Transition state structures and energetics using Gaussian-2 theory

Ab initio molecular orbital and density functional characterization of the potential energy surface of the N 2 O+Br reaction J. Chem. Phys. 109, 9410 (1998); 10.1063/1.477602 An ab initio molecular orbital study of the potential energy surface of the HO 2 +NO reaction J. Chem. Phys. 107, 1872Phys. 107, (1997 The potential energy surface of the NH 2 ϩNO reaction, which involves nine intermediates ͑1-9͒ as well as twenty-three possible transition states (a -w), has been fully characterized at the B3LYP/cc-pVQZ//B3LYP/6-311G (d,p)] and modified Gaussian-2 ͑G2M͒ levels of theory. The reaction is shown to have three different groups of products (HN 2 ϩOH, N 2 OϩH 2 , and N 2 ϩH 2 O denoted as A, B, and C, respectively͒ and a very complicated reaction mechanism. The first reaction path is initiated by the N-N bond association of the reactants to form an intermediate H 2 NNO, 1, which then undergoes a 1,3-H migration to yield an isomer pair HNNOH ͑2,3͒ ͑separated by a low energy torsional barrier͒ which can then proceed along three different paths. Because of the essential role it would play kinetically, the enthalpy of the NH 2 ϩNO→HN 2 ϩOH reaction has been further investigated using various levels of theory. The best theoretical results of this study predicted it to be 0.9 and 2.4 kcal mol Ϫ1 at the B3LYP and CCSD͑T͒ levels, respectively, using a relatively large basis set ͑AUG-cc-pVQZ͒ based on the geometry optimized at the B3LYP/6-311G(d,p) level of theory. It has been found that TS g(4˜B) is expected to be the rate-determining transition state responsible for the NH 2 ϩNO→N 2 OϩH 2 reaction. TS g lies above the reactants by only 2.6 kcal mol Ϫ1 according to the G2M prediction. On the other hand, TS h(3˜7) is a new transition state discovered in this work which may allow some kinetic contribution from the NH 2 ϩNO→N 2 ϩH 2 O reaction under high temperature conditions due to its relatively low energy as well as its loose transition state property. A modified G2 additivity scheme based on the G2͑DD͒ approach has been shown to be necessary for better predicting the energetics for TS h, which gives a value of 2.3 kcal mol Ϫ1 in energy with respect to the reactants. Generally, the cost-effective B3LYP method is found to give very good predictions for the optimized geometries and vibrational frequencies of various species in the system if compare them with those optimized at the QCISD/6-311G(d,p) and 12-in-11 CASSCF/cc-pVDZ levels of theory. Furthermore, it is noticeable in this study that most of the relative energies calculated via the B3LYP method are more close to the G2M results than those predicted at the PMP4 and CCSD͑T͒ levels using the same 6-311G(d,p) basis set.

Section: B Intermediate H 2 Nnomentioning

confidence: 99%

Section: Computational Proceduresmentioning

confidence: 99%

Theoretical investigation of the potential energy surface for the NH2+NO reaction via density functional theory and ab initio molecular electronic structure theory

Diau

Smith

1997

“…The structure and force constants for the oxiryl radical and for the transition state for ring formation have been calculated. 41 The thermodynamic estimates for the transition state were carried out using the G2/G2Q method, 42 and frequencies were derived at the QCISD/6-311G(d,p) level. These frequencies can be used as a qualitative guide for a possible transition state to isomerization, but they do not necessarily represent the actual transition state.…”

Section: ͑14͒mentioning

confidence: 99%

Association and isotopic exchange reactions of CH(CD)[X 2Π]+CO

Taatjes

1997

Potential energy surface for the CH 3 + HBr → CH 4 + Br hydrogen abstraction reaction: Thermal and stateselected rate constants, and kinetic isotope effectsThe high-pressure range of the reaction CH( 2 Π)+CO+MHCCO+M The reaction rates for 12 CH and 12 CD with normal isotopic abundance CO and 13 CO have been studied at 293 K for pressures between 12.5 and 500 Torr and at 100 Torr for temperatures between 293 and 650 K. The pressure and temperature dependence of the addition reaction of CH with CO have been measured. The addition rate coefficient can be fit to the expression 7.2Ϯ0.3ϫ10 Ϫ12 (T/293) Ϫ2.4Ϯ0.2 cm 3 molecule Ϫ1 s Ϫ1 at 100 Torr total pressure ͑He buffer͒. A fit of the pressure dependence to a Troe expression with F c ϭ0.6 yields a low-pressure rate constant (k 0 ) of 2.4Ϯ0.3ϫ10 Ϫ30 cm 6 molecule Ϫ2 s Ϫ1 . The rate for carbon atom exchange has been measured by comparison of the 13 C labeled and unlabeled reaction rates. The isotopic exchange reaction is 1.0Ϯ0.2ϫ10 Ϫ12 cm 3 molecule Ϫ1 s Ϫ1 at 20 Torr. The deuterium isotope effect on the exchange rate is large, with an inverse kinetic isotope effect ͑k H /k D ͒ϭ0.28Ϯ0.08 at 20 Torr. This inverse isotope effect reflects the competition between collisional stabilization and isomerization, and is a convolution of isotope effects for the isomerization, unimolecular dissociation, and stabilization rates. The experimental results are consistent with a mechanism for exchange that involves isomerization of an HCCO adduct via an oxiryl intermediate, and indicate that insertion into the C-O bond is not important in this reaction.

“…The geometry optimizations were performed at the QCISD/6-311G(d,p) level, which is the same as that used by Durant and Rohlfing for their G2Q formulation of G2 theory ͑ZPE corrections for the TS were taken from the QCISD frequency calculations͒. The barrier heights found at the CBS-QCI/APNO level are compared to values from G2Q, 13 G2M ͑where available͒, 4 and 17 which might be an insufficiency in the reference selection in the CCI procedure as discussed by Durant and Rohlfing, 10,14 leading to an error of 3.0 kcal in the energy of the NHϩNO asymptote. When referring the TS energy to the HϩN 2 O, the CBS-QCI/APNO and G2Q results are in excellent agreement with the CASSCF value.…”

Section: B Transition Statesmentioning

confidence: 99%

“…These transition states had been addressed by Durant and Rohlfing when evaluating the reliability of G2 and its G2Q derivative. 10,14 A subset of these species had been reinvestigated by Durant using DFT methods. 15 We included the hydrocarbon radical hydrogen transfer reaction between C 2 H 4 and C 2 H 5 as an additional system that previously had been treated at a high level of theory.…”

Section: B Transition Statesmentioning

confidence: 99%

Prediction of bond dissociation energies and transition state barriers by a modified complete basis set model chemistry

Jungkamp

Seinfeld

1997

The complete basis set model chemistries CBS-4 and CBS-q were modified using density functional theory for the geometry optimization step of these methods. The accuracy of predicted bond dissociation energies and transition state barrier heights was investigated based on geometry optimizations using the B3LYP functional with basis set sizes ranging from 3-21G(d,p) to 6-311G(d, p). Transition state barrier heights can be obtained at CBS-q with B3LYP/6-31G(d, p) geometries with rms error of 1.7 kcal/mol within a test set of ten transition state species. The method should be applicable to molecules with up to eight or more heavy atoms. Use of B3LYP/6-311G(d, p) for geometry optimizations leads to further improvement of CBS-q barrier heights with a rms error of 1.4 kcal/mol. For reference, the CBS-QCI/APNO model chemistry was evaluated and is shown to provide very reliable predictions of barrier heights (rms errorϭ1.0 kcal/mol).