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1.
Liu Y Bersuker IB Garcia-Fernandez P Boggs JE 《The journal of physical chemistry. A》2012,116(28):7564-7570
It is shown that the pseudo Jahn-Teller effect (PJTE) in combination with ab initio calculations explains the origin of instability of the planar configuration of tetrafluorocyclobutadiene, C(4)F(4), with respect to a puckered structure and square-to-rectangle distortion of the carbon ring, and rationalizes its difference from the planar-rectangular geometry of C(4)H(4) and nonplanar (puckered) structure of Si(4)H(4). The two types of instability and distortion of the high-symmetry D(4h) configuration in these systems emerge from the PJT coupling of the ground B(2g) state with the excited A(1g) term producing instability along the b(2g) coordinate (elongation of the carbon or silicon square ring), and with the excited E(g) term resulting in e(g) (puckering) distortion. A rhombic distortion b(1g) of the ring is also possible due to the coupling between excited A(1g) and B(1g) terms. For C(4)F(4), ab initio calculations of the energy profiles allowed us to evaluate the PJTE constants and to show that the two instabilities, square-to-tetragonal b(2g) and puckering e(g) coexist, thus explaining the origin of the observed geometry of this system in the ground state. The preferred cis-trans (e(g) type) puckering in C(4)F(4) versus trans-trans puckering (b(2u) distortion) in Si(4)H(4) follows from the differences in the energy gaps to their excited electronic E(g) and A(1u) terms causing different PJTE in these two cases. 相似文献
2.
We investigated the dynamics of isomerization and multi-channel dissociation of propenal (CH(2)CHCHO), methyl ketene (CH(3)CHCO), hydroxyl propadiene (CH(2)CH(2)CHOH), and hydroxyl cyclopropene (cyclic-C(3)H(3)-OH) in the ground potential-energy surface using quantum-chemical calculations. Optimized structures and vibrational frequencies of molecular species were computed with method B3LYP∕6-311G(d,p). Total energies of molecules at optimized structures were computed at the CCSD(T)∕6-311+G(3df,2p) level of theory. We established the potential-energy surface for decomposition to CH(2)CHCO + H, CH(2)CH + HCO, CH(2)CH(2)∕CH(3)CH + CO, CHCH∕CH(2)C + H(2)CO, CHCCHO∕CH(2)CCO + H(2), CHCH + CO + H(2), CH(3) + HCCO, CH(2)CCH + OH, and CH(2)CC∕cyclic-C(3)H(2) + H(2)O. Microcanonical rate coefficients of various reactions of trans-propenal with internal energies 148 and 182 kcal mol(-1) were calculated using Rice-Ramsperger-Kassel-Marcus and Variational transition state theories. Product branching ratios were derivable using numerical integration of kinetic master equations and the steady-state approximation. The concerted three-body dissociation of trans-propenal to fragments C(2)H(2) + CO + H(2) is the prevailing channel in present calculations. In contrast, C(3)H(3)O + H, C(2)H(3) + HCO and C(2)H(4) + CO were identified as major channels in the photolysis of trans-propenal. The discrepancy between calculations and experiments in product branching ratios indicates that the three major photodissociation channels occur mainly on an excited potential-energy surface whereas the other channels occur mainly on the ground potential-energy surface. This work provides profound insight in the mechanisms of isomerization and multichannel dissociation of the system C(3)H(4)O. 相似文献
3.
Varriale L Bhalla N Tonge NM Ellis AM Wright TG 《The Journal of chemical physics》2011,134(12):124304
Electronic spectra of LiNH(3) and its partially and fully deuterated analogues are reported for the first time. The spectra have been recorded in the near-infrared and are consistent with two electronic transitions in close proximity, the ?(2)E-X(2)A(1) and B(2)A(1)-X(2)A(1) systems. Vibrational structure is seen in both systems, with the Li-N-H bending vibration (ν(6)) dominant in the ?(2)E-X(2)A(1) system and the Li-N stretch (ν(3)) in the B(2)A(1)-X(2)A(1) system. The prominence of the 6(0)(1) band in the ?(2)E-X(2)A(1) spectrum is attributed to Herzberg-Teller coupling. The proximity of the B(2)A(1) state, which lies a little more than 200 cm(-1) above the ?(2)E state, is likely to be the primary contributor to this strong vibronic coupling. 相似文献
4.
