Anharmonic Vibrational Spectroscopy of Molecular Systems

Anharmonic vibrational spectroscopy goes beyond the idealised harmonic oscillator model to characterise molecular vibrations with greater accuracy. By accounting for anharmonicity, researchers can predict fundamental transitions, overtones and combination bands that are absent or misrepresented in harmonic treatments. This approach relies on detailed representations of the potential energy surface and on perturbative or variational methods to correct frequencies and intensities, capturing the interplay of mechanical and electrical anharmonic effects. Advances in computational algorithms, hybrid force fields and selective mode treatment have expanded the applicability of anharmonic simulations to medium- and large-scale molecular systems. These developments enhance spectral assignment, enable the detection of subtle conformational changes and support the identification of molecular species in complex environments, from organometallic catalysts to interstellar media.

Research from Nature Portfolio

A systematic study of nucleobase near-infrared spectra demonstrated the sensitivity of overtone and combination bands to intra- and intermolecular interactions. Using deperturbed second-order vibrational perturbation theory, the simulated spectra of adenine, cytosine, guanine and thymine in polycrystalline form reproduced characteristic bands with high accuracy. The work highlighted the predominance of in-plane deformation modes and the limited influence of out-of-plane interactions, providing the first frequency correlation tables for purine and pyrimidine NIR spectra. This foundational analysis offers a benchmark for interpreting biological NIR hyperspectral data and probing hydrogen-bonding motifs in nucleic acid assemblies.

Anharmonic Vibrational Spectroscopy of Molecular Systems publication trend

The graph below shows the total number of articles in anharmonic vibrational spectroscopy of molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Anharmonicity: departure from the ideal harmonic oscillator model in molecular vibrations, leading to frequency shifts and the appearance of overtones and combination bands.

Potential energy surface: multidimensional representation of molecular energy as a function of nuclear coordinates, essential for calculating vibrational modes.

Vibrational perturbation theory (VPT2): a second-order perturbative method for including anharmonic corrections to vibrational frequencies and intensities.

Overtone: spectral feature arising from transitions involving two or more vibrational quanta in the same mode.

Combination band: spectral feature resulting from simultaneous excitation of two or more different vibrational modes.

Resonance: interaction between vibrational modes when their energies are nearly degenerate, requiring special treatment in perturbative calculations.

References

1. Simulated NIR spectra as sensitive markers of the structure and interactions in nucleobases. Scientific Reports (2019).
2. Scaling-up VPT2: A feasible route to include anharmonic correction on large molecules. Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy (2024).
3. Toward the identification of cyano-astroCOMs via vibrational features: benzonitrile as a test case. Frontiers in Chemistry (2024).