Time-resolved spectroscopic study of the reaction Cl + n-C5H12 --> HCl + C5H11 in solution

We study the hydrogen abstraction reaction from pentane by chlorine radicals using four different experimental approaches. We use two different solvents (CH2Cl2 and CCl4) and two different chlorine atom sources (photodissociation of dissolved Cl2 and two-photon photolysis of the solvent) to investigate their effects on the recombination and reactivity of the chlorine radical. All four experimental schemes involve direct probing of the transient chlorine population via a charge transfer transition with a solvent molecule. In one of the four approaches, photolysis of Cl2 in dichloromethane, we also monitor the nascent reaction products (HCl) by transient vibrational spectroscopy. Probing both the reactants and the products provides a comprehensive view of this bimolecular reaction in solution. Between one-third and two-thirds of the chlorine radicals that initially escape the solvent cage undergo diffusive geminate recombination with their partner radical (either another chlorine atom or the solvent radical). The rest react with pentane with the bimolecular rate constants k(bi) = (9.5 +/- 0.7) x 10(9) M(-1) s(-1) in CH2Cl2 and k(bi) = (7.4 +/- 2) x 10(9) M(-1) s(-1) in CCl4. The recombination yield phi(rec) depends on both the chlorine atom precursor and the solvent and is larger in the more viscous carbon tetrachloride solutions. The bimolecular reaction rate k(bi) depends only on the solvent and is consistent with a nearly diffusion-limited reaction.     

Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d(8)-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl(3) and CD(2)Cl(2)). HCN and DCN products were monitored via their v(1) (C≡N stretching) and v(3) (C-H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5-15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1(1)(0) and 3(1)(0) fundamental bands) decreases with increasing dilution of the THF in CDCl(3) or CD(2)Cl(2), showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5-4.5 M, and a bimolecular rate coefficient of (1.57 ± 0.12) × 10(10) M(-1) s(-1) is derived. Simultaneous analysis of time-dependent HCN 1(1)(0) and 3(1)(0) band intensities following reaction of CN with THF (3.0 M) in CD(2)Cl(2) suggests that C-H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.     

Vibrationally quantum-state-specific dynamics of the reactions of CN radicals with organic molecules in solution

The dynamics of reactions of CN radicals with cyclohexane, d(12)-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ～500 cm(-1) bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v(3)) and H-C-N (D-C-N) bending (v(2)) modes, and for DCN products there is also evidence of vibrational excitation of the v(1) mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ～130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm(-1) and 2065 cm(-1) (in CHCl(3)) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC.     

Infrared spectroscopy of prereactive aluminum-, gallium-, and indium-HCN entrance channel complexes solvated in helium nanodroplets

Prereactive metal atom-HCN entrance channel complexes [M-HCN (M=Al, Ga, In)] have been stabilized in helium nanodroplets. Rotationally resolved infrared spectra are reported for the CH stretching vibration of the linear nitrogen-bound HCN-Ga and HCN-In complexes that show significant perturbation due to spin-orbit coupling of the 2Pi1/2 ground state with the 2Sigma1/2 state which are degenerate at long range. Six unresolved bands are also observed and assigned to the linear hydrogen-bound isomers of Al-HCN, Ga-HCN, and In-HCN corresponding to the fundamental CH stretching vibration and a combination band involving the CH stretch plus intermolecular stretch for each isomer. A nitrogen-bound HCN-Al complex is not observed, which is attributed to reaction, even at 0.37 K. This conclusion is supported by the observation of a weakly bound complex containing two HCN's and one Al atom which, from the analysis of its rotationally resolved zero-field and Stark spectra is assigned to a weakly bound complex of a HCNAl reaction product and a second HCN molecule. Theoretical calculations are presented to elucidate the reaction mechanisms and energetics of these metal atom reactions with HCN.     

