黄嘌呤异构体质子转移异构化反应机理的理论研究

采用量子化学密度泛函B3LYP/6-31G(d,p)和M06-2X/6-311++G(d,p)方法对黄嘌呤两种酮式异构体X(1,3,7)与X(1,3,9)间质子转移引起的互变异构反应机理进行了计算研究,获得了异构化反应过程的反应焓﹑活化吉布斯自由能和质子转移反应的速率常数等参数。水相计算采用极化连续(PCM)模型。结果表明,由于可能的氢迁移顺序差异,分子内由X(1,3,7)向X(1,3,9)异构化可能共有16条反应通道,涉及11个中间体和20个过渡态,其主反应通道速控步骤的活化吉布斯自由能为183.10k J/mol,速率常数为5.17×10-20s-1,其余各通道速控步骤活化吉布斯自由能均较高,而且整体水溶剂效应不利于质子转移的发生。     

Theoretical investigation on multinuclear NMR chemical shifts of some tris(trifluoromethyl)boron complexes

Tris(trifluoromethyl)boron complexes have unusual properties and may find applications in many fields of chemistry, biology, and physics. To gain insight into their NMR properties, the isotropic ¹¹B, ¹³C, and ¹⁹F NMR chemical shifts of a series of tris(trifluoromethyl)boron complexes were systematically studied using the gauge‐included atomic orbitals (GIAO) method at the levels of B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G* and B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p). Solvent effects were taken into account by polarizable continuum models (PCM). The calculated results were compared with the experimental values. The reason that the structurally inequivalent fluorine atoms in a specific species give a same chemical shift in experimental measurements is attributed to the fast rotation of CF₃ group around the B? C(F₃) bond because of the low energy barrier. The calculated ¹¹B, ¹³C(F₃), and ¹⁹F chemical shifts are in good agreement with the experimental measurements, while the deviations of calculated ¹³C(X, X = O, N) chemical shifts are slightly large. For the latter, the average absolute deviations of the results from B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p) are smaller than those from B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G*, and the inclusion of PCM reduces the deviation values. The calculated ¹⁹F and ¹¹B chemical shieldings of (CF₃)₃BCO are greatly dependent on the optimized structures, while the influence of structural parameters on the calculated ¹³C chemical shieldings is minor. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical NMR spectroscopic analysis of the intramolecular proton transfer mechanism in ortho-hydroxyaryl (Un-)substitued Schiff bases

Both NMR spectroscopic parameters are calculated as a function of the distance d(N-H) of the O...H...N subsystem of (un- or Z-) substituted ortho-hydroxyaryl Schiff bases, with Z = 4-OMe and 5-Cl. Typical patterns for NMR J couplings and magnetic shieldings, sigma(N) (or the chemical shift delta(N)), are obtained showing that they are reliable sensors from which one can get a deeper insight on the intramolecular proton transfer mechanism. An inflection point is found by representing each NMR spectroscopic parameter as a function of d(N-H) or when the correlation between both parameters is depicted. The analysis of these (cubic) functions shows whether the proton is bound to the oxygen or to the nitrogen atom or is shared by both atoms. In line with these findings, it is possible to predict the position of the proton in the bridge. These theoretical findings are supported by previous experimental measurements. It is shown that nitrogen chemical shift is quite sensitive to substituent effects though (1) J( (15)NH) is not. This last parameter depends on d(NH). When correlating both spectroscopic parameters, a previous delta(N) vs (1) J( (15)NH) linear dependence is generalized to a cubic dependence which seems to be more reliable. Calculations are based on two state of the art methodologies: DFT-B3LYP and polarization propagators at second order of approach (SOPPA) with large enough basis sets.     

Incorporation of aluminum in the calcium silicate hydrate (C-S-H) of hydrated portland cements: a high-field 27Al and 29Si MAS NMR investigation

