Electric Multipole Moments and (Hyper)Polarizability of X–C≡C–X, X = F, Cl, Br and I

Abstract: We have calculated self-consistent field (SCF) and second-order Møller-Plesset perturbation theory (MP2) for the dihaloethynes X–C≡C–X, X = F, Cl, Br and I. All calculations have been performed with carefully optimized, flexible basis sets of gaussiantype functions. Our best values for the quadrupole moment (Θ/ea₀²) are -0.6524 (FCCF), 3.6612 (ClCCCl), 5.8143 (BrCCBr) and 8.3774 (ICCI). The dipole polarizability is strongly anisotropic. For the mean (α /e²a0²Eh^-1) and the anisotropy (Δα/e²a0²Eh^-1) we obtain 23.58 and 15.09 (FCCF), 51.75 and 48.30 (ClCCCl), 66.53 and 60.04 (BrCCBr), 93.79 and 78.91 (ICCI). The mean dipole hyperpolarizability (γ /e⁴a0⁴Eh^-3) increases rapidly as 2932 (FCCF), 9924 (ClCCCl), 17409 (BrCCBr) and 35193 (ICCI). The transversal component of the hyperpolarizability is larger than the longitudinal one for FCCF, γxxxx > γzzzz but this is reversed for the other molecules in the series. Difluoroethyne is less (hyper)polarizable than ethyne.