Aromatic heterocyclic anion based ionic liquids and electrolytes.

Five new ionic materials comprising fluorine-free aromatic heterocyclic anions based on pyridine and pyrazine combined with a common n-tetrabutylphosphonium cation, (P4444)+, result in two room temperature ionic liquids (RTILs), one semi-solid, and two organic ionic plastic crystals (OIPCs) with melting points >20 °C. The OIPCs showed a plastic crystalline phase, multiple solid-solid transitions, and plastic crystalline and melt phases. For both the neat RTILs and the Li+ conducting electrolytes, the nature and strength of the ion-ion interactions mainly depend on the position of the nitrogen atom with respect to the carboxylate group in the anions. Furthermore, for the RTILs the ionic conductivity is effected by the electronic structure and flexibility of the ions and the anions diffuse faster than the (P4444)+ cation, but are slowed down in the electrolytes due to the strong electrostatic interactions between the carboxylate group of the anions and the Li+, as shown both experimentally and computationally. Overall, this study describes the effect of structural tuning of aromatic anions on the ion-ion interactions and introduces new ionic materials with promising properties to be used as solid and liquid electrolytes in energy storage devices.

Theoretical Appraisal of Cyclopropenone: Aggregation and Complexes with Water.

Cyclopropenone (HCCOCH, "CPN") is an exotic quasi-aromatic cyclic carbene that abounds in the interstellar medium (ISM). Astronomical observations suggest that (i) stagnate CPN exhibits a tendency to polymerize and that (ii) interactions may occur between CPN and water that is also ubiquitous in the ISM. In this light, density functional theory investigations reveal cooperative hydrogen bonding, which leads to stable polymeric conformations of (CPN)n, tracked up to n = 14. Stable agglomerations with water, however, constitute at best only two CPN and two water molecules, signifying that while CPN exhibits remarkable cooperativity for "cohesive" clustering via hydrogen bonding, this tendency is markedly diminished for "hetero"-interactions. Multifaceted data are employed to probe cogent molecular descriptors, such as structure and energetics of various conformers, vibrational spectroscopic response, molecular electrostatic potential (MESP), effective atomic charges: all these, in unison, describe the evolution of the characteristics upon cluster formation. Salient stretching frequency shifts, as well as charge redistribution gleaned from MESP morphology, have a direct bearing on variegated hydrogen bonding patterns: linear, nonlinear, as well as bifurcated. In particular, characteristic C-H, C═O stretching, and O-H vibrations in the water complexes reveal a "softening" (downshift) of frequencies. While small conformers have markedly distinct MESP variations, the differences become less pronounced with incremental clustering, an effect substantiated by corresponding emergent atomic charges.

Noncovalent interactions underlying binary mixtures of amino acid based ionic liquids: insights from theory.

Mixtures of ionic liquids formed by blending a common 1-methyl-3-butylimidazolium [Bmim] cation with the dicarboxylic amino acid anions viz., aspartic acid [Asp], asparagine [Asn], glutamic acid [Glu], and glutamine [Gln], have been investigated by employing dispersion corrected density functional theory. Binary mixtures of [Bmim]2[Asp][Asn] and [Bmim]2[Glu][Gln] ionic liquids emerge with distinct structural patterns. Competition between the constituting anions towards cationic binding sites in acidic and basic (polar) amino acid binary mixtures engenders diverse noncovalent interactions, viz., C-HO hydrogen bonding, π-π stacking, and lpπ and CHπ interactions, which impart local liquid structure to these systems governing the structural and physicochemical properties of such double salt ionic liquids (DSILs). The DSIL conformers reveal distinct structural features arising from the middle, normal and front arrangements of anions combined with parallel, antiparallel, rotated or displaced orientations of the cations. The inclusion of dispersion corrections through the D3 method affects their binding energies significantly bringing forth alteration in their energy rank order. Molecular insights accompanying the ion aggregates provide directives for the use of DSILs with improved performance in tribological applications.

