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 the vibrational spectra of the Li+ complex, the vibrational spectra of the five complexes are so similar that it is concluded that the five complexes exist in the same conformation. B3LYP/6-31+G* force fields were calculated for six of the eight predicted conformations in a previous report (J. Phys. Chem. A 2005, 109, 8041) of the 12c4-Li+, Na+, and K+ complexes that are of symmetries higher than the C-1 symmetry. These six conformations, in energy order, are of C-4, C-s, C-s, C-2V, C-2V, and C-s symmetries. Comparison between the experimental and calculated vibrational frequencies assuming any of the above-mentioned six conformations shows that the five complexes exist in the C-4 conformation. This agrees with the fact that the five alkali metal cations are larger than the 12c4 ring cavity. The B3LYP/6-31+G* force fields of the C-4 conformation of the Li+, Na+ and K+ complexes were scaled using a set of eight scale factors and the scale factors were varied so as to minimize the difference between the calculated and experimental vibrational frequencies. The root-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 the Li+ complex that the Cs conformation is more stable than the C-4 conformation by 0.16 kcal/mol at the MP2/6-31+G* level, optimized geometries of the complex were calculated for the C-4 and Cs conformations at the MP2/6-311++G** level. The C-4 conformation was calculated to be more stable than the Cs conformation by 0.13 kcal/ mol.