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Natural extracts have been widely used to protect metal materials from corrosion. The efficiency of these extracts as corrosion inhibitors is commonly evaluated through electrochemical tests, which include techniques such as potentiodynamic polarization, electrochemical impedance spectroscopy, and weight loss measurement. The inhibition efficiency of different extract concentrations is a valuable indicator to obtain a clear outlook to choose an extract for a particular purpose. A complementary vision of the effectiveness of green extracts to inhibit the corrosion of metals is obtained by means of surface characterizations; atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy analysis are experimental techniques widely used for this purpose. Moreover, theoretical studies are usually addressed to elucidate the nature of the corrosion inhibitor—metal surface interactions. In addition, calculations have been employed to predict how other organic substances behave on metal surfaces and to provide experimental work with fresh proposals. This work reports a broad overview of the current state of the art research on the study of new extracts as corrosion inhibitors on metal surfaces in corrosive media. Most constituents obtained from plant extracts are adsorbed on the metal, following the Langmuir adsorption model. Electron-rich regions and heteroatoms have been found to be responsible for chemisorption on the metal surface, whereas physisorption is due to the polar regions of the inhibitor molecules. The plant extracts compiled in this work obtained corrosion inhibition efficiencies above 60%, most of them around 80–90%. The effect of concentration, extraction solvent, temperature, and immersion time were studied as well. Additional studies regarding plant extracts as corrosion inhibitors on metals are needed to produce solutions for industrial purposes.

The M@C 36 compounds form a family of small endohedral metallofullerenes. Recently, these have been detected as the smallest endohedral compounds formed with Sc, Y, and La. For the first time, these compounds are studied theoretically. Calculations obtained at the dispersion-corrected DFT level PBE-D3(BJ)/def2-TZVP agree admirably with experimental results. The zero-point energy corrected binding energies can explain the lower abundance of La@C 36 in comparison with Sc@C 36 and Y@C 36 . Their small HOMO-LUMO gaps denote high reactivity. The bond between Y and Sc with the cage is mostly covalent. In contrast, La is located at the fullerene's center with an ionic interaction; all metals transferred charge to the cage. Furthermore, La@C 36 was found in doublet state and the others preferred the quartet state. To conclude, according to the analysis of aromaticity performed by the NICS(0) iso index, the insertion of none of these metals increase the aromaticity. K E Y W O R D SC36 fullerene, density functional theory calculation, dispersion-corrected, electronic structure, endohedral fullerenes | I N T R O D U C T I O NSince the same year of their discovery, [1] fullerenes have stood out for their capacity [2] of trapping other species (atoms, [3][4][5] molecules, [3][4][5] or clusters [6] ) inside of them; hence, a large number of studies have been done. [3][4][5] The internally doped fullerenes are known as endohedral fullerenes (EFs); being the endohedral metallofullerenes (EMFs) the most studied EFs, formed by C 60 or bigger cages, which contain lanthanide atoms as their endohedral species. [3][4][5][6] The most notable difference between hollow fullerenes and EFs is that the latter can violate the isolated pentagon rule (IPR). [3][4][5][6] IPR establishes that a cage without adjacent pentagonal rings will form the most energetically favorable isomer of a hollow fullerene, however, EFs can violate this rule [7] and several non-IPR endohedral fullerenes have been synthesized. [3][4][5][6] Despite the fact that, theoretically, C 20 is the smallest possible fullerene, [8] its strained structure is not energetically favorable and the ring geometry is preferred [9] ; as a result, C 28 is the smallest fullerene detected in mass spectra. [10] Moreover, other small fullerenes (smaller than C 60 ) have been experimentally obtained [11,12] and some have been predicted to be stable. [13] Particularly, numerous experiments have been done on C 36 in gas phase. [11,12,[14][15][16][17] The HOMO-LUMO gap of several small fullerenes were measured by anion photoelectron spectroscopy. [15] The gap of 0.8 eV, measured for C 36 , agrees with that calculated (below than 0.5 eV) by a density-functional-based tightbinding method, [15] due to the large error bar obtained in the experimental value. [15] Similarly, C 32 , C 44 , and C 50 show large gaps and high stabilities. [15] The D 6h and D 2d isomers (non-IPR) of C 36 have the minimal number of adjacent pentagonal rings among the 15 possible isomers. [18] Both using density functional ...

Characterization of the structural and electronic properties of binary iron‐carbon clusters composed by six iron atoms and with up to nine carbon atoms was carried out with density functional theory calculations. Neutral, cations (q = +1), and anions (q = −1), some of them experimentally detected, were studied. The formation of dimers and trimers of carbon atoms over the iron surface were preferred. Moreover, some large carbon chains, with up to five atoms, were determined. High spin states emerged for the ground states, with multiplicities above 16, for all clusters independently of the number of carbon atoms attached to the iron core. All neutral clusters were stable because fragmentation (into carbon chains), dissociation (of a single carbon atom), and detachment of all carbons need high amounts of energy. Reactive species were defined by small HOMO‐LUMO gaps. Charge transfer, to the carbon atoms, increased as the carbon content increased, producing, for some cases, an even‐odd behavior for the magnetic moment of the Fe6Cn particles.

Endohedral metallofullerenes M@C44 containing several different endohedral species have been considered as intermediates in the path to form larger species. Such compounds containing interstitial atoms of groups 3 and 4, recently detected in experiments, are studied theoretically for the first time. Calculations carried out at a dispersion-corrected density functional theory level agree admirably well with experimental data for C44 and its endohedral compounds. The most suitable C44 isomer to form endohedral compounds is the D 2 (89) isomer. The binding energy between the endohedral atom and the cage is a good indicator of the abundance found in synthesis. The properties of the endohedral compounds of C44-D 2 (89) can be compared directly with those of the tri- and tetraanions of empty C44. In addition, the electron-richest regions in all of them are the four triple sequentially fused pentagon units. The centroids of the central pentagons of each such unit are approximately disposed in a seesaw structure around the endohedral atom. This structural feature of C44-D 2 (89) accounts for the preferential bonding in almost all cases of these to the endohedral atom. A detailed study of the metal–cage bonding highlights the partially ionic and covalent character of their interaction. The ionic nature of the metal–cage bonding increases for the heavier endohedral atoms. Endohedral species containing group 3 metals are expected to be more reactive than those containing group 4 metals according to their highest occupied molecular orbital–lowest unoccupied molecular orbital gaps. The cage aromaticity evaluated by the NICS(0)iso indices indicates that this property does not play a crucial role in the stabilization of the endohedral species. The evaluated behavior and properties of intermediate M@C44 species can be useful to extend and understand the encapsulation processes of elements as the size of the cage increases toward larger fullerenes.