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

Abstract

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 caff 1 -(H 2 O) 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 S 0 -S 1 transition energy due to the most stable and the least stable conformation of caffeine was found to be $859 cm À1 . One striking feature is the coexistence of the blue and red shift of the vertical excitation energy of the optically bright state S 1 ( 1 pp*) of caffeine upon forming a complex with a water at isolated and conjugated carbonyl sites, respectively. The lowest singlet pp* excited-state of the caff 1 -(H 2 O) 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 caff 1 -(H 2 O) 1 complexes involving conjugated carbonyl, the lowest singlet pp* excited-state is red shifted. The effect of hydration on the S 1 ( 1 pp*) 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 p-electron delocalization extent of the molecule because of hydrogen bond formation. † Electronic supplementary information (ESI) available: The structural parameters, selected harmonic wave number and rotational constants of all six conformers caffeine at MP2/6-311++G(d,p) level. See

Exploring Caffeine–Phenol Interactions by the Inseparable Duet of Experimental and Theoretical Data

“…In such conformers, phenol interacts with caffeine through π⋅⋅⋅π interactions and a hydrogen bond with C=O of caffeine. This assignment would be in agreement with previous studies on caffeine–water, in which water was also found to interact preferentially with the carbonyl groups of caffeine . However, no consensus was found between them for the relative stability of the local minima.…”

“…In such conformers, phenol interacts with caffeine through p···p interactions and ah ydrogen bond with C=Oo fc affeine.T hisa ssignment would be in agreement with previous studies on caffeine-water,i nw hich water was also found to interact preferentially with the carbonyl groups of caffeine. [44,45,65] However,n oc onsensus was found between them for the relative stabilityo ft he local minima. This seems to indicatet hat the methods used are abletoa ccurately model stronger interactions, but they balance other weaker interactions in different ways.…”

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

“…The VEE to the same L b ( 1 pp*) state for Sero 1 -(H 2 O) 1 involving conformer Gph(out)/anti (C) is red shied by 314 cm À1 and for the strongly H-bonded Sero 1 -(H 2 O) 2 complex involving the same conformer C, it (VEE to the L b ( 1 pp*) state) was signicantly red shied by 1287 cm À1 in comparison to monomer. 52 The corresponding red shi (in L b ( 1 pp*) state) for Sero 1 -(H 2 O) 1,2 involving C conformation shows that S 0 -S 1 electronic transition increases the strength of the hydrogen bonds to water. 22 Thus the blue shi in L b ( 1 pp*) state of Sero 1 -(H 2 O) 1 (see Fig.…”

“…A blue shi for a p-p* transition is also attributed to the reduction in the p-electron conjugation length of the molecule due to the hydrogen bond formation. 52 The corresponding red shi (in L b ( 1 pp*) state) for Sero 1 -(H 2 O) 1,2 involving C conformation shows that S 0 -S 1 electronic transition increases the strength of the hydrogen bonds to water. Interestingly the VEE to the optically bright state 1 L a ( 1 pp*) of Sero 1 -(H 2 O) 1 (see Fig.…”

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

“…In the experimental IR spectra 1 the carbonyl stretching modes consists of two major bands; the higher wavenumber band and the lower wavenumber band. Similar to caffeine 42,43 the two C]O groups of theophylline couple into an in phase C]O stretching vibration and an out of phase stretching vibrations. The out of phase (C]O) 2 stretching mode is observed at a lower frequency while the in-phase mode is observed at higher frequency.…”