Thermodynamics of Hydrogen Adsorption on Metal‐Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg²⁺ and Co²⁺ cationic centres in Mg‐MOF‐74 and Co‐MOF‐74, respectively, was studied by means of variable‐temperature infrared (VTIR) spectroscopy. Perturbation of the H₂ molecule by the cationic adsorbing centre renders the H? H stretching mode IR‐active at 4088 and 4043 cm^?1 for Mg‐MOF‐74 and Co‐MOF‐74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79–95 K allowed standard adsorption enthalpy and entropy to be determined. Mg‐MOF‐74 showed ΔH⁰=?9.4 kJ mol^?1 and ΔS⁰=?120 J mol^?1 K^?1, whereas for Co‐MOF‐74 the corresponding values of ΔH⁰=?11.2 kJ mol^?1 and ΔS⁰=?130 J mol^?1 K^?1 were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation‐exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering.     

Ligand Dynamics in the Solid: Lithium‐fluorenide and Lithium‐benzo[b]fluorenide N,N,N′,N′‐Tetramethylethylenediamine (tmeda) Complexes

The dynamic behavior of the N,N,N′,N′‐tetramethylethylenediamine (tmeda) ligand has been studied in solid lithium‐fluorenide(tmeda) ( 3 ) and lithium‐benzo[b]fluorenide(tmeda) ( 4 ) using CP/MAS solid‐state ¹³C‐ and ¹⁵N‐NMR spectroscopy. It is shown that, in the ground state, the tmeda ligand is oriented parallel to the long molecular axis of the fluorenide and benzo[b]fluorenide systems. At low temperature (<250 K), the ¹³C‐NMR spectrum exhibits two MeN signals. A dynamic process, assigned to a 180° rotation of the five‐membered metallacycle (π‐flip), leads at elevated temperatures to coalescence of these signals. Line‐shape calculations yield ΔH^?=42.7 kJ mol^?1, ΔS^?=?5.3 J mol^?1 K^?1, and =44.3 kJ mol^?1 for 3 , and ΔH^?=36.8 kJ mol^?1, ΔS^?=?17.7 J mol^?1 K^?1, and =42.1 kJ mol^?1 for 4 , respectively. A second dynamic process, assigned to ring inversion of the tmeda ligand, was detected from the temperature dependence of T_1ρ, the ¹³C spin‐lattice relaxation time in the rotating frame, and led to ΔH^?=24.8 kJ mol^?1, ΔS^?=?49.2 J mol^?1 K^?1, and =39.5 kJ mol^?1 for 3 , and ΔH^?=18.2 kJ mol^?1, ΔS^?=?65.3 J mol^?1 K^?1, and =37.7 kJ mol^?1 for 4 , respectively. For (D₁₂)‐ 3 , the rotation of the CD₃ groups has also been studied, and a barrier E_a of 14.1 kJ mol^?1 was found.     

Breaking the Semi‐Quinoid Structure: Spin‐Switching from Strongly Coupled Singlet to Polarized Triplet State

2,7‐TMPNO (4,5,9,10‐tetramethoxypyrene‐2,7‐bis(tert‐butylnitroxide)) was found to exist in semi‐quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/k_B=1185 K operating over a distance of 10 Å. Structural transformations with the activation energy of ΔE_eq=949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S=1, g=2.0065, D=?0.0112 cm^?1, and E=?0.0014 cm^?1 by spectral simulation with the hybrid Eigenfield/exact diagonalization method.     

Variable‐Temperature Infrared Spectroscopy Studies on the Thermodynamics of CO Adsorption on the Zeolite Ca–Y

Variable temperature FT–IR spectroscopy (in the range of 298–380 K) is used to study the thermodynamics of formation of Ca²⁺???CO carbonyl species upon CO adsorption on the faujasite‐type zeolite Ca–Y, and also the (temperature‐dependent) isomerization equilibrium between carbonyl and isocarbonyl (Ca²⁺???OC) species. The standard enthalpy and entropy changes involved in formation of the monocarbonyl species resulted to be ΔH⁰=?50.3 (±0.5) kJ mol^?1 and ΔS⁰=?186 (±5) J mol^?1 K^?1, respectively. Isomerization of the (C‐bonded) Ca²⁺???CO carbonyl to yield the (O‐bonded) Ca²⁺???OC isocarbonyl involves an enthalpy change =+11.4 (±1.0) kJ mol^?1. These results are compared with previously reported data for the CO/Sr–Y system; and also, a brief analysis of enthalpy–entropy correlation for CO adsorption on zeolites and metal oxides is given.     

