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1.
Ortiz-Sanchez JM Gelabert R Moreno M Lluch JM 《The journal of physical chemistry. A》2006,110(14):4649-4656
The proton-transfer dynamics in the aromatic Schiff base salicylidene methylamine has been theoretically analyzed in the ground and first singlet (pi,pi) excited electronic states by density functional theory calculations and quantum wave-packet dynamics. The potential energies obtained through electronic calculations that use the time-dependent density functional theory formalism, which predict a barrierless excited-state intramolecular proton transfer, are fitted to a reduced three-dimensional potential energy surface. The time evolution in this surface is solved by means of the multiconfiguration time-dependent Hartree algorithm applied to solve the time-dependent Schr?dinger equation. It is shown that the excited-state proton transfer occurs within 11 fs for hydrogen and 25 fs for deuterium, so that a large kinetic isotope effect is predicted. These results are compared to those of the only previous theoretical work published on this system [Zgierski, M. Z.; Grabowska, A. J. Chem. Phys. 2000, 113, 7845], reporting a configuration interaction singles barrier of 1.6 kcal mol(-1) and time reactions of 30 and 115 fs for the hydrogen and deuterium transfers, respectively, evaluated with the semiclassical instanton approach. 相似文献
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To understand the role of pi-electron delocalization in determining the conformation of the NBA (Ph-N==CH-Ph) molecule, the following three LMO (localized molecular orbital) basis sets are constructed: a LFMO (highly localized fragment molecular orbital), an NBO (natural bond orbital), and a special NBO (NBO-II) basis sets, and their localization degrees are evaluated with our suggesting index D(L). Afterward, the vertical resonance energy DeltaE(V) is obtained from the Morokuma's energy partition over each of three LMO basis sets. DeltaE(V) = DeltaE(H) (one electron energy) + DeltaE(two) (two electron energy), and DeltaE(two) = DeltaE(Cou) (Coulomb) + DeltaE(ex) (exchange) + DeltaE(ec) (or SigmaDeltaE(n)) (electron correction). DeltaE(H) is always stabilizing, and DeltaE(Cou) is destabilizing for all time. In the case of the LFMO basis set, DeltaE(Cou) is so great that DeltaE(two) > |DeltaE(H)|. Therefore, DeltaE(V) is always destabilizing, and is least destabilizing at about the theta = 90 degrees geometry. Of the three calculation methods such as HF, DFT, and MPn (n = 2, 3, and 4), the MPn method provides DeltaE(V) with the greatest value. In the case of the NBO basis set, on the contrary, DeltaE(V) is stabilizing due to DeltaE(Cou) being less destabilizing, and it is most stabilizing at a planar geometry. The LFMO basis set has the highest localization degree, and it is most appropriate for the energy partition. In the NBA molecule, pi-electron delocalization is destabilization, and it has a tendency to distort the NBA molecular away from its planar geometry as far as possible. 相似文献
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The potential energy change during the M --> N process in bacteriorhodopsin has been evaluated by ab initio quantum chemical and advanced quantum chemical calculations following molecular dynamics (MD) simulations. Many previous experimental studies have suggested that the proton transfer from Asp96 to the Schiff base occurs under the following two conditions: (1) the hydrogen bond between Thr46 and Asp96 breaks and Thr46 is detached from Asp96 and (2) a stable chain of four water molecules spans an area from Asp96 --> Schiff base. In this work, we successfully reproduced the proton-transfer process occurring under these two conditions by molecular dynamics and quantum chemical calculations. The quantum chemical computation revealed that the proton transfer from Asp96 to Shiff base occurs in two-step reactions via an intermediate in which an H(3)O(+) appears around Ala215. The activation energy for the proton transfer in the first reaction was calculated to be 9.7 kcal/mol, which enables fast and efficient proton pump action. Further QM/MM (quantum mechanical/molecular mechanical) and FMO (fragment molecular orbital) calculations revealed that the potential energy change during the proton transfer is tightly regulated by the composition and the geometry of the surrounding amino acid residues of bacteriorhodopsin. Here, we report in detail the Asp96 --> Schiff base proton translocation mechanism of bacteriorhodopsin. Additionally, we discuss the effectiveness of combining quantum chemical calculations with truncated cluster models followed by advanced quantum chemical calculations applied to a whole protein to elucidate its reaction mechanism. 相似文献
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研究了salen型席夫结构的荧光探针N,N’-二(2-羟基-1-萘甲醛)-l,2-苯二胺(NAPPDIH)对Cr3+的响应。Cr3+能够使该探针的荧光猝灭。研究表明,在pH3.5(乙酸胺-盐酸)和DMSO/H2O(V/V,8:2)的条件下,8.0×10-6 mol/L的Cr3+可以导致NAPPDIH 65%的荧光猝灭。并且,Cr3+的浓度在5.0×10-7~8.0×10-6 mol/L的范围内,与荧光强度的变化值呈良好的线性关系,检出限为3.7×10-7 mol/L。方法可用于直接检测水样中的Cr3+。 相似文献
