Carmen Herrmann

Exploring local currents in molecular junctions

Electron transfer through molecules is an ubiquitous process underlying the function of biological systems and synthetic devices. The electronic coupling between components varies with the structure of the molecular bridge, often in classically unintuitive ways, as determined by its quantum electronic structure. Considerable efforts in electron-transfer theory have yielded models that are useful conceptually and provide quantitative means to understand transfer rates in terms of local contributions. Here we show how a description of the local currents within a bridging molecule bound to metallic electrodes can provide chemical insight into current flow. In particular, we show that through-space, as opposed to through-bond, terms dominate in a surprising number of instances, and that interference effects can be characterized by the reversal of ring currents. Together these ideas have implications for the design of molecular electronic devices, in particular for the ways in which substituent effects may be used for maximum impact.

Organic Radicals As Spin Filters

Molecular spintronics has received extensive interest in recent years. Due to their favorable properties such as long spin coherence lengths and an amenability to fine-tuning via chemical substituents, organic materials play a prominent role in this field. Here we discuss how organic radicals may act as spin filters in the coherent tunneling regime and how they may be tuned to filter either majority- or minority-spin electrons by adding electron-donating or -withdrawing substituents. For a set of benzene-based model systems, we identify dips in the spin-resolved transmission, which may be caused by destructive interference, as a desirable feature when aiming for efficient spin filtering. Furthermore, the qualitative predictions made for our model systems are shown to be transferable to larger stable radicals.

Comparative analysis of local spin definitions

This work provides a survey of the definition of electron spin as a local property and its dependence on several parameters in actual calculations. We analyze one-determinant wave functions constructed from Hartree-Fock and, in particular, from Kohn-Sham orbitals within the collinear approach to electron spin. The scalar total spin operators S2 and Sz are partitioned by projection operators, as introduced by Clark and Davidson, in order to obtain local spin operators SASB and SzA, respectively. To complement the work of Davidson and co-workers, we analyze some features of local spins which have not yet been discussed in sufficient depth. The dependence of local spin on the choice of basis set, density functional, and projector is studied. We also discuss the results of Sz partitioning and show that SzA values depend less on these parameters than SASB values. Furthermore, we demonstrate that for small organic test molecules, a partitioning of Sz with preorthogonalized Lowdin projectors yields nearly the same results as one obtains using atoms-in-molecules projectors. In addition, the physical significance of nonzero SASB values for closed-shell molecules is investigated. It is shown that due to this problem, SASB values are useful for calculations of relative spin values, but not for absolute local spins, where SzA values appear to be better suited.

The Chameleonic Nature of Electron Transport through π-Stacked Systems

We investigate the electron transport properties of pi-stacked benzene rings and show that when the symmetry of the system is reduced, by the addition of substituents to bind to metallic electrodes, fully eclipsed structures are not necessarily optimal and dislocated structures may show enhanced transport. Conversely, we show the transport through an infinite chain of pi-stacked benzene rings reflects the full 6-fold symmetry of the system and is maximized in the fully eclipsed stack. When utilizing pi-stacked systems for electron transport, the nature of the mechanism for charge injection must be considered to account for these differences in performance.

First-Principles Approach to Vibrational Spectroscopy of Biomolecules

Vibrational spectroscopy of biomolecules like enzymes, nucleic acids, carbohydrates, lipids, and their components, is in most cases the vibrational spectroscopy of large molecules in aqueous solution or in vivo. Since large molecules in solution are likely to yield conventional infrared (IR) and Raman spectra with many close-lying peaks, spectroscopic techniques which filter out information selectively are of special interest in this field. Because of the large size of the investigated molecules and the lack of reliable rules of thumb for many special techniques, accurate first-principles calculations are an important means of interpreting the resulting spectra. First-principle calculations on biomolecules in solution have to cope with the challenges arising from the size of the systems under study, which make selective computational techniques an essential tool in order to be able to investigate biomolecular systems of reasonable size with sufficient computational accuracy. This is why the focus of this work is on the selective first-principles calculation of vibrational spectra of biomolecules obtained by special techniques such as difference IR and Raman, Vibrational Circular Dichroism, Raman Optical Activity, resonance Raman, and also Coherent Anti-Stokes Raman Spectroscopy, two-dimensional IR and Nuclear Resonance Vibrational Spectroscopy. For each of these techniques, a short introduction of their relevance for studies on biomolecules is given. Theoretical as well as practical aspects of calculating the corresponding intensities are discussed and complemented by references to original work on these topics.

