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Neutron diffraction studies of y-glycine, C2HsNO2, have been carried out at 83 K and 298 K using the same crystal. Intensities of 2235 (83 K) and 1426 (298 K) reflections were measured at the Brookhaven National Laboratory High Flux Beam Reactor and the structures were refined by full-matrix least-squares techniques to yield final agreement factors of R(FE)83K = 0-024 and R(F2)E98K : 0.025. The crystal was assigned space group P32 with a cell of dimensions" a --7.046 (3), c = 5.491 (2) A at 298 K and a = 6.975 (2), c = 5.473 (2) A at 83 + 0.5 K. The observed thermal parameters have been analyzed according to a general rigid-body model in order to study the effect of cooling on the molecular dimensions and on the thermal motion of the atoms. Large differences in the hydrogen-bond distances were found. The N...O hydrogen-bond lengths were observed to be 0.02-0.05 A longer at room temperature than at 83 K. These differences are accompanied by substantial changes in the lattice constants. The a and b axes contract more upon cooling than does the c axis, and this observation may be related to the fact that the largest principal axis of libration of the molecules lies nearly along c.

The precise molecular configurations of the amino acid serine (C3H~NO3) in crystals of L-serine monohydrate and DL-serine have been refined by neutron diffraction techniques. The neutron diffraction data collected for DL-serine were used in conjunction with earlier X-ray results to refine the atomic structural parameters. The unknown structure of L-serine. H20 was solved by direct methods, thus confirming that the negative scattering length for hydrogen in neutron diffraction is not an obstacle to the use of such methods even with a comparatively high percentage of scattering by hydrogen atoms [~b2/all ~omsb~ = 0"26]. Both compounds show the characteristics of most of ths crystalline amino acids previously studied: The serine molecules are zwitterionic. There are three-dimensional networks of hydrogen bonds. The calculated barriers to rotati3n of the ammonium groups are 7.3 kcal mole -1 in L-serine. H20 and 10.9 kcal mole -1 in DL-serine. The molecular packing in the two crystals is remarkably similar. There are structurally identical layers parallel to the main faces of the two crystals. In Lserine. H20 these layers are tied together by hydrogen bonds formed through interleaving sheets of water molecules. A previous X-ray study of anhydrous L-serine [Benedetti, Pedone & Sirigo (1972). Cryst. Struct. Commun. 1,[35][36][37] shows that the packing in this structure is quite different. Statistical tests indicat.z that the portion of the serine molecule not involved in hydrogen bonding has the same geometry in both crystals we have studied and in anhydrous L-serine, to the limits of experimental accuracy. In L-serine. H20, a(X-X) and a(X-H) are respectively 0.003, 0.006 A, and in DL-serine 0"001, o.ool A.