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• Dr. Derek Wann
• School of Chemistry
• University of Edinburgh

Structures of small molecules

Methods that we use for determining structures, include for gases:

• electron diffraction
• state-specific electron diffraction
• ultrafast electron diffraction
• rotational spectroscopy

for solutions:

• NMR spectroscopy in liquid crystal solvents

for solids:

• X-ray crystallography

and theoretical methods include:

• ab initio calculations and density functional theory (DFT)
• periodic boundary calculations

Often one method will not provide sufficient information to give a complete and accurate structure for a molecule. To overcome this problem we have developed methods that allow us to analyse simultaneously data obtained by several techniques.

The STRADIVARIUS method uses data from different experimental methods, particularly electron diffraction, rotational spectroscopy and liquid crystal NMR spectroscopy.

The SARACEN method uses experimental data, and also uses flexible restraints derived from theoretical (ab initio or density functional theory) calculations. It thus gives structures which a based on the best available information, both experimental and theoretical.

The DYNAMITE method takes SARACEN one step further by dynamically linking experimental data and theoretical calculations within the least-squares refinement cycles. The core of the molecule is determined by GED and the periphery by low-level (molecular mechanics) calculations, reducing the need to have lots of parameters to describe the ligands in C₁ symmetry.

The SEMTEX method recognises that the use of molecular mechanics in the DYNAMITE method could lead to innacuracies in the final structure. SEMTEX therefore follows the DYNAMITE methodology, but regularly scales the theoretical parameters back using a higher level of theory such as MP2.