Gas-phase Binding Energies Research Articles

Hydration of p-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p-$$\\end{document}aminobenzoic acid: structures and non-covalent bondings of aminobenzoic acid-water clusters

ContextMicro-hydration of the aminobenzoic acid is essential to understand its interaction with surrounding water molecules. Understanding the micro-hydration of the aminobenzoic acid is also essential to study its remediation from wastewater. Therefore, we explored the potential energy surfaces (PESs) of the para-aminobenzoic acid-water clusters, ABWn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_n$$\\end{document}, n=1-10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$n=1-10$$\\end{document}, to study the microsolvation of the aminobenzoic acid in water. In addition, we performed a quantum theory of atoms in molecules (QTAIM) analysis to identify the nature of non-covalent bondings in the aminobenzoic acid-water clusters. Furthermore, temperature effects on the stability of the located isomers have been examined. The located structures have been used to calculate the hydration free energy and the hydration enthalpy of the aminobenzoic acid using the cluster continuum solvation model. The hydration free energy and the hydration enthalpy of the aminobenzoic acid at room temperature are evaluated to be −7.0 kcal/mol and −18.1 kcal/mol, respectively. The hydration enthalpy is in perfect agreement with a previous experimental estimate. Besides, temperature effects on the calculated hydration enthalpy and free energy are reported. Finally, we calculated the gas phase binding energies of the most stable structures of the ABWn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_n$$\\end{document} clusters using twelve functionals of density functional theory (DFT), including empirical dispersion. The DFT functionals are benchmarked against the DLPNO-CCSD(T)/CBS. We have found that the three most suitable DFT functionals are classified in the following order: PW6B95D3 > MN15 > ω\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega $$\\end{document}B97XD. Therefore, the PW6B95D3 functional is recommended for further study of the aminobenzoic acid-water clusters and similar systems.MethodsThe exploration started with classical molecular dynamics simulations followed by complete optimization at the PW6B95D3/def2-TZVP level of theory. Optimizations are performed using Gaussian 16 suite of codes. QTAIM analysis is performed using the AIMAll program.

Effect of electronegative atoms on [formula omitted] – [formula omitted] stacking and hydrogen bonding behavior in simple aromatic molecules — An Ab initio MD study

Ab initio molecular dynamics studies have been performed on fluorobenzene, phenol, and aniline, which have the three most electronegative atoms, fluorine, oxygen, and nitrogen, respectively. Radial distribution functions show strong hydrogen bonding in the phenolic –OH group, whereas it is less prominent in the –NH2 group of aniline. Fluorobenzene does not show strong hydrogen bonds as no solvation shell is found between the fluorine atom and different aromatic hydrogens of the molecule. Spatial distribution functions show that the nitrogen atom of aniline interacts with the aromatic plane, the oxygen atom of phenol is concentrated near the –OH group and fluorobenzene’s fluorine atom interacts with the para hydrogen. Liquid phase dimer structures of these systems reveal that perpendicular orientation (Y-shaped) is preferred over parallel ones. Almost half of the total dimer population tends to prefer 90∘±30° angle. H-bond analyses show that fluorobenzene has the longest mean H-bond lifetime for the H-bond between the aromatic hydrogens and the fluorine atoms, whereas the aniline has the least. The mean lifetime between aromatic hydrogens and electronegative atoms increases steadily from aniline to fluorobenzene. Phenolic –OH and amino –NH2 groups show considerably longer mean H-bond lifetime than the aromatic hydrogens. Gas-phase binding energies obtained from quantum chemical calculations show that aniline and phenol dimers have higher binding energy values than the fluorobenzene dimer. Only the phenol dimer shows a perpendicular structure as a stable one, while aniline and fluorobenzene prefer the parallel orientation.

Benchmarking higher energy collision dissociation (HCD) by investigation of binding energies of gas-phase host-guest complexes of hemicryptophane cages.