The photodissociation dynamics of nitromethane (CH(3)NO(2)) starting at the S(3) excited state has been studied at the complete active space self-consistent field level of theory in conjunction with atomic natural orbital type basis sets. In addition, the energies of all the critical points and the energy profiles connecting them have been recomputed with the multiconfigurational second-order perturbation method. It is found that the key step in the reaction mechanism is a radiationless decay through an S(3)S(2) conical intersection. The branching space spanned by the gradient difference and nonadiabatic coupling vectors of this crossing point comprises dissociation into excited nitromethane plus singlet atomic oxygen [CH(3)NO(1A")+O((1)D)] and S(3)-->S(2) deactivation, respectively. Furthermore, deactivated nitromethane S(n (n<3)) can decompose in subsequent steps into CH(3)+NO(2), where NO(2) is generated at least in two different electronic states (1 (2)B(2) and 1 (2)A(1)). It is shown that formation of excited nitric oxide NO(A (2)Sigma) arises from CH(3)NO(1A") generated in the previous step. In addition, four crossings between singlet and triplet states are localized; however, no evidence is found for a relevant role of such crossings in the photochemistry of CH(3)NO(2) initiated at S(3) state in the gas phase. 相似文献
5.
Han Y Harlan CJ Stoessel P Frost BJ Norton JR Miller S Bridgewater B Xu Q 《Inorganic chemistry》2001,40(13):2942-2952
A series of oxo complexes, Re(O)X(diyne) (X = I, Me, Et), have been prepared from 2,7-nonadiyne and Re(O)I(3)(PPh(3))(2). Addition of B(C(6)F(5))(3) to Re(O)I(2,7-nonadiyne) (5) results in coordination of the oxo ligand to the boron. The protonation of Re(O)(X)(2-butyne)(2) and Re(O)(X)(2,7-nonadiyne)(2) with a variety of acids has been examined. With 5 and HBF(4)/Et(2)O, the ultimate product was [Re(CH(3)CN)(3)(I)(2,7-nonadiyne)](2+) (7). The conversion of 5 to 7 changes the conformation of the diyne ligand from a "chair" to a "boat" and shifts its propargylic protons considerably downfield in the (1)H NMR. The kinetics of the protonation of Re(O)I(2,7-nonadiyne) (5) by CF(3)SO(3)H in CH(3)CN have been monitored by visible spectroscopy, in a stopped-flow apparatus, and by low temperature (1)H NMR. Two second-order rate constants, presumably successive protonations, were observed in the stopped-flow, k(1) = 11.9 M(-)(1) s(-)(1) and k(2) = 3.8 M(-)(1) s(-)(1). Low temperature (1)H NMR spectroscopy indicated that the resulting solution contained a mixture of two doubly protonated intermediates X and Y, each of which slowly formed the product 7 via an acid-independent process. 相似文献
6.
A series of cationic gemini surfactants butanediyl-1,4-bis(dodecyldialkylammonium bromide), C(12)H(25)N(+)(C(m)H(2)(m)(+1))(2)C(4)H(8)N(+)(C(m)H(2)(m)(+1))(2)C(12)H(25)·2Br(-), where m=1, 2, 3, 4, referred to as C(12)C(4)C(12)(Me), C(12)C(4)C(12)(Et), C(12)C(4)C(12)(Pr), and C(12)C(4)C(12)(Bu), respectively, were synthesized, and their thermodynamic properties of micellization were studied by electrical conductivity measurements. There existed a minimum critical micelle concentration (cmc) in the curve of cmc versus temperature, and the temperature of the minimum of cmc (T(min)) increased with increasing the headgroup alkyl chain length. The values of log (cmc) depended linearly on carbon number of the alkyl chains, but that was not true for the carbon number of the headgroup substituents. The temperature dependence of cmc and degree of counterion association (β) were used to calculate the Gibbs free energy (Δ(mic)G°), enthalpies (Δ(mic)H°) and entropies (Δ(mic)S°) of micelle formation for these gemini surfactants, and well correlated enthalpy-entropy compensation was observed. The analyses showed C(12)C(4)C(12)(Me) and C(12)C(4)C(12)(Et) behaved similarly in terms of thermodynamics of micellization, but they behaved differently from C(12)C(4)C(12)(Pr) and C(12)C(4)C(12)(Bu), which could be ascribed to the hydrophobicity and the location of the headgroup alkyl chains in the aggregates. These initial results indicate the headgroup alkyl chain plays an important role in influencing the thermodynamic properties of gemini surfactants. 相似文献
7.
