Quantum chemical studies on electrophilic addition

The geometries of the 2-aminoethyl cation and the isomeric protonated aziridine have been optimized using ab initio molecular orbital calculations employing the split-valence shell 4-31G basis set. The protonated aziridine is computed to be the more stable ion by 46.5 kcal/mole (4-31G level) and 44.9 kcal/mole (double-zeta basis set). The profile to interconversion is found to have a barrier of less than 15 kcal/mole (relative to the 2-aminoethyl cation) and this profile is compared with those computed for the similar ions XCH₂CH ₂ ⁺ where X=OH, F, SH and Cl.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-hydroxyethyl and isomeric oxiranium cations have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set. These species are possible intermediates in both the electrophilic addition of OH to ethylene and in the acid catalysed ring opening of oxirane. The optimized structures were then used to compute more accurate wave functions using Dunning's double-zeta basis set, and with this large basis set the bridged oxiranium ion was found to be the more stable by 7.2 kcal/mole. The barrier to interconversion of these two C₂H₄OH ions was computed to be 25.0 kcal/mole above the oxiranium ion.     

SCF MO LCGO studies on the hydration of ions: The systems H+H2O,Li+H2O,and Na+H2O

The energy surfaces of the systems LiOH ₂ ⁺ and NaOH ₂ ⁺ are studied for a number of different geometries within the SCF MO LCAO framework, using a gaussian basis set to approximate the wavefunction. In the minimum energy geometry of both systems the positive ion is bound to the oxygen atom of the water molecule. The computed binding energies and bond distances are: B ^SCF(LiOH ₂ ⁺ ) = 36.0 kcal/mole, d(LiO) = 3.57 a.u., and B ^SCF(NaOH ₂ ⁺ ) = 25.2 kcal/mole, d(NaO) = 4.23 a.u., resp. The results are compared with those of H₃O⁺ and discussed in view of ion-solvent interaction in aquous solutions.It is a pleasure to thank our technical staff for the careful preparation of the input for the programs and for its enthusiastic and skilful assistance in running the computer.     

Relaxation in torsional motion of triplet oxirane

The rotational barrier height E_rot in the lowest triplet state oxirane molecule was calculated to be 26.3 kcal/mole using a double zeta basis set with partial geometry optimization. This suggest ldrelaxedrd rotation and the computed e(T₁- E(S_o) + E_rot value is commensurate with the enthalpy change for the oxirane-forming O(³P) + C₂H₄ reaction, thus providing a rationale for the stereochemical features of the reaction.     

Conformational study of the structure of free 12-thiacrown-4 and some of its cation metal complexes

Conformational search of 12-thiacrown-4, 12t4, was performed using the CONFLEX method and the MMFF94S force field whereby 156 conformations were predicted. Optimized geometries of the 156 predicted conformations were calculated at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x and M06HF levels using the 6-311G** basis set. The correlation energy was recovered at the MP2 level using the same 6-311G** basis set. Optimized geometries at the MP2/6-311G** level and G3MP2 energies were calculated for some of the low energy conformations. The D ₄ conformation was predicted to be the ground state conformation at all levels of theory considered in this work. Comparison between the dihedral angles of the predicted conformations indicated that for the stability of 12t4, a SCCS dihedral angle of 180° requirement is more important than a gauche CSCC dihedral angle requirement. Conformational search was performed also for the 12t4?CAg⁺, Bi³⁺, Cd²⁺, Cu⁺ and Sb³⁺ cation metal complexes using the CONFLEX method and the CAChe-augmented MM3 and MMFF94S force fields. Conformations with relative energies less than 10?kcal/mol at the MP2/6-31+G*//HF/6-31+G* level, with double zeta quality basis set on the metal cations, were considered for computations at the same levels as those used for free 12t4, using also the 6-311G** basis set. The cc-pVTZ-pp basis set was used for the metal cations. The predicted ground state conformations of the 12t4?CAg⁺, Bi³⁺, Cd²⁺, Cu⁺ and Sb³⁺ cation metal complexes are the C ₄, C ₄, C ₄, C _2v and C ₄ conformations, respectively. This is in agreement with the experimental X-ray data for the 12t4?CAg⁺ and Cd²⁺ cation metal complexes, but experimentally by X-ray, the 12t4?CBi³⁺ and Cu⁺ cation metal complexes have C _s and C ₄ structures, respectively.     

