The proton‐coupled proton transfer mechanism,H2O catalysis,and hydrogen tunneling effects in the reaction of HNCH2 with HCOOH in the interstellar medium

The reaction HNCH₂ + HCOOH → H₂NCH₂COOH is supposed to be an important reaction related to the possible origin of amino acids on the early Earth. We find that it has an energy barrier of 87.37 kcal mol⁻¹ obtained with MP2/6‐311+G** in the gas phase, but it is likely enhanced to occur in the interstellar medium (ISM) through a proton‐coupled proton transfer reaction, initiated by HNCH₂ coupled with H₂⁺, H₃⁺, or H₃⁺O. H₂⁺, H₃⁺, and H₃⁺O serve as a donor of energy in the coupled reactions. H⁺, which is a key species to the coupled reactions, further, plays a catalytic role in reducing a barrier up to 14.14 kcal mol⁻¹. In the coupled reaction with H₃⁺O, H₂O, which can seize, transport, and deliver a proton from HCOOH to H₂NCH₂⁺, reduces a barrier up to 14.96 kcal mol⁻¹. A significant hydrogen‐tunneling pathway is predicted by the temperature dependences of k_H^CVT/SCT, calculated using the small curvature tunneling (SCT) approximation and canonical variational transition state theory (CVT). Hydrogen tunneling is another important mechanism to make the reaction happen in the ISM. The achieved results can be applied to discuss the origin of amino acids from the materials of the Earth itself. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Xenon-nitrogen chemistry: gas-phase generation and theoretical investigation of the xenon-difluoronitrenium ion F2N-Xe+

The xenon–difluoronitrenium ion F₂N? Xe⁺, a novel xenon–nitrogen species, was obtained in the gas phase by the nucleophilic displacement of HF from protonated NF₃ by Xe. According to Møller–Plesset (MP2) and CCSD(T) theoretical calculations, the enthalpy and Gibbs energy changes (ΔH and ΔG) of this process are predicted to be ?3 kcal mol^?1. The conceivable alternative formation of the inserted isomers FN? XeF⁺ is instead endothermic by approximately 40–60 kcal mol^?1 and is not attainable under the employed ion‐trap mass spectrometric conditions. F₂N? Xe⁺ is theoretically characterized as a weak electrostatic complex between NF₂⁺ and Xe, with a Xe? N bond length of 2.4–2.5 Å, and a dissociation enthalpy and free energy into its constituting fragments of 15 and 8 kcal mol^?1, respectively. F₂N? Xe⁺ is more fragile than the xenon–nitrenium ions (FO₂S)₂NXe⁺, F₅SN(H)Xe⁺, and F₅TeN(H)Xe⁺ observed in the condensed phase, but it is still stable enough to be observed in the gas phase. Other otherwise elusive xenon–nitrogen species could be obtained under these experimental conditions.     

A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes

A DFT analysis of the epoxidation of C₂H₄ by H₂O₂ and MeOOH (as models of tert‐butylhydroperoxide, TBHP) catalyzed by [Cp*MoO₂Cl] ( 1 ) in CHCl₃ and by [Cp*MoO₂(H₂O)]⁺ in water is presented (Cp*=pentamethylcyclopentadienyl). The calculations were performed both in the gas phase and in solution with the use of the conductor‐like polarizable continuum model (CPCM). A low‐energy pathway has been identified, which starts with the activation of ROOH (R=H or Me) to form a hydro/alkylperoxido derivative, [Cp*MoO(OH)(OOR)Cl] or [Cp*MoO(OH)(OOR)]⁺ with barriers of 24.9 (26.5) and 28.7 (29.2) kcal mol^?1 for H₂O₂ (MeOOH), respectively, in solution. The latter barrier, however, is reduced to only 1.0 (1.6) kcal mol^?1 when one additional water molecule is explicitly included in the calculations. The hydro/alkylperoxido ligand in these intermediates is η²‐coordinated, with a significant interaction between the Mo center and the O^β atom. The subsequent step is a nucleophilic attack of the ethylene molecule on the activated O^α atom, requiring 13.9 (17.8) and 16.1 (17.7) kcal mol^?1 in solution, respectively. The corresponding transformation, catalyzed by the peroxido complex [Cp*MoO(O₂)Cl] in CHCl₃, requires higher barriers for both steps (ROOH activation: 34.3 (35.2) kcal mol^?1; O atom transfer: 28.5 (30.3) kcal mol^?1), which is attributed to both greater steric crowding and to the greater electron density on the metal atom.     