Synthesis of host molecules that feature well-defined characteristics for molecular recognition of guest molecules is often a major aim of synthetic host-guest (H-G) chemistry. A key consideration in evaluating the selectivity of hosts and the affinities of guests is the measurement of binding energies of obtained H-G complexes. In contrast to nuclear magnetic resonance (NMR) or fluorescence measurements that are capable of measuring binding strengths in solution, mass spectrometry offers the opportunity to measure gas-phase binding energies. Presented in this article is a higher energy collision dissociation (HCD) approach for determining critical energies of dissociation of H-G complexes. Experiments were performed on electrospray ionization (ESI)-generated H-G pairs in an LTQ-XL/Orbitrap hybrid instrument. The presented HCD approach requires preliminary calibration of the internal energy distribution of generated ions that was achieved by the use of activation parameters that were known from previous low-energy collision-induced dissociation (low-energy CID) experiments. Internal energy deposition was modeled based on a truncated Maxwell-Boltzmann distribution and characteristic temperature (Tchar ). Using this method, critical energies of dissociation were determined for 10 H-G biologically relevant complexes of the heteroditopic hemicryptophane cage host (Host). Obtained results are compared with those found previously by low-energy CID. The use of this HCD technique is relatively straightforward, although its implementation does require knowledge (or a presumption) about the Arrhenius pre-exponential factor of the complexes to obtain their critical energies of dissociation.

2-Aminoalkylgold Complexes: The Putative Intermediate in Au-Catalyzed Hydroamination of Alkenes Does Not Protodemetalate.

Au‐catalyzed hydroamination proceeds well for alkynes but not alkenes. We report gas‐phase binding energies of alkenes and alkynes to a cationic Au center, which indicate that differences in binding are not the origin of the disparate chemical behavior. We further report the synthesis and characterization of 2‐aminoalkylgold complexes, which would be the intermediates in a hypothetical Au‐catalyzed hydroamination of styrene. The reactivity of the well‐characterized and isolable complexes reveals that protonation or alkylation of the 2‐aminoalkylgold complexes results in amine elimination in solution, and in the gas phase, indicating that the failure of Au‐catalyzed alkene hydroamination derives from a non‐competitive protodeauration step. We analyze possible transition states for the protodeauration, and identify an insufficiently strong Au‐proton interaction as the reason that the transition states lie too high in energy to compete.

Halogen Bonding Propensity in Solution: Direct Observation and Computational Prediction.

Halogen-bonded complexes are often designed by consideration of electrostatic potential (ESP) predictions. ESP predictions do not capture the myriad variables associated with halogen bond (XB) donors and acceptors; thus, binding interaction cannot be quantitatively predicted. Here, a discrepancy between predictions based on ESP energy difference (ΔVs ) and computed gas phase binding energy (ΔEbind ) motivated the experimental determination of the relative strength of halogen bonding interactions in solution by Raman spectroscopic observation of complexes formed from interacting five iodobenzene-derived XB donors and four pyridine XB acceptors. Evaluation of ΔEbind coupled with absolutely-localized molecular orbital energy decomposition analysis (ALMO-EDA) deconvolutes halogen bonding energy contributions and reveals a prominent role for charge transfer (CT) interactions. Raman spectra reveal ΔEbind accurately predicts stronger interactions within iodopentafluorobenzene (IPFB) complexes than with 1-iodo-3,5-dinitrobenzene (IDNB) complexes even though IPFB has similar electrostatics to IDNB and contains a smaller σ-hole.

Absolute Trends and Accurate and Precise Gas-Phase Binding Energies of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters from Combined Independent and Competitive TCID Measurements.

