Chemphyschem最新文献

The adsorption of the radical α,γ-bisdiphenylene-β-phenylallyl (BDPA) molecule to the Cu(100) surface was studied using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and density functional theory (DFT) calculations accounting for dispersion forces. BDPA on Cu(100) was observed to align preferentially along [[EQUATION]] directions due to weak Cu-C chemisorption between fluorenyl carbons with the underlying copper atoms. The curved shape of the BDPA molecule on Cu(100) can be ascribed to the lack of molecular orbital character on the phenyl substituent. A Kondo-like feature from differential conductance (dI/dV) measurements centered close to the Fermi energy ([[EQUATION]]) suggests the retention of an electron spin-½ state, which is corroborated by hybrid DFT calculations that place the SOMO below and SUMO above EF for Cu(100).

Retinal protonated Schiff base (RPSB), found in its all-trans conformer in Bacteriorhodopsin, undergoes barrier-controlled isomerization upon photoabsorption through polyene- chain torsion. The effects of the protein environment on the active vibrations during photoabsorption and their redistribution are still not understood. This paper reports on femtosecond time-resolved action-absorption measurements of cryogenically cooled gas-phase all-trans RPSB, which exhibit two coherent vibrational oscillations, 167(14) cm-1 and 117(1) cm-1, of the first excited state with dephasing times of ∼ 1 ps. The absence of the high-frequency vibration in solution and the low-frequency vibration in the protein indicates that these vibrations are sensitive to environments. An action-absorption spectrum of cryogenically cold all-trans RPSB, reveals a ∼ 310 cm-1 active vibration when using a hole-burning technique and 1500 cm-1 C=C stretching modes.

The molecular structure of a ferrocene derivative with adjacent centers of chirality, 1,1'-bis(tert - butylphosphino)ferrocene, has been examined in the gas phase using broadband microwave spectroscopy under the isolated and cold conditions of a supersonic jet. The diastereomers of 1,1'-bis(tert-butylphosphino)ferrocene can adopt homo- and hetero-chiral configurations, owing to the P-chiral substituents on the cyclopentadienyl rings. Moreover, the internal ring rotation of each diastereomer gives rise to four conformers with eclipsed ring arrangements, where the two tert-butylphosphino groups were separated by dihedral angles of approximately 72◦, 144◦, 216◦, and 288◦ with respect to the two ring centers. The interconversion barriers between the conformations are below 2 kJ/mol, whereas the pyramidal inversion of the tert-butylphosphino groups is hindered by more than 140 kJ/mol, calculated at the B3LYP-D3(BJ)/def2-QZVP level of theory. In the experimental microwave spectrum, we unambiguously identified the two global-minimum diastereomers with 72◦ conformations. The absence of other conformers can be attributed to the relaxation dynamics in the supersonic jet, which transfers the high-energy conformers to the respective global-minimum geometries. Additionally, we discovered that London dispersion inter- actions between the two tert-butylphosphino groups play a crucial role in stabilizing the structures of this ferrocene complex.

Three types of hydrogen bonds of coordinated glycine and water had been investigated: NH/O of α-amino group, O1/HO involving oxygen coordinated to the metal ion (O1), and O2/HO involving α-carbonyl oxygen (O2). Various glycine complexes were investigated: octahedral cobalt(III) and nickel(II), square pyramidal copper(II), and square planar copper(II), palladium(II), and platinum(II) complexes. Nature of these three hydrogen bond types was analysed using symmetry-adapted perturbation theory (SAPT) and variational energy decomposition analysis (EDA) method (TPSS-D3/def2-TZVPP). The results of the EDA decomposition are in good agreement with the reliable SAPT2+3/def2-TZVPP and its total interaction values with CCSD(T)/CBS energies. Electrostatic interaction is generally the dominant attractive energy term in most of the interactions, followed by orbital relaxation, and lastly dispersion as the weakest. We compared EDA results of various complexes to determine the effects of complex charge, metal oxidation, coordination, and atomic number on the energy decomposition terms. The complex charge influences the values of decomposition terms the most, followed by metal oxidation and coordination number, while atomic number effects them the least. All complex and metal changes have a more significant effect on the results of NH/O and O1/HO then O2/HO interactions, due to its location further away from the metal ion.

Carbon dioxide capture technologies are set to play a vital role in mitigating the current climate crisis. Solid-state 17O NMR spectroscopy can provide key mechanistic insights that are crucial to effective sorbent development. In this work, we present the fundamental aspects and complexities for the study of hydroxide-based CO2 capture systems by 17O NMR. We perform static density functional theory (DFT) NMR calculations to assign peaks for general hydroxide CO2 capture products, finding that 17O NMR can readily distinguish bicarbonate, carbonate and water species. However, in application to CO2 binding in two test case hydroxide-functionalised metal-organic frameworks (MOFs):  MFU-4l and KHCO3-cyclodextrin-MOF, we find that a dynamic treatment is necessary to obtain agreement between computational and experimental spectra. We therefore introduce a workflow that leverages machine-learning forcefields to capture dynamics across multiple chemical exchange regimes, providing a significant improvement on static DFT predictions. In MFU-4l, we parameterise a two-component dynamic motion of the bicarbonate motif involving a rapid carbonyl seesaw motion and intermediate hydroxyl proton hopping. For KHCO3-CD-MOF, we combined experimental and modelling approaches to propose a new mixed carbonate-bicarbonate binding mechanism and thus, we open new avenues for the study and modelling of hydroxide-based CO2 capture materials by 17O NMR.

