Chemical Applications

[Griselda Hamiter]

Wespecialize in manufacturing production, midstream and frac chemicals. We deliver the best treatment solutions and service for our clients in the oil and gas industry. Our Mission is to create and continue long lasting partnerships with our customers by providing unmatched service - not to be just another chemical company.

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Surface-enhanced Raman scattering (SERS) has become a mature vibrational spectroscopic technique during the last decades and the number of applications in the chemical, material, and in particular life sciences is rapidly increasing. This Review explains the basic theory of SERS in a brief tutorial and-based on original results from recent research-summarizes fundamental aspects necessary for understanding SERS and provides examples for the preparation of plasmonic nanostructures for SERS. Chemical applications of SERS are the centerpiece of this Review. They cover a broad range of topics such as catalysis and spectroelectrochemistry, single-molecule detection, and (bio)analytical chemistry.

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Nevertheless, semilocal functionals cannot include long-range correlation effects such as London dispersion interactions.20 To truly judge its applicability, we extensively tested r2SCAN combined with the state-of-the-art D4 dispersion correction,21 which shows unprecedented performance for a range of diverse chemical and physical properties. To investigate the development of the SCAN-type functionals, we include both SCAN-D4 and rSCAN-D4 in the comparison to r2SCAN-D4 and can attribute improvements in non-covalent interactions (NCIs) mainly to the regularization and improvements in thermochemistry and barrier heights to the restoration of the exact constraints.

We give a concise methodological overview (Sec. II) on r2SCAN and D4 before testing the full method against established DFAs over a wide range of benchmarks (Sec. III), with particular focus on molecular geometries, thermochemistry, kinetics, and non-covalent interactions in small and large complexes.

The SCAN functional is constructed as an interpolation between single orbital and slowly varying energy densities designed to maximize exact constraint satisfaction.7 The interpolation is controlled by an iso-orbital indicator

The periodic electronic structure calculations were conducted with vasp 6.157,58 with projector-augmented plane waves (PAWs) with an energy cutoff of 800 or 1000 eV (hard PAWs59,60). Tight self-consistent field settings and large integration (and fine FFT) grids are used. The Brillouin zone sampling has been increased to converge the interaction energy to 0.1 kcal/mol. The non-periodic directions use a vacuum spacing of 12 .

We include r2SCAN-D4, PBE0-D4,66 TPSS-D4,67 and PBE-D468 in the comparison shown in Fig. 2. For organic molecules, we find exceptional performance for all functionals, with errors smaller than 1 pm in the bond distances and half a degree in the bond angles. While all methods reproduce the reference values closely, we observe the best agreement from r2SCAN-D4 with a mean absolute deviation (MAD) of 0.4 pm and 0.3 for the bond distances and angles, respectively. For light main group elements, all methods give a mean absolute deviation of less than 1 pm as well, which was also observed in previous studies.21,25 In comparison with the other methods tested here, r2SCAN-D4 also yields the lowest MAD of only 0.7 pm. Finally, for transition metal complexes, r2SCAN-D4 performs reasonably well with an MAD of 1.9 pm. Overall, the performance of r2SCAN is similar to, and sometimes even better than, the hybrid PBE0-D4, which in turn is one of the best performing hybrid functionals for molecular geometries.61

Weighted mean absolute deviations of r2SCAN-D4 compared to other DFAs for the large database of general main group thermochemistry, kinetics, and non-covalent interactions GMTKN55.69 On the left-hand side panel, r2SCAN-D4 is compared against functionals representative of their respective rungs. On the right-hand side panel, r2SCAN-D4 is compared to other members of the SCAN family, namely, rSCAN-D4 and SCAN-D4.

Comparison of a few non-empirical and empirical dispersion corrected DFAs for the MB16-43 subset (artificial molecules) of GMTKN55. The non-empirical DFAs yield generally lower MADs (in kcal/mol) indicating better transferability across diverse systems.

