title: Guest inclusion in cucurbiturils studied by ESR and DFT: the case of nitroxide radicals and spin adducts of DMPO and MNP.
abstract: We present an ESR and DFT study of the interaction of cucurbiturils CB[6], CB[7], and CB[8] with di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) and with spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2-methyl-2-nitrosopropane (MNP). The primary goal was to understand the structural parameters that determine the inclusion mechanism in the CBs using DTBN, a nitroxide with great sensitivity to the local environment. In addition, we focused on the interactions with CBs of the spin adducts DMPO/OH and MNP/CH(2)COOH generated in aqueous CH(3)COOH. A range of interactions between DTBN and CBs was identified for pH 3.2, 7, and 10. No complexation of DTBN with CB[6] was deduced in this pH range. The interaction between DTBN and CB[7] is evident at all pH values: "in" and "out" nitroxides, with (14)N hyperfine splitting, a(N), values of 15.5 and 17.1 G, respectively, were detected by ESR. Interaction of DTBN with CB[8] was also detected for all pH values, and the only species had a(N) = 16.4 G, a result that can be rationalized by an "in" nitroxide in a less hydrophobic environment compared to CB[7]. Computational studies indicated that the DTBN complex with CB[7] is thermodynamically favored compared to that in CB[8]; the orientations of the NO group are parallel to the CB[7] plane and perpendicular to the CB[8] plane (pointing toward the annulus). Addition of sodium ions led to the ESR detection of a three-component complex between CB[7], DTBN, and the cations; the ternary complex was not detected for CB[8]. The DMPO/OH spin adduct was stabilized in the presence of CB[7], but the effect on a(N) was negligible, indicating that the N-O group is located outside the CB cavity. Computational studies indicated more favorable energetics of complexation for DMPO/OH in CB[7] compared to DTBN. An increase of a(N) was detected in the presence of CB[7] for the MNP/CH(2)COOH adduct generated in CH(3)COOH, a result that was assigned to the generation of the three-component radical between the spin adduct, sodium cations, and CB[7].
authors: Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols, Detroit, Michigan 48221, USA.
authors: Mariana
authors: M
authors: Spulber
authors: Shulamith
authors: S
authors: Schlick
authors: Frederick A
authors: FA
authors: Villamena
mesh_terms: D002073:Butanes / Q000737:chemistry*; D003497:Cyclic N-Oxides / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D047028:Macrocyclic Compounds / Q000737:chemistry*; D015394:Molecular Structure; D009603:Nitroso Compounds / Q000737:chemistry*; D011789:Quantum Theory; D000099246:Cucurbit(n)urils
chemical_list: D002073:Butanes; D003497:Cyclic N-Oxides; D005609:Free Radicals; D047028:Macrocyclic Compounds; D009603:Nitroso Compounds; D000099246:Cucurbit(n)urils; C007539:di-t-butyl nitroxide; C017245:5,5-dimethyl-1-pyrroline-1-oxide; C034526:tert-nitrosobutane