Nitroxide Radicals as Hydrogen Bonding Acceptors. An Infrared and EPR Study.

Abstract

The equilibrium constants for the formation of hydrogen‐bonded complexes with phenol, benzyl alcohol and diphenyl amine of a persistent nitroxide radical (TEMPO) have been measured for the first time by IR spectroscopy in solution. By making use of the data obtained with IR measurements it was possible to quantitatively determine the effect of hydrogen bonding on the nitrogen hyperfine‐splitting constant. On this basis it is shown that EPR spectroscopy can be used as an alternative to IR spectroscopy for the determination of the thermodynamic parameters of hydrogen bond formation with nitroxides by following the dependence of the experimental EPR nitrogen hyperfine splitting of these radicals upon changing the nature of the solvent. The experimental data obtained from both IR and EPR spectroscopy indicate that nitroxides are versatile hydrogen bond acceptors giving hydrogen bonds of strength similar to that of ethers or esters. The corresponding Abraham's β$\rm{_{2}^{H}}$ values have been determined as 0.46.

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

“…To characterize the relaxation behavior of the TEMPO-TMV particles, we performed proton relaxometric measurements at different fields. It is worth noting that nitroxide radicals normally bind noncovalently to at least one water molecule, which is similar to that found with metal-based complexes with one or more water molecules coordinated to the inner sphere of the metal centers. , The overall proton relaxation behavior is determined by the sum of the inner-sphere and outer-sphere contributions . While the outer-sphere contributions cannot be neglected, the inner-sphere contributions are typically dominant .…”

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

“…To characterize the relaxation behavior of the TEMPO-TMV particles, we performed proton relaxometric measurements at different fields. It is worth noting that nitroxide radicals normally bind noncovalently to at least one water molecule, which is similar to that found with metal-based complexes with one or more water molecules coordinated to the inner sphere of the metal centers. , The overall proton relaxation behavior is determined by the sum of the inner-sphere and outer-sphere contributions . While the outer-sphere contributions cannot be neglected, the inner-sphere contributions are typically dominant .…”

The Dimerization of H2NO

“…There has been some mention of neutral radical-molecule hydrogen bonds in the literature, mostly with a neutral proton donating solvent. − The cyclic double hydrogen bonding pattern has been noticed for some simple radical-molecule complexes in a study by Hernández-Soto et al In that study, just as for our dimer 10 , the unpaired electron of the radical-molecule complexes is not directly involved in the hydrogen bond, but resides in an orbital perpendicular to the plane of the hydrogen bonds. Furthermore, they found that the two hydrogen-bonds do not contribute to the total binding energy in a simple additive fashion.…”

Antioxidant Activity of o -Bisphenols: the Role of Intramolecular Hydrogen Bonding

“…7−9 kcal/mol. , It seems likely, on the basis of the present results, that these calculations overestimate the strength of the H-bond to a radical oxygen. Support to this point of view is provided by a recent spectroscopic study on the ability of of nitroxide radicals as hydrogen bond acceptors, indicating that these oxygen centered radicals give with alcohols H-bonds of strength similar to that of ethers and esters, i.e., 4.0−5.5 kcal/mol …”

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

“…To characterize the relaxation behavior of the TEMPO-TMV particles, we performed proton relaxometric measurements at different fields. It is worth noting that nitroxide radicals normally bind noncovalently to at least one water molecule, which is similar to that found with metal-based complexes with one or more water molecules coordinated to the inner sphere of the metal centers. , The overall proton relaxation behavior is determined by the sum of the inner-sphere and outer-sphere contributions . While the outer-sphere contributions cannot be neglected, the inner-sphere contributions are typically dominant .…”

The Dimerization of H2NO

“…There has been some mention of neutral radical-molecule hydrogen bonds in the literature, mostly with a neutral proton donating solvent. − The cyclic double hydrogen bonding pattern has been noticed for some simple radical-molecule complexes in a study by Hernández-Soto et al In that study, just as for our dimer 10 , the unpaired electron of the radical-molecule complexes is not directly involved in the hydrogen bond, but resides in an orbital perpendicular to the plane of the hydrogen bonds. Furthermore, they found that the two hydrogen-bonds do not contribute to the total binding energy in a simple additive fashion.…”

Antioxidant Activity of o -Bisphenols: the Role of Intramolecular Hydrogen Bonding

“…7−9 kcal/mol. , It seems likely, on the basis of the present results, that these calculations overestimate the strength of the H-bond to a radical oxygen. Support to this point of view is provided by a recent spectroscopic study on the ability of of nitroxide radicals as hydrogen bond acceptors, indicating that these oxygen centered radicals give with alcohols H-bonds of strength similar to that of ethers and esters, i.e., 4.0−5.5 kcal/mol …”

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

“…To characterize the relaxation behavior of the TEMPO-TMV particles, we performed proton relaxometric measurements at different fields. It is worth noting that nitroxide radicals normally bind noncovalently to at least one water molecule, which is similar to that found with metal-based complexes with one or more water molecules coordinated to the inner sphere of the metal centers. , The overall proton relaxation behavior is determined by the sum of the inner-sphere and outer-sphere contributions . While the outer-sphere contributions cannot be neglected, the inner-sphere contributions are typically dominant .…”

The Dimerization of H2NO

“…There has been some mention of neutral radical-molecule hydrogen bonds in the literature, mostly with a neutral proton donating solvent. − The cyclic double hydrogen bonding pattern has been noticed for some simple radical-molecule complexes in a study by Hernández-Soto et al In that study, just as for our dimer 10 , the unpaired electron of the radical-molecule complexes is not directly involved in the hydrogen bond, but resides in an orbital perpendicular to the plane of the hydrogen bonds. Furthermore, they found that the two hydrogen-bonds do not contribute to the total binding energy in a simple additive fashion.…”

Antioxidant Activity of o -Bisphenols: the Role of Intramolecular Hydrogen Bonding

“…7−9 kcal/mol. , It seems likely, on the basis of the present results, that these calculations overestimate the strength of the H-bond to a radical oxygen. Support to this point of view is provided by a recent spectroscopic study on the ability of of nitroxide radicals as hydrogen bond acceptors, indicating that these oxygen centered radicals give with alcohols H-bonds of strength similar to that of ethers and esters, i.e., 4.0−5.5 kcal/mol …”