Acid–Base Equilibriums of Lumichrome and its 1-Methyl, 3-Methyl, and 1,3-Dimethyl Derivatives

Lumichrome photophysical properties at different pH were characterized by UV–vis spectroscopy and steady-state and time-resolved fluorescence techniques, in four forms of protonation/deprotonation: neutral form, two monoanions, and dianion. The excited-state lifetimes of these forms of lumichrome were measured and discussed. The results were compared to those obtained for similar forms of alloxazine and/or isoalloxazine, and also to those of 1-methyl- and 3-methyllumichrome and 1,3-dimethyllumichrome. The absorption, emission, and synchronous spectra of lumichrome, 1-methyl- and 3-methyllumichrome, and 1,3-dimethyllumichrome at different pH were measured and used in discussion of fluorescence of neutral and deprotonated forms of lumichrome. The analysis of steady-state and time-resolved spectra and the DFT calculations both predict that the N(1) monoanion and the N­(1,3) dianion of lumichrome have predominantly isoalloxazinic structures. Additionally, we confirmed that neutral lumichrome exists in its alloxazinic form only, in both the ground and the excited state. We also confirmed the existence and the alloxazinic structure of a second N(3) monoanion. The estimated values of p<i>K</i>_a = 8.2 are for the equilibrium between neutral lumichrome and alloxazinic and isoalloxazinic monoanions, with proton dissociation from N(1)–H and N(3)–H groups proceeding at the almost the same pH, while the second value p<i>K</i>_a = 11.4 refers to the formation of the isoalloxazinic dianion in the ground state

Variational Calculation of Highly Excited Rovibrational Energy Levels of H₂O₂

Results are presented for highly accurate ab initio variational calculation of the rotation–vibration energy levels of H₂O₂ in its electronic ground state. These results use a recently computed potential energy surface and the variational nuclear–motion programs WARV4, which uses an exact kinetic energy operator, and TROVE, which uses a numerical expansion for the kinetic energy. The TROVE calculations are performed for levels with high values of rotational excitation, <i>J</i> up to 35. The purely ab initio calculations of the rovibrational energy levels reproduce the observed levels with a standard deviation of about 1 cm^–1, similar to that of the <i>J</i> = 0 calculation, because the discrepancy between theory and experiment for rotational energies within a given vibrational state is substantially determined by the error in the vibrational band origin. Minor adjustments are made to the ab initio equilibrium geometry and to the height of the torsional barrier. Using these and correcting the band origins using the error in <i>J</i> = 0 states lowers the standard deviation of the observed–calculated energies to only 0.002 cm^–1 for levels up to <i>J</i> = 10 and 0.02 cm^–1 for all experimentally known energy levels, which extend up to <i>J</i> = 35