Quantum chemical study of the geometries and stabilities of the two valence-tautomers of C2H2F+

Non-empirical SCF-MO molecular wavefunctions were computed for the two limiting structures of C₂H₂F⁺ with full geometry optimization using double-zeta quality atomic orbital basis sets. The bridged structure (fluorenium ion) was found to be an energy maximum (transition state) about 31 kcal/mole higher than the open structure (fluoro-vinyl cation). The latter, contrary to the unsubstituted vinyl cation, is slightly (4.5 °) bent away from fluorine at the electron deficient centre.     

The study of cis - and trans -2 butene using mass spectrometry

Using mass spectrometric technique, the effect of geometrical isomerism on the first and higher appearance energy values for C₄H₃ ⁺, C₄H₇ ⁺ and C₃H,₃ ⁺ ions obtained from cis-2-butene andtrans-2-butene is reported. The structure in the ionization efficiency curves (studied for 9 eV above threshold) for the same ions obtained from the two isomers is reported and compared. It is believed that at threshold C₄H₇ ⁺ fragment is formed from the two isomers as methallyl ion. For C₃H₃ ⁺ fragment formed from the cw-isomer at threshold the proposed structure is the propargyl ion with ΔH_f equal to 279-4 kcal/mole while for that ion obtained fromtransisomer the proposed structure is the allenyl ion with ΔH_f equal to 296.6 kcal/mole.     

Ab initio and density functional study of substituent effects in halogenated cations of alkenes

A theoretical study of the halogenated cations of mono-, di-, tri- and tetramethyl-substituted ethylenes, C₃H₆X⁺, C₄H₈X⁺, C₅H₁₀X⁺ and C₆H₁₂X⁺, X=F, Cl, Br, have been studied at the ab initio MP2 and density functional B3LYP levels of theory implementing 6-311++G(d,p) basis set. The potential energy surfaces of all molecules under investigation have been scanned and the ¹³C and ¹H NMR chemical shifts for all the bridged halonium ions studied have been calculated using the GIAO method at the B3LYP level. The calculated halogen binding energies in the halonium ions have been correlated with the experimental rates of chlorination and bromination of the corresponding alkenes. The computed hydride affinities and the NICS values for the bridged cations show that the bromo cations are more stable than the analogous chloro and fluoro cations.     

SiH+5 and the proton affinity of monosilane

Tandem mass spectrometric studies show that SiH⁺₅ is formed in bimolecular reactions of SiH₄ and NH⁺₂, C₂H⁺₃, C₂H⁺₆ and C₃H⁺₈ ions. The dependence of the reaction cross sections on ion energy indicates the formation of SiH⁺₅ from NH⁺₂, C₂H⁺₃, and C₂H⁺₆ to be exothermic reactions, while formation from C₃H⁺₈ is endothermic. Using known thermochemical data, these facts permit the assignment of 150 and 156 kcal/mole to the lower and upper limits of the proton affinity of monosilane.     

An ab initio molecular orbital study of the deprotonation of amines

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.     

Ab initio calculations including electron correlation,and mindo/3 calculations on the system C2H+7

Ab initio SCF and CEPA PNO calculations have been performed together with MINDO/3 calculations on the system C₂H⁺₇. In agreement with experimental assignment, but in contradiction to MINDO/3 results, the ab initio methods show the CC protonated structure to be more stable than the CH protonated structure. The energy difference is 8.5 kcal/mol at the SCF level and 6.3 kcal/mol with inclusion of electron correlation. Additionally, ΔH⁰₃₀₀ for the reaction C₂H⁺_s + H₂ = C₂H⁺₇ and the proton affinity of ethane are computed.     

Clustering and activation in reactions of CoCp with hydrogen and methane

Gas-phase clustering reactions of CoCp⁺ with H₂ and with CH₄ were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp⁺ ion was found to form strong interactions with two ligands. The first and second H₂ groups cluster to CoCp⁺ with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH₄ groups cluster to CoCp⁺ with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H₂ and 2.2 kcal/mol for the third CH₄ clustering to CoCp⁺ suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H₂-elimination from CoCp(CH₄)₂⁺ was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H₂-elimination product is (c-C₅H₆)Co⁺---C₂H₅. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH₃ groups to the C₅-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C₅-ring ligand.     

