Theoretical study of the photoinduced intramolecular proton transfer and rotational processes in 2-(2′hydroxyphenyl)-4-methyloxazole in gas phase and embedded in β-cyclodextrin

The intramolecular proton transfer and the internal rotations of the 2-(2′hydroxyphenyl)-4-methyloxazole (HPMO) in the first electronically excited singlet state (S1) have been theoretically studied. Electronic calculations have been carried out within an all-single configuration interaction scheme (CIS). Time-dependent DFT (TDDFT) calculations have been performed to correct the energies of the proton transfer as CIS tends to overestimate the energy barriers. The effect of confinement of the HPMO molecule inside the cavity of β-cyclodextrin (β-CD) has also been studied. The ONIOM hybrid method is used to deal with the large host-guest system. Within the ONIOM procedure two levels of calculation are defined: CIS or TDDFT for the HPMO and the semiempirical PM3 method for the β-CD. A comparison of the electronic energies reveals that the proton-transfer process has a lower energy barrier than the subsequent internal rotation of the keto tautomer, both in the isolated system and in the host-guest complex. However, the initial energy of the wavepacket accessed upon photoexcitation (vertical transition) is high enough to surpass both barriers, so that electronic energies alone are not able to explain the different reaction times found for both processes by means of time-resolved (femtosecond) fluorescence experiments. A dynamic method based on the RRKM statistical theory has been used to account for this difference. The so calculated rate constants also reproduce the increment in the time for the internal rotation process when HPMO is confined inside the β-CD cavity. Analysis of the different factors that contribute to the rate constant disclose that this delay is due to the increment of rigidity of HPMO that takes place upon encapsulation.