Theoretical predictions suggest carbon dioxide phases III and VII are identical.

Solid carbon dioxide exhibits a rich phase diagram at high pressures. Metastable phase III is formed by compressing dry ice above ∼10-12 GPa. Phase VII occurs at similar pressures but higher temperatures, and its stability region is disconnected from III on the phase diagram. Comparison of large-basis-set quasi-harmonic second-order Møller-Plesset perturbation theory calculations and experiment suggests that the long-accepted structure of phase III is problematic. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure agree poorly with experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical

Combining Crystal Structure Prediction and Simulated Spectroscopy to Investigate Challenging High Pressure Phases

Solid carbon dioxide and nitrogen exhibit rich phase diagrams at high pressure. The large number of viable packing motifs stems from their small size and weak, non-polar intermolecular interactions, which make many packing arrangements and orientations energetically competitive. Experimental observation and characterization of high-pressure poly- morphs have proved challenging, not only because of flat energy landscape, but also their kinetic path-dependence and hysteresis in the phase transitions. As a result, high-quality experimental data are difficult to obtain, leaving many high-pressure crystal structures of nitrogen to remain unknown over decades, or creating ambiguities in the nature of some carbon dioxide phases.This thesis employs a combination of high-level fragment-based electronic structure method and Raman simulation to study high-pressure polymorphs in these systems. First, we investigate the nature of carbon dioxide phases III and VII. We provide evidence that the long-accepted structure of phase III is problematic from comparison of large-basis-set quasi-harmonic second-order Møller-Plesset and experimental data. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure contradict experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical.Second, we revisit nitrogen phase λ, one of the high-pressure solid nitrogen forms that was discovered by combining experimental monoclinic lattice parameters with atomic positions from an earlier, computationally predicted structure that had similar unit cell dimensions. Crystal structure prediction is performed to demonstrate that the reported P21/c structure is indeed the likeliest candidate for the λ phase. Furthermore, we provide further evidence for the structural assignment by demonstrating good agreement between its predicted and experimental structural parameters and Raman spectra. Finally, the thermodynamic stability of the λ phase relative to other phases has been uncertain, but the calculations do suggest that it may be the thermodynamically most stable phase for at least part of the pressure range over which it has been observed. Lastly, we perform crystal structure prediction using ab initio random structure searching and density functional theory to identify candidate structures for nitrogen phase ζ, the phase whose structure remains unknown decades after it was first observed spectroscopically, despite numerous experimental and theoretical investigations. The candidates are then analyzed for consistency with experiment in terms of their simulated x-ray diffraction patterns and Raman spectra. While none of the structures generated is a clear match for the phase ζ experimental data, several of the candidates do exhibit features in common with the experiments and could provide an interesting starting point for future studies. The techniques here also rule out several candidate ζ nitrogen structures that have been identified previously. Finally, one of the structures might be considered a candidate for phase κ, whose structure is also unknown

Structural switching in self-assembled metal-ligand helicate complexes via ligand-centered reactions.

Ligand centered reactions are capable of conferring structural switching between a metastable, self-assembled Fe-iminopyridine aggregate and a stable M2L3 helicate. The reactivity is directed and accelerated by the stability of the final product structure. Under aerobic conditions, both substitution and oxidation occurs at the ligand, exploiting atmospheric oxygen as the oxidant. In the absence of air, reaction occurs more slowly, forming the less stable substitution product. Control ligands show a preference for simple substitution, but the self-assembly directs both substitution and oxidation. The metastable nature of the initial aggregate species is essential for the reaction: while the aggregate is "primed" for reaction, other analogous helicate structures are "locked" by self-assembly, preventing reactivity

Structural switching in self-assembled metal-ligand helicate complexes via ligand-centered reactions.

Ligand centered reactions are capable of conferring structural switching between a metastable, self-assembled Fe-iminopyridine aggregate and a stable M2L3 helicate. The reactivity is directed and accelerated by the stability of the final product structure. Under aerobic conditions, both substitution and oxidation occurs at the ligand, exploiting atmospheric oxygen as the oxidant. In the absence of air, reaction occurs more slowly, forming the less stable substitution product. Control ligands show a preference for simple substitution, but the self-assembly directs both substitution and oxidation. The metastable nature of the initial aggregate species is essential for the reaction: while the aggregate is "primed" for reaction, other analogous helicate structures are "locked" by self-assembly, preventing reactivity

Effect of halogen substitution on energies and dynamics of reversible photomechanical crystals based on 9-anthracenecarboxylic acid

9-Anthracene carboxylic acid derivatives comprise a family of thermally reversible photomechanical molecular crystals. The photomechanical response relies on a [4 + 4] photodimerization followed by dissociation that occurs on timescales of seconds to minutes. A combined theoretical and experimental investigation is undertaken to better understand how chemical modification of the anthracene core influences energetics of both the isolated molecule and the crystal lattice. We use both density functional theory and dispersion-corrected Moller–Plesset perturbation theory computational methods to establish orbital energies, photodimerization reaction energies, and lattice energies for a set of substituted 9-anthracene carboxylic acid molecules. The calculations reveal that steric interactions play a dominant role in the ability to form photodimers and indicate an energetic threshold of 80–90 kJ per mole for the dimerization reaction. Examination of intermolecular bonding in a subset of fluorinated 9ACs revealed the absence of H⋯F intermolecular bond formation and energy differences that can explain observed trends in the dissociation kinetics and mechanical reset times. Fluorescence recovery after photobleaching experiments shows that the photodimer dissociation kinetics depend on the amount of initial photodimer, preventing a straightforward correlation between halogen atom substitution and dissociation rates using the Bell–Evans–Polanyi principle. The results clarify how molecular structure affects intermolecular interactions and photoreactivity in this family of molecular crystals, but the origin of the complex photodimer dissociation dynamics remains an open question

Theoretical Study of the Hydrogen Abstraction of Substituted Phenols by Nitrogen Dioxide as a Source of HONO

The mild yet promiscuous reactions of nitrogen dioxide (NO₂) and phenolic derivatives to produce nitrous acid (HONO) have been explored with density functional theory calculations. The reaction is found to occur via four distinct pathways with both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms available. While the parent reaction with phenol may not be significant in the gas phase, electron donating groups in the ortho and para positions facilitate the reduction of nitrogen dioxide by electronically stabilizing the product phenoxy radical. Hydrogen bonding groups in the ortho position may additionally stabilize the nascent resonantly stabilized radical product, thus enhancing the reaction. Catechol (<i>ortho</i>-hydroxy phenol) has a predicted overall free energy change Δ<i>G</i>⁰ = −0.8 kcal mol^–1 and electronic activation energy <i>E</i>_<i>a</i> = 7.0 kcal mol^–1. Free amines at the ortho and para positions have Δ<i>G</i>⁰ = −3.8 and −1.5 kcal mol^–1; <i>E</i>_a = 2.3 and 2.1 kcal mol^–1, respectively. The results indicate that the hydrogen abstraction reactions of these substituted phenols by NO₂ are fast and spontaneous. Hammett constants produce a linear correlation with bond dissociation energy (BDE) demonstrating that the BDE is the main parameter controlling the dark abstraction reaction. The implications for atmospheric chemistry and ground-level nitrous acid production are discussed