A- and B-type hydrogen bond modifications of potassium hydrogen tartronate
ACCORDING to Speakman1 two types of intermolecular hydrogen bonds between carboxyl groups can be discerned in the crystal structures of acid salts of carboxylic acids. The first, type B, occurs when two carboxyl groups are not symmetry related, and have structural differences sufficient to characterise them, separately, as unionised (−CO2H) and ionised (−CO2−). Thus, any hydrogen bond between these groups must be asymmetric. The second, type A, occurs when two carboxyl groups are related by a symmetry element of the crystal, so that they are effectively equivalent. Thus any hydrogen bond between the groups must be, at least formally, symmetrical.
X-ray Evidence for the Acentric Position of the Hydrogen Atom in a Short A Type Hydrogen Bond
IN an X-ray analysis of potassium hydrogen mesotartrate performed with an automatic diffractometer (Mo K radiation; 5,300 independent reflexions) at 120 K (Table 1 and Fig. 1) we have detected three short hydrogen bonds. Two of these connect oxygen atoms that are symmetry related in the assumed space group P1. Speakman et al., who have worked extensively on short hydrogen bonds in acid salts1, call these symmetrical bonds A type H-bonds, whereas the usual asymmetrical arrangement is called B type. During the least squares refinement of potassium hydrogen mesotartrate the A type hydrogen atoms were kept fixed at the crystallographic symmetry elements (1). In order to study in more detail the electron density in the O–H … O planes appropriate difference Fourier sections with coefficients Fobs−FcalcnH were calculated (only atoms other than hydrogen contribute to Fobs−FcalcnH). The result for one of these bonds (A type (1)) is shown in Fig. 2.
Timber, its nature and behaviour
Preface. Acknowledgements. Chapter 1 - Structure of Timber. Chapter 2 - Appearance of Timber in Relation to its Structure. Chapter 3 - Mass Volume Relationships. Chapter 4 - Movement in Timber. Chapter 5 - Flow in Timber. Chapter 6 - Deformation Under Load. Chapter 7 - Strength and Failure in Timber. Chapter 8 - Durability of Timber. Chapter 9 - Processing of Timber. Index.