Mechanism Studies of the Conversion of 13C‐Labeled n‐Butane on Zeolite H‐ZSM‐5 by Using 13C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis

By using ¹³C MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively ¹³C‐labeled n‐butane on zeolite H‐ZSM‐5 at 430–470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective ¹³C‐label in the n‐butane molecule, and 2) oligomerization–cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl‐substituted cyclopentenyl cations and condensed aromatic compounds. In situ ¹³C MAS NMR and complementary ex situ GC–MS data provided evidence for a monomolecular mechanism of the ¹³C‐label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to ¹³C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E_a=75 kJ mol^?1 for the scrambling and 71 kJ mol^?1 for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n‐butane as being the rate‐determining stage of the n‐butane conversion on zeolite H‐ZSM‐5.     

Conversion of CO2 and C2H6 to Propanoic Acid over a Au‐Exchanged MCM‐22 Zeolite

Finding novel catalysts for the direct conversion of CO₂ to fuels and chemicals is a primary goal in energy and environmental research. In this work, density functional theory (DFT) is used to study possible reaction mechanisms for the conversion of CO₂ and C₂H₆ to propanoic acid over a gold‐exchanged MCM‐22 zeolite catalyst. The reaction begins with the activation of ethane to produce a gold ethyl hydride intermediate. Hydrogen transfers to the framework oxygen leads then to gold ethyl adsorbed on the Brønsted‐acid site. The energy barriers for these steps of ethane activation are 9.3 and 16.3 kcal mol^?1, respectively. Two mechanisms of propanoic acid formation are investigated. In the first one, the insertion of CO₂ into the Au?H bond of the first intermediate yields gold carboxyl ethyl as subsequent intermediate. This is then converted to propanoic acid by forming the relevant C?C bond. The activation energy of the rate‐determining step of this pathway is 48.2 kcal mol^?1. In the second mechanism, CO₂ interacts with gold ethyl adsorbed on the Brønsted‐acid site. Propanoic acid is formed via protonation of CO₂ by the Brønsted acid and the simultaneous formation of a bond between CO₂ and the ethyl group. The activation energy there is 44.2 kcal mol^?1, favoring this second pathway at least at low temperatures. Gold‐exchanged MCM‐22 zeolite can therefore, at least in principle, be used as the catalyst for producing propanoic acid from CO₂ and ethane.     

Pressure effects on cation migration in 2,5‐di‐tert‐butyl‐1,4‐benzoquinone radical anion

Pressure effects on the two‐site jumping of sodium and potassium cations in a 2,5‐di‐tert‐butyl‐1,4‐benzoquinone ion pair have been studied using a high‐pressure EPR technique. The rate constants of the intramolecular and intermolecular migrations for Na⁺ and K⁺ were determined from an EPR spectral simulation. The migration rates were found to be accelerated by increasing the external pressure. Using the pressure dependence of the migration rates, we estimated the activation volumes of the intramolecular (ΔV₁^?) and intermolecular (ΔV₂^?) processes for the Na⁺ and K⁺ migrations: ΔV₁^? = ?5.3 cm³ mol^?1 and ΔV₂^? = ?29 cm³ mol^?1 for Na⁺, and ΔV₁^? = ?8.3 cm³ mol^?1 and ΔV₂^? = ?0.85 cm³ mol^?1 for K⁺. Based on the results, the mechanisms for the two‐site jumping of Na⁺ and K⁺ are discussed in terms of volume. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 397–401, 2001     

Formation of a Thymine‐HgII‐Thymine Metal‐Mediated DNA Base Pair: Proposal and Theoretical Calculation of the Reaction Pathway

