首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Geometries have been optimized using molecular-orbital calculations (a) with a 4-31G Gaussian basis set for carbanions CH2X? where X = H, CH3, NH2, OH, F, C?CH, CH?CH2, CHO, COCH3, CN, and NO2; and (b) with an STO -3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the C? X bond. Carbanions containing saturated substituents are pyramidal with the out-of-plane angle α increasing with the electronegativity of the substituent. Double-zeta basis set calculations give proton affinities over the range 449 (for CH3CH2?) to 355 kcal/mol (for CH2NO2?), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.  相似文献   

2.
Using a one-center-method, treating the inner shells statistically, the valence-shell, however, by quantum mechanics, the equilibrium internuclear distances and total molecular energies have been computed for CH4, SiH4, GeH4, SnH4, PbH4, BH 4 ? , AlH 4 ? , GaH 4 ? , InH 4 ? , TlH 4 ? , NH 4 + , PH 4 + , AsH 4 + , SbH 4 + , and BiH 4 + . The results are in good agreement with experimental data as well as with theoretical values.  相似文献   

3.
The entire set of methyl anions, XCH2?, substituted by first-row substituents, Li, BeH, BH2, CH3, NH2, OH, and F, was examined at various ab initio levels. Diffuse orbital-augmented basis sets, such as 4?31+G and 6?31+G *, are needed to describe the energies of these anions adequately. Estimates of proton affinities are further improved by second-order Møller–Plesset (MP 2) electron correlation corrections, but relative energies are less affected. The methyl group in the ethyl anion is destabilizing, the amino substituent is borderline, but all other groups are stabilizing. Very large π effects are exhibited by BH2 and BeH groups; inductive stabilization by the electronegative F and OH groups is less effective. Lithium also is stabilizing, but the best singlet geometry of CH2Li? is not planar. A planar CH2Li? triplet with a π1 configuration may be lower in energy.  相似文献   

4.
Ab initio molecular orbital theory has been used to probe the effect of the substituent X on the structures, strain energies, stabilization energies, inversion barriers, and proton affinities of carbanions CH3CX CH and cis-C3H4X?, where X = H, F, CN, and NC. All geometries have been optimized with a 3-21G basis set, and the parent anions (X = H) were also optimized with the same basis set with a diffuse function added (i.e. the 3-21 + G basis set). The anions, with the exception of the α-cyanoisopropyl anion, are pyramidal. The out-of-plane angle, α, for the pyramidal anions decreases in the order F > H ≈ NC > CN, and the barriers to inversion follow the same order with the cyclopropyl anions consistently having higher barriers than the isopropyl anions. The substituents strongly stabilize the anions with the stabilization energy following the order CN > NC > F. The cyano group slightly reduces the strain energy of cyclopropane, but the isocyano and fluoro substituents are weakly and strongly destabilizing, respectively. The pyramidal cyclopropyl anions are less strained than the cyclopropanes except when the substituent is a cyano group where the strain energies are reversed but are very similar. The planar anions all have higher strain energies than the cyclopropanes.  相似文献   

5.
Gas‐phase anionic reactions X? + CH3SY (X, Y = F, Cl, Br, I) have been investigated at the level of B3LYP/6‐311+G (2df,p). Results show that the potential energy surface (PES) of gas‐phase reactions X? + CH3SY (X, Y = Cl, Br, I) has a quadruple‐well structure, indicating an addition–elimination (A–E) pathway. The fluorine behaves differently in many respects from the other halogens and the reactions F? + CH3SY (Y = F, Cl, Br, I) correspond to deprotonation instead of substitution. The gas‐phase reactions X? + CH3SF (X = Cl, Br, I), however, follow an A–E pathway other than the last two out going steps (COM2 and PR) that proceeds via a deprotonation. The polarizable continuum model (PCM) has been used to evaluate the solvent effects on the energetics of the reactions X? + CH3SY (X, Y = Cl, Br, I). The PES is predicted to be unimodal in the solvents of high polarity. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007  相似文献   

6.
The anomeric effect of the functional groups X = C?N, C?CH, COOH, COO?, O? CH3, NH2, and NH+3 has been studied with ab initio techniques. Geometry effects upon rotation around the central C? O bond in X? CH2? O? CH3 have been compared in the various compounds. The energy differences between the conformers with a gauche and trans (X? C? O? C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non-sp3 hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO?, O? CH3, and NH2(g+) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, C?N, and C?CH (ca. 6 kJ/mol). For X = NH2(t) and NH+3 a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X? C? O? C follows roughly the same trend.  相似文献   

