Infrared spectrum of a spiropyran, 1',3'-dihydro-1',3,3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-[2H]in dole], in a low-temperature argon matrix was measured with a Fourier transformed infrared spectrophotometer. The conformation of this spiropyran was determined by comparison of the observed spectrum with a spectral pattern obtained by the density-functional-theory calculation. This conformation differs from the optimized geometry derived from an HF/6-31G* calculation. The conformations of a merocyanine produced from the spiropyran upon UV irradiation were also determined by the same method. (C) 2003 Elsevier Science B.V. All rights reserved.