CONFORMATIONAL INVESTIGATION OF N-ARALKYLPYRIDINIUM IONS BY COTTON MOUTON EFFECT METHOD

Molar Cotton Mouton constants and refractions are reported at 98 K for the following N-aralkylpyridinium cations as solutes in water or acetonitrile: (C6H5)CH2(+NC5H5), (4-ClC6H4)CH2(+NC5H5), (3-ClC6H4)CH2(+NC5H5), (3-ClC6H4)CH2(2'-Me+NC5H4), (4-NO2C6H4)CH-(+NC5H5), (2,5-Me2C6H3)CH2(+NC5H5), (2,5-Me2C6H3)CH2(2'-Me+NC5H4), (2,4,6-Me3C6H2)CH2(+NC5H5), (2,3,5,6-Me4C6H)CH2(+NC5H5), (2,3,5,6-Me4C6H)CH2(2'-Me+NC5H4),(2,3,4,5,6-Me5C6)CH2(+NC5H5) and (2.3.4.5,6-Me5C6)CH2-(2'-Me+NC5H4). The data are analysed in terms of the preferred conformations adopted by such cations in solution. In most cases these data are consistent with the presence, in solution. of helical or near-helical conformations, where both aryl and pyridinium rings are twisted in the same direction with respect to the C(Ar)-CH2-N(Ar) reference plane. In contrast, tri-ortho-substituted diaryl cations adopt an asymmetric form in which the ortho-substituted aromatic ring lies almost perpendicular to the reference plane, and the mono-ortho-pyridinium ring forms an acute angle with the reference plane.