Alekseev VA van der Burgt PJ McCaffrey JG 《The journal of physical chemistry. A》2012,116(5):1333-1336
The I(2)(D'-A') luminescence in liquid C(7)F(16), C(8)F(18), C(9)F(20), and C(12)F(26) has been observed following irradiation with visible (within the I(2) X → B band region) and UV (λ < 300 nm) lasers. The band is shifted by ~2000 cm(-1) to the red relative to the gas phase and is significantly broader (fwhm ≈ 3000 cm(-1) vs 500 cm(-1) in the gas phase). Two-color excitation of other halogens in C(n)F(2n+2) solvents revealed similarly shifted and strongly broadened D'-A' bands of Br(2), IBr, and BrCl. The stability of the ion-pair states of halogens in room temperature liquids opens new experimental opportunities for condensed phase studies and may be of applied interest considering the increasing industrial applications of perfluorocarbons. 相似文献
8.
Arenas JF Otero JC Peláez D Soto J Serrano-Andrés L 《The Journal of chemical physics》2004,121(9):4127-4132
The doublet potential energy surfaces involved in the decomposition of the nitromethane radical anion (CH(3)NO(2) (-)) have been studied by using the multistate extension of the multiconfigurational second-order perturbation method (MS-CASPT2) in conjunction with large atomic natural orbital-type basis sets. A very low energy barrier is found for the decomposition reaction: CH(3)NO(2) (-)-->[CH(3)NO(2)](-)-->CH(3)+NO(2) (-). No evidence has been obtained on the existence of an isomerization channel leading to the initial formation of the methylnitrite anion (CH(3)ONO(-)) which, in a subsequent reaction, would yield nitric oxide (NO). In contrast, it is suggested that NO is formed through the bimolecular reaction: CH(3)+NO(2) (-)-->[CH(3)O-N-O](-)-->CH(3)O(-)+NO. In particular, the CASSCF/MS-CASPT2 results indicate that the methylnitrite radical anion CH(3)ONO(-) does not represent a minimum energy structure, as concluded by using density functional theory (DFT) methodologies. The inverse symmetry breaking effect present in DFT is demonstrated to be responsible for such erroneous prediction. 相似文献
9.
The IR spectrum of the fluoronium isomer of protonated fluorobenzene (F-C(6)H(6)F(+), phenylfluoronium) is recorded in the vicinity of the C-H and F-H stretch fundamentals to obtain the first structured spectrum of an isolated protonated aromatic molecule in the gas phase. Stable F-C(6)H(6)F(+) ions are produced via proton transfer from CH(5)(+) to fluorobenzene (C(6)H(5)F) in a supersonic plasma expansion. The F-C(6)H(6)F(+) spectrum recorded between 2,540 and 4,050 cm(-1) is consistent with a weakly bound ion-dipole complex composed of HF and the phenyl cation, HF-C(6)H(5)(+). The strongest transition occurs at 3,645 cm(-1) and is assigned to the F-H stretch (sigma(FH)). The antisymmetric C-H stretch of the two ortho hydrogen atoms, sigma(CH) = 3,125 cm(-1), is nearly unshifted from bare C(6)H(5)(+), indicating that HF complexation has little influence on the C-H bond strength of C(6)H(5)(+). Despite the simultaneous production of the more stable ring protonated carbenium isomers of C(6)H(6)F(+) (fluorobenzenium) in the electron ionization source, F-C(6)H(6)F(+) can selectively be photodissociated into C(6)H(5)(+) and HF under the present experimental conditions, because it has a much lower dissociation energy than all carbenium isomers. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311G(2df,2pd) basis support the interpretation of the experimental data and provide further details on structural, energetic, and vibrational properties of F-C(6)H(6)F(+), the carbenium isomers of C(6)H(6)F(+), and other weakly bound HF-C(6)H(5)(+) ion-dipole complexes. The dissociation energy of F-C(6)H(6)F(+) with respect to dehydrofluorination is calculated as D(0) = 4521 cm(-1) (approximately 54 kJ/mol). Analysis of the charge distribution in F-C(6)H(6)F(+) supports the notation of a HF-C(6)H(5)(+) ion-dipole complex, with nearly the whole positive charge of the added proton distributed over the C(6)H(5)(+) ring. As a result, protonation at the F atom strongly destabilizes the C-F bond in C(6)H(5)F. 相似文献
10.
