2 research outputs found
Rotamers or Diastereomers? An Overlooked NMR Solution
The existence of rotamers in a solution of analyte complicates <sup>1</sup>H NMR analysis, especially when the presence of diastereomers
is also possible. Organic chemists have often responded to this problem
by conducting variable-temperature (VT) NMR experiments, changing
NMR solvents, or adding complexing agents. Here, with specific examples,
we illustrate the use of simple yet widely overlooked chemical-exchange
NMR experiments which allow the nonintrusive rapid distinguishment
of rapidly equilibrating small molecules such as rotamers from nonequilibrating
diastereomers
Online Quantification of Criegee Intermediates of α‑Pinene Ozonolysis by Stabilization with Spin Traps and Proton-Transfer Reaction Mass Spectrometry Detection
Biogenic
alkenes, which are among the most abundant volatile organic
compounds in the atmosphere, are readily oxidized by ozone. Characterizing
the reactivity and kinetics of the first-generation products of these
reactions, carbonyl oxides (often named Criegee intermediates), is
essential in defining the oxidation pathways of organic compounds
in the atmosphere but is highly challenging due to the short lifetime
of these zwitterions. Here, we report the development of a novel online
method to quantify atmospherically relevant Criegee intermediates
(CIs) in the gas phase by stabilization with spin traps and analysis
with proton-transfer reaction mass spectrometry. Ozonolysis of α-pinene
has been chosen as a proof-of-principle model system. To determine
unambiguously the structure of the spin trap adducts with α-pinene
CIs, the reaction was tested in solution, and reaction products were
characterized with high-resolution mass spectrometry, electron paramagnetic
resonance, and nuclear magnetic resonance spectroscopy. DFT calculations
show that addition of the Criegee intermediate to the DMPO spin trap,
leading to the formation of a six-membered ring adduct, occurs through
a very favorable pathway and that the product is significantly more
stable than the reactants, supporting the experimental characterization.
A flow tube set up has been used to generate spin trap adducts with
α-pinene CIs in the gas phase. We demonstrate that spin trap
adducts with α-pinene CIs also form in the gas phase and that
they are stable enough to be detected with online mass spectrometry.
This new technique offers for the first time a method to characterize
highly reactive and atmospherically relevant radical intermediates
in situ