Hydrogen Cyanide Exchange on [Al(HCN)6]3+ – A DFT Study

The hydrogen cyanide exchange mechanism of [Al(HCN)₆]³⁺ has been investigated by DFT calculations (B3LYP/6‐311+G**). The calculations provide theoretical evidence that the hydrogen cyanide exchange proceeds via a limiting dissociative (D) mechanism involving a stable five‐coordinate intermediate [Al(HCN)₅ · (HCN)₂]³⁺. The activation energy for the D‐mechanism is 23.4 kcal · mol^–1, which is 2.8 kcal · mol^–1 lower than for the seven‐coordinate transition state [Al(HCN)₇]^3+? for the alternative associative (A) pathway. The difference in stability between the two intermediates [Al(HCN)₅ · (HCN)₂]³⁺ (12.1 kcal · mol^–1) and [Al(HCN)₇]³⁺ (25.7 kcal · mol^–1) in comparison to [Al(HCN)₆ · (HCN)]³⁺ is much more pronounced and further supports a limiting dissociative mechanism.     

Adducts of Niobium (V) and Tantalum (V) Halides. XV. Ligand Exchange of the Adducts with Some Phosphoryl Compounds

The ligand exchange MX₅·L + *L?MX₅·*L + L for the octahedral adducts MX₅·L, in an inert solvent (CH₂Cl₂ or CHCl₃) with neutral ligands, proceeds via a dissociative D mechanism when M = Nb, X = Cl and L = phosphoryl compound. A dissociative interchange I_d mechanism is suggested when M = Nb or Ta, and X = F. A first order rate law and positive values for ΔS* (+4 to +14 cal K^?1 mol^?1) are observed for the exchanges on the pentachloride adducts. However, a second order rate law and large negative values for ΔS* (-15 to -24 cal K^?1 mol^?1) are found for the intermolecular neutral ligand exchange (measured by ¹H-NMR.) and for the intramolecular fluorine exchange (measured by ¹⁹F-NMR.) reactions on the pentafluoride adducts. The fluorine exchange is 2 to 5 times faster than the ligand exchange. The exchanges, on the pentachloride and on the pentafluoride adducts, are slowed down with increasing donor strength of the phosphoryl compound.     

Calculation of Some Thermodynamic Properties and Detonation Parameters of 1‐Ethyl‐3‐Methyl‐H‐Imidazolium Perchlorate, [Emim][ClO4], on the Basis of CBS‐4M and CHEETAH Computations Supplemented by VBT Estimates

This paper estimates some thermochemical (in kcal mol^–1) and detonation parameters for the ionic liquid, [emim][ClO₄] and its associated solid in view of its investigation as an energetic material. The thermochemical values estimated, employing CBS‐4M computational methodology and volume‐based thermodynamics (VBT) include: lattice energy, U_POT([emim][ClO₄]) ≈? 123 ± 16 kcal · mol^–1; enthalpy of formation of the gaseous cation, Δ_fH°([emim]⁺, g) = 144.2 kcal · mol^–1 and anion, Δ_fH°([ClO₄]^–, g) = –66.1 kcal · mol^–1; the enthalpy of formation of the solid salt, Δ_fH°([emim][ClO₄],s) ≈? –55 ± 16 kcal · mol^–1 and for the associated ionic liquid, Δ_fH^o([emim][ClO₄],l) = –52 ± 16 kcal · mol^–1 as well as the corresponding Gibbs energy terms: Δ_fG°([emim][ClO₄],s) ≈? +29 ± 16 kcal · mol^–1 and Δ_fG^o([emim][ClO₄],l) = +24 ± 16 kcal · mol^–1 and the associated standard absolute entropies, of the solid [emim][ClO₄], S°₂₉₈([emim][ClO₄],s) = 83 ± 4 cal · K^–1 · mol^–1. The following combustion and detonation parameters are assigned to [emim][ClO₄] in its (ionic) liquid form: specific impulse (I_sp) = 228 s (monopropellant), detonation velocity (V_oD) = 5466 m · s^–1, detonation pressure (p_C–J) = 99 kbar, explosion temperature (T_ex) = 2842 K.     