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Proton tautomerism of 1,8-dihydroxy-3-methyl-anthraquinone and its analogues were studied using HF and CIS methods with 6-31g(d,p) basis set for the ground and singlet excited states. The calculations indicate that the compound exists two strong intramolecular hydrogen bonds (IHB), and shows similar characters in its proton transfer processes considering the geometries and Mulliken charge population. Calculation results further show that intramolecular proton transfer (IPT) is not favored in view of the energy trend for chryosphanol, which has two hydrogens of hydroxyl groups bond with a common oxygen of carbonyl group and exists two IHBs in the peri region. However, it exhibits normal intramolecular proton transfer for the derivatives of chryosphanol, which have only one pair of adjacent hydroxyl group and carbonyl group existing in the peri region. Hereby, it can be conjectured from a theoretical point of view that IPT is absent in the similar structure such as hypericin’s peri region. Calculation results on the photophysical process show that the isomerization process is competitive with the intersystem crossing process, which facilitates the increase of triplet state quantum efficiency and photosensitive activity. 相似文献
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Fourter transform infrared spectra of a retinyl Schiff base in the presence of one of a series of acids were recorded in solvents containing chloroform and methanol in different proportions. The spectra show that methanol makes it possible for weak acids to achieve complete protonation of the Schiff base. This leads to the hypothesss that water or other OH containing molecules could play a role in bringing about complete protonation of the Schiff base chromophore in rhodopsins. 相似文献
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Liqun Zhang Zhengyu Zhou Dongmei Du Pei Yuan 《International journal of quantum chemistry》2006,106(9):2082-2089
To investigate the tautomerism of glycinamide that is induced by proton transfer, we present detailed theoretical studies on the reaction mechanism of both the isolated gas phase and H2O‐assisted proton transfer process of glycinamide, using density functional theory calculations by means of the B3LYP hybrid functional. Twenty‐six geometries, including 10 significant transition states, were optimized, and these geometrical parameters are discussed in detail. The relative order of the activation energy for hydrogen atom transfer of all the conformers has been systematically explored in this essay. For the amido hydrogen atom transfer process, the relative order of the activation energy is: IV < II < III < I, while in the carbonic hydrogen atom transfer process, the relative order is IV > II > III > I. Meanwhile, the most favorable structure for both the amido hydrogen atom transfer and the carbonic hydrogen atom transfer has been found. The involvement of the water molecule not only can stabilize the transition states and the ground states, but can also reduce the activation energy greatly. The superior catalytic effect of H2O has been discussed in detail. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
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The N---H tautomerism of free base porphyrin is investigated at the semiempirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary structures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensure that realistic delocalized structures are obtained. Both the semiempirical and the ab initio calculations predict that the interconversion between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized as minima in the potential energy surface and are 8–10 kcal mol−1 higher in energy. The activation energy for the trans → cis interconversion is calculated to be approximately 23 kcal mol−1 at the 3-21G level. The activation energy for the synchronous trans → trans interconversion is higher and has a value of 30.5 kcal mol−1. The activation energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values. 相似文献
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Formation constants of lithium ion complexes with 1,5-polymethylenetetrazoles and 3,3-disubstituted glutarimides have been determined in nitromethane solutions by 7Li NMR. Concentration dependence of the obtained values are explained by the competitive ion pair formation. Glutarimide complexes were found to be somewhat more stable than the pentamethylenetetrazole complexes. 相似文献
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Semiempirical SCF-MO studies of tautomerism in alloxan preclude the possibility of direct proton transfer in the gas phase
due to the strain in the four-centred transition state, in which the proton being transferred is forced to come close to the
positively charged carbon atom at the opposite corner of the four-membered ring. However, in aqueous solution, the activation
barrier reduces appreciably, not only due to reduction in strain, but also due to charge separation in the transition state,
which is stabilized due to ionic resonance. The N-H bond is almost broken, while the O-H bond is only partially formed in