An ab initio study of the binding of N2 to Na+ and K+

The binding energies of N₂ to Na⁺ and K⁺ are computed, using the SCF supermolecule approach with extended basis sets together with the counterpoise correction computed in two extreme ways, and supplemented by a perturbation calculation of the dispersion energy. Inclusion of the calculated zero-point energy and the additional correction due to the variation of the correlation in N₂ upon complexation leads to an Na⁺-N₂ binding of ?7.9 to ?8.1 kcal/mole (compared to a measured enthalpy of ?8 ± 0.5) and to a corresponding theoretical value computed for K⁺-N₂ of ?4.6 to ?4.8 kcal/mole.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

Analysis of ToF‐SIMS spectra of poly(2‐vinylpyridine) and poly(4‐vinylpyridine) with density functional theory calculations

The time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) positive and negative ion spectra of poly(2‐vinylpyridine) (P2VP) and poly(4‐vinylpyridine) (P4VP) were analyzed using density functional theory calculations. Most of the ions from these structural isomers shared the same accurate mass, but had different relative abundance. This could be attributed to the fact that from a thermodynamics perspective, the disparity in the molecular structures can affect the ion stability if we assume that they shared the same mechanistic pathway of formation with similar reaction kinetics. The molecular structures of these ions were assigned, and their stability was evaluated based on calculations using the Kohn‐Sham density functional theory with Becke's 3‐parameter Lee‐Yang‐Parr exchange‐correlation functional and a correlation‐consistent, polarized, valence, double‐zeta basis set for cations and the same basis set with a triple‐zeta for anions. The computational results agreed with the experimental observations that the nitrogen‐containing cations such as C₅H₄N⁺ (m/z = 78), C₈H₇N^+· (m/z = 117), C₈H₈N⁺ (m/z = 118), C₉H₈N⁺ (m/z = 130), C₁₃H₁₁N₂⁺ (m/z = 195), C₁₄H₁₃N₂⁺ (m/z = 209), C₁₅H₁₅N₂⁺ (m/z = 223), and C₂₁H₂₂N₃⁺ (m/z = 316) ions were more favorably formed in P2VP than in P4VP due to higher ion stability because the calculated total energies of these cations were more negative when the nitrogen was situated at the ortho position. Nevertheless, our assumption was invalid in the formation of positive ions such as C₆H₇N⁺˙ (m/z = 93) and C₈H₁₀N⁺ (m/z = 120). Their formation did not necessarily depend on the ion stability. Instead, the transition state chemistry and the matrix effect both played a role. In the negative ion spectra, we found that nitrogen‐containing anions such as C₅H₄N^? (m/z = 78), C₆H₆N^? (m/z = 92), C₇H₆N^? (m/z = 104), C₈H₆N^? (m/z = 116), C₉H₁₀N^? (m/z = 132), C₁₃H₁₁N₂^? (m/z = 195), and C₁₄H₁₃N₂^? (m/z = 209) ions were more favorably formed in P4VP, which is in line with our computational results without exception. We speculate that whether anions would form from P2VP and P4VP is more dependent on the stability of the ions.     

A quantum chemical study on electrophilic addition

Non-empirical SCF-MO calculations were carried out on two limiting structures of C₂H₄F⁺, corresponding to the cyclic and open valence tautomers, both of which are possible reaction intermediates of the electrophilic addition reaction of F₂ to CH₂ =CH₂. It was found that both species had thermodynamic stability, corresponding to two distinct minima on the energy surface. However, the 2-fluoroethyl carbonium ion showed a greater stability than the fluoronium ion by about 10 kcal/mole.     

Reactions of Au+ and Au− with benzene and fluorine-substituted benzenes

Reactions of gold anions and cations generated by laser desorption/ionization were studied in the FTICR spectrometer. Au⁻ associated with C₆F₆ to give the novel Au⁻(C₆F₆) complex, whose binding energy was estimated to be 24 ± 4 kcal mol⁻¹ from analysis of the radiative association (RA) kinetics. Au⁺ associated with C₆F₅H to give Au⁺(C₆F₅H), with binding energy estimated to be 31 kcal mol⁻¹. Au⁺ reacted with C₆H₆ to form the well known Au⁺(C₆H₆) and Au⁺(C₆H₆)₂ complexes. The observation of rapid charge transfer from Au⁺(C₆H₆) to C₆H₆ was interpreted as showing that benzene binds more strongly to neutral Au than to Au⁺. The neutral Au–C₆H₆ bond is accordingly concluded to be stronger than about 70 kcal mol⁻¹.     

