Quantitative analysis of hydrogen bonding and atomic thermal motion in the organic non-linear optical material DCNP using X-ray and neutron diffraction

A single-crystal neutron diffraction study of the organic non-linear optical material 3-(1,1-dicyanoethenyl)-1-phenyl-4, 5-dihydro-1H-pyrazole (hereafter DCNP), space group Cc, is presented. The study was conducted in order to relate the structural characteristics of the compound to its physical properties. DCNP exhibits a very large second harmonic generation (SHG) output, an extremely large linear electro-optical effect and photoconductive and pyroelectric properties. The nature of the hydrogen-bonding revealed by the study, in part, accounts for the first two of these phenomena. The neutron study also shows that some rather atypical atomic thermal motion is present in part of the molecule. With the aid of a variable-temperature single-crystal X-ray diffraction study, in conjunction with the neutron study, this thermal motion is attributed to libration and is fully characterized. As a result, suitable corrections to the bond geometry and the anisotropic displacement parameters of DCNP are made. The libration is also shown to enhance the SHG effect. The cell parameters from the variable-temperature X-ray study are also used in order to evaluate the thermal expansivity coefficients of DCNP.