Theoretical Study of the Reaction of Acetylene with B₄H₈. A Proposed Mechanism of Carborane Formation. 2

A uniform computational level ([MP4/6-311+G(d,p)]//MP2/6-31G(d)+ZPC) is used to evaluate 18 intermediates and 12 transition states in the study of the mechanism of carborane formation, beginning with the elimination of H2 from B4H10 and ending with the formation of 1,2-C2B4H6. The calculated activation barrier (33.0 kcal/mol) for the first step (B4H10 → B4H8 + H2) is higher than experiment (23.7 kcal/mol) but in agreement with higher-level theory (CBS-Q, 34.4 kcal/mol). The first stable intermediate is a −CHCH− bridged B4H8 species, which is structurally similar to the known −CH2−CH2− bridged B4H8 structure. The hydroboration pathway for insertion of C2H2 into a BH bond of B4H8 has a slightly lower activation barrier than the addition barrier of C2H2 to B4H8 (10.1 versus 13.1 kcal/mol, respectively). The hydroboration reaction leads, in a series of steps, to 2,5-μ-CH2-1-CB4H7, a known product in the reaction of methylacetylene and B4H10

Ab Initio Study of Rearrangements on the (CH)₂(BR)₂, RH, and NH₂ Potential Energy Surfaces^†

A comprehensive survey of the (CH)2(BH)2 potential energy surface was carried out at the [MP4/6-311+G(d,p)]//MP2/6-31G(d) level. Many of the classical and nonclassical isomers of the carborane surface are separated by high activation barriers, which explains why derivatives of most isomers could be prepared as stable compounds at room temperature. The transition states are grouped into two types, hydrogen migration (terminal-to-bridge and bridge-to-terminal) and group migration (BH, CH, and CH2). The rearrangement of 1,3-diamino-1,3-diboretene (1-NH2) to 1,2-diamino-1,2-diboretene (2-NH2) was computed and compared to the rearrangement in the parent (1 → 2). The effect of the amino group is to substantially increase the barrier height and stabilize the product, 2-NH2

Proposed Fluorination Mechanism of CB₅H₆^- and CB₉H₁₀^- with HF. Evidence of Kinetic Control in the Formation of 2-CB₅H₅F^- and 6-CB₉H₉F^-

Two pathways have been considered in the fluorination of CB5H6- and CB9H10- by HF. In the ionic HF fluorination pathway, the monocarborane anion cage is first protonated in a BBB face followed by H2 elimination and fluoride anion addition. In the covalent HF fluorination pathway, HF is first coordinated through hydrogen to the BBB face. Next, the fluorine can add to either an axial or equatorial boron atom which opens the cage to a nido structure with an endo fluoride substituent. Endo to exo rearrangement occurs with a small activation barrier followed by H2 elimination. In both pathways, fluorination at the equatorial boron position is predicted to have smaller activation barriers even though substitution at the axial position leads to the more stable products

Comparison of Gas-Phase and Solution-Phase Reactions of Dimethyl Sulfide and 2-(Methylthio)ethanol with Hydroxyl Radical

The reaction of the OH radical with dimethyl sulfide (DMS) and 2-(methylthio)ethanol (2-MTE) proceeds with the initial formation of a two-center−three-electron complex. In the gas phase the S−OH binding enthalpies (298 K) are 8.7 and 12.2 kcal/mol for DMS and 2-MTE, respectively. When entropy and aqueous solvation effects (via the CPCM method) are included, the free energies of association (298 K) of hydroxyl to DMS and 2-MTE become 3.0 and 3.2 kcal/mol, respectively. Calculations are based on DFT and/or MP2 optimizations and a G2-like method for evaluating energies. The most favorable (lowest free energy) conformation is often different between the gas phase and solution phase. Electron transfer from 2-MTE/2-MTE+ (1/1+) to OH/OH- has a positive free energy of 4.5 kcal/mol and is in competition with the acid-/base-catalyzed formation of CH3SCH2CH2O (2) plus water. The latter radical (2) undergoes intramolecular hydrogen transfer to form CH2SCH2CH2OH (3) or eliminates formaldehyde to form CH2SCH3+H2CO, where the free energy barriers are 7.9 and 8.3 kcal/mol, respectively. The 2-MTE cation (1+) can eliminate a C−H proton to form three different radicals that are within 2.0 kcal/mol of each other in free energy

Computational Study of the Mono- and Dianions of SO₂, SO₃, SO₄, S₂O₃, S₂O₄, S₂O₆, and S₂O₈

DFT theory (B3LYP/6-311+G(2d)//B3LYP/6-31+G(d)) has been used to characterize sulfoxy anions and dianions as large as S2O82-, while post-HF theory ([QCISD(T)/6-31+G(2df)]//MP2/6-31+G(d)) has been used for systems as large as S2O32-. Adiabatic and vertical ionization potentials have been computed to assess the gas-phase stability of the dianions. Three dianions (S2O62-, S2O82-, and SO42-·4H2O) are predicted to have positive vertical ionization energies. S2O62- is predicted to have a negative (exothermic) adiabatic ionization potential; however, a large predicted geometry change between the dianion and monoanion rationalizes the measurable experimental lifetime of the dianion in the gas phase. Isotropic hyperfine coupling constants for 33S have been calculated for the sulfoxy monoanions and compared with experiment

