Molecular structure and IR spectra of bromomethanes by DFT and post-Hartree-Fock MP2 and CCSD(T) calculations

The molecular parameters (geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of bromomethanes (CH3Br, CH2Br2, CHBr3, CBr4) are predicted by a density func tional theory with the hybrid Becke3-LYP functional (DFT) and post-Hartree- Fock methods (MP2, CCSD(T)) using a 6-311G(2d,2p)-type basis set. The MP2 c alculations an carried out with different numbers of frozen core orbitals t o find how the number of bromine orbitals used for electron correlation inf luences the predicted molecular parameters and IR spectra of the species in question. Three options were used: (a) all electrons (full), with both the core and valence orbitals considered; (b) partial frozen core option (pfc) , when the orbitals up to 3p of bromine were frozen; and (c) full frozen co re option (ffc), when all core orbitals up to 3d were frozen. The CCSD(T) c alculations for geometric parameters were carried out with both the pfc and ffc options, while for the prediction of the IR spectra only the ffc optio n was used. In addition, the calculations at the DFT and MP2(pfc) levels wi th inclusion of f functions on carbon and bromine atoms in bromomethanes (a nd also the CCSD(T)(pfc) calculations for CH3Br) were carried out to predic t the changes in the geometric parameters and/or vibrational IR spectra of the molecules upon inclusion of f functions The geometries of bromomethanes (particularly the CBr bond lengths) are predicted better by the DFT and CC SD(T) calculations when the f functions (in particular on bromine atom) are included, while the MP2 calculations without f functions are good enough f or comet predictions of the molecular geometries. The molecular parameters and vibrational IR spectra of bromomethanes in question and their deuterate d species predicted by the DFT, MP2(ffc) and CCSD(T)(ffc) with the 6-311G(2 d,2p) basis set agree well with the available experimental data.