Calculating spectra: quantum insights into Raman

Quantum first

The first calculations of Raman optical activity spectra using coupled-cluster theory - one of the most reliable quantum chemical methods available - have been used to provide new insights into the scattering of polarized light from chiral molecules.

ROA is a valuable tool for the structural characterization of a wide range of molecules, including large biomolecules such as viruses and proteins. It is particularly useful for the latter. ROA relies on the difference in intensity of Raman scattered right and left circularly polarised light caused by the presence of molecular chirality in a sample and was first developed Laurence D. Barron with Peter Atkins at the University of Oxford and later by Barron with David Buckingham at the University of Cambridge, so a true "blue" technique, one might say.

Now, chemist Daniel Crawford of Virginia Tech, in Blacksburg, Virginia, USA, has carried out simulations with Kenneth Ruud of the Centre for Theoretical and Computational Chemistry at the University of Tromsø, Norway, to explain the underpinnings of ROA. "We have developed the most advanced computer model to-date of the scattering of polarized light from chiral molecules," explains Crawford. Given the prominence of chirality in so many areas of chemistry, pharmaceuticals and other areas, the study represents an important step forward in understanding the relationship between the technology and the materials it is used to analyse.

Long-term calculations

The researchers suggest that one of the long-term goals in this area of study is to allow chemists to easily carry out their own simulations in the lab whether they are studying small molecules of pharmaceutical or agrochemical interest or much larger entities, such as proteins or viral capsule.

"This will allow them to identify which hand of the compound reacts in a desired way - from providing a certain scent to fighting tumours," Crawford adds. The model the team has developed can predict several molecular properties to a greater degree of precision than is commonly available through even the best experimental techniques.

"Our models are built upon an approach to solving the Schrödinger equation that naturally converges towards the exact answer," Crawford told SpectroscopyNOW. "This means that we can systematically improve our models and thus calibrate them even in the absence of experimental data, which makes these models highly predictive." He adds that the application of such models can predict (and has predicted) many molecular properties to such a degree of accuracy that experiment eventually has to catch up. "This has been demonstrated for thermochemical properties and molecular structures, for example," he adds.

Crawford and Ruud have also demonstrated the effectiveness of their method through benchmark computations on (S)-methyloxirane - a compound for which experimental gas-phase data are available. Although rare, gas-phase experimental data have the distinct advantage of being free of the perturbative effects of solvent molecules, which means they can provide a more useful and pristine test environment for validating advanced quantum-chemical methods, the team says.

The next step will be to focus on systematic comparisons between coupled-cluster ROA spectra, density functional theory (DFT) and experiment. "Ultimately, we and the world's other quantum chemists seek to carry out computational experiments that will provide reliable data more quickly, more safely, and with less expense than laboratory analyses," adds Crawford. "For the experimentally relevant frequency region of 400-1600 cm^-1, the Hartree-Fock, B3LYP and coupled-cluster spectra are very similar when the same force field is used, and the results also agree well with experiment," the researchers conclude. "For high-frequency vibrational modes, differences in the ROA difference parameters are observed and are analyzed. The new coupled-cluster ROA code will allow for critical benchmarking of the accuracy of modern exchange?correlation functionals in the calculation of ROA spectra."