Competition between nonclassical hydrogen-bonded acceptor sites in complexes of neutral AH2 Radicals (A = B, Al, and Ga): A theoretical investigation

An ab initio computational study of the properties of the neutral AH2 radicals (A = B, Al, Ga) as hydrogen-bond (HB) acceptors, with H-X (X = F, Cl, Br, CN, and CCH) as HB donors, is carried out at the UMP2/6-311++G(2d,2p) level. Two different minima have been found for each of the 15 possible dimers. One structure corresponds to a single-electron hydrogen-bonded complex (SEHB), with the A atom acting as an HB acceptor. The second corresponds to a dihydrogen bond complex between one of the hydrogen atoms of AH2 and the H-X molecule. Thus, all the atoms of the neutral AH2 molecule can act as HB acceptors and none as donors. The stability of the SEHB complexes decreases as BH2 > AlH2 > GaH2, while for the dihydrogen-bonded complexes the order is AlH2 > GaH2 > BH2. For the BH2 radical the SEHB complexes are stronger than the dihydrogen bonded ones, while the opposite is found for the AlH2 and GaH2 systems. Regarding the HB donors, the order found for the binding energy in the two types of complexes is H2A...HF > H2A...HCl > H2A...HBr > H2A...HCN > H2A...HCCH.     

H+CN体系中平行络合反应的动力学

利用分子反应动力学的经典轨迹法研究化学反应体系已经开展了很多工作,但是,研究长寿命络合物的几何异构体就显得有些困难,主要表现在该方法对几何异构体不容易判别．由于这些原因和计算机时大增,Davis[1]利用经典轨迹法和统计方法相结合,研究了H＋CN和H＋NC体系在固定方位时的长寿命络合物动力学行为,并且采用Murrel等[2]拟合光谱数据的势能函数,该势能函数在HCN和HNC平衡点的频率与实验值相差较大．本文利用经典轨迹法研究了H＋CN产生HCN和HNC平行络合物异构体的分子反应动力学行为,揭示了平行络合反应产生几何异构体的…     

Décomposition du percarbonate de O,O-t-butyle et O-isopropényle en solution dans des cyclanes

Thermolysis of O,O-t-butyl and O-isopropenyl percarbonate in cyclohexane involves free-radical acetonylation of solvent. Free radicals derived from solvent add to the percarbonate double bond and after a double β-scission reaction, cyclohexylacetone carbon dioxide and t-butoxy radicals are formed. Abstracting H atoms from the solvent, t-butoxy radicals regenerate free radicals from solvent, and the reaction becomes a chain process. Extending the study to other cycloalkanes it has been shown that the process is a general synthesis method for cycloalkylacetones. On the other hand, competitive reactions of pairs of solvents have shown that the reactivity of the substrates depends on H atom lability and on more complex phenomena like transfers between hydrocarbons and C-centered free-radicals.     

Self-assembly, structures, and solution dynamics of emissive silver metallacycles and helices

Reactions of 9,10-bis(diphenylphosphino)anthracene (PAnP) and AgX (X = OTf-, ClO4-, PF6-, and BF4-) led to luminescent Ag-PAnP complexes with rich structural diversity. Helical polymers [Ag(mu-PAnP)(CH3CN)X]n (X = OTf-, ClO4-, and PF6-) and discrete binuclear [Ag2(mu-PAnP)2(CH3CN)4](PF6)2, trinuclear [Ag3(mu-PAnP)3 supersetBF4](BF4)2, and tetranuclear [Ag4(mu-PAnP)4 superset(ClO4)2](ClO4)2 metallacycles were isolated from different solvents. The tri- and tetranuclear metallacycles exhibited novel puckered-ring and saddlelike structures. Variable-temperature (VT) 31P{1H}-NMR spectroscopy of the complexes was solvent dependent. The dynamics in CD3CN involve two species, but the exchange processes in CD2Cl2 are more complicated. A ring-opening polymerization was proposed for the exchange mechanism in CD3CN.     

