An ab initio study of the electronic and steric structure of Cl<Subscript>2</Subscript>ZX molecules (Z = P and As)

The electronic and steric structure of the Cl₂ZX molecules [Z = P and As, X = C₂H₅, N(CH₃)₂, and OCH₃] was examined by RHF/6-31G(d) and MP2/6-31G(d) calculations. The data on the electron distribution at the Cl atoms are compared with the published ³⁵Cl NQR data. The main reason for a decrease in the NQR frequency of the molecules with X = N(CH₃)₂ and OCH₃ as compared to the ethyl-substituted compounds is an increase in the population of the 3p components of their p _z(p _σ) orbitals. With X = N(CH₃)₂, electron distribution at two Cl atoms differs significantly.     

Geminal interactions in ClZ(CH3)2X molecules (Z = C, Si, Ge) according to ab initio calculations

The structure and electronic parameters of ClZ(CH₃)₂X molecules (Z = C, Si, Ge, X = CH₃, OCH₃) were calculated by the RHF/6–31G(d) and RHF/6–311G(d,p) methods with full geometry optimization; calculations of ClZ(CH₃)₂OCH₃ molecules were also performed by the RHF/6–31G(d) method with partial geometry optimization. The ³⁵Cl NQR frequencies calculated from the populations of less diffuse 3p constituents of valence p orbitals of chlorine [RHF/6–31G(d)] were in agreement with the experimental values. The ³⁵Cl NQR frequencies for molecules with X = OCH₃ are lower than those for molecules with X = CH₃ (the Z atom being the same), due mainly to direct through-field polarization of the Z-Cl bond, induced by the effect of unshared electron pair of the oxygen atom in the trans position with respect to that bond. The difference in the ³⁵Cl NQR frequencies decreases in going from Z = C to Z = Si, Ge, in parallel with variation of the Z-Cl bond polarization as the size of Z increases.     

Ab Initio Study of the Conformational and Geometrical Isomerism in Heteroallyl and Heteropropenyl Systems

Conformational isomerism has been studied by ab initio methods (RHF/6-31+G*, MP2/6-31+G*) for CH₂=CHCH₂X heteroallyl and CH₃CH=CHX heteropropenyl systems (X = H, Me, NMe₂, OMe, PMe₂, SMe, ONCH₂). In 3-heteroprop-1-enes, substituents preferably occupy the AC position relative to the C=C double bond. The E isomers of 1-methylthio- and 1-methoxyprop-1-enes, which are thermodynamically more stable, have two stable forms, SP and AC; for 1-dimethylamino- and 1-imethylphosphinoprop-1-enes, the stable forms are AP and SC. The molecule of the E isomer of formoxime propenyl ether exists in two stable rotamer forms, SC and AP, the latter being predominant. The Z isomers preferably exist in the form of AC (X = CH₃O, CH₃S) and AP (X = (CH₃)₂N, (CH₃)₂P, CH₂=NO) conformations. Migration of the double bond toward an heteroatom in formoxime allyl ether, forming the E and Z isomers, is energetically favorable, the Z isomer being thermodynamically preferable.     

The anomeric effect: Ab-initio studies on molecules of the type X?CH2?O?CH3

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.     

Quantum-Chemical Study of the Electronic and Steric Structure of Vinyltetrazoles: II. 2-Vinyltetrazoles

The total Mulliken charges on the C and N atoms, populations of the S-trans-(N¹) conformers, and rotation barriers in the molecules of 2-vinyl-5-R-tetrazoles (R = H, CH₃, CH = CH₂, C₆H₅, CH₂Cl, CF₃) were calculated ab initio (HF/6-31G**, MP2/6-31G**). The results were compared with the ¹H and ¹³C NMR data for these compounds.     