Atomistic understanding of the C·T mismatched DNA base pair tautomerization via the DPT: QM and QTAIM computational approaches

It was established that the cytosine·thymine (C·T) mismatched DNA base pair with cis‐oriented N1H glycosidic bonds has propeller‐like structure (|N3C4C4N3| = 38.4°), which is stabilized by three specific intermolecular interactions–two antiparallel N4H…O4 (5.19 kcal mol^?1) and N3H…N3 (6.33 kcal mol^?1) H‐bonds and a van der Waals (vdW) contact O2…O2 (0.32 kcal mol^?1). The C·T base mispair is thermodynamically stable structure (ΔG_int = ?1.54 kcal mol^?1) and even slightly more stable than the A·T Watson–Crick DNA base pair (ΔG_int = ?1.43 kcal mol^?1) at the room temperature. It was shown that the C·T ? C*·T* tautomerization via the double proton transfer (DPT) is assisted by the O2…O2 vdW contact along the entire range of the intrinsic reaction coordinate (IRC). The positive value of the Grunenberg's compliance constants (31.186, 30.265, and 22.166 Å/mdyn for the C·T, C*·T*, and TS_{C·T ? C*·T*}, respectively) proves that the O2…O2 vdW contact is a stabilizing interaction. Based on the sweeps of the H‐bond energies, it was found that the N4H…O4/O4H…N4, and N3H…N3 H‐bonds in the C·T and C*·T* base pairs are anticooperative and weaken each other, whereas the middle N3H…N3 H‐bond and the O2…O2 vdW contact are cooperative and mutually reinforce each other. It was found that the tautomerization of the C·T base mispair through the DPT is concerted and asynchronous reaction that proceeds via the TS_{C·T ? C*·T*} stabilized by the loosened N4? H? O4 covalent bridge, N3H…N3 H‐bond (9.67 kcal mol^?1) and O2…O2 vdW contact (0.41 kcal mol^?1). The nine key points, describing the evolution of the C·T ? C*·T* tautomerization via the DPT, were detected and completely investigated along the IRC. The C*·T* mispair was revealed to be the dynamically unstable structure with a lifetime 2.13·× 10^?13 s. In this case, as for the A·T Watson–Crick DNA base pair, activates the mechanism of the quantum protection of the C·T DNA base mispair from its spontaneous mutagenic tautomerization through the DPT. © 2013 Wiley Periodicals, Inc.     

HCN exchange on [Cu(HCN)4]+: a quantum chemical investigation

Density functional (B3LYP, B3PW91, X3LYP, BP86, PBEPBE, PW91PW91, and M06) and ab initio (MP2, MP4sdq, CCSD, and CCSD(T)) calculations with extended basis sets (6-311+G**, TZVP, LANL2DZ+p, and SDD+p, the latter including extra polarization and diffuse functions) indicate that HCN exchange on [Cu(HCN)₄]⁺ proceeds via an associative interchange (I_a) mechanism and a D_3h transition structure {[Cu(HCN)₅]⁺}^?. The activation barrier, relative to the model complex [Cu(HCN)₄]⁺·HCN, varies modestly, depending on the computational level. Typical values are 8.0?kcal?M^?1 (B3LYP/6-311+G**), 6.0?kcal?M^?1 (M06/6-311+G**), and 4.8?kcal?M^?1 (CCSD(T)/6-311+G**//MP2(full)/6-311+G**). Inclusion of an implicit solvent model (B3LYP(CPCM)/6-311+G**) leads to an activation barrier of 5.8?kcal?mol^?1. Comparison of the HCN exchange mechanisms on [Li(HCN)₄]⁺ (limiting associative, A) and [Cu(HCN)₄]⁺ (associative interchange, I_a) reveals that π back donation in the equatorial Cu–N bonds in the transition state determines the mechanism.     

