The structure of urea. Interatomic distances and resonance in urea and related compounds

Abstract

ASYMMETRIC X-RAY REFLEXION FROM NATURAL QUARTZ CRYSTALSnatural quartz crystals is similar to that predicted by the dynamical theory for perfect, absorbing crystals. Using the results of the theoretical investigation, it has been possible to obtain experimental F values with an accuracy of ~10% from one crystal, without the necessity for making detailed assumptions about the degree of perfection of its texture. From the combination of observations from a series of crystals it may be possible to increase the accuracy of this method. Higher accuracy may be obtained if the absorbing effect of any projections on the crystal surface can be found from the experimental variation of J(fl) with fl for a weak reflexion.A disadvantage of the method is that the crystals used must be quite large, and show faces suitable for mounting. However, wherever applicable the method leads to far more accurate results than any previous investigation.It is a pleasure to thank Prof. Sir Lawrence Bragg and Dr W. H. Taylor, in whose laboratory this work was carried out, for their constant help and encouragement. I am indebted to Dr S. 0. Agrell, Curator of the Museum of the Department of Mineralogy and Petrology, Cambridge, for the provision of the two quartz crystals. I would also like to thank my colleagues, Dr P. B. Hirsch and Dr J. S. Thorp, for their invaluable help and advice throughout this work. This research was carried out while the author was in receipt of a maintenance grant from the Department of Scientific and Industrial Research.References DARWIN, C. G. (1914 Revised values for carbon--oxygen and carbon-nitrogen double bonds are suggested, and these values are used to discuss the effect of resonance on interatomic distances in urea and related compounds.

The crystal structures of L-citrulline hydrochloride and L-homocitrulline hydrochloride

“…These values are in close agreement with those found in the related compounds mentioned above. The shape and the size of the carbamylamino group in both crystals coincide well with those reported for urea (Vaughan & Donohue, 1952) and those for the peptide linkage given by Corey & Pauling (1952). The bond distances of the C(2)-N(2) bonds, 1.455 and 1.457 A, are in close agreement with an average value of 1.455 A for the N-C ~ bonds in the peptide linkages (Marsh & Donohue, 1967).…”

The crystal structure of the 1:1 compound between ammonium P,P-dithiophosphacyanurate and urea

“…The C=O length 1.246 (3) A and the C-N distances of 1-326 (4) and 1-328 (4) A are somewhat shorter than the corresponding values, without correction for librational motion, given by Pryor & Sanger (1970) from a neutron diffraction study. The distances 1"973 (11) 1-987 (9) 1"99 1"992 (7) 2"001 (5) 1"968 (7) 1"991 (9) 1"98 1"969 (7) 1"995 (8) (6) 3.124 (4) 2.28 (7) 165 (5) 0"84 (6) 2"826 (4) 2"06 (6) 152 (5) 0"77 (4) 2"956 (4) 2"31 (4) 142 (4) 0"91 (4) 3"015 (4) 2"11 (4) 172 (4) 0"77 (4) 3"428 (3) 2"67 (4) 174 (4) 0-89 (4) 3"522 (3) 2"64 (4) 174 (3) are also shorter than those reported from various refinements of urea based on X-ray data (Vaughan & Donohue, 1952;Caron & Donohue, 1964;Pryor & Sanger, 1970). On the other hand the present values agree well with those given by Mahmoud & Wallwork (1975) in the 1:1 complex between quinol and urea, where C=O is 1.253 (2) A, and C-N 1-332 (3) and 1.335 (3) A.…”

The crystal structure of sodium bicarbonate

“…The 03 • • • 02 distance of 2.595 A and the hydrogen bond C-On"" 02 of 112 ° 56' are as expected. The bond length of 1.346 A found for the C-03 distance is shorter than the single bond value of 1.43 A found in paraffinic alcohols and corresponds to 14 % double bond character (Vaughan & Donohue, 1952). In the frce ion, both the C-O1 and C-02 bonds would be predicted to have 50% double bond character, or a bond distance of 1.255 J~.…”