Ab initio molecular orbital calculation of the normal frequencies was performed on optimized structures based on the 1:1 and 1:2 complexes of Ag+ and s-trans-1,3-butadiene (ED) of type I, IIa, and IIb. Comparing the results of the calculation with the IR and Raman spectra of the 1:1 and 1:2 complexes of ED and AgNO3, (BD(AgNO3), and BD(AgNO3)(2)), in aqueous solutions, we concluded that BD(AgNO3) takes on a coordination structure of type I, whereas BD(AgNO3)(2) takes on a structure of type IIb. The results of the calculation indicated that the coordination structures are clearly reflected by the frequencies of IR and Raman bands associated with the C=C stretching (v(C=C))and CH2 out-of-plane wagging (omega(CH2)) vibrations. The calculation predicted the following v(C=C)) and omega(CH2) frequencies for the Raman bands : 1631 and 930 cm(-1) (type I), 1607 and 933 cm(-1) (type IIa), and 1614 and 958 cm(-1) (type IIb). The surface-enhanced Raman spectrum of ED adsorbed on a coldly evaporated silver film gave the v(C=C) and omega(CH2) bands at 1628 and 921 cm(-1) at 120 K and at 1602 and 927 cm(-1) at 180 K (Itoh, K. et al. J. Electron Spectrosc. 1990, 54/55, 923). The former set corresponds to the v(C=C)) and omega(CH2) frequencies predicted for type I and the latter set corresponds to the frequencies predicted for type IIa. These results confirmed that the adsorbate takes on a structure with one of the C=C bonds coordinated to a positively charged site on the silver film at 120 K and a structure with both of the C=C bonds coordinated to the positively charged sites at 180 K.