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1.
Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO3(CH2). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]O, C addition which has a barrier of only 2.3 kcal mol−1. The [3+2]O, O addition has a slightly higher barrier of 6.5 kcal mol−1. Four other reactions of OsO3(CH2) with C2H4 have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol−1. Even higher barriers are calculated for the [2+2] addition to the OsO bond (32.6 kcal mol−1) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol−1). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol−1) and for the addition reactions of the latter with C2H4 are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the OsO bond which has an activation barrier of 46.6 kcal mol−1 and the [3+2]O, O addition which is an endothermic process with an activation barrier of 44.3 kcal mol−1.  相似文献   

2.
The structure, stability, and thermochemistry of the H(MF3)+ isomers (M = N-Bi) have been investigated by MP2 and coupled cluster calculations. All the HF-MF2+ revealed weakly bound ion-dipole complexes between MF2+ and HF. For M = N, As, Sb, and Bi they are more stable than the H-MF3+ covalent structures (free energy differences) by 6.3, 14.3, 32.1, and 73.5 kcal mol−1, respectively. H-PF3+ is instead more stable than HF-PF2+ by 21.8 kcal mol−1. The proton affinities (PAs) of MF3 at the M atom range from 91.9 kcal mol−1 (M = Bi) to 156.5 kcal mol−1 (M = P), and follow the irregular periodic trend BiF3 < SbF3 < AsF3 < NF3 < PF3. The PAs at the F atom range instead from 131.9 kcal mol−1 (M = P) to 164.9 kcal mol−1 (M = Bi), and increase in the more regular order PF3 ≈ NF3 < AsF3 < SbF3 < BiF3. This trend parallels the fluoride-ion affinities of the MF2+ cations. For protonated NF3 and PF3, the calculations are in good agreement with the available experimental results. As for protonated AsF3, they support the formation of HF-AsF2+ rather than the previously proposed H-AsF3+. The calculations indicate also that the still elusive H(SbF3)+ and H(BiF3)+ should be viable species in the gas phase, exothermically obtainable by various protonating agents.  相似文献   

3.
It is shown by reliable density functional theory (DFT) calculations that compounds 4 and 6 are very powerful neutral organic superbases as evidenced by the calculated proton affinities in the gas phase 305.4 kcal mol−1 (44.8) and 287.8 kcal mol−1 (37.8), respectively, and the corresponding calculated pKa values in acetonitrile given within parentheses.  相似文献   

4.
The mechanism of the reaction of isothiocyanates with metal-azido complexes of Pt(II), Pd(II), and Sn as well as hydrazoic acid is studied using the density functional theory method. The relative stability between two possible product isomers (S-coordinated tetrazole-thiolato and N-coordinated tetrazolato complexes) does not directly relate to the experimentally synthesized product. The overall reaction proceeds via three steps. The first step is the approach of the S-atom of the organic isothiocyanate to the central metal atom followed by the nucleophilic attack of the coordinated N-atom of the azido group to the C-atom of the isothiocyanate. The activation barrier of this step is 22-24 kcal mol−1, and the resulting intermediate has the imidoyl azide form. In the second reaction step, electrophilic attack of the terminal N-atom of the azido group to the N-atom of the isothiocyanate transforms the intermediate to the S-coordinated tetrazole-thiolato product with a barrier of about 11 kcal mol−1. The N-coordinated tetrazole could be made from the S-coordinated tetrazole-thiolato complex only after the third step, in which the metal coordination migrates from the S- to the N-atom.  相似文献   

5.
X-ray crystallographic analysis and density functional B3LYP/6-31G(d) calculation confirm that dimeric water is embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4]arene and stabilized by hydrogen bondings of aromatic π?H2O(1) and methyl?H2O(2) in the dimeric water inclusion complex with binding energies of 1.4 and 0.9 kcal mol−1 respectively, and by hydrogen bonding formed between H2O(2) and four phenolic OH groups from an adjacent tetra-(p-tert-butyl)thiacalix[4]arene, with binding energy of 3.8-4.2 kcal mol−1.  相似文献   

