Density Functional Theory/Molecular Mechanics Approach for Electronic g-Tensors of Solvated Molecules

Inhomogeneities in PNIPAM Aqueous Solutions: The Inside View by Spin Probe EPR Spectroscopy

Coil to globule transition in poly(N-isopropylacrylamide) aqueous solutions was studied using spin probe continuous-wave electronic paramagnetic resonance (CW EPR) spectroscopy with an amphiphilic TEMPO radical as a guest molecule. Using Cu(II) ions as the “quencher” for fast-moving radicals in the liquid phase allowed obtaining the individual spectra of TEMPO radicals in polymer globule and observing inhomogeneities in solutions before globule collapsing. EPR spectra simulations confirm the formation of molten globules at the first step with further collapsing and water molecules coming out of the globule, making it denser.

o-Semiquinone radical anion isolated as an amorphous porous solid

The use of metal cations is a commonly applied strategy to create S > 1/2 stable molecular systems containing semiquinone radicals. Persistent mono-semiquinonato complexes of diamagnetic metal ions (S = 1/2) have been hitherto less common and mostly limited to the complexes of heavy metal ions. In this work, a mono-semiquinonato complex of aluminum, derived from 1,2-dihydroxybenzene, is obtained using a surprisingly short and uncomplicated procedure. The isolated product is an amorphous and porous solid that exhibits very good stability under ambient conditions. To characterise its molecular and electronic structure, 9.7, 34 and 406 GHz EPR spectroscopy was used in concert with computational techniques (DFT and DLPNO-CCSD). It was revealed that the radical complex is composed of two chemically equivalent aluminum cations and two catechol-like ligands with the unpaired electron uniformly distributed between the two organic molecules. The good stability and porous structure make this complex applicable in heterogeneous aerobic reactions.

Decomposition of molecular properties

We review recent work on property decomposition techniques using quantum chemical methods and discuss some topical applications in terms of quantum mechanics-molecular mechanics calculations and the constructing of properties of large molecules and clusters. Starting out from the so-called LoProp decomposition scheme [Gagliardi et al., J. Chem. Phys., 2004, 121, 4994] for extracting atomic and inter-atomic contributions to molecular properties we show how this method can be generalized to localized frequency-dependent polarizabilities, to localized hyperpolarizabilities and to localized dispersion coefficients. Some applications of the generalized decomposition technique are reviewed - calculations of frequency-dependent polarizabilities, Rayleigh scattering of large clusters, and calculations of hyperpolarizabilities of proteins.

Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix

The g matrices (g tensors) of various phosphinyl radicals (R2 P(.) ) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless PO or PS bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with PO bonds, this contribution to the g2 component was dominant.

Frequency-dependent force fields for QMMM calculations

The frequency-dependent localized polarizabilities are calculated for the first time using analytical response theory and benchmarked for different water clusters and the tryptophan residue embedded in a protein.

A gauge invariant multiscale approach to magnetic spectroscopies in condensed phase: general three-layer model, computational implementation and pilot applications

Analytical equations to calculate second order electric and magnetic properties of a molecular system embedded into a polarizable environment are presented. The treatment is limited to molecules described at the self consistent field level of theory, including Hartree-Fock theory as well as Kohn-Sham density functional theory and is extended to the Gauge-Including Atomic Orbital method. The polarizable embedding is described by means of our already implemented polarizable quantum mechanical/molecular mechanical (MM) methodology, where the polarization in the MM layer is handled by means of the fluctuating charge (FQ) model. A further layer of description, i.e, the polarizable continuum model, can also be included. The FQ(/polarizable continuum model) contributions to the properties are derived, with reference to the calculation of the magnetic susceptibility, the nuclear magnetic resonance shielding tensor, electron spin resonance g-tensors, and hyperfine couplings.

π-Stacking effects on the EPR parameters of a prototypical DNA spin label

The character and value of spin labels for probing environments like double-stranded DNA depend on the degree of change in the spin Hamiltonian parameters of the spin label induced by the environment. Herein we provide a systematic theoretical investigation of this issue, based on a density functional theory method applied to a spin labeled DNA model system, focusing on the dependence of the EPR properties of the spin label on the π stacking and hydrogen bonding that occur upon incorporating the spin label into the selected base pair inside DNA. It is found that the EPR spin Hamiltonian parameters of the spin label are only negligibly affected by its incorporation into DNA, when compared to its free form. This result gives a theoretical ground for the common empirical assumption regarding the behaviour of spin Hamiltonian parameters made in EPR based measurements of the distance between spin labels incorporated into DNA.