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1.
采用MP4/6-311++G(d,p)和B3LYP/6-311++G(d,p)对磷叶立德CH2PH3和类磷叶立德自由基∙CHPH3进行构型优化,从电子密度拓扑分析的角度对C—P键的键结构进行了探讨。得到如下结论:类磷叶立德自由基和磷叶立德的C—P键性质类似,但磷叶立德中π键由两个电子形成,类磷叶立德自由基中π键由一个电子形成,所以前者的π性明显,而后者的π性不明显。类磷叶立德自由基中的这个单电子在碳原子附近,垂直于对称面的方向上运动,有p(C→P)配键的特征,所以类磷叶立德自由基∙CHPH3中的C—P键比相应的产物∙CH2PH2中的C—P键要弱一些。  相似文献   

2.
Deprotonation of the 1‐isopropyl‐3‐(phenylamino)pyridin‐1‐ium iodide gives the corresponding neutral betaine, which is formalised as a pyridinium‐amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo‐amide and is a strong donor akin to alkyl phosphines and N‐heterocyclic carbenes. Furthermore, rotation about both N substituent C? N bonds occurs, which is in contrast to the two alternative pyridinium positional isomers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norharman were prepared, which contain an additional C? C bond supporting conjugation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norharman‐derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to dioxygen and water.  相似文献   

3.
The title compound, alternatively known as 3‐acetoxy‐2‐(acetylamino)pyridinium betaine of squaric acid, C13H10N2O6, has been synthesized. The bond distances within the squarate ring indicate two possible resonance structures. The mean planes of the pyridinium and squarate systems are inclined at an angle of 24.0 (2)° with respect to one another due to a strong intramolecular hydrogen‐bonding interaction between the amide NH group and a squarate O atom. In the extended structure, there are additional weak π–π and π–ring interactions, which also stabilize the crystal structure.  相似文献   

4.
A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur–nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight‐membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular S?S bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit. The final structures depend on the number of π electrons in the bridges. Intermediate biradicals are stabilized by Jahn–Teller distortion, giving transannular S?S bonds between the NSN units. This procedure may be summarized by two rules, which provide a rationale for the structures of a large number of sulfur–nitrogen‐based molecules. The long bonds between the NSN units show a p character of >95 %. The qualitative results have been compared with known molecular structures and the results of B3LYP/cc‐pVTZ calculations as well as CASSCF and CASVB calculations. B3LYP/cc‐pVTZ calculations have also provided the UV/Vis spectra and the NICS values of the planar 10π systems.  相似文献   

5.
Is one electron sufficient to bring about significant σ bonding between two atoms? The chemist’s view on the chemical bond is usually tied to the concept of shared electron pairs, and not too much experimental evidence exists to challenge this firm belief. Whilst species with the unusual one‐electron σ‐bonding motif between homonuclear atoms have so far been identified mainly by spectroscopic evidence, we present herein the first crystallographic characterization, augmented by a detailed quantum‐chemical validation, for a radical anion featuring a B?B one‐electron‐two‐center σ bond.  相似文献   

6.
The molecular structure and intramolecular hydrogen bond energy of 32 conformers of 4‐methylamino‐3‐penten‐2‐one were investigated at MP2 and B3LYP levels of theory using the standard 6–31G** basis set and AIM analyses. Furthermore, calculations for all the possible conformations of 4‐methylamino‐3‐penten‐2‐one in water solution were also carried out at B3LYP/6–31G** level of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the ketoamine conformers of this compound are more stable than the other conformers (i.e., enolimine and ketoimine). This stability is mainly due to the formation of a strong N? H···O intramolecular hydrogen bond, which is assisted by π‐electrons resonance. Hydrogen bond energies for all conformers of 4‐methylamino‐3‐penten‐2‐one were obtained from the related rotamers method. The nature of intramolecular hydrogen bond existing within 4‐methylamino‐3‐penten‐2‐one has been investigated by means of the Bader theory of atoms in molecules, which is based on topological properties of the electron density. The results of these calculations support the results which obtained by related rotamers method. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007  相似文献   

7.
The title compound, C13H6F5NO, exists in the enol form and adopts the E configuration about the enol double bond. It is the first example of an enol‐type pyridinium ylide. The enol structure was unambiguously determined on the basis of the significantly longer C—O bond and shorter C—C bond. Intramolecular C—H...O and C—H...F hydrogen bonds are responsible for promotion of the enol form and for the stability of this compound.  相似文献   

8.
A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.  相似文献   

9.
The syntheses of sulfur‐ and selenium‐bridged cyclic compounds containing boron stabilized by N‐heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron–boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six‐electron reduction reactions, resulting in the formation of [2.2.1]‐bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron–boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC‐stabilized trisulfidodiborolane. The demonstration of these six‐ and four‐electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.  相似文献   

10.
11.
A series of N‐heterocyclic carbene‐stabilized silanechalcogenones 2 a , b (Si?O), 3 a , b (Si?S), 4 a , b (Si?Se), and 5 a , b (Si?Te) are described. The silanone complexes 2 a , b were prepared by facile oxygenation of the carbene–silylene adducts 1 a , b with N2O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1 a , b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2 b , 3 a , 4 a , 4 b , and 5 b have been confirmed by single‐crystal X‐ray crystallography. Due to the NHC→Si donor–acceptor electronic interaction, the Si?E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide‐like resonance structures. Nevertheless, these species also exhibit considerable Si?E double‐bond character, presumably through a nonclassical Si?E π‐bonding interaction between the chalcogen lone‐pair electrons and two antibonding Si? N σ* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si–E distances (between 5.4 and 6.3 %) compared with the corresponding Si? E single‐bond lengths.  相似文献   