The calcium silicate hydrate (C-S-H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO(2) solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by (27)Al and (29)Si MAS NMR spectroscopy. (27)Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the (27)Al isotropic chemical shift (delta(iso)) and quadrupolar product parameter (P(Q) = C(Q)) for tetrahedrally coordinated Al incorporated in the C-S-H phase and for a pentacoordinated Al site. The latter site may originate from Al(3+) substituting for Ca(2+) ions situated in the interlayers of the C-S-H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (delta(iso) = 5.0 ppm and P(Q) = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C-S-H phase has been quantitatively determined from (27)Al MAS spectra at 14.09 T and indirectly observed quantitatively in (29)Si MAS NMR spectra by the Q(2)(1Al) resonance at -81.0 ppm. A linear correlation is observed between the (29)Si MAS NMR intensity for the Q(2)(1Al) resonance and the quantity of Al incorporated in the C-S-H phase from (27)Al MAS NMR for the different samples of hydrated wPc. This correlation supports the assignment of the resonance at delta(iso)((29)Si) = -81.0 ppm to a Q(2)(1Al) site in the C-S-H phase and the assignment of the (27)Al resonance at delta(iso)((27)Al) = 74.6 ppm, characterized by P(Q)((27)Al) = 4.5 MHz, to tetrahedrally coordinated Al in the C-S-H. Finally, it is shown that hydration of wPc in a NaAlO(2) solution results in a C-S-H phase with a longer mean chain length of SiO(4) tetrahedra and an increased quantity of Al incorporated in the chain structure as compared to the C-S-H phase resulting from hydration of wPc in water.     

Theoretical investigation on transformation of Cr(II) to Cr(V) complexes bearing tetra-N-heterocyclic carbene and group transfer reactivity

The mechanism for the transformation among a series of Cr(II) to Cr(V) complexes bearing tetra-N-heterocyclic carbene macrocycle is investigated. The oxidation and aziridination of Cr(II) monomer are studied by local density functional M06L. The former generates Cr(IV) oxo and further oxidizes to cationic Cr(V) oxo. The latter proceeds via two paths with different multiplicity, forming Cr(IV) imide. The Cr(IV) oxo cannot transfer its oxygen atom neither to phosphine nor to alkene because of the high energy barrier and endothermic process. The group transfer reactions are explored for Cr(V) oxo and Cr(IV) imide. The doublet Cr(V) oxo can also exist as quartet Cr(IV)-oxyl radical and promote oxygen transfer to phosphine, resulting in phosphine oxide in one exergonic step. The macrocyclic ligand effect of imido group transfer from Cr(IV) imide is verified by Multiwfn analysis. For 18-ringed imide, the matched orbital type and same-phase overlap reduce the barrier of its 16-ringed analog and facilitate the formation of phosphorus imine.     

Triphenylamine-substituted 2-pyridyl-1,2,3-triazole copper(I) complexes: an experimental and computational investigation

Abstract

Four new heteroleptic copper(I) complexes bearing either 2-pyridyl-1,2,3-triazole (pytri) or the related triphenylamine (TPA) substituted (TPA-tripy) ligands and the ancillary ligands 6,6′-dimesityl-2,2′-bipyridine (diMesbpy) or bis[(2-diphenylphosphino)phenyl] ether (POP) were synthesized in good yields (75-95%). All the complexes were extensively characterized using nuclear magnetic resonance (NMR) spectroscopies and electrospray ionization mass spectrometry (ESIMS) and in the case of the two pytri compounds the solid state structures were determined via X-ray crystallography. The pytri complexes showed MLCT absorption bands which shift from 433?nm for the diMesbpy complex to 347?nm for POP. TPA-pytri complexes introduce an ILCT band resulting in improved visible absorption (376?nm, 26,400 M^?1 cm^?1 for [Cu(TPA-pytri)(diMesbpy)](PF₆)). Emission from this ILCT state (470?nm, Φ?=?0.08) was red-shifted compared to the free ligand with negligible effects from ancillary ligands. Band assignments were confirmed with resonance Raman spectroscopy and TD-DFT calculations.     

(119)Sn MAS NMR and first-principles calculations for the investigation of disorder in stannate pyrochlores

The local structure and cation disorder in Y(2)Ti(2-x)Sn(x)O(7) pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using (119)Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how (119)Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift; an upfield shift resulting from the incorporation of the more polarizing Ti(4+) cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the (119)Sn spectra hinders the extraction of the detailed structural information previously obtained using (89)Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations.     

Theoretical and experimental investigation of the basicity of phosphino(silyl)carbenes

[reaction: see text] The reactivity of phosphino(trimethylsilyl)carbenes 1 with several organic acids has been examined in order to evaluate the pKa values of the conjugate acids. Carbenes 1 react efficiently with C-organic acids such as 1,3-dimesitylimidazolium chloride, phenylacetylene, acetonitrile, and acetyltrimethylsilane, which have pKa's in DMSO in the range 18-31. However, the reaction of the conjugate acids 1H+ with the anion perturbs the determination of the genuine basicity of 1. Theoretical calculations have been performed in order to quantify the basicity of phosphino(trimethylsilyl)carbenes 1 and to compare them with that of N-heterocyclic carbenes 2. The pKa of 1H+ in DMSO has been computed to be in the 23.0-23.4 range, so that 1 is not strong enough as a base to spontaneously deprotonate organic acids such as phenylacetylene, acetonitrile, or acetyltrimethylsilane. However, its conjugate acid 1H+ is a strong electrophile and easily reacts with the nucleophilic conjugate bases of these acids leading to the formation of the corresponding phosphorus ylides.     

