Fourier transform Raman spectrum of trans isoindigo was measured in polycrystalline phase for the first time. Ground state geometry and vibrational frequencies together with Raman intensities were computed by density functional theory (DFT: B3LYP/6-311G++(d,p)). All intense observed Raman frequencies were identified as totally symmetric. The difference of carbon-carbon bond lengths of trans isoindigo compared to trans stilbene, relates well with the shifts of C=C stretching modes frequencies. C=O stretching and N-H bending modes are significantly affected by intermolecular hydrogen bonding. (c) 2009 Elsevier B.V. All rights reserved.