The N(4S) + O2(X3Sigma) O(3P) + NO(X2Pi) reaction: thermal and vibrational relaxation rates for the 2A', 4A' and 2A'' states

The kinetics and vibrational relaxation of the N(4S) + O2(X3Sigma-g) O(3P) + NO(X2Pi) reaction is investigated over a wide temperature range based on quasiclassical trajectory simulations on 3-dimensional potential energy surfaces (PESs) for the lowest three electronic states. Reference energies at the multi reference configuration interaction level are represented as a reproducing kernel and the topology of the PESs is rationalized by analyzing the CASSCF wavefunction of the relevant states. The forward rate matches one measurement at 1575 K and is somewhat lower than the high-temperature measurement at 2880 K whereas for the reverse rate the computations are in good agreement for temperatures between 3000 and 4100 K. The temperature-dependent equilibrium rates are consistent with results from JANAF and CEA results. Vibrational relaxation rates for O + NO(nu = 1) O + NO(nu = 0) are consistent with a wide range of experiments. This process is dominated by the dynamics on the 2A' and 4A' surfaces which both contribute similarly up to temperatures T 3000 K, and it is found that vibrationally relaxing and non-relaxing trajectories probe different parts of the potential energy surface. The total cross section depending on the final vibrational state monotonically decreases which is consistent with early experiments and previous simulations but at variance with other recent experiments which reported an oscillatory cross section

Conformer-specific polar cycloaddition of dibromobutadiene with trapped propene ions

Identifying a concerted or stepwise mechanism in Diels-Alder reactions is experimentally challenging. Here the authors demonstrate the coexistence of both mechanisms in the reaction of 2,3-dibromobuta-1,3-diene with propene ions, using a conformationally controlled molecular beam reacting with trapped ions and ab initio computations Diels-Alder cycloadditions are efficient routes for the synthesis of cyclic organic compounds. There has been a long-standing discussion whether these reactions proceed via stepwise or concerted mechanisms. Here, we adopt an experimental approach to explore the mechanism of the model polar cycloaddition of 2,3-dibromo-1,3-butadiene with propene ions by probing its conformational specificities in the entrance channel under single-collision conditions in the gas phase. Combining a conformationally controlled molecular beam with trapped ions, we find that both conformers of the diene, gauche and s-trans, are reactive with capture-limited reaction rates. Aided by quantum-chemical and quantum-capture calculations, this finding is rationalised by a simultaneous competition of concerted and stepwise reaction pathways, revealing an interesting mechanistic borderline case

Conformational and state-specific effects in reactions of 2,3-dibromobutadiene with Coulomb-crystallized calcium ions

Recent advances in experimental methodology enabled studies of the quantum-state- and conformational dependence of chemical reactions under precisely controlled conditions in the gas phase. Here, we generated samples of selected gauche and s-trans 2,3-dibromobutadiene (DBB) by electrostatic deflection in a molecular beam and studied their reaction with Coulomb crystals of laser-cooled Ca + ions in an ion trap. The rate coefficients for the total reaction were found to strongly depend on both the conformation of DBB and the electronic state of Ca + . In the (4p) 2 P 1/2 and (3d) 2 D 3/2 excited states of Ca + , the reaction is capture-limited and faster for the gauche conformer due to long-range ion-dipole interactions. In the (4s) 2 S 1/2 ground state of Ca + , the reaction rate for s-trans DBB still conforms with the capture limit, while that for gauche DBB is strongly suppressed. The experimental observations were analysed with the help of adiabatic capture theory, ab initio calculations and reactive molecular dynamics simulations on a machine-learned full-dimensional potential energy surface of the system. The theory yields near-quantitative agreement for s-trans -DBB, but overestimates the reactivity of the gauche -conformer compared to the experiment. The present study points to the important role of molecular geometry even in strongly reactive exothermic systems and illustrates striking differences in the reactivity of individual conformers in gas-phase ion-molecule reactions

The C(P-3) + O-2((3)sigma(-)(g)) -> CO2 CO((1)sigma(+)) + O(D-1)/O(P-3) reaction: thermal and vibrational relaxation rates from 15 K to 20 000 K

