ELECTRONEGATIVITY EQUALIZATION IN POLYYNE CARBON CHAINS

Electronegativity equalization in polyyne carbon chains with the compo sition X-(-C=C-)(N)-Y, where X, Y = H, F, or Na, and 1 less than or eq ual to N less than or equal to 5, has been studied at the MP2/6-31G* level of theory. In contrast to charges of individual atoms, charges o f the X-C=C-, -C=C-, and -C=C-Y chain fragments are found to be highly additive. The observed additivity of fragment charges stems from the underlying additivity of the corresponding electronegativity vectors a nd softness matrices. Although the latter quantities are poorly transf erable among polyyne carbon chains, they satisfy additive combination rules to a high degree of accuracy. Approximate models that involve ei ther one-electron effective Hamiltonians or screened electrostatic int eractions fail to reproduce the rigorously computed atomic softness ma trices, indicating the complex nature of phenomena that control the el ectron flow in molecules.