Force Constants and Normal Vibrations of the Propargyl Halides

Several modifications of a Urey—Bradley field combined with a valence force field have been used for the calculation of the normal coordinates of the propargyl halides CH2X–C≡C–H (X=F, Cl, Br, I). The force constants obtained by a least-squares method have been compared in terms of the structural changes introduced by the halogen atom. A kinetic effect seems to account for the splitting of the –C≡C–H in-plane and out-of-plane bending modes.