Thermal rates for the C(P-3) + O-2((3)sigma(-)(g)) CO((1)sigma(+))+ O(D-1)/O(P-3) reaction are investigated over a wide temperature range based on quasi classical trajectory (QCT) simulations on 3-dimensional, reactive potential energy surfaces (PESs) for the (1)A ', (2)(1)A ', (1)A '', (3)A ' and (3)A '' states. These five states are the energetically low-lying states of CO2 and their PESs are computed at the MRCISD+Q/aug-cc-pVTZ level of theory using a state-average CASSCF reference wave function. Analysis of the different electronic states for the CO2 -> CO + O dissociation channel rationalizes the topography of this region of the PESs. The forward rates from QCT simulations match measurements between 15 K and 295 K whereas the equilibrium constant determined from the forward and reverse rates is consistent with that derived from statistical mechanics at high temperature. Vibrational relaxation, O + CO(nu = 1,2) -> O + CO(nu = 0), is found to involve both, non-reactive and reactive processes. The contact time required for vibrational relaxation to take place is tau >= 150 fs for non-reacting and tau >= 330 fs for reacting (oxygen atom exchange) trajectories and the two processes are shown to probe different parts of the global potential energy surface. In agreement with experiments, low collision energy reactions for the C(P-3) + O-2((3)sigma(-)(g), nu = 0) -> CO((1)sigma(+)) + O(D-1) lead to CO((1)sigma(+), nu ' = 17) with an onset at E-c similar to 0.15 eV, dominated by the (1)A ' surface with contributions from the (3)A ' surface. Finally, the barrier for the COA((1)sigma(+)) + O-B(P-3) -> COB((1)sigma(+)) + O-A(P-3) atom exchange reaction on the (3)A ' PES yields a barrier of similar to 7 kcal mol(-1) (0.300 eV), consistent with an experimentally reported value of 6.9 kcal mol(-1) (0.299 eV)

N-3(+): Full-dimensional ground state potential energy surface, vibrational energy levels, and dynamics

The fundamental vibrational frequencies and higher vibrationally excited states for the N3+ ion in its electronic ground state have been determined from quantum bound state calculations on three-dimensional potential energy surfaces (PESs) computed at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ-f12 and multireference configuration interaction singles and doubles with quadruples (MRCISD+Q)/aug-cc-pVTZ levels of theory. The vibrational fundamental frequencies are 1130 cm(-1) (nu(1), symmetric stretch), 807 cm(-1) (nu(3), asymmetric stretch), and 406 cm(-1) (nu(2), bend) on the higher-quality CCSD(T)-F12b surface. Bound state calculations based on even higher level PESs [CCSD(T)-F12b/aug-cc-pVQZ-f12 and MRCISD+Q-F12b/aug-cc-pVTZ-f12] confirm the symmetric stretch fundamental frequency as similar to 1130 cm(-1). This compares with an estimated frequency from experiment at 1170 cm(-1) and previous calculations [Chambaud et al., Chem. Phys. Lett. 231, 9-12 (1994)] at 1190 cm(-1). The remaining disagreement with the experimental frequency is attributed to uncertainties associated with the widths and positions of the experimental photoelectron peaks. Analysis of the reference complete active space self-consistent field wave function for the MRCISD+Q calculations provides deeper insight into the shape of the PES and lends support for the reliability of the Hartree-Fock reference wave function for the coupled cluster calculations. According to this, N-3(+) has a mainly single reference character in all low-energy regions of its electronic ground state ((3)A '') PES

Scalable Electron Correlation Methods. 4. Parallel Explicitly Correlated Local Coupled Cluster with Pair Natural Orbitals (PNO-LCCSD-F12)

We present an efficient explicitly correlated pair natural orbital local coupled cluster (PNO-LCCSD-F12) method. The method is an extension of our previously reported PNO-LCCSD approach (Schwilk et al., J. Chem. Theory Comput. 2017, 13, 3650−3675). Near linear scaling with the molecular size is achieved by using pair, domain, and projection approximations, local density fitting and local resolution of the identity, and by exploiting the sparsity of the local molecular orbitals as well as of the projected atomic orbitals. The effect of the various domain approximations is tested for a wide range of chemical reactions and intermolecular interactions. In accordance with previous findings, it is demonstrated that the F12 terms significantly reduce the domain errors. The convergence of the reaction and interaction energies with respect to the parameters that determine the domain sizes and pair approximations is extensively tested. The results obtained with our default thresholds agree within a few tenths of a kcal mol^–1 with the ones computed with very tight options. For cases where canonical calculations are still feasible, the relative energies of local and canonical calculations agree within similar error bounds. The PNO-LCCSD-F12 method needs only 25–40% more computer time than a corresponding PNO-LCCSD calculation while greatly improving the accuracy. Our program is well parallelized and capable of computing accurate correlation energies for molecules with more than 150 atoms using augmented triple-ζ basis sets and more than 5000 basis functions. Using several nodes of a small computer cluster, such calculations can be carried out within a few hours