Fascinating interaction of the ammonium cation with [2.2.2]paracyclophane: experimental and theoretical study

By means of electrospray ionisation mass spectrometry, it was evidenced experimentally that the ammonium cation (NH₄⁺) reacts with the electroneutral [2.2.2]paracyclophane ligand (C₂₄H₂₄) to form the cationic complex [NH₄(C₂₄H₂₄)]⁺. Moreover, applying quantum chemical calculations, the most probable conformation of the proven [NH₄(C₂₄H₂₄)]⁺ complex was solved. In the complex [NH₄(C₂₄H₂₄)]⁺ having a symmetry very close to C₃, the ‘central’ cation NH₄⁺ is coordinated by three strong bifurcated intramolecular hydrogen bonds to the corresponding six carbon atoms from the three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, E(int), of the considered complex [NH₄(C₂₄H₂₄)]⁺ was evaluated as ?625.8 kJ/mol, confirming the formation of this fascinating complex species as well. It means that the [2.2.2]paracyclophane ligand can be considered as an effective receptor for the ammonium cation in the gas phase.     

Assessment of the nature of interactions of cations with cycloheptatriene derivatives using change in the aromaticity: Comparison with electron density and NBO results

ABSTRACT

Interactions of cycloheptatriene derivatives, C₇H₆X, (X?=?NH, PH, AsH, O, S, Se) with the cations H⁺, CH₃⁺, Cu⁺, Al⁺, Li⁺, Na⁺, and K⁺ are studied using B3LYP functional and 6-311++G(d,p) basis set. The calculated gas-phase cation affinities (CA) and cation basicities (CB) for all molecules decrease as H⁺ > CH₃⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. We used the induced aromaticity in the 7-membered ring of C₇H₆X upon interaction with the cations, M⁺, as a measure of C₇H₆X/M⁺ interaction. Nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) were used as two indices of aromaticity. The highest and lowest induced aromaticities were observed for interactions of H⁺ and K⁺, respectively. Also, the aromaticity induced by interaction with a cation in C₇H₆AsH and C₇H₆PH was larger than that in C₇H₆NH and C₇H₆O. Hence, the aromaticity was considered as a measure of covalency for the C₇H₆X/M⁺ interactions showing a rational dependence on both the molecule and cation. The nature of the interactions was also assessed using electron density, charge distribution analysis and NBO calculations. The results of the aromaticity indices, NICS and HOMA, were compared with the electron density and NBO results.     

Ab initio study of synergetic effects of two strong interactions of cation–π interaction and lithium bond in M+ ··· phenyl lithium ··· N (M = Li,Na, K; N = H2O and NH3) complex

Quantum chemical calculations have been performed to investigate the interplay between the cation–π interaction and lithium bonding in the M⁺?···?phenyl lithium?···?OH₂ and M⁺?···?phenyl lithium?···?NH₃ (M?=?Li, Na, K) complexes. The cation–π interaction and lithium bonding in the trimers become stronger relative to the dimers. The interaction energy of cation–π interaction is increased by about 4.4–6.3%, while that of lithium bonding is increased by about 5.2–15.9%. The cooperative energy becomes larger for the stronger cation–π interaction and lithium bond. The F atom and methyl group in the phenyl ring impose a reverse effect on the cation–π interaction and lithium bond. The interaction mechanism in the complexes has been understood with the many-body interaction analysis, electrostatic potentials, and energy decomposition.     

The role of metal cation in electron-induced dissociation of tryptophan

The fragmentation of tryptophan (Trp) – metal complexes [Trp+M]⁺, where M = Cs, K, Na, Li and Ag, induced by 22 eV energy electrons was compared to [Trp+H]⁺. Additional insights were obtained through the study of collision-induced dissociation (CID) of [Trp+M]⁺ and through deuterium labelling. The electron-induced dissociation (EID) of [Trp+M]⁺ resulted in the formation of radical cations via the following pathways: (i) loss of M to form Trp^+?, (ii) loss of an H atom to form [(Trp-H)+M]^+?, and (iii) bond homolysis to form C₂H₄NO₂M^+?. Deuterium labelling suggests that H atom loss can occur from heteroatom and/or C–H positions. Other types of fragment ions observed include: C₉H₇NM⁺, C₉H₈N⁺, M⁺, C₂H₃NO₂M⁺, CO₂M⁺, C₁₀H₁₁N₂M⁺, C₁₀H₉NOM⁺. Formation of C₂H₄NO₂M^+? and C₉H₇NM⁺ cations suggests that the metal interacts with both the backbone and aromatic side chain, thus implicating π-interactions for all M. CID of [Trp+M]⁺ resulted in: loss of metal cation (for M = Cs and K); successive loss of NH₃ and CO as the dominant channel for M = Na, Li and Ag; formation of C₂H₃NO₂M⁺. Preliminary DFT calculations were carried out on [Trp+Na]⁺ and [(Trp-H)+Na]^+? which reveal that: the most stable conformation involves chelation by the backbone together with a $\pi $ -interaction with the indole side chain; loss of H atom from $\alpha $ -CH of the side chain is thermodynamically favoured over losses from other positions, with the resultant radical cation maintaining a (N, O, ring) chelated structure which is stabilized by conjugation.     

