di-t-butyl-nitroxide and tert-nitrosobutane

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].

Topics: Butanes; Cyclic N-Oxides; Electron Spin Resonance Spectroscopy; Free Radicals; Macrocyclic Compounds; Molecular Structure; Nitroso Compounds; Quantum Theory

We present electron spin resonance (ESR) experiments that describe the interaction of beta-cyclodextrin (beta-CD) with spin adducts of three spin traps: 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butyl-alpha-phenylnitrone (PBN), and 2-methyl-2-nitrosopropane (MNP). The focus was on spin adducts of oxygen-centered radicals trapped by DMPO and PBN and on carbon-centered radical adducts trapped by MNP. The radicals were generated by reaction with hydroxyl radicals and the spin adducts studied were DMPO/OH and PBN/OH, MNP/CH(2)COOH generated in CH(3)COOH, and MNP/CF(2)COOH in CF(2)HCOOH. Di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) was also detected in experiments with MNP as the spin trap. A range of interactions of the spin adducts and DTBN with beta-CD was identified. The presence of beta-CD led to significant stabilization of DMPO/OH and PBN/OH but to a negligible effect on the (14)N hyperfine splitting of the adducts, a(N), indicating that the N-O group is outside the beta-CD cavity. An increase of a(N) was detected for DTBN and MNP/CH(2)COOH in CH(3)COOH in the presence of beta-CD, a result we assigned to bonding at the rim of the host. Experiments with methylated beta-CD (Me beta-CD) provided support for this conclusion. A different type of complexation was detected for DTBN and MNP/CF(2)COOH in CF(2)HCOOH: for specific host concentrations both "in" and "out" species were detected. We suggest that the hydrophobicity of the fluorinated adduct leads to insertion of the adduct inside the host cavity. Calculation of the association constant K(a) indicated the competition between DTBN and the adduct for inclusion in the host. For MNP as spin trap, the two nitroxide radicals (adduct and DTBN) have the same type of interaction with the host: at the rim in acetic acid, and inside the host cavity in CF(2)HCOOH. Experiments with DTBN in the absence of the spin trap and of adducts illuminated the effect of the local polarity and of the pH on the hyperfine splittings and indicated that the presence of acetic acid encourages rim complexation.

Topics: Acetic Acid; Butanes; Carbon; Cyclic N-Oxides; Cyclodextrins; Free Radicals; Hydrogen-Ion Concentration; Hydrophobic and Hydrophilic Interactions; Models, Molecular; Molecular Conformation; Nitroso Compounds; Oxygen; Spin Trapping