An experimental charge density study has been carried out on proton-transfer complexes exhibiting nonlinear optical (NLO) properties-melaminium tartrate monohydrate and L-asparaginium picrate employing high-resolution X-ray diffraction at 100 K. Both the complexes crystallize in non-centric space group P2₁ and the structures exhibit interesting patterns of N-H…O and O-H…O hydrogen bonding. Experimental determination of the dipole moment (𝜇) for the asymmetric unit reveals that for both the crystals, there is a large cooperative enhancement in the crystalline 𝜇 arising essentially due to hydrogen bond mediated charge transfer between the melaminium ion and the L-tartrate in one case, between the Lasparaginium ion and the picrate in the other complex. We have additionally performed theoretical calculations at the density functional theory (DFT) level to understand the origin of enhancement of the dipole moments in the two systems.

We have modelled for the first time a chemodosimeter for As(III) detection in water. The chemodosimeter modelled is a 1,3-dithiole-2-thione derivative with an anthracene unit which has been previously described as a chemodosimeter for Hg(II) detection. Quantum chemical calculations at the DFT level have been used to describe the binding energies and selectivity of the chemodosimeter. We find that the dosimeter action is intrinsically dependent on the thiophillic affinity and the coordination sphere of the metal ion. Binding studies for a series of metal ions: Pb(II), Cd(II), Hg(II), Ni(II) and As(III) followed by an analysis of the complete reaction pathway explains the high selectivity of the dosimeter towards As(III). The dosimeter efficiency is calculated as 66% for As(III)-ion.

Structures of planar and bowl-shaped circulenes as well as their stacks with G-quartet (G4) have been investigated through dispersion-corrected Density Functional Theory (DFT-D). The binding energies are substantial $\tilde ∼$10 kcal/mol with d ∼3.5 Å between the stacking rings. The calculations show that G4 binds much more effectively to planar circulenes as compared to bowl shaped molecules. The strength of binding between a G-quartet and a non-planar circulene molecule depends on the orientation of the circulene (concave or convex) with respect to G-quartet. An AIM analysis of the M05-2X wave-functions has also been performed to confirm the presence of weak intermolecular interactions between guanine quartets and circulenes. Apart from 𝜋-stacking interactions, the concave bowl-shaped circulenes also interact with G4 through C-H$\cdots \pi$ interactions. The charge transport properties between the two moieties have also been analysed through effective transport integral. The calculations provide an understanding for the basis of molecular recognition by G4 for non-planar systems.

The pronounced effect of quantum mechanical tunneling in chemical reactions involving light atoms like hydrogen is well established. Recent studies have found that tunneling can also play a significant role for common organic transformations, where if the participating atom is carbon/nitrogen, etc. For thesecases, various reaction parameters like reaction barrier, barrier width and temperature play a crucial role in determining the efficiency of tunneling. In this review, we have focused on all those organic transformations where the influential role of tunneling has been documented either computationally or experimentally