CaF_2微晶玻璃的一步法制备及Eu~(3+)离子探针特性

以组成为n(SiO_2)∶n(Al2O3)∶n(CaO)∶n(CaF_2)∶n(NaF)∶n(B_2O_3)=40∶20∶10∶10∶15∶5的微晶发光玻璃为基质,采用一步析晶法制备了CaF_2析晶相.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、X射线能量散射谱仪(EDS)和荧光分光光度计等对样品结构、组成及光谱性能进行分析,探讨了Eu3+掺杂浓度和析晶温度对微晶玻璃发光性能的影响.实验结果表明,在850℃下处理可获得分布均匀、粒径尺寸为200 nm的CaF_2析晶相,微晶玻璃的发光强度是基质玻璃的1.7倍.微晶玻璃的发射光谱在590,614,652和700 nm出现发射峰,分别对应Eu3+的5D0-7FJ(J=1,2,3,4)跃迁.通过对5D0-7F1和5D0-7F2跃迁强度的分析以及Judd-Ofelt理论参数Ω2值的计算可知Eu3+周围晶体场在析晶前后对称性发生变化.机理分析表明,析晶处理后Eu3+从高声子能量的Si-O环境进入低声子能量的Ca-F环境中,说明Eu3+可作为荧光探针研究微晶玻璃晶体结构的变化.     

Formation constants beta(2) of calcium and magnesium fluorides at 25 degrees C

Until now, scientific work on the reactions of the complexation in water of fluoride ions with calcium or magnesium ions considered only soluble order 1 complexes (CaF(+) and MgF(+)). The precipitation of the compounds CaF(2) and MgF(2) did not take into account the preliminary formation in solution of these order 2 complexes (CaF(2)(0) and MgF(2)(0)). We therefore studied their formation using a direct potentiometric method in the F(-) ion-selective electrode at 25 degrees C. This experiment, together with a computerized thermodynamic study, allowed us to determine the values of formation constants at ionic strength equal to zero: log beta (CaF (2)(0))=5.7 and log beta (CaF (2)(0))=3.2 , as well as limiting solubilities: S(CaF(2))=10(-4.8) and S(MgF(2))=10(-5) mol l(-1). Then, some examples were studied showing the necessity to take into account the formation constant of CaF(2): the supersaturation of solutions, the validity of results from the fluoride selective electrode, the difference between released fluoride in distilled water and in artificial saliva in restorative dentistry, the fluoride concentration in hard waters and its health consequences.     

Dynamics of the F atom reaction with propene

The F+C2H3CH3 reaction has been investigated using the crossed molecular beam technique. Three reaction channels have been observed in this reaction: H+C3H5F, CH3+C2H3F, and HF+C3H5. Time-of-flight spectra as well as product laboratory angular distributions have been measured for the HF, C2H3F, and C3H5F products from these three channels. Relative branching ratios of the three observed reaction channels have also been estimated. Experimental results indicate that these different channels exhibit significantly different reaction dynamics.     

A study of the radical-radical reaction dynamics of O(3P)+t-C4H9 --> OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.     

Unprecedented ROMP activity of low-valent rhenium-nitrosyl complexes: Mechanistic evaluation of an electrophilic olefin metathesis system

The reaction of [Re(H)(NO)2(PR3)2] complexes (1 a: R = PCy3; 1 b: R = PiPr3) with [H(OEt2)2][BAr(F)4] ([BAr(F)4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) in benzene at room temperature gave the corresponding cations [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b). The addition of phenyldiazomethane to benzene solutions of 2 a and 2 b afforded the moderately stable cationic rhenium(I)-benzylidene-dinitrosyl-bis(trialkyl)phosphine complexes 3 a and 3 b as [BAr(F)4]- salts in good yields. The complexes 2 a and 2 b catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights and over 80 % Z configuration of the double bonds in the chain backbone. However, these complexes do not show metathesis activity with acyclic olefins. The benzylidene derivatives 3 a and 3 b are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave the first hints of the initial formation of carbene complexes from [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) and norbornene. In a detailed mechanistic study ESI-MS/MS measurements provided further evidence that the carbene formation is initiated by a unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylide-carbene complex, capable of undergoing metathesis with alternating rhenacyclobutane formation and cycloreversion reactions ("ylide" route). However, even at an early stage the ROMP propagation route is expected to merge into an "iminate" route by attack by the ylide function on one of the N(NO) atoms followed by phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. The proposed mechanism is supported further by detailed DFT calculations.     

Troposelective substitutions in microsolvated systems.