7.
Ab initio molecular orbital calculations have been carried out for the neutrals X? NH2, X? OH, and X? F and the anions X? NH? and X? O? with substituents X = Li, BeH, BH2, CH3, NH2, OH, and F. All structures have been fully optimized with the 4-31G basis set which is found to perform considerably better than the minimal STO-3G basis in predicting the lengths of strongly polar bonds. A quantitative analysis of interactions between the directly bonded groups, utilizing energy changes in hydrogenation reactions, is presented and rationalized with the aid of perturbation molecular orbital theory. Favorable interactions occur when electron-donor groups bond to electron-acceptor groups. This applies to both σ and π interactions, the relative importance of which depends on the particular substituents.  相似文献   

8.
CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N‐haloammonium cation, X? + NH3X+ (X = F, Cl, Br, and I), with comparison of classic anionic SN2 reactions, X? + CH3X. The described SN2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X? + NH3X+, charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE for X? + NH3X+ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE relative to the reactants are negative for all halogens: ?626.0 kJ/mol (F), ?494.1 kJ/mol (Cl), ?484.9 kJ/mol (Br), and ?458.5 kJ/mol (I). Stability energies of the ion–ion complexes ΔEcomp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R2 = 0.972) and proton affinity of halogen anions X? (R2 = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X? + CH3X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011  相似文献   

9.
Extensive semiempirical SCF-MO calculations confirm that the exo-anomeric effect in methyl O-, N- and S-glycosides deals with an interaction of π-character along the C1(SINGLE BOND)Y1 bond in a X5(SINGLE BOND)C1(SINGLE BOND)Y1(SINGLE BOND)Me moiety (where X = O, S; Y = O, NH, S). The bond-order between orbitals of pπ symmetry on C1 and Y1 serves as a measure of all significant molecular orbital interactions responsible for the exo-anomeric stabilization. The set of simpler compounds X(SINGLE BOND)CH2(SINGLE BOND)Y (X = OH, SH, SeH, TeH; Y = OH, SH, SeH, TeH, NH2) on which the anomeric effect has been well studied was also calculated and it is noticeable that the π-bond-orders accord with the results of other analyses of the ab initio wave function accounting for the anomeric effect. Although the AM1 and the PM3 parameterizations of MNDO do not accurately reproduce the anomeric effect energetic, they do reproduce accordingly the expected variations in the molecular conformations of complex carbohydrates, and thus it follows that there are maximal π-bond-orders for the synclinal arrangement around the C1(SINGLE BOND)Y1 bond. In addition, the π-bond-orders show the same behavior for conformational preferences around the C1(SINGLE BOND)C′1 and the C5(SINGLE BOND)C6 bonds in methyl C-glycosides and in the hydroxymethyl group of α-D -glucose, respectively. © 1996 by John Wiley & Sons, Inc.  相似文献   

10.
The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro-methane (CF2Br2) and iodomethane (CH3I)/methane (CH4) as reagent gases is examined. The NICI mass spectrum of CF2Br2 contains Br?, [HBr2]? and [CF2Br3]? anions. Weak acids (i.e. those acids with approximately ΔH°(acid) values between 1674 and 1464 kJ mol?1) react with Br? to produce minor yields of the hydrogen?bonded bromide attachment [MH + Br]? anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°(acid) > 1464 kJ mol?1) produce primarily [MH + Br]? anions with a minor yield of proton transfer [M ? H]? anion. The NICI spectrum of CH3I/CH4 is dominated by I?. Weak acids react with I? to yield minor amounts of [MH + 1]? or are unreactive. Strong acids produce only [MH + l]? anions. From a consideration of the gas-phase basicity of the halide anion and the binding energy of the hydrogen-bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.  相似文献   

