If X-ray crystallography is so powerful, why do we bother with other methods? There are two reasons: • X-ray crystallography works by the scattering of X-rays from electrons and requires crystalline solids. If an organic compound is a liquid or is a solid but does not form good crystals, its structure cannot be determined in this way.

• X-ray crystallography is a science in its own right, requiring specialized skills, and a structure determination can take a long time. Modern methods have reduced this time to a matter of hours or less, but nonetheless by contrast a modern NMR machine with a robot attachment can run more than 100 spectra overnight. We normally use NMR routinely and reserve X-rays for difficult unknown structures and for determining the detailed shape of important molecules.

X-ray crystallography is not infallible! Because it cannot usually ‘see’ H atoms, it is important to appreciate that X-ray crystallography is not infallible: it can still get things wrong. A famous example is the antibiotic diazonamide A, which from 1991 (when it was isolated from a marine organism) until 2001 (when the error was realized) was thought to have the structure shown on the right. It has the same mass as the real structure on the left, and X-ray crystallography was unable to tell the O and the N apart. Only when the compound was synthesized did the error become apparent, and the fact that the correct structure was indeed that on the left was confirmed by the fact that synthetic material of this structure made in 2002 was identical with the natural product.

مواضيع ذات صلة

Curly arrows show the movement of electrons

Curly arrows represent reaction mechanisms

Identifying an electrophile

Nucleophiles and electrophiles

Orbital overlap is essential for successful reaction

Reactions happen when electrons flow between molecules

Orbital overlap brings molecules together

Charge attraction brings molecules together

Not all collisions between molecules lead to chemical change

Chemical reactions

Rotation and rigidity

The carbonyl group