2,3‐Dihydro‐1,3‐benzothiazol‐2‐iminium monohydrogen sulfate and 2‐iminio‐2,3‐dihydro‐1,3‐benzothiazole‐6‐sulfonate: a combined structural and theoretical study

The 2‐aminobenzothiazole sulfonation intermediate 2,3‐dihydro‐1,3‐benzothiazol‐2‐iminium monohydrogen sulfate, C₇H₇N₂S⁺·HSO₄⁻, (I), and the final product 2‐iminio‐2,3‐dihydro‐1,3‐benzothiazole‐6‐sulfonate, C₇H₆N₂O₃S₂, (II), both have the endocyclic N atom protonated; compound (I) exists as an ion pair and (II) forms a zwitterion. Intermolecular N—H...O and O—H...O hydrogen bonds are seen in both structures, with bonding energy (calculated on the basis of density functional theory) ranging from 1.06 to 14.15 kcal mol⁻¹. Hydrogen bonding in (I) and (II) creates DDDD and C(8)C(9)C(9) first‐level graph sets, respectively. Face‐to‐face stacking interactions are observed in both (I) and (II), but they are extremely weak.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Halogen,hydrogen and electrostatic interactions in 2‐amino‐5‐chloro‐1,3‐benzoxazol‐3‐ium nitrate and 2‐amino‐5‐chloro‐1,3‐benzoxazol‐3‐ium perchlorate

In the title compounds, C₇H₆ClN₂O⁺·NO₃⁻ and C₇H₆ClN₂O⁺·ClO₄⁻, the ions are connected by N—H...O hydrogen bonds and halogen interactions. Additionally, in the first compound, co‐operative π–π stacking and halogen...π interactions are observed. The energies of the observed interactions range from a value typical for very weak interactions (1.80 kJ mol⁻¹) to one typical for mildly strong interactions (53.01 kJ mol⁻¹). The iminium cations exist in an equilibrium form intermediate between exo‐ and endocyclic. This study provides structural insights relevant to the biochemical activity of 2‐amino‐5‐chloro‐1,3‐benzoxazole compounds.     

Hydrogen bonding in 1:1 proton‐transfer compounds of 5‐sulfosalicylic acid with 4‐X‐substituted anilines (X = F,Cl or Br)

The crystal structures of three proton‐transfer compounds of 5‐sulfosalicylic acid (3‐carboxy‐4‐hydroxy­benzene­sulfonic acid) with 4‐X‐substituted anilines (X = F, Cl and Br), namely 4‐fluoro­anilinium 5‐sulfosalicylate (3‐carboxy‐4‐hydroxybenzenesulfonate) monohydrate, C₆H₇FN⁺·C₇H₅O₆S⁻·H₂O, (I), 4‐chloro­anilinium 5‐sulfosalicylate hemihydrate, C₆H₇ClN⁺·C₇H₅O₆S⁻·0.5H₂O, (II), and 4‐bromo­anilinium 5‐sulfosalicylate monohydrate, C₆H₇BrN⁺·C₇H₅O₆S⁻·H₂O, (III), have been determined. The asymmetric unit in (II) contains two formula units. All three compounds have three‐dimensional hydrogen‐bonded polymeric structures in which both the water molecule and the carboxylic acid group are involved in structure extension. With both (II) and (III), which are structurally similar, the common cyclic (8) dimeric carboxylic acid association is present, whereas in (I), an unusual cyclic (8) association involving all three hetero‐species is found.     

Theoretical study of the C3Cl radical and its cation

A theoretical study of C₃Cl and C₃Cl⁺ isomers has been carried out. The global minimum for C₃Cl is a cyclic C_2V species (a three-membered ring with an exocyclic chlorine atom). However, a quasi-linear CCCCl structure is predicted to lie only 3-5 kcal mol⁻¹ higher. This quasi-linear structure is floppy, since the linear arrangement lies only 2-3 kcal mol⁻¹ higher in energy. The cyclic and open-chain isomers have dipole moments of 1.986 and 3.363 D, respectively. In C₃Cl⁺ the global minimum is a linear singlet species, the singlet cyclic isomer lying about 19 kcal mol⁻¹ higher. The ionization potentials of cyclic and open-chain C₃Cl are estimated to be 9.17 and 8.21 eV, respectively, suggesting that these species should be easily ionized if present in the interstellar medium.     

