Jozef Hodyl PhD Thesis - Theses - Flinders University

Percent inclusion values for these two sets of guest molecules are tabulated inTable 6.6. The expected trend with aminobenzoate guest species was partiallyobserved with a lower percent value (22%, pK a 2.38) for p-aminobenzoate than for o-aminobenzoate (43%, pK a 2.14).Table 6.6Percent guest inclusion of aminobenzoate and nitrobenzoate guest species with Si-GPS-[Cd(Trac)](ClO 4 ) 2 , 94, in aqueous solution with a guest concentration of 5 x 10 -3 moldm -3 at pH 7 in 0.01 mol dm -3 HEPES buffer at 298 K.Guest Species % pK aaGuest Species % pK aap-aminobenzoate 22 2.38 p-nitrobenzoate 16 3.44m-aminobenzoate 12 3.07 m-nitrobenzoate 23 3.46o-aminobenzoate 43 2.14 o-nitrobenzoate 12 2.47a Values obtained in H 2 O from PHYSPROP database for their respective conjugate acids. 202These values demonstrate an increase in binding for o-aminobenzoate, 43%,compared with of p-aminobenzoate, 22%, despite the lower basicity of the former.This suggests that a degree of non-classical hydrogen bonding with the host pendantarms is contributing to the higher value for o-aminobenzoate. However, inclusionwith m-aminobenzoate appears to oppose this suggestion with a value of 12%.Considering that m-aminobenzoate has the highest pK a (3.07) of the aminobenzoatesits percent inclusion value is anomalously low.Inclusion with nitrobenzoate guest species conforms with binding as predicted bythe basicity of the guest molecule. The guest with the highest pK a (3.46), m-nitrobenzoate, also shows the highest inclusion value (23%) with p-nitrobenzoatenext with 16% followed by o-nitrobenzoate (12%). These percent inclusion valuesdemonstrate that nitro substituents do not significantly assist in binding of guestspecies. Amino substituents are somewhat helpful, but not as effective as hydroxysubstituents.108