MEP: a tool for interpretation and prediction. From molecular structure to solvation effects

Structure and properties of nitrogen-rich 1,4-dicyanotetrazine, C4N6: a comparative study with related tetracyano electron acceptors

The crystal structure, redox electrochemical stability, and reaction chemistry of 1,4-dicyanotetrazine (DCNT) has been experimentally characterized. These experimental results were rationalized by the results of theoretical calculations of the electronic structure, spin and charge distributions, electronic absorption spectra, and electron affinity and compared with the results for related the tetracyano electron acceptors tetracyanoethylene (TCNE), 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), and 2,3,5,6-tetracyanopyrazine (TCNP). DCNT is made from the dehydration of 1,2,4,5-tetrazine-3,6-dicarboxamide, and because of the unusual deep-magenta color of the dicarboxamide in the solid state, its hydrogen-bonded layered structure, electronic structure, and electronic absorption spectra were determined. The magenta color is attributed to its absorptions at 532 nm (18 800 cm(-1)), and this corresponds to normalized chromaticity coordinates of x = 0.42 and y = 0.31 in the pink/red/orange part of the 1931 CIE chromaticity diagram. In contrast with previous reports, DCNT exhibits an irreversible one-electron reduction at -0.09 V vs SCE (MeCN), and reduced forms of DCNT have yet to be isolated and characterized. In addition, the reactions of DCNT with V(CO)6, Fe(II)(C5Me5)2, and I(-) are discussed.

Anion receptors based on highly fluorinated aromatic scaffolds

Mono-, di-, and tri-pentafluorobenzyl-substituted hexafluorobenzene (HFB) scaffolds, viz., R(I), R(II), and R(III) are proposed as promising receptors for molecules of chemical, biological, and environmental relevance, viz., N2, O3, H2O, H2O2, F(-), Cl(-), BF4(-), NO3(-), ClO(-), ClO2(-), ClO3(-), ClO4(-), and SO4(2-). The receptor-guest complexes modeled using M06L/6-311++G(d,p) DFT show a remarkable increase in the complexation energy (E(int)) with an increase in the number of fluorinated aromatic moieties in the receptor. Electron density analysis shows that fluorinated aromatic moieties facilitate the formation of large number of lone pair-π interactions around the guest molecule. The lone pair strength of the guest molecules quantified in terms of the absolute minimum (V(min)) of molecular electrostatic potential show that E(int) strongly depends on the electron deficient nature of the receptor as well as strength of lone pairs in the guest molecule. Compared to HFB, R(I) exhibits 1.1-2.5-fold, R(II) shows 1.6-3.6-fold, and the bowl-shaped R(III) gives 1.8-4.7-fold increase in the magnitude of E(int). For instance, in the cases of HFB···F(-), R(I)···F(-), R(II)···F(-), and R(III)···F(-) the E(int) values are -21.1, -33.7, -38.1, and -50.5 kcal/mol, respectively. The results strongly suggest that tuning lone pair-π interaction provides a powerful strategy to design receptors for small molecules and anions.

Through-Space Effects of Substituents Dominate Molecular Electrostatic Potentials of Substituted Arenes

Model systems have been studied using density functional theory to assess the contributions of π-resonance and through-space effects on electrostatic potentials of substituted arenes. The results contradict the widespread assumption that changes in molecular ESPs reflect only local changes in the electron density. Substituent effects on the ESP above the molecular plane are commonly attributed to changes in the aryl π-system. We show that ESP changes for a collection of substituted benzenes and more complex aromatic systems can be accounted for mostly by through-space effects, with no change in the aryl π-electron density. Only when π-resonance effects are substantial do they influence changes in the ESP above the aromatic ring to any extent. Examples of substituted arenes studied here are taken from the fields of drug design, host-guest chemistry, and crystal engineering. These findings emphasize the potential pitfalls of assuming ESP changes reflect changes in the local electron density. Since ESP changes are frequently used to rationalize and predict intermolecular interactions, these findings have profound implications for our understanding of substituent effects in countless areas of chemistry and molecular biology. Specifically, in many non-covalent interactions there are significant, often neglected, through-space interactions with the substituents. Finally, the present results explain the perhaps unexpectedly good performance of many molecular mechanics force-fields applied to supramolecular assembly phenomena and π-π interactions in biological systems despite the neglect of the polarization of the aryl π-system by substituents.

New Method for Direct Linear-Scaling Calculation of Electron Density of Proteins

A new scheme for direct linear-scaling quantum mechanical calculation of electron density of protein systems is developed. The new scheme gives much improved accuracy of electron density for proteins than the original MFCC (molecular fractionation with conjugate caps) approach in efficient linear-scaling calculation for protein systems. In this new approach, the error associated with each cut in the MFCC approach is estimated by computing the two neighboring amino acids in both cut and uncut calculations and is corrected. Numerical tests are performed on six oligopeptide taken from PDB (protein data bank), and the results show that the new scheme is efficient and accurate.