Michael L. McKee

Ab initio study of rearrangements on the nitromethane potential energy surface.

A theoretical study has been performed for the thermal rearrangements connecting nitromethane, methyl nitrite, nitrosomethanol, and aci-nitromethane by using the 6-31G* basis set to optimize geometries and introducing correlation at the MP2 level. The lowest calculated rearrangement pathways from nitromethane are to methyl nitrite (..delta..H/sup double dagger/ = 73.5 kcal/mol) and aci-nitromethane (..delta..H/sup double dagger/ = 75.0 kcal/mol). Nitrosomethanol, although predicted to be only 1.6 kcal/mol less stable, is separated by two high barriers from nitromethane. An elimination transition structure connecting methyl nitrite and formaldehyde plus nitroxyl is found to have an enthalpic barrier of 44.1 kcal/mol. Fragmentation reactions have enthalpies of reaction less than calculated barrier heights, suggesting that concerted rearrangements on the CH/sub 3/NO/sub 2/ surface in general will not be observed.

Mechanistic Study of LINH2BH3 Formation from (LiH)4 + NH3BH3 and Subsequent Dehydrogenation

The formation of LiNH(2)BH(3) from (LiH)(4) and NH(3)BH(3) and the subsequent dehydrogenation have been studied computationally at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(2d,p) level. A cubic unit of (LiH)(4) is predicted to react readily with NH(3)BH(3) to form LiNH(2)BH(3) plus H(2). The (LiH)(4) tetramer enables dehydrogenation through the exchange of a hydride vertex of (LiH)(4) and NH(2)BH(3)(-) where NH(2)BH(3)(-) is formed when the hydride vertex of (LiH)(4) abstracts a proton from NH(3). The free energy of activation for loss of H(2) is reduced from 37.2 kcal/mol in NH(3)BH(3) to 11.0 kcal/mol in (LiH)(4) + NH(3)BH(3). Further, H(2) elimination from the (LiNH(2)BH(3))(2) dimer is predicted to be much easier than from the monomer which may suggest a cooperative H(2)-loss mechanism is possible in solid LiNH(2)BH(3). While two molecules of H(2) can be lost reversibly from (LiNH(2)BH(3))(2), loss of further H(2) molecules is more difficult but could occur if the lattice energy stabilization accompanying H(2) loss is sufficiently large.

The Stabilities of N−Cl Bonds in Biocidal Materials

N-halamine chemistry has been a research topic of considerable importance in these laboratories for over two decades. N-halamine compounds are useful in preparing biocidal materials. There are three N-Cl moieties available in cyclic N-halamine compounds:  imide, amide, and amine. The stabilities toward the release of free halogen have been experimentally shown to decrease in the order amine > amide > imide. In this work, this generalization has been tested theoretically at the level of B3LYP/6-31+G(d) and using the conductor-like polarizable continuum aqueous solvation model with UAKS cavities. Excellent accord was observed between theory and experiment. It was also found that the imide and amide N-halamine stabilities on hydantoin rings could be reversed with substitution patterns at the 5 position.

Endohedral Hydrogen Exchange Reactions in C60 (nH2@C60, n = 1−5): Comparison of Recent Methods in a High-Pressure Cooker

The interactions of several H(2) molecules [(H(2))(n), n = 1-5] within C(60), C(70), and C(82) have been studied with several DFT methods as well as with MP2 and SCS-MP2. As expected, B3LYP significantly underestimates dispersion interactions, while the M05-2X and M06-2X methods are in much better agreement with MP2 and SCS-MP2 results. Degenerate hydrogen exchange reactions were calculated for 3H(2) --> 3H(2) inside C(60), C(70), and C(82). The free-energy barrier at 298 K for the hydrogen exchange reaction 3H(2) --> 3H(2) is reduced from 88.8 kcal/mol for the free reaction to 36.2 kcal/mol for the reaction within C(60), corresponding to a k(cat)/k(uncat) ratio of 10(36). Steric compression, dispersion, and a favorable entropy contribute similar increments to the reduction in the free-energy barrier.

Calculated properties of C60 isomers and fragments

Abstract MNDO calculations have been carried out for three C 60 isomers, buckminsterfullerene ( I h ), graphitene ( D 6h ), and a planar ring ( D 60h ). Buckminsterfullerene is 880.2 kcal mol −1 more stable than the ring structure while graphitene is 648.7 kcal mol −1 more stable (1 kcal mol −1 = 4.184 kJ mol −1 ). Optimized geometric parameters and orbital energies are also reported. A novel mechanism for formation of the I h structure is presented which involves a minimum of rearrangement.

