title: Near-silence of isothiocyanate carbon in (13)C NMR spectra: a case study of allyl isothiocyanate.
abstract: (1)H and (13)C NMR spectra of allyl isothiocyanate (AITC) were measured, and the exchange dynamics were studied to explain the near-silence of the ITC carbon in (13)C 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 (13)C chemical shifts are small (νM1 - νM2 < 3 ppm). Isotropic chemical shifts, ICS(γ), were determined for sp, sp(x), and sp(2) 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 (13)C 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 (13)C NMR spectra of organic isothiocyanates.
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States.
authors.forename: Rainer
authors.initials: R
authors.lastname: Glaser
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States.
authors.forename: Roman
authors.initials: R
authors.lastname: Hillebrand
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States.
authors.forename: Wei
authors.initials: W
authors.lastname: Wycoff
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States.
authors.forename: Cory
authors.initials: C
authors.lastname: Camasta
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States.
authors.forename: Kent S
authors.initials: KS
authors.lastname: Gates
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D002247:Carbon Isotopes; D017879:Isothiocyanates / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory
chemical_list: D000498:Allyl Compounds; D002247:Carbon Isotopes; D017879:Isothiocyanates; C037152:isothiocyanic acid; D002244:Carbon