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 × 10–4 to 20 × 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 × 10–4 a.u.; For
C–H stretching vibrations with benzene ring, the vibrations
of symmetry and asymmetry were uncoupled when IEFs are −20
× 10–4, −10 × 10–4, 0 × 10–4, and 30 × 10–4 a.u. In addition, DAH+ undergoes rotational isomerization only when
IEF ranges from −20 × 10–4 to 10 ×
10–4 a.u.