Deciphering Noncovalent Interactions Accompanying 7,7,8,8-Tetracyanoquinodimethane Encapsulation within Biphene[n]arenes: Nucleus-Independent Chemical Shifts Approach.

Binding of novel biphene[n]arene hosts to antiaromatic 7,7,8,8-tetracyanoquinodimethane (TCNQ) are investigated by DFT. Biphene[4]arene favors the inclusion complex through noncovalent interactions, such as hydrogen bonding, π-π stacking, C-H⋅⋅⋅π, and C-H⋅⋅⋅H-C dihydrogen bonding. Donor-acceptor complexation renders aromatic character to the guest through charge transfer. The formation of TCNQ anionic radicals through supramolecular π stacking significantly influences its chemical and photophysical behavior. Electron density reorganization consequent to encapsulation of TCNQ reflects in the shift of characteristic vibrations in the IR spectra. The accompanying aromaticities arising from the induced ring currents are analyzed by employing nucleus-independent chemical shifts based profiles.

CO2 Absorption Using Fluorine Functionalized Ionic Liquids: Interplay of Hydrogen and σ-Hole Interactions.

Use of ionic liquids (ILs) for CO2 capture offers certain advantages over currently used methodologies and is of growing interest. With this perspective, ILs composed of S-ethyl-N,N,N',N'-tetramethylthiouronium ([ETT]) and 1-hexyl-3-methylimidazolium ([Hmim]) cations and tris(pentafluoroethyl)trifluorophosphate ([FEP]) anion have been investigated. The present work unravels the noncovalent interactions accompanying CO2 capture by these ILs. Electronic structure of ion pairs and their CO2 absorbed [ETT][FEP]·n(CO2) and [Hmim][FEP]·n(CO2) (n up to 30) complexes are derived. The anisotropy in molecular electrostatic potential dictates the binding of CO2 through the interplay of (i) halogen bonding (O···F) between electron deficient σ-holes on fluorines, (ii) electrostatic C···F interactions between electron deficient carbons of CO2 and the electron-rich fluorine atoms, and the (iii) hydrogen bonding (O···H) interactions from the cation. The manifestations of these interactions on binding energies, polarizabilities, and vibrational spectra of CO2 absorbed complexes are presented. Consequent "frequency shift" accompanying hydrogen and halogen bonding exhibit complementary characteristics in the infrared spectra of CO2 absorbed complexes. Correlation of binding energies to absorbed CO2 molecules further demonstrate that [Hmim] based ILs are more efficient for CO2 capture applications.

Electronic Structure, NMR, Spin-Spin Coupling, and Noncovalent Interactions in Aromatic Amino Acid Based Ionic Liquids.

Noncovalent interactions accompanying phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) amino acids based ionic liquids (AAILs) composed of 1-methyl-3-butyl-imidazole and its methyl-substituted derivative as cations have been analyzed employing the dispersion corrected density functional theory. It has been shown that cation-anion binding in these bioionic ILs is primarily facilitated through hydrogen bonding in addition to lp---π and CH---π interactions those arising from aromatic moieties which can be probed through (1)H and (13)C NMR spectra calculated from the gauge independent atomic orbital method. Characteristic NMR spin-spin coupling constants across hydrogen bonds of ion pair structures viz., Fermi contact, spin-orbit and spin-dipole terms show strong dependence on mutual orientation of cation with the amino acid anion. The spin-spin coupling mechanism transmits spin polarization via electric field effect originating from lp---π interactions whereas the electron delocalization from lone pair on the carbonyl oxygen to antibonding C-H orbital is facilitated by hydrogen bonding. It has been demonstrated that indirect spin-spin coupling constants across the hydrogen bonds correlate linearly with hydrogen bond distances. The binding energies and dissected nucleus independent chemical shifts (NICS) document mutual reduction of aromaticity of hydrogen bonded ion pairs consequent to localization of π-character. Moreover the nature and type of such noncovalent interactions governing the in-plane and out-of-plane NICS components provide a measure of diatropic and paratropic currents for the aromatic rings of varying size in AAILs. Besides the direction of frequency shifts of characteristic C═O and NH stretching vibrations in the calculated vibrational spectra has been rationalized.