Ionic liquid (IL) development efforts have focused on achieving desired properties via tuning of the IL through variation of the cations and anions. However, works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been rather limited such that the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation approaches supported and enhanced by electronic structure calculations to characterize the nature of the cation-anion interactions in and determine the bond dissociation energies (BDEs) of a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [2Cnmim:BF4]+. The cation was varied over the series: 1-ethyl-3-methylimidazolium, [C2mim]+, 1-butyl-3-methylimidazolium, [C4mim]+, 1-hexyl-3-methylimidazolium, [C6mim]+, and 1-octyl-3-methylimidazolium, [C8mim]+, to determine the structural and energetic effects of the size of the 1-alkyl substituent on the binding. The variation in the strength of binding determined for these [2Cnmim:BF4]+ clusters was found to be similar in magnitude to the average experimental uncertainty in these determinations. To definitively establish an absolute order of binding among these [2Cnmim:BF4]+ clusters, we extend this work here to include competitive binding studies of three mixed 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [Cn-2mim:BF4:Cnmim]+ for n = 4, 6, and 8. Importantly, the results of the present work simultaneously provide the absolute BDEs of these mixed [Cn-2mim:BF4:Cnmim]+ clusters and the absolute relative order of the intrinsic binding interactions as a function of the cation with significantly improved precision. Further, by combining the thermochemical results of the previous and present studies, the BDEs of the [2Cnmim:BF4]+ clusters are more accurately and precisely determined.

Beyond Size Complementary Factors in Anion-Tetralactam Macrocycle Complexes: From Intrinsic Gas-Phase to Solvent-Predicted Stabilities.

The gas-phase affinities of different types of anions X– (halogen anions, oxoanions, and hydrogenated anions) toward a model tetralactam-based macrocycle receptor (1), defined in terms of stability of an anion–receptor complex (1 + X–) against its disintegration, were evaluated by dissociation studies using a mass spectrometry-based methodology and supported by theoretical calculations (density functional theory–PBE0). The gas-phase complex with Cl– was found to be tailor-made for the macrocycle 1, while 1 + SA– (SA– = salicylate anion) and 1 + HSO4– were the weakest ones. Other complexes displayed a relatively low-stability dispersion (<1.2 kcal·mol–1). The 1/εr approach of the electrostatic contribution scaling method was used to predict the stability trends in a dimethyl sulfoxide solvent from the gas-phase binding energy partition using the symmetry-adapted perturbation theory. High deformation energy and differences in solvation energies were suggested to be the main sources of inconsistency in the predicted and experimental stabilities of 1 + F– and 1 + H2PO4– complexes.

Selectivity of 18-crown-6 ether to alkali ions by density functional theory and molecular dynamics simulation

Molecular dynamics (MD) simulation was employed to explore the structural and thermodynamic characteristics of 18-crown-6 ether selectivity to alkali metal ions in aqueous solution. The radial distribution functions (RDFs), potentials of mean force (PMFs) and binding free energies between Li+, Na+, K+, Rb+, Cs+ and 18-crown-6 ether with SPC/E and TIP4P water models in aqueous solutions at 278.15 K, 298.15 K and 318.15 K were obtained. It shows that K+ is selected over other alkali metal ions by 18-crown-6 ether in aqueous solution at all water models and temperatures we considered, and increasing temperature is beneficial for the binding of 18-crown-6 ether to alkali metal ions. The calculated selective sequence (Li+ ≪ Na+ < K+ > Rb+ > Cs+) in aqueous solution is close to the experimental results. The gas-phase binding energies of 18-crown-6 to alkali metal ions and the minimum potential energies with corresponding minimum positions of alkali ions complexes were got by density functional theory (DFT). The results reveal that the selectivity of 18-crown-6 to alkali metal ions in gas-phase follows the sequence, Li+ > Na+ > K+ > Rb+ > Cs+, and follows their ionic potentials. The effect of solvent on the selectivity of 18-crown-6 ether to alkali ions is confirmed by comparing the selective sequences in gas-phase and aqueous solution. The noncovalent interactions sites and types between alkali ions and 18-crown-6 ether were determined by electrostatic potential (ESP) and the average reduced density gradient (aRDG), which shows that the interactions between alkali ions and 18-crown-6 are electrostatic dominated.