Supramolecular hydrogen‐bonding patterns in salts of the antifolate drugs trimethoprim and pyrimethamine

Nine salts of the antifolate drugs trimethoprim and pyrimethamine, namely, trimethoprimium [or 2,4‐diamino‐5‐(3,4,5‐trimethoxybenzyl)pyrimidin‐1‐ium] 2,5‐dichlorothiophene‐3‐carboxylate monohydrate (TMPDCTPC, 1:1), C₁₄H₁₉N₄O₃⁺·C₅HCl₂O₂S⁻, ( I ), trimethoprimium 3‐bromothiophene‐2‐carboxylate monohydrate, (TMPBTPC, 1:1:1), C₁₄H₁₉N₄O₃⁺·C₅H₂BrO₂S⁻·H₂O, ( II ), trimethoprimium 3‐chlorothiophene‐2‐carboxylate monohydrate (TMPCTPC, 1:1:1), C₁₄H₁₉N₄O₃⁺·C₅H₂ClO₂S⁻·H₂O, ( III ), trimethoprimium 5‐methylthiophene‐2‐carboxylate monohydrate (TMPMTPC, 1:1:1), C₁₄H₁₉N₄O₃⁺·C₆H₅O₂S⁻·H₂O, ( IV ), trimethoprimium anthracene‐9‐carboxylate sesquihydrate (TMPAC, 2:2:3), C₁₄H₁₉N₄O₃⁺·C₁₅H₉O₂⁻·1.5H₂O, ( V ), pyrimethaminium [or 2,4‐diamino‐5‐(4‐chlorophenyl)‐6‐ethylpyrimidin‐1‐ium] 2,5‐dichlorothiophene‐3‐carboxylate (PMNDCTPC, 1:1), C₁₂H₁₄ClN₄⁺·C₅HCl₂O₂S⁻, ( VI ), pyrimethaminium 5‐bromothiophene‐2‐carboxylate (PMNBTPC, 1:1), C₁₂H₁₄ClN₄⁺·C₅H₂BrO₂S⁻, ( VII ), pyrimethaminium anthracene‐9‐carboxylate ethanol monosolvate monohydrate (PMNAC, 1:1:1:1), C₁₂H₁₄ClN₄⁺·C₁₅H₉O₂⁻·C₂H₅OH·H₂O, ( VIII ), and bis(pyrimethaminium) naphthalene‐1,5‐disulfonate (PMNNSA, 2:1), 2C₁₂H₁₄ClN₄⁺·C₁₀H₆O₆S₂²⁻, ( IX ), have been prepared and characterized by single‐crystal X‐ray diffraction. In all the crystal structures, the pyrimidine N1 atom is protonated. In salts ( I )–( III ) and ( VI )–( IX ), the 2‐aminopyrimidinium cation interacts with the corresponding anion via a pair of N—H…O hydrogen bonds, generating the robust R₂²(8) supramolecular heterosynthon. In salt ( IV ), instead of forming the R₂²(8) heterosynthon, the carboxylate group bridges two pyrimidinium cations via N—H…O hydrogen bonds. In salt ( V ), one of the carboxylate O atoms bridges the N1—H group and a 2‐amino H atom of the pyrimidinium cation to form a smaller R₂¹(6) ring instead of the R₂²(8) ring. In salt ( IX ), the sulfonate O atoms mimic the role of carboxylate O atoms in forming an R₂²(8) ring motif. In salts ( II )–( IX ), the pyrimidinium cation forms base pairs via a pair of N—H…N hydrogen bonds, generating a ring motif [R₂²(8) homosynthon]. Compounds ( II ) and ( III ) are isomorphous. The quadruple DDAA (D = hydrogen‐bond donor and A = hydrogen‐bond acceptor) array is observed in ( I ). In salts ( II )–( IV ) and ( VI )–( IX ), quadruple DADA arrays are present. In salts ( VI ) and ( VII ), both DADA and DDAA arrays co‐exist. The crystal structures are further stabilized by π–π stacking interactions [in ( I ), ( V ) and ( VII )–( IX )], C—H…π interactions [in ( IV )–( V ) and ( VII )–( IX )], C—Br…π interactions [in ( II )] and C—Cl…π interactions [in ( I ), ( III ) and ( VI )]. Cl…O and Cl…Cl halogen‐bond interactions are present in ( I ) and ( VI ), with distances and angles of 3.0020 (18) and 3.5159 (16) Å, and 165.56 (10) and 154.81 (11)°, respectively.     

