Ion Pairs In Gas Phase Research Articles

The cohesive properties and pyrolysis mechanism of an aprotic ionic liquid tetrabutylammonium bis(trifluoromethanesulfonyl)imide.

As the cohesive properties (such as the enthalpy of sublimation) of solid organic salts (or ionic liquids, ILs) are unmeasurable, a method of their indirect determination is proposed in this paper. For this purpose, the thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) were carried out over a wide range of temperatures. In this study, the mathematical relationship of the thermodynamic properties between the liquid and solid phases of ILs is established using the Born-Fajans-Haber cycle, in which the sum of the vaporization enthalpy of ILs, melting enthalpy and the enthalpy of solid-solid phase transition is regarded as the sublimation enthalpy of solid organic salts. With this method, the cohesive properties of tetrabutylammonium bis(trifluoromethanesulfonyl)imide ([N4444][NTf2]), which is an aprotic IL, were successfully obtained. Additionally, the difference between the lattice energy and the cohesive energy was employed to quantitatively calculate the charge separation distance of single ion pair (r12) in the gas phase of ionic liquids for the first time, which can serve as a standard methodology to measure the closeness in distance between the anion and the cation in a gas phase ion pair. The pyrolysis mechanism of [N4444][NTf2] was also explored.

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods.

Ionic liquids are described by a delicate balance of Coulomb interaction, hydrogen bonding and dispersion forces. Dissecting the different types of interactions from thermodynamic properties is still a challenge. Here, we show that comparison of vaporization enthalpies of tetra-alkyl-ammonium ionic liquids with bis(trifluoromethylsulfonyl)imide [NTf2]- anions and the related molecular liquids, trialkylamines, allows for determining dispersion interactions in the gas phase ion-pairs. For this purpose, we measured vapor pressures and vaporization enthalpies of these ionic and molecular liquids by using a quartz-crystal microbalance. For supporting these data, we determined the vaporization enthalpies additionally from experimental activity coefficients at infinite dilution. Characteristic alkyl chain length dependences of the vaporization enthalpies have been established and were used for quantifying the dispersion forces in the gas phase species. The dissected dispersion contributions suggest that the alkyl chains do not show star-like topologies but embrace the anion maximizing the dispersion interactions. For the longest alkyl chains with eight carbon atoms, the dispersion interaction is as strong as two and a half hydrogen bonds. The proportion of dispersion in the gas phase species depending on the number of methylene groups in the ammonium cations is strongly supported by quantum chemical calculations.

Photoinduced Intermolecular Electron Transfer in Gas Phase Ion Pairs of the 1-Ethyl-3-methylimidazolium Cation and the Bis(trifluoromethylsulfonyl)imide Anion.

In this study, the UV photodissociation of gas phase ion pairs of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim]+[tf2n]-, is shown to proceed primarily through radical intermediates. [emim]+[tf2n]- ion pairs have been shown previously to undergo two-photon-dependent dissociation, but the mechanisms of this have not been probed in detail. By employing a two-laser pump probe spectroscopy and time-dependent density functional theory (TD-DFT) calculations, we have illustrated that one of the major UV photodissociation pathways in [emim]+[tf2n]- ion pairs is an intermolecular electron transfer wherein the anion transfers an electron to the cation resulting in two neutral open-shelled products. These products were observable for at least 1.6 μs post photodissociation, the experimental limit, via detection of the [emim]+ cation. This data demonstrates that the likely photoproducts of [emim]+[tf2n]- UV photodissociation are two neutral species that separate spatially, demonstrated through lack of observed relaxation pathways such as electron recombination. TD-DFT and frontier molecular orbital analysis calculations at the MN15/6-311++G(d,p) level are employed to aid in identifying excited state characteristics and support the interpretations of the experimental data. The energetic onset of the intermolecular electron transfer is consistent with previously observed [emim]+[tf2n]- absorption spectra in the bulk and gas phases. The similarities between bulk and gas phase UV spectra imply that this electron-transfer pathway may be a major photodissociation channel in both phases.