Collision-induced dissociation of HS-(HCN): unsymmetrical hydrogen bonding in a proton-bound dimer anion

The energy-resolved competitive collision-induced dissociation of the proton-bound complex [HS.H.CN](-) is studied in a guided ion beam tandem mass spectrometer. H(2)S and HCN have nearly identical gas-phase acidities, and therefore, the HS(-) + HCN and the CN(-) + H(2)S product channels exhibit nearly the same threshold energies, as expected. However, the HS(-) + HCN channel has a cross section up to a factor of 50 larger than CN(-) + H(2)S at higher energies. The cross sections are modeled using RRKM theory and phase space theory. The complex dissociates to HS(-)+ HCN via a loose transition state, and it dissociates to CN(-) + H(2)S via a tight transition state. Theoretical calculations show that the proton-transfer potential energy surface has a single minimum and that the hydrogen bonding in the complex is strongly unsymmetrical, with an ion-molecule complex of the form HS(-)..HCN rather than CN(-)..H(2)S or an intermediate structure. The requirement for proton transfer before dissociation and curvature along the reaction path impedes the CN(-) + H(2)S product channel.     

Steric effects on nascent vibrational distributions of the HCN product produced in CN radical reactions with ethane, propane and chloroform

Time-resolved IR emission spectroscopy (TRIES) has been used to study infrared emission in the 3400–3100 cm⁻¹ region from HCN molecules produced when CN radicals abstract a hydrogen atom from ethane, propane, and chloroform. From these observations the nascent vibrational distributions of the HCN produced were derived. The nascent vibrational population distributions of the product HCN in all of the reactions are non-statistical and inverted in both the pure CH stretch (00p) and CH stretch—bend (0np) series.     

Molybdenum(VI) catalysts obtained from η3-allyl dicarbonyl precursors: synthesis, characterization and catalytic performance in cyclooctene epoxidation

The oxidative decarbonylation of the η(3)-allyl dicarbonyl complexes [Mo(η(3)-C(3)H(5))Cl(CO)(2)(L)] (L = 2,2'-bipyridine (bipy) (1), 4,4'-di-tert-butyl-2,2'-bipyridine (di-tBu-bipy) (2)) by reaction with aqueous tert-butylhydroperoxide (TBHP) or H(2)O(2) gave the following compounds in good to excellent yields: the oxo-bridged dimers [MoO(2)Cl(L)](2)O (L = bipy (3), di-tBu-bipy (6)) using TBHP(10 equiv.)/CH(3)CN/r.t.; the molybdenum oxide/bipyridine hybrid material {[MoO(3)(bipy)][MoO(3)(H(2)O)]}(n) (4) and the octanuclear complex [Mo(8)O(24)(di-tBu-bipy)(4)] (7) using TBHP(50 equiv.)/H(2)O/70 °C; the oxodiperoxo complexes MoO(O(2))(2)(L) (L = bipy (5), di-tBu-bipy (8)) using H(2)O(2)(10 equiv.)/CH(3)CN/r.t. The structure of 7·x(solvent) (where solvent = CH(2)Cl(2) and/or diethyl ether) was determined by single crystal X-ray diffraction. Despite possessing the same windmill-type complex as that described previously for 7·10CH(2)Cl(2), the crystal structure of 7·x(solvent) is unique due to differences in the crystal packing. Compounds 1-8 were examined as catalysts or catalyst precursors for the epoxidation of cyclooctene using aqueous TBHP or H(2)O(2) as oxidant at 55 or 70 °C. Reactions were performed without co-solvent or with the addition of water, ethanol or acetonitrile. Cyclooctene oxide was always the only reaction product. Solids recovered after 24 h reaction at 70 °C were identified by FT-IR spectroscopy as the hybrid 4 from (1,3-5)/TBHP, complex 5 from (1,3-5)/H(2)O(2), and complex 8 from (2,6-8)/H(2)O(2). With TBHP as oxidant, the highest epoxide yields (for 24 h reaction at 70 °C) were obtained using excess H(2)O as solvent (28-38% for 1,3-5; 87-98% for 2,6-8), while with H(2)O(2) as oxidant, the highest epoxide yields were obtained using CH(3)CN as solvent (54-81% for 3-8).     