Degenerate lithium-hydrogen exchange reactions: Ab initio models for metallation mechanisms involving H2, CH4, NH3, H2O,and HF

Hydrogen exchange reactions between lithium and sodium compounds, MX (M=Li: X=H, CH₃, NH₂, OH, F; M=Na: X=CH₃), and the corresponding hydrides, HX, have been modelled by means of ab initio calculations including electron correlation and zero point energy (ZPE) corrections. Small or no activation barriers (from the initial complexes) are encountered in systems involving lone pairs (10.8, 2.4, 0.0 kcal/mol for X=NH₂, OH, F, respectively). Since the association energies of the initial complexes are much larger (21.0, 20.4, 23.5 kcal/mol, respectively; MP2/6–31+G*/6–31+G* + ZPE), such exchange reactions should occur spontaneously in the gas phase. The methyl systems (X=CH₃) have the largest barriers: 26.7 (M=Li) and 31.7 (M=Na) kcal/mol (MP2/6–31+G*/6–31G* + ZPE), and the initial complexes are only weakly bound. The significance of these systems as models for hydrogen exchange reactions in complexes of electropositive transition metals is discussed. However, the gegenion-free exchange of hydrogen between CH₃ and CH₄ has a much lower, 11.8 kcal/mol barrier (MP2/6–31+G*/6–31+G* + ZPE). All the transition structures are highly ionic (charges on the metals > +0.8). The effect of aggregation has been considered by examining the hydrogen exchange between (LiX)₂ and HX(X=H, CH₃, NH₂, OH). Although these dimer reactions formally involve six, instead of four electrons, no “aromatic” preference is observed.     

A theoretical approach to substituent effects: Interaction between directly bonded groups in the isoelectronic series XNH3+, XCH3, and XBH3−

Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH₂, CH₃, NH₂, OH and F) and substrates Y(Y = NH₃⁺, CH₃, BH₃^?) in the isoelectronic series XNH₃⁺, XCH₃ and XBH₃^?. The results indicate that the interaction energies are dominated by σ-effects. NH₃⁺ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH₂) and unfavorably with the σ-acceptors (e.g. F, OH, NH₂). The reverse pattern a observed for XBH₃^?. The range of interaction energies for XCH₃ is considerably smaller than for XNH₃⁺ and XBH₃^?.     

A theoretical approach to substituent effects. Interactions between directly bonded groups in the neutrals X?NH2, X?OH,and X?F and the anions X?NH− and X?O−

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.     

Die Reaktionen von M[BF4] (M = Li,K) und (C2H5)2O·BF3 mit (CH3)3SiCN. Bildung von M[BFx>(CN)4—x] (M = Li,K; x = 1, 2) und (CH3)3SiNCBFx(CN)3—x, (x = 0, 1)

The Reactions of M[BF₄] (M = Li, K) and (C₂H₅)₂O·BF₃ with (CH₃)₃SiCN. Formation of M[BF_x(CN)_4—x] (M = Li, K; x = 1, 2) and (CH₃)₃SiNCBF_x(CN)_3—x, (x = 0, 1) The reaction of M[BF₄] (M = Li, K) with (CH₃)₃SiCN leads selectively, depending on the reaction time and temperature, to the mixed cyanofluoroborates M[BF_x(CN)_4—x] (x = 1, 2; M = Li, K). By using (C₂H₅)₂O·BF₃ the synthesis yields the compounds (CH₃)₃SiNCBF_x(CN)_3—x x = 0, 1. The products are characterized by vibrational and NMR‐spectroscopy, as well as by X‐ray diffraction of single‐crystals: Li[BF₂(CN)₂]·2Me₃SiCN Cmc2₁, a = 24.0851(5), b = 12.8829(3), c = 18.9139(5) Å V = 5868.7(2) ų, Z = 12, R₁ = 4.7%; K[BF₂(CN)₂] P4₁2₁2, a = 13.1596(3), c = 38.4183(8) Å, V = 6653.1(3) ų, Z = 48, R₁ = 2.5%; K[BF(CN)₃] P1¯, a = 6.519(1), b = 7.319(1), c = 7.633(2) Å, α = 68.02(3), β = 74.70(3), γ = 89.09(3)°, V = 324.3(1) ų, Z = 2, R₁ = 3.6%; Me₃SiNCBF(CN)₂ Pbca, a = 9.1838(6), b = 13.3094(8), c = 16.840(1) Å, V = 2058.4(2) ų, Z = 8, R₁ = 4.4%     