Dispersion XDM with Hybrid Functionals: Delocalization Error and Halogen Bonding in Molecular Crystals

The accurate calculation of relative lattice energies of molecular crystals is important in polymorph ranking and crystal structure prediction. Delocalization error has been shown to affect calculated intermolecular binding energies in DFT and is similarly expected to affect the lattice energies of some classes of molecular crystals. In this work, we explore the use of dispersion-corrected hybrid functionals in the planewave-pseudopotentials approach to reduce delocalization error. We combine several hybrid functionals with the exchange-hole dipole moment (XDM) model for dispersion and show that they generally outperform GGA functionals in the calculation of both gas-phase binding energies and molecular crystal lattice energies. We apply the resulting XDM-corrected functionals to four halogen-bonded crystals: Cl2, Br2, I2, and ICl. GGA functionals severely overestimate their lattice energies, while hybrid functionals give accurate values. The preference of GGA functionals for monatomic structures in the Br2 and Cl2 crystals is also explained. Finally, we apply a recently developed method to calculate Bader's delocalization indices to examine the extent of intermolecular delocalization in the halogen molecular crystals. It is shown that intermolecular delocalization indices can be used to measure the strength of halogen bonds within the crystal, as well as detect the presence of delocalization error.

Investigation of activation energies for dissociation of host-guest complexes in the gas phase using low-energy collision induced dissociation.

A low-energy collision induced dissociation (CID) (low-energy CID) approach that can determine both activation energy and activation entropy has been used to evaluate gas-phase binding energies of host-guest (H-G) complexes of a heteroditopic hemicryptophane cage host (Zn (II)@1) with a series of biologically relevant guests. In order to use this approach, preliminary calibration of the effective temperature of ions undergoing resonance excitation is required. This was accomplished by employing blackbody infrared radiative dissociation (BIRD) which allows direct measurement of activation parameters. Activation energies and pre-exponential factors were evaluated for more than 10 H-G complexes via the use of low-energy CID. The relatively long residence time of the ions inside the linear ion trap (maximum of 60s) allowed the study of dissociations with rates below 1s-1 . This possibility, along with the large size of the investigated ions, ensures the fulfilment of rapid energy exchange (REX) conditions and, as a consequence, accurate application of the Arrhenius equation. Compared with the BIRD technique, low-energy CID allows access to higher effective temperatures, thereby permitting one to probe more endothermic decomposition pathways. Based on the measured activation parameters, guests bearing a phosphate (-OPO3 2- ) functional group were found to bind more strongly with the encapsulating cage than those having a sulfonate (-SO3 - ) group; however, the latter ones make stronger bonds than those with a carboxylate (-CO2 - ) group. In addition, it was observed that the presence of trimethylammonium (-N(CH3 )3 + ) or phenyl groups in the guest's structure improves the strength of H-G interactions. The use of this technique is very straightforward, and it does not require any instrumental modifications. Thus, it can be applied to other H-G chemistry studies where comparison of bond dissociation energies is of paramount importance.

On the estimation of the strength of supramolecular complexes of fullerenes

The accurate estimation of the free energy changes in solution for supramolecular complexes is one of the most important challenges for quantum chemistry in this century. In general, there is a good correlation between the association constants determined experimentally and the gas phase binding energies calculated using the most popular density functional methods. However, when the receptors adopt very different structures in the gas and condensed phase, this approach does not work, leading to unreliable predictions. We discuss this problem in detail using three popular fullerene receptors: Aida's dimeric porphyrins, buckycatchers I, and II. We found that it is possible to obtain a reliable order of affinities using gas phase binding energies when a special attention is put on analyzing the structure adopted by the receptor in the gas and condensed phases.

To Anion-π or not to Anion-π: The Case of Anion-Binding to Divalent Fluorinated Pyridines in the Gas Phase.

