Uniform quality gaussian basis sets for molecular calculations,I. C1 hydrocarbons

The optimality of MO basis sets of Gaussian functions, when constructed from AO basis sets optimized for the neutral atom or for atom ions, is investigated. A formal charge parameter Q is defined and used to adjust the AO basis sets to the molecular environment, by virtue of a simple quadratic expression. Calculations on a series of C₁ hydrocarbons (CH₂, CH₃, CH₃⁺, CH₃^?, CH₄) using 3G basis sets indicate considerable variations in the optimum Q value with the molecular species. The proposed method offers a simple alternative technique to a full molecular basis set optimization.     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

The minimal basis set MINI-1—powerful tool for calculating intermolecular interactions. II. Ionic complexes

Optimum geometries and stabilization energies are determined for complexes of H₂O, NH₃, CH₄, C₂H₄, CO, and N₂ with metal cations including Li⁺, Na⁺, K⁺, Rb⁺, Be²⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺, for the complex (HO)₂PO ₂ ^– ...Mg²⁺ and for the complexes of water with F^–, Cl^–, and Br^– by SCF calculations employing the MINI-1 minimal gaussian basis sets. The Boys-Bernardi method was used to evaluate the superposition error. Comparison with the extended basis set results revealed that the MINI-1 set gives uniformly good results for a broad variety of ionic complexes and therefore should be preferred to other small basis sets.     

A photo-ionization study of organosulfur ring compounds: Thiirane,thietane and tetrahydrothiophene

The gas phase heats of formation of several organosulfur cations were determined from thiirane, thietane and tetrahydrothiophene precursor molecules by photoionization mass spectrometry. Heats of formation at 0 K and 298 K are reported for the following ions: [H₂CS]^+˙, [H₃CS]⁺, [C₂H₃S]⁺, [C₂H₄S]^+˙, [C₃H₅S]⁺, [C₃H₆S]^+˙, [C₄H₇S]⁺ and [C₄H₈S]^+˙. The [C₄H₇S]⁺ (m/z 87), [C₂H₄S]^+˙ (m/z 60), [C₂H₃S]⁺ (m/z 59), [C₄H₇]⁺ (m/z 55), [C₄H₆]^+˙ (m/z 54) and [CH₂S]^+˙ (m/z 46) ions are produced from metastable tetrahydrothiophene ions at photon energies between 10.2 and 10.7 eV. Decay rates of internal energy selected parent ions to the m/z 60, 59, 55 and 54 fragments were measured by threshold photoelectron-photoion coincidence, the results of which are compared to statistical theory (RRKM/QET) calculations. The [C₂H₄S]^+˙ ion from tetrahydrothiophene is found to have the thioacetaldehyde structure. From the measured [C₂H₄S]^+˙ onset a ΔH = 50±8 kJ mol^?1 was calculated for the thioacetaldehyde molecule.     

Proton‐transfer compounds of isonipecotamide with the aromatic dicarboxylic acids 4‐nitrophthalic, 4,5‐dichlorophthalic, 5‐nitroisophthalic and terephthalic acid

The structures of the 1:1 proton‐transfer compounds of isonipecotamide (piperidine‐4‐carboxamide) with 4‐nitrophthalic acid [4‐carbamoylpiperidinium 2‐carboxy‐4‐nitrobenzoate, C₆H₁₃N₂O₈⁺·C₈H₄O₆⁻, (I)], 4,5‐dichlorophthalic acid [4‐carbamoylpiperidinium 2‐carboxy‐4,5‐dichlorobenzoate, C₆H₁₃N₂O₈⁺·C₈H₃Cl₂O₄⁻, (II)] and 5‐nitroisophthalic acid [4‐carbamoylpiperidinium 3‐carboxy‐5‐nitrobenzoate, C₆H₁₃N₂O₈⁺·C₈H₄O₆⁻, (III)], as well as the 2:1 compound with terephthalic acid [bis(4‐carbamoylpiperidinium) benzene‐1,2‐dicarboxylate dihydrate, 2C₆H₁₃N₂O₈⁺·C₈H₄O₄²⁻·2H₂O, (IV)], have been determined at 200 K. All salts form hydrogen‐bonded structures, viz. one‐dimensional in (II) and three‐dimensional in (I), (III) and (IV). In (I) and (III), the centrosymmetric R₂²(8) cyclic amide–amide association is found, while in (IV) several different types of water‐bridged cyclic associations are present [graph sets R₄²(8), R₄³(10), R₄⁴(12), R₃³(18) and R₆⁴(22)]. The one‐dimensional structure of (I) features the common `planar' hydrogen 4,5‐dichlorophthalate anion, together with enlarged cyclic R₃³(13) and R₄³(17) associations. In the structures of (I) and (III), the presence of head‐to‐tail hydrogen phthalate chain substructures is found. In (IV), head‐to‐tail primary cation–anion associations are extended longitudinally into chains through the water‐bridged cation associations, and laterally by piperidinium–carboxylate N—H...O and water–carboxylate O—H...O hydrogen bonds. The structures reported here further demonstrate the utility of the isonipecotamide cation as a synthon for the generation of stable hydrogen‐bonded structures. An additional example of cation–anion association with this cation is also shown in the asymmetric three‐centre piperidinium–carboxylate N—H...O,O′ interaction in the first‐reported structure of a 2:1 isonipecotamide–carboxylate salt.     

