Ab Initio and DFT Predictions of Infrared Intensities and Raman Activities

Abstract

Relative infrared (IR) intensities and relative Raman activities have been computed for vibrations of test molecules, including from two to nine heavy atoms, using second-order Moller-Plesset perturbation theory (MP2), and three hybrid density functionals (B3LYP, M05, and M05-2X). The basis set convergence of vibrational properties is discussed. Our results demonstrate that B3LYP offers the most cost-effective choice for the prediction of molecular vibrational properties, but the predictions of another two tested hybrid functionals are very similar and in very good agreement with experimental data. MP2 shows good performance for the IR intensities, whereas the quality of prediction of the relative Raman activities should be characterized as only moderate. B3LYP calculations of the relative IR intensities using highly compact Sadlej's Z3PolX basis set retain the high accuracy of the more CPU expensive Sadlej's pVTZ and much more expensive aug-cc-pVTZ calculations. Relative Raman activities are more sensitive to basis set effects and require at least Sadlej's pVTZ to obtain quantitative results.

On the Origin of the 3.3 μm Unidentified Infrared Emission Feature

“…The accuracy of the theoretical calculations can be tested by comparison with gas-phase laboratory infrared spectra (Zvereva et al 2011). Previously, we have estimated errors of 0.12-0.13 µm for our density functional theory calculations (Sadjadi et al 2015a).…”

“…We find average errors of 0.03316 µm and 53 kcal/mol in wavelength and absolute intensity, respectively. It should be noted that although DFT calculations show large error in predicting the absolute intensity values, they can predict good relative intensities (Zvereva et al 2011;Sadjadi et al 2015a). Table 1).…”

Vibrational analysis of [4-[(E)-phenylazo] phenyl]ethanol based on the comparison between the experimental and DFT calculated Raman spectra

“…The criteria used to sort these different methods were mainly based on the comparison between the experimental and numerical frequency modes. It is commonly accepted that the Raman intensities are best predicted by aug-cc-pVTZ or Sadlej's pVTZ [61,62]. Yet, for our compounds the 6-311++G(2d,2p) basis set seemed to have work very well.…”

Unbranched n-Alkanes

“…B3LYP was shown to reproduce absolute intensities of methane and ethane (the two most interesting test cases for this work) commonly within 20 % when combined with basis sets optimized to reproduce polarizabilities accurately [65,66]. Pople basis sets approach the quality of these specialized basis sets when diffuse (+) and polarization functions (*) are added [67,68], because the description of the more Table 4.4 B3LYP-D3/6-311++G** single gauche energies relative to all-trans ( E 0 ) in kJ mol −1 and associated gauche/trans abundance ratio…”

“…Several recent computational studies summarized experimental Raman intensities of small molecules and compared them to ab initio and DFT calculations [65][66][67]. B3LYP was shown to reproduce absolute intensities of methane and ethane (the two most interesting test cases for this work) commonly within 20 % when combined with basis sets optimized to reproduce polarizabilities accurately [65,66].…”