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1.
Prof. Dr. Robert A. Rouse 《Theoretical chemistry accounts》1976,41(2):149-156
Despite the large number of experimental and theoretical studies on the size, shape, and orientation of lone pairs and their resulting stereochemical character, lone pairs still remain poorly defined in terms of quantitative observable properties of a molecule. Using the conformation of saturated molecules and barriers to internal rotation, experimental chemists have arrived at conflicting sizes and orientations for lone pairs. Most theoretical attempts to define lone pair properties have centered on such non-observables as localized molecular orbitals or have been based on studies on isolated molecules.The use of observable properties to construct a consistent set of physical models to analyze the physical nature of lone pairs is discussed. Much as one probes an electric field with a test charge, probes such as H+, H–, He and H could be used to probe regions of molecules such as NH3 and H2O where lone pairs are often postulated to exist.Ab initio quantum mechanical studies can be analyzed using electron density (and resulting changes during interaction), total pair density of electrons, the electrostatic potential about the molecule and bond energy analysis to study lone pair properties. A simple study of NH3 using an H+ probe is presented to clarify the approach. 相似文献
2.
Seven different optimized conformers of α‐fluoroglycine (H2NCHFCOOH) were obtained from ab initio calculations. Some of these conformers are exceptionally stable compared to similar conformers of glycine. Conformers in which the lone pair of electrons on the nitrogen atom are antiperiplanar to the C F bond are more stable than conformers that do not have such an arrangement. The stability difference between conformers with such an arrangement and conformers that have the lone pair of electrons synperiplanar to the C F bond is about 27 kJ/mol (calculated at the MP2/6‐31+G* level). Conformers that have the lone pair of electrons antiperiplanar to the C F bond possess a longer C F bond, a shorter C N bond, and sp2‐like amino bond angles. For some conformers an unusual hydrogen bond involving the acidic carboxylic acid hydrogen and the electronegative fluorine atom is observed. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 426–431, 2000 相似文献
3.
U. Fleischer M. Schindler W. Kutzelnigg 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4)
Abstract The IGLO (individual gauge for localized orbitals) method1, has been applied successfully to organic molecules.2 The application to phosphorous compounds is more laborious, mainly since larger basis sets are required. By the IGLO method one obtains the chemical shielding as a sum of contributions of localized orbitals. For 31P the dominant contributions come from the K-shell (well transferable), the L-shell (depending somewhat on the bonding situation), the bonds attached to P (large differences between single and multiple bonds), and the lone pair on P (large variations), the contributions of distant bonds and lone pairs being small, but often not negligible. We find good agreement with experiments for those molecules for which experimental data are available (e.g. PH3, P2H4, P4, CH3PH2, OPF3) and we can make predictions for others (e.g. P2, P2H2, HSiP). The interpretation of the variation of is more complicated than in the case of hydrocarbons, since bond contributions are usually not transferable between different molecules, and it is hard to justify an increment system. An interesting example is the dependence of the 31P-shift in RC≡P on R. 相似文献
4.
5.
Hartree-Fock (HF) and molecular mechanics calculations were performed on linear azine oligomers and model compounds. The rotational energy curves for the model compounds formaldazine, H2C = N? N = CH2, ethenyl diazene, H2C = CH? N = NH, and ethanediimine, HN = CH? CH = NH were calculated for a variety of basis sets at the HF and MP2 level. In all of these cases the rotational energy barriers are quite different from butadienes or aza-substituted butadienes because of the lone pair–lone pair interaction of the adjacent nitrogen atoms. The results on the model compounds were used to generate a set of molecular mechanics (MM) parameters that are appropriate for linear oligo- and polyazines. Comparison of the geometries of the HF results and MM results for the oligoazines showed that the two methods gave comparable results. © 1994 by John Wiley & Sons, Inc. 相似文献
6.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1992,48(11-12):1795-1799
The HeI photoelectron spectrum of the hydrogen bonded hetero-dimer H2S⋯HCl shows two vertical ionization energies at 10.91 and 12.16 eV. Ab initio MO calculations reveal that these features are due to the sulphur and chlorine lone pair ionizations respectively. Results show that while the ground ionic state is repulsive the first excited ionic state is strongly bound. The photoelectron spectrum of the diethyl sulphide⋯HCl complex is similar to that of H2S⋯HCl 相似文献
7.
