The Harwell electron linear acceleratorhelios has been used for studying the crystal structures of polycrystalline materials by neutron diffraction. Atomic positions and atomic vibrational amplitudes can be determined equally well with the pulsed method as with the more conventional method employing a high-flux nuclear reactor as source. The powder pattern can be indexed more easily with pulsed neutrons.

The molecular structure of the predominant monocarboxylic acid, E2, obtained by mild acid hydrolysis of cyanocobalamin has been determined by neutron diffraction with some support from x-ray diffraction. The undried crystals, formula probably C₆₃H₈₇O₁₅N₁₃PCo l6H₂O, have unit cell parametersa = 14.91(1) A, b = 17.49(l)A, c = 16.41(1)A, β = 104.11(5)°; space group P2₁, Z = 2.

The analysis was carried out in two stages with data extending to 1.3 A (1531 terms) and to 1.0 A (2993 terms) respectively. It was initiated by the use of coordinates of 84 atoms from the parallel x-ray analysis to phase the first Fourier series. The atomic positions derived from the 1.3 A data set appeared in good agreement with the chemical evidence both on the corrin structure and on the position of the acid group ate. However the analysis by Fourier and least squares methods on the extended data defined the atomic positions much more clearly and showed that diffraction ripples had distorted some of the hydrogen atom positions in the low resolution map. The acid group appeared clearly placed in the higher resolution map at theb position. The positions of disordered atoms in thee chain and some water molecules were checked with the parallel x-ray analysis.

It seems most likely to us therefore that this acid is α-(5,6-dimethylbenzimidazolyl)cobamic acida,c,d,g,e pentamide cyanide.