Temperature Dependence of the Deuterium Quadrupole Coupling Constants and the Correlation Times for Neat Formamide

Abstract

High-level ab initio calculations yield a linear relationship between the proton chemical shift, δH, and the deuterium quadrupole coupling constant, χD, for the amide protons in neat, liquid formamide. An experimental measurement of δH in combination with the linear relationships provides an accurate value for χD in the hydrogen-bonded liquid; subsequent measurements of the deuterium relaxation rate provide a means to obtain effective rotational correlation times in the neat liquid. Temperature-dependent proton chemical shifts and relaxation rates for the three nuclei in formamide are presented. The chemical shift data and the ab initio calculations indicate that both amide protons hydrogen bond, whereas the C−H proton does not participate in hydrogen bonding in the neat liquid. The effective correlation times at room temperature were found to be 9.2 ps for the trans N−D vector, 6.3 ps for the cis N−D vector, and 5.2 ps for the C−D vector. This indicates that in the neat liquid formamide rotates anisotropically.