Molecular structure and vibrational spectra of piperidine and 4-methylpiperidine by density functional theory and ab initio Hartree–Fock calculations

The molecular geometry and vibrational frequencies of piperidine and 4-methylpiperidine in the ground state have been calculated by using the Hartree–Fock and density functional methods (B3LYP and BLYP) with 6-31 G(d) as the basis set. The optimized geometric bond lengths have been obtained by DFT (B3YLP) show the best agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of piperidine and 4-methylpipiperidine, and calculated results by density functional B3LYP, BLYP and Hartree–Fock methods indicates that B3LYP is superior to the scaled BLYP, Hartree–Fock approach for molecular vibrational problems.