77Se-77Se spibspin coupling constants of unsymmetrically and, for the first time, of symmetrically substituted diselenides have beem measured for CF,SeSeCH, , CF,SeSeCF, , CF,SeSeCF2CI and CF,CISeSeCF,CI. A correlation of the 'J(SeSe) values with the average "Se shielding is found for perhalomethyl diselenides and organyl diselenides. "Se-' 'C coupling constants are also reported for the perhalomethyl diselenides. The coupling constants are much larger than in organyl diselenides. NMR data for the thioselenides CF,SeSR (R = CF, , CF,CI, CFCI, , CCl,) are reported for comparison. It can he concluded that n-type interactions occur in the M or Ses bonds of diselenides and thioselenides, respectively, containing perhalomethyl groups. KEY WORDS 77Se-77Se coupling constants 77Se-1'C coupling constants "Se chemical shifts Diselenides Thioselenides * Part VIII of the series NMR Spectroscopic Studies on Chalcogen Compounds. For Parts VI and VII, see Refs l a and lb, respectively.
phenyl diselenide, 'J(SeSe) values of 15.1 and 13.7 Hz, respectively, were reported,' and in a paper on selenium isotope effects on adjacent "Se nuclei two Se-Se coupling constants of 10.4 and 11.5 Hz were reported for CF,SeSeCH, . These values, derived from the highly resolved 77Se NMR spectrum of the compound, were assigned to different species8 This interpretation was criticized, and the observed fine splitting of the signals was attributed to thermal inhomogeneity of the ample.^ However, several arguments which rule out this possibility were discussed.* Recently, 'J(SeSe) values of alkyl methyl and alkyl phenyl diselenides" and dialkyl polyselenides' ranging from 2.7 to 36.3 Hz were published. Hence it appears that typical 'J(SeSe) values of organyl diselenides range from about 2 to 36 Hz.
Attempts to detect Se-Se coupling constants over two or three bonds in selenophene derivatives were unsuccessful;' however, such values could be determined in the cations TeSe:' and Te,Sei', in tetraselenafulvalenes,' ' in dialkylp~lyselenides~ and in mono-, di-and tri-seleno-substituted alkenes.'
EXPERIMENTAL NMR spectra
Samples of diselenides were neat liquids containing 10% C,D, as a deuterium lock. The NMR spectra were recorded on a Bruker WM 250 spectrometer operating at 47.71, 235.36, 62.90 and 250.13 MHz for 77Se, 19F,