A series of mono- and divalent fluorinated pyridine derivatives is investigated by electrospray ionization (tandem) mass spectrometry and quantum chemical calculations with respect to their capability to bind anions in the gas phase. The pyridine derivatives differ not only in valency, but also with regard to the degree of fluorination of the pyridine rings, the positions of the fluorine atoms, the rigidity of the spacers connecting the two pyridines in the divalent compounds, and the relative configuration. While the monovalent compounds did not form anion complexes, the divalent analogues exhibit anion binding even to weakly coordinating anions such as tetrafluoroborate. Three different tandem mass spectrometric experiments were applied to rank the gas-phase binding energies: (i) collision-induced dissociation (CID) experiments in a Fourier transform ion-cyclotron-resonance (FTICR) mass spectrometer on two different, simultaneously mass-selected complexes with different receptors, (ii) determination of the collision energy required to fragment 50 % of the mass-selected complexes in an ESI-QToF mass spectrometer, and (iii) CID of heterodimers formed from two different, competing pyridine receptors and indigo carmine, a dianion with two identical binding sites. All three experiments result in consistent binding energy ranking. This ranking reveals surprising features, which are not in agreement with binding through anion-π interactions. Density functional theory (DFT) calculations comparing different potential binding modes provide evidence that the ranking can instead nicely be explained, when C-H⋅⋅⋅anion interactions with the spacers are invoked. These results are supported by gas-phase IR spectroscopy and ion mobility-mass spectrometry (IM-MS) on a selected set of chloride pyridine complexes.

Cation-π Interactions between Quaternary Ammonium Ions and Amino Acid Aromatic Groups in Aqueous Solution.

Cation-π interactions play important roles in the stabilization of protein structures and protein-ligand complexes. They contribute to the binding of quaternary ammonium ligands (mainly RNH3+ and RN(CH3)3+) to various protein receptors and are likely involved in the blockage of potassium channels by tetramethylammonium (TMA+) and tetraethylammonium (TEA+). Polarizable molecular models are calibrated for NH4+, TMA+, and TEA+ interacting with benzene, toluene, 4-methylphenol, and 3-methylindole (representing aromatic amino acid side chains) based on the ab initio MP2(full)/6-311++G(d,p) properties of the complexes. Whereas the gas-phase affinity of the ions with a given aromatic follows the trend NH4+ > TMA+ > TEA+, molecular dynamics simulations using the polarizable models show a reverse trend in water, likely due to a contribution from the hydrophobic effect. This reversed trend follows the solubility of aromatic hydrocarbons in quaternary ammonium salt solutions, which suggests a role for cation-π interactions in the salting-in of aromatic compounds in solution. Simulations in water show that the complexes possess binding free energies ranging from -1.3 to -3.3 kcal/mol (compared to gas-phase binding energies between -8.5 and -25.0 kcal/mol). Interestingly, whereas the most stable complexes involve TEA+ (the largest ion), the most stable solvent-separated complexes involve TMA+ (the intermediate-size ion).

Correction: Developing force fields when experimental data is sparse: AMBER/GAFF-compatible parameters for inorganic and alkyl oxoanions.

Correction for 'Developing force fields when experimental data is sparse: AMBER/GAFF-compatible parameters for inorganic and alkyl oxoanions' by Sadra Kashefolgheta et al., Phys. Chem. Chem. Phys., 2017, 19, 20593-20607.

Developing force fields when experimental data is sparse: AMBER/GAFF-compatible parameters for inorganic and alkyl oxoanions.

