Editing Kinetic isotope effect (section)

==== Single-pulse NMR ====
Quantitative single-pulse [[nuclear magnetic resonance spectroscopy]] (NMR) is a method amenable for measuring [[kinetic fractionation]] of [[isotope]]s for natural abundance KIE measurements. Pascal et al. were inspired by studies demonstrating dramatic variations of deuterium within identical compounds from different sources and hypothesized that NMR could be used to measure {{sup|2}}H KIEs at natural abundance.<ref>{{Cite journal| vauthors = Martin GJ, Martin ML |title=Deuterium labelling at the natural abundance level as studied by high field quantitative 2H NMR |journal=Tetrahedron Letters|language=en|year=1984|volume=22|issue=36|pages=3525–3528|doi=10.1016/s0040-4039(01)81948-1}}</ref><ref name=":0" /> Pascal and coworkers tested their hypothesis by studying the [[insertion reaction]] of dimethyl diazomalonate into [[cyclohexane]]. Pascal et al. measured a KIE of 2.2 using {{sup|2}}H NMR for materials of natural abundance.<ref name=":0">{{cite journal| vauthors = Pascal Jr RA, Baum MW, Wagner CK, Rodgers LR |title=Measurement of deuterium kinetic isotope effects in organic reactions by natural-abundance deuterium NMR spectroscopy|journal=Journal of the American Chemical Society|date=September 1984|volume=106|issue=18|pages=5377–5378|doi=10.1021/ja00330a071 |bibcode=1984JAChS.106.5377P }}</ref>

[[File:Chemical reaction 1.png|frameless|none|500px]]

Singleton and coworkers demonstrated the capacity of {{sup|13}}C NMR based natural abundance KIE measurements for studying the mechanism of the [4 + 2] [[cycloaddition]] of [[isoprene]] with [[maleic anhydride]].<ref name="Singleton_1995" /> Previous studies by Gajewski on isotopically enrich materials observed KIE results that suggested an asynchronous transition state, but were always consistent, within error, for a perfectly synchronous [[reaction mechanism]].<ref>{{cite journal| vauthors = Gajewski JJ, Peterson KB, Kagel JR, Huang YJ |date=December 1989|title=Transition-state structure variation in the Diels-Alder reaction from secondary deuterium kinetic isotope effects. The reaction of nearly symmetrical dienes and dienophiles is nearly synchronous|journal=Journal of the American Chemical Society|volume=111|issue=25|pages=9078–9081|doi=10.1021/ja00207a013 |bibcode=1989JAChS.111.9078G }}</ref>

[[File:Chemical reaction 2.png|frameless|none|500px]]

This work by Singleton et al. established the measurement of multiple {{sup|13}}C KIEs within the design of a single experiment. These {{sup|2}}H and {{sup|13}}C KIE measurements determined at natural abundance found the "inside" hydrogens of the diene experience a more pronounced {{sup|2}}H KIE than the "outside" hydrogens and the C1 and C4 experience a significant KIE. These key observations suggest an asynchronous [[reaction mechanism]] for the [[cycloaddition]] of [[isoprene]] with [[maleic anhydride]].

[[File:KIE measurements.png|frameless|274x274px]]

The limitations for determining KIEs at natural abundance using NMR are that the recovered material must have a suitable amount and purity for NMR analysis (the signal of interest should be distinct from other signals), the reaction of interest must be irreversible, and the [[reaction mechanism]] must not change for the duration of the [[chemical reaction]].

Experimental details for using quantitative single pulse NMR to measure KIE at natural abundance as follows: the experiment needs to be performed under quantitative conditions including a relaxation time of 5 T{{sub|1}}, measured 90° flip angle, a digital resolution of at least 5 points across a peak, and a signal:noise greater than 250. The raw FID is zero-filled to at least 256K points before the Fourier transform. NMR spectra are phased and then treated with a zeroth order baseline correction without any tilt correction. Signal integrations are determined numerically with a minimal tolerance for each integrated signal.<ref name="Singleton_1995" />{{clarify|date=November 2018}}