Hydrogen Bonding, (1)H NMR, and Molecular Electron Density Topographical Characteristics of Ionic Liquids Based on Amino Acid Cations and Their Ester Derivatives.

Amino acid ionic liquids (AAILs) have attracted significant attention in the recent literature owing to their ubiquitous applications in diversifying areas of modern chemistry, materials science, and biosciences. The present work focuses on unraveling the molecular interactions underlying AAILs. Electronic structures of ion pairs consisting of amino acid cations ([AA(+)], AA = Gly, Ala, Val, Leu, Ile, Pro, Ser, Thr) and their ester substituted derivatives [AAE(+)] interacting with nitrate anion [NO3(-)] have been obtained from the dispersion corrected M06-2x density functional theory. The formation of ion pair is accompanied by the transfer of proton from quaternary nitrogen to anion facilitated via hydrogen bonding. The [Ile], [Pro], [Ser], and [Thr] and their esters reveal relatively strong inter- as well as intramolecular hydrogen-bonding interactions. Consequently, the hierarchy in binding energies of [AA][NO3] ion pairs and their ester analogues turns out to be [Gly] > [Ala] > [Ser] ∼ [Val] ∼ [Ile] > [Leu] ∼ [Thr] > [Pro]. The work underlines how the interplay of intra- as well as intermolecular hydrogen-bonding interactions in [AA]- and [AAE]-based ILs manifest in their infrared and (1)H NMR spectra. Substitution of -OCH3 functional group in [AA][NO3] ILs lowers the melting point attributed to weaker hydrogen-bonding interactions, making them suitable for room temperature applications. As opposed to gas phase structures, the presence of solvent (DMSO) does not bring about any proton transfer in the ion pairs or their ester analogues. Calculated (1)H NMR chemical shifts of the solvated structures agree well with those from experiment. Correlations of decomposition temperatures in [AA]- and [AAE]-based ILs with binding energies and electron densities at the bond critical point(s) in molecular electron density topography, have been established.

Distinct photophysical behaviour and transport of cell-impermeable [Ru(bpy)2dppz]2+ in live cells using cucurbit[7]uril as a delivery system.

Here, we report the delivery of a cell-impermeable [Ru(bpy)2dppz]2+ complex across a cell membrane using a cucurbit[7]uril molecular container. Encapsulation of complex 1 in the cucurbit[7]uril cavity showed an 830-fold enhancement in the luminescence intensity of the non-emissive complex in aqueous solution. This molecular light-switch effect stems from the incorporation of the dppz ligand of 1 inside the CB7 cavity and can be attributed to long range coulombic forces between Ru2+ and the carbonyl portal of CB7 via CHO interactions. This is reflected in the 1H-NMR experiments, and further corroborated by theoretical calculations.

Density functional investigations on 2-naphthalenecarbonitrile dimerization within cucurbit[8]uril cavitand.

Dimerization of 2-naphthalenecarbonitrile (2-NpCN) mediated by cucurbit[8]uril (CB[8]) has been investigated employing the density functional theory. Different structures of 2-NpCN dimers were generated by combining monomers in anti-head-to-head (A), anti-head-to-tail (B) and syn-head-to-tail (C) fashion. All these dimeric structures possess rigid cube-like architecture. On confinement within the CB[8] dimer A turns out to be the lowest energy structure. Calculated (1)H NMR spectra revealed that the 2-NpCN dimer exhibits large shielding for aromatic protons consistent with the experiment. The protons attached to cubane moiety on the other hand, led to down-field signals. Dimerization mediated with CB[8] cavitand is further accompanied by the frequency up-shift (blue shift) of methylene stretching vibration in its infrared spectra.