the transition state. The other stabilizing effect in aqueous solution is due to bulk solvent dielectric effects, which stabilize
the transition state to a greater extent due to its higher dipole moment. Although the transition states for proton transfer
to the neighbouring oxygen atoms on either side have comparable energies, as the mechanisms of proton transfer leading to
the formation of the 2-hydroxy and 4-hydroxy tautomers are similar, bulk solvent effects are larger in the latter due to the
higher dipole moment of the transition state. The reason is the almost complete separation of the two entities, i.e. the alloxan
anion and the hydronium ion in the latter case, indicating that in this case a dissociative mechanism of the kind encountered
in acid-base equilibria is operating. 相似文献
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To elucidate the mechanism of reaction M+ + SCO, the reaction of Cr+ + SCO has been investigated using density functional theory (DFT) with the popular hybrid functional, B3LYP, in conjunction with 6‐311+G* basis set on both the sextet and quartet potential energy surfaces (PESs). To obtain an accurate evaluation of the activation barrier and reaction energy, the coupled cluster single‐point calculations using the B3LYP structures is performed. The crossing points (CPs) of the different PESs have been localized with the approach suggested by Yoshizawa and colleagues. The involving potential energy curve‐crossing dramatically affects reaction mechanism. The present results show that the reaction mechanism is insertion‐elimination mechanism both along the C? S and C? O bond activation branches, but the C? S bond activation is much more favorable than the C? O bond activation in energy. All theoretical results not only support the existing conclusions inferred from early experiment study, but also complement the pathway and mechanism for this reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
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I. G. Bikchantaev R. M. Galimov I. V. Ovchinnikov 《Theoretical and Experimental Chemistry》1988,24(3):360-364
The structural arrangement and the structure of the mesogenic copper compound {C7H15O-C6H3(O)CH-C6H4OC8H17}2Cu(II) were investigated in the dilute solution, the isotropic melt, the mesophase, and the crystalline state by ESR in conjunction with calorimetry and polarization microscopy. The complex exhibits smectic polymorphism with the following transitions: K 415 SC 430 SA 438 I. It was found by ESR that the complexes in the solution and in the isotropic melt have a square-planar trans configuration. In the mesophase part the planar molecules undergo tetrahedral distortion; the planar molecules form the smectic phase, while the distorted molecules are concentrated into microscopic isotropic drops, i.e., the system becomes heterogeneous in the mesophase. In the SA SC transition the mesophase is oriented anomalously in the magnetic field (compared with the diamagnetic analogs) with the director orthogonal to the direction of the field.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 3, pp. 370–375, May–June, 1988. 相似文献
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Harger MJ 《Chemical communications (Cambridge, England)》2004,(24):2896-2897
The phosphonochloridate R(2)CHP(O)(YEt)Cl (R(2)CH = fluoren-9-yl, Y = O or S) reacts with Pr(i)(2)NH largely or exclusively by an elimination-addition mechanism; the three-coordinate phosphene intermediate R(2)C=P(O)YEt is formed ca. 10(3) times more easily when Y = S than when Y = O. 相似文献
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The Floquet states of N(2) (2+) created by the interactions of the six lowest singlet (1 (1)Sigma(g) (+), 1 (1)Delta(g), 2 (1)Sigma(g) (+), 1 (1)Pi(u), 1 (1)Pi(g), and 1 (1)Sigma(u) (-)) states of the dication with intense (0.4 x 10(13) Wcm(2)) radiation have been studied using the recently developed multireference configuration interaction method with single and double excitations (MRCISD)-based approach. The adiabatic Floquet state coinciding near its minimum with the initial X (1)Sigma(g) (+) ground state and asymptotically correlating with A (1)Pi(u) (m = -1), i.e., with one less photon in the dressed state, is expected to be metastable, as is the ground state in the absence of a field, but to support up to the v(max) = 12 quasibound vibrational level in comparison with v(max) = 11 in the parent field-free X (1)Sigma(g) (+) ground state. The tunneling lifetimes of the highest vibrational levels in this adiabatic Floquet state are predicted to be several orders longer than those in the parent field-free state. Analysis of the complete basis set limit extrapolated MRCISD potential energy curve of the field-free X (1)Sigma(g) (+) state of N(2) (2+) calculated in the present work (R(e) = 1.130 A, omega(e) = 2011 cm(-1), omega(e)x(e) = 26.1 cm(-1)) is in good agreement with spectroscopic experimental data. Calculations on the field-free A (1)Pi(u) state (T(e) = 12 106 cm(-1), R(e) = 1.252 A, omega(e) = 1438 cm(-1), omega(e)x(e) = 23.5 cm(-1)) generally support earlier theoretical work and do not support reported experimental values. 相似文献