Comments on 'Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers'.

Hans-Beat Bürgi,Piero Macchi

doi:10.1107/s2052252518007406

Abstract

The recent paper by Belo, Pereira, Freire, Argyriou, Eckert & Bordallo [(2018 ▸), IUCrJ, 5, 6-12] reports observations that may lead one to think of very strong and visible consequences of the parity-violation energy difference between enantiomers of a molecule, namely alanine. If proved, this claim would have an enormous impact for research in structural chemistry. However, alternative, more realistic, explanations of their experiments have not been ruled out by the authors. Moreover, the theoretical calculations carried out to support the hypothesis are unable to differentiate between enantiomers (molecules or crystals). Therefore, the conclusions drawn by Belo et al. (2018 ▸) are deemed inappropriate as the data presented do not contain sufficient information to reach such a conclusion.

Highlights

In a recent paper, entitled ‘Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers’, Belo et al (2018) report polarized Raman spectra collected on hydrogenated d-alanine single crystals (C3H7NO2, d-ala-h7), neutron powder diffraction (NPD) measurements on fully deuterated d-ala-d7 and ab initio calculations of the harmonic vibrational frequencies of an isolated d-ala molecule and of d- and l-ala crystals
While Wang et al (2002) claimed that differences exist between the enantiomers, Sullivan et al (2003) found no unusual behaviour in their X-ray diffraction and NMR experiments in the temperature range expected for the putative phase transition ($270 K)
There is no mention of the energy difference between the two, nor of that between the structure optimized for d-ala and the inverted density functional theory (DFT)-optimized structure of l-ala and vice versa

Summary

Introduction

In a recent paper, entitled ‘Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers’ (our emphasis), Belo et al (2018) report polarized Raman spectra collected on hydrogenated d-alanine single crystals (C3H7NO2, d-ala-h7), neutron powder diffraction (NPD) measurements on fully deuterated d-ala-d7 and ab initio calculations of the harmonic vibrational frequencies of an isolated d-ala molecule and of d- and l-ala crystals. The far-reaching interpretation of their experimental data by Belo et al (2018) represents a refutation of the basic tenet implicit in the usual quantum chemical Schrodinger equation that currently represents so-called ‘normal science’ according to Kuhn (1962) It could represent the beginning of a scientific revolution that might lead to the formulation of a new paradigm (and perhaps a revision of the currently accepted values of parity-violation energy) and, by accumulation of additional evidence, to a new ‘normal science’. We comment on the paper by Belo et al (2018) with these thoughts in mind

Some general comments on comparing and interpreting data of enantiomers

Data from the literature

Unusual Raman spectroscopic behaviour of one enantiomer

Comments on theoretical calculations

Conclusion