The kinetics of dissociation of the mono, bis, and tris complexes of Tiron (1,2-dihydroxy-3,5-benzenedisulfonate) have been studied in acidic aqueous solutions in 1.0 M HClO(4)/NaClO(4), as a function of [H(+)] and temperature. In general, the kinetics can be explained by two reactions, (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) + H(+) (k(n), k(-n)) and (HO)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) (k(n)', k(-n)'), a rapid equilibrium, (H(2)O)Fe(L(n)H) right arrow over left arrow (H(2)O)Fe(L)(n) + H(+) (K(cn)), and the formation constant (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L)(n) + 2H(+). For n = 1, the reaction was observed at 670 nm, and at [H(+)] of 0.05-0.5 M at temperatures of 2.0, 14.0, 25.0, and 36.7 degrees C. For n = 2, the analogous conditions are 562 nm, at [H(+)] of 1.5 x 10(-3) to 1.4 x 10(-2) M at temperatures of 2.0, 9.0, and 14.0 degrees C. For n = 3, the conditions are 482 nm, at pH 4.5-5.7 in 0.02 M acetate buffer at temperatures of 1.8, 8.0, and 14.5 degrees C. The rate or equilibrium constants (25 degrees C) with DeltaH or DeltaH degrees (kcal mol(-1)) and DeltaS or DeltaS degrees (cal mol(-1) K(-1)) in brackets are as follows: for n = 1, k(1) = 2.3 M(-1) s(-1) (8.9, -27.1), k(-1) = 1.18 M(-1) s(-1) (4.04, -44.8), K(c1) = 0.96 M (-9.99, -33.6), K(f1) = 2.01 M (-5.14, -15.85); for n = 2, k(-2)/K(c2) = 1.9 x 10(7) (19.9, 41.5) and k(-2)'/K(c2) = 1.85 x 10(3) (1.4, -38.8) and a lower limit of K(c2) > 0.015 M; for n = 3, k(3) = 7.7 x 10(3) (15.8, 12.3), k(-3) = 1.7 x 10(7) (16.2, 28.9), K(c3) = 7.4 x 10(-5) M (4.1, -5.1), and K(f3) = 3.35 x 10(-8) (3.7, -21.7). From the variations in rate constants and activation parameters, it is suggested that the Fe(L)(2) and Fe(L)(3) complexes undergo substitution by dissociative activation, promoted by the catecholate ligands. 相似文献
11.
Yamada Y Maeda K Ohkubo K Karlin KD Fukuzumi S 《Physical chemistry chemical physics : PCCP》2012,14(27):9654-9659
The catalytic durability of an organic photocatalyst, 9-mesityl-10-methyl acridinium ion (Acr(+)-Mes), has been dramatically improved by the addition of [{tris(2-pyridylmethyl)amine}Cu(II)](ClO(4))(2) ([(tmpa)Cu(II)](2+)) in the photocatalytic oxygenation of p-xylene by molecular oxygen in acetonitrile. Such an improvement is not observed by the addition of Cu(ClO(4))(2) in the absence of organic ligands. The addition of [(tmpa)Cu](2+) in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu(II)](2+). In the photocatalytic oxygenation, a stoichiometric amount of H(2)O(2) formation was observed in the absence of [(tmpa)Cu(II)](2+), however, much less H(2)O(2) formation was observed in the presence of [(tmpa)Cu(II)](2+). The photocatalytic mechanism was investigated by laser flash photolysis measurements in order to detect intermediates. The reaction of O(2)˙(-) with [(tmpa)Cu(II)](2+) monitored by UV-vis spectroscopy in propionitrile at 203 K suggested formation of [{(tmpa)Cu(II)}(2)O(2)](2+), a transformation which is crucial for the overall 4-electron reduction of molecular O(2) to water, and a key in the observed improvement in the catalytic durability of Acr(+)-Mes. 相似文献
12.
