Infrared spectra of gas-phase metal-nitrate anions M(NO3)3-, where M = Mg2+, Ca2+, Sr2+ and Ba2+, were recorded by infrared multiple-photon dissociation (IRMPD) spectroscopy. Photodissociation of each of the precursors produces NO3- through the elimination of a neutral M(NO3)2 unit. An absorption pattern characteristic of metal nitrates is observed in the IRMPD spectra, including the symmetric and antisymmetric NO3 stretches. The latter is split into high-and low-frequency components as a result of perturbation of the nitrate symmetry by complexation to the metal ion, and the magnitude of the splitting decreases following the trend Mg2+ > Ca2+ > Sr2+ ≅ Ba2+. The experimental spectra are in good general agreement with those obtained from density functional theory (DFT) calculations. © 2008 Elsevier B.V. All rights reserved.