TY - JOUR
T1 - Experimental evidence for long-range stabilizing and destabilizing interactions between charge and radical sites in distonic ions
AU - Marshall, David L.
AU - Gryn'ova, Ganna
AU - Poad, Berwyck L.J.
AU - Bottle, Steven E.
AU - Trevitt, Adam J.
AU - Coote, Michelle L.
AU - Blanksby, Stephen J.
PY - 2019/1
Y1 - 2019/1
N2 - Polarizable radical sites in distonic radical anions are stabilized by ostensibly remote negative charges. Computational evidence suggests bond dissociation energies of closed-shell precursors are significantly lowered by through-space interactions with a proximate negative charge, however direct experimental confirmation has proved challenging. Herein, we exploit two complementary tandem mass spectrometry strategies to probe the influence of a remote charge on the stability of nitroxyl radicals, and vice versa. Dissociation of negatively charge-tagged alkoxyamines reveals that the energetic onset of radical formation is dependent on the proximity and basicity of the charged group, thus providing direct evidence for a charge-induced stabilization of the nitroxyl radical. Complementary kinetic method experiments on a series of proton-bound dimers demonstrate that the presence of a nitroxyl radical decreases the proton affinity for a selection of proximate ionic groups. These data show excellent agreement with quantum-chemical calculations and provide a general framework to explore the magnitude and direction of charge-radical interactions through systematic exploration of the identity, polarity and the proximity of the ion to the radical site. These findings expand our fundamental understanding of radical ion energetics that underpin the application of distonic ions as models for neutral radical reactivity, and open new avenues for exploiting these interactions as chemical switches.
AB - Polarizable radical sites in distonic radical anions are stabilized by ostensibly remote negative charges. Computational evidence suggests bond dissociation energies of closed-shell precursors are significantly lowered by through-space interactions with a proximate negative charge, however direct experimental confirmation has proved challenging. Herein, we exploit two complementary tandem mass spectrometry strategies to probe the influence of a remote charge on the stability of nitroxyl radicals, and vice versa. Dissociation of negatively charge-tagged alkoxyamines reveals that the energetic onset of radical formation is dependent on the proximity and basicity of the charged group, thus providing direct evidence for a charge-induced stabilization of the nitroxyl radical. Complementary kinetic method experiments on a series of proton-bound dimers demonstrate that the presence of a nitroxyl radical decreases the proton affinity for a selection of proximate ionic groups. These data show excellent agreement with quantum-chemical calculations and provide a general framework to explore the magnitude and direction of charge-radical interactions through systematic exploration of the identity, polarity and the proximity of the ion to the radical site. These findings expand our fundamental understanding of radical ion energetics that underpin the application of distonic ions as models for neutral radical reactivity, and open new avenues for exploiting these interactions as chemical switches.
KW - Distonic ion
KW - Free radicals
KW - Gas-phase ion chemistry
KW - Kinetic method
KW - Tandem mass spectrometry
UR - http://www.scopus.com/inward/record.url?scp=85056212966&partnerID=8YFLogxK
UR - http://purl.org/au-research/grants/ARC/CE0561607
UR - http://purl.org/au-research/grants/ARC/FL170100041
UR - http://purl.org/au-research/grants/ARC/DP140101237
U2 - 10.1016/j.ijms.2018.10.031
DO - 10.1016/j.ijms.2018.10.031
M3 - Article
AN - SCOPUS:85056212966
SN - 1387-3806
VL - 435
SP - 195
EP - 203
JO - International Journal of Mass Spectrometry
JF - International Journal of Mass Spectrometry
ER -