Rhenium(V) and technetium(V) nitrido complexes with mixed tridentate π-donor and monodentate π-acceptor ligands

Mixed-ligand [M(N)(SNS)(PPh(3))] complexes (M = Tc, Re) (1, 2) were prepared by reaction of the precursor [M(N)Cl(2)(PPh(3))(2)] with ligand 2,2'-dimercaptodiethylamine [H(2)SNS = NH(CH(2)CH(2)SH)(2)] in refluxing dichloromethane/ethanol mixtures. In these compounds, 2,2'-dimercaptodiethylamine acts as a dianionic tridentate chelating ligand bound to the [M≡N](2+) group through the two π-donor deprotonated sulfur atoms and the protonated amine nitrogen atom. Triphenylphosphine completes the coordination sphere, acting as a monodentate ligand. [M(N)(NS(2))(PPh(3))] complexes can assume two different isomeric forms depending on the syn and anti orientations of the hydrogen atom bound to the central nitrogen atom of the SNS ligand with respect to the M≡N moiety. X-ray crystallography of the syn isomer of complex 2 demonstrated that it has a distorted trigonal bipyramidal geometry with the nitrido group and the two sulfur atoms defining the equatorial plane, the phosphorus atom of the monophosphine and the protonated amine nitrogen of the tridentate ligand spanning the two reciprocal trans positions along the axis perpendicular to the trigonal plane. Synthesis of the analogous Tc derivatives with tris(2-cyanoethyl)phosphine, [Tc(N)(SNS)(PCN)] [(PCN = P(CH(2)CH(2)CN)(3)], required the preliminary preparation of the new precursor [Tc(N)(PCN)(2)Cl(2)](2) (3), which was prepared by reacting [n-NBu(4)][Tc(N)Cl(4)] with a high excess of PCN. The crystal structure of compound 3 consists of a noncrystallographic centrosymmetric dimer of Tc(V) nitrido complexes having an octahedral geometry. In this arrangement, the apical positions are occupied by two tris(2-cyanoethyl)phosphine groups and the equatorial positions by the nitrido group whereas the two Cl(-) anions and one cyano ligand belong to the other octahedral component of the dimer. By reacting the new precursor [Tc(N)(PCN)(2)Cl(2)](2) with the ligand H(2)SNS the complex [Tc(N)(SNS)(PCN)] (5) was finally obtained in acetonitrile solution. The new Tc(III) complex trans-[Tc(PCN)(2)Cl(4)][n-NBu(4)] (4) was also isolated from the reaction solution used for preparing complex 3 as side product and characterized by X-ray diffraction. The crystal structure of 4 consists of independent trans-[TcCl(4)(PCN)(2)](-) anions situated on crystallographic centers of symmetry and tetrabutylammonium cations in general positions.     

A high-resolution infrared spectroscopic investigation of the halogen atom-HCN entrance channel complexes solvated in superfluid helium droplets

Rotationally resolved infrared spectra are reported for the X-HCN (X = Cl, Br, I) binary complexes solvated in helium nanodroplets. These results are directly compared with those obtained previously for the corresponding X-HF complexes [J. M. Merritt, J. Küpper and R. E. Miller, Phys. Chem. Chem. Phys., 2005, 7, 67]. For bromine and iodine atoms complexed with HCN, two linear structures are observed and assigned to the (2)Sigma(1/2) and (2)Pi(3/2) ground electronic states of the nitrogen and hydrogen bound geometries, respectively. Experiments for HCN + chlorine atoms give rise to only a single band which is attributed to the nitrogen bound isomer. That the hydrogen bound isomer is not stabilized is rationalized in terms of a lowering of the isomerization barrier by spin-orbit coupling. Theoretical calculations with and without spin-orbit coupling have also been performed and are compared with our experimental results. The possibility of stabilizing high-energy structures containing multiple radicals is discussed, motivated by preliminary spectroscopic evidence for the di-radical Br-HCCCN-Br complex. Spectra for the corresponding molecular halogen HCN-X(2) complexes are also presented.     