《Tetrahedron》1987,43(17):3863-3874
Ah initio molecular orbital calculations have been carried out on adducts of trihydroxy phosphine, P(OH)3, and formaldehyde, H2C=0. Stationary points were located and a reaction surface calculated. One stationary point exists as a stable pentacovalent phosphorane, and the other as a 1,3-dipolar transition state. Calculations differing in the conformation about the P-OH bonds of the phosphite reveal that an antiperiplanar (app) lone pair on oxygen to the phosphorus lone pair (acyclic analogue) raises the energy of the molecule by 1.7 kcal/mol relative to a phosphite conformation with no app lone pairs to the phosphorus lone pair (bicyclic analogue). In the transition state, the relative energy between the two conformations reverses with the acyclic analogue transition state 5 kcal/mol lower energy than the bicyclic analogue transition states. The lower energy for the acyclic analogue in the transition state is attributed to the mixing of the app lone pairs on the oxygens of the phosphite mixing with the σ orbital of the newly formed bond between phosphorus and carbon. This kinetic Stereoelectronic effect can explain why acyclic phosphites react much faster in nucleophilic reactions than bicyclic phosphites. This phenomenon suggests that the origin of the α-effect, the enhanced nucleophilicity of a base possessing a heteroatom with an adjacent unshared electron pair arises from the stereoelectronic effect. 相似文献
8.
14N and 35Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N? P bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the N? P bond due to the pπ? dπ interaction and pπ? σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N? P bond populations is observed in X ? PRn (R2N)3-n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P?Se > P?S > P?O > P. 相似文献
9.
Paul G. Mezey Raymond Daudel Imre G. Csizmadia 《International journal of quantum chemistry》1979,16(5):1009-1019
Size and shape parameters for the core, bonding, and lone electron pairs of the ten-electron hydrides (CH4, NH3, H2O, HF) were determined from ab initio MO wave functions using various Gaussian basis sets. The fundamental features of approximate electron pair loge representation are somewhat more sensitive to the quality of the basis functions than the molecular total energy. The total size of the molecular electron distribution is less affected by basis set variations than its components: the core, bonding, and lone pair sizes. There is an apparent tendency to “preserve” the total size of molecular distribution. 相似文献
10.
Dr. Friedrich Wossidlo Daniel S. Frost Jinxiong Lin Dr. Nathan T. Coles Katrin Klimov Manuela Weber Dr. Tobias Böttcher Prof. Dr. Christian Müller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(50):12788-12795
The synthesis and isolation of a phosphinine selenide was achieved for the first time by reacting red selenium with 2,6-bis(trimethylsilyl)phosphinine. The rather large coupling constant of 1JP,Se=883 Hz is in line with a P−Se bond of high s-character. The σ-electron donating Me3Si-substituents significantly increase the energy of the phosphorus lone pair and hence its basicity, making the heterocycle considerably more basic and nucleophilic than the unsubstituted phosphinine C5H5P, as confirmed by the calculated gas phase basicities. NBO calculations further reveal that the lone pairs of the selenium atom are stabilized through donor-acceptor interactions with antibonding orbitals of the aromatic ring. The novel phosphinine selenide shows a distinct reactivity towards hexafluoro-2-butyne, Au(I)Cl as well as iPrOH. Our results pave the way for new perspectives in the chemistry of phosphorus in low coordination. 相似文献
11.
Steinar Husebye Karl W. Trnroos 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(10):1242-1244
The structure of the title compound, [TeCl4(C5H10N2S)2] or C10H20Cl4N4S2Te, has been solved in order to study the stereochemical activity of the lone pair of electrons on TeIV. The two crystallographically independent molecules in the asymmetric unit both show a distorted octahedral coordination of the Te atom. The two Te—S bonds are trans to each other in both molecules and are greatly asymmetric, with bond lengths of 2.5686 (7) versus 2.8557 (8) Å and 2.5859 (7) versus 2.8165 (9) Å. The Te—Cl bond lengths lie in the range 2.5236 (7)–2.5589 (8) Å. The asymmetric Te—S bonds and a large S—Te—Cl angle of ca 97° involving the long Te—S bonds indicate stereochemical activity of the lone pair of electrons on Te. 相似文献
12.