Frontier orbitals and quasiparticle energy levels in ionic liquids

Ionic liquids play an important role in many technological applications and a detailed understanding of their frontier molecular orbitals is required to optimize interfacial barriers, reactivity and stability with respect to electron injection and removal. In this work, we calculate quasiparticle energy levels of ionic liquids using first-principles many-body perturbation theory within the GW approximation and compare our results to various mean-field approaches, including semilocal and hybrid density-functional theory and Hartree–Fock. We find that the mean-field results depend qualitatively and quantitatively on the treatment of exchange–correlation effects, while GW calculations produce results that are in excellent agreement with experimental photoelectron spectra of gas phase ion pairs and ionic liquids. These results establish the GW approach as a valuable tool for understanding the electronic structures of ionic liquids.

The electronic structure of ionic liquids based on the TFSI anion: A gas phase UPS and DFT study

The valence bands of [EMIM][TFSI], [DEME][TFSI] and [PYR1,4][TFSI] gas-phase ion pairs have been investigated using ultraviolet photoelectron spectroscopy (UPS). The photoelectron spectra are interpreted by using several density functional and ab initio calculation methods. Although the experimental vapor phase spectra are similar, the different calculation methods make different predictions about the HOMO molecular state of the ion-pairs of the ionic liquids. The HOMO state of the [DEME][TFSI] ion-pair is due to the TFSI anion, while in [EMIM][TFSI] it is due to the EMIM cation. However, it is difficult to make conclusive assignments for the [PYR1,4][TFSI] ionic liquid. All calculation methods predict the LUMO to be of cationic origin in all the studied ion-pairs.

Identification of multiple conformers of the ionic liquid [emim][tf2n] in the gas phase using IR/UV action spectroscopy.

In this study we investigate the effect of deuteration and molecular beam temperature on the hydrogen bond in the ionic liquid [emim][tf2n]. Using IR/UV double resonance spectroscopy, we probe the microscopic structure of the [emim][tf2n] ion pair and its mono-deuterated, [emim-d1][tf2n], analog. Comparisons of the infrared absorption frequencies between these two species show that there are multiple conformers of the ion pair present in the gas phase and trapped through the molecular beam cooling process. Furthermore, each conformer has a characteristic red shift in the frequency of its C2-H group that reveals the variation in strength of a hydrogen bond between the cation and anion.

Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids

In this paper we have explored the structural and energetic landscape of potential π(+)-π(+) stacked motifs, hydrogen-bonding arrangements and anion-π(+) interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C1C1im]Cl. We classify cation-cation ring stacking as an electron deficient π(+)-π(+) interaction, and a competitive anion on-top IP motif as an anion-donor π(+)-acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0-126 kJ mol(-1). The structures have been found to exhibit a complex interplay of structural features. We have found that low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled range of IP-dimers exhibits new structural forms that cannot be recovered by examining the ion-pairs alone, moreover the IP-dimers are recovering additional key features of the local liquid structure. Including dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.

Insight into Hydrazinium Nitrates, Azides, Dicyanamide, and 5-Azidotetrazolate Ionic Materials from Simulations and Experiments

A transferrable, polarizable, quantum chemistry (QC) based force field has been developed for hydrazinium (N(2)H(5)(+)), monomethylhydrazinium ((CH(3))N(2)H(4)(+)), and dimethylhydrazinium ((CH(3))(2)N(2)H(3)(+)) cations in combination with the nitrate (NO(3)(-)), azide (N(3)(-)), dicyanamide (N(CN)(2)(-)), and 5-azidotetrazolate (CN(7)(-)) anions. Inclusion of the off-atom charge center to represent a lone pair on the hydrazinium-based cations significantly improved the electrostatic potential description around cations and led to overall a more accurate prediction of ionic crystal cell parameters in molecular dynamics (MD) simulations. Seven different ionic systems have been investigated: [N(2)H(5)][NO(3)], [(CH(3))N(2)H(4)][NO(3)], [(CH(3))(2)N(2)H(3)][NO(3)], [N(2)H(5)][CN(7)], [(CH(3))N(2)H(4)][N(3)], [(CH(3))(2)N(2)H(3)][N(3)], [N(2)H(5)][N(CN)(2)]. For all but [(CH(3))(2)N(2)H(3)][NO(3)] and [N(2)H(5)][N(CN)(2)], QC calculations of a single, gas-phase ion pair predicts spontaneous deprotonation of the cation. Crystal lattice parameters obtained from MD simulations for these seven ionic crystals were compared with the previously published experimental data as well as the crystal structure of [N(2)H(5)][N(CN)(2)] determined in this work from X-ray data. In general, MD simulations predicted crystal lattice vectors/angles (volumes) within a 5% (3%) absolute margin of error from experiments, with outlying volume deviations of 5-6.6% for three crystals [(CH(3))N(2)H(4)][N(3)], [N(2)H(5)][NO(3)], and [(CH(3))N(2)H(4)][NO(3)] with combinations of particularly small anions and/or cations. Structural comparisons between ionic materials in the liquid and crystalline states are made, including the observation of two crystalline systems where the crystalline state induces conformational changes in the methylated hydrazinium cations between the gas-phase and liquid states. Elastic constants and estimated shear and bulk moduli were extracted from MD simulations for all seven ionic crystals and correlated with the structural motifs of ion interactions in the crystals.

