Spectroscopic signatures and structural motifs in isolated and hydrated caffeine: a computational study

The conformational landscapes of neutral caffeine and its hydrated complex have been investigated by MP2 and DFT methods. The ground state geometry optimization yields six lowest energy structures for bare caffeine and five lowest energy conformers of the caff₁–(H₂O)₁ complex at the MP2/6-311++G(d,p) level of theory for the first time. We investigated the low-lying excited states of bare caffeine by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra (Phys. Chem. Chem. Phys., 2012, 14, 10677–10682) is obtained. The difference between the S₀–S₁ transition energy due to the most stable and the least stable conformation of caffeine was found to be ∼859 cm⁻¹. One striking feature is the coexistence of the blue and red shift of the vertical excitation energy of the optically bright state S₁ (¹ππ*) of caffeine upon forming a complex with a water at isolated and conjugated carbonyl sites, respectively. The lowest singlet ππ* excited-state of the caff₁–(H₂O)₁ complex involving isolated carbonyl is strongly blue shifted which is in agreement with the result of R2PI spectra of singly hydrated caffeine (J. Chem. Phys., 2008, 128, 134310). While for the most stable and the second most stable caff₁–(H₂O)₁ complexes involving conjugated carbonyl, the lowest singlet ππ* excited-state is red shifted. The effect of hydration on the S₁ (¹ππ*) excited state due to the bulk water environment was mimicked by a combination of a polarizable continuum solvent model (PCM) and a conductor like screening model (COSMO), which also shows a blue shift in accordance with the result of electronic absorption spectra in an aqueous solution (Phys. Chem. Chem. Phys., 2012, 14, 10677–10682). This hypsochromic shift is expected to be the result of the changes in the π-electron delocalization extent of the molecule because of hydrogen bond formation.