An ab initio study of the properties of the linear hydrogen-bonded complex, OC. . .HArF, was undertaken at the MP2/6-311++G(2d,2p) level of theory. The complex has a binding energy of 21 kJ mol(-1) and exhibits a harmonic vibrational frequency blue shift,of 10 cm(-1) for the Ar-H stretching mode, a 7-fold decrease in the IR intensity of this stretching mode and contraction of the Ar-H bond. These results, which are the opposite of what is expected for normal hydrogen bonds, are explained in terms of charge transfer and electrostatic effects. (C) 2002 Elsevier Science B.V. All rights reserved.