A reaction mechanism that describes the substitution of two imino protons in a thymine:thymine (T:T) mismatched DNA base pair with a Hg^II ion, which results in the formation of a (T)N3‐Hg^II‐N3(T) metal‐mediated base pair was proposed and calculated. The mechanism assumes two key steps: The formation of the first Hg^II? N3(T) bond is triggered by deprotonation of the imino N3 atom in thymine with a hydroxo ligand on the Hg^II ion. The formation of the second Hg^II? N3(T) bond proceeds through water‐assisted tautomerization of the remaining, metal‐nonbonded thymine base or through thymine deprotonation with a hydroxo ligand of the Hg^II ion already coordinated to the thymine base. The thermodynamic parameters ΔG_R=?9.5 kcal mol^?1, ΔH_R=?4.7 kcal mol^?1, and ΔS_R=16.0 cal mol^?1 K^?1 calculated with the ONIOM (B3LYP:BP86) method for the reaction agreed well with the isothermal titration calorimetric (ITC) measurements by Torigoe et al. [H. Torigoe, A. Ono, T. Kozasa, Chem. Eur. J. 2010 , 16, 13218–13225]. The peculiar positive reaction entropy measured previously was due to both dehydration of the metal and the change in chemical bonding. The mercury reactant in the theoretical model contained one hydroxo ligand in accord with the experimental pK_a value of 3.6 known for an aqua ligand of a Hg^II center. The chemical modification of T:T mismatched to the T‐Hg^II‐T metal‐mediated base pair was modeled for the middle base pair within a trinucleotide B‐DNA duplex, which ensured complete dehydration of the Hg^II ion during the reaction.     

Alleno‐Acetylenic Cage (AAC) Receptors: Chiroptical Switching and Enantioselective Complexation of trans‐1,2‐Dimethylcyclohexane in a Diaxial Conformation

Four enantiopure 1,3‐diethynylallenes (DEAs) with OH termini were attached to the rim of a resorcin[4]arene cavitand. The system undergoes conformational switching between a cage form, closed by a circular H‐bonding array, and an open form, with the tertiary alcohol groups reaching outwards. The cage form is predominant in apolar solvents, and the open conformation in small, polar solvents. Both states were confirmed in solution and in X‐ray co‐crystal structures. ECD spectra of the alleno‐acetylenic cages (AACs) are highly conformation sensitive, the longest wavelength Cotton effect at 304 nm switches from Δ?=+191 m ^?1 cm^?1 for open (P)₄‐AAC?acetonitrile to Δ?=?691 m ^?1 cm^?1 (ΔΔ?=882 m ^?1 cm^?1) for closed (P)₄‐AAC?cyclohexane. Complete chiral resolution of (±)‐trans‐1,2‐dimethylcyclohexane was found in the X‐ray structures, with (P)₄‐AAC exclusively bound to the (R,R)‐ and (M)₄‐AAC to the (S,S)‐guest. Guest inclusion occurs in a higher energy diaxial conformation.     

Thermal Behavior of N,N＇Bis[N-（2,2,2-trinitroethyl）-N-nitro]ethylenediamine

The thermal behavior and kinetic parameters of the exothermic decomposition reaction of N‐N‐bis[N‐(2,2,2‐tri‐nitroethyl)‐N‐nitro]ethylenediamine in a temperature‐programmed mode have been investigated by means of differential scanning calorimetry (DSC). The results show that kinetic model function in differential form, apparent activation energy E_a and pre‐exponential factor A of this reaction are 3(1 ‐α)^2/3, 203.67 kJ·mol^?1 and 10^20.61s^?1, respectively. The critical temperature of thermal explosion of the compound is 182.2 °C. The values of ΔS^≠ ΔH^≠ and ΔG^≠ of this reaction are 143.3 J·mol^?1·K^?1, 199.5 kJ·mol^?1 and 135.5 kJ·mol^?1, respectively.     

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

The synthesis of a bithiophene‐bridged 34π conjugated aromatic expanded porphycene 1 and a cyclopentabithiophene bridged 32π conjugated anti‐aromatic expanded porphycene 2 by a McMurry coupling strategy is presented. Magnetic measurements and theoretical calculations reveal that both 1 and 2 exhibit an open‐shell singlet ground state with significant radical character (y₀=0.63 for 1 ; y₀=0.68, y₁=0.18 for 2 ; y₀: diradical character, y₁: tetraradical character) and a small singlet–triplet energy gap (ΔE_S‐T=?3.25 kcal mol^?1 for 1 and ΔE_S‐T=?0.92 kcal mol^?1 for 2 ). Despite the open‐shell radical character, both compounds display exceptional stability under ambient air and light conditions owing to effective delocalization of unpaired electrons in the extended cyclic π‐conjugation pathway.     

