## Scaled Quantum Chemical Force Fields for 1,1-Difluorocyclopropane and the Influence of Vibrational Anharmonicity

journal contribution

posted on 24.07.2008, 00:00 by Donald C. McKean, Norman C. Craig, Mark M. LawPotential functions and harmonic (ω

_{i}) and anharmonic (ν_{i}) fundamental frequencies have been calculated for 1,1-difluorocyclopropane (DFCP) and its*d*_{4}and*d*_{2}isotopomers using the program Gaussian 03. B3LYP and MP2 models were employed, each with the bases 6-311++G** and cc-pVTZ. Anharmonicity corrections Δ_{i}= ω_{i}− ν_{i}are listed and shown to be different for symmetric and antisymmetric CH stretching modes in situations where Fermi resonance appears to be absent. The same effect is missing in C_{2}H_{4}, for which similar calculations were made. The quadratic force fields for DFCP have been scaled in symmetry coordinate space with 15 scale factors both to observed frequencies ν_{obsd}and also to ω_{obsd}, where ω_{obsd}= ν_{obsd}+ Δ. With ν_{obsd}especially, different scale factors are needed for the symmetric and antisymmetric CH stretching force constants due to their differing anharmonicities. The source of the latter in the quartic and cubic force field is explored. MP2 calculations of valence interaction force constants involving the stretching of bonds on a common carbon atom are preferred to those from a B3LYP model. In either model, scaling to ω_{obsd}rather than to ν_{obsd}does not remove the necessity of varying scale factors for differing types of motion in the same group. Theoretical values of the five quartic centrifugal distortion constants are listed for the normal species and compared with new experimental data. The predictions are sufficiently good to be useful in fitting pure rotational transitions. A weakness is identified in the current Gaussian 03 code for the calculation of vibration−rotation quantities, and limitations are noted in the manner in which Fermi resonance is handled.#### Read the peer-reviewed publication

### Categories

### Keywords

2HAnharmonicity corrections Δ i15 scale factorsVibrational AnharmonicityPotential functionsB 3LYP modelScaled Quantum Chemical Force FieldsMP 2 calculationsω obsdν obsdFermi resonanceDFCPfrequencies ν obsdMP 2 modelsvalence interaction force constantsscale factorsΔ.Gaussian 03 code03. B 3LYPantisymmetric CH

### History

### Licence

### Exports

#### Read the peer-reviewed publication

### Categories

### Keywords

2HAnharmonicity corrections Δ i15 scale factorsVibrational AnharmonicityPotential functionsB 3LYP modelScaled Quantum Chemical Force FieldsMP 2 calculationsω obsdν obsdFermi resonanceDFCPfrequencies ν obsdMP 2 modelsvalence interaction force constantsscale factorsΔ.Gaussian 03 code03. B 3LYPantisymmetric CH