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.

Cyclopropenylidene: Clustering and Interaction with Water Molecules.

Cyclopropenylidene (c-C3H2, abbreviated CPD) is a highly reactive, planar, partially aromatic carbene discovered in the interstellar medium, and, also recently, in the outer solar system. It is demonstrated herein on cogent quantum chemical grounds that CPD which possesses an electric dipole moment of 3.4 D is capable of forming stable dimer and trimer clusters through hydrogen-bonding. These attributes of CPD are conducive to the formation of stable hydrogen-bonded conformations with one- and two-water molecules. Having determined its consistency with the second-order Møller-Plesset perturbation theory MP2, we employ the ωB97xD hybrid density functional theory in conjunction with a 6-311++G(2d,2p) basis set for a credible description of noncovalent interactions involved in clustering. Molecular electrostatic potential (MESP) and characteristic vibrational frequency shifts upon clustering are presented.

Design and synthesis of piezochromic materials exploring intermolecular charge transfer: chalconoids bound to the p-sulfonatocalix[6]arene macrocycle.

Solid-state systems composed of chalconoid encapsulated within p-sulfonatocalix[6]arene (SCX6) scaffolds that exhibit mechanochromism and thermochromism have been developed. An introduction of a supramolecular host promises a variety of applications in diverse areas, which makes them fascinating. Largely hydrogen bonding as well as π···π interactions are responsible for the host-guest complexation. The complex shows partial encapsulation of the guest with one of the phenyl rings of chalcone (guest) is held inside the SCX6 cavity, whilst other phenyl rings that exclude the cavity are hydrogen-bonded to sulfonate portals of the host. The hydrogen bonding conducing such complexation triggers proton transfer engendering a mechanochromic switch. The complexes are further characterized by a variety of experiments such as cyclic voltammetry (CV), steady-state fluorescence, vibrational spectroscopy, and 1H or 2D NMR (NOESY) spectroscopy experiments. Detailed structure furnished through the NMR shows deshielding of the Ha-e (guest) protons whereas, the hydroxyl protons from the host experience shielding as evidenced from the 1H NMR spectra. These inferences have further been corroborated through the density functional theory. Electrochemical investigations suggested an irreversible one-electron transfer in the host-guest binding. The characteristic 'frequency shift' for the intense carbonyl vibration in the infrared spectra, which can be correlated to the kinetic energy density parameter, G(r), in the quantum theory of atoms in molecules (QTAIM).

1H and 13C NMR chemical shifts of 2-n-alkylamino-naphthalene-1,4-diones.

1H as well as 13C chemical shifts of 32 compounds of C (3) substituted 2-(n-alkylamino)-3R-naphthalene-1,4-dione (where n-alkyl: methyl, to octyl, R = H, Cl, Br, and CH3) are investigated through 1H, 13C, DEPT, gDQCOSY, and gHSQCAD NMR experiments and M06-2X/6-311++G (d,p) density functional theory are discussed. Single crystal X-ray structure of Br-3, as well as 18 different derivatives of naphthalene-1,4-diones, are revealed for its inter and intra-molecular hydrogen bonding interactions.

Regioselectivity in nonsymmetric methyl pentyl Pillar[5]arene bound to non-symmetric axles.

The present work illustrates regioselective binding of nonsymmetric axle BuX (X = F, Cl, Br, CN) and 5-bromovaleronitrile (BVN) to the non-symmetric methyl pentyl pillar[5]arene (MPP5). Theoretical calculations reveal that the guest encapsulation within MPP5 is spontaneous and the conformer showing X weakly bound to pentyl rim of MPP5 is favoured over its other conformer wherein it interacts with methyl rim of the host. The noncovalent interactions namely C-H---π, C-H---X and H-H prevail over C-H⋯O hydrogen bonding in the complexes of MPP5. The manifestations of these to vibrational spectra obtained from the present theory are discussed. The strength of host-guest binding further is shown to correlate well with weakening of the C-X bond through natural bond orbital analyses.

Perethylated pillar[n]arenes versus pillar[n]arenes: theoretical perspectives.

