The Planarity of the Urea Molecule

Abstract

Arguments based on infra-red observations were recently presented by W. E. Keller for the planarity of the urea molecule in the crystalline state. Though plausible, we have not found these completely convincing since the statement that in a nonplanar model one of the A_1 hydrogen bending fundamentals should be essentially inactive is based on certain assumptions, and not on a selection rule required by symmetry. The really conclusive evidence is to be found in the 3µ region where the resolving power of the spectrometer employed by Keller seems to have been inadequate. In the planar model four N-H valence frequencies may be active, two of which have the character A_1. In the nonplanar C_(2v) model only three of these fundamentals can be active, and only one of these has the character A_1.