A computational study of interplay between hydride bonding and cation–π interactions: H-Mg-H···X···Y triads (X = Li+, Na+, Y = C2H2, C2H4, C6H6) as model systems

In the present study, H-Mg-H···X···Y (X = Li⁺, Na⁺ and Y = C₂H₂, C₂H₄, C₆H₆) triads have been investigated at MP2/6-311++G(2d,2p) computational level to characterise cooperative effects between hydride bonding and cation–π interactions. Molecular geometries, binding energies, cooperative energies and many-body interaction energies were evaluated. The diminutive energy values in triads with Li⁺ are larger than respective values in triads with Na⁺. The electronic properties of the complexes are analysed using parameters derived from the quantum theory of atoms in molecules methodology.     

Cation-mediated sandwich formation between benzene and pillar[5]arene: a DFT study

Cation–π interactions in alkali metal ion (Li⁺, Na⁺ and K⁺)–pillar[5]arene complexes and sandwiches of pillar[5]arene and benzene formed via alkali metal ions are studied in the light of density functional theory. Several possible modes of interaction between metal ions and pillar[5]arene have been studied. Results suggest that interaction is stronger in the complexes with the metal ion present inside the cavity of the pillar[5]arene as compared to that where the metal ion is outside the cavity. The calculated interaction energy further reveals that though cation–π complexes with larger number of alkali metal ions are unstable, however, corresponding sandwiches are stable, which further support the fact that pillar[5]arene–metal ion complexes can interact with other π–electron-rich species. Absorption spectra of the complexes formed undergo both blue and red shifts as compared to the pillar[5]arene.     

Relationship between cation–π and anion–π interactions: individual binding energies in the π–Mz+–π–X−–π system

The mutual influence of cation–π and anion–π interactions in the π–M^z+–π–X^?–π system (M^z+ = Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺ and X^? = F^?, Cl^?) has been studied by quantum mechanical calculations. Both geometric parameters and energy data reveal that cation–π and anion–π interactions enhance each other in the π–M^z+–π–X^?–π system. Individual binding energies (E_ion···π) have been estimated in the quintuplet system using a simple new method from electron charge densities calculated at the bond critical points (BCPs) of the ion···π interaction by the atoms in molecules (AIM) method at the M062X/6-31+G(d) level of theory. With respect to the obtained individual binding energies, the strength of an ion···π interaction depends on the cooperative effects of other components.     

MCsn cluster formation on organic surfaces: A novel way to depth-profile organics?

In this work, we investigated the bombardment of polymer surfaces (PC, PMMA) with very low energy (250 eV) Cs⁺ ions.In the positive mode, we observed many cations combining a molecular fragment of the polymer (M) with two Cs atoms, similar to the well-known MCs₂⁺ clusters commonly observed in inorganic depth profiling with Cs. For example, Cs₂COOH⁺ or Cs₂C₆H₅O⁺ were measured on PC and Cs₂CH₃O⁺ or Cs₂C₄H₅O₂⁺ were measured on PMMA. In fact, most of the polymer characteristic fragments measured in negative spectra also appear in the positive spectra, combined with two Cs atoms. It is remarkable that stable negative ions tend to combine not with one Cs, but with two Cs atoms to create the MCs₂⁺ cluster.This effect is, to some extent, similar to the well-known cationisation of polymers (with Ag or Au) although here few clusters with only one Cs atom are detected (MCs⁺ clusters), like in classical cationisation. The most abundant clusters are the MCs₂⁺ clusters, but heavier clusters (MCs₃⁺, MCs₄⁺ and above) are also measured with high yields in the spectrum.Surprisingly, some of those MCs₂⁺ clusters were still detected even after a very long sputtering fluence (above 10¹⁷ ions/cm²), proving that some molecular depth profiling is also possible in this “Cs₂-cationisation” mode. In other words, this work could open the way to an extension of the MCs_n⁺ cluster analysis, commonly used in inorganic depth profiling, to the in-depth molecular analysis of organic layers.     

