Second-order NMR spectra at high field of common organic functional groups

Abstract

The proton NMR spectra of aryl n-propyl sulfides gave rise to what may appear to be first-order proton NMR spectra. Upon oxidation to the corresponding sulfone, the spectra changed appearance dramatically and were clearly second-order. A detailed analysis of these second-order spectra, in the sulfone series, provided vicinal coupling constants which indicated that these compounds had a moderate preference for the anti-conformer, reflecting the much greater size of the sulfone over the sulfide. It also emerged, from this study, that the criterion for observing large second-order effects in the proton NMR spectra of 1,2-disubstituted ethanes was that the difference in vicinal coupling constants must be large and the difference in geminal coupling constants must be small. n-Propyl triphenylphosphonium bromide and 2-trimethylsilylethanesulfonyl chloride, and derivatives thereof, also exhibited second-order spectra, again due to the bulky substituents. Since these spectra are second-order due to magnetic nonequivalence of the nuclei in question, not chemical shifts, the proton spectra are perpetually second-order and can never be rendered first-order by using higher field NMR spectrometers.

Hyperpolarization Effects in Parahydrogen Activation with Pnictogen Biradicaloids: Metal‐free PHIP and SABRE

“…The activation products 10 ‐H 2 and 8 ‐H 2 form AA'XX’ spin systems (A= 1 H, X= 75 As or 31 P), and this feature complicates corresponding NMR signal shapes significantly. Since the spin pairs of the same kind are strongly coupled, the resulting NMR spectra are expected to have complex signal structures due to the second‐order effects [14] . The observed resonances correspond to superposition states of the spin pairs rather than to the individual spins.…”

“…The reversibility of activation was used to hyperpolarize biradicaloids themselves via a high‐field SABRE effect, providing another example of SABRE in metal‐free systems in addition to ansa‐aminoboranes [11c] . Symmetrical P−P and As−As four‐membered biradicaloid adducts ( 8 ‐H 2 and 10 ‐H 2 ) form symmetrical AA'XX’ spin systems (A= 1 H, X=P,As) resulting in complex NMR multiplets having features of second‐order spectra [14] . This can be viewed as a consequence of chemical equivalence of 1 H and pnictogen spin pairs at the simultaneous magnetic inequivalence of spins in these pairs.…”

“…Since the spin pairs of the same kind are strongly coupled, the resulting NMR spectra are expected to have complex signal structures due to the secondorder effects. [14] The observed resonances correspond to superposition states of the spin pairs rather than to the individual spins.…”

The Future of Retrosynthesis and Synthetic Planning: Algorithmic, Humanistic or the Interplay?

“…[191] In an effort to conceptualise retrosynthetic planning based on complexity, there are three general parameters that have been used by the lead author over the years to categorise synthetic approaches to targets. These three categories, which are essentially a ranking system of synthetic difficulty, are best imagined as analogies to proton NMR splitting patterns, which include first-order, [197] second-order, [198] and non-first-order [197] interpretations.…”

Long-term thermal stabilities of ammonium ionic liquids designed as potential absorbents of ammonia

“…2). According to literature, 13 (9) was the same than that employed for salts 1-4 but starting from N,N-di-(2-hydroxyethyl)-N-methylamine instead of N,N-dimethylethanolamine. 12 These couplings appear very clearly in the 1 H NMR spectrum of [EM 2 NCH 2 CH 2 OH][DCA] (3).…”