2Central South University, College of Chemistry
and Chemical Engineering, Lushan South Road
Changsha, Hunan, CN 410083
email:
steven.kirk@cantab.net
email:
samanthajsuman@gmail.com
We investigate the presence of helical character and chirality using a
vector-based charge density perspective instead of energetic or
structural measures. The vector-based perspective of the chemical
bonding, constructed using the most preferred direction of charge
density accumulation, finds the presence of induced symmetry-breaking
for α,ω -disubstituted [4]cumulenes as the end groups are
torsioned. The stress tensor trajectories Tσ(s )
are used to provide the additional symmetry-breaking required to
quantify the degree and nature of the chirality and helical character.
We find an absence of chirality for [4]cumulene but a very
significant degree of axiality as demonstrated by the purely axial form
of the Tσ(s ) indicating a lack of helical
character. The S-1,5-dimethyl-[4]cumulene contains a very low degree
of chiral character but significant axiality(helicity) resulting in a
weakly helical morphology of the corresponding
Tσ(s ). The (-)S(-), (+)S(-) and (+)S(+)
conformations of S-1,5-diamino-[4]cumulene contain very significant
degrees of both chirality and helical character resulting in helical
morphology of the corresponding Tσ(s ). The
chirality assignments are in agreement with the Cahn–Ingold–Prelog
(CIP) classifications for the (-)S(-), (+)S(-) and (+)S(+) conformations
of S-1,5-diamino-[4]cumulene. We discuss the consequences for the
Tσ(s ) in locating chiral character in these
molecules in future experiment investigations.