第三族金属氧化物离子对二氧化碳转化的红外光谱研究

本文利用红外光解离光谱研究了第三族金属氧化物离子对二氧化碳分子的转化机制. 研究表明,对于[ScO(CO₂)_n]⁺体系,在n≤4时,形成了溶剂化结构;在n=5时,形成了碳酸盐结构,实现了二氧化碳的转化. 对于[YO(CO₂)_n]⁺体系,需要4个二氧化碳分子就可以实现二氧化碳的转化. 而在[YO(CO₂)_n]⁺体系中,只发现了溶剂化结构,没有观察到碳酸盐结构. 理论计算表明,[YO(CO₂)_n]⁺体系拥有最小的溶剂化结构向碳酸盐结构转化能垒,[LaO(CO₂)_n]⁺体系拥有最大的溶剂化结构向碳酸盐结构转化能垒. 本文从分子水平揭示了不同金属氧化物离子对二氧化碳分子转化的影响规律.     

Electronic structures of imidazolium-based ionic liquids

Electronic structures of ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [C_nmim]⁺ (n = 4 and 8) with various inorganic and organic anions have been investigated by ultraviolet photoemission, X-ray photoemission, inverse photoemission and soft X-ray emission spectroscopies (SXES). The comparison of the calculated density of states with the observed spectra revealed that the molecular orbital energies of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions. The SXES results clearly show that the both highest occupied and lowest unoccupied states of [C₄mim]⁺PF₆⁻ are derived from the cation as a result of strong Madelung potential. On the other hand, the SXES results show the valence electronic structures of ionic liquids with larger anion molecules, [C_nmim]⁺Tf₂N⁻ and [C_nmim]⁺OTf⁻ are contributed from the both cation and anion.     

A quantum mechanical explanation of the structure of vinyl cation based on a CASSCF/CASMP2 study

An ab-initio CASSCF/CASMP2 study on the structures and energies of the classical and bridged forms of the vinyl cation is presented. Our calculations which are in agreement with the experimental results predict that the bridged form of the vinyl cation (C₂H₃ ⁺) is more stable than its classical counterpart and is the unique species on the potential energy surface. A quantum mechanical explanation based in the notion of electron correlation energy and in Boltzmann distribution shows that the classical form is considered to be a transient species having a fleeting existence.     

Self‐assembly of a [2]pseudorotaxane complex by the threading of a nitrobenzimidazol‐based guest on dibenzo‐24‐crown ether host

A new derivative of the previously reported 1,2‐bis(benzimidazol‐2‐yl)ethane motif, cation [1H₂]²⁺, was synthesized under microwave irradiation and fully characterized by solution NMR, high‐resolution mass spectrometry, cyclic voltammetry and X‐ray crystallography. This cation presents a linear geometry and incorporates nitro substituents as electrochemical handles. In solution, cation [1H₂]²⁺, is capable of threading the cavity of dibenzo‐24‐crown‐8 ether host (DB24C8) giving rise to a [2]pseudorotaxane complex [1H₂?DB24C8]²⁺, regardless of the counterion, [CF₃SO₃]^? or [CF₃COO] ^?. The interpenetrated structure of [1H₂?DB24C8]²⁺ was proven by solution NMR and X‐ray crystallography. This host–guest complex is held together by several non‐covalent interactions, such as hydrogen bonding and ion‐dipole. An electrochemical study of [1H₂]²⁺ in the presence of variable amounts of DB24C8 was performed; due to the irreversible redox behavior of cation [1H₂]²⁺, it was not possible to electrochemically control the association/dissociation process with DB24C8. Copyright © 2014 John Wiley & Sons, Ltd.     

Structure and bonding of the (C6H6)+ 2 radical

To elucidate the relative stability of various structures of the benzene dimer cation radical, (C₆H₆)⁺ ₂ in its ground and low-lying excited states, ab initio complete active space self-consistent field (CASSCF), multi-reference singly and doubly excited configuration interaction (MRSDCI), and multi-reference coupled pair approximation (MRCPA) calculations were performed. Full optimization was performed at the CASSCF level for various structures of the dimer cation, followed by MRSDCI and MRCPA calculations. It was found that the global minimum of the cation is at a slipped C_2h sandwich structure but there are some other sandwich structures with almost the same stability, being within about kcal mol^?1. T-shape structures are less stable than the sandwich structures, by more than 5 kcal mol^?1 by MRCPA calculations. Low lying electronic excited states in various structures are also discussed.     

