Conformational and vibrational analysis of 18-crown-6-alkali metal cation complexes.

Conformational analysis was performed for the 18-crown-6-alkali metal cation complexes, 18c6-AMCCs, using the CONFLEX method. The number of predicted conformations of the 18c6-Li+, Na+, K+, Rb+ and Cs+ complexes was 10, 24, 15, 9 and 4 conformations, respectively. Electronic and geometrical structures were calculated for the predicted conformations at the HF, B3LYP, CAM-B3LYP, M06 and MP2 levels. Binding energies and enthalpies of the ground state conformations were also calculated. Vibrational, IR and Raman, spectra of free 18c6 and 18c6-AMCCs were measured. Comparison between the calculated vibrational frequencies using multi-scale-factor scaling of the B3LYP force field and the experimental vibrational frequencies predicted that the 18c6-K+, Rb+ and Cs+ complexes exist in the D3d, C3v and C3v conformations, respectively. It was also predicted that the 18c6-Na+ complex exists in a D3d-like conformation. It was not possible to identify in what conformation the 18c6-Li+ complex exists.