Substituent effects on nuclear spin–spin carbon–carbon coupling constants in derivatives of acetylene

Abstract

Abstract 1 J ( 13 C 13 C) nuclear spin–spin coupling constants in derivatives of acetylene have been measured from natural abundance 13 C NMR spectra and in one case (triethylsilyllithiumacetylene) from the 13 C NMR spectrum of a 13 C‐enriched sample. It has been found that the magnitude of J (CC) depends on the electronegativity of the substituents at the triple bond. The equation 1 J ( 13 C 13 C) = 43.38 E x + 17.33 has been derived for one particular series of the compounds Alk 3 SiCCX, where X denotes Li, R 3 Sn, R 3 Si, R 3 C, I, Br or Cl. The 1 J (CC) values found in this work cover a range from 56.8 Hz (in Et 3 SiCLi) to 216.0 Hz (in PhCCCI). However, the 1 J (CC) vs E x equation combined with the Egli–von Philipsborn relationship allows the calculation of the coupling constants in Li 2 C 2 (32 Hz) and in F 2 C 2 (356 Hz). These are probably the lowest and the highest values, respectively, which can be attained for 1 J (CC) across a triple bond. The unusually large changes of the 1 J (CC) values are explained in terms of substituent effects followed by a re‐hybridization of the carbons involved in the triple bond. INDO FPT calculations performed for two series of acetylene derivatives, with substituents varied along the first row of the Periodic Table, corroborate the conclusions drawn from the experimental data.