  Hybrid QM(DFT)/MM molecular dynamics simulations have been carried out for
the Watson-Crick base pair of 9-ethyl-8-phenyladenine and 1-cyclohexyluracil in
deuterochloroform solution at room temperature.
  Trajectories are analyzed putting special attention to the geometric
correlations of the $\NHN$ and $\NHO$ hydrogen bonds in the base pair. Further,
based on empirical correlations between the hydrogen bond bond length and the
fundamental NH stretching frequency its fluctuations are obtained along the
trajectory. Using the time dependent frequencies the infrared lineshape is
determined assuming the validity of a second order cumulant expansion. The
deviations for the fundamental transition frequencies are calculated to amount
to less than 2% as compared with experiment. The width of the spectrum for the
$\NHN$ bond is in reasonable agreement with experiment while that for the
$\NHO$ case is underestimated by the present model. Comparing the performance
of different pseudopotentials it is found that the Troullier-Martins
pseudopotential with a 70 Ry cut-off yields the best agreement.
