The position of the nu(XH) stretching band for more than 100 proton donors (X = O, N, C, S, Se, P, Si, B, F, Cl, Br) complexed with CO2 was calculated at the MP2/6-311++G(d,p) level. Complexation energies and natural bond orbital characteristics were also determined. These calculations revealed that the direction of the m(XH) band shift was related to the charge born by the hydrogen atom of the proton donor. XH center dot center dot center dot O=C=O blue-shifting hydrogen bonds were found for compounds characterised by a small charge, while red-shifting was observed for those compounds with a high positive charge on the hydrogen atom. (C) 2011 Elsevier B.V. All rights reserved.