Ab initio and density functional study of substituent effects in halogenated cations of alkenes

A theoretical study of the halogenated cations of mono-, di-, tri- and tetramethyl-substituted ethylenes, C₃H₆X⁺, C₄H₈X⁺, C₅H₁₀X⁺ and C₆H₁₂X⁺, X=F, Cl, Br, have been studied at the ab initio MP2 and density functional B3LYP levels of theory implementing 6-311++G(d,p) basis set. The potential energy surfaces of all molecules under investigation have been scanned and the ¹³C and ¹H NMR chemical shifts for all the bridged halonium ions studied have been calculated using the GIAO method at the B3LYP level. The calculated halogen binding energies in the halonium ions have been correlated with the experimental rates of chlorination and bromination of the corresponding alkenes. The computed hydride affinities and the NICS values for the bridged cations show that the bromo cations are more stable than the analogous chloro and fluoro cations.     

Bis(tetra‐n‐butylammonium) (a redetermination at 150 K) and bis(tetraphenylarsonium) bis(1,3‐dithiole‐2‐thione‐4,5‐dithiolato)zinc(II) (at 300 K)

The title compounds are salts of the general form (Q⁺)₂[Zn(dmit)₂]^2?, where dmit corresponds to the ligand (C₃S₅)^? present in both and Q⁺ to the counter‐cations (ⁿBu₄N)⁺ [or C₁₆H₃₆N⁺] and (Ph₄As)⁺ [or C₂₄H₂₀As⁺], respectively. In the first case, Zn is in the 4e special positions of space group C2/c and hence the [Zn(dmit)₂]^2? dianion possesses twofold axial crystallographic symmetry. Including these, there are now 11 known examples of [Zn(dmit)₂]^2? or its analogues, with O replacing the exocyclic thione S, and [Zn(dmio)₂]^2? dianions in nine structures with various Q. Comparison of these reveals a remarkable variation in details of the conformation which the dianion may adopt in terms of Zn coordination, equivalence of the Zn—S bond lengths, displacement of Zn from the plane of the ligand and overall dianion shape.     

Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

This study undertakes a theoretical investigation into uncommon hydrogen bonds between the ethyl cation (C₂H₅ ⁺) and π hydrocarbons. Firstly, it considers the hyperconjugation effect of the ethyl cation, in which the non-localized hydrogen (H⁺) is taken to be a pseudoatom bound to the carbons of the methyl groups. The goal of the research is to use this electronic phenomenon to gain a better understanding of the (H⁺···π) and (H⁺···p-π) hydrogen bonds, which are considered uncommon because they are formed through the interaction of the H⁺ of the ethyl cation with the π bonds of the acetylene (C₂H₂) and ethene (C₂H₄), as well as with the pseudo-π bond of the cyclopropane (C₃H₆). In view of this, B3LYP/6-311++G(d,p) calculations were used to determine the geometries of the C₂H₅ ⁺···C₂H₂, C₂H₅ ⁺···C₂H₄, and C₂H₅ ⁺···C₃H₆ hydrogen-bonded complexes. Deformations of the bond lengths and bond angles of these systems were analyzed geometrically. Examination of the stretch frequencies and absorption intensities of the (H⁺···π) and (H⁺···p-π) hydrogen bonds has revealed red-shifts in π and p-π bonds. After structural modeling and vibrational characterization, analysis of the charge transfer following the ChelpG approach and subsequently quantification of the hydrogen bond energies (basis sets superpostition error and zero point vibrational energies being considered) were used to predict the strength of the (H⁺···π) and (H⁺···p-π) hydrogen bonds. In addition, the molecular topography was estimated using the quantum theory of atoms in molecules (QTAIM). QTAIM was chosen because of a desire to understand the (H⁺···π) and (H⁺···p-π) hydrogen bonds chemically on the basis of the quantity of charge density and interpretation of Laplacian fields. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Theoretical description of dihydrogen/hydride and trihydride molybdocene complexes: An insight from static and molecular dynamics simulations

