A study of intramolecular hydrogen bonding in benzoylacetone (1-phenyl-1,3-butadione) has been carried out with 8.4(4)K X-ray data and 20(1)K neutron data. Analysis of the neutron data shows that the hydrogen, between the two oxygens in the keto-enol part of the molecular structure, is asymmetrically placed in a large flat potential well. The charge density obtained from X-ray and neutron data has been analyzed by using multipolar functions and topological methods, which gave evidence of extensive π-delocalization in the keto-enol group. The multipole populations show that there are large formal charges on the oxygens and the enol hydrogen, which impart polar character to the hydrogen bond. This effect is also evident in the Laplacian and in the electrostatic potential calculated from the X-ray data and it is found that the hydrogen position is stabilized by both electrostatic and covalent bonding contributions at each side of the hydrogen atom. The resonance assisted hydrogen bonding model has been refined to reflect the redistribution of charge.

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Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort

The first (so-called) lambda transition in solids was found in the specific heat measurements for NH(4)Cl at 242 K by F. Simon in 1922 [Simon (1922). Ann. Phys. 68, 241-280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys. 7, 313-321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of "hybrid crystals" and "smeared transitions" can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4'-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' "hybrid crystal".

Crystal structures of the two polymorphs of perylene have been determined at low temperatures (150 and 200 K), and the molecular geometry was compared with earlier crystallographic and NMR measurements. Cell dimensions have been measured for both polymorphs over the ranges 150–320 K for (EI‐β) and 150–420 K for (EII‐α). Combination of thermodynamic and crystallographic measurements has permitted inference of pressuretemperature phase diagrams for perylene and pyrene that are compatible with available measurements up to ca. 50 kbar. Perylene and pyrene are both enantiotropic systems, while phenazine appears to be monotropic.

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