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1.
Weihua Wang Ping Li Xiaojun Tan Qiufen Wang Gengxiu Zheng Yuxiang Bu 《Structural chemistry》2008,19(3):527-533
The geometries and isomerization of the imine silylenoid HN=SiNaF as well as its insertion reactions with some R–H molecules
have been systematically investigated theoretically, where R=F, OH, NH2, and CH3, respectively. The barrier heights for the four insertion reactions are 67.7, 115.6, 153.5, and 271.5 kJ/mol at the B3LYP/6-311+G*
level of theory, respectively. Here, all the mechanisms of the four reactions are identical to each other, i.e., a stable
intermediate has been formed during the insertion reaction. Then, the intermediate could dissociate into the substituted silylene
(HN=SiHR) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies
for the four reactions are 71.8, 95.5, 123.3, and 207.6 kJ/mol, respectively, which are linearly correlated with the calculated
barrier heights. Furthermore, the effects of halogen substitutions (F, Cl, and Br) on the reaction activity have also been
discussed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH2 > H–CH3. 相似文献
2.
Xiaojun Tan Weihua Wang Ping Li Xiuli Yang Gengxiu Zheng Yuxiang Ma Zhuangdong Yuan 《Theoretical chemistry accounts》2007,118(2):357-362
The formations of the phosphinidene derivative HPNaF and its insertion reactions with R–H (R=F, OH, NH2, CH3) have been systematically investigated employing the density functional theory (DFT), such as the B3LYP and MPW1PW91 methods.
A comparison with the results of MP2 calculations shows that MPW1PW91 underestimates the barrier heights for the four reactions
considered. Similarly, the same is also true for the B3LYP method depending on the selected reactions, but by much less than
MPW1PW91, where the barrier heights of the four reactions are 25.2, 85.7, 119.0, and 142.4 kJ/mol at the B3LYP/6-311+G* level
of theory, respectively. All the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been
located during the insertion reaction. Then, the intermediate could dissociate to substituted phosphinidane(H2RP) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies
for the four reactions are −92.2, −68.1, −57.2, and −44.3 kJ/mol at the B3LYP/6-311+G* level of theory, respectively, where
both the B3LYP and MPW1PW91 methods underestimate the reaction energies compared with the MP2 results. The linear correlations
between the calculated barrier heights and the reaction energies have also been observed. As a result, the relative reactivity
among the four insertion reactions should be as follows: H–F > H–OH > H–NH2 > H–CH3. 相似文献
3.
A systematic way to derive all thea priori possible mechanisms classified according to the numbers, ϱ, of elementary or molecular reaction steps was presented recently.
The method is now applied to overall reactions of the type A +B ⇒ C+D. The “laminar” (elementary steps' stoichiometric coefficients
unity) mechanisms that turn out possible are derived and listed for the important ϱ = 2 case. The ϱ = 3 three step mechanisms
were reported in another paper. Many examples from actual chemical, biochemical mechanisms, such as SE1, SN1, SE2, SN2, activated complex theory of abstraction reactions, free radical chain propagation and biochemical electron transport chains
are given. By far most mechanisms usually encountered are of the laminar type, although the method also gives the “turbulent”
ones, i.e. with some stoichiometric coefficients in elementary steps larger than one or some species occurring in many of
the steps.
Alexander von Humboldt Senior Scientist Awardee, West Germany.
Predoctoral Research Assistant, Yale University 1973–74. Then at the Roswell Park Memorial Institute, Department of Experimental
Therapeutics, Buffalo, New York 14203. 相似文献
4.
Jacques Cazaux 《Mikrochimica acta》2000,132(2-4):173-177
To account for the published yield curves, δ = f ( E
0), of some insulators such as diamond, an elementary theory of secondary electron emission (s.e.e) is used. This theory permits
the estimation of the effective attenuation lengths of the emitted electrons during their transport and to explain the significant
differences between the s.e.e. from metals and that from insulators. Some practical consequences related for instance to the
lack of topographic contrast in LVSEM and to charging mechanisms in electron beam techniques are then deduced. 相似文献
5.
