Conformers

In chemistry, conformers, or conformational isomers, are species that share the same molecular formula and connectivity but differ in the three-dimensional arrangement of atoms produced by rotation about single (sigma) bonds. Because such rotations occur without breaking bonds, conformers can interconvert readily and rapidly at room temperature for many molecules. The relative stability of conformers is governed by torsional strain, steric interactions, and nonbonded repulsions, giving rise to a landscape of local energy minima separated by barriers associated with eclipsed or otherwise strained arrangements.

Alkanes illustrate basic ideas: ethane has staggered conformations that are lower in energy than eclipsed forms; for longer chains, low-energy conformers such as anti and gauche forms arise due to steric interactions among substituents. The barrier to rotation around a C–C bond sets the rate of interconversion; small barriers yield rapid averaging in NMR, while larger barriers can freeze certain conformers below certain temperatures.

Cyclic systems show characteristic conformational behavior. Cyclohexane, for instance, exists predominantly as a pair of chair conformers that interconvert through ring flipping; substituents prefer axial or equatorial positions depending on the conformation and steric demands. In rigid systems or when rotation is hindered, distinct conformers may be isolable and observed spectroscopically or crystallographically.

Conformational analysis uses experimental techniques such as NMR, which can reveal population distributions via chemical shifts and coupling constants, and computational methods to map potential energy surfaces and estimate Boltzmann populations. Conformational preferences affect physical properties, reactivity, and the binding of ligands in medicinal chemistry.

Conformers that interconvert slowly at a given temperature may be treated as separate stereoisomers, a phenomenon known as atropisomerism when rotation around a bond creates stable, isolable forms.