The IR spectra and normal-mode analysis of the adamantane-like compound [Mn4O6(bpea)(4)](n+) (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn-4(IV) and (MnMn3IV)-Mn-III, that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, O-16-->O-18, of the mono-mu-oxo bridging atoms in the complex. IR spectra of the (MnMn3IV)-Mn-III species are obtained by electrochemical reduction of the Mn4IV species using a spectro-electrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. 2001, 73, 4374-4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn-O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the Mn-4(IV) species at 745 and 707 cm(-1), and a weaker band is observed at 510 cm(-1). Upon reduction, the (MnMn3IV)-Mn-III species exhibits two strong IR bands at 745 and 680 cm(-1), and several weaker bands are observed in the 510-425 cm(-1) range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized Mn-4(IV) and reduced (MnMn3IV)-Mn-III species. The calculated force constants for the Mn-4(IV) species are f(r)(IV) = 3.15 mdyn/Angstrom, f(rOr) = 0.55 mdyn/Angstrom, and f(rMnr) = 0.20 mdyn/Angstrom. The force constants for the (MnMn3IV)-Mn-III species are f(r)(IV) = 3.10 mdyn/Angstrom, f(r)(III) = 2.45 mdyn/Angstrom, f(rOr) = 0.40 mdyn/Angstrom, and f(rMnr) = 0.15 mdyn/Angstrom. This study provides insights for the identification of Mn-O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.