Calculated molecular properties for different alkanoic acid–alkylamine complexes: A comparison with measured FTIR and Raman spectra Original Research Article

In this work the charges, geometries, changes in geometries due to complexation, stabilization energies and vibrational frequencies have been calculated for three different propanoic acid (A)–propylamine (B) complexes. The complexes are A3B1, A1B3 and A2B2. Furthermore, emphasis was given to clarifying which complex is the most stable one in binary solutions of alkanoic acid and alkylamine. The theoretically predicted vibrational spectra are compared with IR and Raman spectra for the binary system heptanoic acid–heptylamine. In theoretical calculations we have used Hartree–Fock and density function theory approximations. Calculations show the A3B1 complex to be the most stable one, which is also confirmed by the experiments at low amine concentrations. At equimolecular mixing ratios a A2B2 complex seems more likely.