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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium-Based Dicationic Ionic Liquids.

Electronic structure, binding energies, and spectral characteristics of functionalized asymmetric dicationic ionic liquids (DILs) composed of quaternary ammonium cations substituted with the ethoxyethyl and allyl/3-phenylpropyl/methoxyethoxyethyl/pentyl functionalities on two different nitrogen centers of the dication and the bis(trifluoromethanesulfonyl)imide (Tf2N-) anion were derived employing the dispersion-corrected density functional theory. DILs based on methoxyethoxyethyl-substituted cation reveal stronger binding toward the Tf2N- anion. The measured glass transition temperatures are found to be strongly dependent on the cation-anion binding facilitated through noncovalent interactions with dominant contributions from the electrostatics and hydrogen bonding. The manifestations of these interactions to vibrational spectra, in particular, to SO2 and CF3 stretchings in the complexes are presented. It has been demonstrated that the frequency down (red)-shift of the SO2 stretching in these DILs with varying substituent follows the order: methoxyethoxyethyl (35 cm-1) > allyl (23 cm-1) > pentyl (20 cm-1) > 3-phenylpropyl (5 cm-1), which is consistent with the strength of cation-anion binding. The CF3 stretching of the anion exhibits the frequency shift in the opposite direction with its hierarchy being reversed to that of SO2 stretchings; the largest upshift (blue shift) of 60 cm-1 was predicted for the DILs composed of 3-phenlpropyl substituted dications. The direction of such frequency shift has been rationalized through the difference molecular electron density maps in conjunction with the electron density at the bond critical point in the quantum theory of atoms in molecules. The underlying cation-anion binding has been analyzed through charge distribution analysis characterized in terms of molecular electrostatic potential topography. Furthermore, the observed decomposition temperatures of DILs are shown to correlate well with the maximum surface electrostatic potential parameters in quantum theory of atoms in molecules.

Encaged molecules in external electric fields: A molecular "tug-of-war".

Response of polar molecules CH3OH and H2O2 and a non-polar molecule, CO2, as "guests" encapsulated in the dodecahedral water cage (H2O)20 "host," to an external, perturbative electric field is investigated theoretically. We employ the hybrid density-functionals M06-2X and ωB97X-D incorporating the effects of damped dispersion, in conjunction with the maug-cc-pVTZ basis set, amenable for a hydrogen bonding description. While the host cluster (cage) tends to confine the embedded guest molecule through cooperative hydrogen bonding, the applied electric field tends to rupture the cluster-composite by stretching it; these two competitive effects leading to a molecular "tug-of-war." The composite remains stable up to a maximal sustainable threshold electric field, beyond which, concomitant with the vanishing of the HOMO-LUMO gap, the field wins over and the cluster breaks down. The electric-field effects are gauged in terms of the changes in the molecular geometry of the confined species, interaction energy, molecular electrostatic potential surfaces, and frequency shifts of characteristic normal vibrations in the IR regime. Interestingly, beyond the characteristic threshold electric field, the labile, distorted host cluster fragmentizes, and the guest molecule still tethered to a remnant fragment, an effect attributed to the underlying hydrogen-bonded networks.