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.