Density functional theory studies on molecular structure and IR spectra of 9-methyladenine: A scaled quantum mechanical force field approach

Abstract

Density functional theory with the combined Becke's three-parameter exchange functional in combination with the Lee, Yang, and Parr correlation functional (B3LYP) exchange–correlation energy functions using the 6-31G* basis set was applied to study the structures and vibrational infrared (IR) spectra of 9-methyladenine and the related compounds purine and adenine. The calculated force fields were scaled with the scale factors determined from purine. The predicted vibrational frequencies and isotopic shifts of IR bands of adenine and 9-methyladenine were compared with the available observed matrix-isolated IR spectra with a mean deviation of about 6.4 cm−1. The results made it possible to give reliable assignments of the IR spectra of these molecules. This study shows that the scaled density functional force field approach enables, through the transferability of scale factors, good interpretation of vibrational spectra of large molecules. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 76: 686–699, 2000