Thermal investigation of diamine complexes of Zn(II) in the solid state

The complexes [Zn(en)₃]X2·n H₂O, where en = ethylenediamine, X = Cl^?, Br^? or $\text{1}\text{2}$SO^2?₄, n = 1 or 0.5, and [Zn(tn)₂]X₂·n H₂O, where tn=1,3-diaminopropane, X=Cl^?, Br^? or $\text{1}\text{2}$SO^2?₄, n = 0 or 0.25, have been synthesized and their thermal investigations carried out. The complexes were characterized by elemental analysis and IR spectral data. These complexes have been observed to decompose through several isolable as well as non-isolable complex species as intermediates during heating. [Zn(tn)₂]SO₄ undergoes solid-state phase transition in the temperature range 126–145°C. ZnenSO₄ and ZntnX₂ (X = Cl^?, Br^? or $\text{1}\text{2}$SO^2?₄) have been synthesized pyrolytically in the solid state from their corresponding mother diamine complexes. ZnenSO₄ and ZntnX₂ (X = Cl^?, Br^? or $\text{1}\text{2}$SO^2?₄) complexes decompose through non-isolable hemidiamine species. ZnX₂ (X = Cl^? or Br^?) complexes of tn undergo melting after formation of the monodiamine species. In contrast, the corresponding en complexes undergo melting at non-stoichiometric composition. Diamine (en or tn) is found to be bridging in all monodiamine (en or tn) complexes; whilst their mother complexes possess chelated en or tn. The thermal stability sequence of en and tn complexes of Zn(II) is ZnCl₂ < ZnBr₂ < ZnSO₄. ΔH values are reported for some steps of decomposition. Possible mechanistic paths have been reported for each step of decomposition.     

^29Si（^27Al）MAS NMR考察镧离子在LaDAlY型沸石中的作用

用~(29)Si(~(27)Al)MAS NMR和辅加乙酰丙酮(acac)处理样品的静态~(27)Al NMR研究了镧离子对脱铝Y型沸石(DAIY)骨架硅、铝和非骨架铝(EFAL)的影响.结果表明,~(29)Si MAS谱的化学位移及其形状不仅取决于连接[SiO_4]四面体的[AlO]-四面体数目,而且还与引入镧离子的量有关.镧离子的引入导致~(27)Al MAS谱的明显宽化和不对称形变.另外,还讨论了镧离子对非骨架铝的影响.     

Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g)

In this paper, we report the results of equilibrium pressure measurements designed to identify the volatile species in the Cr-O-H system and to resolve some of the discrepancies in existing experimental data. In addition, ab initio calculations were performed to lend confidence to a theoretical approach for predicting the thermochemistry of chromium-containing compounds. Equilibrium pressure data for CrO2(OH)2 were measured by the transpiration technique for the reaction 0.5Cr2O3(s) + 0.75O2(g) + H2O(g) = CrO2(OH)2(g) over a temperature range of 573 to 1173 K at 1 bar total pressure. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the condensate in order to quantify the concentration of Cr-containing volatile species. The resulting experimentally measured thermodynamic functions are compared to those computed using B3LYP density functional theory and the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)].     

In search of excited-state proton transfer in the lumichrome dimer in the solid state: theoretical and experimental approach

Quantum chemical density functional theory (DFT) calculations and spectral data were employed to investigate the possibility of the excited-state double proton transfer (ESDPT) in lumichrome crystals. The calculations in a lumichrome dimer predict a transfer of a proton in the first excited state, leading to a cation-anion pair. The presently reported X-ray structure of 1,3-dimethyllumichrome and its complex solid-state luminescence indicate that also in this molecule intermolecular hydrogen bonds might be involved in the photophysics. The long-wavelength emission in lumichrome crystals peaked at 530 nm is attributed to excited-state proton transfer, whereas a wider emission band in methylated lumichrome derivatives peaked at 560 nm is attributed to ions formed upon photoexcitation of the crystals.     