The mechanism of addition reaction between carbene and epoxyethane has been investigated employing the MP2 and B3LYP/6-311+G*
levels of theory. Geometry optimization, vibrational analysis, and energy property for the involved stationary points on the
potential energy surface have been calculated. Based on the calculated results at the MP2/6-311+G* level of theory, it can
be predicted that there are two reaction mechanisms (1) and (2). In the first reaction carbene attacks the atom O of epoxyethane
to form an intermediate 1a (IM1a), which is a barrier-free exothermic reaction. Then, IM1a can isomerize to IM1b via a transition
state 1a (TS1a), where the potential barrier is 48.6 kJ/mol. Subsequently, IM1b isomerizes to a product epoxypropane (Pro1)
via TS1b with a potential barrier of 14.2 kJ/mol. In the second carbene attacks the atom C of epoxyethane firstly to form
IM2 via TS2a. Then IM2 isomerizes to a product allyl alcohol (Pro2) via TS2b with a potential barrier of 101.6 kJ/mol. Correspondingly,
the reaction energies for the reactions (1) and (2) are −448.4 and −501.6 kJ/mol, respectively. Additionally, the orbital interactions are also discussed for the leading intermediate.
The results based on the B3LYP/6-311+G* level of theory are paralleled to those on the MP2/6-311+G* level of theory. Furthermore,
the halogen and methyl substituent effects of H2C: on the two reaction mechanisms have been investigated. The calculated results indicate that the introductions of halogen
or methyl make the addition reaction difficult to proceed. 相似文献
6.
Bruno Medronho Anabela Romano Maria Gra?a Miguel Lars Stigsson Bj?rn Lindman 《Cellulose (London, England)》2012,19(3):581-587
Despite being the world’s most abundant natural polymer and one of the most studied, cellulose is still challenging researchers.
Cellulose is known to be insoluble in water and in many organic solvents, but can be dissolved in a number of solvents of
intermediate properties, like N-methylmorpholine N-oxide and ionic liquids which, apparently, are not related. It can also be dissolved in water at extreme pHs, in particular
if a cosolute of intermediate polarity is added. The insolubility in water is often referred to strong intermolecular hydrogen
bonding between cellulose molecules. Revisiting some fundamental polymer physicochemical aspects (i.e. intermolecular interactions)
a different picture is now revealed: cellulose is significantly amphiphilic and hydrophobic interactions are important to
understand its solubility pattern. In this paper we try to provide a basis for developing novel solvents for cellulose based
on a critical analysis of the intermolecular interactions involved and mechanisms of dissolution. 相似文献
7.
Yi-Zhen Tang Jing-Yu Sun Hao Sun Ya-Ru Pan Rong-Shun Wang 《Theoretical chemistry accounts》2008,119(4):297-303
The mechanisms of the reaction of NCCO with molecular oxygen are investigated at the G3MP2//B3LYP/6-311G(d,p) levels for the
first time. The calculation results show that two mechanisms are involved, namely, O attack on α atom mechanism and O attack on β atom mechanism, with six products yielded. The most feasible channel is the addition of O2 to β atom in NCCO radical leading to the energy-rich intermediate IM1, NCC(O)OO, which can isomerize to a four-center-structure
IM3, and then undergoes C–C and O–C bond fission to form P1(NCO + CO2) finally. The barriers are 27.3 and 25.4 kcal/mol, respectively. For other channels involved in the two mechanisms, with
less stable initial adducts and higher barrier, they are less conceivable dynamically and thermochemically. 相似文献
8.