In this study ab initio Car–Parrinello molecular dynamics simulations, extended transition state (ETS)‐natural orbitals for chemical valence (NOCV) and QTAIM bonding analyses, were performed to characterize the ansa‐bridged molybdocene complexes [(C₅H₄)₂XMe₂MoH₃]⁺ for X = C, Si, Ge, Sn, Pb, and nonbridged Cp₂MoH system. The results have shown that the [(C₅H₄)₂CMe₂MoH(H₂)]⁺ complex exhibits nonclassical dihydrogen/hydride (H₂/H) conformation (97.6% of time of simulation), contrary to trihydride (H₃) structure noted for nonbridged Cp₂MoH (86.9%) and ansa‐bridged [(C₅H₄)₂SnMe₂MoH₃]⁺ (84.8%), [(C₅H₄)₂PbMe₂MoH₃]⁺ (84.9%) systems. Further, [(C₅H₄)₂SiMe₂MoH₃]⁺ and [(C₅H₄)₂GeMe₂MoH₃]⁺ complexes, appeared to exist in both conformations (H₂/H—55.4%, H₃—44.6% for Si‐based system and H₂/H—36.2%, H₃—63.8 % for germanium congener). It has been proven that the “steric availability” of the metal center, measured by the changes in the Cp? Mo? Cp angle (α), determines the existence of a given conformation—namely, the smaller value of the angle (molybdenum is sterically more accessible) the larger preference for the formation of dihydrogen/hydride structure. ETS‐NOCV method allowed to conclude that increase in the Cp? Mo? Cp angle (from α ca. 120° to α ca. 150°) leads to the enhancement of donation from H₂ and back‐donation from Mo to the σ*(H? H), what consequently leads to breaking of the H? H bond and formation of the trihydride structure. Systematical increase in the charge depletion from the σ‐bonding orbital of H₂ can be related to the reduction of the energy gap between the major orbitals involved in this contribution, namely highest occupied molecular orbital (HOMO) of H₂ with lowest unoccupied molecular orbital (LUMO) of [MoHL]⁺; ΔE = 0.0868 a.u. [for L =(C₅H₄)₂C], ΔE = 0.0827a.u. [for L = (C₅H₄)₂Si] ΔE = 0.0638 a.u. [for L = Cp₂]. Further, the relatively low energetic barrier to hydrogen exchange (ΔE^# = 3.3 kcal/mol) for carbon‐bridged complex, [(C₅H₄)₂CMe₂MoH_c(H_aH_b)]⁺ → [(C₅H₄)₂ CMe₂MoH_a(H_bH_c)]⁺, is related to strengthening of the Mo–H bonds when going from the substrate to the transition state (TS). Notably higher barrier to hydrogen rotation (ΔE^# = 10.1 kcal/mol) in [(C₅H₄)₂CMe₂MoH(H₂)]⁺ is due to lowering in the electrostatic stabilization as well as weakening of the donation (H₂ → Mo charge transfer) and practically lack‐of back‐donation (Mo → H₂) in the rotated TS. © 2012 Wiley Periodicals, Inc.     

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

Chromium(III)-isonicotinate complexes, cis-[Cr(C₂O₄)₂(N-inic)(H₂O)]^- and [Cr(C₂O₄)(H₂O)₃-OH-Cr(C₂O₄)₂(O-inic)]^-(N-inic)(H₂ (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The dependencies of the pseudo-first order rate constant on [H⁺] were established: k_obs = k₀+k_HQ_H[H⁺] and k_obs = k_HQ_H[H⁺] for the N-inic and O-inic complex, respectively, where k₀ and k_H are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and Q_H is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.     

