The structures of liquid pyridine and naphthalene: the effects of heteroatoms and core size on aromatic interactions

Abstract

Total neutron scattering has been used in conjunction with H/D and *N/ 15 N isotopic substitution to determine the detailed liquid-state structures of pyridine and naphthalene. Analysis of the data via an empirical potential-based structure refinement method has allowed us to interrogate the full six-dimensional spatial and orientational correlation surfaces in these systems, and thereby to deduce the fundamental effects of a heteroatom and aromatic core-size on intermolecular p-p interactions. We find that the presence of a nitrogen heteroatom, and concomitant dipole moment, in pyridine induces surprisingly subtle departures from the structural correlations observed in liquid benzene: in both cases the most probable local motif is based on perpendicular (edge-to-face) intermolecular contacts, while parallel-displaced configurations give rise to a clear shoulder in the correlation surface. However, the effect of the heteroatom is revealed through detailed analysis of the intermolecular orientational correlations. This analysis shows a tendency for neighbouring pyridine molecules to direct one meta-and one para-hydrogen towards the neighbouring aromatic p-orbitals in edge-to-face configurations, while head-to-tail alignment of adjacent nitrogen atoms is favoured in faceto-face configurations. In contrast to this, increasing aromatic core size from one to only two rings has a clear and profound effect on the p-p interactions and liquid structure. Our experiments show that naphthalenenaphthalene contacts are dominated by parallel-displaced configurations, akin to those found in graphite. This marks a fundamental difference with the structure of liquid benzene, in which perpendicular geometries are favoured. Furthermore, it is remarkable to note that in the systems studied, the most favoured spatioorientational configurations observed in the liquid state are not predicted from ab initio calculations and/or solid state crystallographic studies. This highlights the need for caution when extrapolating the results of crystallographic and computational studies to aromatic interactions in liquids and disordered systems.

Structural features of selected protic ionic liquids based on a super-strong base

“…9.5 Å, and encompassing approximately 13 neighbour molecules. This scenario is very much resembling the case of naphthalene, a bicyclic aromatic molecule that has been recently explored by joint neutron scattering and Reverse Monte Carlo (EPSR) simulation approaches, where exactly the same coordination shell has been reported, 49 thus suggesting that this organization should not depend upon the aromaticity of the bicyclic compound, but has prevalently steric origin. Driven by these analogies we explored the orientational organization of this first solvation shell, aiming at understanding how a neighbour DBU molecule orients itself around the reference one, as a function of the distance between their CoMs.…”

“…This trend is qualitatively identical to the organization reported in Figure 12 of the mentioned paper on liquid structure in naphthalene. 49 The microscopic structural scenario encountered in the protic ionic liquids based on DBU is drastically different from the case of neat DBU. The existence of a net charge on both the anion and the cation introduces coulombic interactions, leading to long-range correlations that have no counterpart in the neutral molecular system; furthermore the possibility of establishing doubly ionic hydrogen bonding 50,51 correlations between cation and anion leads to directional interactions that again have no counterpart in neat DBU.…”

Temperature dependent structural changes in liquid benzene studied using neutron diffraction

“…Our previous experiments [15,16], alongside those of Falkowska and co-workers [17,18], have successfully used Neutron Diffraction with Isotopic Substitution (NDIS) interpreted using Empirical Potential Structure Refinement (EPSR) to understand the liquid structure of a number of small aromatics, including benzene, in unprecedented detail. The key feature of the liquid structure of benzene is perpendicular 'Y-stacking' molecules with two hydrogen atoms pointing towards the aromatic ring of the central molecule (Figure 1).…”

Predicting Asphaltene Aggregate Structure from Molecular Dynamics Simulation: Comparison to Neutron Total Scattering Data

“…The extension of these force-fields (e.g., OPLS 17 ) to larger aromatic Polyaromatic Hydrocarbons (PAHs) remains relatively untested, save for some preliminary studies. 18 Neutron and X-ray scattering can provide a strong experimental benchmark for solution structure as it can be directly calculated from molecular simulation trajectories from Fourier inversion of the weighted sum of the radial distribution functions in the system 19 . Additionally, molecular simulation is also a valuable tool in analysis of liquid 5 structure from scattering data 20,21 .…”

“…Results from EPSR can then be used to benchmark force-fields in the ability to predict liquid structure. 21,23 Our recent studies of liquid naphthalene 18 have indicated that standard "aromatic carbon" potentials may not be sufficient to correctly predict liquid structure of polyaromatic hydrocarbons. These EPSR studies of liquid PAHs are a "bottom-up" approach, testing the underlying methods using simple systems.…”