Self-association of organic solutes in solution: a NEXAFS study of aqueous imidazole.

M J Thomason,S L M Schroeder,C R Seabourne,E F Aziz,J S Stevens,G A Hembury,B M Sattelle,A J Scott

doi:10.1039/c5fd00005j

Abstract

N K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of imidazole in concentrated aqueous solutions have been acquired. The NEXAFS spectra of the solution species differ significantly from those of imidazole monomers in the gas phase and in the solid state of imidazole, demonstrating the strong sensitivity of NEXAFS to the local chemical and structural environment. In a concentration range from 0.5 to 8.2 mol L(-1) the NEXAFS spectrum of aqueous imidazole does not change strongly, confirming previous suggestions that imidazole self-associates are already present at concentrations more dilute than the range investigated here. We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure. This suggests that such computational modelling of experimental NEXAFS will permit an incisive analysis of the molecular interactions of organic solutes in solutions. It is confirmed that microhydrated clusters with a single imidazole molecule are poor models of imidazole in aqueous solution. Our analysis indicates that models including both a hydrogen-bonded network of hydrate molecules, and imidazole-imidazole interactions, are necessary to explain the electronic structure evident in the NEXAFS spectra.

Highlights

Volmer's concept of the nucleation stage of crystallization[1] resulted in the development of what is known as classical nucleation theory (CNT)
We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure
Due to its sensitivity to unoccupied valence orbitals, near-edge X-ray absorption fine-structure (NEXAFS) is chemically and structurally more incisive than core level binding energy measurements by X-ray photoelectron spectroscopy (XPS), and we have recently demonstrated the level of detail that can be obtained by combining XPS, NEXAFS and density functional theory, to examine local bonding both in the organic solid state[29] and by solute species in solutions.[30]

Summary

Introduction

Volmer's concept of the nucleation stage of crystallization[1] resulted in the development of what is known as classical nucleation theory (CNT). Molecules in a supersaturated solution aggregate into nuclei, thereby developing an interface with the surrounding solution. The stability of these nuclei is size-dependent, re ecting the free energy balance between an interfacial tension penalty and cohesive energy stabilisation. Once the nuclei have grown beyond a critical size, above which the cohesive term outweighs the interfacial destabilisation, the total free energy decreases continuously as a function of size and crystal growth becomes the favourable process.[2,3] Whilst a proven and useful concept for describing and predicting nucleation and crystal growth phenomena, o en quantitatively, CNT remains a description of nucleation that does not explicitly consider intermolecular interactions or the precise structural nature of the pre-crystalline state; this limits its predictive power for many systems.[4,5] Researchers are using a variety of experimental techniques to obtain molecular level information about the solute species in solution, how they associate and assemble during nucleation, and whether the pre-crystalline structures relate to those in the crystalline products.[4,6,7,8,9,10] Examples of techniques commonly employed include nuclear magnetic resonance (NMR),[11,12,13] small- and wide-angle X-ray scattering (SAXS/WAXS),[14] optical microscopy,[15] and vibrational spectroscopies (infrared and Raman),[16] as well as grazing incidence X-ray diffraction (GIXD) of interfacial species.[17]

Methods

Results

Conclusion