Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

/pdf/thermochemistry-of-proton-coupled-electron-transfer-reagents-v04esq6den.pdf

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

The self-consistent-charge density-functional tight-binding method (SCC-DFTB) is an approximate quantum chemical method derived from density functional theory (DFT) based on a second-order expansion of the DFT total energy around a reference density. In the present study we combine earlier extensions and improve them consistently with, first, an improved Coulomb interaction between atomic partial charges, and second, the complete third-order expansion of the DFT total energy. These modifications lead us to the next generation of the DFTB methodology called DFTB3, which substantially improves the description of charged systems containing elements C, H, N, O, and P, especially regarding hydrogen binding energies and proton affinities. As a result, DFTB3 is particularly applicable to biomolecular systems. Remaining challenges and possible solutions are also briefly discussed.

/pdf/dftb3-extension-of-the-self-consistent-charge-density-3fwp09yfl7.pdf

DFTB3: Extension of the self-consistent-charge density-functional tight-binding method (SCC-DFTB).

Pulsed laser ablation of graphite targets combined with an intense, atomic nitrogen source has been used to prepare C-N thin film materials. The average nitrogen content in the films was systematically varied by controlling atomic nitrogen flux. Rutherford backscattering measurements show that up to 40 percent nitrogen can be incorporated on average into these solids under the present reaction conditions. Photoelectron spectroscopy further indicates that carbon and nitrogen form an unpolarized covalent bond in these C-N materials. Qualitative tests indicate that the C-N solids are thermally robust and hard. In addition, strong electron diffraction is observed from crystallites within the films. Notably, analysis of these diffraction data show that the only viable structure for the C-N crystallites is that of β-C3N4, a material predicted theoretically to exhibit superhardness. The experimental synthesis of this new C-N material offers exciting prospects for both basic research and engineering applications.

https://science.sciencemag.org/content/sci/261/5119/334.full.pdf

Experimental Realization of the Covalent Solid Carbon Nitride

An investigation of the mobility separation of some peptide and protein ions using a new hybrid quadrupole/travelling wave IMS/oa-ToF instrument

Recent interest in the application of density functional theory prompted us to test various functionals for the van der Waals interactions in the rare-gas dimers, the alkaline-earth metal dimers, zinc dimer, and zinc-rare-gas dimers. In the present study, we report such tests for 18 DFT functionals, including both some very recent functionals and some well-established older ones. We draw the following conclusions based on the mean errors in binding energies and complex geometries: (1) B97-1 gives the best performance for predicting the geometry of rare-gas dimers, whereas M05-2X and B97-1 give the best energetics for rare-gas dimers. (2) PWB6K gives the best performance for the prediction of the geometry of the alkaline-earth metal dimers, zinc dimers, and zinc-rare-gas dimers. M05-2X gives the best energetics for the metal dimers, whereas B97-1 gives the best energetics for the zinc-rare-gas dimers. (3) The M05 functional is unique in providing good accuracy for both covalent transition-metal dimers and van der Waals metal dimers. (4) The combined mean percentage unsigned error in geometries and energetics shows that M05-2X and MPWB1K are the overall best methods for the prediction of van der Waals interactions in metal and rare-gas van der Waals dimers.

/pdf/comparative-dft-study-of-van-der-waals-complexes-rare-gas-vcmremm1oy.pdf

Comparative DFT Study of van der Waals Complexes: Rare-Gas Dimers, Alkaline-Earth Dimers, Zinc Dimer, and Zinc-Rare-Gas Dimers

A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ion−molecule reactions involving Ar+(2P3/2,1/2), O+(4S, 2D, 2P), and organosulfur ions (CH3SH+, CH3CH2SH+, and CH3SCH3+) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) and those produced in photoionization of CO2 (CO, N2, O2) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociat...

State-Selected and State-to-State Ion−Molecule Reaction Dynamics

This monograph reviews the recent progress in vacuum ultraviolet (VUV) photoionization and photodissociation processes. Photoionization, photoelectron, and fluorescence spectroscopic techniques have played an important role in revealing the photoionization and photodissociation dynamics of molecules in the vacuum ultraviolet region and in providing accurate energetic and spectroscopic information of ions as well as neutral molecules. The book represents the first detailed review of major experimental developments in the studies of single vacuum ultraviolet photon ionization and dissociation processes of gaseous molecules and clusters.

Vacuum ultraviolet photoionization and photodissociation of molecules and clusters

H 2 + (X,v + =0~15,N + =1)+Heプロトン移動反応のパルス電界イオン化光電子二次イオンコインシデンス研究

At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high-flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular-beam experiments. A photon flux of 1016 photon/s has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionoization studies. At this second end station a peak flux of 3×1011 was observed for 25 000 resolving power. This monochromator has achieved a resolving power of 70 000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a vacuum ultraviolet monochromator.

Performance of the vacuum ultraviolet high-resolution and high-flux beamline for chemical dynamics studies at the Advanced Light Source

The recent developments of vacuum ultraviolet (VUV) laser and third generation synchrotron radiation sources, together with the introduction of pulsed field ionization (PFI) schemes for photoion-photoelectron detection, have had a profound impact on the field of VUV spectroscopy and chemistry. Owing to the mediation of near-resonant autoionizing states, rovibronic states of ions with negligible Franck-Condon factors for direct photoionization can be examined by VUV-PFI measurements with rotational resolutions. The VUV-PFI spectra thus obtained have provided definitive ionization energies (IEs) for many small molecules. The recent synchrotron-based PFI-photoelectron-photoion coincidence experiments have demonstrated that dissociative photoionization thresholds for a range of molecules can be determined to the same precision as in PFI-photoelectron measurements. Combining appropriate dissociation thresholds and IEs measured in PFI studies, thermochemical data for many neutrals and cations can be determined with unprecedented precision. The further development of two-color excitation-ionization schemes promises to expand the scope of spectroscopic and chemical applications using the photoionization-photoelectron method.

Cheuk-Yiu Ng

Papers

State-Selected and State-to-State Ion−Molecule Reaction Dynamics

Vacuum ultraviolet photoionization and photodissociation of molecules and clusters

H 2 + (X,v + =0~15,N + =1)+Heプロトン移動反応のパルス電界イオン化光電子二次イオンコインシデンス研究

Performance of the vacuum ultraviolet high-resolution and high-flux beamline for chemical dynamics studies at the Advanced Light Source

Vacuum ultraviolet spectroscopy and chemistry by photoionization and photoelectron methods