PROTON MAGNETIC RESONANCE MEASUREMENTS OF FORMAMIDE

Abstract

The high-resolution proton resonance spectrum of formamide has been measured and analyzed. Quadrupole broadening of the resonance signals due to the N 14 nucleus was eliminated by substitution of the isotopic nucleus N 15 . Twenty-four lines were resolved in the spectrum, which is consistent with restricted rotation about the C—N bond. The non-equivalence of the two protons in the N 15 H 2 group is characterized by a smaller screening constant and a greater spin–spin interaction of the proton which is in the trans position relative to the carbonyl group. It is suggested this difference in the spin coupling of the two protons may be associated with different N—H bond lengths. Pronounced changes in the chemical shifts and spin-coupling constants result when formamide is dissolved in dilute solution in water and acetone. In water solution the relative magnitudes of the spin coupling of the two protons in the NH 2 group is reversed from that observed in pure formamide and the coupling of the aldehyde proton to N 15 varies by nearly 50%. Heating of formamide above room temperature causes rotation of the NH 2 group as well as proton exchange between neighboring molecules. The two effects could be separated. From the observed changes in the proton spectra as a function of temperature the barrier hindering rotation (measured in a 10 mole% solution of formamide in acetone) was found to be 18 ± 3 kcal/mole. The activation energy for proton exchange in pure liquid formamide was found to be 10 ± 3 kcal/mole.