The mechanism and the stereochemistry of the intracomplex "solvolysis" of the proton-bound complexes I(X)() between CH(3)(18)OH and (R)-(+)-1-aryl-ethanol (1(R)()(X)(); aryl = phenyl (X = H); pentafluorophenyl (X = F)) have been investigated in the gas phase in the 25-100 degrees C temperature range. The results point to intracomplex "solvolysis" as proceeding through the intermediacy of the relevant benzyl cation III(X)() (a pure S(N)1 mechanism). "Solvolysis" of I(H)() leads to complete racemization at T > 50 degrees C, whereas at T < 50 degrees C the reaction displays a preferential retention of configuration. Predominant retention of configuration is also observed in the intracomplex "solvolysis" of I(F)(). This picture is rationalized in terms of different intracomplex interactions between the benzylic ion III(X)() and the nucleophile/leaving group pair, which govern the timing of their reorientation within the electrostatic complex. The obtained gas-phase picture is discussed in the light of related gas-phase and solution data. It is concluded that the solvolytic reactions are mostly governed by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature of the solvent cage. Their rigid subdivision into the S(N)1 and S(N)2 mechanistic categories appears inadequate, and the use of their stereochemistry as a mechanistic probe can be highly misleading.     

Communication: non-adiabatic coupling and resonances in the F + H2 reaction at low energies

Quantum reactive scattering calculations on accurate potential energy surfaces predict that at energies below ～5 meV, the reaction of F atoms with H(2) is dominated by the Born-Oppenheimer (BO) forbidden reaction of the spin-orbit excited F((2)P(1∕2)) atom. This non-BO dominance is amplified by low-energy resonances corresponding to quasi-bound states of the HF(v = 3, j = 3) + H product channel. Neglect of non-adiabatic coupling between the electronic states of the F atom leads to a qualitatively incorrect picture of the reaction dynamics at low energy.     

ansa-Zirconocene ester enolates: synthesis, structure, reaction with organo-lewis acids, and application to polymerization of methacrylates

The synthesis, structural characterization, and abstraction chemistry of ansa-zirconocene ester enolate complexes relevant to the isospecific polymerization of methacrylates are reported. Reactions of rac-(EBI)ZrMe(OTf) and rac-(EBI)Zr(OTf)(2) [EBI = C(2)H(4)(Ind)(2)] with 1 and 2 equiv of lithium isopropylisobutyrate in toluene produce the first examples of ansa-zirconocene mono- and diester enolate complexes: rac-(EBI)ZrMe[OC(O(i)Pr)=CMe(2)] (1) and rac-(EBI)Zr[OC(O(i)Pr)=CMe(2)](2) (2) in 89% and 50% isolated yields, respectively. The reaction of 1 with B(C(6)F(5))(3) was investigated in six different organic solvents; in THF at ambient temperature, this reaction cleanly produces the isolable cationic ansa-zirconocene ester enolate complex rac-(EBI)Zr(+)(THF)[OC(O(i)Pr)=CMe(2)][MeB(C(6)F(5))(3)](-) (3) in quantitative yield. The analogous reaction of 1 with Al(C(6)F(5))(3) in toluene, however, proceeds through a proposed novel, intramolecular proton transfer process in which propylene is eliminated from the isopropoxy group, subsequently producing a carboxylate-bridged tight ion pair rac-(EBI)Zr(+)(Me)OC((i)Pr)OAl(C(6)F(5))(3)(-) (4). In addition to standard spectroscopic and analytical characterizations for the isolated complexes 1-4, complexes 2 and 4 have also been structurally characterized by X-ray diffraction studies. Polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA) has been investigated using complexes 1, 3, and 4. Both the isolated cationic 3 and neutral 1 (the latter combined with B(C(6)F(5))(3) in situ) are highly active (10 min for quantitative MMA conversion) and highly isospecific ([mm] > 95% for PMMA; [mm] > 99% for PBMA) via enantiomorphic-site control, producing polymethacrylates with extremely narrow molecular weight distributions (M(w)/M(n) = 1.03). The aluminate complex 4, however, produces syndiotactic PMMA predominantly via chain-end control.     

SN2-like reaction in hydrogen-bonded complexes: a theoretical study

S(N)2-like reactions in hydrogen-bonded complexes have been investigated in this paper at a correlated MP2(full)/6-311++G(3df,3pd) level, employing FH...NH(3)...HF and ClH...NH(3)...HCl as model systems. The unconventional F(Cl)-H...N noncovalent bond and the conventional F(Cl)-H...N hydrogen bond can coexist in one complex which is taken as the reactant of the S(N)2-like reaction. The S(N)2-like reaction occurs along with the inversion of NH(3) and the interconversion of the unconventional F(Cl)-H...N noncovalent bond and the conventional F(Cl)-H...N hydrogen bond. In comparison with that of the isolated NH(3), the inversion barriers of the two complexes both are significantly reduced. The effect of carbon nanotube confinement on the inversion barrier is also discussed.     

