A density functional study of the vibrations of three oligomers of thiophene

The accuracy of two density functional derived models-BLYP/6-31G* and B3LYP/6-31G*-is tested to calculate the molecular response to slow neutrons and infrared photons in a series of oligomers of thiophene. In the first type of experiment, the response is a function of the vibrational frequencies and the shapes of the normal modes; in the second, knowledge of the dipole moment surface is also necessary. The combination of the two simulations allows one to conclude that both models give fairly accurate vibrational frequencies and normal modes but may overestimate the infrared response in large systems. For this spectroscopy, BLYP/ 6-31G* and B3LYP/6-31G* find all the modes present in the experiment to be active, A few modes with modest activity are also calculated to appear strongly in the spectrum. Scaling of the force fields shows the complementary roles of the two methods. BLYP/6-31G* is very accurate-scaling factor of 1.00-in the calculation of the C-alpha-C-beta, C-alpha-C-alpha, and HCC force constants, and B3LYP/6-31G* does not require scaling of CS, SCC, CCC, and CSC force constants. On the basis of the combined use of the two models, a simple procedure is proposed that should give good agreement with experimental results of conjugated systems.