Identifying adenine–thymine base pairing by anharmonic analysis of the hydrogen-bonded NH stretching vibrations

Infrared spectra in the frequency range of hydrogen-bonded NH stretching vibrations of three different adenine–thymine gas-phase isomers are studied. This includes the adenine symmetric and antisymmetric NH2 vibrations as well as the thymine NH vibration. The ab initio Hamiltonian combines single mode potentials on a grid with up to fourth order force constants to account for mode coupling. Infrared absorption spectra are obtained from dipole autocorrelation functions using the multiconfiguration time-dependent Hartree-approach. In the range of the thymine NH vibration the spectra are sufficiently different to allow for a clear assignment of the experimentally observed isomer.