Spin states in polynuclear clusters: The [Fe2O2] core of the methane monooxygenase active site

The ability to provide a correct description of different spin states of mono‐ and polynuclear transition metal complexes is essential for a detailed investigation of reactions that are catalyzed by such complexes. We study the energetics of different total and local spin states of a dinuclear oxygen‐bridged iron(IV) model for the intermediate Q of the hydroxylase component of methane monooxygenase by means of spin‐unrestricted Kohn–Sham density functional theory. Because it is known that the spin state total energies depend systematically on the density functional, and that this dependence is intimately connected to the exact exchange admixture of present‐day hybdrid functionals, we compare total energies, local and total spin values, and Heisenberg coupling constants calculated with the established functionals BP86 and B3LYP as well as with a modified B3LYP version with an exact exchange admixture ranging from 0 to 24%. It is found that exact exchange enhances local spin polarization. As the exact exchange admixture increases, the high‐spin state is energetically favored, although the Broken‐Symmetry state always is the ground state. Instead of the strict linear variation of the energy splittings observed for mononuclear complexes, a slightly nonlinear dependence is found. The Heisenberg coupling constants JFe1Fe2—evaluated according to three different proposals from the literature—are found to vary from −129 to −494cm−1 accordingly. The experimental finding that intermediate Q has an antiferromagnetic ground state is thus confirmed.

MOVIPAC: Vibrational spectroscopy with a robust meta‐program for massively parallel standard and inverse calculations

We present the software package MOVIPAC for calculations of vibrational spectra, namely infrared, Raman, and Raman Optical Activity (ROA) spectra, in a massively parallelized fashion. MOVIPAC unites the latest versions of the programs SNF and AKIRA alongside with a range of helpful add‐ons to analyze and interpret the data obtained in the calculations. With its efficient parallelization and meta‐program design, MOVIPAC focuses in particular on the calculation of vibrational spectra of very large molecules containing on the order of a hundred atoms. For this purpose, it also offers different subsystem approaches such as Mode‐ and Intensity‐Tracking to selectively calculate specific features of the full spectrum. Furthermore, an approximation to the entire spectrum can be obtained using the Cartesian Tensor Transfer Method. We illustrate these capabilities using the example of a large π‐helix consisting of 20 (S)‐alanine residues. In particular, we investigate the ROA spectrum of this structure and compare it to the spectra of α‐ and 310‐helical analogs.

Relevance of the electric-dipole--electric-quadrupole contribution to Raman optical activity spectra.

We examine the importance of the electric-dipole--electric-quadrupole polarizability tensor in the intensity theory of Raman optical activity (ROA) spectra. Using density functional theory, ROA spectra of organic cyclic compounds, alanine, oligoalanines, and examples from the literature are analyzed in detail, and a statistical investigation is performed. It is found that the contribution of the electric-dipole--electric-quadrupole tensor is often small, except for some special cases that involve C-H stretching vibrations.

Finding a needle in a haystack: direct determination of vibrational signatures in complex systems

Vibrational spectroscopy is a powerful tool to investigate the structure and dynamics of molecular systems. When large molecules are studied, quantum chemical calculations are used to interpret the spectra. In many cases, experimentally driven questions are related to specific regions of a vibrational spectrum, so that an assignment is only required for a subset of vibrations. This holds true in particular for biomolecules, inorganic compounds which are stabilized by many bulky ligands, or other extended systems. In standard quantum chemical calculations of the vibrational spectrum, all normal modes and frequencies of the molecule under study are determined. However, the selective calculation of only relevant information can be made much more efficient by using mode-selective techniques as provided by the mode-tracking algorithm. A critical point for the performance of the mode-tracking scheme is the preparation of a guess vibration, which is then iteratively refined. The guess defines the scientific problem which is to be studied. Various examples are presented to highlight this aspect and the features of the mode-tracking algorithm in general.

Designing organic spin filters in the coherent tunneling regime

Spin filters, that is, systems which preferentially transport electrons of a certain spin orientation, are an important element for spintronic schemes and in chemical and biological instances of spin-selective electronic communication. We study the relation between molecular structure and spin filtering functionality employing a theoretical analysis of both model and stable organic radicals based on substituted benzene, which are bound to gold electrodes, with a combination of density functional theory and the Landauer-Imry-Büttiker approach. We compare the spatial distribution of the spin density and of the frontier central subsystem molecular orbitals, and local contributions to the transmission. Our results suggest that the delocalization of the singly occupied molecular orbital and of the spin density onto the benzene ring connected to the electrodes, is a good, although not the sole indicator of spin filtering functionality. The stable radicals under study do not effectively act as spin filters, while the model phenoxy-based radicals are effective due to their much larger spin delocalization. These conclusions may also be of interest for electron transfer experiments in electron donor-bridge-acceptor complexes.