Hydrogen-bonding dynamics of photoexcited coumarin 138 and 339 in protic methanol solution: Time-dependent density functional theory study

Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.

DFT study and topological analysis of the bonding in DNA Hoogsteen-type base pairs

The purpose of our work is to characterize and present a theoretical comparative study of a variety of compounds based on DNA base pairs linked with some transition metal ions in gas phase: C–M–G (Cytosine–metal–Guanine) where [Formula: see text](I), Zn(II), Cd(II) and A–M–T (Adenine–metal–Thyminate) where [Formula: see text](II), Ru(I), Ni(I), Y(II), Zn(I), Cd(I), Cu(II). Geometry optimization and frequency calculations were carried out at DFT/ZORA/BLYP-D/TZ2P level. M–N and M–O bonds were investigated with the quantum chemical topology (QCT): Quantum theory of atoms in molecules (QTAIM) and electron localization functions (ELF). The hydrogen bonds: N10–H[Formula: see text]O7 for A–M–T complexes and N7–H[Formula: see text]O10 for C–M–T ones were visualized and discussed, QTAIM and ELF prove the existence of O7–H[Formula: see text]N10 hydrogen bond for some A–M–T systems, since the bond critical point (BCP) of N7–H having [Formula: see text], so it has a covalent character confirming the existence of a tautomer process of these complexes. Bonding energy [Formula: see text], Pauli repulsion [Formula: see text], electrostatic [Formula: see text], and orbital [Formula: see text] interactions were represented and compared together. Hirschfeld’s charges showed the existence of charge transfer process in the bridge moieties. It seems that, in contrast to natural base pairs that are stabilized by hydrogen bonding, Hoogsteen-type base pairs are held together by coordinative bond with metal ions.