Simulations of photoemission and equilibrium redox processes of ionic liquids: the role of ion pairing and long-range polarization

We have studied oxidation and reduction of ionic liquids using methods of theoretical chemistry. In particular, we have modeled photoelectron spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf(2)N]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf(2)N]) ionic liquids and their components. We have considered ion pairs in the gas phase and solvated in the water represented by a dielectric continuum. We have also characterized the isolated cation and anion of the ionic liquids. The calculated quantities have been compared with available experiments. The photoelectron spectra were modeled within linearized reflection approximation, using a composite ab initio approach based on combination of the MP2 method for the ground state, the PMP2 method for the first ionized electronic state and the TDDFT method to estimate the higher ionization energies. We have also briefly explored energetics of processes following the vertical electron attachment/detachment. Our calculations were able to reproduce the measured photoelectron spectra. We have shown that the valence photoelectron spectrum results from simultaneous ionization from many molecular orbitals. In the threshold region, the electron is ejected from the anion in the gas phase ion pair while the cationic moiety is ionized when the ion pair is solvated. We have investigated the effect of both short-range and long-range interactions on the ionization characteristics, showing that the long-range polarization of the solvent effectively screens the specific short-range interactions. The large difference between the HOMO-LUMO gap and the electrochemical window has been rationalized in terms of relaxation following the redox processes, i.e. solute and solvent geometric relaxation and reactions, e.g. dimerization of the imidazolium radical formed during the reduction.

X-ray Crystal Structures of [XeF][MF6] (M = As, Sb, Bi), [XeF][M2F11] (M = Sb, Bi) and Estimated Thermochemical Data and Predicted Stabilities for Noble-Gas Fluorocation Salts using Volume-Based Thermodynamics

The crystal structures of the xenon(II) salts, [XeF][SbF(6)], [XeF][BiF(6)], and [XeF][Bi(2)F(11)], have been determined for the first time, and those of XeF(2), [XeF][AsF(6)], [XeF][Sb(2)F(11)], and [XeF(3)][Sb(2)F(11)] have been redetermined with greater precision at -173 °C. The Bi(2)F(11)(-) anion, which has a structure analogous to those of the As(2)F(11)(-) and Sb(2)F(11)(-) anions, has been structurally characterized by single crystal X-ray diffraction for the first time as its XeF(+) salt. The fluorine bridge between the bismuth atoms is asymmetric with Bi---F(b) bond lengths of 2.092(6) and 2.195(6) A and a Bi---F(b)'---Bi bridge bond angle of 145.3(3)°. The XeF(+) cations interact with their anions by means of Xe---F(b)---M bridges. Consequently, the solid-state Raman spectra of [XeF][MF(6)] (M = As, Sb, Bi) were modeled as the gas-phase ion pairs and assigned with the aid of quantum-chemical calculations. Relationships among the terminal Xe-F(t) and bridge Xe---F(b) bond lengths and stretching frequencies and the gas-phase fluoride ion affinities of the parent Lewis acid that the anion is derived from are considered. The analogous krypton ion pairs, [KrF][MF(6)] (M = As, Sb, Bi) were also calculated and compared with their previously published X-ray crystal structures. The calculated cation-anion charge separations indicate that the [XeF][MF(6)] salts are more ionic than their krypton analogues and that XeF(2) is a stronger fluoride ion donor than KrF(2). The lattice energies, standard enthalpies, and free energies of formation for salts containing the NgF(+), Ng(2)F(3)(+), XeF(3)(+), XeF(5)(+), Xe(2)F(11)(+), and XeOF(3)(+) (Ng = Ar, Kr, Xe) cations were estimated using volume-based thermodynamics (VBT) based on crystallographic and estimated ion volumes. These estimated parameters were then used to predict the stabilities of noble-gas salts. VBT is used to examine and predict the stabilities of, inter alia, the salts [XeF(m)][Sb(n)F(5n+1)] and [XeF(m)][As(n)F(5n+1)] (m = 1, 3; n = 1, 2). VBT also confirms that XeF(+) salts are stable toward redox decomposition to Ng, F(2), and MF(5) (M = As, Sb), whereas the isolable krypton compounds and the unknown ArF(+) salts are predicted to be unstable by VBT with the ArF(+) salts being the least stable.