Different catalysis role of in‐loop and out‐of‐loop waters in assisting HNS/HSN proton transfer isomerizations: Bridging vs. surrounding effect

In this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out‐of‐loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol^?1 at the B3LYP/6‐311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol^?1 compared with the no‐water‐assisting isomerization barrier T (65.52/43.87 kcal · mol^?1). But when adding one water molecule on the HNS, there is another special proton‐transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol^?1. Clearly, this process is more difficult to follow than the R–T–P process. For the two‐water‐assisting mechanism, the heat of reaction is 19.61 kcal · mol^?1, and the forward/backward barriers are 32.27/12.66 kcal · mol^?1, decreased by 33.25/31.21 kcal · mol^?1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol^?1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol^?1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol^?1 than the trihydration situation. In the second mode, the outer‐sphere water effect from the other water molecules directly H‐bonded to the loop is considered. When one to three water molecules attach to the looped water in one‐water in‐loop‐assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol^?1 compared with the original monohydration‐assisting case. When adding one or two water molecules on the dihydration‐assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in‐loop water‐assisting case are the lowest, and the surrounding water molecules (out‐of‐loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

The thermodynamic stability of P8, A CBS‐Q study

The thermodynamic stabilities of P₂, P₄, and three P₈ cage structure were investigated through high‐precision CBS‐Q calculations. The CBS‐Q values for the bond energy of P₂ (ΔE_o: +115.7 kcal mol⁻¹) and the formation of P₄ from P₂ (Δ E_o:‐56.6 kcal mol⁻¹) were in excellent agreement with the experimental values (E_o: +117 and ‐56.4 kcal mol⁻¹ respectively). Among the P₈ cages, the cubane structure was the least stable (Δ E_o +37 kcal vs. 2×P₄). The most stable P₈ isomer adopts a cuneane structure resembling S₄N₄, and is more stable than white phosphorus at T = 0 K (Δ E_o −3.3 kcal mol⁻¹), but still unstable under standard conditions for entropic reasons (Δ G^o of +8.1 kcal mol⁻¹ vs. 2×P₄). The CBS‐Q energies represent significant revisions (6–20 kcal mol⁻¹) of previous computational predictions obtained by high‐level single method calculations. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:453–457, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20119     

Preparation and Diels-Alder Reactivity of 2,3,5,6,7,8-Hexamethylidenebicyclo [2.2.2]octane (‘[2.2.2]Hericene’). Force-field calculations of exocyclic dienes as a moiety of bicyclic skeletons

