I. The Crystal Structures of Mono- and the Two Dimethylureas. II. The Determination and Use of Crystallographic Parameters

Citation

Hybl, Albert (1961) I. The Crystal Structures of Mono- and the Two Dimethylureas. II. The Determination and Use of Crystallographic Parameters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/MCYS-5G51. https://resolver.caltech.edu/CaltechETD:etd-06142006-101757

Abstract

Part I: The crystal structures of mono-, and the two dimethylureas were determined by x-ray crystallographic methods. The positional and anisotropic temperature parameters of all heavy atoms were refined by the method of least-squares. All of the hydrogen atoms bonded to the amide nitrogen atoms were found in difference map sections computed in the planes of the molecules. Preferred orientations were found for the two methyl groups on N,N-dimethylurea, but the methyl groups in N-methylurea and N, N'-dimethylurea appear to be rotating. The preferred methyl orientations are neither the perfectly staggered nor eclipsed arrangements and appear to depend, in part, on the manner of packing of the molecules to form hydrogen bonded chains in the crystal. The average bond distances were found to be: C=0, 1.253 A; c1-N (carbonyl C), 1.336 A and C2-N (methyl group C), 1.447 A. The partial double bond character is calculated to be 40% for the C=O bond and 30% for the C1-N bonds. The methyl groups appear to make little change in the contributions of the resonance structures to the hybrid structure. The average out of plane distance for the heavy atoms is 0.02 A, the maximum being 0.05 A. The various types and configurations of cis- and trans-amide hydrogen bonds are reviewed and classified by the type of hydrogen bonded chain structure formed. Part II: Expressions employable for the calculation of structure factors and their derivatives for any orthorhombic space group are developed. Complementing this work is a general description of the least-squares method as used to determine a parameter set. Several methods for the conversion of a parameter set into more perceptible quantities, such as interatomic distances, bond angles, planarity of groups of atoms, magnitudes and direction cosines of the principal axes of the vibrational ellipsoids of each atom and the rigid body representation of the thermal displacements and vibrations, are reviewed.

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