Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization
Nargiz B. Asanbaeva,
Larisa Yu. Gurskaya,
Yuliya F. Polienko,
Tatyana V. Rybalova,
Maxim S. Kazantsev,
Alexey A. Dmitriev,
Nina P. Gritsan,
Nadia Haro-Mares,
Torsten Gutmann,
Gerd Buntkowsky,
Evgeny V. Tretyakov,
Elena G. Bagryanskaya
Affiliations
Nargiz B. Asanbaeva
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Larisa Yu. Gurskaya
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Yuliya F. Polienko
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Tatyana V. Rybalova
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Maxim S. Kazantsev
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Alexey A. Dmitriev
V.V. Voevodsky Institute of Chemical Kinetics and Combustion, SB RAS, 3 Institutskaya Str., Novosibirsk 630090, Russia
Nina P. Gritsan
V.V. Voevodsky Institute of Chemical Kinetics and Combustion, SB RAS, 3 Institutskaya Str., Novosibirsk 630090, Russia
Nadia Haro-Mares
TU Darmstadt, Eduard-Zintl-Institute for Inorganic and Physical Chemistry, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany
Torsten Gutmann
TU Darmstadt, Eduard-Zintl-Institute for Inorganic and Physical Chemistry, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany
Gerd Buntkowsky
TU Darmstadt, Eduard-Zintl-Institute for Inorganic and Physical Chemistry, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany
Evgeny V. Tretyakov
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prosp., Moscow 119991, Russia
Elena G. Bagryanskaya
N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS), 9 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia
Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.