Synthesis of phthalide derivatives using nickel-catalyzed cyclization of o-haloesters with aldehydes
Rayabarapu DK Chang HT Cheng CH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(12):2991-2996
The reaction of o-bromobenzoate (1 b) with benzaldehyde (2 a) in the presence of [NiBr(2)(dppe)] (dppe=1,2-bis(diphenylphosphino)ethane) and zinc powder in THF (24 hours, reflux temperature), afforded 3-phenyl-3H-isobenzofuran-1-one (3 a) in an 86 % yield. Similarly, o-iodobenzoate reacts with 2 a to give 3 a, but in a lower yield (50 %). A series of substituted aromatic and aliphatic aldehydes (2 b, 4-MeC(6)H(4)CHO; 2 c, 4-MeOC(6)H(4)CHO; 2 d, 3-MeOC(6)H(4)CHO; 2 e, 2-MeOC(6)H(4)CHO; 2 f, 4-CNC(6)H(4)CHO; 2 g, 4-(Me)(3)CC(6)H(4)CHO; 2 h, 4-C(6)H(5)C(6)H(4)CHO; 2 i, 4-ClC(6)H(4)CHO; 2 j, 4-CF(3)C(6)H(4)CHO; 2 k, CH(3)(CH(2))(5)CHO; 2 l, CH(3)(CH(2))(2)CHO) also underwent cyclization with o-bromobenzoate (1 b) producing the corresponding phthalide derivatives in moderate to excellent yields and with high chemoselectivity. Like 1 b, methyl 2-bromo-4,5-dimethoxybenzoate (1 c) reacts with tolualdehyde (2 b) to give the corresponding substituted phthalide 3 m in a 71 % yield. The methodology can be further applied to the synthesis of six-membered lactones. The reaction of methyl 2-(2-bromophenyl)acetate (1 d) with benzaldehyde under similar reaction conditions afforded six-membered lactone 3 o in a 68 % yield. A possible catalytic mechanism for this cyclization is also proposed. 相似文献
13.
rac-Bis[{(diphenylphosphino)ethyl}-phenylphosphino]methane (DPPEPM) reacts with iron(II) and ruthenium(II) halides to generate complexes with folded DPPEPM coordination. The paramagnetic, five-coordinate Fe(DPPEPM)Cl(2) (1) in CD(2)Cl(2) features a tridentate binding mode as established by (31)P{(1)H} NMR spectroscopy. Crystal structure analysis of the analogous bromo complex, Fe(DPPEPM)Br(2) (2) revealed a pseudo-octahedral, cis-α geometry at iron with DPPEPM coordinated in a tetradentate fashion. However, in CD(2)Cl(2) solution, the coordination of DPPEPM in 2 is similar to that of 1 in that one of the external phosphorus atoms is dissociated resulting in a mixture of three tridentate complexes. The chloro ruthenium complex cis-Ru(κ(4)-DPPEPM)Cl(2) (3) is obtained from rac-DPPEPM and either [RuCl(2)(COD)](2) [COD = 1,5-cyclooctadiene] or RuCl(2)(PPh(3))(4). The structure of 3 in both the solid state and in CD(2)Cl(2) solution features a folded κ(4)-DPPEPM. This binding mode was also observed in cis-[Fe(κ(4)-DPPEPM)(CH(3)CN)(2)](CF(3)SO(3))(2) (4). Addition of an excess of CO to a methanolic solution of 1 results in the replacement of one of the chloride ions by CO to yield cis-[Fe(κ(4)-DPPEPM)Cl(CO)](Cl) (5). The same reaction in CH(2)Cl(2) produces a mixture of 5 and [Fe(κ(3)-DPPEPM)Cl(2)(CO)] (6) in which one of the internal phosphines has been substituted by CO. Complexes 2, 3, 4, and 5 appear to be the first structurally characterized monometallic complexes of κ(4)-DPPEPM. 相似文献
14.