6.
When a benzene ring bears two 2-methyl-1-naphthyl moieties in the para, meta or ortho positions as in 1,4-bis(2-methyl-1-naphthyl)benzene, 1, 1,3-bis(2-methyl-1-naphthyl)benzene, 2 and 1,2-bis(2-methyl-1-naphthyl)benzene 3, two rotational isomers (atropisomers) are generated, with the two naphthyl substituents in a syn or anti relationship. In the case of the para and meta derivatives (1 and 2, respectively) these atropisomers could not be separated but were detected by NMR spectroscopy, that also allowed the determination of their syn-anti interconversion barriers in solution (19.5 and 20.4 kcal mol−1, respectively) and, in the case of 2, also in the solid state (26.7 kcal mol−1). In the more hindered ortho derivative 3, the syn (meso) and anti (racemic) atropisomers interconvert in solution with a barrier (31.2 kcal mol−1) sufficiently high to allow their physical separation. The racemic form could also be separated (by enantioselective HPLC) into the PP and MM enantiomers. Analysis of the corresponding CD spectra allowed the assignment of the absolute configuration. When three such naphthyl substituents are bonded to the phenyl in a meta relationship, two atropisomers in statistical proportions were observed: the anti (Cs symmetry) and the syn (C3v symmetry) display a 3:1 ratio at the equilibrium in solution. This ratio is different in the solid state, as is the interconversion barrier (22.1 and 32.1 kcal mol−1 in solution and in the solid, respectively).  相似文献   

7.
Theoretical calculations of the Michael addition of diethylamine, pyrrolidine, and benzylamine to DMAD at the DFT (B3LYP/6-31+G) level indicate that the reaction follows a stepwise mechanism via a zwitterionic intermediate. The reactions have low activation barriers, 13–15 kcal mol−1 and are exothermic, ΔH° = −29 to −44 kcal mol−1. The detailed investigation of the reaction of benzylamine with DMAD reveals participation of the reactant-, transition structure-, and the product-complexes and that the 1,3-prototropic shift occurs through the benzylamine molecule. It also predicts formation of dimethyl 2-(N-benzylimino)butane-1,4-dicarboxylate as one of the products, which has been duly isolated and characterized experimentally.  相似文献   

8.
An experimental and theoretical DFT study was carried out on the solution behavior in [D7]DMF for bis-chelate complex [Pd(L)2](BF4)2·2CH3CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)2]2+, the central square-planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)2]2+ and syn-trans-[Pd(L)2]2+ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) 1H NMR spectroscopy in the temperature range of 223-353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)2]2+ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol−1 and 0.8 ± 1 cal mol−1 K−1, respectively. From the full-line-shape analysis of resonances in the VT 1H NMR spectra, the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol−1 and 2.7 ± 1.6 cal mol−1 K−1, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol−1. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.  相似文献   

9.
Thermal rearrangement of (±)-spiro{naphthalene-1(2H),4′-(naphtho-[2′,1′:2,3]pyrano[4,5-c]furazan)}-2-one-11′-oxides in DMF or acetic anhydride at 140 °C gave an isomeric mixture of (±)-spiro{naphthalene-1(2H),1′-(5′-hydroxyphenalene[1,2-c]furazan)}-2-one-2′-oxides and 4′-oxides. The rearranged structure was confirmed from X-ray analysis and was consistent with the through space NOE data. The rearrangement is suggested to be an overall tandem isomerization process. Using variable temperature 1H NMR spectroscopy the lower limit for the isomerisation barrier for a pair of tautomers was calculated to be 22 kcal mol−1 at 423 K. The isomerisation equilibrium for a pair of isomers was studied by variable temperature 1H NMR. The lower limit for the isomerisation barrier was calculated to be 22 kcal mol−1 at 423 K. This low value may be indicative of the difficulty encountered in separating the isomers by chromatography. Semi-empirical AM1 and molecular mechanics calculations suggest that the (±)-spiro{naphthalene-1(2H),1′-(5′-hydroxyphenalene[1,2-c]furazan)}-2-one-2′-oxides are more stable than their 4′-oxide counterparts, in accordance to the X-ray structure. The lower population of the 4′-oxide isomers relative to that of the 2′-oxide isomers was explained in terms of an unfavourable intramolecular steric interaction found in the low energy structure of the former.  相似文献   

