The absorption spectrum of the title compound in the spectral range of the Hydrogen-bonded OH-stretching vibration has been investigated using a five-dimensional gas phase model as well as a QM/MM classical molecular dynamics simulation in solution.
The gas phase model predicts a Fermi-resonance between the OH-stretching fundamental and the first OH-bending overtone transition with considerable oscillator strength redistribution. The anharmonic coupling to a low-frequency vibration of the Hydrogen bond leading to a vibrational progression is studied within a diabatic potential energy curve model. The condensed phase simulation of the dipole-dipole correlation function results in a broad band in the 3000 cm region in good agreement with experimental data. Further, weaker absorption features around 2600 cm have been identified as being due to motion of the Hydrogen within the Hydrogen bond.
A hybrid Car-Parrinello QM/MM molecular dynamics simulation has been carried out for the Watson-Crick base pair of 9-ethyl-8-phenyladenine and 1-cyclohexyluracil in deuterochloroform solution at room temperature. The resulting trajectory is analyzed
putting emphasis on the N-H$...$N Hydrogen bond geometry. Using an empirical correlation between the $NN$-distance and the fundamental NH-stretching frequency, the time-dependence of this energy gap along the trajectory is obtained. From the gap-correlation function we determine the infrared absorption spectrum using lineshape theory in combination with a multimode oscillator model. The obtained average transition frequency and the width of the spectrum is in reasonable agreement with recent experimental data.