The [2.2.2]hericene ( 6 ), a bicyclo[2.2.2]octane bearing three exocyclic s-cis-butadiene units has been prepared in eight steps from coumalic acid and maleic anhydride. The hexaene 6 adds successively three mol-equiv. of strong dienophiles such as ethylenetetracarbonitrile (TCE) and dimethyl acetylenedicarboxylate (DMAD) giving the corresponding monoadducts 17 and 20 (k₁), bis-adducts 18 and 21 (k₂) and tris-adducts 19 and 22 (k₃), respectively. The rate constant ratio k₁/k₂ is small as in the case of the cycloadditions of 2,3,5,6-tetramethylidene-bicyclo [2.2.2]octane ( 3 ) giving the corresponding monoadducts 23 and 27 (k₁) and bis-adducts 25 and 29 (k₂) with TCE and DMAD, respectively. Constrastingly, the rate constant ratio k₂/k₃ is relatively large as the rate constant ratio k₁/k₂ of the Diels-Alder additions for 5,6,7,8-tetramethylidenebicyclo [2.2.2]oct-2-ene ( 4 ) giving the corresponding monoadducts 24 and 28 (k₁) and bis-adducts 26 and 30 (k₂). The following second-order rate constants (toluene, 25°) and activation parameters were obtained for the TCE additions: 3 +TCE→ 23 : k₁ = 0.591±0.012 mol^?1·l·s^?1, ΔH^≠=10.6±0.4 kcal/mol, and ΔS^≠ = ?24.0±1.4 cal/mol·K (e.u.); 23 +TCE→ 25 : k₂=0.034±0.0010 mol^?1·l·s^?1, ΔH^≠ = 10.6±0.6 kcal/mol, and ΔS^≠ = ?29.7±2.0 e.u.; 4 +TCE→ 26 : k₁ = 0.172±0.035 mol^?1·l·s^?1, ΔH^≠ 11.3±0.8 kcal/mol, and ΔS^≠ = ?24.0±2.8 e.u.; 24 +TCE→ 26 : k₂ = (6.1±0.2)·10^?4 mol^?1·l·s^?1, ΔH^≠ = 13.0±0.3 kcal/mol, and ΔS^≠ = ?29.5±0.8 e.u.; 6 +TCE→ 17 : k₁ = 0.136±0.002 mol^?1·l·s^?1, ΔH^≠ = 11.3±0.2 kcal/mol, and ΔS^≠ = ?24.5±0.8 e.u.; 17 +TCE→ 18 : k₂ = 0.0156±0.0003 mol^?1·l·s^?1, ΔH^≠ = 10.9±0.5 kcal/mol, and ΔS^≠ = ?30.1 ± 1.5 e.u.; 18 +TCE→ 19 : k₃=(5±0.2) · 10^?5 mol^?1 mol^?1 ·l·s^?1, ΔH^≠ = 15±3 kcal/mol, and ΔS^≠ = ?28 ± 8 e.u. The following rate constants were evaluated for the DMAD additions (CD₂Cl₂, 30°): 6 +DMAD→ 20 : k₁ = (10±1)·10^?4 mol^?1 · l·s^?1; 20 +DMAD→ 21 : k₂ = (6.5±0.1) · 10^?4 mol^?1 ·l·^?1; 21 +DMAD→ 22 : k₃ = (1.0±0.1) · 10^?4 mol^?1 ·l·s^?1. The reactions giving the barrelene derivatives 19, 22, 26 and 30 are slower than those leading to adducts that are not barrelenes. The former are estimated less exothermic than the latter. It is proposed that the Diels-Alder reactivity of exocyclic s-cis-butadienes grafted onto bicycle [2.2.1]heptanes and bicyclo [2.2.2]octanes that are modified by remote substitution of the bicyclic skeletons can be affected by changes inthe exothermicity of the cycloadditions, in agreement with the Dimroth and Bell-Evans-Polanyi principle. Force-field calculations (MMPI 1) of 3, 4, 6 and related exocyclic s-cis-butadienes as a moiety of bicyclo [2.2.2]octane suggested single minimum energy hypersurfaces for these systems (eclipsed conformations, planar dienes). Their flexibility decreases with the degree of unsaturation of the bicyclic skeleton. The effect of an endocyclic double bond is larger than that of an exocyclic diene moiety.     

Tautomerism of hydrophosphoryl compounds and their features as ligands in metal complex catalysis. Quantum-chemical simulations by the density functional method

The density functional method (gradient-corrected nonempirical functional PBE, basis TZ2p) was used to perform a large-scale study of the mechanism of tautomerization of hydrophosphoryl compounds RR′P (H)O ? RR′POH (R,R′ = Alk, Ar, OR, NR₂). It was shown that intramolecular proton transfer in this rearrangement is forbidden (activation barriers 43.3–60 kcal mol^?1), and, in the absence of carrier molecules, it occurs as synchronous transfer of two protons in fairly strong dimeric associates (2.50–10.5 kcal mol^?1) formed due to O-H···O, O-H···P, and C-H···O hydrogen bonding. The process involves six-membered transition states with activation barriers of 5–15 kcal mol^?1. The contribution of tunneling into the rate constants at 300–400 K, according to estimates in terms of the reaction-path Hamiltonian formalism, reaches 20–40% and increases as the temperature decreases. The mechanism of ethylene hydroformylation in a model complex of a hydrophosphoryl compound with Pt(II) [(H₂PO)₂H]Pt(PH₃)(H)] was considered to reveal factors responsible for the high efficiency of such complexes in the reaction studied. It was found that the key stages of the catalytic cycle involve reversible proton migration in the ?PH₂OH··· O=P chain of the quasi-chelate ring, which provides fine tuning of the electron distribution in the catalytic node and thus functions as a molecular switcher.     

