Vibrational Spectroscopy of Large Molecules

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Theoretical spectroscopy is a unique means for us to relate structural predictions to experimental observations. Vibrational spectroscopy is a variant that has attracted our continuous attention as it, in the harmonic approximation, becomes a challenge for large molecules of more than about 200 atoms. We have developed inverse-quantum-chemical methods to meet this challenge. Developments comprise methods tailored for large molecules (parallelized calculation of vibrational spectra, Mode-Tracking of pre-selected vibrations, and the invention of Intensity-Tracking algorithms for IR, Raman, resonance Raman, and ROA spectroscopy). For the understanding of vibrations in large molecules, the concept of localized modes defined by unitary transformations was developed.

We proposed the so-called Mode-Tracking approach for the exact calculation of normal modes of pre-selected molecular vibrations in very large molecules, which is an efficient substitute for the a posteriori inspection of all vibrations of a molecule. This latter procedure is the current standard procedure in static quantum chemical calculations. The vibrational spectroscopy program packages developed in our group - including the massively parallel code SNF and the Mode-Tracking program AKIRA which were recently joined into the MOVIPAC package - are available free of charge from our internet pages.

For reviews see:

C. Herrmann, J. Neugebauer, M. Reiher, Finding a Needle in a Haystack: Direct Determination of Vibrational Signatures in Complex Systems (PDF, 1.5 MB), New J. Chem., 2007, 31, 818-831. 

C. Herrmann, M. Reiher, First Principles Approach to Vibrational Spectroscopy of Biomolecules in: Atomistic Approaches in Modern Biology; M. Reiher (Ed.),Top. Curr. Chem., Vol. 268, p. 85-132, Springer-Verlag: Heidelberg, Berlin, ISBN 978-3-540-38082-5 (DOI 10.1007/978-3-540-38085-6, 2007

Complementary to the Mode-Tracking approach, Intensity-Tracking was developed as an efficient method to predict the fingerprint of a spectrum by converging only those normal modes that pick up intensity of a specific spectroscopic technique.

For a review see:

K. Kiewisch, S. Luber, J. Neugebauer, M. Reiher, Intensity Tracking for Vibrational Spectra of Large Molecules (PDF, 299 KB), Chimia, 2009, 63, 270-274. 

Raman Optical Activity (ROA) as the chiral variant of Raman spectroscopy allows one to determine the absolute configuration of a chiral molecule by comparison of calculated and measured spectra. Our efficient implementation of this type of spectroscopy allowed us to study ROA spectra of large biomolecules up to the size of a small protein. We developed the theoretical basis of Raman and ROA intensity-carrying modes and proposed and implemented the ROA intensity-tracking algorithm. Recent studies concentrated on uncovering structural fingerprints of biomolecules. Furthermore, we predicted ROA spectra of chiral transition metal complexes, a class of substances for which no experimental results have become available yet.

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