Using (FH)2 and (FH)3 to Bridge the σ‐Hole and the Lone Pair at P in Complexes with H2XP,for X=CH3, OH,H, CCH,F, Cl,NC, and CN
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Ab initio MP2/aug′‐cc‐pVTZ calculations are used to investigate the binary complexes H2XP:HF, the ternary complexes H2XP:(FH)2, and the quaternary complexes H2XP:(FH)3, for X=CH3, OH, H, CCH, F, Cl, NC, and CN. Hydrogen‐bonded (HB) binary complexes are formed between all H2XP molecules and FH, but only H2FP, H2ClP, and H2(NC)P form pnicogen‐bonded (ZB) complexes with FH. Ternary complexes with (FH)2 are stabilized by F?H???P and F?H???F hydrogen bonds and F???P pnicogen bonds, except for H2(CH3)P:(FH)2 and H3P:(FH)2, which do not have pnicogen bonds. All quaternary complexes H2XP:(FH)3 are stabilized by both F?H???P and F?H???F hydrogen bonds and P???F pnicogen bonds. Thus, (FH)2 with two exceptions, and (FH)3 can bridge the σ‐hole and the lone pair at P in these complexes. The binding energies of H2XP:(FH)3 complexes are significantly greater than the binding energies of H2XP:(FH)2 complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone‐pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge‐transfer energies across the pnicogen bond correlate with the intermolecular P?F distance, while charge‐transfer energies across F?H???P and F?H???F hydrogen bonds correlate with the distance between the lone‐pair donor atom and the hydrogen‐bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron‐donor atom and the hydrogen‐bonded F atom. EOM‐CCSD spin‐spin coupling constants 2hJ(F–P) across F?H???P hydrogen bonds, and 1pJ(P–F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F–P coupling data than pnicogen bonds, despite the longer F???P distances in F?H???P hydrogen bonds compared to P???F pnicogen bonds. There is a correlation between the two bond coupling constants 2hJ(F–F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes. 相似文献
13.
Summary The NCH
2CH3 region of the1H-n.m.r. spectrum oftrans-PtCl2(-C2H4)(NHEt2) has been investigated and shown to contain inequivalent protons. A study has also been made in related amine complexes of the conditions under which Pt-N-CH
2 protons are equivalent, NH-CH coupling takes place and195Pt-N-CH coupling occurs.The lability of the N-H bond intrans-olefin-amine, carbonyl-amine, ethanide-amine and phosphine-amine complexes is also investigated and compared with that of the platinum-amine link.Trans-labilizing powers of-C2H4, CO, CH2¯CH2 and Ph3P are compared. 相似文献
14.
Amrita DasSomnath Ray Choudhury Prankrishna MannaDominic Baxter Madeleine HelliwellSubrata Mukhopadhyay 《Polyhedron》2011,30(12):2121-2126
One Mg(II) malonate complex with protonated 2-aminopyridine and hexafluoridophosphate as counterions, (C5H7N2)4[Mg(C3H2O4)2(H2O)2](PF6)2 (1) [C5H7N2 = protonated 2-aminopyridine, C3H4O4 = malonic acid] has been synthesized from purely aqueous media just by mixing the reactants in their stoichiometric proportion and its crystal structure has been determined by single-crystal X-ray diffraction. The role of weak forces like lone pair?π and anion?π interactions in influencing the self-assembly process appears to be of importance. A rare combination of lone pair?π and anion?π interactions in 1, of the type lone pair?π/π?π/π?anion, is observed, and this unusual supramolecular network is fully described here. 相似文献
15.
The dipole moments of the (C2H5)nPX3?n (X = Cl, Br, I; n = 0, 1, 2, 3) and P{EIVB(CH3)3}3 compounds (EIVB = C, Si, Sn) have been determined by dielectric measurements in benzene at 25°C by Higasi's method. The results are interpreted in terms of an additive vector model from bond moment calculations and a maximum phosphorus lone pair contribution of 1.75 D as calculated for P(C2H5)3. The high dipole moment of the mixed PA2B-type compounds in comparison with PA3 moments is mainly ascribed to a loss of C3v symmetry, characterized by a quite asymmetric orientation of the phosphorus lone pair with respect to the phosphorus bonding orbitals. 相似文献
16.