Platinum(II)-mediated coupling reactions of acetonitrile with the exocyclic nitrogen of 9-methyladenine and 1-methylcytosine. Synthesis, NMR characterization, and X-ray structures of new azametallacycle complexes

The hydroxo complex cis-[L2Pt(mu-OH)]2(NO3)2, (L = PMePh2, 1a), in CH3CN solution, deprotonates the NH2 group of 9-methyladenine (9-MeAd) to give the cyclic trinuclear species cis-[L2Pt[9-MeAd(-H)]]3(NO3)3, (L = PMePh2, 2a), in which the nucleobase binds the metal centers through the N(1), N(6) atoms. In solution at room temperature, 2a slowly reacts with the solvent to form quantitatively the mononuclear azametallacycle cis-[L2PtNH=C(Me)[9-MeAd(-2H)]]NO3 (L = PMePh2, 3a), containing as anionic ligand the deprotonated form of molecule N-(9-methyl-1,9-dihydro-purin-6-ylidene)-acetamidine. In the same experimental conditions, the hydroxo complex with PPh3 (1b) forms immediately the insertion product 3b. Single-crystal X-ray analyses of 3a and 3b show the coordination of the platinum cation at the N(1) site of the purine moiety and to the N atom of the inserted acetonitrile, whereas the exocyclic amino nitrogen binds the carbon atom of the same CN group. The resulting six-membered ring is slightly distorted from planarity, with carbon-nitrogen bond distances for the inserted nitrile typical of a double bond [C(3)-N(2) = 1.292(7) Angstroms in 3a and 1.279(11) Angstroms in 3b], while the remaining CN bonds of the metallocycle are in the range of 1.335(8)-1.397(10) Angstroms. A detailed multinuclear 1H, 31P, 13C, and 15N NMR study shows that the nitrogen atom of the inserted acetonitrile molecule binds a proton suggesting for 3a,b an imino structure in solution. In DMSO and chlorinated solvents, 3a slowly releases the nitrile reforming the trinuclear species 2a, whereas 3b forms the mononuclear derivative cis-[L2Pt[9-MeAd(-H)]]NO3 (L = PPh3, 4b), in which the adeninate ion chelates the metal center through the N(6) and N(7) atoms. Complex 4b is quantitatively obtained when 1b reacts with 9-MeAd in DMSO and can be easily isolated if the reaction is carried out in CH(2)Cl(2). In CH(3)CN solution, at room temperature, 4b slowly converts into 3b indicating that the insertion of acetonitrile is a reversible process. A similar metal-mediated coupling reaction occurs when 1a,b react with 1-methylcytosine (1-MeCy) in CH(3)CN. The resulting complexes, cis-[L(2)PtNH=C(Me)[1-MeCy(-2H)]]NO3, (L = PMePh2, 5a and PPh3, 5b), contain the deprotonated form of the ligand N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine. The X-ray analysis of 5a shows the coordination of the metal at the N(3) site of the pyrimidine cycle and to the nitrogen atom of the acetonitrile, with features of the six-membered metallocycle only slightly different from those found in 3a and 3b. In CD3CN/CH3(13)CN solution complexes 5a,b undergo exchange of the inserted nitrile, while in DMSO or chlorinated solvents they irreversibly release CH3CN to form species not yet fully characterized. No insertion of CH3CN occurs when the hydroxo complexes are stabilized by PMe3 and PMe2Ph.     