Gary Tresadern Paul F. Faulder M. Paul Gleeson Zubeir Tai Grant MacKenzie Neil A. Burton Ian H. Hillier 《Theoretical chemistry accounts》2003,109(3):108-117
Hybrid quantum mechanical (QM)/molecular mechanical (MM) calculations are used to study two aspects of enzyme catalysis,
Kinetic isotope effects associated with the hydride ion transfer step in the reduction of benzyl alcohol by liver alcohol
dehydrogenase are studied by employing variational transition-state theory and optimised multidimensional tunnelling. With
the smaller QM region, described at the Hartree–Fock ab initio level, together with a parameterised zinc atom charge, good
agreement with experiment is obtained. A comparison is made with the proton transfer in methylamine dehydrogenase. The origin
of the large range in pharmacological activity shown by a series of α-ketoheterocycle inhibitors of the serine protease, elastase,
is investigated by both force field and QM/MM calculations. Both models point to two different inhibition mechanisms being
operative. Initial QM/MM calculations suggest that these are binding, and reaction to form a tetrahedral intermediate, the
latter process occurring for only the more potent set of inhibitors.
Recieved 3 October 2001 / Accepted: 6 September 2002 / Published online: 31 January 2003
Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American
Chemical Society, 2001
Correspondence to: I. H. Hillier
Acknowledgements. We thank EPSRC and BBSRC for support of the research and D.G. Truhlar for the use of the POLYRATE code. 相似文献
9.
Nilsson C Nilsson F Turner P Sixtensson M Nordberg Karlsson E Holst O Cohen A Gorton L 《Analytical and bioanalytical chemistry》2006,385(8):1421-1429
In this work, a real-time sampling/analytical method for on-line measurements of two newly discovered cyclomaltodextrinases
(CDases) has been developed and evaluated. This novel methodology not only allows the final products to be investigated, but
it also reveals enzyme-specific differences in the degradation pathways during the hydrolysis of different substrates, which
is a great advantage in the important tasks of investigating the mechanisms of and classifying new hydrolases, and is an advantage
that conventional techniques cannot offer. Two different enzymes, one CDase from Laceyella
sacchari (LsCda13) and one from Anoxybacillus flavithermus (AfCda13), were investigated during the hydrolysis of α-, β- and γ-cyclodextrin, and the hydrolysis products were sampled via
a microdialysis probe and injected on-line every 30 min into a high-performance anion exchange chromatography system equipped
with a pulsed amperometric detector (HPAEC–PAD), where they were identified. The enzymes yielded the same end-products, maltose
and glucose, in an approximate molar ratio of 2:1, but they exhibited distinctly different patterns of intermediate product
formation before reaching the end-point. LsCda13 had a more random distribution of the intermediate products, whereas AfCda13 showed the distinct intermediate production of maltotriose, which in some cases accumulated. 相似文献
10.
Rita Prosmiti Alexei A. Buchachenko Pablo Villarreal Gerardo Delgado-Barrio 《Theoretical chemistry accounts》2001,106(6):426-433
Ab initio molecular electronic structure calculations are performed for H5
+ at the QCISD(T) level of theory, using a correlation-consistent quadruple-zeta basis set. Structures, vibrational frequencies
and thermochemical properties are evaluated for ten stationary points of the H5
+ hypersurface and are compared with previous calculations. The features of the H3
+…H2 interaction at intermediate and large intermolecular distances are also investigated. Furthermore, an analytical functional
form for the potential-energy surface of H5
+ is derived using a first-order diatomics-in-molecule perturbation theory approach. Its topology is found to be qualitatively
correct for the short-range interaction region.
Received: 15 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001 相似文献
11.
The many-body diagrammatic perturbation theory is used for calculation of the correlation energy of closed-shell molecular
systems. We apply Brueckner's concept of the two-particle renormalized interaction defined by a non-linear diagrammatic expression
containing all possible (diagonal and/or non-diagonal) particle-particle, hole-hole and particle-hole intermediate elementary
processes. Then, a “second-order” simple diagrammatic expression for the correlation energy can be formed, where the correlation
energy is approximated by all the diagrams with biexcited intermediate states. An illustrative numerical application for the
LiH molecule is presented.
This article is dedicated to the memory of our friends and colleagues Dr. Jarka Surá and Dr. Marta Černayová, who tragically
died in July 1976. 相似文献
12.