We have investigated the nature of the interactions of ns2‐cations and the possible structure‐determining role of the ns2electron pair at ambient and high pressure in several AB2X5 (A = K, Rb, Cs, In, Tl; B = Sn, Pb, Sr; X = Cl, Br, I) compounds. Structural parameters are obtained by high pressure x‐ray diffraction as well as by quantum mechanical methods (DFT‐GGA‐calculations). The structural parameters at ambient and high pressure are discussed and compared to those of Tl5Se2I crystallising in the antitype structure. Short cation—cation distances in the NH4Pb2Br5 type structure enable direct cation—cation interactions and the existence of an ns2‐cation in the B‐position is crucial for the stability of these structures. The effect of pressure on the structural parameters of these compounds gives new insights into the interactions of lone pair cations. The pronounced decrease of the cation—cation distances with pressure points to strongly increasing bonding interactions between the lone pair cations. 相似文献
17.
The geometries and the energies of conformers of PnHn + 2 (n = 2−9) have been studied with PM3 method. To test the quality of the semiempirical results, ab initio calculations have been carried out on P3H5. All results of P2H4 and P3H5 with PM3 are consistent with the experimental and ab initio data. According to the analysis of P3H5 and P4H6 results, it is concluded that gauche interaction between adjacent lone electron pairs and gauche interaction between polar P-H bond with adjacent polar P-P bond are important for predicting the stable conformer of open-chain phosphoanes. The calculations of PnHn + 2 (n > 4) give further support to this conclusion. © 1996 John Wiley & Sons, Inc. 相似文献
18.
I. I. Zakharov O. V. Voroshina A. N. Startsev 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2006,80(7):1083-1087
The density functional theory (DFT) with the B3P86 hybrid exchange-correlation functional was used to calculate the molecular and electronic structure of the Mo12S24 macromolecule as a single MoS2 layered structure slab. Calculations with geometry optimization are indicative of insignificant relaxation of the coordinatively unsaturated Mo and S atoms, which corresponds with the literature DFT data on the MoS2 single slab obtained with periodic boundary conditions. The calculated forbidden band width (0.85–0.98 eV) is comparable with its experimental value (1.30 eV) and the results of DFT calculations of MoS2 with periodic boundary conditions (0.89 eV). An analysis of the electronic state of the surface Mo centers in the Mo12S24 macromolecule showed that these centers were reduced to a greater degree than the Mo(IV) atoms in the bulk. The adsorption complex between the Mo12S24 macromolecule and six H2S molecules was calculated to characterize the adsorption ability of the coordinatively unsaturated Mo centers. The structure and energy characteristics of the adsorption complex corresponded to weak donor-acceptor interaction between the π lone pair of H2S and the surface (reduced) Mo centers. The suggestion was made that the active center of the catalytic cycle of thiophene hydrodesulfurization should induce the oxidative addition of H2 followed by the occlusion of hydrogen into the MoS2 matrix. 相似文献
19.
PE Spectra and Molecular Properties. 89. Ionisation Patterns and Conformation of Cyclo Polythianes (H2CS)n Cyclopolythianes (H2CS)n (n = 3, 4, 5) show characteristic low-energy ionization patterns, which are assigned to radical cation states with predominant sulfur lone pair contributions, and the correlation of which with topological eigenvalues ? of a perimeter model exhibit the rather low standard deviation of only 0.08 eV. This finding suggests that the sulfur lone pairs should be equivalent on time-averaging. Geometry-optimized SCF calculations concerning preferred conformations as well as molecular flexibility of cyclopolythianes (H2CS)3 and (H2CS)4 in the gaseous phase, yield low barriers for transition between tub and chair conformers and also eigenvalue-sets which correlate with the PE data. 相似文献
20.
The structures of title compounds, [TeBr2(C8H6Br)(C6H5)] (I) and [TeBr2(C8H6Br)(C7H9)](H2O)(CT3OH) (II), have been determined by X‐ray diffraction. The structures confirm that E‐ or Z‐type configuration of vinylic telluride depends on the polarity of solvent employed. In either structure, Te atom is in a trigonal dipyramide configuration with the lone pair of electrons in the equatorial position. 相似文献