In this article we obtain the electronic structure, 1H NMR, and the vibrational spectra of perethylated pillar[n]arene (EtPn) (n = 5-8, 10) macrocycles using the ωB97x-based density functional theory. A single cavitand structure endowed with the robust pillar-like architecture was derived for the EtPn (n = 5-7) hosts while for the EtP8 receptor, two conformers one possessing the single cavitand and other double cavitand structure having four hydroquinone monomeric units in each cavity were obtained as the local minima. A double cavitand conformer of the EtP8 was found to be ~ 50 kJ mol-1 lower in energy than the single cavitand one which was attributed to the increased hydrogen bonding cooperativity and attractive H-H interactions. Further increase of hydroquinone monomers as in EtP10 host engenders only the double cavitand structure, composed of five hydroquinone monomeric units in each cavity. The molecular electrostatic potential investigations along the EtPn series of hosts reveal large electron-rich aromatic moieties compared to unsubstituted pillararenes which suggest the EtPn to be better artificial receptors toward cationic or neutral guests than their unsubstituted analogs. Calculated vibrational frequencies of EtPn (n = 5-8) show the cooperative stretching vibrations in a narrow span of 3241 cm-1 to 3228 cm-1. Besides, the most deshielded signals from aromatic protons along the EtPn (n = 5-8) series show up near 7.35 ppm in their 1H NMR spectra. An increased cooperativity in the lowest energy of the EtP8 conformer further is evident from up-field signal corresponding to methylene protons of ethyl substituent compared with the single cavitand structure. Graphical abstractMolecular electrostatic potential for EtP5 and correlations obtained for δH VsρBCP and ν Vs G(r) plots.

Probing Binding of Ethylated Pillar[5]arene with Pentene and Chlorobutane Positional Isomers.

Host guest binding from alkyl-modified pillararene macrocycles has been of significant interest in a variety of applications in the domains of supramolecular chemistry. In this work, we analyze the selectivity in binding of the ethylated pillar[5]arene (EtP5) macrocycle with 1-pentene, cis and trans 2-pentene, and the 1- and 2-chlorobutane isomer guests employing the ωB97x-based density functional theory. EtP5 reveals stronger binding with 1-pentene the accompanying change of energy upon the complexation being 85.5 kJ mol-1 compared to 71.3 and 75.8 kJ mol-1 for the cis and trans-2-pentene isomers, respectively. The complexation of EtP5 with pentene isomers is governed by the interplay of CH···π, H-H, and O···H noncovalent interactions. The inferences on the guest binding rationalized through the quantum theory of atoms in molecules are in consonance with data on relative uptake of pentene isomers observed from the gas chromatography experiments reported earlier. A stronger binding of EtP5 with 1-pentene is borne out from a large C-H···π and H-H interactions. The chlorobutane isomers are held together within the EtP5 cavity via C-H···π as well as Cl···H interactions those prevail over the O-H···O hydrogen bonding in such complexes. The host-guest binding emerges with the signature in "frequency shifts" of the characteristic alkyl vibrations of the host and corroborated with the natural bond orbital analyses.

Modeling protic dicationic ionic liquids based on quaternary ammonium, imidazolium or pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion: Structure and spectral characteristics.

Protic dicationic ionic liquids (PDILs) have attracted growing attention owing to their applications in domains of electrochemistry, proton conducting materials and other diverse areas. In the present work protic dicationic ionic liquids (PDILs) comprising of quaternary ammonium-, imidazolium- or pyrrolidinium-dications and bis(trifluoromethanesulfonyl)imide (Tf2N‾) anion have been modelled as the dication-(Tf2N)2 complexes. Electronic structure, vibrational and 1H NMR spectra of these complexes have been derived employing the M06-2x density functional theory. Theoretical calculations have shown that the strength of cation-anion binding follows the order: methylpyrrolidinum > quaternary ammonium > butylpyrrolidinium > imidazolium, which can be attributed to number and strength of N-H---O and C-H---O interactions. The dication-(Tf2N)2 complexes emerge with signature in frequency up-shift of the characteristic N-H stretching in their infrared spectra. Underlying molecular interactions are unveiled through natural bond orbital analyses, Quantum theory of atoms in molecules (QTAIM) and noncovalent interaction reduced density gradient method. The calculations have shown that cation-anion binding energies increase linearly with kinetic energy density component G(r) in QTAIM analysis and proton affinities in the PDILs. A correlation between change in free energies accompanying the dication-(Tf2N)2 complexes and proton affinities has also been established.

Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium, or Quaternary Ammonium Cation and Acetate Anion Based Protic Ionic Liquids: Structure and Spectral Characteristics.

In the present work protic ionic liquids (PILs) composed of imidazolium-, quaternary ammonium-, or pyrrolidinium-dications and acetate (OAc-) anion have been modeled as the dication-anion complexes through the M06-2x based density functional theory. It has been shown that cation-anion interaction energies are larger for the PILs containing the quaternary ammonium cation, which can be attributed to strong hydrogen bonding from the terminal ammonium protons. Underlying N-H···O and C-H···O hydrogen bonding, electrostatic, and van der Waals interactions are unraveled using the natural bond orbital analyses in conjunction with the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction index reduced density gradient methods. The ramifications of noncovalent binding to 1H NMR and vibrational spectra are presented. The calculations further demonstrate a linear correlation of the kinetic energy density parameter G( r) in QTAIM analysis with the characteristic frequency shift of -NH3+ stretching in the dication-anion complexes. Moreover, the chemical shifts (δH) in 1H NMR spectra from theory reveal larger deshielding; the corresponding δH value correlates well with proton affinities and cation-anion binding energies as well. Effect of solvent (DMSO) on structure, binding energies, and 1H NMR are presented. The shifts of the characteristic carbonyl and the terminal ammonium stretching vibrations accompanying the dication-anion complexes from gas phase calculations are in consonance with the self-consistent reaction field theory.

Understanding the Atmospheric Oxidation of HFE-7500 (C3F7CF(OC2H5)CF(CF3)2) Initiated by Cl Atom and NO3 Radical from Theory.

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 ( n-C3F7CF(OCH2CH3)CF(CF3)2) initiated by Cl atom and NO3 radical have been studied using density functional theory. Oxidative degradation pathways facilitated by H-abstraction from the -OCH2 or -CH3 groups in HFE-7500 have been considered. It has been shown that H-abstraction from the α-site (-OCH2) is favored over other reaction pathways. The rate constants were computed employing transition-state theory and canonical variation transition-state theory incorporating small curvature tunnelling correction, over the temperature range of 250-450 K at atmospheric pressure. Calculated rate constants at 298 K and 1 atm compare well with earlier experiments. Temperature dependence of the rate constants and branching ratios for these pathways contributing to overall reaction are described. It has been shown that the rate constants over the studied temperature range was found to fit well to the modified Arrhenius equation (in cm3 molecule-1 s-1) kCl = 1.10 × 10-14  T0.04 exp(-69.87 ± 1.41/T) and kNO3 = 7.66 × 10-26 T3.30 exp(596.40 ± 1.22/T). Standard enthalpies of formation for the reactant (C3F7CF(OCH2CH3)CF(CF3)2) and the products [C3F7CF(OC•HCH3)CF(CF3)2 and C3F7CF(OCH2C•H2)CF(CF3)2] during H-abstraction are derived using the isodesmic approach. Atmospheric implications of the titled molecule are presented.

Exploring Chimeric Calix[4]tetrolarene Molecular Scaffolds: Theoretical Investigations.

The structure and spectral characteristics of the chimeric mixture of calixarene and pyrogallolarene (usually referred to as calix[4]tetrolarene) and its derivatives are studied employing the M06-2X-based density functional theory. Different conformers, viz., cone, partial cone, 1,2-alternate, and 1,3-alternate, were identified as the stationary point structures on their potential energy surfaces. Among these, the symmetric C4 v cone conformer is found to be energetically favorable, which can be attributed to the cyclic array of hydrogen-bonding network in the calix[4]tetrolarene or its thia analogue. The substitution of methoxy groups at the upper rims of calix[4]tetrol- and thicalix[4]tetrol-arenes significantly influences the cooperative hydrogen-bonding network and conformational behavior of these hosts. The methoxy-substituted macrocycles show lowering in symmetry from C4 v to C2 v, engendering the pinched cone conformer to be the lowest energy structure. The enhanced solubility of the modified calix[4]tetrolarene macrocycles has been further explained from diminutive cooperative hydrogen bonding in its top rim compared to the pyrogallolarene, which is evidenced from the quantum theory of atoms in molecule and noncovalent interaction reduce density gradient method. Discernibly, the underlying cooperative hydrogen bonding emerges its signature in the characteristic vibrational patterns of the calixarene-based molecular scaffolds. The chemical shift parameters in their 1H NMR spectra have further been characterized.

