Theoretical modeling of molecular spectra parameters of disubstituted diacetylenes

Symmetrically disubstituted diacetylenes, X-C≡C-C≡C-X, were studied computationally by using the DFT B3LYP/aug-cc-pVDZ method. For more than 35 substituents the bond lengths, charge density and Laplacian in bond critical points, C≡C stretching vibrational frequencies, (13)C NMR chemical shifts and spin-spin CC coupling constants through diacetylene moiety were calculated and examined by using the substituent sEDA and pEDA descriptors. It is demonstrated that in disubstituted diacetylenes the triple bond length increases with the electron donating and decreases with the electron withdrawing properties of the substituents. The σ-electron repulsion is likely to be responsible for this phenomenon. The electron density of the C-X bond critical point decreases linearly with an increase of the σ-electron donating properties, whereas the Laplacian of electron density in the C≡C bond critical point increases as the sEDA descriptor i.e., σ-electron donating properties of the substituent, are increased. Thus, ρ(C≡C) is locally reduced with an increase of sEDA. The ν(as)(C≡C) and ν(s)(C≡C) mode frequencies decrease with the electron donating and increase with the electron withdrawing properties of the substituents. The calculated chemical shift of the C1-atom, to which the substituent is attached, does not correlate with the substituent descriptors, whereas the δ(C2) deshielding increases when the σ-electron donating properties and C≡C distance are increased. The calculated (1)J(CC) coupling constants decrease with an increase of the triple bond length, the sEDA parameter, and ρ(C-X), whereas they decrease with Laplacian in the C≡C BCP.