1H NMR utilization of through-space effects in configurational assignments. I—application to nitriles; determination of the separate effects of the electric field and anisotropy of the CN group

The changes in chemical shift induced by isomerization for all the ring protons of the Z- and E-5,5-dimethyl-2-cyclohexenylidene acetonitriles depend only on the through-space effects of the cyano group. The configurational assignments were made taking into consideration the anisotropic and electric field effects, either separately or together. In the first case, the total effects are Δ_XCN^T=?14.7×10^?6cm³ mol^?1 and bμ_CN^T=14.7×10^?30 cm³, respectively. The second approach allows the estimation of the values Δ_XCN=?4.9 × 10^?6 cm³ mol^?1 and bμ_CN=9.8 × 10^?3 cm³, reflecting the combined contributions of magnetic anisotropy and electric field to the total effect.     

Antiferromagnetic Interactions in 1D Heisenberg Linear Chains of 7‐(4‐Fluorophenyl) and 7‐Phenyl‐Substituted 1,3‐Diphenyl‐1,4‐dihydro‐ 1,2,4‐benzotriazin‐4‐yl Radicals

7‐(4‐Fluorophenyl) and 7‐phenyl‐substituted 1,3‐diphenyl‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl radicals were characterized by X‐ray diffraction analysis and variable‐temperature magnetic susceptibility studies. The radicals pack in 1D π stacks of equally spaced slipped radicals with interplanar distances of 3.59 and 3.67 Å and longitudinal angles of 40.97 and 43.47°, respectively. Magnetic‐susceptibility studies showed that both radicals exhibit antiferromagnetic interactions. Fitting the magnetic data revealed that the behavior is consistent with 1D regular linear antiferromagnetic chain with J=?12.9 cm^?1, zJ′=?0.4 cm^?1, g=2.0069 and J=?11.8 cm^?1, zJ′=?6.5 cm^?1, g=2.0071, respectively. Magnetic‐exchange interactions in benzotriazinyl radicals are sensitive to the degree of slippage, and inter‐radical separation and subtle changes in structure alter the fine balance between ferro‐ and antiferromagnetic interactions.     

Magnetic Properties of a Single‐Molecule Lanthanide–Transition‐Metal Compound Containing 52 Gadolinium and 56 Nickel Atoms

Monodisperse metal clusters provide a unique platform for investigating magnetic exchange within molecular magnets. Herein, the core–shell structure of the monodisperse molecule magnet of [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]@SiO₂ ( 1 a @SiO₂) was prepared by encapsulating one high‐nuclearity lanthanide–transition‐metal compound of [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]?(NO₃)₁₈?164 H₂O ( 1 ) (IDA=iminodiacetate) into one silica nanosphere through a facile one‐pot microemulsion method. 1 a @SiO₂ was characterized using transmission electron microscopy, N₂ adsorption–desorption isotherms, and inductively coupled plasma‐atomic emission spectrometry. Magnetic investigation of 1 and 1 a revealed J₁=0.25 cm^?1, J₂=?0.060 cm^?1, J₃=?0.22 cm^?1, J₄=?8.63 cm^?1, g=1.95, and z J=?2.0×10^?3 cm^?1 for 1 , and J₁=0.26 cm^?1, J₂=?0.065 cm^?1, J₃=?0.23 cm^?1, J₄=?8.40 cm^?1 g=1.99, and z J=0.000 cm^?1 for 1 a @SiO₂. The z J=0 in 1 a @SiO₂ suggests that weak antiferromagnetic coupling between the compounds is shielded by silica nanospheres.     