The synthesis of a range of alkyl/chloro-gallium alkoxide and amido/alkoxide compounds was achieved via a series of protonolysis and alcoholysis steps. The initial reaction involved the synthesis of [Me(Cl)Ga{N(SiMe(3))(2)}](2) (1) via methyl group transfer from the reaction of GaCl(3) with two equivalents of LiN(SiMe(3))(2). Reaction of 1 with varying amounts of ROH resulted in the formation of [Me(Cl)Ga(OR)](2) (2, R = CH(2)CH(2)OMe; 3, CH(CH(3))CH(2)NMe(2)), [Me(Cl)Ga{N(SiMe(3))(2)}(μ(2)-OR)Ga(Cl)Me] (4, R = CH(2)CH(2)NMe(2)), or [MeGa(OR)(2)] (5, R = CH(CH(3))CH(2)NMe(2)). Compound 4 represents an intermediate in the formation of dimeric complexes, of the type [Me(Cl)Ga(OR)](2), when formed from compound [Me(Cl)Ga{N(SiMe(3))(2)}](2). A methylgallium amido/alkoxide complex [MeGa{N(SiMe(3))(2)}(OCH(2)CH(2)OMe)](2) (6) was isolated when 2 was further reacted with LiN(SiMe(3))(2). In addition, reaction of 2 with HO(t)Bu resulted in a simple alcohol/alkoxide exchange and formation of [Me(Cl)Ga(O(t)Bu)](2) (7). In contrast to the formation of 1, the in situ reaction of GaCl(3) with one equivalent of LiN(SiMe(3))(2) yielded [Cl(2)Ga{N(SiMe(3))(2)}](2) in low yield, where no methyl group transfer has occurred. Reaction of alcohol with [Cl(2)Ga{N(SiMe(3))(2)}](2) was then found to yield [Cl(2)Ga(OR)](2) (8, R = CH(2)CH(2)NMe(2)), and further reaction of 8 with LiN(SiMe(3))(2) yielded the gallium amido alkoxide complex, [ClGa{N(SiMe(3))(2)}(OR)](2) (9, R = CH(2)CH(2)NMe(2)), similar to 6. The structures of compounds 4, 5, 7, and 8 have been determined by single-crystal X-ray diffraction. 相似文献
15.
The Co(III) complexes of N,N'-bis(2-mercaptophenyl)pyridine-2,6-dicarboxamide (PyPSH(4)), a designed pentadentate ligand with built-in carboxamide and thiolate groups, have been synthesized and studied to gain insight into the role of Cys-S oxidation in Co-containing nitrile hydratase (Co-NHase). Reaction of [Co(NH(3))(5)Cl]Cl(2) with PyPS(4)(-) in DMF affords the thiolato-bridged dimeric Co(III) complex (Et(4)N)(2)[Co(2)(PyPS)(2)] (1). Although the bridged structure is quite robust, reaction of (Et(4)N)(CN) with 1 in acetonitrile affords the monomeric species (Et(4)N)(2)[Co(PyPS)(CN)] (2). Oxidation of 2 with H(2)O(2) in acetonitrile gives rise to a mixture which, upon chromatographic purification, yields K(2)[Co(PyPSO(2)(OSO(2))(CN] (3), a species containing asymmetrically oxidized thiolates. The Co(III) metal center in 3 is coordinated to a S-bound sulfinate and an O-bound sulfonate (OSO(2)) group. Upon oxidation with H(2)O(2), 1 affords an asymmetrically oxidized dimer (Et(4)N)(2)[Co(2)(PyPS(SO(2)))(2)] (4) in which only the terminal thiolates are oxidized to form S-bound sulfinate groups while the bridging thiolates remain unchanged. The thiolato-bridge in 4 is also cleaved upon reaction with (Et(4)N)(CN) in acetonitrile, and one obtains (Et(4)N)(2)[Co(PyPS(SO(2)))(CN)] (5), a species that contains both coordinated thiolate and S-bound sulfinate around Co(III). The structures of 1-4 have been determined. The spectroscopic properties and reactivity of all the complexes have been studied to understand the behavior of the Co(III) site in Co-NHase. Unlike typical Co(III) complexes with bound CN(-) ligands, the Co(III) centers in 2 and 5 are labile and rapidly lose CN(-) in aqueous solutions. Since 3 does not show this lability, it appears that at least one thiolato sulfur donor is required in the first coordination sphere for the Co(III) center in such species to exhibit lability. Both 2 and 5 are converted to the aqua complexes [Co(PyPS)(H(2)O)](-) and [Co(PyPS(SO(2))(H(2)O)](-) in aqueous solutions. The pK(a) values of the bound water in these two species, determined by spectrophotometry, are 8.3 +/- 0.03 and 7.2 +/- 0.06, respectively. Oxidation of the thiolato sulfur (to sulfinate) therefore increases the acidity of the bound water. Since 2 and 5 promote hydrolysis of acetonitrile at pH values above their corresponding pK(a) values, it is also evident that a metal-bound hydroxide is a key player in the mechanism of hydrolysis by these model complexes of Co-NHase. The required presence of a Cys-sulfinic residue and one water molecule at the Co(III) site of Co-NHase as well as the optimal pH of the enzyme near 7 suggests that (i) modulation of the pK(a) of the bound water molecule at the active site of the enzyme could be one role of the oxidized Cys-S residue(s) and (ii) a cobalt-bound hydroxide could be responsible for the hydrolysis of nitriles by Co-NHase. 相似文献
16.