We present a set of Lennard-Jones parameters for classical, all-atom models of acetate and various alkylated and non-alkylated forms of sulfate, sulfonate and phosphate ions, optimized to reproduce their interactions with water and with the physiologically relevant sodium, ammonium and methylammonium cations. The parameters are internally consistent and are fully compatible with the Generalized Amber Force Field (GAFF), the AMBER force field for proteins, the accompanying TIP3P water model and the sodium model of Joung and Cheatham. The parameters were developed primarily relying on experimental information - hydration free energies and solution activity derivatives at 0.5 m concentration - with ab initio, gas phase calculations being used for the cases where experimental information is missing. The ab initio parameterization scheme presented here is distinct from other approaches because it explicitly connects gas phase binding energies to intermolecular interactions in solution. We demonstrate that the original GAFF/AMBER parameters often overestimate anion-cation interactions, leading to an excessive number of contact ion pairs in solutions of carboxylate ions, and to aggregation in solutions of divalent ions. GAFF/AMBER parameters lead to excessive numbers of salt bridges in proteins and of contact ion pairs between sodium and acidic protein groups, issues that are resolved by using the optimized parameters presented here.

Symmetric ethereal hosts for lithium: factors for high affinity and selectivity upon ion recognition

Since the host-guest chemistry field has been formulated, organic hosts for various ions have been studied extensively. It is now widely accepted that structural and electrostatic complementarity is very important in host-guest chemistry. However, the detailed understanding which enables the chemists to design useful hosts is still in paucity. Although the calculated gas-phase binding energy is not of direct use for many practical applications, this could provide an objective measure for the intrinsic affinity. Here, we have calculated approximate binding energies for famous organic hosts for lithium. The selectivity between lithium and sodium was also investigated. 6-311++G(d,p) basis set with B3LYP functional was used to calculate binding energy of Li+ and Na+ with four famous hosts. They are spherand, [2,1,1]cryptand, 12-crown-4 and [16]starand. We examined several factors such as coordination number, partial charge of oxygen, distance from metal to its coordinating oxygen and metal-dipolar moiety interactions. Within the boundary of the chosen four symmetric hosts in this work, charge–dipolar moiety orientation was the factor that roughly explains the calculated binding affinity.

Comprehensive Benchmark of Association (Free) Energies of Realistic Host-Guest Complexes.

The S12L test set for supramolecular Gibbs free energies of association ΔGa (Grimme, S. Chem. Eur. J. 2012, 18, 9955-9964) is extended to 30 complexes (S30L), featuring more diverse interaction motifs, anions, and higher charges (-1 up to +4) as well as larger systems with up to 200 atoms. Various typical noncovalent interactions like hydrogen and halogen bonding, π-π stacking, nonpolar dispersion, and CH-π and cation-dipolar interactions are represented by "real" complexes. The experimental Gibbs free energies of association (ΔGa exp) cover a wide range from -0.7 to -24.7 kcal mol-1. In order to obtain a theoretical best estimate for ΔGa, we test various dispersion corrected density functionals in combination with quadruple-ζ basis sets for calculating the association energies in the gas phase. Further, modern semiempirical methods are employed to obtain the thermostatistical corrections from energy to Gibbs free energy, and the COSMO-RS model with several parametrizations as well as the SMD model are used to include solvation contributions. We investigate the effect of including counterions for the charged systems (S30L-CI), which is found to overall improve the results. Our best method combination consists of PW6B95-D3 (for neutral and charged systems) or ωB97X-D3 (for systems with counterions) energies, HF-3c thermostatistical corrections, and Gibbs free energies of solvation obtained with the COSMO-RS 2012 parameters for nonpolar solvents and 2013-fine for water. This combination gives a mean absolute deviation for ΔGa of only 2.4 kcal mol-1 (S30L) and 2.1 kcal mol-1 (S30L-CI), with a mean deviation of almost zero compared to experiment. Regarding the relative Gibbs free energies of association for the 13 pairs of complexes which share the same host, the correct trend in binding affinities could be reproduced except for two cases. The MAD compared to experiment amounts to 1.2 kcal mol-1, and the MD is almost zero. The best-estimate theoretical corrections are used to back-correct the experimental ΔGa values in order to get an empirical estimate for the "experimental", zero-point vibrational energy exclusive, gas phase binding energies. These are then utilized to benchmark the performance of various "lowcost" quantum chemical methods for noncovalent interactions in large systems. The performance of other common DFT methods as well as the use of semiempirical methods for structure optimizations is discussed.