10.
The mechanisms underlying the hydrolysis of methyl acetate and acetamide under acidic conditions were investigated using the MP2/6-311+G(d,p)//MP2/6-31+G(d,p) level of theory. It was necessary to include two water molecules as reactants to obtain a tetrahedral (TD) intermediate for the AAC2 mechanism that Ingold classified for the hydrolysis of methyl acetate. This mechanism includes two TS structures, one for the formation of the TD intermediate and the other for its decomposition. Since the activation energies were calculated to be 15.7 and 18.3 kcal mol−1, the second step determines the rate of hydrolysis. The calculated value was close to that observed at ∼16 kcal mol−1. It was confirmed that the AAC2 mechanism had a barrier lower by 9.9 kcal mol−1 than the AAL2 mechanism. The AAC2 mechanism is also applicable to the acid-catalyzed hydrolysis of acetamide. It is not the TD intermediate with which the NH3+ moiety forms, but one further step is required to produce the final products, acetic acid and ammonium ion.  相似文献   

11.
The formation of the complex between tetrandrine and the calcium ion, in solution, was studied using FTIR, UV-Vis, 1H NMR, 13C NMR and electrospray mass spectroscopy spectroscopic methods and molecular modeling. The calcium salts used were: Ca(ClO4)2·4H2O and Ca(Picrate)2 in the solvents: acetonitrile (CH3CN), deuterated acetonitrile (CD3CN) and tetrahydrofurane (THF). The determined complex stability constant was: 20277±67 dm3 mol−1 and corresponding free energy ΔG0=−5.820±0.002 kcal mol−1. The molecular simulation of the complex formation with the MM3 Augmented force field integrated in CAChe provided useful data about its energy. Combining the experimental results and molecular modeling we propose a model for the structure of tetrandrine-Ca complex with an eight coordinated geometry.  相似文献   

12.
Single crystal X-ray diffraction studies and a theoretical analysis indicate a preferred conformation for O-β-fluoroethyl esters, where the CF and CO(CO) bonds are gauche rather than anti to each other. The OCCF dihedral angles for three compounds and five independent structures indicate a range of only 63.4-69.6°. Evaluation of a rotational energy profile around this bond in a model system (β-fluoroethyl acetate) predicted a similar dihedral angle and the gauche conformation to be the minimum on the rotational energy profile. High level ab initio calculations measured the gauche conformer to be 0.95 kcal mol−1 lower in energy than the anti conformer and application of a solvation model further increased this differential to 1.6 kcal mol−1, consistent with a previous solution state (NMR) evaluation of this system.  相似文献   

13.
Jian-Wei Xu  Wei-Ling Wang 《Tetrahedron》2005,61(39):9248-9256
Crown-tetrathia[3.3.3.3]metacyclophanes 3a-c were synthesized via intermolcular coupling reaction in 22-30% yields. X-ray crystal analysis of 3b revealed that it adopted a perpendicular conformation (3b-B or 3b-C) in which two aromatic rings were inclined to be perpendicular to the opposite aromatic rings, driving two internal methyl groups into the π-cloud of the corresponding benzene rings. Furthermore, this perpendicular structural feature led to benzylic protons of thia-bridges being in close proximity to the adjacent aromatic rings. As a result, the induced upfield shifts for the two internal methyl protons and four benzylic protons were clearly observed in dynamic 1H NMR spectra at low temperature, indicating that the intramolecular C-H?π interaction became increasingly important at low temperature. The energy barrier for inter-conversion between 3b-B and 3b-C was estimated to be 12.1 kcal mol−1 by using a coalescence method. The total stabilization enthalpy of the C-H?π interactions was quantitatively calculated to be 7.9±0.8 kcal mol−1 by the dynamic NMR spectroscopy. In contrast, 3a showed two non-interconvertible conformers at room temperature, which tended to interconvert at elevated temperature, however, many conformers co-existed at low temperature.  相似文献   

