Transmission of electronic substituent effects along polyenic chains: a quantum chemical study based on structural variation and π-charge distribution

Abstract

The transmission of electronic substituent effects along chains of conjugated double bonds has been investigated by analyzing the small structural changes induced by a variable substituent X in the phenyl group of Ph–(CH=CH) _n –X molecules (n = 2, 3, and 4). The structures of many such molecules with charged or dipolar substituents have been determined from quantum chemical calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed S ^PEn_F (n = 2, 3, and 4). Multiple regression analysis of the S ^PEn_F parameters using appropriate explanatory variables reveals a composite electronic effect, the main component of which is the field effect of the variable substituent, enhanced by field-induced π-polarization of the polyenic chain. Also important is the role of resonance-induced field effects. An electronegativity term contributes significantly to the structural variation of the phenyl probe in (E)-β-substituted styrenes, Ph–CH=CH–X, but is marginally significant in Ph–(CH=CH)₂–X molecules and not significant at all in Ph–(CH=CH)₃–X and Ph–(CH=CH)₄–X molecules. The structural substituent parameters S ^PE2_F , S ^PE3_F , S ^PE4_F , as well as S ^STY_F from (E)-β-substituted styrenes, are all correlated to each other. However, even though the correlation coefficients are high, it appears unequivocally that the data points corresponding to dipolar substituents and those corresponding to charged groups are aligned along slightly different straight lines. An analysis of π-charge distribution in Ph–(CH=CH) _n –X molecules (n = 1–4) has also been carried out. It appears that as the number of double bonds increases, the π-charge transmitted from the variable substituent to the hydrocarbon frame becomes larger, while the π-charge transmitted to the phenyl probe becomes smaller. In each of the three series of Ph–(CH=CH) _n –X molecules (n = 2, 3, and 4), the π-charge of the phenyl probe is linked by an excellent nonlinear relationship to the corresponding structural substituent parameter S ^PEn_F (n = 2, 3, and 4). The effect of the variable substituent on the geometry of the polyenic chain has been studied by analyzing the alternation of C–C bond lengths along the chain in Ph–(CH=CH)₄–X molecules. The alternation is most pronounced and regular when the variable substituent X is an uncharged group, irrespective of whether it is a π-acceptor or a π-donor. For the five strongest resonant substituents in our data set (namely, the charged groups CH₂ ⁺, CH₂ ⁻(c), NH⁻, O⁻, and N₂ ⁺), there is a region in the chain where the alternation between adjacent C–C bonds decreases and inverts, a structural feature known as geometric soliton.