The dynamical role of solvent on the ICN photodissociation reaction: connecting experimental observables directly with molecular dynamics simulations

The ICN photodissociation reaction is the prototype system for understanding energy disposal and curve crossing in small molecule bond-breaking. The wide knowledge base on this reaction in the gas phase makes it an excellent test case to explore and understand the influence of a liquid solvent on the photo-induced reaction dynamics. Molecular dynamics simulations that include surface-hopping have addressed numerous aspects of how the solvent should influence non-adiabatic transitions and energy flow and ultimately determine product branching for this reaction system. In this paper, we report femtosecond transient absorption work directly combined with new molecular dynamics simulations that make direct connection with the spectroscopic observables. The full spectral evolution after initiating ICN photodissociation at 266 nm in water and ethanol is recorded with unprecedented time resolution, fast enough to see the nascent products emerge before interacting with the solvent cage. Use of a 266 nm pump maximizes the probability of subsequent caging on the upper diabat while launching large rotational energy release for trajectories emerging on the lower diabat. The 2D dataset yields a map of the different products and how they interconvert. In particular, information on the branching ratio and spectral evolution of the product bands is revealed as the products relax their electronic and rotational degrees of freedom. An evolution from rotationally hot gas-phase like CN (sharp band, at 390 nm) to equilibrated and solvated CN radicals (broad, at 326 nm in water and 415 nm in ethanol) is clearly observed in both solvents, and signals assignable to I* are also captured. The non-adiabatic molecular dynamics simulations focus on identifying when trajectories curve cross, filtering the trajectory ensemble into spectroscopically distinct sub-populations and analyzing the rotational energy for the CN product population. The experimental results, taken together with the MD simulations, establish the initial surface crossing probability and suggest multiple passes through the curve crossing region determine the final product yields and provide a source of freshly torqued CN radicals that continues to top up the population of rotationally hot photoproduct over the first few picoseconds.     

Cyanides,Isocyanides, and Hydrides of Zn,Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H-atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to ¹³C and ¹⁵N isotopic substitution for comparison with experimental data.     

Precursors to water-soluble dinitrogen carriers. Synthesis of water-soluble complexes of iron(II) containing water-soluble chelating phosphine ligands of the type 1,2-bis(bis(hydroxyalkyl)phosphino)ethane

The reactions of the water-soluble chelating phosphines 1,2-bis(bis(hydroxyalkyl)phosphino)ethane (alkyl = n-propyl, DHPrPE; n-butyl, DHBuPE; n-pentyl, DHPePE) with FeCl(2).4H(2)O and FeSO(4).7H(2)O were studied as routes to water-soluble complexes that will bind small molecules, dinitrogen in particular. The products that form and their stereochemistry depend on the solvent, the counteranion, and the alkyl chain length on the phosphine. In alcoholic solvents, the reaction of FeCl(2).4H(2)O with 2 equiv of DHBuPE or DHPePE gave trans-Fe(L(2))(2)Cl(2). The analogous reactions in water with DHBuPE and DHPePE gave only cis products, and the reaction of FeSO(4).7H(2)O with any of the phosphines gave only cis-Fe(L(2))(2)SO(4). These results are interpreted as follows. The trans stereochemistry of the products from the reactions of FeCl(2).4H(2)O in alcohols is suggested to be the consequence of the trans geometry of the Fe(H(2)O)(4)Cl(2) complex, i.e., substitution of the water molecules by the phosphines retains the geometry of the starting material. The formation of cis-Fe(DHPrPE)(2)Cl(2) is an exception to this result because the coordination of two -OH groups forms two six-membered rings, as shown in the X-ray structure of the molecule. DHBuPE and DHPePE reacted with FeSO(4).7H(2)O in water to initially yield cis-Fe(P(2))(2)SO(4) compounds, but subsequent substitution reactions occurred over several hours to give sequentially trans-Fe(DHBuPE)(2)(H(2)O)(SO(4)) and then trans-[Fe(DHBuPE)(2)(H(2)O)(2)]SO(4). The rate constants and activation reactions for these aquation reactions were determined and are consistent with dissociatively activated mechanisms. The cis- and trans-Fe(L(2))(2)X (X = (Cl)(2) or SO(4)) complexes react with N(2), CO, and CH(3)CN to yield trans complexes with bound N(2), CO, or CH(3)CN. The crystal structures of the cis-Fe(DHPrPE)(2)SO(4), trans-Fe(DHPrPE)(2)(CO)SO(4), trans-Fe(DHBuPE)(2)Cl(2), trans-[Fe(DHBuPE)(2)(CO)(Cl)][B(C(6)H(5))(4)], trans-Fe(DMeOPrPE)(2)Cl(2), trans-Fe(DMeOPrPE)(2)Br(2), and trans-[Fe(DHBuPE)(2)Cl(2)]Cl complexes are reported. As expected from using water-soluble phosphines, the complexes reported herein are water soluble (generally greater than 0.5 M at 23 degrees C).     

