The molecules benzo[a]pyrene (BaP) and 1-, 3-, and 6-nitrobenzo[a]pyrene (1-NBaP, 3-NBaP, 6-NBaP) are currently of significant interest due to their presence in respirable combustion exhaust particulates and their mutagenic and carcinogenic properties. Structure-function correlations as well as spectroscopic signatures for trace analysis are necessary for these benzo[a]pyrene derivatives. In this paper, detailed infrared and Raman spectroscopic data of BaP and its three mononitrated isomers are provided for the first time. By utilizing density functional theory (DFT, B3LYP method with 6-311+G basis set), the molecular geometries and the vibrational spectra are calculated. Good agreement is noted between the calculated and experimental geometry for BaP, and predictions of the vibrational data for all compounds are within approximately 5 cm-1 of the experimental data. Normal mode assignments are proposed with particular emphasis on the nitro group vibrations. The geometrical distortions of the BaP structure upon nitro group substitution and correlations between structural parameters and vibrational data as well as structure-function relationships related to the mutagenicity of this important class of polycyclic aromatic hydrocarbons are discussed.