Water exchange on beryllium complexes: part VIII – influence of neutral electron pair donors

In this article, water exchange reactions on [Be(L)(H₂O)₃]²⁺ (L?=?NH_{3? x} (CH₃) _x , PH_{3? x} (CH₃) _x , AsH_{3? x} (CH₃) _x , OH_{2? x} (CH₃) _x , SH_{2? x} (CH₃) _x , SeH_{2? x} (CH₃) _x , pyridine, 4-fluoropyridine, 4-bromopyridine, 4-chloropyridine, 4-hydroxypyridine, 4-thiolopyridine, 4-selenidopyridine, 4-nitrilopyridine, 1,4-diazine, 1,3,5-triazine, HCN, acetonitrile, and benzonitrile) are examined, utilizing the B3LYP//6-311?+?G** density functional for geometry optimizations, and B3LYP//6-311?+?G** both with and without the CPCM solvent model as well as MP2(full)//6-311?+?G** for subsequent single-point energy calculations. In all examined cases, the results prove that these complexes show associative interchange mechanisms for water exchange. With the exception of the NH _x (CH₃)_{3? x} series of ligands, activation energy barriers vary little, making these ligands mostly spectator ligands. Geometrical parameters vary mainly with the ligand size.     

Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−

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.     

Excitonic Bands in the Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units

Summary.  The optical absorption, photoluminescence, and photoconductivity spectra of some compounds of the formulas [R(CH₂) _n NH₃] _x M _y X _z , [R(CH₂) _n NH(CH₃)₂] _x M _y X _z , [R(CH₂) _n S(CH₃)₂] _x M _y X _z , [R(CH₂) _n SC(NH₂)₂] _x M _y X _z , and [R(CH₂) _n SeC(NH₂)₂] _x M _y X _z (R = organic residue; M = Bi(III), Pb(II), Sn(II), Cu(I), Ag(I) etc; X = I, Br, Cl; n, x, y, z = 0, 1, 2, 3, …) are briefly reviewed, and some new results are reported. The position, intensity, and shape of the excitonic bands depend on the dimensionality and size of the inorganic network as well as on the nature of the M, X, R, and onium moieties. Received June 23, 2000. Accepted August 1, 2000     

Excitonic Bands in the Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units

 The optical absorption, photoluminescence, and photoconductivity spectra of some compounds of the formulas [R(CH₂) _n NH₃] _x M _y X _z , [R(CH₂) _n NH(CH₃)₂] _x M _y X _z , [R(CH₂) _n S(CH₃)₂] _x M _y X _z , [R(CH₂) _n SC(NH₂)₂] _x M _y X _z , and [R(CH₂) _n SeC(NH₂)₂] _x M _y X _z (R = organic residue; M = Bi(III), Pb(II), Sn(II), Cu(I), Ag(I) etc; X = I, Br, Cl; n, x, y, z = 0, 1, 2, 3, …) are briefly reviewed, and some new results are reported. The position, intensity, and shape of the excitonic bands depend on the dimensionality and size of the inorganic network as well as on the nature of the M, X, R, and onium moieties.     

