The foreword to this special issue of the Journal provides an overview of the papers that have been contributed by participants at the 2019 RACI Australian Conference on Physical Chemistry, which was held at the University of Western Australia from 11 to 14 February 2019.

Studies of isolated reaction centre preparations of PS II have been pivotal in the development of models of both the coupling between its pigments and the process of charge separation. Several key puzzles remain in our understanding of these important systems. We apply new and powerful optical techniques (CPL and MCPL) and, by means of an exciton analysis, point to key new ideas.

Super-resolution microscopy continues to develop: super-resolution optical fluctuation imaging (SOFI) is well suited for application in live cells and expansion microscopy (ExM) allows for imaging sub-diffraction detail with conventional microscopes. Here, they are utilised along with single molecule localisation microscopy (dSTORM) to visualise microtubule remodelling by rabies virus P proteins.

The formation of complexes between water molecules and radicals are important in dictating the processes of a number of atmospheric reactions. Here, a combination of mass spectrometry, phototelectron spectroscopy, and ab initio calculations are used to assign the form of an experimental I⁻…H₂O…CH₃CH₂ gas-phase radical ion complex.

Delayed fluorescence allows detection of single molecules of a naphthalene diimide–tertiary amine system with efficient electron transfer operative. Calculation of forward and reverse rate constants for individual molecules reveals contributions from both coupling and driving force to the distribution of behaviour from molecule to molecule when embedded in a polymer film.

A combination of ion-trap mass spectrometry and gas-phase reactions are used to study radical ions derived from protonated cyanobenzene reacting with ethylene (the archetypal olefin). Molecular weight growth is observed, from barrierless entrance channels – thus, these are viable reactions in the cold environment of space.

This work develops the theory of density-driven correlations in ensemble density functional theory, to help produce accurate low-cost theories of low-lying excitations.

Molecular modelling, through the calculation of replacement energies, provides insights on how organic species will impact the growth rates and resultant morphology of crystals.

Rotational dynamics of molecules is surprisingly complicated even in apparently simple liquids. We present an analysis of the rotational dynamics of hydrogen sulfide and compare it with that of water. The two liquids have similar molecular geometries but different hydrogen bonding properties. Neither liquid follows the standard Debye model that is traditionally used in the NMR literature to describe molecular reorientation.

To improve constraints on the variation of the proton-to-electron mass ratio and provide better guidance to theories of new physics, new molecular probes will be useful. From our investigation, we consider criteria to screen potential astrophysical molecules as probes. For rovibronic transitions in diatomic molecules, we find that low-lying electronic states allow for enhanced transitions.

Atoms in molecules (AIM) and natural bond orbital (NBO) analyses have been used to investigate three-centre two-electron bonds and classify them based on their arrangements into either V, L, Δ, Y, T, or I types. These differences in their arrangements lead to differences in their properties.

The physical behaviour of the methyl group during internal rotation (torsion) is explored. Precession of the methyl group is shown to give rise to a number of the constants required to fit experimental rotational line data. Quantum chemistry calculations of the optimised molecular structures at key torsional angles provide further evidence that methyl precession occurs.

Theoretical investigation of phosphorus heterocyclic carbenes (PHCs): do they behave like NHCs, which undergo ring-insertion by element hydrides (SiH₂Ph₂, BH₂NMe₂)?

The molecular structure of morpholine borane complex, a candidate for chemical hydrogen storage, has been studied in the solid state and gas phase using single-crystal X-ray diffraction and gas electron diffraction, and its thermochemistry studied computationally. Evolution of hydrogen from the complex is calculated to be close to thermoneutral and spontaneous, thus introducing a new amine borane complex with potential for future on-board hydrogen generation.

Molecular spectroscopy is a branch of quantum mechanics. Spectroscopic analysis depends on structural information at the molecule level. The earlier experimental measurements of IR spectra of ferrocene (Fc) could neither identify the fingerprints of the Fc conformer nor explain why the gas phase and solution IR spectra have different stereoisomer fingerprints without theoretical support. This article indicates that contemporary molecular spectroscopy needs guidance as well as support from computational spectroscopy.