NMR Properties of Formamide: A First Principles and Experimental Study

The tensors corresponding to the second-rank NMR observables, nuclear shielding, quadrupole coupling, and spin-spin coupling of formamide (HCONH2, FA) were determined using several first principles quantum chemical methods. The changes induced on the shielding and quadrupole coupling tensors by intermolecular hydrogen bonding were examined computationally. Liquid crystal NMR experiments were performed for dissolved FA in the SDS and CTAB lyotropic mesophases and their isotropic phases and in the gas phase. We report experimental data on shielding, quadrupole coupling, and spin-spin coupling constants. The convergence of the calculations with the basis set completeness and the treatment of electron correlation were investigated. The calculated and experimental data on the anisotropic properties of the C, N, and O shielding tensors are found to be in good agreement, given the large error limits of the latter caused by the low degree of order of FA in these systems. The medium effects on the observables are found to be readily understood by comparison of structurally relaxed FA monomer and chain trimer calculations. The calculated spin-spin coupling constants are in good agreement with the experimental ones. The anisotropic properties of the corresponding tensors are calculated to be small enough to prevent experimental detection and not to disturb structure determinations by using experimental dipolar couplings. The principal components and the orientation of the principal axis systems of each of the NMR tensors are specified.