We propose three reasons for the simplicity of vibrationally resolved electronic spectra of complex chain molecules: vibr(onic) state localization, insensitivity of chromophores to conformational changes outside the chromophore, and an 'energy gap law' for low-lying conformers. Here we use alkanals as simple models of chain molecules to study the third proposal. A disconnectivity tree and scaling analysis reveals an energy gap creating just two 'special' conformational minima in the torsional energy landscape of alkanals. We also derive a simple scaling formula to connect the energy spanned by all torsional minima in the alkanal energy landscape with molecular size. (C) 2011 Elsevier B.V. All rights reserved.