12.
Valence bond (VB) calculations using a double‐zeta D95 basis set have been performed for borazine, B3N3H6 and for benzene, C6H6 in order to determine the relative weights of individual standard Lewis structures. In the delocalized resonance scheme of borazine, the structure ( I ) with no double bonds and three lone pairs of electrons at the three nitrogen atoms is the major contributor with a structural weight of 0.17, followed by six equivalent Lewis structures with one double bond and two lone pairs at two nitrogen atoms ( II ) with weights of 0.08 each. In the case of benzene, the two Kekulé structures ( III ) contribute with structural weights of 0.15 each, followed by 12 equivalent ionic structures ( IV ) with weights of 0.03 each, followed by the three equivalent Dewar‐type structures ( V ) with structural weights of 0.02 each. The values of 54.1 and 45.8 kcal mol−1 for the delocalization energies of borazine and benzene were estimated. Therefore, B3N3H6 is calculated to have substantial aromatic character, similar to benzene, when we assume that the resonance energy can provide a criterion for aromaticity. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:311–315, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20095  相似文献   

13.
A novel rhodium‐catalyzed dearomatization of O‐substituted pyridines to access N‐substituted 2‐pyridones has been developed. A computational study suggests a mechanism involving the formation of a pyridinium ylide followed by an unprecedented 1,4‐acyl migratory rearrangement from O to C. Furthermore, the chiral dirhodium complexes serve as the catalyst for the asymmetric transformation with excellent enantioselective control. DFT calculations indicate the chirality is transferred from axial chirality to the central stereogenic centre. The stronger π–π interaction and CH–π interaction account for the high enantioselectivity.  相似文献   

14.
Two salts ( 2 and 4 ) containing the radical cations of complexed diphosphenes have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, IR spectroscopy, and single‐crystal X‐ray diffraction. The P?P bond is coordinated to the Cr center either in an end‐on (in 2 ) or a side‐on (in 4 ) fashion. The spin density of the radical is delocalized over the Cr atom and the two P atoms in 2 whereas the unpaired electron is mainly localized on the Cr atom in 4 . This work provides the first example of a complexed diphosphene radical ( 2 ) featuring novel three‐center three‐electron (3c‐3e) π‐bonding in the Cr‐P‐P unit, and the first example of a 17 e Cr radical with a side‐on π‐bonded ligand ( 4 ).  相似文献   

15.
The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C15H27NO11·H2O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cistrans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.  相似文献   

16.
The title ylides, 3‐(triphenyl­phospho­ranyl­idene)pentane‐2,4‐dione, C23H21O2P, (I), and diethyl 2‐(triphenyl­phospho­ranyl­idene)malonate, C25H25O4P, (II), differ in the conformations adopted by their extended ylide moieties. In (I), one carbonyl O atom is syn and the other is anti with respect to the P atom, the ylide group is nearly planar, with a maximum P—C—(C=O) angle of 18.2 (2)°, and the P—C, C—C and C=O bond lengths are consistent with electronic delocalization involving the O atoms. In (II), both carbonyl O atoms are anti and the ester groups are twisted out of the plane of the near trigonal ylide C atom, reducing delocalization, the largest P—C—(C=O) angle being 30.2 (2)°.  相似文献   

17.
The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O2 molecule, the ground state 3Σ? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet 1Δ and 1Σ+ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol?1 to the total bonding energies of the 3Σ?, 1Δ and 1Σ+ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.  相似文献   

18.
19.
硫叶立德化合物优势构型和键结构的量子拓扑研究   总被引:1,自引:4,他引:1  
曾艳丽  郑世钧  孟令鹏 《化学学报》2002,60(9):1564-1570
采用MP4(SDTQ)/6-311++G(d,p)和B3LYP/6-311++G(d,p)对所选四种化合 物进行构型优化,从量子拓扑学的角度对各稳定构型进行电子密度拓扑分析,讨论 了C-S键的特性。研究发现:(1)类硫叶立德自由基(·CHSH_2)和硫叶立德( CH_2SH_2)基态的稳定构型都不具有C_s对称性;(2)类硫叶立德自由基和硫叶立 德中C-S键的性质类似,硫叶立德中π键由两个电子形成,类硫叶立德自由基中π 键由一个电子形成,所以前者的π键性质明显,后者的π键性质不明显;(3)类 硫叶立德自由基(·CHSH_2)中单电子π键中的电子主要在碳原子附近运动,属于 单电子π(C → S)配键,所以其C-S键的强度比相应的产物要弱。  相似文献   

20.
Nitroxyl–aziridine hybrid, a candidate for a magnetic resonance imaging (MRI) probe and an anticancer drug, was synthesized by aziridine formation reaction of nitroxyl‐introduced aldehyde and guanidinium ylide in good diastereoselectivity. The relative configuration at aziridine C(2) C(3) bond of the major diastereoisomer was determined to be cis by X‐ray crystallographic analysis. Application of chiral guanidinium ylide resulted in the formation of the corresponding optically active aziridine in 84% ee. Reversible one‐electron redox potential and quantitative spin yield of the hybrid were observed in cyclic voltammogram and electron spin resonance, respectively. However, cytotoxicity of the hybrid against cancer cell lines used was not observed.  相似文献   

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