A Hirshfeld-I interpretation of the charge distribution, dipole and quadrupole moments of the halogenated acetylenes FCCH, ClCCH, BrCCH, and ICCH.

We report the dipole and quadrupole moments of the halogenated acetylenes calculated using large basis sets and the SCF, DFT(B3LYP), and CCSD methods, and we analyze the charge density using the Hirshfeld and Hirshfeld-I techniques. The atomic charges, dipoles, and quadrupoles resulting from the Hirshfeld-I analysis are used to interpret the unusually small molecular dipole moments in the sequence as well as the molecular quadrupole moments. The very small dipoles obtain for two reasons. First, the dipole moment associated with the σ and π electron densities is comparable in magnitude and opposite in direction. Second, the charge and induced dipole contributions for ClCCH, BrCCH, and ICCH have opposite signs further reducing the molecular dipoles. The molecular quadrupole moments are the sum of a charge, atomic dipole, and in situ quadrupole terms, and are dominated by the atomic dipoles and in situ quadrupoles with the charge contributions playing an unexpectedly minor role.