Comments on the carbon cluster C60 and on its complexes with alkaline elements

We have analyzed in the Hartree–Fock approximation the carbon cluster C₆₀ with a single-zeta [(9,5)/(2,1)] basis set and a double-zeta [(9,5)/(4,2)] basis set, the latter with and without 3d polarization functions. Estimates of the correlation energy correction were obained either using Becke's density functional theory or the Clementi–Chakravorty's electron–pair density approximation. The cluster's positive ion and singly and doubly charged negative ions have also been studied (doublets for C and C and singlet and triplet for C) and computed both with a doublezeta basis set and defferent geometries or a double-zeta plus polarization basis set. The geometries considered include the one obtained by quantum molecular dynamics using the Car–Parrinello approximation and two additional near this minimum. The computed ionization potential and electron affinity are in reasonable agreement with the experiments considering the basis sets adopted. A lithium, a sodium, or a potassium atom or the corresponding positive ions have been placed at the center of the cluster and have been shown to form stable complexes: C₆₀Li⁺, C₆₀Li, C₆₀Na⁺, C₆₀Na, C₆₀K⁺, and C₆₀K. In addition, preliminary data with a calcium atom are reported. Computations on model cluster C₅, C₆, and C₉ are also reported to show that one needs large basis sets, extended use of polarization functions, and correlation corrections for quantitative results, more accurate than ～5 kcal/mol per carbon atom, in the total energy, as in this work.     

Bis(1‐methyl­guanidinium) tetra­chloro­cuprate(II) and 4‐(2‐pyrimidin‐1‐io)­piperazinium tetra­chloro­cuprate(II)

The crystal structures of the title compounds, (C₂N₃H₈)₂[CuCl₄], (I), and (C₈H₁₄N₄)[CuCl₄], (II), have been studied by X‐ray diffraction. The structures consist of discrete [CuCl₄]^2? anions with two monoprotonated (C₂N₃H₈)⁺ cations for (I) and a diprotonated (C₈N₄H₁₄)²⁺ cation for (II). The [CuCl₄]^2? anions of both compounds have flattened tetrahedral geometries. There are several N—H?Cl weak bonds that join the [CuCl₄]^2? anions and the organic cations helping retain the pseudo‐tetrahedral geometries of the anions.     

ab initio calculations and thermochemical analysis on C1 atom abstractions of chlorine from chlorocarbons and the reverse alkyl abstractions: Cl2 + R· ↔ Cl· + RCl

Thermodynamic properties (ΔH°_f(298), S°₍₂₉₈₎ and Cp(T) from 300 to 1500 K) for reactants, adducts, transition states, and products in reactions of CH₃ and C₂H₅ with Cl₂ are calculated using CBSQ//MP2/6‐311G(d,p). Molecular structures and vibration frequencies are determined at the MP2/6‐311G(d,p), with single‐point calculations for energy at QCISD(T)/6‐311 + G(d,p), MP4(SDQ)/CbsB4, and MP2/CBSB3 levels of calculation with scaled vibration frequencies. Contributions of rotational frequencies for S°₍₂₉₈₎ and Cp(T)'s are calculated based on rotational barrier heights and moments of inertia using the method of Pitzer and Gwinn [1]. Thermodynamic parameters, ΔH°_f(298), S°₍₂₉₈₎, and C_P(T), are evaluated for C₁ and C₂ chlorocarbon molecules and radicals. These thermodynamic properties are used in evaluation and comparison of Cl₂ + R· → Cl· + RCl (defined forward direction) reaction rate constants from the kinetics literature for comparison with the calculations. Data from some 20 reactions in the literature show linearity on a plot of Ea_fwd vs. ΔH_rxn,fwd, yielding a slope of (0.38 ± 0.04) and intercept of (10.12 ± 0.81) kcal/mole. A correlation of average Arrhenius preexponential factor for Cl· + RCl → Cl₂ + R· (reverse rxn) of (4.44 ± 1.58) × 10¹³ cm³/mol‐sec on a per‐chlorine basis is obtained with Ea_Rev = (0.64 ± 0.04) × ΔH_rxn,Rev + (9.72 ± 0.83) kcal/mole, where Ea_Rev is 0.0 if ΔH_rxn,Rev is more than 15.2 kcal/mole exothermic. Kinetic evaluations of literature data are also performed for classes of reactions. Ea_fwd = (0.39 ± 0.11) × ΔH_rxn,fwd + (10.49 ± 2.21) kcal/mole and average A_fwd = (5.89 ± 2.48) × 10¹² cm³/mole‐sec for hydrocarbons: Ea_fwd = (0.40 ± 0.07) × ΔH_rxn,fwd + (10.32 ± 1.31) kcal/mole and average A_fwd = (6.89 ± 2.15) × 10¹¹ cm³/mole‐sec for C₁ chlorocarbons: Ea_fwd = (0.33 ± 0.08) × ΔH_rxn,fwd + (9.46 ± 1.35) kcal/mole and average A_fwd = (4.64 ± 2.10) × 10¹¹ cm³/mole‐sec for C₂ chlorocarbons. Calculation results on the methyl and ethyl reactions with Cl₂ show agreement with the experimental data after an adjustment of +2.3 kcal/mole is made in the calculated negative Ea's. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 548–565, 2000