Distortions, deviations and alternative facts: reliability in crystallography.

William Clegg

doi:10.1107/s205225252001458x

Abstract

This feature article is derived from the author's presentation of the Lonsdale lecture at the BCA Spring Meeting in 2018. One of the research results for which Kathleen Lonsdale is best known was her 1929 demonstration that the benzene ring in crystalline hexa-methyl-benzene is planar and has essentially hexagonal symmetry, resolving decades of dispute among organic chemists. More recent crystallographic studies of hexa-methyl-benzene have shown that there are actually small deviations from planarity. Such deviations for aromatic compounds may be due to electronic, steric, and/or intermolecular factors. Some substituted benzene molecules display remarkably large deviations, both from a planar ring structure and from regular hexagonal angular geometry around the ring. Starting from this specific connection with Kathleen Lonsdale's research, a number of stories are recounted of structural distortions and deviations from expected results and explanations that have been suggested for them, across a wide range of chemical topics including macrocycles, metal clusters, unusual coordination geometry and isomerism. On the way we find genuine surprises and results that have led to new understanding, but also examples of poor experiments, misinterpretation of data, scientific bias and preconceived ideas, incompetence and even deliberate fraud. Some aspects of structure validation are discussed. While showcasing some interesting research in its own right, this account also serves an educational purpose.

Highlights

This article is based on the Lonsdale lecture given by invitation at the British Crystallographic Association Spring Meeting at Warwick University in March 2018; by tradition, the Lonsdale Lecturer, nominated by the BCA Young Crystallographers’ Group, is expected to combine aspects of original research with an educational approach
The lecture in 2018, with a title partly inspired by a current political catchphrase, took its starting point from work carried out by Professor Dame Kathleen Lonsdale (Fig. 1), in whose honour the annual lecture was created
Her best-known published work was as joint editor of Volume I (Symmetry Groups) of International Tables for X-Ray Crystallography in 1952

Summary

Introduction

This article is based on the Lonsdale lecture given by invitation at the British Crystallographic Association Spring Meeting at Warwick University in March 2018; by tradition, the Lonsdale Lecturer, nominated by the BCA Young Crystallographers’ Group, is expected to combine aspects of original research with an educational approach. In addition to scientific works, she wrote books expressing her Christian faith and pacifism (as a Quaker), including an account of her time in prison as a conscientious objector, and based on her own balance of scientific research and family life; she was a fulltime mother of three children in the early 1930s. She saw no conflicts in these diverse aspects of her life. The entry HMBENZ04 (Le Magueres et al, 2001) has a maximum internal torsion angle (C–C–C–C involving only ring atoms) of 2 and a maximum external torsion angle (Me–C–C–Me) of 5, rounding to integer values

Angular distortions in and around benzene rings

Findings

Crystal structure validation and some selected errors