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

• Conformational analysis of 18-crown-6–alkali metal cation complexes.
• CONFLEX conformational search method.
• Vibrational analysis of 18-crown-6 and its alkali metal cation complexes.
• Scaled quantum mechanical force fields.
• Prediction in what conformation 18-crown-6–alkali metal cation complexes exist.

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.

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