Kinetics and mechanism of one dipicolinato ligand substitution in the chromium(III)–bis(dipicolinato) complex: an unusual rate dependence on acid concentration

The Na[Cr(PDA)₂] · 2H₂O complex (PDA¹ = dipicolinic acid anion) and its aquation product, [Cr(PDA)(H₂O)₃]⁺, were prepared and characterized. The electronic spectra demonstrate that the bis(dipicolinato) complex undergoes very fast partial dechelation during dissolution. In acidic media, pH controlled, rapid protolytic and ring opening processes lead to coexistence of complexes with one tridentate (PDA) and the other bi- or mono-dentate (PDA). The kinetics of PDA ligand liberation were followed spectrophotometrically within the 0.1–2.0 M HClO₄ range at I = 2.0 M. The observed first-order rate constant depends on [H⁺] according to the equation: k _obs = A[H⁺]/(1 + B[H⁺] + C[H⁺]²). A reaction course via the uncharged [Cr(PDA)(HPDA)(H₂O)₂]⁰ complex is proposed. The observed rate increase, followed by rate retardation with [H⁺] increase, is attributed to the unreactive [Cr(PDA)(H₂PDA)(H₂O)₂]⁺ complex. In terms of the proposed mechanism, A, B, C parameters have been defined as: A = k ₁ Q ₁, B = Q ₁, C = Q ₁ Q ₂ where k ₁ is the rate constant of the Cr^III-carboxylato oxygen bond-breaking in the monodentate HPDA ligand, Q ₁ is a composite value describing protolytic and dechelation processes and Q ₂ is the protonation constant of the uncharged [Cr(PDA)(HPDA)(H₂O)₂]⁰ complex.     

Theoretical study of the structure and unimolecular decomposition pathways of ethyloxonium, [CH3CH2OH2]+

Ab initio molecular orbital calculations with split-valence plus polarization basis sets and incorporating valence-electron correlation have been performed to determine the equilibrium structure of ethyloxonium ([CH₃CH₂OH₂]⁺) and examine its modes of unimolecular dissociation. An asymmetric structure (1) is predicted to be the most stable form of ethyloxonium, but a second conformational isomer of C_s symmetry lies only 1.4 kJ mol^?1 higher in energy than 1. Four unimolecular decomposition pathways for 1 have been examined involving loss of H₂, CH₄, H₂O or C₂H₄. The most stable fragmentation products, lying 65 kJ mol^?1 above 1, are associated with the H₂ elimination reaction. However, large barriers of 257 and 223 kJ mol^?1 have to be surmounted for H₂ and CH₄ loss, respectively. On the other hand, elimination of either C₂H₄ or H₂O from ethyloxonium can proceed without a barrier to the reverse associations and, with total endothermicities of 130 and 160 kJ mol^?1, respectively, these reactions are expected to dominate at lower energies. A second important equilibrium structure on the surface is a hydrogen-bridged complex, lying 53 kJ mol^?1 above 1. This complex is involved in the C₂H₄ elimination reaction, acts as an intermediate in the proton-transfer reaction connecting [C₂H₅]⁺ +H₂O and C₂H₄ + [H₃O]⁺ and plays an important role in the isotopic scrambling that has been observed experimentally in the elimination of either H₂O or C₂H₄ from ethyloxonium. The proton affinity of ethanol was calculated as 799 kJ mol^?1, in close agreement with the experimental value of 794 kJ mol^?1.     

Supramolecular association in proton‐transfer adducts containing benzamidinium cations. I. Four molecular salts with uracil derivatives

Four organic salts, namely benzamidinidium orotate (2,6‐dioxo‐1,2,3,6‐tetrahydropyrimidine‐4‐carboxylate) hemihydrate, C₇H₉N₂⁺·C₅H₃N₂O₄⁻·0.5H₂O (BenzamH⁺·Or⁻), (I), benzamidinium isoorotate (2,4‐dioxo‐1,2,3,4‐tetrahydropyrimidine‐5‐carboxylate) trihydrate, C₇H₉N₂⁺·C₅H₃N₂O₄⁻·3H₂O (BenzamH⁺·Isor⁻), (II), benzamidinium diliturate (5‐nitro‐2,6‐dioxo‐1,2,3,6‐tetrahydropyrimidin‐4‐olate) dihydrate, C₇H₉N₂⁺·C₄H₂N₃O₅⁻·2H₂O (BenzamH⁺·Dil⁻), (III), and benzamidinium 5‐nitrouracilate (5‐nitro‐2,4‐dioxo‐1,2,3,4‐tetrahydropyrimidin‐1‐ide), C₇H₉N₂⁺·C₄H₂N₃O₄⁻ (BenzamH⁺·Nit⁻), (IV), have been synthesized by a reaction between benzamidine (benzenecarboximidamide or Benzam) and the appropriate carboxylic acid. Proton transfer occurs to the benzamidine imino N atom. In all four acid–base adducts, the asymmetric unit consists of one tautomeric aminooxo anion (Or⁻, Isor⁻, Dil⁻ and Nit⁻) and one monoprotonated benzamidinium cation (BenzamH⁺), plus one‐half (which lies across a twofold axis), three and two solvent water molecules in (I), (II) and (III), respectively. Due to the presence of protonated benzamidine, these acid–base complexes form supramolecular synthons characterized by N⁺—H...O⁻ and N⁺—H...N⁻ (±)‐charge‐assisted hydrogen bonds (CAHB).     