Syntheses, Raman Spectra, and X-ray Crystal Structures of [XeF5][μ-F(OsO3F2)2] and [M][OsO3F3] (M = XeF5+, Xe2F11+)

Stoichiometric amounts of XeF(6) and (OsO(3)F(2))(infinity) react at 25-50 degrees C to form salts of the known XeF(5)(+) and Xe(2)F(11)(+) cations, namely, [XeF(5)][mu-F(OsO(3)F(2))(2)], [XeF(5)][OsO(3)F(3)], and [Xe(2)F(11)][OsO(3)F(3)]. Although XeF(6) is oxophilic toward a number of transition metal and main-group oxides and oxide fluorides, fluoride/oxide metathesis was not observed. The series provides the first examples of noble-gas cations that are stabilized by metal oxide fluoride anions and the first example of a mu-F(OsO(3)F(2))(2)(-) salt. Both [XeF(5)][mu-F(OsO(3)F(2))(2)] and [Xe(2)F(11)][OsO(3)F(3)] are orange solids at room temperature. The [XeF(5)][OsO(3)F(3)] salt is an orange liquid at room temperature that solidifies at 5-0 degrees C. When the salts are heated at 50 degrees C under 1 atm of N(2) for more than 2 h, significant XeF(6) loss occurs. The X-ray crystal structures (-173 degrees C) show that the salts exist as discrete ion pairs and that the osmium coordination spheres in OsO(3)F(3)(-) and mu-F(OsO(3)F(2))(2)(-) are pseudo-octahedral OsO(3)F(3)-units having facial arrangements of oxygen and fluorine atoms. The mu-F(OsO(3)F(2))(2)(-) anion is comprised of two symmetry-related OsO(3)F(2)-groups that are fluorine-bridged to one another. Ion pairing results from secondary bonding interactions between the fluorine/oxygen atoms of the anions and the xenon atom of the cation, with the Xe...F/O contacts occurring opposite the axial fluorine and from beneath the equatorial XeF(4)-planes of the XeF(5)(+) and Xe(2)F(11)(+) cations so as to avoid the free valence electron lone pairs of the xenon atoms. The xenon atoms of [XeF(5)][mu-F(OsO(3)F(2))(2)] and [Xe(2)F(11)][OsO(3)F(3)] are nine-coordinate and the xenon atom of [XeF(5)][OsO(3)F(3)] is eight-coordinate. Quantum-chemical calculations at SVWN and B3LYP levels of theory were used to obtain the gas-phase geometries, vibrational frequencies, and NBO bond orders, valencies, and NPA charges of the ion pairs, [Xe(2)F(11)][OsO(3)F(3)], [XeF(5)][OsO(3)F(3)], and [XeF(5)][mu-F(OsO(3)F(2))(2)], as well as those of the free ions, Xe(2)F(11)(+), XeF(5)(+), OsO(3)F(3)(-), and mu-F(OsO(3)F(2))(2)(-). The Raman spectra (-150 degrees C) of the salts have been assigned based on the ion pairs observed in the crystal structures and the calculated vibrational frequencies and intensities of the gas-phase ion pairs.

Study on the relationship between the interaction energy and the melting point of amino acid cation based ionic liquids

The DFT method B3LYP together with basis set 6-311++G(d,p) was used to study the interaction energy between cation and anion in a series of new generation ionic liquids whose cations are derived from amino acids. The geometry optimization of gas-phase ion pairs of over 30 chosen ionic liquid and related analogues (some salts with high melting points) was performed first, then the interaction energies of the series of molecules between cation and anion were calculated and the basis set superposition error (BSSE) was corrected by counterpoise method (CP). Combining the experimental melting point data, it was found that for the amino acid cation based ionic liquids, there exists good correlation between the melting points and the interaction energies, and the corresponding linear equation of melting point on interaction energy was obtained. The results show the probability of designing ILs with the help of quantum chemistry study in the future exploitation.

