Mechanochemically Triggered Topology Changes in Expanded Porphyrins

A hitherto unexplored class of molecules for molecular force probe applications are expanded porphyrins. This work proves that mechanical force is an effective stimulus to trigger the interconversion between Hückel and Möbius topologies in [28]hexaphyrin, making these expanded porphyrins suitable to act as conformational mechanophores operating at mild (sub-1 nN ) force conditions. A straightforward approach based on distance matrices is proposed for the selection of pulling scenarios that promote either the planar Hückel topology or the three lowest lying Möbius topologies. This approach is supported by quantum mechanochemical calculations. Force distribution analyses reveal that [28]hexaphyrin selectively allocates the external mechanical energy to molecular regions that trigger Hückel–Möbius interconversions, explaining why certain pulling scenarios favor the Hückel two-sided topology and others favor Möbius single-sided topologies. The meso-substitution pattern on [28]hexaphyrin determines whether the energy difference between the different topologies can be overcome by mechanical activation.     

First principles NMR calculations by fragmentation

The nuclear magnetic shielding tensor is a molecular property that can be computed from first principles. In this work we show that by utilizing the fragmentation approach, one is able to accurately compute this property for a large class of molecules. This is of great significance because the computational expense required in the evaluation of the shielding tensor for all nuclei in a large molecule is now subject to near linear scaling. On the basis of previous studies and this work, it is also very likely that all molecular properties that can be expressed as derivatives of the total energy of the system are also amenable to accurate evaluation via fragmentation. If only the chemical shifts for nuclei in a small part of a large molecule are of interest, then only those molecular fragments containing those nuclei need to have their shielding tensors evaluated. Further, the fragmentation approach allows one to construct a database of molecular fragments that could, in principle, be used in the NMR characterization of molecules and at the same time provide possible three-dimensional representations of these molecules.     

Improving on equilibrium isotropic nuclear shielding constants

Isotropic nuclear shielding constants at the equilibrium molecular structure σ_eq and zero‐point vibrational corrections (ZPVCs) to σ_eq are evaluated using the B3LYP/aug‐cc‐pVTZ level of theory, as well as the KT2/aug‐cc‐pVTZ level of theory. Various scaling factors and systematic corrections are obtained by linear regression to experimental shielding constants. Comparisons of the scaled and systematically corrected equilibrium and vibrationally averaged shielding constants reveal that, at the 99% confidence level, the ZPVCs via second‐order perturbation theory do not improve the agreement of B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ calculated shielding constants with experiment. This holds true when the same analysis is applied to CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq of Teale et al. [Journal of Chemical Physics 2013, 138, 024111]. In addition, at the 99% confidence level, B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ scaled and systematically corrected shielding constants are found to be statistically no different from CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq. The use of scaling factors and systematic corrections could thus provide a cheaper but yet reasonably accurate alternative for the study of nuclear shielding constants of larger systems.     

Alkaline Earth Metals Activate N2 and CO in Cubic Complexes Just Like Transition Metals: A Conceptual Density Functional Theory and Energy Decomposition Analysis Study

Following the recent discovery of stable octa-coordinated alkaline earth metals with N₂ and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT-based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N₂ and CO become much more nucleophilic and electrophilic in all complexes, relevant for N₂ fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO–LUMO) orbital interactions.     

Estimation of vocal dysperiodicities in disordered connected speech by means of distant-sample bidirectional linear predictive analysis

The article presents an analysis of vocal dysperiodicities in connected speech produced by dysphonic speakers. The processing is based on a comparison of the present speech fragment with future and past fragments. The size of the dysperiodicity estimate is zero for periodic speech signals. A feeble increase of the vocal dysperiodicity is guaranteed to produce a feeble increase of the estimate. No spurious noise boosting occurs owing to cycle insertion and omission errors, or phonetic segment boundary artifacts. Additional objectives of the study have been investigating whether deviations from periodicity are larger or more commonplace in connected speech than in sustained vowels, and whether sentences that comprise frequent voice onsets and offsets are noisier than sentences that comprise few. The corpora contain sustained vowels as well as grammatically- and phonetically matched sentences. An acoustic marker that correlates with the perceived degree of hoarseness summarizes the size of the dysperiodicities. The marker values for sustained vowels have been highly correlated with those for connected speech, and the marker values for sentences that comprise few voiced/unvoiced transients have been highly correlated with the marker values for sentences that comprise many.