The physical nature of the lone pair

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.     

Theoretical analysis of fluoroglycine conformers

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     

Theory of 31P Chemical Shifts

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.     

A computational study of azine,azoethene, and diimine linkages in the poly/oligoazine system

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.     

Ultraviolet photoelectron spectroscopy of the hydrogen bonded heterodimer H2S⋯HCl

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     

Stereoelectronic Effects on the Nucleophilic Addition of Phosphite to the Carbonyl Double Bond: Ab Initio Molecular Orbital Calculations on Reaction Surfaces and the α-Effect

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.     

14N and 35Cl NQR studies of organophosphorus compounds

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 p_π? d_π interaction and p_π? 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 ? PR_n (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.     

Dependence of approximate ab initio molecular loge sizes on the quality of basis functions

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.     

Making Aromatic Phosphorus Heterocycles More Basic and Nucleophilic: Synthesis,Characterization and Reactivity of the First Phosphinine Selenide

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 ¹J_P,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.     

trans‐Tetrachlorobis(N,N′‐dimethyl­imidazolidine‐2‐thione)tellurium(IV), a thiourea complex of tellurium with asymmetric Te—S bonds

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 Te^IV. 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.     

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

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.     

Proton lability in the amino ligand of and the1H-n.m.r. spectra oftrans-dichloro(diethylamine)(π-ethene)platinum and related complexes

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.     

Associative combination of lone pair-π, π-π and anion-π interactions observed in a ternary system comprising Mg(II)-malonate-2-aminopyridine-hexafluoridophosphate

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.     

Study of the electric dipole moments of the (C2H5)n PX3−n and P{EIVB (CH3)3}3 compounds (X = Cl,Br, I; EIVB = C,Si, Sn; n = 0, 1, 2, 3)

The dipole moments of the (C₂H₅)_nPX_3?n (X = Cl, Br, I; n = 0, 1, 2, 3) and P{E^IVB(CH₃)₃}₃ compounds (E^IVB = 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 C_3v symmetry, characterized by a quite asymmetric orientation of the phosphorus lone pair with respect to the phosphorus bonding orbitals.     

High Pressure Study of NH4Pb2Br5 Type Compounds. I Structural Parameters and Their Evolution under High Pressure

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.     

Gauche effect and PM3 calculation of open-chain polyphosphorus hydrides

The geometries and the energies of conformers of P_nH_{n + 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 P_nH_{n + 2} (n > 4) give further support to this conclusion. © 1996 John Wiley & Sons, Inc.     

The molecular and electronic structure of the Mo12S24 macromolecule as a model of the active component of a hydrodesulfurization catalyst

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.     

PE-Spektren und Moleküleigenschaften. 89. Ionisationsmuster und Konformation von Cyclo-Polythianen (H2CS)n

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.     

Molecular Structures of Dibromo[(E)2‐bromo‐2‐phenylvinyl]‐(phenyl) tellurium (IV) and Dibromo [(Z)‐2‐bromo‐2‐phenyl‐vinyl] (p‐tulyl) tellurium (IV) hydrate methanolate

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.