Optical rotation refers to when a beam of polarized light passes through a substance,the plane of polarized light rotates at a certain angle relative to its own direction of motion. The property of rotating polarized light that a substance has is called optical rotation. Materials with optical rotation include crystals, spin-polarized molecules and chiral molecules. For the same molecule, when the wavelength of the incident polarized light changes, the observed optical rotation will also change accordingly. This change is called optical rotation dispersion (ORD). ORD is a kind of dispersion effect in which the polarization angle of optically active materials changes with wavelength. It is usually studied with the monochromatic light of the xenon lamp light source in the 200-700 nm spectral region.
Figure 1. ORD of (R)-methyloxirane in gas phase as calculated by exploiting different functionals and the aug-cc-pVDZ basis set. Top panel: fixed B3LYP/aug-cc-pVDZ/PCM geometry; bottom panel: optimized geometry in vacuo with each functional. Experimental values in gas phase and the experimental ORD in aqueous solution are also shown. (Egidi, F.; et al. 2015)
Application of ORD
- Distinguish the structure of different conformations.
- Determine the position of substituents in macromolecules such as steroids.
At Alfa Chemistry, we are capable of applying field applied molecular dynamics computer simulation, ab initio calculations, time dependent density functional theory and QM/FQ/PCM approach to the calculation of chiroptical properties and spectroscopies by predicting ORD curves.
- Field applied molecular dynamics (FMD) computer simulation
We apply FMD computer simulation using second-order rising transients (RS) and a series of time-correlated functions (CF) to investigate ORD in the far infrared frequency interval in chiral molecules. Our experts use a set of novel pseudoscalar to express the FMD results, and the first far infrared ORD spectrum is isolated in terms of a novel rotational velocity difference CF for different applied laser intensities.
- Ab Initio calculation
At Alfa Chemistry, both Hartree-Fock (HF) and density functional theory (DFT) methods have been used to perform ab initio calculations of the optical rotatory power of selected molecules at several wavelengths.
We are capable of using only small basis sets to predict optical rotatory dispersion curve which reproduces (below 400 nm) the experimental trend well, allowing for the correct configurational assignment.
In order to achieve a reliable configurational assignment, we compare the experimental and predicted rotation values at different wavelengths and not at a single frequency.
Coupling the use of an inexpensive instrument (a polarimeter working at a few wavelengths) with the use of a DFT-calculation method allows for a rapid and reliable assignment of the molecular absolute configuration.
- Time dependent density functional theory (TD-DFT)
TDDFT along with the COSMO has been applied to model the optical rotatory dispersion of the aromatic amino acids.
Solution structures at low, neutral, and high pH are determined. Both the anomalous dispersion absorbing (resonance) region and the lower energy (transparent) region of the ORD of the compounds are modeled.
Moreover, we have compared the linear response calculation of the ORD with experimental data from the literature to prove the accuracy of our prediction.
- QM/FQ/PCM approach
Our scientists have established a fully polarizable QM/MM/PCM approach based on Fluctuating Charges (FQ) combined with different density functionals for the description of the optical rotation at different wavelengths. The basic steps of the computational protocol are:
1. The geometry of molecule is optimized accounting for the effects of the aqueous environment by means of the PCM at the B3LYP/aug-cc-pVDZ level. We have validated this combination of functional and basis set to provide good geometries and vibrational properties for various isolated and solvated systems.
2. Thousands of snapshots are extracted from an MD simulation performed by using a fixed molecular geometry.
3. For each snapshot, a spherical cluster centered on the solute is cut and surrounded by a spherical PCM cavity. Thus system is then used to calculate the OR at the QM/FQ/PCM level.
4. The calculated OR is then averaged over the snapshots, and vibrational corrections calculated at the B3LYP/aug-cc-pVDZ/PCM level are added to the average value.
5. Finally, we use the OR to predict the ORD curve.
Alfa Chemistry's Advantages
- We have designed a new, simple, and reliable method to assign the molecular absolute configuration, at least for rigid molecules through our ORD simulation prediction.
- All of our computational methods have been proved reliable through the comparison with experimental ORD curves.
ORD simulation prediction can be used to determine the structure of different conformations. Our ORD simulation prediction services remarkably reduce the cost, promote further experiments, and enhance the understanding of chemical process for customers worldwide. Our personalized and all-around services will satisfy your innovative study demands. If you are interested in our services, please don't hesitate to contact us. We are glad to cooperate with you and witness your success!
- Egidi, F.; et al. Optical rotatory dispersion of methyloxirane in aqueous solution: assessing the performance of density functional theory in combination with a fully polarizable QM/MM/PCM approach. Optical Materials Express. 2015, 5(1): 509-514.