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.
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