Atomic charge and atomic dipole modeling of gas-phase infrared intensities of fundamental bands for out-of-plane CH and CF bending vibrations

Author(s)

Richter, Wagner E. ^{[1, 2]} ; Duarte, Leonardo J. ^[1] ; Bruns, Roy E. ^[1]

Affiliation

^[1] Univ Estadual Campinas, Inst Chem, Campinas, SP - Brazil

^[2] Univ Tecnol Fed Parana, Dept Chem Engn, Ponta Grossa, Parana - Brazil

• Document type: Journal article
• Source: SPECTROCHIMICA ACTA PART A-MOLECULAR AND BIOMOLECULAR SPECTROSCOPY; v. 251, APR 15 2021.
• Web of Science Citations: 0

Abstract

Out-of-plane CH group bending vibrational bands have long been known to be more intense than those for CF groups in similar molecular environments. This contrasts with expectations derived from charge models for which equilibrium atomic charge displacements are considered dominant contributions to dipole moment change on vibration. For this reason, the Charge, Charge Transfer, Dipolar Polarization (CCTDP) model based on the Quantum Theory for Atoms in Molecules (QTAIM) has been applied to the ethylene, tetrafluoroethylene and difluoro- and dichloroethylene molecules. Atomic charges and atomic dipoles from QTAIM and infrared intensities were calculated at the M06-2X/aug-cc-pVTZ level. The CH out-of-plane bending vibrations with relatively high intensities between 48.0 and 82.1 km/-mol are characterized by small atomic charge and large polarization contributions having the same sign resulting in large net dipole moment contributions. Large charge and polarization dipole moment derivative contributions with opposite signs cancel each other producing very small intensities between 0.3 and 12.7 km/mol for the CF bends. Intensity variations can be successfully modeled by only their carbon atomic contributions with smaller contributions from the terminal atoms. Both CH and CF bending vibrations have large polarization contributions. Their charge contributions are usually small except for carbon atoms bonded to two fluorine atoms. The terminal atoms as well as the carbons have charge and polarization contributions of opposite sign. Comparison to benzene and hexafluorobenzene reveals that changes in these molecules' electronic densities caused by the out-of-plane atomic displacements are characteristic for each bond. In conclusion, successful modeling of the ethylene intensities must include atomic dipole parameters.Models based only on charges are doomed to failure. (C) 2021 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 17/22741-3 - Using atomic multipoles and developing machine learning models to investigate transition states
• Grantee: Leonardo José Duarte
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)
• FAPESP's process: 18/08861-9 - Application of the QTAIM / CCTDP model and machine learning for the forecast of chemical reactivities
• Grantee: Roy Edward Bruns
• Support Opportunities: Regular Research Grants