Exploring the Reaction Mechanism of C–H Oxidation by Copper–Salen Complexes

The mechanism of C–H oxidation of propylene (C3H6) and 1-phenyl-1-pentyne (C3H7–CC–Ph) by HOOR (RMe, tBu) and 3O2 by a copper–salen complex was explored by computations. The most noteworthy step is the complexation of two Cu salens to the peroxide to form either the LCuOH/LCuOR pair or an OH-bridged complex LCu­(μ-OH)­CuL plus OR. The latter pathway involves an avoided crossing of two triplet electronic states. The LCuOH complex can abstract a hydrogen atom from C3H6 and the C3H5 radical plus 3O2 forms the complex LCuOOC3H5. Migration of a hydrogen to the proximal oxygen atom reforms LCuOH and acrolein HC­(O)­CHCH2

Density Functional Theory Study of Anionic and Neutral Per-Substituted 12-Vertex Boron Cage Systems, B₁₂X₁₂<i>ⁿ</i>^- (<i>n</i> = 2, 1, 0)

The 12(12) closomers form a rapidly expanding class of compounds where a 12-vertex cage is surrounded by 12 identical substituents. Density functional theory (B3LYP/6-31G(d)) is used to study a number of these closomers in different states of oxidation (dianion, radical anion, and neutral cages). The cage stability increases as the group electronegativity of the substituent increases. Also, the 12(12) closomer becomes easier to oxidize as the Hammett σp parameter becomes more negative (electron-donating). As the closomer is oxidized, the size of the cage increases and the B−B distances become more asymmetric. The Raman-active breathing mode in the 404−434 cm-1 range moves to lower frequency as the cage is oxidized, which is caused by removing one or two electrons from a cage-bonding molecular orbital

Theoretical Study of the Reaction of Acetylene with B₄H₈. A Proposed Mechanism of Carborane Formation. 2

A uniform computational level ([MP4/6-311+G(d,p)]//MP2/6-31G(d)+ZPC) is used to evaluate 18 intermediates and 12 transition states in the study of the mechanism of carborane formation, beginning with the elimination of H2 from B4H10 and ending with the formation of 1,2-C2B4H6. The calculated activation barrier (33.0 kcal/mol) for the first step (B4H10 → B4H8 + H2) is higher than experiment (23.7 kcal/mol) but in agreement with higher-level theory (CBS-Q, 34.4 kcal/mol). The first stable intermediate is a −CHCH− bridged B4H8 species, which is structurally similar to the known −CH2−CH2− bridged B4H8 structure. The hydroboration pathway for insertion of C2H2 into a BH bond of B4H8 has a slightly lower activation barrier than the addition barrier of C2H2 to B4H8 (10.1 versus 13.1 kcal/mol, respectively). The hydroboration reaction leads, in a series of steps, to 2,5-μ-CH2-1-CB4H7, a known product in the reaction of methylacetylene and B4H10

Theoretical Study of Methane Storage in Cu₂₄(<i>m</i>‑BDC)₂₄

Calculations on the Cu24(m-BDC)24 (m-BDC = 1,3-benzenedicarboxylate) polyoxometalate (POM) cage with 0, 12, 24, and 40 methane molecules inside were made using the M06 exchange/correlation functional. During filling of the cage with 40 CH4 molecules, the 12 strongest binding CH4 molecules are those to the coordination unsaturated sites (CUS) to the inwardly directed Cu­(+2) centers via agostic interactions. The next 12 CH4 molecules are less tightly bound followed by the next 16 CH4 molecules with average binding energies of 8.27, 7.88, and 7.36 kcal/mol per CH4, respectively. A section of the Cu24(m-BDC)24 cage was taken with the formula Cu4(m-BDC)­(BC)6 (BC = benezenecarboxylate) in order to estimate zero-point, thermal, and entropy corrections of the larger cage. Estimating free energies at 1 bar, the Cu24(m-BDC)24 POM is predicted to lose 16, 12, and 12 CH4 molecules at 67, 123, and 171 °C, respectively. The 40CH4@Cu24(m-BDC)24 cage, which is isostructural to the main cavity of HKUST-1 with 40 CH4 molecules inside, is predicted to have a loading of 224 cm3(STP) cm–3 at 1 bar

Ab Initio Study of B<i>_n</i>H<i>_n</i> and B<i>_n</i>(NH₂)<i>_n</i> (<i>n</i> = 3−6) Species. A Comparison of Classical and Nonclassical Structures

For early members of the hypercloso boron hydride family, BnHn (n = 3−5), the lowest energy isomer contains one or more three-membered aromatic BBB rings. Not until B6H6 do cage structures become more stable. When hydrogens are replaced by amino groups, the classical nonplanar ring structure is more stable than the nonclassical cage, Bn(NH2)n (n = 4−6). A disagreement of over 20 kcal/mol is found between MP2/6-31G(2d,p)//MP2/6-31G(d) and B3LYP/6-31G(d)//B3LYP/6-31G(d) for the relative energy of ring and cage structures of B6(NH2)6. Calculations on B4(NH2)4 including additional electron correlation indicate B3LYP/6-31G(d) is more reliable than MP2/6-31G(2d,p) for relative energies. The lowest energy B6(NH2)6 classical structure is a D3d symmetry chair, while a D3d cage is predicted to be 15.0 kcal/mol higher in energy