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This represents a proof-of-concept study of the appropriateness of vibrational and NMR spectroscopy for predicting the molecular structure of large molecules on the basis of a library of small molecules. Density Functional Theory (DFT) B3LYP/6-311G was used generate all spectra. 20 model compounds comprising two multiple-ringed polynuclear aromatic hydrocarbons (PAHs) connected by varying aliphatic chain-lengths were investigated. A least squares optimization algorithm was developed to determine the contribution of molecular subunits in the model compounds. 1H and 13C NMR spectroscopy failed to identify subunits unambiguously even with a constrained library. By contrast, IR and Raman results independently identified 40% and 65% respectively and jointly more than 80 % of the aromatic groups present; however, the aliphatic chain-length was poorly defined in general. IR and Raman spectroscopy are a suitable basis for spectral decomposition and should play a greater role in the identification of ringed subunits present in ill-defined hydrocarbons / Chemical Engineering