X-ray and neutron diffraction studies of the non-linear optical compounds MBANP and MBADNP at 20 K: charge-density and hydrogen-bonding analyses.

Neutron-diffraction studies of the compounds 5-nitro-2-[[1-phenylethyl]amino]pyridine (methylbenzylaminonitropyridine, hereafter MBANP) and 3,5-dinitro-2-[[1-phenylethyl]amino]pyridine (methylbenzylaminodinitropyridine, hereafter MBADNP) are presented, and a charge-density study of the latter is reported. The studies were conducted in order to relate the structural attributes of these materials to their physical properties. MBANP exhibits a second-harmonic generation (SHG) output, chi(2), over eight times higher than that of MBADNP, despite their very similar molecular characteristics and the seemingly more SHG-favourable molecular features present in MBADNP. The neutron-diffraction studies show that intramolecular hydrogen bonding is responsible for this apparent discrepancy. The charge-density study on MBADNP confirms this and reveals that the pyridine group is the principal moiety responsible for the SHG effect on the molecular scale. Moreover, the strong intramolecular hydrogen bond present in MBADNP is proven to result from an electrostatic interaction. The dipole moment of MBADNP is also deduced from the charge-density study.