Near-Silence of Isothiocyanate Carbon in ¹³C NMR Spectra: A Case Study of Allyl Isothiocyanate

1H and 13C NMR spectra of allyl isothiocyanate (AITC) were measured, and the exchange dynamics were studied to explain the near-silence of the ITC carbon in 13C NMR spectra. The dihedral angles α = ∠(C1–C2–C3–N4) and β = ∠(C2–C3–N4–C5) describe the conformational dynamics (conformation change), and the bond angles γ = ∠(C3–N4–C5) and ε = ∠(N4–C5–S6) dominate the molecular dynamics (conformer flexibility). The conformation space of AITC contains three minima, Cs-M1 and enantiomers M2 and M2′; the exchange between conformers is very fast, and conformational effects on 13C chemical shifts are small (νM1 – νM2 < 3 ppm). Isotropic chemical shifts, ICS(γ), were determined for sp, spx, and sp2 N-hybridization, and the γ dependencies of δ(N4) and δ(C5) are very large (10–33 ppm). Atom-centered density matrix propagation trajectories show that every conformer can access a large region of the potential energy surface AITC(γ,ε,...) with 120° < γ < 180° and 155° < ε < 180°. Because the extreme broadening of the 13C NMR signal of the ITC carbon is caused by the structural flexibility of every conformer of AITC, the analysis provides a general explanation for the near-silence of the ITC carbon in 13C NMR spectra of organic isothiocyanates.