11.
The 1H NMR chemical shifts of the C(α)? H protons of arylmethyl triphenylphosphonium ions in CD2Cl2 solution strongly depend on the counteranions X?. The values for the benzhydryl derivatives Ph2CH? PPh3+ X?, for example, range from δH=8.25 (X?=Cl?) over 6.23 (X?=BF4?) to 5.72 ppm (X?=BPh4?). Similar, albeit weaker, counterion‐induced shifts are observed for the ortho‐protons of all aryl groups. Concentration‐dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl? > Br? ? BF4? > SbF6?. For the less bulky derivatives PhCH2? PPh3+ X?, we also find C? H???Ph interactions between C(α)? H and a phenyl group of the BPh4? anion, which result in upfield NMR chemical shifts of the C(α)? H protons. These interactions could also be observed in crystals of (p‐CF3‐C6H4)CH2? PPh3+ BPh4?. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the C? H???X? hydrogen bonds between the phosphonium ion and anions, in particular Cl? or Br?. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh4? anions. The concentration dependence of the 1H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD2Cl2 solution. The cation–anion interactions increase with the acidity of the C(α)? H protons and the basicity of the anion. The existence of C? H???X? hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X‐ray analyses of the crystals. The IR spectra of the Cl? and Br? salts in CD2Cl2 solution show strong red‐shifts of the C? H stretch bands. The C? H stretch bands of the tetrafluoroborate salt PhCH2? PPh3+ BF4? in CD2Cl2, however, show a blue‐shift compared to the corresponding BPh4? salt.  相似文献   

12.
The kinetics of base hydrolysis of (αβ S)-(o -methoxy benzoato) (tetraethylenepentamine)cobalt(III) obeyed the rate law: kobs = kOH[OH?], in the range 0.05 ? [OH?]T, mol dm?3 ? 1.0, I = 1.0 mol dm?3, and 20.0–40.0°C. At 25°C, kOH = 13.4 ± 0.4 dm3 mol?1 s?1, ΔH = 93 ± 2 kJ mol?1 and ΔS = 90 ± 5 JK?1 mol?1. Several anions of varying charge and basicity, CH3CO2?, SO32?, SO42?, CO32?, C2O42?, CH2(CO2)22?, PO43?, and citrate3? had no effect on the rate while phthalate2?, NTA3?, EDTA4?, and DTPA5? accelerated the process via formation of the reactive ion pairs. The anionic (SDS), cationic (CTAB), and neutral (Triton X-100) micelles, however, retarded the reaction, the effect being in the order SDS> CTAB > Triton X-100. The importance of electrostatic and hydrophobic effects of the micelles on the selective partitioning of the reactants between the micellar and bulk aqueous pseudo-phases which control the rate are discussed. © 1994 John Wiley & Sons, Inc.  相似文献   

13.
The complex formation of PdII with tris[2-(dimethylamino)ethyl]amine (N(CH2CH2N(CH3)2)3, Me6tren) was investigated at 25° and ionic strength I = 1, using UV/VIS, potentiometric, and NMR measurements. Chloride, bromide, and thiocyanate were used as auxiliary ligands. The stability constant of [Pd(Me6tren)]2+ in various ionic media was obtained: log β([Pd(Me6tren)] = 30.5 (I = 1(NaCl)) and 30.8 (I = 1(NaBr)), as well as the formation constants of the mixed complexes [Pd(HMe6tren)X]2+ from [Pd(HMe6tren)(H2O)]3+:log K = 3.50 = Cl?) and 3.64 (X? = Br?) and [Pd(Me6tren)X]+ from [Pd(Me6tren)(H2O)]2+: log K = 2.6 (X? = Cl?), 2.8(Br?) and 5.57 (SCN?) at I = 1 (NaClO3). The above data, as well as the NMR measurements do not provide any evidence for the penta-coordination of PdII, proposed in some papers.  相似文献   

14.
The tetrachlorocuprate(II) ethylenediammonium and tetrachlorocadmate(II) ethylenediammonium were synthesized. Chemical analysis, elemental analysis, and X‐ray crystallography were applied to characterize the compositions and crystal structures of the two complexes. The lattice potential energies and the radiuses of the anions of two complexes were calculated to be UPOT[(C2H10N2)CuCl4]=1810.19 kJ·mol?1, UPOT[(C2H10N2)CdCl4]=1784.39 kJ·mol?1, r[(CuCl4)2?]=0.308 nm, and r[(CdCl4)2?]=0.321 nm from the data of the crystal structure, respectively. Low‐temperature heat capacities of the two complexes were measured by a precision automatic adiabatic calorimeter with the small sample over the temperature range from 78 to 400 K, respectively. Two polynomial equations of heat capacities against the temperatures were fitted by least square method: Cp,m[(C2H10N2)CuCl4, s] =213.553+118.578X?5.816X2+4.392X3+0.276X4 and Cp,m[(C2H10N2)CdCl4, s] =190.927+98.501X?7.931X2+0.657X3+3.834X4, in which X= (T?239)/161. Based on the fitted polynomial equations, the smoothed heat capacities and thermodynamic functions of the two complexes relative to the standard reference temperature 298.15 K were calculated at intervals of 5 K.  相似文献   