When VSEPR Fails: Experimental and Theoretical Investigations of the Behavior of Alkaline‐Earth‐Metal Acetylides

The synthesis, structural, and spectral characterization as well as a theoretical study of a family of alkaline-earth-metal acetylides provides insights into synthetic access and the structural and bonding characteristics of this group of highly reactive compounds. Based on our earlier communication that reported unusual geometry for a family of triphenylsilyl-substituted alkaline-earth-metal acetylides, we herein present our studies on an expanded family of target derivatives, providing experimental and theoretical data to offer new insights into the intensively debated theme of structural chemistry in heavy alkaline-earth-metal chemistry.

A new mechanism for methane production from methyl-coenzyme M reductase as derived from density functional calculations.

We propose a new DFT-based mechanism for methane production using the full F430 cofactor of MCR (methyl-coenzyme M reductase) along with a coordinated O=CH2CH2C(H)NH2C(H)O (surrogate for glutamine) as a model of the active site for conversion of CH3SCoM(-) (CH3SCH2CH2SO3(-)) + HSCoB to methane plus the corresponding heterodisulfide. The cycle begins with the protonation of F430, either on Ni or on the C-ring nitrogen of the tetrapyrrole ring, both of which are nearly equally favorable. The C-ring protonated form is predicted to oxidatively add CH3SCoM(-) to give a 4-coordinate Ni center where the Ni moves out of the plane of the four ring nitrogens. The movement of Ni (and the attached CH3 and SCH2CH2SO3(2-) ligands) toward the SCoB(-) (deprotonated HSCoB) cofactor allows a 2c-3e interaction to form between the two sulfur atoms. The release of the heterodisulfide yields a Ni(III) center with a methyl group attached. The concerted elimination of methane, where the methyl group coordinated to Ni abstracts the proton from the C-ring nitrogen, has a very small calculated activation barrier (5.4 kcal/mol). The NPA charge on Ni for the various reaction steps indicates that the oxidation state changes occur largely on the attached ligands.

Configuration interation calculations of structures, vibrational frequencies, and heats of formation for HHNO species

Abstract We have used ab initio Hartee—Fock (HF) and configuration interaction (CI) methods to determine the structure, harmonic vibrational frequencies, and heats of formation for several stationary points on the potential energy surface of HHNO. All structures were fully optimized and vibrational frequencies determined at the frozen-core singles and doubles CI level (CISD) using a correlation consistent basis set with polarization functions. Using isogyric energy comparisons and single-point CISD energy calculations incorporating contracted Gaussian basis sets as large as (5s4p2d1f) on nitrogen and oxygen and (4s2p1d) on hydrogen, we predict the following: Δ H 298 f (H 2 NO( 2 A′))=16.2 kcal/mol, Δ H 298 f (H 2 NO( 2 A″))=47.1 kcal/mol,Δ H 298 f (cis-HNOH)=27.3 kcal/mol, Δ H 298 f (trans-HNOH)=22.0 kcal/mol.

Theoretical study of the reaction of OH with HNO

Abstract A complete-active-space (CAS) multiconfiguration self-consistent field (MCSCF) wavefunction with a polarized correlation-consistent basis set was used to determine the stationary points on the OH + HNO potential energy surface. The single-point energies were determined with a multireference configuration interaction (MRCI) wavefunction consisting of single and double excitations from selected configurations chosen from the reference wavefunction. Saddle points were confirmed at the five-electron, five-active-orbital CASSCF level by calculating analytical second derivatives. Segments of the minimum-energy paths (MEPs) for the hydrogen abstraction reaction and for the radical addition to nitrogen were calculated. It was found that the zero-point energy contribution for the abstraction pathway increases as one moves away from the MCSCF saddle point toward the reactants. An evaluation of the MRCI energy and the MCSCF zero-point energy contribution on the MEP at a few points towards the reactants suggests that the true transition state is earlier than the computed MCSCF saddle point, and that abstraction occurs with little or no activation barrier. At the MRCI level, the maximum in the vibrationally adiabatic potential along the MEP for the addition of OH to the nitrogen of HNO occurs at 5.6 kcal mol −1 above the reactants and, like the abstraction, this structure is earlier than the MCSCF saddle point. The resulting product of the OH addition, HN(O)OH, is not expected to decompose to H + HONO because the barrier to H elimination (31.4 kcal mol −1 ) is significantly higher than the barrier for the reaction back to OH + HNO (20.3 kcal mol −1 ). Statistical rate calculations have been performed for the abstraction and addition reactions, and the results are fitted to standard three-parameter temperature-dependent rate expressions.

The carbonyl oxide-aldehyde complex: a new intermediate of the ozonolysis reaction

MP4(SDQ)/6-31G (d,p) calculations suggest that the ozonolysis of alkenes in solution phase does not proceed via carbonyl oxide, but via a dipole complex between aldehyde and carbonyl oxide, which is 9 kcal/mol more stable than the separated molecules. The dipole complex is probably formed in the solvent cage upon decomposition of primary ozonide to aldehyde and carbonyl oxide. Rotation of either aldehyde or carbonyl oxide in the solvent cage leads to an antiparallel alignment of molecular dipole moments and dipole-dipole attraction.