Hydrogen bonded complexes and the HF vibrational energy distributions from the reaction of F atoms with NH2 and NH3

The gas phase reactions of fluorine atoms with amino radicals and ammonia molecules: F(²P)+NH₂(²B₁) → HF(¹Σ⁺)+NH(³Σ⁻) and F(²P)+NH₃(¹A₁) → HF(¹Σ⁺)+NH₂(²B₁) produce hydrogen fluoride with very different primary vibrational energy distributions as determined by low-pressure chemiluminescence studies. The reaction with NH₂ yields HF with an inverted primary vibrational energy distribution, P(v′=1:2:3:4)=0.23:0.68:0.08:0.01. The HF from the reaction with ammonia is cold (non-inverted), P(v′=1:2)=0.60:0.40. Recent experimental work on these reactions is critically assessed and some discrepancies between low-pressure chemiluminescence results and fast-flowing afterglow studies are resolved. The results of high-level ab initio calculations (up to 6–311G^** CISD) on reactants, products, and the hydrogen bonded complexes FH … NH and FH … NH₂ in the exit channels are reported. The most reliable of the computations predict that FH … NH₂ is significantly more bound than FH … NH (8.1 versus 4.1 kcal mol⁻¹ in comparison with products at the 6-311G^** MP2 level).Also, the calculated vibrational frequencies for the two hydrogen bonded complexes indicate that the FH stretch and NH₂ asymmetric stretch are much closer in frequency in FH … NH₂ than are the FH and NH stretches in FH … NH. The strong interaction and the close match of vibrational frequencies in the FH … NH₂ case both will lead to fast internal vibrational relaxation (IVR) of the reaction exoergicity from the FHN bonds, where it is released, to the NH₂ fragment in the F/NH₃ reaction. Thus, the HF produced in this reaction is expected to have less vibrational excitation than that created in the F/NH₂ reaction, for which these IVR mechanisms are not as important, and simple direct abstraction dynamics are expected.     

Probing state-to-state reaction dynamics using H-atom Rydberg tagging time-of-flight spectroscopy

In the last decade or so, the H-atom Rydberg tagging time-of-flight (HRTOF) technique has made a significant impact in the study of state-to-state reaction dynamics, and especially in the study of transition state dynamics of elementary chemical reactions and quantum state resolved dynamics of molecular photodissociation of important molecules. In this perspective, we will discuss mainly the state-to-state dynamics of three important elementary reactions: H + H(2), O((1)D) + H(2) and F + H(2) that have been studied in our laboratory in recent years using the HRTOF method. In addition, we will also mention briefly the experimental results of other reactive systems. In the end, we will also present a brief research outlook in the study of molecular reaction dynamics using this powerful experimental method.     

Molecular beam studies of the F atom reaction with propyne: site specific reactivity

The dynamics of the F atom reaction with propyne (CH(3)CCH) has been investigated using a universal crossed molecular beam apparatus. Two reaction channels have been clearly observed: H+C(3)H(3)F and HF+C(3)H(3). The substitution of F for H occurs mainly via a complex formation mechanism, producing reaction products with some contribution from a direct reaction mechanism. The HF product, however, appears to be dominantly forward scattered relative to the F atom beam direction, suggesting that the HF formation occurs via a direct abstraction mechanism. Branching ratios for the two observed reaction channels are also determined. The H formation channel is found to be the major reaction pathway, while the HF formation channel is also significant. From the measurements of DF versus HF products from the F atom reaction with deuterated propyne, the H atom picked up by the F atom in the reaction with normal propyne seems to come mostly from the CH(3) group. In addition, the H atom produced in the H atom formation channel appears to be mostly from the CH(3) group with some contribution from the CCH group.     

Ab initio study of the F + CH3NHNH2 reaction mechanism

The F + CH(3)NHNH(2) reaction mechanism is studied based on ab initio quantum chemistry methods as follows: the minimum energy paths (MEPs) are computed at the UMP2/6-311++G(d,p) level; the geometries, harmonic vibrational frequencies, and energies of all stationary points are predicted at the same level of theory; further, the energies of stationary points and the points along the MEPs are refined by UCCSD(T)/6-311++g(3df,2p). The ab initio study shows that, when the F atom approaches CH(3)NHNH(2), the heavy atoms, namely N and C atoms, are the favorable combining points. For the two N atoms, two prereaction complexes with C(s) symmetry are generated and there exists seven possible subsequent reaction routes, of which routes 1, 2, 5, and 7 are the main channels. Routes 1, 2, and 5 are associated with HF elimination, with H from the amino group or imido group, and route 7 involves the N-N bond break. Routes 3 and 6 with relation to HF elimination with H from methyl, and route 4 involved the C-N bond break, are all energetically disfavored. For the C atom, the attack of F results in the break of the C-N bond and the products are CH(3)F + NHNH(2). This route is very competitive.     