Short intermolecular N—Br...O=C contacts in 1,3‐dibromo‐5,5‐dimethylimidazolidine‐2,4‐dione

In the title compound, C₅H₆Br₂N₂O₂, all atoms except for the methyl group lie on a mirror plane in the space group Pnma (No. 62). All bond lengths are normal and the five‐membered ring is planar by symmetry. Two short intermolecular N—Br...O=C contacts [Br...O = 2.787 (2) and 2.8431 (19) Å] are present, originating primarily from the O‐atom lone pairs donating electron density to the antibonding orbitals of the N—Br bonds (delocalization energy transfers 3.27 and 2.11 kcal mol⁻¹). The total stabilization energies of the Br...O interactions are 3.4828 and 2.3504 kcal mol⁻¹.     

An investigation on the reaction mechanism of the F2+Cl2→2ClF using the B3LYP method

The reaction mechanism of F₂+Cl₂→2ClF has been investigated with the density functional theory at the B3LYP/6‐311G* level. Six transition states have been found for the three possible reaction paths and verified by the normal mode vibrational and IRC analyses. Ab initio MP2/6‐311G* geometry optimizations and CCSD(T)/6‐311G(2df)//MP2/6‐311G* single‐point energy calculations have been performed for comparison. It is found that when the F₂ (or Cl₂) molecule decomposes into atoms first and then the F (or Cl) atom reacts with the molecule Cl₂ (or F₂) nearly along the molecular axis, the energy barrier is very low. The calculated energy barrier of F attacking Cl₂ is zero and that of Cl attacking F₂ is only 15.57 kJ?mol^?1 at the B3LYP level. However, the calculated dissociation energies of F₂ and Cl₂ are as high as 145.40 and 192.48 kJ?mol^?1, respectively. When the reaction proceeds through a bimolecular reaction mechanism, two four‐center transition states are obtained and the lower energy barrier is 218.69 kJ?mol^?1. Therefore, the title reaction F₂+Cl₂→2ClF is most probably initiated from the atomization of the F₂ molecule and terminated by the reaction of F attacking Cl₂ nearly along the Cl? Cl bond. MP2 calculations lead to the same conclusion, but the geometry of TS and the energy barrier are somewhat different. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

Kinetic and thermochemical parameters of chlorine atom transfer reactions from CF3Cl,CF3CF2Cl,CF2ClCF2Cl,and CF2ClCFCl2 in gas phase

The following reactions: (1) were studied over the temperature ranges 533–687 K, 563–663 K, and 503–613 K for the forward reactions respectively and over 683–763 K, for the back reaction. Arrhenius parameters for chlorine atom transfer were determined relative to the combination of the attacking radicals. The ΔH_r°(1) = ?3.95 ± 0.45 kcal mol^?1 was calculated and from this value the ΔH∮(C₂F₅Cl) = ?2.66.3 ± 2.5 kcal mol^?1 and D(C₂F₅-Cl) = 82.0 ± 1.2 kcal mol^?1 were obtained. Besides, the ΔH_r°(2) was estimated leading to D(CF₂ClCF₂Cl) = 79.2 ± 5 Kcal mol^?1. The bond dissociation energies and the heat of formation are compared with those of the literature. The effect of the halogen substitutents as well as the importance of the polar effects for halogen transfer processes are discussed.     

Formic acid catalyzed gas-phase reaction of H2O with SO3 and the reverse reaction: a theoretical study

The formic acid catalyzed gas‐phase reaction between H₂O and SO₃ and its reverse reaction are respectively investigated by means of quantum chemical calculations at the CCSD(T)//B3LYP/cc‐pv(T+d)z and CCSD(T)//MP2/aug‐cc‐pv(T+d)z levels of theory. Remarkably, the activation energy relative to the reactants for the reaction of H₂O with SO₃ is lowered through formic acid catalysis from 15.97 kcal mol^?1 to ?15.12 and ?14.83 kcal mol^?1 for the formed H₂O ??? SO₃ complex plus HCOOH and the formed H₂O ??? HCOOH complex plus SO₃, respectively, at the CCSD(T)//MP2/aug‐cc‐pv(T+d)z level. For the reverse reaction, the energy barrier for decomposition of sulfuric acid is reduced to ?3.07 kcal mol^?1 from 35.82 kcal mol^?1 with the aid of formic acid. The results show that formic acid plays a strong catalytic role in facilitating the formation and decomposition of sulfuric acid. The rate constant of the SO₃+H₂O reaction with formic acid is 10⁵ times greater than that of the corresponding reaction with water dimer. The calculated rate constant for the HCOOH+H₂SO₄ reaction is about 10^?13 cm³ molecule^?1 s^?1 in the temperature range 200–280 K. The results of the present investigation show that formic acid plays a crucial role in the cycle between SO₃ and H₂SO₄ in atmospheric chemistry.     