Complexation and abstraction channels in the Al + CO2 reaction

The AI + CO₂ reaction is studied in the gas phase at 296 K by laser-induced fluorescence monitoring of Al and AlO. Pressure dependence of the effective bimolecular rate constant in the range 10–600 Torr (Ar+CO₂) indicates a complex formation channel yielding a stable Al·CO₂ adduct. Observation of AlO confirms the presence of an abstraction channel. A simple chemical activation mechanism is used to interpret the pressure dependence of the effective bimolecular rate constant. The activation energy for Al·CO₂ complex formation is estimated at ⪢ 1.0 kcal mol⁻¹, and the binding energy is estimated at ⪢ 9 kcal mol⁻¹.     

Kinetics and Mechanism of Radical‐Chain Oxidation of 1,2‐Substituted Ethylene and 1,4‐Substituted Butadiene‐1,3

A combination of microvolumetry, the rotating sector method, ESR, ¹H NMR, and IR allowed to establish a detailed mechanism of liquid‐phase oxidation of vinyl compounds X₁CH=CHX₂ and X₁CH=CH–CH=CHX₂ (X₁ and X₂—a polar substitute: С₆Н₅–, CO–, СOO–) initiated by azobisisobutyronitrile. A distinctive feature of the mechanism is the fact that the oxidation chain is carried out by a low‐molecular hydroperoxide radical joining the π‐bond. For nine compounds in the temperature range of 303–353 K, relative chain propagation and termination rate constants were measured (k ₂•k ₃^−0.5). Absolute values of k ₂ were obtained for diphenylethylene (110 L·mol⁻¹·s⁻¹), ethyl ether of trans‐phenyl‐pentadiene acid (13 L·mol⁻¹·s⁻¹), and methyl ether of trans‐phenyl‐pentadiene acid (14.2 L·mol⁻¹·s⁻¹) at T = 323 K. For the same conditions, 10⁻⁸k ₃ were calculated for diphenylethylene (0.87 L·mol⁻¹·s⁻¹) and methyl ether of trans‐phenyl‐pentadiene acid (1.21 L·mol⁻¹·s⁻¹). A cyclic mechanism of the oxidation chain termination on introduced antioxidants (stable nitroxyl radicals of the piperidine series ( > NO^●) and the transition metal compounds (Meⁿ )) was established. The inhibition factor (f ) showing how many reaction chains are terminated by the one particle of the antioxidant is equal to 10². The cyclic chain termination is caused by the following reactions: HO₂^● + > NO^● → NOH + O₂, HO₂● + NOH → >NO^● + H₂O₂ (for >NO^●) and HO₂^● + Meⁿ → Me^{n +1} + HO₂^●−, HO₂^● + Me^{n +1} → Meⁿ + H⁺ + O₂ (for Meⁿ ).     

