Lowest Doublet Potential Energy Surface Research Articles

Ab Initio Molecular Orbital Calculations for the N(2D) + Ethylene Reaction

The lowest doublet potential energy surface for the N(2D) + C2H4 reaction has been characterized using ab initio molecular orbital theory. The CASSCF/cc-pVDZ calculations predict that the dominant mechanism is the addition of N(2D) to the CC π-bond of C2H4 to form a cyclic three-membered intermediate radical rather than the insertion into the CH bond in C2H4. Reaction pathways have also been discussed on the basis of the PMP4(full,SDTQ)/cc-pVTZ//MP2/cc-pVDZ level calculations. The reaction is shown to have several possible products via somewhat complicated reaction mechanisms. The results of RRKM calculations predict that the main product channel is cyclic-CH(N)CH2 (azirine) + H under collision-free conditions.

Measurements of Thermal Rate Constants and Theoretical Calculations for the N(2D, 2P) + C2H2 and C2D2 Reactions

Rate constants for the reactions N(2D,2P) + C2H2 and C2D2 have been measured using a technique of pulse radiolysis−resonance absorption between 220 and 293 K. Arrhenius parameters have been determined from the temperature dependence of the measured rate constants; the activation energies for the reactions of N(2D) were about 0.5 kcal/mol, while those for N(2P) were about 0.9 kcal/mol. The H/D isotope effect was found to be very small for both the N(2D) + C2H2 and N(2P) + C2H2 reactions. The rate constants for N(2D) + C2H2 were found to be about 3 times as large as those for N(2P) + C2H2. To understand the overall reaction mechanism of the N(2D) + C2H2 reaction, ab initio molecular orbital calculations of the lowest doublet potential energy surface have been performed. It has been found that the initial step of the reaction is the addition of the N atom to the π bond of acetylene. The rate constants have been calculated using conventional transition-state theory and compared to the experimental results. Possible reaction pathways are discussed on the basis of the ab initio results.

Theoretical Study of the H2 + NO and Related Reactions of [H2NO] Isomers

Mechanisms of the reactions occurring on the multichannel lowest doublet potential energy surface of the [H2NO] system containing H2NO, HNOH, and NOH2 intermediates and including the H2 + NO, NH2 + O, NH + OH, and H + HNO entry channels have been probed using unrestricted coupled-cluster formalism. Also, the transition structure on the doublet energy surface for the direct hydrogen abstraction from H2 + NO yielding H + HNO has been identified. The energetic and molecular parameters derived from coupled cluster singles and doubles with triples correction (CCSD(T)) calculations using the 6-311++G(3df,3pd) basis set, based on their respective optimized geometries obtained with a 6-311++G(d,p) basis set, have then been utilized to compute the apparent rate constants of the different competitive channels in the [H2NO] system within the framework of a quantum version of Rice−Ramsperger−Kassel theory (QRRK). The H2 + NO reaction is found to be a rather slow reaction, and it occurs via bimolecular hydrogen abstraction. The calculated total rate constant for this reaction at 2000 K and 1 atm is 2.35 × 108 cm3 mol-1 s-1. Stabilization of NH2O or NHOH intermediates plays a minor role in the kinetics of NH2 + O reaction which is dominated by the dissociation products, HNO + H, NH + OH and H2 + NO. The H + HNO reaction kinetics has only 10% contribution from addition/isomerization/dissociation paths at 2000 K and 1 atm. H2 + NO and H + HNO are expected to be the competitive products in the reaction of OH with nitrene. The calculated standard heats of formation are ΔHf298(NH2O) = 17.5 ± 2 and ΔHf298(HNOH) = 23.7 ± 2 kcal/mol.

Binding energies of Ti+(H2)1–6 clusters: Theory and experiment

Formation of Ti+(H2)n clusters (n=1–6) has been studied by both temperature-dependent equilibrium measurements and density functional theory (DFT). The successive binding energies (BDEs) were measured to be 7.5±0.5, 9.7±0.6, 9.3±0.7, 8.5±0.4, 8.2±0.4, and 8.7±0.4 kcal/mol for n=1–6, respectively. The relatively low value of the n=1 BDE is due to a curve crossing from the Ti+[a4F(sd2)] ground state to the Ti+[b4F(d3)] first excited asymptote with the addition of the first ligand. The first BDE is 10 kcal/mol when measured with respect to the excited state asymptote. This series of almost constant BDEs is unlike any other M+(H2)n series. The present DFT calculations show these relatively constant BDE values for the Ti+(H2)n clusters are due to an electronic occupation which allows the Ti+ ion to interact equally with up to six H2 ligands. Bond lengths, geometries, and vibrational frequencies from the DFT calculations are reported here for all clusters. The influence of basis set size and computational method on the first two clusters was also examined. It was determined that a multireference wave function was required to describe these first two clusters accurately. A possible crossing to the lowest doublet potential energy surface was examined for Ti+(H2)4 and found to be endoergic.

The mechanism of the reaction CH(X 2Π) + N 2 (X 1Σ +g) → HCN (X 1Σ +) + N ( 4S). : II. The intermediate complex region

The spin-forbidden reaction CH(X 2Π) + N 2(X 1Σ + g) → HCN (X 1Σ +) + N( 4S) involves a doublet → quartet intersystem crossing. It has been postulated that although the probability for a single pass intersystem crossing is small the existence of a bound intermediate on the lowest doublet potential energy surface increases the overall transition probability by enabling multiple crossings of the doublet—quartet surface intersection. This work considers this proposition, using large scale ((3.2 − 3.8) × 10 6 term), multireference configuration interaction wavefunctions. Two local minima on the lowest doublet potential energy surface are reported, a datively bonded structure N 2 → CH, and a C 2v structure, with HC along the perpendicular bisector of the N 2 bond, which resembles the recently reported geometrical arrangement at the transition state for this reaction, the minimum energy point on doublet—quartet intersection surface. These results enable a clear conceptual picture for the mechanism of this reaction to be presented.

Potential energy surfaces for the (ArCO)+ system

The five lowest doublet potential energy surfaces for the (ArCO)+ system have been determined using the procedure of Archirel and Levy. These states correlate at infinite separation, in order of increasing energy, to Ar+CO+(X 2Σ+), Ar+(2P3/2)+CO, Ar+(2P1/2)+CO, and Ar+CO+(A 2Π). The potential energy curves are shown at several values of the orientation angle. In addition, contour maps of the two lowest surfaces are presented. Both of these surfaces are quite anisotropic, and each has a deep potential well. Adiabatic vibronic potential energy surfaces have also been computed. These give insight into the charge transfer process at low collision energies. For the purpose of comparison the five lowest surfaces for the isoelectronic system (ArN2)+ are also shown.

Chercher le croisement

An intersection of the two lowest doublet potential energy surfaces of LiNaK has been established by ab-initio calculations.