Supramolecular structures of rhenium(I) complexes mediated by ligand planarity via the interplay of substituents

The crystal and molecular structures of two Re^I tricarbonyl complexes, namely fac‐tricarbonylchlorido[1‐(4‐fluorocinnamoyl)‐3‐(pyridin‐2‐yl‐κN)pyrazole‐κN²]rhenium(I), [ReCl(C₁₇H₁₂FN₃O)(CO)₃], (I), and fac‐tricarbonylchlorido[1‐(4‐nitrocinnamoyl)‐3‐(pyridin‐2‐yl‐κN)pyrazole‐κN²]rhenium(I) acetone monosolvate, [ReCl(C₁₇H₁₂ClN₄O₃)(CO)₃]·C₃H₆O, (II), are reported. The complexes form centrosymmetric dimers that are linked into one‐dimensional columns by C—H…Cl and N—O…H interactions in (I) and (II), respectively. C—H…Cl interactions in (II) generate two R₂¹(7) loops that merge into a single R₂¹(10) loop. These interactions involve the alkene, pyrazole and benzene rings, hence restricting the ligand rotation and giving rise to a planar conformation. Unlike (II), complex (I) exhibits a twisted conformation of the ligand and a pair of molecules forms a centrosymmetric dimer with an R₂²(10) loop via C—H…O interactions. The unique supramolecular structures of (I) and (II) are determined by their planarity and weak interactions. The planar conformation of (II) provides a base for appreciable π–π stacking interactions compared to (I). In addition, an N—O…π interaction stabilizes the supramolecular structure of (II). We report herein the first n→π* interactions of Re^I tricarbonyl complexes, which account for 0.33 kJ mol⁻¹. Intermolecular C—H…Cl and C—H…O interactions are present in both complexes, with (II) showing a greater preference for these interactions compared to (I), with cumulative contributions of 48.7 and 41.5%, respectively. The influence of inductive (fluoro) and/or resonance (nitro) effects on the π‐stacking ability was further supported by LOLIPOP (localized orbital locator‐integrated π over plane) analysis. The benzene ring of (II) demonstrated a higher π‐stacking ability compared to that of (I), which is supported by the intrinsic planar geometry. The HOMA (harmonic oscillator model of aromaticity) index of (I) revealed more aromaticity with respect to (II), suggesting that NO₂ greatly perturbed the aromaticity. The Hirshfeld fingerprint (FP) plots revealed the preference of (II) over (I) for π–π contacts, with contributions of 6.8 and 4.4%, respectively.     

Difluorophosphoryl Nitrene F2P(O)N: Matrix Isolation and Unexpected Rearrangement to F2PNO

Triplet difluorophosphoryl nitrene F₂P(O)N (X³A′′) was generated on ArF excimer laser irradiation (λ=193 nm) of F₂P(O)N₃ in solid argon matrix at 16 K, and characterized by its matrix IR, UV/Vis, and EPR spectra, in combination with DFT and CBS‐QB3 calculations. On visible light irradiation (λ>420 nm) at 16 K F₂P(O)N reacts with molecular nitrogen and some of the azide is regenerated. UV irradiation (λ=255 nm) of F₂P(O)N (X³A′′) induced a Curtius‐type rearrangement, but instead of a 1,3‐fluorine shift, nitrogen migration to give F₂PON is proposed to be the first step of the photoisomerization of F₂P(O)N into F₂PNO (difluoronitrosophosphine). Formation of novel F₂PNO was confirmed with ¹⁵N‐ and ¹⁸O‐enriched isotopomers by IR spectroscopy and DFT calculations. Theoretical calculations predict a rather long P? N bond of 1.922 Å [B3LYP/6‐311+G(3df)] and low bond‐dissociation energy of 76.3 kJ mol^?1 (CBS‐QB3) for F₂PNO.     

