The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

We present reaction pathways for adsorption reactions of the O atom and O 2 molecule in the pristine and monovacancy defective graphite (0001) based on quantum chemical potential energy surfaces (PESs) obtained by the dispersion-augmented densityfunctional tight-binding (DFTB-D) method. We use a dicircumcoronene C 96 H 24 (L0D) graphene slab as the pristine graphite (0001) model and dicircumcoronene C 95 H 24 (LIV) as the graphite (0001) monovacancy defect model. We found that the adsorption reactions of O and O 2 on the L0D surface can produce defects on the graphite surface. O can yield CO, while O 2 can yield both CO and CO 2 molecules. The adsorption reactions of the O and O 2 on the LIV surface can produce a 2-C defective graphite surface and CO, and CO and CO 2 , respectively. The O and O 2 more readily oxidize the defected surface, LIV, than the defect-free surface, L0D. On the basis of the computed reaction pathways, we predict reaction rate constants in the temperature range between 300 and 3000 K using RiceÀRamspergerÀKasselÀMarcus (RRKM) theory. High-temperature quantum chemical molecular dynamics simulations at 3000 K based on on-the-fly DFTB-D energies and gradients support the results of our PES studies.

A spin-polarized density functional theory calculation was carried out to study the adsorption of NH(x) species (x = 1-3) on a TiO2 anatase (101) surface with and without hydroxyl groups by using first-principles calculations. It was found that the present hydroxyl group has the effect of significantly enhancing the adsorption of monodentate adsorbates H2N-Ti(a) compared to that on a bare surface. The nature of the interaction between the adsorbate (NH(x)) and the hydroxylated or bare surface was analyzed by the Mulliken charge and density of states (DOS) calculations. This facilitation of NH2 is caused by the donation of coadsorbed H filling the nonbonding orbital of NH2, resulting in an electron gain in NH2 from the bonding. In addition, the upper valence band, which originally consisted of the mixing of O 2p and Ti 3d orbitals, has been broadened by the two adjacent H 1s and NH2 sigma(y)(b) orbitals joined to the bottom of the original TiO2 valence band. The results are important to understand the OH effect in heterogeneous catalysis.

Spin-polarized density functional theory calculation was carried out to characterize the adsorption and dissociation of CO(2) molecule on the Fe(111) surface. It was shown that the barriers for the stepwise CO(2) dissociation reaction, CO(2(g)) --> C(a) + 2O(a), are 21.73 kcal/mol (for OC-O bond activation) and 23.87 kcal/mol (for C-O bond activation), and the entire process is 35.73 kcal/mol exothermic. The rate constants for the dissociative adsorption of CO(2) have been predicted with variational RRKM theory, and the predicted rate constants, k(CO(2)) (in units of cm(3) molecule(-1) s(-1)), can be represented by the equations 2.12 x 10(-8)T(-0.842) exp(-0.258 kcal mol(-1)/RT) at T = 100-1000 K. To gain insights into high catalytic activity of the Fe(111) surface, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.

The mechanism for sublimation of NH(4)N(NO(2))(2) (ADN) has been investigated quantum-mechanically with generalized gradient approximation plane-wave density functional theory calculations; the solid surface is represented by a slab model and the periodic boundary conditions are applied. The calculated lattice constants for the bulk ADN, which were found to consist of NH(4)(+)[ON(O)NNO(2)](-) units, instead of NH(4)(+)[N(NO(2))(2)](-), agree quite well with experimental values. Results show that three steps are involved in the sublimation/decomposition of ADN. The first step is the relaxation of the surface layer with 1.6 kcal/mol energy per NH(4)ON(O)NNO(2) unit; the second step is the sublimation of the surface layer to form a molecular [NH(3)]-[HON(O)NNO(2)] complex with a 29.4 kcal/mol sublimation energy, consistent with the experimental observation of Korobeinichev et al. (10) The last step is the dissociation of the [H(3)N]-[HON(O)NNO(2)] complex to give NH(3) and HON(O)NNO(2) with the dissociation energy of 13.9 kcal/mol. Direct formation of NO(2) (g) from solid ADN costs a much higher energy, 58.3 kcal/mol. Our calculated total sublimation enthalpy for ADN(s) → NH(3)(g) + HON(O)NNO(2)) (g), 44.9 kcal/mol via three steps, is in good agreement with the value, 42.1 kcal/mol predicted for the one-step sublimation process in this work and the value 44.0 kcal/mol computed by Politzer et al. (11) using experimental thermochemical data. The sublimation rate constant for the rate-controlling step 2 can be represented as k(sub) = 2.18 × 10(12) exp (-30.5 kcal/mol/RT) s(-1), which agrees well with available experimental data within the temperature range studied. The high pressure limit decomposition rate constant for the molecular complex H(3)N···HON(O)NNO(2) can be expressed by k(dec) = 3.18 × 10(13) exp (-15.09 kcal/mol/RT) s(-1). In addition, water molecules were found to increase the sublimation enthalpy of ADN, contrary to that found in the ammonium perchlorate system, in which water molecules were shown to reduce pronouncedly the enthalpy of sublimation.