We have performed quantum chemical calculations for the optimized structure and Raman vibrational frequencies-based on density functional theory at the B3LYP level and using the 6-31G(dp) basis set-for the cyanine dye cation 1,1'-diethyl-2,2'-quinocyanine, also referred to as pseudoisocyanine (PIC). We have ascertained that the equilibrium structure of ground-state PIC has near C-2 syn-unetry, with a 46degrees twist between the planes of the two quinoline moieties that are positioned about the central methine carbon. Vibrational mode analysis of the calculated Raman spectrum suggests that many of the experimentally observed Raman bands between 500 and 1800 cm(-1) for monomeric PIC are associated with totally symmetric in-plane deformations of phenyl and/or pyridyl rings, while several weak bands below 500 cm(-1) are attributed to out-of-plane doming and ruffling of quinoline macrocycles. We further have noted that (1) upon comparison of the Raman spectrum of PIC monomer with the Raman spectrum measured for aggregated PIC, under nonresonant condition, changes reveal that aggregation results in enhanced scattering for specific vibrational modes that contain principal contributions from in-plane deformation of the phenyl ring in the quinoline moiety; (2) with resonant excitation for the aggregate, observed vibrational modes associated with out-of-plane distortions of the quinoline macrocycle are found to exhibit even greater enhancement. Analysis of our Raman measurements for monomeric and aggregated PIC provides details about the structure of the molecular aggregate. Additionally, calculation of charge distribution, utilizing the Mulliken population analysis approach, indicates that positive and negative charges are alternately and symmetrically distributed over a conjugated ring system, and the positive charges among the peripheral hydrogens and those in the N-ethyl side chains are asymmetrically distributed.