[{(CH3)3Si}3C–Li–C{Si(CH3)3}3][Li · 3(OC4H8)] und {(CH3)3Si}3C–Li · O=C(Si(CH3)3)2, zwei neue Addukte des Lithium‐trisylmethanids

[{(CH₃)₃Si}₃C–Li–C{Si(CH₃)₃}₃][Li · 3(OC₄H₈)] and {(CH₃)₃Si}₃C–Li · O=C(Si(CH₃)₃)₂, two New Adducts of Lithium Trisylmethanide Sublimation of (Tsi–Li) · 2 THF (Tsi = –C(Si(CH₃)₃)₃) at 180 °C and 10^–4 hPa gives (Tsi–Li) · 1.5 THF in very low yield. The X‐ray structure determination shows an almost linear [Tsi–Li–Tsi]^– anion connected by short agostic Li…C contacts with the threefold THF‐coordinated Li‐cation. Base‐free Tsi–Li, solved in toluene is decomposed by oxygen, forming the strawberry‐colored ketone O=C(SiMe₃)₂, which forms an 1 : 1 adduct with undecomposed Tsi–Li. The X‐ray structure elucidation of this compound is also discussed.     

Stable conformations of CH3CH2OCH2CH2OH: A comparison of theoretical methods

This article presents the results of an extensive examination of the stable conformations of CH₃CH₂OCH₂CH₂OH at various levels of theory. In particular, 41 initial conformations are optimized using the MM2 force field in BIGSTRN-3; the MINDO/3, MNDO, and AM1 Hamiltonians in AMPAC 2.2; the PM3 Hamiltonian in MOPAC 7.0; and at the HF/STO-3G and HF/3-21G levels using Gaussian 92. The optimized HF/3-21G structures are reoptimized at the HF/6-31G(d) level, and the unique structures are optimized again at the MP2 = FULL/6-31G(d) level. In addition, single-point MP2/6-31G(d) calculations are performed using the HF/6-31G(d) geometries. The goal is to determine the relative accuracy of each method and discuss their strengths and weaknesses. © 1994 by John Wiley & Sons, Inc.     

Substituted Diorganotin(IV) O,O’-Alkylene Dithiophosphates: Synthesis and Spectral Aspects

Six new substituted diphenyltin(IV) O,O′-alkylene dithiophosphates, (C₆H₅)₂Sn(X)S(S) POGO [G = —CH₂C(CH₃)₂CH₂—, X = Cl (1), SCN (3), ClO₄ (5); G = —CH₂C (C₄H₉)(C₂H₅)CH₂—, X = Cl (2), SCN (4), ClO₄ (6)], were synthesized by the reaction of the corresponding ammonium salts of the O,O’-alkylene dithiophosphates with an appropriate organotin(IV) chloride. The compounds were characterized on the basis of elemental and spectral analyses (ESI mass spectrometry, IR, ¹H, ¹³C, ³¹P, and ¹¹⁹Sn NMR). The presence of a four-coordinated Sn atom and monodentate O,O’-alkylene dithiophosphate moiety in compounds 1–4 as well as bidentate O,O’-alkylene dithiophosphate unit in compounds 5,6 is established.     

Structures and stabilization energies of methyl anions with main group substituents from the first five periods

The effects of substituents (X) on the structures and stabilities of CH₂X^? anions for groups comprised of fourth- and fifth-period main group elements (X = K, CaH, GaH₂, GeH₃, AsH₂, SeH, Br, Rb, SrH, InH₂, SnH₃, SbH₂, TeH, and I) have been investigated by ab initio pseudopotential calculations. Full geometry optimizations have been carried out on the CH₂X^? anions and the corresponding neutral parent molecules, CH₃X, at HF/DZP + and MP2/DZP + levels. Results for substituents from the second (X = Li? F) and third (X = Na? Cl) periods provide comparisons of substituent effects of the main group elements of the first four rows of the periodic table on methyl anions. Frequency calculations characterize the nature of stationary points and show pyramidal CH₂X^? anion structures to be the most stable unless π acceptor interactions (e.g., with BH₂, AlH₂, GaH₂, and InH₂ favor planar geometries. The CH₂X^? stabilization energies [at QCISD(T)/DZP + /MP2/DZP + + ZPE level for X = K? I and QCISD(T)/6?31 + G*/MP2/6?31 + G* + ZPE level] for X = Li? Cl) also show strong π-stabilizing effects for the same substituents. With the exception of CH₃ and NH₂, all substituents stabilize methyl anions, although the σ stabilization by OH and F is small. The SiH₃? PH₂? SH? Cl, GeH₃? AsH₂? SeH? Br, and SnH₃? SbH₂? TeH? I sets of substituents give stabilization energies between 19 and 30 kcal/mol. The stability of methyl anions substituted by the halogens and the chalcogens (X = OH, SH, SeH, and TeH) increases down a group in accord with the increasing substituent polarizability, while for π acceptors (BH₂, AlH₂, GaH₂, and InH₂) the stability decreases down a group in line with their π-accepting ability. © 1994 by John Wiley & Sons, Inc.     

