Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(C n F 2n ) (L = CO, PMe 3 ; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C 3 F 6 ) (L = CO, PMe 3 ) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol −1 , and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol −1 . For the two metal carbene structures Cp(L)CoC(CF 3 ) 2 and Cp(L)CoCF(C 2 F 5 ), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C 4 F 8 ) (L = CO, PMe 3 ) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcal mol −1 , and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol −1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh 2 Me)(–CFR–CF 2 –CF 2 –) (R = F, CF 3 ) to the perfluoroolefin complexes CpCo(PPh 2 Me)(RCFCF 2 ) in the presence of catalytic quantities of HN(SO 2 CF 3 ) 2 . Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above.
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Hidden complexities in the reaction of H 2 O 2 and HNO revealed by ab initio quantum chemical investigations
Nitroxyl (HNO) and hydrogen peroxide have both been implicated in a variety of reactions relevant to environmental and physiological processes and may contribute to a unique, unexplored, pathway for the production of nitrous acid (HONO) in soil. To investigate the potential for this reaction, we report an in-depth investigation of the reaction pathway of H 2 O 2 and HNO forming HONO and water. We find the breaking of the peroxide bond and a coupled proton transfer in the first step leads to hydrogen nitryl (HNO 2 ) and an endogenous water, with an extrapolated NEVPT2 (multireference perturbation theory) barrier of 29.3 kcal mol −1 . The first transition state is shown to possess diradical character linking the far peroxide oxygen to the bridging, reacting, peroxide oxygen. The energy of this first step, when calculated using hybrid density functional theory, is shown to depend heavily on the amount of Hartree–Fock exchange in the functional, with higher amounts leading to a higher barrier and more diradical character. Additionally, high amounts of spin contamination cause CCSD(T) to significantly overestimate the TS1 barrier with a value of 36.2 kcal mol −1 when using the stable UHF wavefunction as the reference wavefunction. However, when using the restricted Hartree–Fock reference wavefunction, the TS1 CCSD(T) energy is lowered to yield a barrier of 31.2 kcal mol −1 , in much better agreement with the NEVPT2 result. The second step in the reaction is the isomerization of HNO 2 to trans -HONO through a Grotthuss-like mechanism accepting a proton from and donating a proton to the endogenous water. This new mechanism for the isomerization of HNO 2 is shown to have an NEVPT2 barrier of 23.3 kcal mol −1 , much lower than previous unimolecular estimates not including an explicit water. Finally, inclusion of an additional explicit water is shown to lower the HNO 2 isomerization barrier even further.
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- Award ID(s):
- 1665427
- NSF-PAR ID:
- 10059406
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 19
- Issue:
- 43
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 29549 to 29560
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The mechanism of isomerization of hydroxyacetone to 2‐hydroxypropanal is studied within the framework of reaction force analysis at the M06‐2X/6‐311++G(d,p) level of theory. Three unique pathways are considered: (a) a step‐wise mechanism that proceeds through the formation of the Z‐isomer of their shared enediol intermediate, (b) a step‐wise mechanism that forms the E‐isomer of the enediol, and (c) a concerted pathway that bypasses the enediol intermediate. Energy calculations show that the concerted pathway has the lowest activation energy barrier at 45.7 kcal mol−1. The reaction force, chemical potential, and reaction electronic flux are calculated for each reaction to characterize electronic changes throughout the mechanism. The reaction force constant is calculated in order to investigate the synchronous/asynchronous nature of the concerted intramolecular proton transfers involved. Additional characterization of synchronicity is provided by calculating the bond fragility spectrum for each mechanism.
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Highly reactive benzyl radicals are generated by electron dissociative attachment to benzyl chloride doped into a neon–hydrogen–helium discharge and immediately cooled to T rot = 15 K in a high density, supersonic slit expansion environment. The sub-Doppler spectra are fit to an asymmetric-top rotational Hamiltonian, thereby yielding spectroscopic constants for the ground ( v = 0) and first excited ( v = 1, ν 3 , ν 4 ) vibrational levels of the ground electronic state. The rotational constants obtained for the ground state are in good agreement with previous laser induced fluorescence measurements (LIF), with vibrational band origins ( ν 3 = 3073.2350 ± 0.0006 cm −1 , ν 4 = 3067.0576 ± 0.0006 cm −1 ) in agreement with anharmonically corrected density functional theory calculations. To assist in detection of benzyl radical in the interstellar medium, we have also significantly improved the precision of the ground state rotational constants through combined analysis of the ground state IR and LIF combination differences. Of dynamical interest, there is no evidence in the sub-Doppler spectra for tunneling splittings due to internal rotation of the CH 2 methylene subunit, which implies a significant rotational barrier consistent with partial double bond character in the CC bond. This is further confirmed with high level ab initio calculations at the CCSD(T)-f12b/ccpVdZ-f12 level, which predict a zero-point energy corrected barrier to internal rotation of Δ E tun ≈ 11.45 kcal mol −1 or 4005 cm −1 . In summary, the high-resolution infrared spectra are in excellent agreement with simple physical organic chemistry pictures of a strongly resonance-stabilized benzyl radical with a nearly rigid planar structure due to electron delocalization around the aromatic ring.more » « less
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High-resolution direct absorption infrared spectra of metastable cis-formic acid (HCOOH) trapped in a cis-well resonance behind a 15 kcal/mol barrier are reported for the first time, with the energetically unstable conformer produced in a supersonic slit plasma expansion of trans-formic acid/H 2 mixtures. We present a detailed high-resolution rovibrational analysis for cis-formic acid species in the OH stretch ( ν 1 ) fundamental, providing first precision vibrational band origin, rotational constants, and term values, which in conjunction with ab initio calculations at the couple-cluster with single, double, and perturbative triple [CCSD(T)]/ANOn (n = 0, 1, 2) level support the experimental assignments and establish critical points on the potential energy surface for internal rotor trans-to-cis isomerization. Relative intensities for a- and b-type transitions observed in the spectra permit the transition dipole moment components to be determined in the body fixed frame and prove to be in good agreement with ab initio CCSD(T) theoretical estimates but in poor agreement with simple bond-dipole predictions. The observed signal dependence on H 2 in the discharge suggests the presence of a novel H atom radical chemical mechanism for strongly endothermic “up-hill” internal rotor isomerization between trans- and cis-formic acid conformers.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c7cp05883g
