Natural atomic charge

In general, the -dependent variations of natural atomic charges in dative bonds are significantly larger than those in covalent bonds. Indeed, the Q (R) variations in dative bonds resemble those in ionic bonds (cf. Fig. 2.9), to which they are evidently related by similarities in donor-acceptor character. The strong AQ /AR dependence tends to be associated with enhanced infrared vibrational intensity and other spectroscopic signatures characteristic of ionic bonding. 

The exaggerated ionic picture (3.189) is doubtless unrealistic, but it is interesting to note that the calculated natural atomic charges in PF5,... 

HB-5) unusually high cationic character of central atom B and anionic character of terminal atoms A and C, as reflected in natural atomic charges Qa, Qb, Qc), ionic bond orders (6AB(ion) and Aiic<1011)), and electrovalencies (TA(lon), IV1011 . Vc(l0n)). 

Table 3.29. The NBO descriptors of XYZ triatomic anions (see Table 3.28), showing natural atomic charges (Q), percentage polarization of oxy and ayz NBOs toward terminal atoms, second-order nz— oxy and nx->OYZ stabilizations (A/s(2)), and occupancies of valence antibonds (oxy and ayz ) and extra-valent Rydberg orbitals (ry ) parenthesized values refer to Lewis structure of lower accuracy...

Table 3.35. Natural atomic charges (Q), d-orbital occupancies (d f), and NRT bond orders and valencies (with percentage ionic character) for ArF species...

Let us now consider some general aspects of the atomic charge distributions. For the H atoms, the calculated natural atomic charges Qw are found to depend most strongly on whether the atom appears at a bridging H(p), terminal H(t), or BH2 extra H(x) position. Typical Ou values fall within the disjoint ranges... 

Table 4.26. Geometrical (Rxy) and NBO descriptors ofXYn ligands (NH3, PH3, and CO), illustrating natural atomic charges (Qy) and NBO occupancy (Occ ), polarity (%pol = lOOcx2), and orbital energy (e ) of Unh, apn or rtco acceptor orbitals in the free ligands...

Figure 5.56 illustrates the variation of NRT bond orders bef for reactant and product C—F bonds along the reaction coordinate, and Fig. 5.57 illustrates the corresponding variations of natural atomic charge on the two F atoms. Despite the numerical scatter, one can see in Fig. 5.56 that the reactant (6a0 and product (6cp) bond orders respectively diminish and increase while preserving approximately constant total bond order in the shifting toF Cp hyperbond,... 

Another important feature of the extracoordinate silicon compounds (Scheme 7.14) is the increase in natural atomic charge at the central atom compared to the tetracoordinate precursors [69]. The counter-intuitive increase in the positive charge on silicon, which becomes even more substantial in the case of anionic nucleophiles, such as F , is compensated by a more negative character of the surrounding groups (X), and this results in an enhanced ionic nature of the Si-X bond. This polarization then favors intermolecular charge-dipole interaction, which results in an increased Lewis acidity of the hypercoordinate silicon [70]. 

Pyridine is the prototypical electron-poor six-membered ring heterocycle. The aromaticity originally found in the benzene framework is maintained in pyridine via overlap with the unhybridized p orbital found on the sp hybridized nitrogen atom that is parallel to the Ji-system of the carbon framework. The resonance pictures, as well as, the natural atomic charges of pyridine (Fig. 4.2), predict its electron deficient nature. 

Table 1 Net natural atomic charges on some reactive atoms of the species involved in the H2 activation step...

Figure 2.2 Computed energy profile (kcalmoP ) for the addition of 2 to 34 with potential energy curves for variation of the Ru-O-O angle, 0, in 35a, 35b and 35 (L = " liPr). Values for the Ru-O-O angle and the 0-0 distances (A, degrees, plain text) and natural atomic charges at Ru and O (italics) also shown. Reproduced, with permission, from Burling et al. ...

FIGURE 29.5 Natural atomic charges for [CBnHi2] anion. In this and subsequent figures, an unsubstituted corner of the polyhedron without symbol or letter represents a BH unit, a substituted corner without symbol or letter, a B atom. (Adapted from I. Zharov et al., J. Am. Chem. Soc. 2004, 126, 12033.)... 

Figure 8.5 Natural atomic charge (0 variations with torsional angle (p in aminoborane, showing zwitterion-like pattern of charges on B (gg, circles, left scale) and N (0i, crosses, right scale) as the Tig-N coordinate bond is broken by 90° twisting.

Figure 10.8 Natural atomic charges ( h) of proximal H (squares) and distal H (triangles) along the IRC of reaction in (10.8) and (10.9), showing the increasingly cationic (H) versus anionic (H ) character resulting from heterolytic push-pull dissociation interactions (cf. Fig. 10.7).

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