Microhydration of 2-Naphthol at Ground, First Excited Triplet, and First Excited Singlet States: A Case Study on Photo Acids.

Molecular interactions of 2-naphthol (nap) with water molecules are studied at the ground, first excited triplet and first excited singlet states, applying DFT and TD-DFT methods. The minimum energy structure of hydrated clusters of 2-naphthol up to four water molecules are selected from several possible input geometries. It is observed that the minimum energy conformer of the tetra-hydrate of 2-naphthol has proton transfer occurring from nap to solvent water molecules, in its first excited singlet state. This is however not observed in case of its ground or first excited triplet state. It is consistent with the fact that the pKa of nap in the first excited singlet state is very much lower compared to the ground and first excited triplet state. This is also reflected in the O-H potential energy profile of tetrahydrate of nap, obtained by performing a rigid potential energy scan of the dissociating O-H bond of nap at ground, first excited triplet and first excited singlet states. Frequency of O-H stretching vibration of 2-napthol and its hydrated clusters in the ground (S0) as well as in the first excited singlet (S1) state are calculated and compared with the available experimental data. The performance of macroscopic solvation model is also examined in the ground and these excited states.

Microhydration of Neutral and Charged Acetic Acid.

A systematic theoretical study has been carried out on the effect of sequential addition of water molecules to neutral and mono positively charged acetic acid molecules by applying first principle based electronic structure theory. Geometry, dipole moment, and polarizability of hydrated clusters of neutral and mono positively charged acetic acid of the type CH3COOH·nH2O (n = 1-8) and [CH3COOH·nH2O]+ (n = 1, 2) are calculated at the ωB97X-D/aug-cc-pVDZ level of theory. Free energies of formation of the hydrated acid clusters, at different temperatures and pressures are determined. Solvent stabilization energy and interaction energy are also calculated at the CCSD(T)/6-311++G(d,p) level of theory. It is observed that in the case of neutral acetic acid, proton transfer from the acid molecule to solvent water molecules does not occur even with eight water molecules and the acid molecule remains in the undissociated form. High-energy equilibrium structures showing dissociation of acetic acid are obtained in case of hexahydrated and larger hydrated clusters only. However, dissociation of mono positively charged acetic acid occurs with just two water molecules. Interestingly, it is noted that in the case of dissociation, calculated bond dipole moments of the dissociating bonds of acetic acid in microhydated clusters shows a characteristic feature. IR spectra of CH3COOH·nH2O (n = 1-8) and [CH3COOH·nH2O]+ (n = 1-3) clusters are simulated and compared with the available experimental data.

Effect of microhydration on dissociation of trifluoroacetic acid.

First-principle-based electronic structure calculations were carried out on microhydrated trifluoroacetic acid clusters (CF3COOH, tfa) to understand its molecular level interaction with water and subsequent ionic dissociation to form CF3COO(-) ion. From several geometrical inputs, the global minimum energy structure of hydrated cluster, tfa · nH2O (n = 1-7), was obtained adopting dispersion-corrected density functional, namely, ωB97X-D, and a set of correlated atomic basis function, aug-cc-pVDZ. It was predicted that tfa requires at least six H2O molecules to dissociate. Energy parameters of these hydrated clusters were improved by applying MP2 as well as CCSD(T) methods. A linear variation was observed for calculated solvent stabilization energy profile with the number of solvent H2O molecules present in the hydrated cluster. However, the calculated interaction energy profile showed the characteristic feature indicating the formation of contact ion-pair on the addition of six H2O molecules to tfa. On the basis of energy decomposition analysis, it was observed that the major interaction between tfa and H2O molecules was of electrostatic nature. On successive addition of water molecules, the electrostatic component of the interaction between solute and solvent molecules depicted a sudden increase when moving from penta- to hexahydrated cluster. This observed nature of energy profile coincided with the formation of hydronium ion in the case of hexahydrated cluster. The formation of H3O(+) was manifested in simulated IR spectra of tfa·6H2O and tfa · 7H2O clusters. A large red shift in IR peak positions corresponding to O-H stretching of tfa was predicted on microhydration.

Supramolecularly Assisted Modulation of Optical Properties of BODIPY-Benzimidazole Conjugates.

Supramolecular host-guest interaction of neutral and cationic (protonated) forms of two boron-dipyromethane (BODIPY)-benzimidazole (mono- and di-benzimidazole) conjugate dyes with the macrocyclic host cucurbit[7]uril (CB7) has been investigated using photophysical and density functional theory studies. Expectedly, cationic forms of the dyes show exceptionally stronger binding than that of the neutral forms with CB7, which can be ascribed to the strong ion-dipole interaction between the positive charge of the dye and the highly polarizable carbonyl portals of the host. The formation of dye-host inclusion complexes is supported by the significant changes in the photophysical properties and longer rotational relaxation times of the dye in the presence of CB7. Job's plot studies indicate the formation of a 1:1 inclusion complex for the mono and a 1:2 inclusion complex for the dibenzimidazole BODIPY dyes. Quantum chemical calculations are in good agreement with the inferences outlined from photophysical measurements. Findings from the studied dye-CB7 systems are of direct relevance to applications such as drug delivery, aqueous dye lasers, sensors, and so on.