Correlation Times and Quadrupole Coupling Constants in Liquid N-Methylformamide and N-Methylacetamide

Abstract

Longitudinal ¹H, ¹³C and ¹⁵N nmr relaxation rates were measured in the liquid phase for N-methylformamide (NMF) over the temperature range 228–328 K and for N-methylacetamide (NMA) over the range 300–378 K. Rotational correlation times were determined for the formyl C-H and the N-H bonds. The motion of both molecules is anisotopic, particularly that of N-methylacetamide. The activation energies are 5.6 kJ/mol for C-H and 19.7 kJ/mol for N-H in NMF and 20.4–22.5 kJ/mol for N-H in NMA. Quadrupole coupling constants were determined by relaxation measurements for the nuclei ¹⁴N and ²H. The quadrupole coupling constant for the amide deuteron in NMF is temperature dependent and varies from 193 to 206 kHz. For NMA this quadrupole coupling constant falls into the range 178 to 238 kHz. The quadrupole coupling constant for the formyl ²H in NMF (151–169 kHz) is temperature independet while for the ¹⁴N (NMF: 1.85–2.40 MHz; NMA: 1.65–2.45 MHz) it is temperature dependent.