Nuclear Magnetic Resonance Relaxation Study of the Phase Transitions of Rb2CuCl4·2H2O and Cs2MnCl4·2H2O Single Crystals

The physical properties and phase transitions of Rb₂CuCl₄·2H₂O and Cs₂MnCl₄·2H₂O crystals were investigated by performing ⁸⁵Rb, ⁸⁷Rb, and ¹³³Cs nuclear magnetic resonance relaxation analyses. The temperature dependences of the spectra and the spin–lattice relaxation times T ₁ near T _C are related to changes in the symmetry of the octahedrons consisting of four Cl⁻ ions and two water molecules around Rb⁺ or Cs⁺; the forms of these octahedrons are disrupted by the loss of H₂O. The difference between the relaxation times of the two crystals is possibly due to the difference between the electron structures of the Cu and Mn ions. Cu²⁺ has nine valence electrons in its 3d orbital, whereas Mn²⁺ has five valence electrons in its 3d orbital.     

Interplay between the intramolecular hydrogen bonds and cation–π interactions in various complexes of salicylaldehyde,thiosalicylaldehyde and selenosalicylaldehyde with Li+, Na+, K+, Mg2+ and Ca2+ cations

For the first time, the mutual influences of the intramolecular hydrogen bond (IMHB) and cation–π interactions in various complexes of salicylaldehyde, thiosalicylaldehyde and selenosalicylaldehyde with Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺ cations were studied. First, the strength of IMHB and cation–π interactions of the mentioned complexes by energetic, geometrical, spectroscopic, topological and molecular orbital parameters was evaluated and compared with the corresponding results of benzene–cation complexes and salicylaldehyde analogues. The results show that the coexistence of IMHB and cation–π interactions increases the IMHB strength and decreases the cation–π interactions. Second, the significance of π–electron delocalisation (π–ED) within the resonance-assisted hydrogen bond (RAHB) unit and aromaticity of benzene ring in the studied complexes were estimated by using the harmonic oscillator model of aromaticity and compared with the respective amounts of references. The results indicated that the mentioned coupling decreases the π–ED of RAHB unit and aromaticity of the benzene ring. In addition, it was found that variations in the strength of the interactions, π–ED and aromaticity, depend on the charge-to-radius ratio of cations. Finally, the effects of replacement of O by S and Se atoms in both of the mentioned cases were explored.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

Magnetic shielding studies on the alkali molecules Na2 and Cs2 by NMR

NMR in the alkali molecules Na₂ and Cs₂ is performed by the atom-molecule exchange optical pumping method. The shielding differences σ(Na)?σ(Na₂)=(29±16)·10^?6 and σ(Cs)?σ(Cs₂)=(221±12)·10^?6 are obtained. The investigation of the contribution of the valence electron to the magnetic shielding is supported by a NMR experiment in free Cs⁺ ions, which yields the shielding difference σ(Cs)?σ(Cs⁺)=(14±12)·10^?6. These measurements allow an estimation of the spin rotation interaction constant in these molecules.     

Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane

Extending our earlier findings for [3.3]paracyclophane, NMR line shape studies of the conformational dynamics in [3.2] and [4.3]paracyclophanes are reported, of which the former is conformationally homogeneous and the latter occurs in two enantiomeric forms. For [3.2]paracyclophane, the Arrhenius activation energy E_a = 11.6 ± 0.1 kcal/mol and preexponential factor log (A/s^?1) = 12.92 ± 0.07 were found. In [4.3]paracyclophane, the conformational dynamics are quite complicated because, apart from interconversions of each enantiomer into itself proceeding via inversion of the propano bridge with rate constant k₁, the enantiomers mutually rearrange with rate constant k₂ due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic ¹H spectra of the aromatic protons for these two conformational processes (E_a = 11.2 ± 0.5 kcal/mol and log (A/s^?1) = 13.6 ± 0.5 for the former, and E_a = 9.7 ± 0.4 kcal/mol and log (A/s^?1) = 13.2 ± 0.4 for the latter) is the highlight of this work. In the investigated temperature range, in [4.3]paracyclophane, the occurrence of other conformational processes beyond those mentioned above can be excluded, because they would produce different line shape patterns than those actually observed. Copyright © 2013 John Wiley & Sons, Ltd.