Triel–hydride triel bond between ZX3 (Z = B and Al; X = H and Me) and THMe3 (T = Si,Ge and Sn)

Complexes between THMe₃ (T = Si, Ge and Sn) and ZX₃ (Z = B and Al; X = H and Me) have been characterized using MP2/aug‐cc‐pVTZ calculations. These complexes are chiefly stabilized by a triel–hydride triel bond with the T–H bond pointing to the π‐hole on the triel atom. The triel–hydride interaction is mainly attributed to the charge transfer from the T–H bond orbital to the empty p orbital of the triel atom. These complexes are very stable with a large interaction energy (>10 kcal mol^?1) excluding THMe₃···BMe₃ (T = Si and Ge), indicating that the sp²‐hydridized triel atom has a strong affinity for the T–H bond. The formation of THMe₃···BH₃ results in proton transfer, characterized by conversion of orbital interaction and large charge transfer (ca 0.5e). The large deformation is primarily responsible for the abnormally greater interaction energy in THMe₃···BH₃ (>30 kcal mol^?1) than in the AlH₃ analogue. Methyl substitution on the triel atom weakens the triel–hydride interaction and causes a larger interaction energy in THMe₃···AlMe₃ with respect to its BMe₃ counterpart. Most of these interactions possess characteristics of covalent bonds. Polarization makes a contribution to the stability of most complexes nearly equivalent to the electrostatic term.     

Kinetics of the mercury(II)‐catalyzed substitution of coordinated cyanide ion in hexacyanoruthenate(II) by nitroso‐R‐salt

The kinetics and mechanism of Hg²⁺‐catalyzed substitution of cyanide ion in an octahedral hexacyanoruthenate(II) complex by nitroso‐R‐salt have been studied spectrophotometrically at 525 nm (λ_max of the purple‐red–colored complex). The reaction conditions were: temperature = 45.0 ± 0.1°C, pH = 7.00 ± 0.02, and ionic strength (I) = 0.1 M (KCl). The reaction exhibited a first‐order dependence on [nitroso‐R‐salt] and a variable order dependence on [Ru(CN)₆^4?]. The initial rates were obtained from slopes of absorbance versus time plots. The rate of reaction was found to initially increase linearly with [nitroso‐R‐salt], and finally decrease at [nitroso‐R‐salt] = 3.50 × 10^?4 M. The effects of variation of pH, ionic strength, concentration of catalyst, and temperature on the reaction rate were also studied and explained in detail. The values of k₂ and activation parameters for catalyzed reaction were found to be 7.68 × 10^?4 s^?1 and E_a = 49.56 ± 0.091 kJ mol^?1, ΔH^≠ = 46.91 ± 0.036 kJ mol^?1, ΔS^≠ = ?234.13 ± 1.12 J K^?1 mol^?1, respectively. These activation parameters along with other experimental observations supported the solvent assisted interchange dissociative (I_d) mechanism for the reaction. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 215–226, 2009     

Solvent effects on the activation parameters of the reaction between an α‐tocopherol analogue and dpph•: The role of H‐bonded complexes

The analysis of the activation parameters for the formal H‐atom transfer reaction between 2,2,5,7,8‐pentamethyl‐6‐chromanol (ChrOH) and 2,2‐diphenyl‐1‐picrylhydrazyl (dpph^?) reveals that these parameters are effective probes of the actual reaction mechanism. Indeed, the A factors measured in various polar and apolar solvents are localized in three distinct domains according to whether the reaction occurs via outer‐sphere electron transfer (ET) from the anion ChrO^? or hydrogen atom transfer (HAT). For instance, A = 5.9 × 10⁵ M^?1 s^?1 and E_a = 2.5 kcal mol^?1 in cyclohexane where the reaction proceeds by HAT, whereas in methanol, ethanol, and their mixtures with water where there is a substantial ET contribution A > 10⁹ M^?1s^?1 and E_a > 7 kcal mol^?1. Interestingly, in nonhydroxylic polar solvents, A～ 10⁷ M^?1s^?1 and the E_a values reflect the H‐bond accepting ability of the solvent in agreement with the “standard” kinetic solvent effects on HAT reactions. Addition of small quantities of pyridine accelerates the reaction rates in these solvents. This suggests that the H‐bonded complex (ChrOH···Py) is able to react via intermolecular ET with dpph^?. It is known, in fact, that pyridine lowers the oxidation potential of phenols by ～0.5 V and the ΔG_ET of ChrOH + dpph^? consequently decreases by about 10 kcal mol^?1. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 524–531, 2012     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Ligand Exchange Processes on Solvated Beryllium Cations. II [Be(solvent)(12‐Crown‐4)]2+