Structural and Electronic Properties of Amino Acid Based Ionic Liquids: A Theoretical Study

The gas-phase ion pairs of the ionic liquids containing 1-ethyl-3-methylimidazolium ([emim](+)) and 20 natural amino acids ([AA](-)) are studied at the B3LYP/6-311+G (d,p) level. The optimized structures, energies, and natural population analysis are presented and analyzed in terms of their possible correlation with the interaction energies and the H-bond separations. It is found that all the ion pairs of [emim][AA] can form strong H-bond interactions, which are dominated by the side-chain structure and the functional group of amino acid anions. The calculations indicate that an increase of the alkyl side-chain length coincides with a gradual decrease of H-bond energy, while the functional groups lead to the different localized charges on the anions, consequently affecting the electrostatic force. In addition, the intramolecular H bond in [AA](-) can weaken the interaction, due to the decrease of the proton-accepting ability of the carbonyl O atoms. The H-bond chemical nature of [emim][AA] is investigated by atoms in molecules and natural bond orbital analyses. The preliminary analysis of 20 kinds of [emim][AA] ion pairs provides some initial hints as to the relationship between the interaction energy and the experimental glass transition temperature.

Interaction between 1-Ethyl-3-Methyl-Imidazolium Cation and Asparagine Anion

利用密度泛函理论B3LYP方法,在6-311＋G（d,p）水平上,对1-乙基-3-甲基咪唑阳离子[Emim]＋与天冬酰胺阴离子[Asn]-形成的氨基酸离子液体气态阴阳离子对（[Emim][Asn]）进行理论研究.通过几何结构优化和频率分析得到势能面上的五个稳定构型.[Emim]＋和[Asn]-之间能够形成较强的氢键相互作用,零点能校正后的能量在-373.96至-326.28kJ·mol^-1之间.其稳定化能主要来源于[Asn]-中羰基O的孤对电子lp（O）与[Emim]＋中C—H反键轨道σ＊（C—H）之间的相互作用：lp（O）→σ＊（C—H）.红外光谱特征和自然布居分析（NPA）计算表明咪唑阳离子中参与形成氢键的C—H键振动的红移值、阴阳离子间的电荷转移与氢键相互作用能成正比关系.分子中的原子（AIM）理论分析得到[Emim]＋和[Asn]-之间的氢键相互作用以静电作用为主.通过计算结果初步探讨影响氨基酸离子液体玻璃化温度Tg的结构因素.

Vapourisation of ionic liquids

Eight common imidazolium based ionic liquids have been successfully evaporated in ultra-high vacuum, their vapours analysed by line of sight mass spectrometry and their heats (enthalpy) of vapourisation determined. They were found to evaporate as ion pairs, with heats of vapourisation which depend primarily on the coulombic interactions within the liquid phase and the gas phase ion pair. An electrostatic model is presented relating the heats of vapourisation to the molar volumes of the ionic liquids.

Structural Characterization of the 1-Butyl-3-methylimidazolium Chloride Ion Pair Using ab Initio Methods

Ab initio theoretical methods are used to investigate the gas-phase ion pairs of the ionic liquid 1-butyl-3-methylimidazolium chloride. Multiple stable conformers with the chloride anion positioned (in-plane) around the imidazolium ring or above the C2-H bond are determined. The relative energy ordering of the conformers is examined at the B3LYP, MP2, and CCSD(T) levels. Zero-point energies, BSSE, and basis set effects are examined. For accurate results, correlation (dispersion) effects must be included. The most stable conformers are essentially degenerate and have the chloride H-bonding to, or lying above, the C2-H bond. Other conformers are found to lie approximately 30 and approximately 60 kJ mol(-1) higher in energy. Results are compared with those from recent simulations and experimental studies. The effect of the chloride anion on rotation of the butyl chain is investigated and found to lower some rotational barriers while enhancing others. The origin of the rotational barriers is determined. The number and type of hydrogen bonds formed between the imidazolium cation and chloride anion is found to vary significantly among conformers. No evidence of a possible intra C(alkyl)-H...pi interaction is obtained; however, hints of a Cl...pi interaction are found. The vibrational spectrum of each conformer is examined, and the origin of multiple (H-bonding) features in the vibrational spectrum of the ionic liquid explained.

