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Org. Biomol. Chem., 2005, 3, 1481 - 1488, DOI: 10.1039/b415594g
Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4
-oxyazobenzene: UV-visible, 1H NMR and ab initio studiesBruce L. May, Jacobus Gerber, Philip Clements, Mark A. Buntine, David R. B. Brittain, Stephen F. Lincoln and Christopher J. Easton
-Cyclodextrin, 
-cyclodextrin, N-(6A-deoxy--cyclodextrin-6A-yl)-N
-(6A-deoxy--cyclodextrin-6A-yl)urea and N,N-bis(6A-deoxy--cyclodextrin-6A-yl)urea (CD, CD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4-oxyazobenzene, E- 3–. In aqueous solution at pH 10.0, 298.2 K and I= 0.10 mol dm–3(NaClO4) spectrophotometric UV-visible studies yield the sequential formation constants: K11=(2.83 ± 0.28)× 105 dm3 mol–1 for CD·E- 3–, K21=(6.93 ± 0.06)× 103 dm3 mol–1 for (CD)2·E- 3–, K11=(1.24 ± 0.12)× 105 dm3 mol–1 for CD·E- 3–, K21=(1.22 ± 0.06)× 104 dm3 mol–1 for (CD)2·E- 3–, K11=(3.08 ± 0.03)× 105 dm3 mol–1 for 1·E- 3–, K11=(8.05 ± 0.63)× 104 dm3 mol–1 for 2·E- 3– and K12=(2.42 ± 0.53)× 104 dm3 mol–1 for 2·(E- 3–)2. 1H ROESY NMR studies show that complexation of E- 3– in the annuli of CD, CD, 1 and 2 occurs. A variable-temperature 1H NMR study yields k(298 K)= 6.7 ± 0.5 and 5.7 ± 0.5 s–1, 
H
= 61.7 ± 2.7 and 88.1 ± 4.2 kJ mol–1 and S=–22.2 ± 8.7 and 65 ± 13 J K–1 mol–1 for the interconversion of the dominant includomers (complexes with different orientations of CD) of CD·E- 3– and (CD)2·E- 3–, respectively. The existence of E- 3– as the sole isomer was investigated through an ab initio study.
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