Ab initio based calculations of force fields and atomic polar tensors are used to simulate amide I infrared absorption spectra for a series of isotopically substituted (Ac-A(12)-NH-CH3)(n) peptides clustered in an antiparallel beta -sheet conformation having a varying number of strands, n = 2-5. The results demonstrate that the anomalous intensity previously reported for the isotopically shifted amide 1 in C-13 labeled peptides is due to formation of multistranded beta -sheet structures in this conformation. Computations show that the characteristic widely split amide 1 mode for beta -sheet polypeptides as well as this anomalous intensity enhancement in isotopically substituted beta -sheet peptides grows with increasing sheet size. For sheets of five strands, qualitative and near quantitative agreement with experimental amide I intensity patterns is obtained for both labeled and unlabeled peptides. The strongest transitions primarily represent in-phase coupled modes of the C-13 labeled, next nearest neighbor amides on the inner strands of the multistranded beta -sheet. Long-range transition dipole coupling interactions do not promote the C-13 amide I intensity enhancement. Understanding of the IR intensity mechanisms with this level of detail for the isotopically labeled peptides permits design of site-specific probes of beta -sheet folding and unfolding dynamics.