水杨醛亚胺钒（III）化合物催化乙烯聚合反应机理

钒系烯烃聚合催化剂在工业上有着不可替代的位置,它可用于制备高活性窄分布的聚合物、乙烯与α-烯烃共聚物和间规聚丙烯等。但由于实验手段难以确定钒催化剂活性物种的结构,进一步对催化机理的确认及催化剂结构的改进十分困难。本文运用密度泛函方法对水杨醛亚胺钒配合物催化乙烯聚合的活性物种结构进行了理论研究。对多种活性物种模型的比较研究结果表明,对此催化反应最有利的活性物种为中性双金属物种a1, a1结构中包含两个连接铝原子与钒中心的氯桥结构。研究同时表明,助催化剂AlEt2Cl的存在不仅加速了钒配合物前体的烷基化反应,同时其对活性物种a1结构中氯桥的形成至关重要。最后还研究了该催化体系的链终止反应机理。     

Theoretical investigation on electron transport through an organic molecule: effect of the contact structure

Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.     

Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states

The ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5-hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP) are studied at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level. The calculated results show that the PES of GSIPT for HNA, HNQ and HAQ exhibit a single minimum in the enol zone, while for 7-HIN, DHNQ and DHP exhibit a double minimum and a high barrier between the two minima. The barrierless ESIPT for HNA is predicted, however, the PES of ESIPT for HNQ, HAQ, 7HIN, DHNQ and DHP exhibit a high barrier in the S₁ tautomerism.     

Thermal investigation of some heterocyclic ligand complexes of tin(IV) in the solid state

1,10-phenanthroline (phen), 2,2′-bipyridyl (bipy), pyridine (py) and 4-picoline (4-pic) complexes of dibutyltindichloride (Bu₂SnCl₂) and dimethyltindichloride (Me₂SnCl₂) were synthesized. The complexes were characterized with the help of elemental analyses, IR spectra and thermal analyses. The complexes were found to have the compositions [Bu₂SnCl₂·phen], [Bu₂SnCl₂·bipy], [Me₂SnCl₂·phen], [Me₂SnCl₂·bipy], [Me₂SnCl₂·2py] and [Me₂SnCl₂·2(4-pic)]·H₂O. All these complex compounds appeared to posses octahedral structures. Thermodynamic parameters, such as activation energyE _a ^* enthalpy change ΔH and entropy change ΔS, for the dehydration and sublimation of the complexes were evaluated using some standard methods.     

Water formation reaction on Pt(111): role of the proton transfer

The catalytic water formation reaction on Pt(111) was investigated by kinetic Monte Carlo simulations, where the interaction energy between reaction species and the high mobility of H(2)O molecule was considered. Results obtained clearly reproduce the scanning tunneling microscopy images which show that the reaction proceeds via traveling the reaction fronts on the O-covered Pt(111) surface by creating H(2)O islands backwards. The reaction front is a mixed layer of OH and H(2)O with a (square root 3 x square root 3)R30(o) structure. Coverage change during the reaction is also reproduced in which the reaction consists of three characteristic processes, as observed by the previous experiments. The simulation also revealed that the proton transfer from H(2)O to OH plays an important role to propagate the water formation.     

Electrochromic properties of poly(alkoxy-terthiophenes): an experimental and theoretical investigation

Much effort has been made to synthesize novel compounds and enhance the optical properties of poly(terthiophenes). The electrochromic properties of poly(4,4′′-dimetoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDMMT) and poly(4,4′′-dipentoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDPMT) have been studied focusing on the differences in the alkoxy-group length. Theoretical calculations were employed to elucidate the structural and thermodynamic stability of the monomers and dimmers. The results showed that in this type of thiophenes large alkoxy chains assist positive charge dispersion through hyperconjugative effect. Thus, PDPMT is thermodynamically more stable than PDMMT in the oxidized state, leading to better electrochromic stability and optical memory.     

Tautomerism in bis(oxazolines): an experimental and theoretical study of proton transfer in 1,1-bis(4,4-dimethyl-1,3-oxazolin-2-yl)-ethane

The serendipitous isolation of an unusual protonated bis(oxazoline) prompted us to discuss the role of H⁺ on the kinetics and thermodynamics of the equilibrium between its ketimine and enamine forms. X-ray diffraction analysis revealed that the protonated bis(oxazoline) is in the Z-enamine form, the unipositive charge is counterbalanced by [(FeCl₃)₂O]²⁻ and [FeCl₄]⁻ anions. DFT calculation at the BP86/TZVP level showed that relative stability of enamine tautomer versus ketimine tautomer increases with the protonation of the nitrogen atom of the oxazolidine ring. At the same time, the barrier energy of tautomerism decreases.