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

H-1 and C-13 NMR spectra of allyl isothiocyanate (AITC) were measured, and the exchange dynamics were studied to explain the near-silence of the ITC carbon in C-13 NMR spectra. The dihedral angles alpha = angle(C1-C2-C3-N4) and beta = angle(C2-C3-N4-C5) describe the conformational dynamics (conformation change), and the bond angles gamma = angle(C3-N4-C5) and epsilon = angle(N4-C5-S6) dominate the molecular dynamics (conformer flexibility). The conformation space of AITC contains three minima, C-s-M1 and enantiomers M2 and M2'; the exchange between conformers is very fast, and conformational effects on C-13 chemical shifts are small (nu(M1) - nu(M2) < 3 ppm). Isotropic chemical shifts, ICS(gamma), were determined for sp, sp(x), and sp(2) N-hybridization, and the gamma dependencies of delta(N4) and delta(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(gamma,epsilon,...) with 120 degrees < gamma < 180 degrees and 155 degrees < epsilon < 180 degrees. Because the extreme broadening of the C-13 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 C-13 NMR spectra of organic isothiocyanates.