The (13)C-mixed triacylglcerol (MTG, 1,3-distearyl, 2-[1-(13)C]octanoyl glycerol) breath test is a non-invasive measure of intraluminal fat digestion. Recovery of (13)C in breath CO(2) is incomplete (<50%) owing to sequestration of (13)C into organic molecules via the tricarboxylic acid (TCA) cycle. In addition lack of knowledge of CO(2) production rate (VCO(2)) during the test leads to errors in the calculated percentage dose recovered (PDR). (2)H sequestration into organic molecules is low ( approximately 4%) and is not influenced by factors that affect VCO(2) such as food intake or physical activity. After oxidation of (2)H-labelled macromolecules, the label appears in body water, which can be sampled non-invasively in urine or saliva. After an overnight fast, two healthy adults consumed [(2)H]MTG (1,3-distearyl, 2-[(2)H(15)]octanoyl glycerol) and [(13)C]MTG (1,3 distearyl, 2-[1-(13)C]octanoyl glycerol) simultaneously. Total body water (TBW) was measured by (18)O dilution and also estimated from height and weight. Urine and saliva were sampled at baseline and for 10 h after consumption of the test meal. The abundance of (2)HOH and H(2) (18)O in urine and saliva was measured by continuous-flow isotope-ratio mass spectrometry. Cumulative PDR of (2)H and (18)O was calculated from the plateau enrichment, which was reached by 6 h in both saliva and urine. Recovery of (2)H calculated using measured TBW was compared with that using an estimated value of TBW. Mean recovery of (2)H in saliva was 99.3% and in urine was 96.4%. Errors introduced by estimating TBW were <5%. [(2)H]MTG could provide a simpler, more robust, indirect test of intraluminal fat digestion compared with the (13)C-breath test. Further studies are required in pancreatic insufficient patients. 相似文献
17.
Diogo HP Kiyobayashi T Minas da Piedade ME Burlak N Rogers DW McMasters D Persy G Wirz J Liebman JF 《Journal of the American Chemical Society》2002,124(9):2065-2072
In this work, the aromaticity of pyracylene (2) was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3) at 298.15 K was determined to be minus sign(114.5 +/- 4.2) kJ x mol(-1) in toluene solution and minus sign(107.9 +/- 4.2) kJ x mol(-1) in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene at 298.15 K gave -(249.9 plus minus 4.6) kJ x mol(-1). The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(o)m(cr) and DeltaH(o)m(sub) values obtained in this work for 2 and 4, was -(236.0 +/- 7.0) kJ x mol(-1). Molecular mechanics calculations (MM3) led to Delta(hyd)H(o)m(1,g) = -110.9 kJ x mol(-1) and Delta(hyd)H(o)m(2,g) = -249.3 kJ x mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(o)m(2,g) = -(244.6 +/- 8.9) kJ x mol(-1) at 298.15 K. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that, on energetic grounds, pyracylene is a borderline case in terms of aromaticity/antiaromaticity character. 相似文献
18.
