The structure of histidine hydrochloride monohydrate has been refined by three-dimensional least-squares methods. The amino acid part of the molecule is in the zwitter ion form; the imidazole ring is positively charged. The distances in the imidazole ring, combined with some previous results, are satisfactorily interpreted with the valence bond theory, and imply a carbon-nitrogen double-bond distance of 1.265 /~. All available hydrogen atoms form hydrogen bonds.

The molecular structure of B5H11 has been determined from 299 x-ray diffraction maxima which have been analyzed by three-dimensional Fourier and least-squares methods. There are four molecules in a monoclinic unit cell, in the space group P21/n, having parameters a=6.76, b=8.51, c=10.14A, and β=94.3°. Although no molecular symmetry is demanded by the space group, the molecular dimensions indicate C8 symmetry for the isolated molecule. The boron skeleton is a fragment of the icosahedronlike arrangement in B10H14, and can also be derived from the tetragonal pyramid in B5H9 by opening up one of the basal B–B bonds. The apex boron BI is ``singly'' bonded to two hydrogens, one of which HVII, is very weakly bonded to the two outer borons BIII. Each BIII is ``singly'' bonded to two hydrogen atoms. Each of the remaining two borons BII is ``singly bonded'' to one hydrogen atom, and there are three bridge hydrogens, one bridging the BII–BII bond and two bridging the BII–BIII bonds. Molecular parameters are two BI–BIII=1.865, two BII–BIII=1.75, two BI–BII=1.72, one BII–BII=1.77A, seven B–H=1.07, six B–H (bridge) =1.24A, one BI–HVII=1.00 and two BIII–HVII=1.67A. The BIII–BI–BIII angle is 107°. Bond angles to hydrogen strongly resemble those in the other known boron hydrides.

Two programs have been coded and used for least-squares refinement by the ElectroData digital computer with three-dimensional data for crystal structures of classes 2/m and 222. Relevant details of the computer are given. The method of calculation and the forms of input and output are described. The rate of operation of each program is stated. An expression for predicting the value of the minimisation function has been tested.

No abstract