Intramolecular Charge Distribution Research Articles

Polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystal

The polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystals containing structurally asymmetric NHN + bridges were studied at room and liquid nitrogen temperatures. It was shown that the broad intense absorption over the frequency range 200–3200 cm −1 should be assigned to the protonic ν(NHN) stretching vibrations. The transmission windows can be ascribed most probably to a coupling of ν(NHN) vibrations with deformation δ(NHN) and γ(NHN) modes and their overtones as well as with internal vibrations of the pyridine ring which lead to a modulation of the bridge geometry. The behaviour of NH 2 groups in polarized i.r. spectra confirms the inequivalence of 4-aminopyridine moieties in homoconjugated cations both from the point of view of intramolecular charge distribution and environment of NH 2 groups in the lattice, in agreement with the diffraction data.

On the representation of the electric charge distribution in ethane for calculations of the molecular quadrupole moment and intermolecular electrostatic energy

It is shown that a simple atom-site monopole model is inferior to one which includes higher-order local multipoles to represent the intramolecular charge distribution of ethane. Unlike the latter model, the local monopole representation predicts the wrong sign for the molecular quadrupole moment and gives a repulsive rather than an attractive electrostatic interaction at typical intermolecular distances. In the local multipole model, the atom-site dipoles give the largest contribution to both the molecular quadrupole moment and the intermolecular interaction.