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.

Molecular Recognition, Conformational Behavior, and Spectral Characteristics of Oxatub[4]arene Macrocycle.

In the present work, we analyze molecular recognition behavior of synthetic hydroxylated oxatub[4]arene (TA4) receptor toward the methyl viologen in different redox states. The supramolecular binding of methyl viologen guest toward TA4 macrocyclic scaffold has been studied employing the dispersion corrected ωB97X-D based density functional theory. The methyl viologen in dicationic and neutral forms revealed distinct features in electronic, 1H nuclear magnetic resonance, and infrared spectra. Quantum theory of atoms in molecules in conjunction with the noncovalent interaction reduced density gradient in real space have been used as tools to characterize the underlying host-guest binding.

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.

Host-Guest Interactions Accompanying the Encapsulation of 1,4-Diazabicyclo[2.2.2]octane within endo-Functionalized Macrocycles.

The binding of novel endofunctionalized bis-urea/thiourea molecular receptors toward neutral 1,4-diazabicyclo[2.2.2]octane (DABCO) demonstrates stronger binding of the bis-thiourea macrocycles than for their urea analogues by employing M06-2X/6-31+G(d,p)-based density functional theory. The formation of such inclusion complexes is spontaneous, thermodynamically favorable, and facilitated via bifurcated N-H···N···H-N hydrogen bonding and C-H···π, dipole-dipole, and other noncovalent interactions, which are reflected in the frequency shift of their characteristic N-H vibrations in the calculated vibrational spectra of these complexes. The underlying noncovalent interactions are analyzed using the molecular electrostatic potential topography and quantum theory of atoms in molecules in conjunction with the noncovalent interactions reduced density gradient method. It has also been shown that the encapsulation of DABCO within the π-electron-rich cavity of such hosts brings about shielding of the guest protons confined within the host cavity whereas those facilitating hydrogen bonding engender the downfield signals in their calculated 1H NMR spectra.

Noncovalent Interactions Accompanying Encapsulation of Resorcinol within Azacalix[4]pyridine Macrocycle.

Electronic structure and noncovalent interactions within the inclusion complexes of resorcinol and calix[4]pyridine (CXP[4]) or azacalix[4]pyridine (N-CXP[4]) macrocycles have been analyzed by employing hybrid M06-2X density functional theory. It has been demonstrated that substitution of a heteroatom (-NH-) at the bridging position of the CXP[4] alters the shape of the cavity from a "box-shaped" to funnel-like one. Penetration of resorcinol guest within the CXP[4] cavity renders a "butterfly-like" structure to the complex, whereas the N-CXP[4] complex reveals distorted geometry with the guest being nearer to one of the pyridine rings at the upper rim of the host. Underlying hydrogen bonding, π···π, and other weak interactions are characterized using the Quantum Theory of Atoms in Molecules (QTAIM) and Noncovalent Interactions Reduced Density Gradient (NCI-RDG) methods. The coexistence of multiple intermolecular interactions is envisaged through the frequency shifts of the characteristic -NH and -OH vibrations in their calculated vibrational spectra. The guest protons confined to the host cavity exhibit shielding, while those facilitating hydrogen bonding engender the downfield signals in their calculated 1H NMR spectra.

Density Functional Investigations on the Selective Binding of an endo-Functionalized Bis-urea Macrocycle.

The preferential binding of syn and anti configurational isomers of endo-functionalized bis-urea molecular receptor to 1,2-dinitrobenzene (G1) and 1,4-dioxane (G2) guests has been explained using dispersion-corrected M06-2X-based density functional theory. The host-guest binding is facilitated via hydrogen bonding, C-H···π, dipole-dipole, C···C and O···O (chalcogen-chalcogen) interactions. The formation of an inclusion complex is spontaneous and thermodynamically favorable. The molecular electrostatic potential and quantum theory of atoms in molecules in conjunction with the noncovalent interactions reduced density gradient have been employed to characterize the noncovalent interactions. The encapsulation of G1 or G2 within the π-electron-rich cavity of the bis-urea macrocycle reflects the frequency shift of the characteristic N-H and C-H vibrations of their vibrational spectra. It has also been shown that binding of the bis-urea isomers to G1 and G2 emerges with a signature in the upfield signals of the guest protons confined to the host cavity in 1H NMR spectra.

