Molecular and Supramolecular Origins of Optical Nonlinearity in N-Methylurea

The delicate balance between solid-state intermolecular interactions and electron-donating methyl-group influences in N -methylurea (NMU) is shown to distinguish its nonlinear optical properties, relative to those of urea, a standard reference material for second harmonic generation (SHG). The solid-state intermolecular interactions in NMU are identified using neutron diffraction data, showing that hydrogen bonding generates an extensive 3D supramolecular network of NMU molecules with secondary and tertiary nonbonded contacts helping to hold this network in a closely packed form. The undulating “urea tape” motif within this network renders an overall packing arrangement that is less SHG-favorable than that of urea, which exhibits a more head-to-tail molecular alignment. The primary, secondary, and tertiary nonbonded contacts are classified using graph-sets, Hirshfeld surfaces, and fingerprint plots. H···H contacts in NMU contribute to the overall Hirshfeld surface area much more than in urea, forming at the expense of O···H interactions. However, SHG-contributing electronic effects of the methyl group in NMU provide some compensation to these hydrogen-bonding influences. This methyl group is also shown to librate, which could augment SHG. Our experimental results offer a direct response to previous density functional theory calculations on NMU and urea,1 corroborating their predictions as well as enabling a better relationship between the molecular and bulk optical nonlinearity of NMU. To that end, crystal engineering options are discussed with a view to balancing these seemingly conflicting structural attributes, so that one can produce an SHG-active form of NMU that is superior to urea.