The solvation and solvent exchange mechanism of [Be(12‐crown‐4)]²⁺ in water and ammonia was studied by DFT calculations (RB3LYP/6‐311+G**). In solution, five‐fold coordinated Be²⁺ species of quadratic pyramidal [Be(H₂O)(12‐crown‐4)]²⁺ and [Be(NH₃)(12‐crown‐4)]²⁺ exist. The water and ammonia exchange reactions follow an associative interchange mechanism, similar to that found for the pure solvent complexes [Be(H₂O)₄]²⁺ and [Be(NH₃)₄]²⁺. The activation barriers are clearly smaller than for the pure solvent complexes, viz. [Be(H₂O)(12‐crown‐4)]²⁺: 6.0 kcal/mol and [Be(NH₃)(12‐crown‐4)]²⁺: 15.3 kcal/mol.     

Water Exchange on Hexaaquavanadium(III): a Variable-Temperature and Variable-Pressure 17O-NMR Study at 1.4 and 4.7 Tesla

Water exchange on hexaaquavanadium (III) has been studied as a function of temperature (255 to 413 K) and pressure (up to 250 MPa, at several temperatures) by ¹⁷O-NMR spectroscopy at 8.13 and 27.11 MHz. The samples contained V³⁺ (0.30–1.53 m), H⁺ (0.19–2.25 m) and ¹⁷O-enriched (10–20%) H₂O. The trifluoromethanesulfonate was used as counter-ion, and, contrary to the previously used chloride or bromide, CF₃SO is shown to be non-coordinating. The following exchange parameters were obtained: k = (5.0 ± 0.3) · 10² s^?1, ΔH* = (49.4 ± 0.8) kJ mol^?1, ΔS* = ?(28 ± 2) JK^?1 mol^?1, ΔV* = ?(8.9 ± 0.4) cm³ mol^?1 and Δβ* = ?(1.1 ± 0.3) · 10^?2 cm³ mol^?1 MPa^?1. They are in accord with an associative interchange mechanism, I_a. These results for H₂O exchange are discussed together with the available data for complex formation reactions on hexaaquavanadium(III). A semi-quantitative analysis of the bound H₂O linewidth led to an estimation of the proportions of the different contributions to the relaxation mechanism in the coordinated site: the dipole-dipole interaction hardly contributes to the relaxation (less than 7%); the quadrupolar relaxation, and the scalar coupling mechanism are nearly equally efficient at low temperature (～ 273 K), but the latter becomes more important at higher temperature (75–85% contribution at 360 K).     

Ab initio study of the OH + CH2O reaction: The effect of the OH··OCH2 complex on the H‐abstraction kinetics

Kinetics for the reaction of OH radical with CH₂O has been studied by single‐point calculations at the CCSD(T)/6‐311+G(3df, 2p) level based on the geometries optimized at the B3LYP/6‐311+G(3df, 2p) and CCSD/6‐311++G(d,p) levels. The rate constant for the reaction has been computed in the temperature range 200–3000 K by variational transition state theory including the significant effect of the multiple reflections above the OH··OCH₂ complex. The predicted results can be represented by the expressions k₁ = 2.45 × 10^‐21 T^2.98 exp (1750/T) cm³ mol^?1 s^?1 (200–400 K) and 3.22 × 10^‐18 T^2.11 exp(849/T) cm³ mol^?1 s^?1 (400–3000 K) for the H‐abstraction process and k₂ = 1.05 × 10^‐17 T^1.63 exp(?2156/T) cm³ mol^?1 s^?1 in the temperature range of 200–3000 K for the HO‐addition process producing the OCH₂OH radical. The predicted total rate constants (k₁ + k₂) can reproduce closely the recommended kinetic data for OH + CH₂O over the entire range of temperature studied. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 322–326, 2006     

