Molecular geometry

Lore & Background

Molecular geometry is determined by various spectroscopic and diffraction methods. IR, microwave, and Raman spectroscopy provide information from vibrational and rotational absorbance. X-ray crystallography, neutron diffraction, and electron diffraction give molecular structure for crystalline solids based on nuclear distances and electron density. Gas electron diffraction is used for small molecules in the gas phase. NMR and FRET methods determine complementary information including relative distances, dihedral angles, angles, and connectivity. Geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries. Larger molecules often exist in multiple stable geometries (conformational isomerism) that are close in energy on the potential energy surface. Geometries can also be computed by ab initio quantum chemistry methods to high accuracy. The molecular geometry can be different as a solid, in solution, and as a gas.

Reader's Guide

Molecular geometry is fundamental to understanding chemical behavior. The angles between bonds that an atom forms depend only weakly on the rest of a molecule, meaning they can be understood as approximately local and hence transferable properties. The position of each atom is determined by the nature of the chemical bonds by which it is connected to its neighboring atoms. Molecular geometry can be described by bond lengths of two joined atoms, bond angles of three connected atoms, and torsion angles (dihedral angles) of three consecutive bonds. Thermal excitation influences molecular geometry: at absolute zero all atoms are in their vibrational ground state and show zero point quantum mechanical motion. At higher temperatures vibrational modes may be thermally excited, but molecules oscillate still around the recognizable geometry. Typical rotational excitation energies are on the order of a few cm−1, and many spectroscopic observations can only be expected to yield reliable molecular geometries at temperatures close to absolute zero, because at higher temperatures too many higher rotational states are thermally populated.

Did You Know?

• Molecular geometry includes bond lengths, bond angles, torsional angles, and any other geometrical parameters that determine the position of each atom.
• The angles between bonds that an atom forms depend only weakly on the rest of a molecule and can be understood as approximately local and hence transferable properties.
• At absolute zero all atoms are in their vibrational ground state and show zero point quantum mechanical motion, with the wavefunction of a single vibrational mode approximately a Gaussian function.