Silica materials represent a promising material for the application in heterogeneous organocatalysis due to their readily modifiable surface and chemical inertness. To achieve high catalyst loadings, usually, porous carriers with high surface areas are used, such as special silica monoliths or spherical particles for backed bed reactors.  Yet, their synthesis is elaborate, and thus less complex and cheaper alternatives are of interest, especially considering scaling up. In this work, two commercial silica materials functionalized with the organocatalyst 4-(dimethylamino)pyridine (DMAP) were used in catalytic acylation reactions: a mesoporous silica gel (Siliabond®-DMAP) and non-porous silica nanoparticles (Ludox®). Both were successfully used in the acylation of phenylethanol, but the latter required significantly longer reaction times, presumably due to mass-transfer limitations as a consequence of substantial agglomeration that limits the accessible amount of catalyst. Furthermore, it was shown that the influence of the linker molecule is negligible, since both reaction yields and the activation energy remain largely similar. As main result the commercial material Siliabond-DMAP, despite the non-uniform particles, exhibited significant yield in a flow setup, thus demonstrating the potential as support material for application in heterogeneous organocatalysis.

The use of oriented external electric fields (OEEFs) shows promise as an alternative approach to chemical catalysis. The ability to target a specific bond by aligning it with a bond-weakening electric field may be beneficial in mechanochemical reactions, which use mechanical force to selectively rupture bonds. Previous computational studies have focused on a static description of molecules in OEEFs, neglecting to test the influence of thermal oscillations on molecular stability. Here, we performed ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) to investigate the behaviour of a model mechanophore under the simultaneous influence of thermal and electric field effects. We show that the change in bond length caused by a strong electric field is largely independent of the temperature, both without and with mechanical stretching forces applied to the molecule. The amplitude of thermal oscillations increases with increasing field strength and temperature, but at low temperatures, the application of mechanical force leads to an additional increase in amplitude. Our research shows that methods for applying mechanical force and OEEFs can be safely combined and included in an AIMD simulation at both low and high temperatures, allowing researchers to computationally investigate mechanochemical reactions in realistic application scenarios.

3-tolunitrile (3-TN) can form three different dimers, which differ in the relative orientation of the methyl groups. We determined the geometry changes of two of these conformers of 3-TN upon electronic excitation via a Franck-Condon fit of the vibronic intensities in the fluorescence emission spectra. Both aromatic rings expand upon electronic excitation, showing a delocalized one-photon excitation of the cluster. The conformer with the smaller center-of-mass distance shows an unusual order of the split components of the electronic origin. We attribute this changed order to the stronger charge transfer contributions to the splitting and a partial breakdown of the point dipole approximation, made in the Frenkel type interaction.

Mixing different aliphatic polyamides provides opportunities to tune and optimize the properties of these semicrystalline polycondensates. Combining experiment and theory, we predict and explain the miscibility of aliphatic polyamide mixtures. Visual inspection and Raman spectroscopy of polyamide mixtures show that liquid/liquid phase demixing occurs in the melt due to limited miscibility. The large number of potential polyamide mixtures makes it challenging to test all miscibilities experimentally. Moreover, the dependence of miscibility on dispersity and the presence of water implies further challenges to a systematic experimental approach. Our theory predicts polyamide miscibility, while accounting for amide content, non-uniformity, and moisture content, using generalizations of Flory-Huggins theory. Predicted miscibilities align with experimental results obtained on tested mixed polyamides. The gained insights guide the optimization of functional polyamide blends.

The development of peroxidase mimic nanocatalysts is relevant for oxidation reactions in biosensing, environmental monitoring and green chemical processes. Several nanomaterials have been proposed as peroxidase mimic, the majority of which consists of noble metals and oxide nanoparticles (NPs). Yet, there is still limited information about how the change in the composition influences their catalytic activity. Here, the peroxidase mimic behaviour of gold NPs is compared to a traditional nanoalloy as Au-Ag and to the Au-Fe and the Au-Co nanoalloys, which were not tested before as oxidation catalysts. Since the alloys of gold with iron and cobalt are thermodynamically unstable, laser ablation in liquid (LAL) is exploited for the synthesis of these NPs. Using LAL, no chemical stabilizers or capping agents are present on the NPs surface, allowing the evaluation of the oxidation behaviour as a function of the alloy composition. The results point to the importance of surface gold atoms in the catalytic process, but also indicate the possibility of obtaining active nanocatalysts with a lower content of Au by alloying it with iron, which is earth-abundant, non-toxic and low cost. Overall, Au nanoalloys are worth consideration as a more sustainable alternative to pure Au nanocatalysts for oxidation reactions.