High-resolution infrared spectra of Ne-(CO2)-C-12-O-18, and Xe-(CO2)-C-12-O-18 have been observed in the v(3) band (2314 cm(-1)) region of (CO2)-C-12-O-18 by diode laser absorption spectroscopy of a pulsed molecular beam. The vibrational band origins and rotational constants for these complexes are obtained. The observed shifts of the v3 band origin for Rg-CO2 (Rg = Ne, Ar, Kr, Xe) with respect to that of CO2 exhibit a systematic O-16-O-18 isotope effect excepting a significant deviation for Ne-CO2 from a regular trend. This apparent anomaly is interpreted in terms of a change in the intramolecular potential function Of CO2 caused by complex formation. (c) 2006 Elsevier B.V. All rights reserved.