Figure 3. Median values of a) NBO Total charges, b) NBO Lewis charges andc) NBO non Lewis charges for carbon atoms according to their backbone length, comparing helical and zigzag geometries.
NPA calculated total charges are the opposite of expected values based on atom electronegativities: C atoms are negatively and Cl atoms are positively charged, respectively. This effect has already been observed by Neumann,[15] and could be an indication of unexpected high C electronegativity. Furthermore, NPA non Lewis charges indicate that C atoms receive more electron density in the zigzag geometries, while Lewis charges indicate that C atoms are also the ones that experience higher electronic density withdrawals. These results confirm that there is a backdonation in perchloroalkanes, in a manner that NL mechanisms promote Cl to C electron donation through hyperconjugation, but Lewis mechanisms promote C to Cl electron donation according to electronegativity difference. Both mechanisms are more intense in zigzag geometries.
The NPA total charge graph in Figure 3 is complemented by Figure S3, where individual charges are plotted according to the atomic sequence. The central part of both conformers is well-behaved and indicate that C atoms are negatively charged and Cl atoms positively charged in the zigzag geometry. Therefore, the balance between NL and Lewis charges indicate that the NL term governs the total charge, both in positive/negative sign as in trend. The NL donation is coherent with the argument of Cl to C electron donation caused by intense steric repulsions.
2.4 Hyperconjugative Interactions. There are two important hyperconjugative interactions that provide donation from Cl lone pairs to C atoms: n Cl → σ*CC andn Cl → σ*CCl. These hyperconjugative interactions increase according to the backbone length, always favouring zigzag geometries and sum up to 72 kcal mol-1 for perchloroicosane (Tables S8 and S9). This result reinforces and complement NL charges results: NL charges indicate more charge donation from Cl to C in zigzag geometries and NBO analysis confirm that this occurs via hyperconjugation.
Also, there is geometric and electronic data supporting the NBO results, showing that there is high electron donation from Cl to C atoms in the zigzag geometries; and there are steric interactions that would favour the helical geometry in case of higher electron density in Cl atoms. However, as the Natural Steric Analysis indicates that zigzag geometries have lower steric effects overall, the QTAIM method was applied in order to evaluate steric effects from a different perspective.
2.5 Quantum Theory of Atoms in Molecules. QTAIM results are the opposite from NBO, exhibiting positively charged C atoms and negatively charged Cl atoms (Figure 4a). However, the difference between QTAIM and NBO in calculated charges are expected[37], [38] and can be useful. In this case, QTAIM charges agree with the calculated NBO Lewis charges, which is an indication that QTAIM calculation methodology is not affected by Cl to C backdonation represented by NBO non Lewis charges.