How Accessible Is Atomic Charge Information from Infrared Intensities? A QTAIM/CCFDF Interpretation

Abstract

Infrared fundamental intensities calculated by the quantum theory of atoms in molecules/charge-charge flux-dipole flux (QTAIM/CCFDF) method have been partitioned into charge, charge flux, and dipole flux contributions as well as their charge-charge flux, charge-dipole flux, and charge flux-dipole flux interaction contributions. The interaction contributions can be positive or negative and do not depend on molecular orientations in coordinate systems or normal coordinate phase definitions, as do CCFDF dipole moment derivative contributions. If interactions are positive, their corresponding dipole moment derivative contributions have the same polarity reinforcing the total intensity estimates whereas negative contributions indicate opposite polarities and lower CCFDF intensities. Intensity partitioning is carried out for the normal coordinates of acetylene, ethylene, ethane, all the chlorofluoromethanes, the X(2)CY (X = F, Cl; Y = O, S) molecules, the difluoro- and dichloroethylenes and BF(3). QTAIM/CCFDF calculated intensities with optimized quantum levels agree within 11.3 km mol(-1) of the experimental values. The CH stretching and in-plane bending vibrations are characterized by significant charge flux, dipole flux, and charge flux-dipole flux interaction contributions with the negative interaction tending to cancel the individual contributions resulting in vary small intensity values. CF stretching and bending vibrations have large charge, charge-charge flux, and charge-dipole flux contributions for which the two interaction contributions tend to cancel one another. The experimental CF stretching intensities can be estimated to within 31.7 km mol(-1) or 16.3% by a sum of these three contributions. However, the charge contribution alone is not successful at quantitatively estimating these CF intensities. Although the CCl stretching vibrations have significant charge-charge flux and charge-dipole flux contributions, like those of the CF stretches, both of these interaction contributions have opposite signs for these two types of vibrations.

An atom in molecules study of infrared intensity enhancements in fundamental donor stretching bands in hydrogen bond formation

“…Some features noticed before in a group of mostly organic molecules can also be detected in our results. 37 First, the predominantly negative values for the charge flux-dipole flux interaction contribution (A CFÂDF a ) are expected because a negative correlation between both of these contributions to dipole moment derivatives was detected in previous studies with the QTAIM/CCFDF model. 31 HNC is the only exception presenting a small positive A CFÂDF a contribution (11 km mol À1 ).…”

Quantum theory of atoms in molecules/charge-charge flux-dipole flux models for fundamental vibrational intensity changes on H-bond formation of water and hydrogen fluoride

“…Rotational spectra of the gas phase HF:H 2 O complex have been used to partially determine its equilibrium geometry. 29,30 In this work, the infrared intensity enhancements observed in these dimers are investigated at the QCISD level with correlation consistent Dunning basis sets using the Quantum Theory of Atoms In Molecules (QTAIM) 31,32 /Charge-Charge Flux-Dipole Flux (CCFDF) model 33,34 recently developed by our group. The CCFDF model results in intensity contributions owing to electronic structure changes for vibrations that can all be interpreted classically as movements of equilibrium atomic charges and changes in atomic charges and atomic dipoles during the vibration.…”

Latin American contributions to quantum chemical topology

“…Substitution of Equation in Equation yields the IR intensities as a sum of C, CF, DF, and crossed contributions among them that can be traced to the atomic components to the polar tensor. This model has been extensively used . The atomic charges, dipoles and their fluxes have been calculated for inorganic and organic molecules using MP2, DFT, and Quadratic Configuration‐Interaction including single and double substitutions (QCISD) calculations.…”