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two Cr^III–picolinato complexes were obtained and characterized in solution. The [Cr(C₂O₄)(pyac)₂]^– and [Cr(C₂O₄)₂(pyac)]^2– ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end Cr^III–picolinato chelate ring opening via Cr^III—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO₄ range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H⁺] according to the equation: k _obs = a + b[H⁺] + c/[H⁺]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k ₁, b = k ₂ Q ₁, c = k _–1/Q ₂, where k ₁, k _–1,k ₂ are rate constants for the forward and reverse processes and Q ₁, Q ₂ are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.     

Kinetics and Mechanism of Chromium(III)-oxalates-2-hydroxynicotinate and Chromium(III)-oxalates-3-hydroxypicolinate Aquation in HClO4 Solutions

New chromium(III) complexes, [Cr(C₂O₄)₂(2-hnic)]²⁻ and [Cr(C₂O₄)₂(3-hpic)]²⁻ (where 2-hnic = O,O′-bonded 2-hydroxynicotinic acid and 3-hpic = N,O-bonded 3-hydroxypicolinic acid), were obtained and characterized in solution. The acid-catalyzed aquation of the both complexes leads to liberation of the appropriate pyridinecarboxylic acid and formation of cis-[Cr(C₂O₄)₂(H₂O)₂]⁻. Kinetics of these reactions were studied spectrophotometrically in the 0.1–1.0 M HClO₄ range, at I = 1.0 M. In the case of [Cr(C₂O₄)₂(2-hnic)]²⁻, a slow chelate-ring opening at the Cr–O (phenolate) bond is followed by a fast Cr–O (carboxylate) bond breaking. The rate law: k_obs = k_HQ_H[H⁺] was established, where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the coordinated phenolate oxygen atom. In the case of [Cr(C₂O₄)₂(3-hpic)]²⁻, the reversible chelate-ring opening at Cr–N bond is followed by the rate determining step – the one-end bonded ligand liberation. The rate law for the first step was determined: k_obs = k₁+k₋₁/Q₁[H⁺], where k₁ and k₋₁ are the rate constants of the chelate-ring opening and closure and Q₁ is the protonation constant of the pyridine nitrogen atom. The aquation mechanisms are proposed and the effect of ligand coordination mode on complex reactivity is discussed.     

Low‐dimensional compounds containing bioactive ligands. IV. Unusual ionic forms of 5‐chloroquinolin‐8‐ol

Bis(5‐chloro‐8‐hydroxyquinolinium) tetrachloridopalladate(II), (C₉H₇ClNO)₂[PdCl₄], (I), catena‐poly[dimethylammonium [[dichloridopalladate(II)]‐μ‐chlorido]], {(C₂H₈N)[PdCl₃]}_n, (II), ethylenediammonium bis(5‐chloroquinolin‐8‐olate), C₂H₁₀N₂²⁺·2C₉H₅ClNO⁻, (III), and 5‐chloro‐8‐hydroxyquinolinium chloride, C₉H₇ClNO⁺·Cl⁻, (IV), were synthesized with the aim of preparing biologically active complexes of Pd^II and Ni^II with 5‐chloroquinolin‐8‐ol (ClQ). Compounds (I) and (II) contain Pd^II atoms which are coordinated in a square‐planar manner by four chloride ligands. In the structure of (I), there is an isolated [PdCl₄]²⁻ anion, while in the structure of (II) the anion consists of Pd^II atoms, lying on centres of inversion, bonded to a combination of two terminal and two bridging Cl⁻ ligands, lying on twofold rotation axes, forming an infinite [–μ₂‐Cl–PdCl₂–]_n chain. The negative charges of these anions are balanced by two crystallographically independent protonated HClQ⁺ cations in (I) and by dimethylammonium cations in (II), with the N atoms lying on twofold rotation axes. The structure of (III) consists of ClQ⁻ anions, with the hydroxy groups deprotonated, and centrosymmetric ethylenediammonium cations. On the other hand, the structure of (IV) consists of a protonated HClQ⁺ cation with the positive charge balanced by a chloride anion. All four structures are stabilized by systems of hydrogen bonds which occur between the anions and cations. π–π interactions were observed between the HClQ⁺ cations in the structures of (I) and (IV).     