An Argon–Oxygen Covalent Bond in the ArOH+ Molecular Ion

The OH⁺ cation is a well‐known diatomic for which the triplet (³Σ^?) ground state is 50.5 kcal mol^?1 more stable than its corresponding singlet (¹Δ) excited state. However, the singlet forms a strong donor–acceptor bond to argon with a bond energy of 66.4 kcal mol^?1 at the CCSDT(Q)/CBS level, making the singlet ArOH⁺ cation 3.9 kcal mol^?1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH⁺ is the predicted singlet with a covalent Ar?O bond.     

Si‐Doped B12N12 Nanocage as an Adsorbent for Dissociation of N2O to N2 Molecule

Adsorption of N₂O molecule by using density functional theory calculations at the B3LYP/6–31G* level onto pristine and Si‐doped B₁₂N₁₂ nanocage in terms of energetic, geometric, and electronic properties was investigated. The results of calculations showed that the N₂O molecule is physically adsorbed on the pristine and Si‐doped B₁₂N₁₂ (Si_N) models, releasing energies in the range of –1.13 to –2.02 kcal mol⁻¹. It was found that the electronic properties of the models have not changed significantly upon the N₂O adsorption. On the other hand, the adsorption energy of N₂O on the Si‐doped B₁₂N₁₂ (Si_B model) was about –67.20 kcal mol⁻¹and the natural bond orbital charge of 0.58|e| is transferred from the nanocage to the N₂O molecule. In the configuration, the O atom of N₂O molecule is bonded to the Si atom of the nanocage, so that an N₂ molecule escapes from the wall of the nanocage. The results showed that the Si_B model can be an adsorbent for dissociation of the N₂O molecule.     

6‐Chloroisocytosine and 5‐bromo‐6‐methylisocytosine: again,one or two tautomers present in the same crystal?

It is well known that pyrimidin‐4‐one derivatives are able to adopt either the 1H‐ or the 3H‐tautomeric form in (co)crystals, depending on the coformer. As part of ongoing research to investigate the preferred hydrogen‐bonding patterns of active pharmaceutical ingredients and their model systems, 2‐amino‐6‐chloropyrimidin‐4‐one and 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4‐one have been cocrystallized with several coformers and with each other. Since Cl and Br atoms both have versatile possibilities to interact with the coformers, such as via hydrogen or halogen bonds, their behaviour within the crystal packing was also of interest. The experiments yielded five crystal structures, namely 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (1/3), C₅H₇N₂⁺·C₄H₃ClN₃O⁻·3C₄H₄ClN₃O, (Ia), 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one–2‐aminopyridine (2/10/1), 2C₅H₇N₂⁺·2C₄H₃ClN₃O⁻·10C₄H₄ClN₃O·C₅H₆N₂, (Ib), the solvent‐free cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one (1/1), C₅H₆BrN₃O·C₅H₆BrN₃O, (II), the solvate 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–N‐methylpyrrolidin‐2‐one (1/1/1), C₅H₆BrN₃O·C₅H₆BrN₃O·C₅H₉NO, (III), and the partial cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (0.635/1/0.365), C₅H₆BrN₃O·C₅H₆BrN₃O·C₄H₄ClN₃O, (IV). All five structures show R₂²(8) hydrogen‐bond‐based patterns, either by synthon 2 or by synthon 3, which are related to the Watson–Crick base pairs.     

Reactions of Au+ and Au− with benzene and fluorine-substituted benzenes

Reactions of gold anions and cations generated by laser desorption/ionization were studied in the FTICR spectrometer. Au⁻ associated with C₆F₆ to give the novel Au⁻(C₆F₆) complex, whose binding energy was estimated to be 24 ± 4 kcal mol⁻¹ from analysis of the radiative association (RA) kinetics. Au⁺ associated with C₆F₅H to give Au⁺(C₆F₅H), with binding energy estimated to be 31 kcal mol⁻¹. Au⁺ reacted with C₆H₆ to form the well known Au⁺(C₆H₆) and Au⁺(C₆H₆)₂ complexes. The observation of rapid charge transfer from Au⁺(C₆H₆) to C₆H₆ was interpreted as showing that benzene binds more strongly to neutral Au than to Au⁺. The neutral Au–C₆H₆ bond is accordingly concluded to be stronger than about 70 kcal mol⁻¹.