The solubilization of triglycerides [1,2,3-tributanoylglycerol (TBG) and 1,2,3-trihexanoylglycerol (THG)] in water/octa(oxyethylene) dodecyl ether (C(12)EO(8)) systems has been investigated. Oil-induced changes in the structure of liquid crystals in water/C(12)EO(8) system have been studied by optical observation and small-angle X-ray scattering (SAXS) measurements. In the water/C(12)EO(8)/oil systems, solubilization of THG and TBG induces a transition between H(1) (hexagonal) and L(alpha) (lamellar) liquid crystals at high C(12)EO(8) concentrations, whereas at low surfactant concentrations a H(1)-I(1) (discontinuous micellar cubic phase) transition occurs. This anomalous behavior is attributed to the partitioning of solubilized oil in the micelles. At low surfactant concentrations THG is mainly solubilized into the hydrophobic cores of the surfactant micelles, indicating high swelling or low penetration tendency, resulting in a steep increase in the radius of the aggregates (r(H)), thereby inducing a rod-sphere transition. At high surfactant concentrations, THG is not mainly solubilized into the core but distributed between the palisade layer and the core of the aggregates. The TBG is considerably solubilized into the surfactant palisade layer, indicating a high penetration tendency, resulting in an increase in the effective cross-sectional area per surfactant molecule, a(s). The thermal stability of the I(1) phase increases with the solubilization of THG into the aggregate cores. The percentage deviation of the experimental interlayer spacings (P(d)) from complete swelling was also evaluated for different triglycerides in the H(1) and L(alpha) phases or different surfactant concentrations. It is found that the penetration tendency of triglycerides could be used as a tuning parameter for I(1) phase formation depending on the surfactant concentration and the molecular weight of the oil. 相似文献
19.
T. S. Urbański M. Chojecki R. Kaczyńska 《Journal of Radioanalytical and Nuclear Chemistry》1976,30(2):369-376
The solvent extraction of molybdenum(VI) from sulphuric acid solutions with di-(2-ethylhexyl)-phosphoric acid (HDEHP) and
monododecylphosphoric acid (HDDP) in n-heptane has been studied (a) as a function of the concentration of sulphuric acid,
molybdenum and the extractant; (b) in the presence of copper and zinc in the aqueous phase and (c) in the presence of tri-n-butylphosphate
(TBP) in the organic phase. The distribution of the sulphuric acid between aqueous and organic phase has also been studied. 相似文献
20.
The infrared (IR) spectra of cuboidic titanium carbide (TiC) nanocrystals have been studied at the density-functional-theory (DFT) level using the Becke-Perdew (BP) functional and triple-zeta quality basis sets augmented by one set of polarization functions (TZVP). The accuracy of the calculations was checked by DFT calculations using the Perdew-Burke-Ernzerhof hybrid functional (PBE0) and up to quadruple-zeta quality basis sets augmented by one set of polarization functions (QZVP). The calculated IR spectrum for Ti(14)C(13) (3 x 3 x 3) is found to be in fair agreement with the experimental IR spectrum obtained by infrared resonance-enhanced multiphoton ionization (IR-REMPI) measurements, whereas, for Ti(18)C(18) (4 x 3 x 3) and Ti(32)C(32) (4 x 4 x 4), the calculated IR spectra differ significantly from the experimental ones. The smallest TiC cluster (Ti(4)C(4), 2 x 2 x 2) considered has not been reported in any mass-spectrometer studies. The present DFT calculations show that the vibrational modes related to the in-plane vibrations of solid TiC are not observed in the IR-REMPI spectra of nanocrystals larger than Ti(14)C(13). Contrary to solid TiC, the studied TiC nanocrystals are nonmetallic with optical gaps of 0.62 eV (0.55 eV) and 0.028 eV (0.027 eV) for Ti(32)C(32) and Ti(108)C(108) (6 x 6 x 6), calculated at the time-dependent density-functional-theory (TDDFT) level using the BP functional. The HOMO-LUMO gaps obtained in the BP DFT calculations are given within parentheses. At the PBE0 DFT level, the HOMO-LUMO gaps for Ti(32)C(32) and Ti(108)C(108) are 1.74 and 0.32 eV, respectively. 相似文献