Coupling between C-D and C=O vibrational modes of formamide has been characterized by dual-frequency 2D IR spectroscopy. The signals are consistent with those computed assuming an optically dense system at the C=O frequency. A negative off-diagonal anharmonicity of similar to 0.85 +/- 0.20 cm(-1) was obtained and the coupled modes have anticorrelated frequency distributions. Other features including relaxation times, diagonal anharmonicities for the individual C=O and C-D modes and the dephasing dynamics of the cross peak have been determined to characterize the coupling between modes. (c) 2006 Elsevier B.V. All rights reserved.