Intermolecular H-bond of solvent molecule with nitroxide radical using ab initio MO calculations

Otsuka, Takao; Motozaki, W.; Nishikawa, Kiyoshi; Endo, Kazunaka

doi:10.1016/s0022-2860(02)00222-3

Cited by 7 publications

(8 citation statements)

References 14 publications

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“…However, they claimed that the low preexponential factors cannot be attributed to the tunnel effect, and they mentioned an intermediate formation of a hydrogen-bonded complex or the triplet repulsion in the transition state . The existence of such a hydrogen-bonded complex is supported by several studies of ( tert -Bu) 2 NO • or TEMPO • by spectroscopic data and theoretical calculations. − …”

Section: Resultsmentioning

confidence: 98%

“…51 The existence of such a hydrogen-bonded complex is supported by several studies of (tert-Bu) 2 NO • or TEMPO • by spectroscopic data and theoretical calculations. [52][53][54] It is often proposed that unusually low preexponential factors might be attributed to the formation of a hydrogenbonded intermediate in hydrogen abstraction from phenols. [55][56][57] When k -a , k b , the rate-determining step is k a and the Arrhenius preexponential factor will be high (≈10 11 L mol -1 s -1 ), which is a normal value for a diffusioncontrolled reaction in normal viscosity solvents.…”

Section: Resultsmentioning

confidence: 99%

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Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols

2003

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The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO* is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degrees C. The reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma+ (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degrees C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E(a)(D) - E(a)(H), was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.

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Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols

2003

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“…The NO⋅⋅⋅I angle (137.0°) and C‐N‐O⋅⋅⋅I torsion angle (8.20°) are consistent with electron donation from the NO . oxygen lone pair, with limited contribution from the π‐type arrangement observed for the 4‐amino‐TEMPO/DITFB complex11 and some nitroxides participating in hydrogen bonding 43…”

Section: Resultsmentioning

confidence: 99%

Halogen Bonding between an Isoindoline Nitroxide and 1,4‐Diiodotetrafluorobenzene: New Tools and Tectons for Self‐Assembling Organic Spin Systems

Hanson

Jensen

McMurtrie

et al. 2009

Chemistry A European J

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Radical assembly: Halogen bonding has been observed for the first time between an isoindoline nitroxide and an iodoperfluorocarbon (see figure), which cocrystallize to form a discrete 2:1 supramolecular compound in which N--O(.)I halogen bonding is the dominant intermolecular interaction. This illustrates the potential use of halogen bonding and isoindoline nitroxide tectons for the assembly of organic spin systems.The isoindoline nitroxide 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO) and 1,4-diiodotetrafluorobenzene readily form a discrete 2:1 complex that shows evidence of relatively strong N--O(.)I halogen bonding. This interaction was characterized in the solid state by single-crystal X-ray analysis, thermal analysis, and vibrational spectroscopy (IR and Raman), backed by density functional theory calculations. EPR spectroscopy performed on a solution of TMIO in pentafluoroiodobenzene, a halogen-bonding donor, indicates that halogen bonding induces an increase in electron density at the nitroxide nitrogen nucleus and an increase in the nitroxide rotational correlation time. Our findings demonstrate the potential of utilizing halogen-bonding interactions to promote the self-assembly of new isoindoline nitroxide tectons for the preparation of organic spin systems.

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“…This conclusion, however, is in contradiction to several previous reports on nitroxide radicals in the literature. For example, liquid-solution NMR studies of nitroxide radicals dissolved in alcohols revealed proton hyperfine couplings interpreted with a π-type structure of the complex [66,67]. Also, the recent W-band ENDOR study of the nitrosodisulfonate anion (Fremy salt) in binary methanol/water solution favored a π-type complex [84].…”

Section: Nitroxide R2 the Six-membered Ring Systemmentioning

confidence: 99%

“…The aim is to elucidate the structure of hydrogen-bonded complexes. Several structural models of hydrogen-bonded nitroxide-alcohol complexes have been proposed previously, see for instance [66,67]. They include two types of the models: (i) an in-plane σ-type model in which the H-bond is placed in the σ-orbital plane of the oxygen and directed toward an oxygen lone-pair orbital (with an N-O-H bond angle of ~ 120°), and (ii) an out-of-plane π-type model in which the orbital of the hydrogen-bonded proton contacts the p z orbital of either oxygen, π(O), or nitrogen, π(N), of the nitroxide atoms.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

et al. 2021

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The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

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Intermolecular H-bond of solvent molecule with nitroxide radical using ab initio MO calculations

Cited by 7 publications

References 14 publications

Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols

Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols

Halogen Bonding between an Isoindoline Nitroxide and 1,4‐Diiodotetrafluorobenzene: New Tools and Tectons for Self‐Assembling Organic Spin Systems

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

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