Characterization of a Carbon−Nitrogen Network Solid with NMR and High Field EPR

The preparation and properties of a g-C3N4/AgBr nanocomposite photocatalyst based on protonation pretreatment

Protonation pretreatment promotes the formation of a uniform g-C₃N₄/AgBr nanocomposite, thereby efficiently enhancing the photocatalytic activity through the Z-scheme charge transfer.

Ab Initio Calculation of Solid-State NMR Spectra for Different Triazine and Heptazine Based Structure Proposals of g-C3N4

We present a comprehensive theoretical study of the structure and NMR parameters of a large number of triazine and heptazine based structure proposals for g-C3N4 in different condensation states. This approach includes a detailed investigation of cyclic melon which tends toward the formation of densely packed hydrogen bonded meshes. In all of the investigated systems, we found planar layers to represent saddlepoints on the energy surface, whereas corrugated species were identified as minima. The corrugation source was linked to the repulsion of nitrogen lone pairs in close NN contacts. A linear dependency of the corrugation energy from the number of NN interactions in the investigated clusters was found. Heptazine based systems gain about twice as much energy per NN close contact in comparison to triazine structures which could be understood in terms of the distortion mechanism in the investigated structures. Furthermore, a full study of the 15N and 13C chemical shift tensors was performed for the different C/N layers. The description of the NMR parameters required dividing the investigated systems into subclusters for which the NMR tensors were calculated with density functional theory (DFT) methods. A statistical analysis of these entities allowed for the investigation of the change in the chemical shift upon corrugation and, in the case of the cyclic melon system, hydrogen bonding. With the here presented study, the most prominent structure models for g-C3N4 are characterized in terms of the 15N and 13C NMR parameters which now can directly be compared to experimental spectra.

The role of carbon in the nitridic high performance ceramics in the system Si-B-N-C

Using a multitude of modern solid-state NMR techniques including 11B-MQMAS-NMR, 11B-13C-REDOR NMR, 13C-11B-REAPDOR NMR, 15N-11B-REAPDOR NMR and 13C-2D-RFDR NMR experiments, the network organization in the quaternary high performance ceramic SiBN3C has been studied. Carbon is found to agglomerate into domains with predominant carbon-carbon bonding. The NMR results are compatible with carbon atoms involved in a graphite-like bonding scheme. The network adopted is quite similar to the corresponding one in the related ternary ceramic Si3B3N7, consisting of domains characterized by predominant Si-N bonding and domains of predominant B-N bonding with the carbon agglomerates located within the B-N domains of the network. The homogeneity with respect to the elemental distribution down to the nm regime, observed using electron spectroscopic imaging techniques puts an upper limit of ca. 1 nm to these agglomerates.

Improved narrowband dipolar recoupling for homonuclear distance measurements in rotating solids

Recovery of the magnetic dipolar interaction between nuclei bearing the same gyromagnetic ratio in rotating solids can be promoted by synchronous rf irradiation. Determination of the dipolar interaction strength can serve as a tool for structural elucidation in polycrystalline powders. Spinning frequency dependent narrow-band (nb) RFDR and SEDRA experiments are utilized as simple techniques for the determination of dipolar interactions between the nuclei in coupled homonuclear spin pairs. The magnetization exchange and coherence dephasing due to a fixed number of rotor-synchronously applied pi-pulses is monitored at spinning frequencies in the vicinity of the rotational resonance (R(2)) conditions. The powder nbRFDR and nbSEDRA decay curves of spin magnetizations and coherences, respectively, as a function of the spinning frequency can be measured and analyzed using simple rate equations providing a quantitative measure of the dipolar coupling. The effects of the phenomenological relaxation parameters in these rate equations are discussed and an improved methodology is suggested for analyzing nbRFDR data for small dipolar couplings. The distance between the labeled nuclei in the 1,3-(13)C(2)-hydroxybutyric acid molecule is rederived using existing nbRFDR results and the new simulation procedure. A nbSEDRA experiment has been performed successfully on a powder sample of singly labeled 1-(13)C-L-leucine measuring the dipolar interaction between the labeled carboxyl carbon and the natural abundant beta-carbon. Both narrowband techniques are employed for the determination of the nuclear distances between the side-chain carbons of leucine and its carbonyl carbon in a tripeptide Leu-Gly-Phe that is singly (13)C-labeled at the leucine carbonyl carbon position.