Two chiral conformers of the optically active molecule naproxen have been identified in its high resolution electronic spectrum, differing only in the orientation of the attached isopropionic acid group relative to the naphthalene ring. Both conformers exhibit b-type spectra, indicating that the S-1 states of the isolated molecules are similar to those of other 2-substituted naphthalenes. The measured rotational constants of the left-and right-handed forms of both conformers are identical to within the experimental error, +/- 0.1 MHz (similar to 1:10(9)), indicating that they are enantiomers of each other, not diastereomers. The naphthyl ring with its attached methoxy group is insufficient to provide chiral discrimination. (C) 2011 Elsevier B.V. All rights reserved.