Cover Feature: Are the Accompanying Cations of Doping Anions Influential in Conducting Organic Polymers? The Case of the Popular PEDOT (Chem. Eur. J. 63/2019)

Conducting organic polymers (COPs) are made of a conjugated polymer backbone supporting a certain degree of oxidation. These positive charges are compensated by the doping anions that are introduced into the polymer synthesis along with their accompanying cations. In this work, the influence of these cations on the stoichiometry and physicochemical properties of the resulting COPs have been investigated, something that has previously been overlooked, but, as here proven, is highly relevant. As the doping anion, metallacarborane [Co(C₂B₉H₁₁)₂]⁻ was chosen, which acts as a thistle. This anion binds to the accompanying cation with a distinct strength. If the binding strength is weak, the doping anion is more prone to compensate the positive charge of the polymer, and the opposite is also true. Thus, the ability of the doping anion to compensate the positive charges of the polymer can be tuned, and this determines the stoichiometry of the polymer. As the polymer, PEDOT was studied, whereas Cs⁺, Na⁺, K⁺, Li⁺, and H⁺ as cations. Notably, with the [Co(C₂B₉H₁₁)₂]⁻ anions, these cations are grouped into two sets, Cs⁺ and H⁺ in one and Na⁺, K⁺, and Li⁺ in the second, according to the stoichiometry of the COPs: 2:1 EDOT/[Co(C₂B₉H₁₁)₂]⁻ for Cs⁺ and H⁺, and 3:1 EDOT/[Co(C₂B₉H₁₁)₂]⁻ for Na⁺, K⁺, and Li⁺. The distinct stoichiometries are manifested in the physicochemical properties of the COPs, namely in the electrochemical response, electronic conductivity, ionic conductivity, and capacitance.     

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

OH⁺ is an extraordinarily strong oxidant. Complexed forms (L? OH⁺), such as H₂OOH⁺, H₃NOH⁺, or iron–porphyrin‐OH⁺ are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L? OH⁺ towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L? OH⁺. With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas‐phase pseudo‐first‐order reaction rates of H₂OOH⁺, H₃NOH⁺, and protonated 4‐picoline‐N‐oxide towards isobutane and different halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂ have been determined by means of Fourier transform ion cyclotron resonance meaurements. Reaction rates for H₂OOH⁺ are generally fast (7.2×10^?10–3.0×10^?9 cm³ mol^?1 s^?1) and only in the cases HCF₃ and CF₄ no reactivity is observed. In contrast to this H₃NOH⁺ only reacts with tC₄H₉Cl (k_obs=9.2×10^?10), while 4‐CH₃‐C₅H₄N‐OH⁺ is completely unreactive. While H₂OOH⁺ oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH₂OH, a series of alkanes C_nH_2n+2 (n=1–4), and halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂, were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C_nH_2n+1⁺ (n=1–4) and C_nH_2nCl⁺ (n=1–4). The hypersurface for the reaction of H₂OOH⁺ with CH₃Cl and C₂H₅Cl was calculated at the B3 LYP, MP2, and G3_m* level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

Collisional activation study of cyclic polysulfides

Cyclic polysulfides isolated from higher plants, model compounds and their electron impact induced fragment ions have been investigated by various mass spectrometric methods. These species represent three sets of sulfur compounds: C₃H₆S_x (x=1?6), C₂H₄S_x (x=1?5) and CH₂S_x (x=1?4). Three general fragmentation mechanisms are discussed using metastable transitions: (1) the unimolecular loss of structural parts (CH₂S, CH₂ and S_x); (2) fragmentations which involve ring opening reactions, hydrogen migrations and recyclizations of the product ions ([M? CH₃]⁺, [M? CH₃S]⁺, [M? SH]⁺ and [M? CS₂]^+˙); and (3) complete rearrangements preceding the fragmentations ([M? S₂H]⁺ and [M? C₂H₄]^+˙). The cyclic structures of [M]^+˙ and of specific fragment ions have been investigated by comparing the collisional activation spectra of model ions. On the basis of these results the cyclic ions decompose via linear intermediates and then recyclizations of the product ions occur. The stabilities of the fragment ions have been determined by electron efficiency vs electron energy curves.