Two-Dimensional Infrared Spectra of Cationic Dopamine under Different Electric Fields: Theoretical Studies from the Density Function Theory Anharmonic Potential

The geometry optimizations and vibrational frequencies of cationic dopamine (DAH+) under different intensity of electric fields (IEFs) are investigated using density function theory (DFT)/B3LYP/6-311G(d,p) and two-dimensional infrared spectra are pictured based on anharmonic approximation. In spite of some differences, the optimized structural parameters calculated by DFT/B3LYP/6-311G(d,p) can well reproduce the experimental ones. Anharmonic approximation greatly improves the agreement between experimental and calculated fundamental frequencies. This study mainly focuses on the potential energy distributions and rotational isomerization of DAH+ under different electric fields. The results showed that there were very strong absorption peaks at 3571 and 3683 cm(-1) for O-H stretching vibrations. The symmetric and asymmetric peaks of C-H stretching vibrations with benzene ring and amidogen were calculated by anharmonic approximation and observed at 2921, 2904, 2997, and 3007 cm(-1). In addition, strong absorption peaks were also observed at 3266, 3313, and 3316 cm(-1) for N-H stretching vibrations. For C-H stretching vibrations with amidogen, when IEF is increasing from -30 x 10(-4) to 20 x 10(-4) a.u., the vibrations of symmetry and asymmetry are coupled. However, the two vibrations were no longer coupled while IEF goes on increasing to 30 x 10(-4) a.u.; For C-H stretching vibrations with benzene ring, the vibrations of symmetry and asymmetry were uncoupled when IEFs are 20 x 10(-4), 10 x 10(-4), 0 x 10(-4), and 30 x 10(-4) a.u. In addition, DAH+ undergoes rotational isomerization only when IEF ranges from 20 x 10(-4) to 10 x 10(-4) a.u.