Quantum-Chemical Study of Alkyl Carbenium Ions in 100% Sulfuric Acid

A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO₄and [RH₂SO₄]⁺(where R = CH₃, C₂H₅, iso-C₃H₇, and tert-C₄H₉) is carried out. Calculations are performed using the Hartree–Fock method in the 6-31G** and 6-31++G** basis sets taking into account electron correlation according to the Müller–Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonated tert-butyl sulfate was also calculated by the DFT B3LYP/6-31++G** method. It was found that monoalkyl sulfates are covalent compounds, and the complete abstraction of alkyl carbenium ions from them has substantial energy cost: 196.4, 161.7, 150.8 and 136.0 kcal/mol, respectively. Protonated methyl and ethyl sulfates are also covalent compounds according to the calculation. They have lower but still high energies of heterolytic dissociation (65.0 and 33.5 kcal/mol, respectively). The energy of R⁺abstraction from protonated isopropyl sulfate is much lower: 23.6 kcal/mol. The main covalent state and the ion–molecular pair, which is a carbenium ion [C(CH₃)₂H]⁺solvated by the H₂SO₄molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion–molecular complex [C(CH₃)⁺ ₃H₂SO₄] with still lower energy of carbenium ion [C(CH₃)₃]⁺abstraction, which is equal to 10.0 kcal/mol. Calculation according to the DFT B3LYP/6-31++G** method shows the absence of a minimum for the protonated tert-butyl sulfate with a covalent structure on the potential energy surface.     

Quantum-Chemical Study of Electronic and Steric Structure of Vinyltetrazoles: I. 2-Substituted 5-Vinyltetrazoles

The total Mulliken charges on the carbon atoms of the vinyl group, populations of S-trans-(N¹)conformers, and internal rotation energies were calculated ab initio (HF/6-31G**, MP2/6-31G**, and MP2/6-31G**//AM1) for a series of 2R-5-vinyltetrazoles (R = CH₃, C₂H₅, i-C₃H₇, t-C₄H₉, C₆H₅). The calculation results were compared to the available experimental data.     

The remarkably invariant interaction energies of lithium first-row compounds with water and with ammonia

Solvation energies of lithium first-row compounds LiX (X ? H, Li, BeH, BH₂, CH₃, NH₂, OH, F) and of the lithium cation with the model solvents, water and ammonia, have been calculated ab inito (MP2/6-31 + G^*//6-31G^* with zero-point vibrational energy corrections at 3-21G//3-21G). The solvation energies are found to be remarkably constant: ?18.0 ± 1.2 and ?21.5 ± 1.3 kcal/mol for the hydrates and ammonia solvates, respectively. This independence on the nature of X is due largely to the ionic character of the LiX compounds (dipole moments 4.7–6.6 debye). The unexpectedly high solvation energies of the lithium molecule (?14.3 and ?17.8 kcal/mol, respectively) are due to the polarizability of Li₂. At the same level, the lithium cation has interaction energies with H₂O and NH₃ of ?34.1 and ?39.7 kcal/mol, respectively. For the hydrates of LiOH and LiF cyclic structures with hydrogen bonds and somewhat increased solvation energies also are described.