Aromatic measurements

The numerous aromaticity measures proposed in recent decades have widened the number of descriptors in the literature considerably [2,8,9,44,45], Aromaticity descriptors can be classified as magnetic, energetic, electronic, and structural [46]. 

Table 1 Collected aromaticity measures for cyclopentadiene and phosphole (34)...

Further reports on the aromaticity investigated in conjunction with new synthesized heterophospholes are also in accord with their aromaticity (Scheme 46). The l-pheny-3,5-di-tert-butyl-l,2,4-azadiphosphole 88 (R t-Bu, R Ph) exhibited equalized bond lengths (PC bond lengths were between 1.697 and 1.746 A). A comparison of the B3LYP/6-311+G computed NICS(O), BI, and BDSHRT values of 88, 89, and 90 (R H, R H) with the corresponding parameters for pyrrole, furan, and thipohene, respectively, has shown a small, but consistent, decrease in all these aromaticity measures of the phosphorus derivatives [264], The reactivity of 89 (R Mes) has been compared to that of 90 (R /-Hu) in a [4 + 2] cycloaddition... 

The aromaticity of the six-membered ring phosphinine is similar to that of benzene (different aromaticity measures all exhibit about 90% of the benzene values), as reviewed previously [4, 278]. These characteristics reflect in the frequently used term phosphabenzene (for a recent review on its chemistry, coordination, and catalytic behavior see [279]). The numerical values for some aromaticity indicators... 

Table 3 Aromaticity measures of benzene and phosphinine (phosphabenzene)...

PM2 5 PM10 PMA PMC PNA Pour point Particulate matter less than 2.5 ptm and 10 jam in diameter. Polymetacrylate viscosity index improver or viscosity modifier. Pensky-Martin closed cup-flash point test. Polynuclear aromatic. Measure of lubricant low-temperature flow which is 3°C above the temperature at which a normally liquid petroleum product maintains fluidity. Oil forms a honeycomb or crystals at low... 

As stated above, a new scale is desirable to describe quantitatively the global aromaticity in the VB model. Since the global aromaticity measures the average benzene character of a conjugated system, resonance energy per hexagon (REPH), as defined by Eq. (48), may be a natural choice for BHs [22],... 

Schleyer demonstrated the utility of NICS as an aromaticity metric by comparing NICS values with the values of ASE for a series of five-member ring systems. The correlation is quite acceptable, suggesting that NICS might be useful for rank-ordering relative aromatic character. An example of NICS used in this way was in the study of a series of dimethyldihydropyrene derivatives. NICS and other aromaticity measures were found to correlate. ... 

A method of checking whether all protons in a sample have Tj (H) s long enough for carbons to cross polarize, is to measure the T. (H) s Independently of carbon and directly through protons. Data of thls sort is presented in Table III. The shortest Tj (H) can be as small as 0.15 ms. Clearly these protons will not be Sble to transfer polarization quantitatively to carbons. Whether these protons are associated with organic as well as inorganic matter has, however, yet to be established. This is important because it is only the organically bound protons which may influence aromaticity measurements. 

One way to check whether the cross polarization method is responsible for introducing errors in aromaticity measurements, is to perform experiments using single pulse excitation in much the same way as solution NMR measurements are made. This method is time... 

Significant relationships between aromaticity measured by diamagnetic susceptibility exaltation and derivatives of the total molecular valence for molecule and HOMO orbital, calculated ab initio in the framework the Density Function Theory, were also found [Balawender, Komorowski et al., 1998]. 

Discrepancy between common local aromaticity measures in a series of carbazole derivatives. Phys. Chem. Chem. Phys., 6, 314—318. 

The application of the electron delocalization indices as an aromaticity measure has been recently amply reviewed by Sola et al. (2010SY1156) and Feixas et al. (2015CSR6434). 

Now, there are some well accepted categories identified as aromaticity measures (Table 1). They include structural (both electronic and geometrical), energetic, magnetic and reactivity criteria. 

Chesnut differences in ring proton shieldings between the fully unsaturated species and its monoene counterpart recommended as aromaticity measure ... 

The Al4 system is both a-aromatic and Ti-aromatic. However, a conflicting nature has been exhibited by the rectangular which is simultaneously TT-antiaromatic and a-aromatic The LisAljcluster, containing the Al4 unit, has been synthesized by laser vaporization and analyzed by photoelectron spectroscopy and ab initio calculations Conflicting results have been reported through different aromaticity measures, e.g., the and magnetic field induced current density ... 

Although known for almost a couple of centuries there exists no direct measure of aromaticity mainly because it is neither an experimental observable nor it is a theoretically defined quantity. In this chapter various types of aromatic systems with a special emphasis to all-metal aromatic clusters have been discussed. Different indirect aromaticity measures have been analyzed with special emphasis to some chemical reactivity based aromaticity indices within a conceptual density functional theory... 

Computational DFT Performance of DFT Methods in the Calculation of Aromaticity Measures... 

In this section, it is shown that some properties and quantities arising from the noncomputational or conceptual side of density functional theory can be proposed as aromaticity measures. 

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