High-dimensional ab initio potential energy surfaces for reaction dynamics calculations

There has been great progress in the development of potential energy surfaces (PESs) for reaction dynamics that are fits to ab initio energies. The fitting techniques described here explicitly represent the invariance of the PES with respect to all permutations of like atoms. A review of a subset of dynamics calculations using such PESs (currently 16 such PESs exist) is then given. Bimolecular reactions of current interest to the community, namely, H + CH(4) and F + CH(4), are focused on. Unimolecular reactions are then reviewed, with a focus on the photodissociation dynamics of H(2)CO and CH(3)CHO, where so-called "roaming" pathways have been discovered. The challenges for electronically non-adiabatic reactions, and associated PESs, are presented with a focus on the OH* + H(2) reaction. Finally, some thoughts on future directions and challenges are given.     

State-to-state dynamics analysis of the F + CHD3 reaction: a quasiclassical trajectory study

An exhaustive state-to-state dynamics study was performed to analyze the F + CHD3 --> FD(nu', j') + CHD2(nu) gas-phase abstraction reaction. Quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were performed at different collision energies on an analytical potential energy surface (PES-2006) recently developed by our group. Whereas the CHD2 coproduct appears vibrationally and rotationally cold, most of the available energy appears as FD(nu') product vibrational energy, peaking at nu' = 2 and nu' = 3, with the population in the latter level growing as the energy increases. The excitation function rises from the threshold of the reaction and then levels off at higher energies, with the maximum contribution from the FD(nu' = 3) level. The state-specific FD(nu') scattering distributions correlated with the coproduct CHD2 in the nu4 = 2 and nu3 = 1 states, at different collision energies, show a steady change from backward to forward scattering as the energy increases. This similar behavior for the two coproduct vibrational states, nu4 = 2 and nu3 = 1, agrees qualitatively with the experimental measurements. Comparison with theoretical and experimental results for the isotopic analogues, F + CH4 and F + CD4, shows that the title reaction presents a direct mechanism, similar to the perdeuterated reaction, but contrasts with that of the F + CH4 reaction. These results for the dynamics of different isotopic variants, always in qualitative and sometimes in quantitative agreement with experiment, show the capacity of the PES-2006 surface to correctly describe the title reaction, even though there are differences that could be due to deficiencies of the PES but also to the known limitations of the classical treatment in the QCT method.     

Some comments on valence bond representations for the radical exchange reaction X*+R:Y-->X:R+Y*

Qualitative valence bond formulations by Hiberty and co-workers (Hiberty, P. C.; Megret, C.; Song, L.; Wu, W.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 2836) of mechanisms for the radical exchange reactions H*+F:H-->H:F+H* and F*+H:F-->F:H+F* are compared to a previously published formulation of the generalized radical exchange reaction X*+R:Y-->X:R+Y*. The former formulation uses covalent-ionic VB complexes, and the latter formulation, which is more general, involves the formation of reactant-like and product-like complexes at intermediate stages along the reaction coordinate.     

A quantum defect model for the s, p, d, and f Rydberg series of CaF

We present an improved quantum defect theory model for the "s," "p," "d," and "f" Rydberg series of CaF. The model, which is the result of an exhaustive fit of high-resolution spectroscopic data, parameterizes the electronic structure of the ten ("s"Σ, "p"Σ, "p"Π, "d"Σ, "d"Π, "d"Δ, "f"Σ, "f"Π, "f"Δ, and "f"Φ) Rydberg series of CaF in terms of a set of twenty μ(ll('))(Λ) quantum defect matrix elements and their dependence on both internuclear separation and on the binding energy of the outer electron. Over 1000 rovibronic Rydberg levels belonging to 131 observed electronic states of CaF with n? ≥ 5 are included in the fit. The correctness and physical validity of the fit model are assured both by our intuition-guided combinatorial fit strategy and by comparison with R-matrix calculations based on a one-electron effective potential. The power of this quantum defect model lies in its ability to account for the rovibronic energy level structure and nearly all dynamical processes, including structure and dynamics outside of the range of the current observations. Its completeness places CaF at a level of spectroscopic characterization similar to NO and H(2).     

Uncovering an oxide ion substitution for the OH− + CH3F reaction

Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH⁻ + CH₃F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (S_N2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH₃O⁻ products. This exothermic reaction pathway occurs via the CH₃OH⋯F⁻ deep potential well of the S_N2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH⁻ + CH₃F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.

Reaction dynamics simulations on a high-level ab initio analytical potential energy surface reveal a novel oxide ion substitution channel for the OH⁻ + CH₃F reaction.