K. I. Patrilyak L. K. Patrilyak Yu. G. Voloshina I. A. Manza S. V. Konovalov 《Theoretical and Experimental Chemistry》2011,47(4):205-214
The mechanisms of formation are proposed on the basis of the distribution of the intermediate and final products of zeolite
alkylation of isobutane with butenes. The mechanisms are based on activation of the isobutane molecule at the methyl groups,
simultaneous intermolecular and intramolecular hydride transfer, and β dissociation during skeletal isomerization of the carbonium
ions. 相似文献
13.
Quantum chemical modeling of but-1-ene isomerization to cis-but-2-ene and trans-but-2-ene in the presence of a gold atom has been carried out in the framework of the density functional theory with an extended
basis set, the PBE functional, and a pseudopotential with relativistic corrections included. Two possible mechanisms have
been considered, viz., with the formation of an intermediate σ-complex with one C(sp2)—Au bond and with gold insertion into the C—H bond. In the former case, the calculated energy barriers to two isomerization
stages are higher than 30 kcal mol−1. In the latter case, the reaction involves three stages and proceeds via a metal hydride complex with low barriers.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1330–1337, July, 2008. 相似文献
14.
Chi-Kit Siu Junfang Zhao Julia Laskin Ivan K. Chu Alan C. Hopkinson K. W. Michael Siu 《Journal of the American Society for Mass Spectrometry》2009,20(6):996-1005
Fragmentations of tautomers of the α-centered radical triglycine radical cation, [GGG•]+, [GG•G]+, and [G•GG]+, are charge-driven, giving b-type ions; these are processes that are facilitated by a mobile proton, as in the fragmentation of protonated triglycine
(Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006–3012). By contrast, radical centers are less mobile. Two mechanisms have been examined theoretically utilizing density
functional theory and Rice-Ramsperger-Kassel-Marcus modeling: (1) a direct hydrogen-atom migration between two α-carbons,
and (2) a two-step proton migration involving canonical [GGG]•+ as an intermediate. Predictions employing the latter mechanism are in good agreement with results of recent CID experiments
(Chu, I. K. et al. J. Am. Chem. Soc. 2008, 130, 7862–7872). 相似文献
15.
The mechanism of the chloride ion-induced catalytic decomposition of methyl(trifluoromethyl)dioxirane in trifluoroacetone
was studied at the MP4//MP2/6-31+G(d) level of theory. The solvated chloride ion interacts with dioxirane to form an ion-dipole
pair, which is transformed into the key intermediate ClO—C(Me)(CF3)—O− acting as a chain carrier in the catalytic decomposition of dioxirane. The generation of singlet oxygen occurs during the
transformations of this intermediate on the singlet potential energy surface. 相似文献
16.
Recently a basis-set-superposition-error-free second-order perturbation theory was introduced based on the “chemical Hamiltonian
approach” providing the full antisymmetry of all wave functions by using second quantization. Subsequently, the “Heitler–London”
interaction energy corresponding to the sum of the zero- and first-order perturbational energy terms was decomposed into different
physically meaningful components, like electrostatics, exchange and overlap effects. The first-order wave function obtained
in the framework of this perturbation theory also consists of terms having clear physical significance: intramolecular correlation,
polarization, charge transfer, dispersion and combined polarization–charge transfer excitations. The second-order energy,
however, does not represent a simple sum of the respective contributions, owing to the intermolecular overlap. Here we propose
an approximate energy decomposition scheme by defining some “partial Hylleraas functionals” corresponding to the different
physically meaningful terms of the first-order wave functions. The sample calculations show that at large and intermediate
intermolecular distances the total second-order intermolecular interaction energy contribution is practically equal to the
sum of these “physical” terms, while at shorter distances the overlap-caused interferences become of increasing importance.
Received: 18 June 2001 / Accepted: 28 August 2001 / Published online: 16 November 2001 相似文献
17.