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2‐electron oxygen‐atom‐transfer (OAT) chemistry. The 6‐coordinate [Mn^V(O)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? to the 5‐coordinate Mn^V(O) precursor. The cyanide‐ligated complex was characterized for the first time by Mn K‐edge X‐ray absorption spectroscopy (XAS) and gives Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000‐fold versus the same reaction for the parent 5‐coordinate complex. An Eyring analysis gives ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

1，1－二氨基2，2－二硝基乙烯（DADE）的热化学性质、热行为和热分解机理

The constant-volume combustion energy, △cU （DADE, s, 298.15 K）, the thermal behavior, and kinetics and mechanism of the exothermic decomposition reaction of 1,1-diamino-2,2-dinitroethylene （DADE） have been investigated by a precise rotating bomb calorimeter, TG-DTG, DSC, rapid-scan fourier transform infrared （RSFT-IR） spectroscopy and T-jump/FTIR, respectively. The value of △cHm （DADE, s, 298.15 K） was determined as （-8518.09±4.59） j·g^-1. Its standard enthalpy of combustion, △cU （DADE, s, 298.15 K）, and standard enthalpy of formation, △fHm （DADE, s, 298.15 K） were calculated to be （-1254.00±0.68） and （- 103.98±0.73） kJ·mol^-1, respectively The kinetic parameters （the apparent activation energy Ea and pre-exponential factor A） of the first exothermic decomposition reaction in a temperature-programmed mode obtained by Kissinger＇s method and Ozawa＇s method, were Ek=344.35 kJ·mol^-1, AR= 1034.50 S^-1 and Eo=335.32 kJ·mol^-1, respectively. The critical temperatures of thermal explosion of DADE were 206.98 and 207.08 ℃ by different methods. Information was obtained on its thermolysis detected by RSFT-IR and T-jump/FTIR.     

3‐[4′‐(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer

The crystallization of 3‐[4′‐(diethylboryl)phenyl]pyridine ( 1 ), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 °C afforded a cyclic tetramer that co‐crystallized with solvent molecules. Similarly, solutions of compound 1 in toluene at 10 °C and in benzene at 8 °C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetramer was the minor component. ¹³C CP/MAS NMR spectroscopy of precipitates of compound 1 suggested that precipitation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipitation from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D₆]benzene, as determined from a van′t Hoff plot, were ΔH°=?8.8 kcal mol^?1 and ΔS°=?23.7 cal mol^?1 K^?1, which were coincident with previously reported calculations and observations.     

Activation of the aromatic system by the SO2CF3 group: Kinetics study and structure–reactivity relationships

The rate constants for the reaction of 2,6‐bis(trifluoromethanesulfonyl)‐4‐nitroanisole with some substituted anilines have been measured by spectrophotometric methods in methanol at various temperatures. The data are consistent with the S_NAr mechanism. The effect of substituents on the rate of reaction has been examined. Good linear relationships were obtained from the plots of log k₁ against Hammett σ_para constants values at all temperature with negative ρ values (?1.68 to ?1.11). Activation parameters Δ^≠H varied from 41.6 to 54.3 kJ mol^?1 and Δ^≠S from ?142.7 to ?114.6 J mol^?1 K^?1. The δΔ^≠H and δΔ^≠S reaction constants were determined from the dependence of Δ^≠H and Δ^≠S activation parameters on the σ substituent constants, by analogy with the Hammett equation. A plot of Δ^≠H versus Δ^≠S for the reaction gave good straight line with 177°C isokinetic temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 203–210, 2010     

Phosphatation of Zeolite H‐ZSM‐5: A Combined Microscopy and Spectroscopy Study

A variety of phosphated zeolite H‐ZSM‐5 samples are investigated by using a combination of Fourier transfer infrared (FTIR) spectroscopy, single pulse ²⁷Al, ²⁹Si, ³¹P, ¹H‐³¹P cross polarization (CP), ²⁷Al‐³¹P CP, and ²⁷Al 3Q magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, scanning transmission X‐ray microscopy (STXM) and N₂ physisorption. This approach leads to insights into the physicochemical processes that take place during phosphatation. Direct phosphatation of H‐ZSM‐5 promotes zeolite aggregation, as phosphorus does not penetrate deep into the zeolite material and is mostly found on and close to the outer surface of the zeolite, acting as a glue. Phosphatation of pre‐steamed H‐ZSM‐5 gives rise to the formation of a crystalline tridymite AlPO₄ phase, which is found in the mesopores of dealuminated H‐ZSM‐5. Framework aluminum species interacting with phosphorus are not affected by hydrothermal treatment. Dealuminated H‐ZSM‐5, containing AlPO₄, retains relatively more framework Al atoms and acid sites during hydrothermal treatment than directly phosphated H‐ZSM‐5.