The reaction of nitrogen atoms with methyl radicals: Are spin-forbidden channels important?

A computational study of the N(4S) + CH3 reaction has been carried out. The reactants approach through an attractive potential surface leading to an intermediate, H3CN, whose formation does not involve any barrier. In agreement with the experimental results, the dominant channel for this reaction is H2CN+H. The theoretically estimated rate coefficient for the overall process at 298 K is 9.1 x 10(-12) cm3 s(-1) molecule(-1), which is nearly 1 order of magnitude lower than the experimental result, but also much larger than those computed for the reactions of ground-state nitrogen atoms with halomethyl radicals. The analysis of the singlet potential energy surface, and the corresponding computational kinetic study, shows that for the reaction of excited nitrogen atoms with methyl radicals, the preferred product from the kinetic point of view is also H2CN+H, but in this case production of HCN is significant (with branching ratios around 0.185). According to our calculations, spin-forbidden processes are highly unlikely for the N(4S) + CH3 reaction. However, further evolution of the preferred products, H2CN+H, might explain the experimental observation of hydrogen cyanide as a minor product in this reaction.     

Factors affecting the catalytic epoxidation of olefins by iron porphyrin complexes and H2O2 in protic solvents

The catalytic epoxidation of cyclohexene by iron(III) porphyrin complexes and H2O2 has been investigated in alcohol solvents to understand factors affecting the catalyst activity in protic solvents. The yields of cyclohexene oxide and the Fe(III/II) reduction potentials of iron porphyrin complexes were significantly affected by the protic solvents, and there was a close correlation between the product yields and the reduction potentials of the iron porphyrin catalysts. The role of alcohol solvents was proposed to control the electronic nature of iron porphyrin complexes that determines the catalyst activity in the epoxidation of olefins by H2O2. We have also demonstrated that an electron-deficient iron porphyrin complex can catalyze the epoxidation of olefins by H2O2 under conditions of limiting substrate with high conversion efficiency in a solvent mixture of CH3OH and CH2Cl2.     

卤代苯溶剂对镓咔咯配合物光物理性质的影响:外重原子效应

利用紫外-可见光谱、稳态和时间分辨荧光光谱以及飞秒瞬态吸收光谱探测了不同卤代苯溶剂对三种五氟苯基取代的镓咔咯(1-Ga、2-Ga、3-Ga)光物理性质的影响,结果表明卤代苯溶剂的色散力对于镓咔咯电子光谱吸收峰位置的影响起着主要作用;溶剂外重原子效应能显著降低镓咔咯的荧光量子产率。飞秒瞬态吸收光谱表明,光激发下,镓咔咯与卤代苯溶剂之间可发生电子转移反应,溶剂的重原子效应可以减缓电荷分离态复合物电荷重组速率。