Martin Albrecht Peter Reinhardt Jean-Paul Malrieu 《Theoretical chemistry accounts》1998,100(1-4):241-252
A size-consistent ab initio formalism to calculate correlation corrections to ionization potentials as well as electron affinities
of periodic systems is presented. Our approach is based on a Hartree-Fock scheme which directly yields local orbitals without
any a posteriori localization step. The use of local orbitals implies non-zero off-diagonal matrix elements of the Fock operator,
which are treated as an additional perturbation and give rise to localization diagrams. Based on the obtained local orbitals,
an effective Bloch Hamiltonian is constructed to second order of perturbation theory with all third-order localization diagrams
included. In addition, the summation of certain classes of diagrams up to infinite order in the off-diagonal Fock elements
as well as the Epstein-Nesbet partitioning of the full Hamiltonian are discussed. The problem of intruder states, frequently
encountered in many-body perturbation theory, is dealt with by employing the theory of intermediate Hamiltonians. As model
systems we have chosen cyclic periodic structures up to an oligoethylene ring in double-zeta basis; however, the theory presented
here straightforwardly carries over to infinite periodic systems.
Received: 30 April 1998 / Accepted: 27 July 1998 / Published online: 7 October 1998 相似文献
18.
Ab initio molecular dynamics simulations at the Hartree-Fock/6-31G level of theory are performed on methyl chloride hydrolysis
with explicit consideration of one solute and two solvent water molecules at a temperature of 298 K. The reaction involves
the formation of a reactant complex and the energy surface to the transition state is found to be simple. Two types of trajectories
toward the product are observed. In the first type, the system reaches an intermediate complex (complex-P1) region after two
nearly concerted proton transfers involving the attacking water molecule and the solvent water molecules. These trajectories
resemble the intrinsic reaction coordinate trajectory. The thermal motion of the atoms leads the system to another intermediate
complex (complex-P2) region. A second type of trajectory is found in which the system reaches the complex-P2 region directly
after the proton transfers. In both of these forward trajectories, back proton transfers lead the system to a final complex-F
region which resembles protonated methanol.
Received: 3 July 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999 相似文献
19.
Three mechanistic possibilities for the formation of the oxetane (D ring) of Taxol were examined at various levels of theory
[B3LYP/6-31+G(d,p), mPW1PW91/6-31+G(d,p), and MP2/6-31+G(d,p)] including one mechanism involving an unusual oxabicyclobutonium
ion intermediate. The mechanisms examined differ considerably in terms of their predicted inherent activation barriers, and
the requirements for acceleration of each by an enzyme active site are outlined. Our calculations provide an important starting
point for future studies in this area. Also examined were previously published calculations on simple oxabicyclobutonium ions,
as well as the all-carbon analog of the pathway involving the oxabicyclobutonium ion.
Published in Rossiiskii Khimicheskii Zhurnal, 2007, Vol. 51, No. 5, pp. 49–55. 相似文献
20.
Sosale Chandrasekhar 《Research on Chemical Intermediates》1998,24(6):625-642
It is proposed that molecular phenomena may only be described within the framework of the Complementarity Principle (‘CP’),
and that scientific controversy may originate in the essential incompatibility of complementary representations. Complementarity
based on the temporal Uncertainty Principle leads to new insights into transition state theory, microscopic reversibility
and the Curtin-Hammett Principle. An empirical application of the ‘CP’ to the structural theory leads to a revision of present
concepts of ‘reaction dynamics’, with the Principle of Least Nuclear Motion (‘PLNM’) emerging as a general alternative to
electronic theories of reactivity. In fact, it is argued that the ‘PLNM’ is a better basis for the Woodward-Hoffmann rules
than is orbital symmetry. A more flexible approach to organic reaction mechanisms is thus indicated. Also, as the basis of
the structural theory is fundamentally uncertain, and the present theory of X-ray diffraction apparently incompatible with
the ‘UP’, a reinterpretation of the Bragg equation has been attempted. 相似文献