Computational evaluation of unsaturated carbonitriles as neutral receptor model for beryllium(II) recognition.

Design of neutral receptor molecules (ionophores) for beryllium(II) using unsaturated carbonitrile models has been carried out via density functional theory, G3, and G4 calculations. The first part of this work focuses on gas phase binding energies between beryllium(II) and 2-cyano butadiene (2-CN BD), 3-cyano propene (3-CN P), and simpler models with two separate fragments; acrylonitrile and ethylene. Interactions between beryllium(II) and cyano nitrogen and terminal olefin in the models have been examined in terms of geometrical changes, distribution of charge over the entire π-system, and rehybridization of vinyl carbon orbitals. NMR shieldings and vibrational frequencies probed charge centers and strength of interactions. The six-membered cyclic complexes have planar structures with the rehybridized carbon slightly out of plane (16° in 2-CN BD). G3 results show that in 2-CN BD complex participation of vinyl carbon further stabilizes the cyclic adduct by 16.3kcal mol(-1), whereas, in simpler models, interaction between beryllium(II) and acetonitrile is favorable by 46.4kcal mol(-1) compared with that of ethylene. The terminal vinyl carbon in 2-CN BD rehybridizes to sp (3) with an increase of 7% of s character to allow interaction with beryllium(II). G4 calculations show that the Be(II) and 2-CN BD complex is more strongly bound than those with Mg(II) and Ca(II) by 98.5 and 139.2kcal mol(-1) (-1), respectively. QST2 method shows that the cyclic and acyclic forms of Be(II)-2-CN BD complexes are separated by 12.3kcal mol(-1) barrier height. Overlap population analysis reveals that Ca(II) can be discriminated based on its tendency to form ionic interaction with the receptor models.

Nano Cavity Induced Isotope Separation of Zinc: Density Functional Theoretical Modeling

Density functional theoretical calculation was performed to understand the isotopic fraction of zinc in the nano cavity of different crown ethers and crown ethers anchored on solid resin matrix. The structure, energetic, thermodynamics, and vibration IR spectra were also evaluated. The gas phase binding energy of Zn2+ with crown ether of varied cavity can be correlated with NPA charge transfer but no such correlation is obtained with the fraction of electron transfer, ΔN. The reduced partition function ratio (RPFR) of complexes of Zn2+ ion with different solvents, crown ethers, and grafted crown ethers was also computed to predict the isotope separation factor of the Zn2+ ion. The present study reveals that with increasing cavity size of the crown ether the RPFR was found to be decreased leading to a high value of isotope separation factor as observed in the experiments. An attempt was made to correlate the calculated RPFR and separation factor with binding energy of different zinc-crown ether systems. Fu...

Inclusion Complexes and Solvates of Buckycatcher, a Versatile Molecular Host with Two Corannulene Pincers

X-ray crystal structure study of cocrystallites of buckycatcher (2, C60H28) demonstrates a remarkable versatility of the molecular clip to accommodate guest molecules of various sizes and shapes. As demonstrated by the B97-D calculations, concave–concave conformation of the clip seems to be preferred for the gas-phase 1:1 inclusion complexes of 2 with a series of potential guests. However, the unprecedented concave–convex conformations were found in two of the reported solvates [i.e., 2·0.75(p-xylene) and 2·2PhNO2]. Three of the studied inclusion complexes with the highest calculated gas-phase binding energies (1,3,5-trinitrobenzene@2, and the previously reported C60@2 and C70@2) exhibit the analogous 1:1 supramolecular arrangements in the crystal state. On the other hand, p-xylene and PhNO2 solvates as well as DDQ/MeCN cocrystallite exhibit quite different stoichiometries and supramolecular arrangements, presumably enforced by the crystal packing forces. Dramatic variations of the intercentroid distances...