Density functional theory studies on molecular structure and IR spectra of9-methyladenine: A scaled quantum mechanical force field approach

Density functional theory with the combined Becke's three-parameter exchang e functional in combination with the Lee, Yang, and Parr correlation functi onal (B3LYP) exchange-correlation energy functions using the 6-31G* basis s et was applied to study the structures and vibrational infrared (IR) spectr a of 9-methyladenine and the related compounds purine and adenine. The calc ulated force fields were scaled with the scale factors determined from puri ne. The predicted vibrational frequencies and isotopic shifts of LR bands o f adenine and 9-methyladenine were compared with the available observed mat rix-isolated IR spectra with a mean deviation of about 6.4 cm(-1). The resu lts made it possible to give reliable assignments of the LR spectra of thes e molecules. This study shows that the scaled density functional force fiel d approach enables, through the transferability of scale factors, good inte rpretation of vibrational spectra of large molecules. (C) 2000 John Wiley & Sons, Inc.