Diamagnetic Anisotropy Effects of C≡C and C≡N Bonds in Nuclear Magnetic Resonance Spectroscopy

Publisher Website
Google Scholar
Abstract
The previously reported empirical method for determining diamagnetic anisotropy contributions to chemical shifts in nuclear magnetic resonance spectroscopy, based upon a linear relationship between chemical shifts and C13–H coupling constants, has been applied to a series of compounds containing the C≡C and C≡N bonds. The anisotropy effects upon the protons in HC≡CH and HC≡N have been found to be 160 cps and 170 cps, both ±20 cps, respectively, corresponding to a value of Δχ = —16.5×10—6 cm3/mole for both bonds, if the centers of anisotropy are located at the midpoints of the bonds. These values account satisfactorily for the anisotropy effects on methyl and methylene protons in all acetylenic and nitrilic compounds studied. The value of Δχ for C≡C bond obtained by this procedure agrees well with the theoretical estimate of Pople. An attempt is made to separate the C–C σ‐bond contribution from the total C≡C bond anisotropy and thus obtain the ring current effect in acetylene. The value thus obtained compares reasonably well with Pople's theoretical estimate.