Probing the robustness of the charge-charge transfer-dipolar polarization model and infrared intensities.

The robustness of the QTAIM charge-charge transfer-dipolar polarization parameters for the CH, CF, and CCl stretching and bending distortions of the fluoro- and chloromethanes was determined comparing results calculated at three quantum levels, MP2/6-311G++(3d,3p), QCISD/cc-pVTZ, and QCISD/aug-cc-pVTZ. The correlation coefficients between the MP2/6-311G++G(d,p) and QCISD/cc-pVTZ results with those of QCISD/aug-cc-pVTZ intensities are excellent, 0.934 and 0.988, respectively, showing that the parameters converge with increasing quality of the quantum levels. In spite of numerical differences, the interpretation of the electronic structure changes occurring for these vibrations is the same for all three quantum levels. Accurate determination of charge transfer-counterpolarization effects is important for properly describing electron density changes for small molecular distortions.