Vibrational anharmonicity and harmonic force fields for dichloromethane from quantum-chemical calculations
Donald C McKean1, Norman C Craig, Mark M Law
1School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH93JJ, UK.
The Journal of Physical Chemistry. A
|September 9, 2008
Summary
Anharmonic constants for dichloromethane isotopomers were computed using advanced quantum chemical models. The MP2/cc-pVTZ force field was preferred for accurate vibrational and centrifugal distortion analysis.
Area of Science:
- Computational Chemistry
- Molecular Spectroscopy
- Quantum Mechanics
Background:
- Accurate calculation of molecular properties requires sophisticated theoretical models.
- Vibrational anharmonicity and centrifugal distortion are crucial for understanding molecular dynamics and spectra.
- Dichloromethane (CH2Cl2) and its isotopomers are important model systems for studying these effects.
Purpose of the Study:
- To calculate anharmonic and related constants for CH2Cl2, CD2Cl2, and CHDCl2.
- To investigate the performance of different quantum chemical models (B3LYP, MP2) and basis sets (6-311++G**, cc-pVTZ).
- To explore the impact of Fermi resonance on vibrational anharmonic constants and refine force fields for accurate property prediction.
Main Methods:
- Utilized Gaussian03 program with B3LYP and MP2 theoretical models.
- Employed 6-311++G** and cc-pVTZ basis sets for calculations.
- Calculated anharmonic frequencies, cubic constants, centrifugal distortion constants (CDCs), and vibrational dependence of rotational constants (alphas).
Main Results:
- The MP2/cc-pVTZ force field was identified as the most reliable.
- Minor Fermi resonances were found to influence cubic constants in different isotopomers.
- Computed anharmonic constants for CH and CD motions showed good agreement with experimental data.
- Differences in computed harmonic frequencies necessitated different scaling factors for symmetric and antisymmetric C-H and C-Cl stretching force constants.
Conclusions:
- The MP2/cc-pVTZ model provides accurate anharmonic and related constants for dichloromethane isotopomers.
- The study highlights the importance of basis set choice and the need for careful force field scaling.
- Calculated centrifugal distortion constants are influenced by the frequencies used in the scaling process.
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