| Abstract
| - The vibrational, Raman, and IR, spectra of the five 12-crown-4 (12c4) complexes with Li+, Na+, K+, Rb+,and Cs+ alkali metal cations were measured. Except for a small shift of the position of some bands in thevibrational spectra of the Li+ complex, the vibrational spectra of the five complexes are so similar that it isconcluded that the five complexes exist in the same conformation. B3LYP/6-31+G* force fields were calculatedfor six of the eight predicted conformations in a previous report (J. Phys. Chem. A2005, 109, 8041) of the12c4−Li+, Na+, and K+ complexes that are of symmetries higher than the C1 symmetry. These sixconformations, in energy order, are of C4, Cs, Cs, C2v, C2v, and Cs symmetries. Comparison between theexperimental and calculated vibrational frequencies assuming any of the above-mentioned six conformationsshows that the five complexes exist in the C4 conformation. This agrees with the fact that the five alkalimetal cations are larger than the 12c4 ring cavity. The B3LYP/6-31+G* force fields of the C4 conformationof the Li+, Na+ and K+ complexes were scaled using a set of eight scale factors and the scale factors werevaried so as to minimize the difference between the calculated and experimental vibrational frequencies. Theroot-mean-square (rms) deviations of the calculated frequencies from the experimental frequencies were 7.7,5.6, and 5.1 cm-1 for the Li+, Na+, and K+ complexes, respectively. To account for the earlier results of theLi+ complex that the Cs conformation is more stable than the C4 conformation by 0.16 kcal/mol at the MP2/6-31+G* level, optimized geometries of the complex were calculated for the C4 and Cs conformations at theMP2/6-311++G** level. The C4 conformation was calculated to be more stable than the Cs conformation by0.13 kcal/mol.
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