To stress the importance of including a dispersion correction, we test the plain dispersion-uncorrected r2SCAN, which yields a significantly worse WTMAD2 of 8.8 kcal/mol, a difference similar in magnitude to the improvement from SCAN-D4 to r2SCAN-D4. In summary, r2SCAN-D4 shows a systematic improvement over its predecessor SCAN-D4 in all categories of GMTKN55 and can preserve improvements present in rSCAN-D4. This makes r2SCAN-D4 one of the best non-empirical meta-GGAs that have been broadly benchmarked so far.

With the improved description of non-covalent interactions (NCIs), while retaining the computational efficiency of a meta-GGA, r2SCAN-D4 is a promising choice for interaction and association energies of large complexes. The results for the S30L,35 L7,74 and X40 1075 benchmark set are shown in Fig. 4.

We choose the recently revised L7 benchmark74 set to assess the performance of r2SCAN-D4 against local natural orbital based coupled cluster including singles, doubles, and perturbative triple excitations in the complete basis set limit [LNO-CCSD(T)/CBS].76 Close agreement with an MAD of 0.9 kcal/mol is reached for r2SCAN-D4. This is a significant improvement over other meta-GGAs such as SCAN-D4 and TPSS-D4 with MADs of 1.3 kcal/mol and 1.4 kcal/mol, respectively.

We also investigated the description of association energies for large supramolecular complexes using the S30L benchmark set.35 SCAN-D4 proved to be one of most accurate meta-GGAs in the previous benchmarks,21 giving a remarkable MAD of 2.0 kcal/mol, close to the uncertainty of the provided reference interactions; r2SCAN-D4 further improves upon this.

In particular, the association energies of the halogen-bonded complexes (15 and 16) are improved with r2SCAN-D4. The same trend can be observed in the HAL59 benchmark set of the GMTKN55, which shows an MAD of 1.0 kcal/mol with SCAN-D4 and improves with r2SCAN-D4 to an MAD of 0.8 kcal/mol. To confirm this trend, we additionally evaluated the X40 10 benchmark75 containing 40 halogen bond dissociation curves with SCAN-D4 and r2SCAN-D4. Again, r2SCAN-D4 gives the lowest MAD of 0.36 kcal/mol, showcasing on overall improved description of this kind of NCIs.

We have presented an accurate and robust combination of the non-empirical r2SCAN DFA with the state-of-the-art D4 dispersion correction. The resulting r2SCAN-D4 electronic structure method shows exceptional performance across several diverse categories of chemical problems assessed by thousands of high-level data points in a number of comprehensive benchmark sets. Included in the assessment were molecular thermochemistry for both main group and transition metal compounds, barrier heights, structure optimizations, lattice energies of molecular crystals, and both inter- and intramolecular non-covalent interactions of small to large systems, creating an extensive coverage of chemically relevant problems.

We found r2SCAN-D4 to be an accurate and (more importantly) consistent DFA for a large variety of problems and chemical systems. The already good performance of the original SCAN functional is kept and systematically improved in r2SCAN, while the numeric stability is almost on par with the established GGA functionals. We were able to gain some insight into the improvement from SCAN over rSCAN to r2SCAN, where we can attribute the improved description of non-covalent interactions to the regularization in the step from SCAN to rSCAN and the improved thermochemistry and barrier heights to the constraint restoration in the step from rSCAN to r2SCAN. Like SCAN, r2SCAN is not fitted to molecules, so its accuracy in extensive molecular tests demonstrates the predictive power of its exact constraints and appropriate norms.

Anthocyanins are a group of phenolic compounds widely found in nature, occurring in all tissues of higher plants. Currently, there are over 600 identified anthocyanins, and their activity is related to the protection of plants against insect attacks and to the animals attraction for pollination and seed dispersal. Red fruits such as blueberries and cranberries are among the main sources of anthocyanins and can supply large quantities of this compound in a single meal.

Several studies have shown the beneficial effects of anthocyanins on health due to its high antioxidant action through neutralizing free radicals by the donation of hydrogen atoms. These beneficial effects include, among others, the anti-carcinogenic and anti-inflammatory activities, the protective effect against degenerative and chronic diseases, the risk reduction of cardiovascular diseases, and vision improvement. In addition to discussing the health benefits of anthocyanins, it also discusses different food sources for anthocyanins and the chemical applications. (Imprint: Nova)

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