Volume, percent buried

In the following years, the percent buried volume, % has emerged as a standard and intuitive parameter able to describe reasonably well the steric properties of NHCs [75]. The is the fraction of the volume of the fust coordination sphere around the metal occupied by a given ligand, see Fig. 1.17 [75]. 

Figure 76 Schematic depiction of percent buried volume f%...

Figure 3 Percent buried volume (%VgJ, steric parameter of some common NHC ligands computed from the DFT-optimized geometries of [(NHC)lr(CO)fl] complexes in which d=2. iO A and R = 3.5 For comparison purposes, the values of two...

To address this difficulty, Nolan, Cavallo, and co-workers introduced the percent buried volume a steric parameter specifically tailored to the steric topology of... 

Table 1 Comparison of the percent buried volume (%V J values in [Pd(NHC)(cin)CI] complexes...

FIGURE 14.1 (a) Cone angle and (b) Percent Buried Volume. In both cases, the M—P distance is defined as 228 pm. 

This reference also provides percent buried volumes for a variety of N-heterocyclic carbenes. 28. 

The initial intuition that steric properties of NHCs could be modulated and could impact catalytic behavior stimulated the development of steric descriptors to quantify the steric requirement of different NHCs and, possibly, to compare them with tertiary phosphines. For obvious reasons, the Tolman cone angle [23] and related descriptors [24-36], largely used to measure the steric property of phosphines, could not be used for NHCs since they are not cone shaped. After some initial attempts such as the fence model [37], work in the field mostly converged on the percent buried volume, %14mr> 21s a standard and intuitive descriptor (in line with the main philosophy used by Tolman to develop the cone angle) able to describe reasonably well the steric bulkiness of NHCs [9,22,38-40]. 

Cavallo and coworkers have shown that the stability of a singlet carbene (with respect to dimerization) can be ascertained via calculation of two key properties of a carbene [23]. The steric bulk is quantified using percent of buried volume, %Vbur (shown later) [24], while the electronic nature is quantified using the singlet-triplet energy gap (Es-x)- It was shown that a linear correlation = 0.93 or... 

Figure 1.17 Percent of buried volume for characterizing the steric properties of NHC ligands [24].

While the Al angle and the percent of buried volume do not correlate with the P-( )/a ratio, the Ah angle proves to be nicely related to the regioselectivity of the process (Figure 5.17) [32]. To the best of our knowledge, it is the first time that this structural descriptor is correlated successfully to an observed reactivity... 

The Voronoi volume calculation was also carried out for protein atoms at protein-DNA interfaces in 28 high-resolution structures of protein-DNA complexes by Nadassy et al. (1999). The coordinate files also contain water positions, and volume calculations were performed in the presence of these water molecules. Fifty-seven percent of the interface protein atoms could be included and the V/Vq ratios were found to spread over a narrow range of 0.97-1.04, with a mean of 1.01 (Fig. 5, bottom). Thus, the atomic packing is very similar to that found for protein-protein interfaces. Omitting water molecules from the calculations yielded a broader range (0.94-1.1) and many fewer buried interface atoms. Thus, water molecules once more play a key role in fostering efficient packing at protein-nucleic acid interfaces. 

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