15.
The first vertical electron affinities EA of 13 series of molecules and free radicals D(X i ) n are related to the inductive (σ I ), resonance (σ R ? ), and polarization (σα) parameters of substituents X i by the dependences EA = EA H + aΣσ I + bΣσ R/? + cΣσα: In radical anions D(X i ) n , compared to radical cations D(X i ) n , the polarization interaction is weaker or similar in magnitude but has an opposite sign. The previously unknown resonance parameters σ R ? of substituents SiMe3 and CH2SiMe3 bound to the radical anion center H2C=CH were calculated.  相似文献   

16.
The dinuclear cyclopalladated complexes [Pd(L1 or L2)(µ-X)]2 (HL1=5-alkyl-2-(4′-alkoxyphenyl)pyrimidine, HL2=3-(4′-alkoxyphenyl)-6-alkoxypyridazine, X=Cl?, CH2ClCOO?, CH2BrCOO?, CH3CHBrCOO?, CH2BrCH2COO?, CH3COO?) have been synthesized and characterized; their mesogenic properities were determined by DSC and polarizing microscopy. The effect of the bulk and the polarity of the bridging ligands on their mesogenic properties is discussed. The effect of the length of the alkyl chains on the mesogenic properties of these organometallic complexes has also been investigated.  相似文献   

17.
According to the X-ray diffraction data, the crystal and molecular structure of tris(2-hydroxyethyl) ammonium fluoride (F?N+H(CH2CH2OH)3, fluoroprotatrane, substantially differs from other halo protatranes X?N+H(CH2CH2OH)3 (X = Cl, Br, and I). At X = F, to the endo-molecular LP of the nitrogen atom the HF molecule having the minimum ionic radius in a series of X? anions is bonded. The geometry of fluoroprotatrane and the cation packing in the crystal are analyzed.  相似文献   

18.
Eric Magnusson 《Tetrahedron》1985,41(14):2945-2948
Substituent interaction energies are calculated by ab initio molecular orbital methods for the two series SiH2X- and SiH3X for the directly bound substituents X = BH2, CH3, NH2, OH, F and the results compared with those for the corresponding first row species. Interactions with the groups NH2, OH, F are as large in the neutral as in the anionic series and this is attributed to the presence of important π-bonding interactions, supplementing the effects of inductive withdrawal of σ-electrons. The restoration of charge neutrality by π-donation to silicon is more important in the neutral molecules, σ-electron transfer from silicon in the anions. π-Bonding with the π-acceptor substitutent BH2 is favourable, as it is in the CH3X and CH2X- systems, but with π-donor substituents the interactions are always destabilizing.  相似文献   

19.
The geometries of the amines NH2X and amido anions NHX?, where X = H, CH3, NH2, OH, F, C2H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH3NH?, NH2NH?, and HONH? are very similar to those for the isoprotic and isoelectronic neutral compounds CH3OH, NH2OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH2X is X = CN > HCO > F ≈ C2H > OH > NH2 > CH3 > H and for the reaction NHX? + H+ → NH2X the computed energies vary over the range 373–438 kcal/mol.  相似文献   

20.
Preparation of μ-Sulfurdisulfonium Salts [(CH3)2S? Sx? S(CH3)2]2+2A? (x = 1–3, A? = AsF6?, SbF6?, SbCl6?). On the Analogy of the Reactivity of Sulfanes and Sulfonium Salts The preparation of the μ-sulfurdisulfonium salts [(CH3)2S? Sx? S(CH3)2]2+(A?)2 with x = 1–3 and A? = AsF6?, SbF6?, SbCl6? is reported. The salts are formed by reaction of (CH3)2SH+A? and (CH3)2SSH+A? with SCl2 and S2Cl2, resp. They are characterized by vibrational spectroscopic measurements. [(CH3)2S? S2? S(CH3)2]2+(SbF6?)2 crystallizes in the space group C2/c with a = 1 884.5(7) pm, b = 1 302.8(5) pm, c = 1 477.2(5) pm, β = 98.62(3)° und Z = 8.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号