14.
Resorcin[4]arene-based tetramidocavitands containing four secondary amide groups on their upper rim showed strong (R = methyl or ethyl) binding properties. The caviplex formation through hydrogen bonds of -(CO)N-H?X was supported by 1H NMR and crystal structure analyses. In a mixture of C2D2Cl4/DMSO/D2O = 5:15:2 at 25 °C, the thermodynamic parameters for caviplex @1, ΔG (kcal mol−1), ΔH (kcal mol−1), and ΔS (cal K−1 mol−1), are −3.7, −8.6, and −16.7, respectively.  相似文献   

15.
First-order rate constants of Brook-type isomerization of acylpolysilanes (Me3Si)3SiCOR (R = iso-Pr, tert-Bu, Ad, 2,6-xylyl, and Mes) leading to silenes (Me3Si)2SiC(OSiMe3)R at various temperatures were determined. Their Eyring plots gave kinetic parameters of ΔH = 26.6-29.4 kcal mol−1 and ΔS = −11.5 to −14.6 cal mol−1 K−1. The isomerization was accelerated by introducing an electron-donating alkyl substituent on the carbonyl carbon. These results are in accordance with a concerted mechanism involving a four-centered transition state.  相似文献   

16.
A combined experimental and computational approach towards the development of Pd-NHC catalysts is described. A range of benzimidazolylidinium ligands incorporating electron-rich and electron-poor substituents were prepared and evaluated in the Suzuki reaction. The most electron-rich ligand showed the highest catalytic activity. Based on this information, the first alkyl-alkyl Negishi cross-coupling reaction protocol was developed. Evaluation of N,N′-diaryl-(4,5-dihydro)imidazolylilidinium ligands showed a strong dependence on the steric topography around the metal centre. A computational study of the most active ligand in the Negishi reaction, its Pd(0) and PdCl2-complexes and related structures were modelled at the B3LYP/DZVP and HF/3-21G levels of theory. The potential energy hypersurfaces flattened with increase in ligand size. Binding energies were computed for carbene/Pd(0) adducts (in the range ∼31-40 kcal mol−1), roughly double that for PH3 (∼16 kcal mol−1). Weak intramolecular interactions were found using AIM analyses.  相似文献   

17.
Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol−1, are much smaller than the value (32.57 kcal mol−1) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.  相似文献   

18.
The energies and structures of possible intermediates in the dinitrogen extrusion from diazidophenylborane 4a to give phenylborylene 11a were determined using density functional (B3LYP), multiconfigurational (CASSCF and MRMP2), and coupled cluster (CCSD(T)) computations in conjunction with basis sets of up to cc-pVTZ quality. Formation of 11a and nitrogen from 4a is an exothermic process (−21 kcal mol−1). The triplet electronic ground state of azidophenylborylnitrene 5a (PhBN4) is only 26 kcal mol−1 higher in energy than 4a and the phenyl shift in 5a to yield N-azidophenyliminoborane 7a is highly exothermic.  相似文献   

19.
It is shown by the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) calculations that the hexacyano derivative of aza-acepentalene is an extremely powerful superacid both in the gas phase and in DMSO as evidenced by the ΔHacid = 255.1  kcal mol−1 and pKa (DMSO) = −26.5. Its synthesis is strongly recommended, in particular, since the related conjugate base hexachloro aza-acepentalenide anion was prepared recently.  相似文献   

20.
Density functional theory has been used to explore the mechanism of cleavage of H2 at a sulfido-bridged molybdenum cluster, CpMo(μ-SH)(μ-S)(μ-S2CH2)MoCp. The addition occurs across a single Mo-S bond, and the disruption of the strong Mo-S π bonding in the ground state leads to a very high-lying transition state (+43 kcal mol−1). Once formed, the adsorbed hydrogen migrates over the cluster via a series of hops from metal to sulphur, formally corresponding to a switch from hydridic to protic character. The low barrier (+15 kcal mol−1) for migration leads to facile hydrogenolysis of coordinated substrates.  相似文献   

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