We present the first gas phase Fourier transform infrared (FTIR) spectra of flavone, 3-hydroxyflavone, 5-hydroxyflavone, 6-hydroxyflavone and 3-methoxyflavone by using a fast thermal heating technique to vaporize them without decomposition. The intramolecular OH center dot center dot center dot O = C hydrogen bond strength increases from 3HF (5-ring) to 5HF (6-ring) as evidenced by the increasing red-shift and band width of the OH stretch vibration. C = O stretch and OH in-plane bend motions are coupled in the 5- and 6-rings and give rise to two intense IR bands in the range 1600-1700 cm(1). 6HF is not able to form an intramolecular hydrogen bond. (C) 2008 Elsevier B.V. All rights reserved.