Kinetics and Mechanistic Investigations of Atmospheric Oxidation of HFO-1345fz by OH Radical: Insights from Theory.

HFO-1345fz (CF3CF2CH═CH2 or 3,3,4,4,4-pentafluoro-1-butene) belongs to a class of hydrofluoro-olefins and represents a new generation of potential foam expansion agents. Its atmospheric impact and environmental acceptability can be estimated from the studies of kinetics and mechanism of its oxidative degradation. The molecular insights accompanying the reaction pathways in terms of the characterization of intermediates or products and radiative properties should prove useful for large-scale industrial applications. Systematic mechanistic gas-phase kinetics investigations on the reactivity of HFO-1345fz with the •OH facilitating a variety of degradation routes have been carried out employing the M06-2x-based density functional theory. Structure and energetics of different reaction pathways such as hydrogen abstraction, •OH addition, isomerization-dissociation, or interaction with atmospheric O2 have been analyzed. The formation of gaseous products from the interaction of HFO-1345fz with •OH in the absence and presence of NOx atmospheric conditions has been reported. Calculated branching ratios have shown that the addition channel dominates such oxidative degradation, whereas the abstraction channel contributes negligibly to the global rate constant and addition of •OH to the terminal carbon is favored over the nonterminal one. The rate constants for all reaction channels were computed by conventional transition state theory (TST) and canonical variation transition state theory (CVT) including small curvature tunneling (SCT) over the temperature range of 200-1000 K at atmospheric pressure. The CVT calculated rate constant for the reaction at 298 K was shown to be 1.17 × 10-12 cm3 molecule-1 s-1, which compares well with the 1.24 × 10-12 cm3 molecule-1 s-1 as obtained from TST and is in excellent agreement with the experiments reported earlier. The atmospheric lifetime, radiative efficiency, and global warming potential (GWP) have also been obtained.

Understanding Binding of Cyano-Adamantyl Derivatives to Pillar[6]arene Macrocycle from Density Functional Theory.

The ωB97X-based density functional theory has been employed to characterize molecular interactions between adamantane carbonitrile (ACN) or adamantane methyl carbonitrile (AMCN) and the ethylated pillar[6]arene (EtP[6]) molecular receptor. The inclusion complexes in 1:1 stoichiometry are stabilized through noncovalent interactions such as hydrogen bonding, C-H···π and dipole-dipole interactions. Gibbs free energies accompanying the encapsulation of ACN or AMCN within EtP[6] revealed that the formation of complex is spontaneous and thermodynamically favorable. Underlying interactions are unraveled through quantum theory of atoms in molecules and molecular electrostatic potential topography. Structural changes consequent to guest encapsulation have been rationalized through characteristic infrared and NMR spectra. The frequency downshifts for -C≡N stretching of the guest accompanying the complexation has been attributed to hydrogen bonding and C-H···π interactions. The methylene vibrations of ACN reveal the frequency shifts in opposite directions consequent to distinct binding features with EtP[6] host. The selective binding of AMCN further brings about a significant distortion of the host cavity. Calculated 1H NMR spectra of ACN and AMCN complexes show shielded signals for the adamantyl protons in consonance with experiment.

Cooperative Hydrogen Bonding, Molecular Electrostatic Potentials, and Spectral Characteristics of Partial Thia-Substituted Calix[4]arene Macrocycles.

Structure and spectral characteristics of the 2,14-dithiacalix[4]arene and its homooxa derivatives are obtained employing the dispersion-corrected ωB97X-D-based density functional theory. The conformational behavior of these receptors is governed by the nature and number of substituents at the bridging positions. The partial thia-substituted calix[4]arene scaffolds reveal the electron-rich regions reside near heteroatoms which emerge with deeper minima in molecular electrostatic potential topography. Underlying cooperativity of intramolecular hydrogen bonding manifests in characteristic OH vibrations of their infrared spectra. Moreover, the (1)H NMR reveals that hydrogen-bonded protons are deshielded, unlike those from tertiary butyl substituents. These inferences are in agreement with the experimental data.