Computational Spectroscopy

Computational spectroscopy predicts molecular spectra from quantum-chemical calculations, linking theory to experiment by computing the energies and intensities that spectrometers measure.

Definition

The use of quantum-chemical methods to predict the spectroscopic properties of molecules, including vibrational, electronic, and magnetic-resonance spectra.

Scope

Covers the calculation of spectroscopic observables: vibrational frequencies and infrared and Raman intensities from the Hessian, electronic excitation energies and UV-visible spectra, nuclear magnetic resonance chemical shifts and coupling constants, and other response properties obtained as derivatives of the energy. Connects computed properties to experimental assignment.

Core questions

• How are vibrational frequencies and infrared intensities obtained from a calculation?
• How are electronic absorption spectra predicted?
• How are NMR chemical shifts and couplings computed?
• How do computed spectra aid the assignment and interpretation of experiments?

Key theories

Harmonic vibrational analysis

Diagonalizing the mass-weighted Hessian at a stationary point yields vibrational frequencies and normal modes, from which infrared and Raman spectra are predicted.

Molecular response properties

Spectroscopic observables are computed as derivatives of the energy with respect to perturbations such as fields and nuclear magnetic moments, within response theory.

Clinical relevance

Computed spectra are widely used to assign and interpret experimental infrared, Raman, UV-visible, and NMR data, to confirm structures and reaction intermediates, and to design molecules with target spectroscopic signatures.

History

As analytic derivative techniques and response theory matured from the 1980s onward, routine calculation of vibrational, electronic, and magnetic-resonance properties became a standard complement to experimental spectroscopy.

Key figures

• Trygve Helgaker
• Kenneth Ruud
• Christopher Cramer
• Frank Jensen

Seminal works

• helgaker2012

Frequently asked questions

Why are computed vibrational frequencies often scaled?

Harmonic frequencies systematically overestimate experimental fundamentals because of neglected anharmonicity and method error, so empirical scaling factors are commonly applied for better agreement.

How does computed spectroscopy help experimentalists?

By predicting the positions and intensities of spectral features for candidate structures, calculations help assign observed bands and discriminate between possible structures or intermediates.