Alkaliisocyanacetate. Synthese und Struktur von [K(18-Krone-6)](O2CCH2NC)

Alkali-isocyanoacetates. Synthesis and Structure of [K(18-crown-6)](O2CCH2NC) The alkali isocyanoacetates M+[O2CCH2NC]− (M = Li,Na,K,Cs) (1a-d) are synthesized by reaction of ethyl isocyanoacetate with the respective alkali hydroxides in ethanol and characterized by IR, NMR (1H, 13C), and mass spectrometry (FAB). In alcoholic solution as well as in the gas phase ion pairs and higher aggregated species are observed. In contrast, [K(18-crown-6)][O2CCH2NC] (2) which is obtained from 1c and 18-crown-6, turns out to be a 1:1 electrolyte in solution (acetone); in the solid, the isocyanoacetate anion binds to K+ via the two carboxylate oxygen atoms resulting in an O8-coordinated metal atom.

Ab Initio Molecular Dynamics Simulation of a Room Temperature Ionic Liquid

Ab initio molecular dynamics simulations have been performed for the first time on the room-temperature organic ionic liquid dimethyl imidazolium chloride [DMIM][Cl] using density functional theory. The aim is to compare the local liquid structure with both that obtained from two different classical force fields and from neutron scattering experiments. The local structure around the cation shows significant differences compared to both the classical calculations and the neutron results. In particular, and unlike in the gas-phase ion pair, chloride ions tend to be located near a ring C-H proton in a position suggesting hydrogen bonding. The results are used to suggest ways in which the classical potentials may be improved.

Simple means to alleviate sensitivity loss by trifluoroacetic acid (TFA) mobile phases in the hydrophilic interaction chromatography–electrospray tandem mass spectrometric (HILIC–ESI/MS/MS) bioanalysis of basic compounds

Trifluoroacetic acid (TFA) is a commonly used additive in HPLC and LC-MS analysis of basic compounds. It is also routinely added to aqueous-organic mobile phases utilized in the hydrophilic interaction chromatography-electrospray tandem mass spectrometry (HILIC-ESI/MS/MS) technique used in our laboratories for bioanalysis. However, TFA is known to suppress the ESI signals of analytes due to its ability to form gas-phase ion pairs with positively-charged analyte ions. The most common method to overcome this problem involves the post-column addition of a mixture of propionic acid and isopropanol. However the post-column addition setup requires additional pumps and is not desirable for continuous analysis of large amounts of samples. In this paper we present a simple yet very effective means of minimizing the negative effect of TFA in bioanalysis by direct addition of 0.5% acetic acid or 1% propionic acid to mobile phases containing either 0.025 or 0.05% TFA. A factor of two- to five-fold signal enhancement was achieved for eight basic compounds studied. Furthermore, chromatography integrity was maintained even with the addition of acetic acid and propionic acid to existing TFA mobile phases. This method has been successfully applied to the HILIC-ESI/MS/MS high-throughput analysis of extracted biological samples to support pre-clinical and clinical studies.

Ion Pair SN2 Reactions at Nitrogen: A High-Level G2M(+) Computational Study

The gas-phase ion pair SN2 reactions at nitrogen, LiY + NH2X → NH2Y + LiX (Y, X = F, Cl, Br, or I), with inversion mechanisms are investigated at the level of modification of G2 theory, and the results are compared with two related reactions (Y- + NH2X and LiY + CH3X). Calculated results show that the LiY + NH2X reactions are exothermic only when the nucleophile is a heavier lithium halide. The complexation enthalpies are found to depend primarily on the identity of nucleophile LiY and only to a small extent on the identity of NH2X, decreasing in the order LiI > LiBr > LiCl > LiF. Including a Li cation in the anionic SN2 model will raise the overall barriers for the LiY + NH2X (Y, X = F−Br) reactions and lower the overall barriers for the LiI + NH2X (X = F−I) reactions. Another interesting feature of the ion pair reactions at nitrogen is the good correlation between the reaction barriers with the geometrical looseness of Li−Y and N−X bonds in the transition-state structures. The data for the reaction barr...