Mechanistic investigation on oxygen transfer with the manganese-salen complex

The best-known application of salen complexes is the use of a chiral ligand loaded with manganese to form the Jacobsen complex. This organometallic catalyst is used in the epoxidation of unfunctionalized olefins and can achieve very high selectivities. Although this application was proposed many years ago, the mechanism of oxygen transfer remains a question until now. In this paper, the epoxidation mechanism is investigated by an ab initio kinetic modeling study. First of all a proper DFT functional is selected that yields the correct ordering of the various spin states. Our results show that the epoxidation proceeds via a radical intermediate. If we start from the radical intermediate, these results can explain the experiments with radical probes. The subtle influences in the transition state using the full Jacobsen catalyst explain the product distribution observed experimentally

Silicon-on-insulator polarization rotating micro-ring resonator

We propose a novel micro-ring resonator which uses quasi-TE polarized light in the bus waveguide to excite the quasi-TM polarized modes in a micro-ring. An all-pass filter is demonstrated on Silicon-on-Insulator

DMRG-SCF study of the singlet, triplet, and quintet states of oxo-Mn(Salen)

We use CheMPS2, our free open-source spin-adapted implementation of the density matrix renormalization group (DMRG) [Wouters et al., Comput. Phys. Commun. 185, 1501 (2014)], to study the lowest singlet, triplet, and quintet states of the oxo-Mn(Salen) complex. We describe how an initial approximate DMRG calculation in a large active space around the Fermi level can be used to obtain a good set of starting orbitals for subsequent complete-active-space or DMRG self-consistent field (CASSCF or DMRG-SCF) calculations. This procedure mitigates the need for a localization procedure, followed by a manual selection of the active space. Per multiplicity, the same active space of 28 electrons in 22 orbitals (28e, 22o) is obtained with the 6-31G*, cc-pVDZ, and ANO-RCC-VDZP basis sets (the latter with DKH2 scalar relativistic corrections). Our calculations provide new insight into the electronic structure of the quintet.Comment: 5 pages, 4 figures, 2 tables; submitted to J. Chem. Phy

Experimental extraction of effective refractive index and thermo-optic coefficients of silicon-on-insulator waveguides using interferometers

We propose and demonstrate an accurate method of measuring the effective refractive index and thermo-optic coefficient of silicon-on-insulator waveguides in the entire C-band using three Mach-Zehnder interferometers. The method allows for accurate extraction of the wavelength dispersion and takes into account fabrication variability. Wafer scale measurements are performed and the effective refractive index variations are presented for three different waveguide widths: 450, 600, and 800 nm, for the TE polarization. The presented method is generic and can be applied to other waveguide geometries and material systems and for different wavelengths and polarizations

Tuning the lateral leakage loss of TM-like modes in shallow-etched waveguides using liquid crystals

We examine the tuning effect a liquid crystal (LC) cladding has on the lateral leakage loss of TM-like modes in shallow-etched waveguides. For such waveguides with an air cladding, the guided TM-like mode and the unguided cladding TE-like mode can only be phase matched at precisely one angle. We find that for an anisotropic cladding such as an LC, this phase matching is now possible for a range of angles. Each of these angles corresponds to a given orientation of the molecules in the LC cladding. We show that the waveguide width at which the minimum in leakage loss occurs can be changed by varying the orientation of the LC cladding. We find different tuning regimes, identify a suitable tuning range, and discuss the feasibility of tunable leakage loss experiments