Unexpected A·T(WC)↔A·T(rWC)/A·T(rH) and A·T(H)↔A·T(rH)/A·T(rWC) conformational transitions between the classical A·T DNA base pairs: A QM/QTAIM comprehensive study

In this study, using QM/QTAIM calculations in the continuum with ε = 1 under normal conditions, we have revealed for the first time the nondissociative A·T(WC)↔A·T(rWC)/A·T(rH) and A·T(H)↔A·T(rH)/A·T(rWC) conformational transitions. It was established that they proceed via the essentially nonplanar transition states (С₁ symmetry) through the intermediates, which are wobbled conformers (С₁ symmetry) theoretically predicted in our previous work (Brovarets’ et al., Frontiers in Chemistry, 2018, 6:8, 10.3389/fchem.2018.00008) of the classical А·Т DNA base pairs—Watson–Crick А·Т(WC), reverse Watson–Crick А·Т(rWC), Hoogsteen А·Т(Н) and reverse Hoogsteen А·Т(rН). At this, the A·T(H)↔A·T(rWC) and A·T(WC)↔A·T(rH) conformational transformations are controlled by the transition states (TSs) stabilized by the participation of the intermolecular (T)N3H···N6(A) H‐bond (∼3.70 kcal·mol⁻¹) between the imino group N3H of T and pyramidilized amino group N6H₂ of A. Gibbs free energies of activation for these processes consist 12.22 and 11.11 kcal·mol⁻¹, accordingly, under normal conditions. TSs, which control the A·T(WC)↔A·T(rWC) and A·T(H)↔A·T(rH) conformational transitions are stabilized by the participation of the intermolecular (T)N3H···N6(A) H‐bond (5.82 kcal·mol⁻¹) and bifurcating intermolecular (T)N3H···N6(A) (5.00) and (T)N3H···N7(A) (0.61 kcal·mol⁻¹) H‐bonds, accordingly. Notably, in these two TSs amino group N6H₂ of A is significantly pyramidilized; Gibbs free energies of activation for these reactions are 19.07 and 19.71 kcal·mol⁻¹, accordingly.     

Strukturen ionischer Di(arensulfonyl)amide. 8. Natrium‐bis[di(4‐fluorbenzolsulfonyl)amido‐N]argentat: Ein Heterobimetallkomplex mit lamellarer Schichtstruktur und kurzen Zwischenschichtkontakten C–H···F–C

Structures of Ionic Di(arenesulfonyl)amides. 8. Sodium Bis[di(4‐fluorobenzenesulfonyl)amido‐N]argentate: A Heterobimetallic Complex Exhibiting a Lamellar Layer Structure and Short C–H···F–C Interlayer Contacts Na[Ag{N(SO₂–C₆H₄–4‐F)₂}₂] (monoclinic, C2/c, Z′ = 1/2) is the first heterobimetallic representative in a well‐documented class of layered inorgano‐organic solids where the inorganic component is comprised of metal cations and coordinating N(SO₂)₂ groups and the outer regions are formed by the aromatic rings of the di(arenesulfonyl)amide entities, which adopt a folded conformation approximating to mirror symmetry. The inversion‐symmetric bis(amido)argentate unit of the novel compound displays an exactly linear N–Ag–N core and short Ag–N bonds of 217.55(17) pm (at ?140 °C); the coordination number of the silver ion is extended to 2 + 6 by four internal and two external Ag···O secondary interactions. The polar lamella is constructed from rows of Na⁺ ions located on twofold axes, alternating with bis(amido)argentate strands reinforced by Ag···O interactions and weak C–H···O hydrogen bonds; Na⁺ is embedded in an O₆ environment. Adjacent layers are cross‐linked via short C–H···F–C contacts suggestive of weak hydrogen bonding enhanced by cooperativity.