Stability and Anharmonic N–H Stretching Frequencies of 1‑Methylthymine Dimers: Hydrogen Bonding versus π‑Stacking

Stability of three hydrogen-bonded and six stacked 1-methylthymine (1 mT) dimers was studied with the DFT-D3 method at various temperatures. It was demonstrated that the stacked dimers are slightly less stable than the hydrogen-bonded counterparts. Existence of T-shaped dimers is addressed. Anharmonic couplings that involve N–H stretching modes of the nine species are studied. Surprisingly, we find that N–H stretching modes of the two 1 mT molecules are significantly coupled in four stacked dimers. The presented results shed light on existence of strong mode couplings between the two N–H stretching modes in stacked aromatic species. Our calculations support the proposal (J. Phys. Chem. A 2011, 115, 9429−9439) that presence of several dimers is responsible for appearance of wide and structured bands in 1 mT homodimers’ jet-cooled spectra above 2900 cm^–1.