Rotational Isomerism in Chloroacetyl Halides

Infrared and Raman spectra are reported for the acid fluoride, chloride, and bromide derivatives of chloroacetic acid. Data from vapor, liquid, and solid spectra are given. The results show that these acid halides exist in the vapor and liquid states as a mixture of two isomers, only one of which remains in the solid state. The more stable isomer was found to have the trans conformation. Calculations of the normal vibrations using a Urey–Bradley potential function serve to indicate that the other isomer has a staggered conformation with an azimuthal angle close to 180°. The measured enthalpy differences ${\mathrm{(H°}}_{\mathrm{trans}}{\mathrm{\hspace{0.17em}-\hspace{0.17em}H°}}_{\mathrm{staggered}})$ between the isomers in the vapor state were $\text{−\hspace{0.17em}0.76\hspace{0.17em}±\hspace{0.17em}0.2}\mathrm{kcal}/\mathrm{mole}\text{, −\hspace{0.17em}0.97\hspace{0.17em}±\hspace{0.17em}0.2}\mathrm{kcal}/\mathrm{mole},\mathrm{and}\text{−\hspace{0.17em}1.86\hspace{0.17em}±\hspace{0.17em}0.27}\mathrm{kcal}/\mathrm{mole}$ for the acid fluoride, chloride, and bromide, respectively.