecular chemical shifts as well as solution 'behavior in general. We wish to thank Professor L. A. Currie for assistance in preparing the gas eamples. DISPLACEMENT REACTIONS AT THE SULFUR ATOM. 11.' THE REACTION OF CYANIDE WITH TETRATHIONATE BYROBERT EARL DAVIS~ Converse MemoricE Laboratory of HarvGrd Universitu. Cambridge, M a s s Received June 18, 1968
Cyanide ion reacts quantitatively with tetrathionate in aqueous solution producing sulfate, thiosulfate and thiocyanate . CN-
+
5406-2
k2 + HzO + + SzO3-2 + SCN- + 2H+ Sod-2
(1)
Indeed all of the higher polythionates react quantitatively and the reaction has been used as an (1) Part I. R. E. Davis, J . A m . Chem. Soc., 80, 3565 (1958). (2) Public Health Service Research Fellow of the National Cancer Institute. Massachusetts Institute of Technology, Cambridge, Mass.
