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.
R. Glaser et al., "Near-Silence of Isothiocyanate Carbon in ¹³C NMR Spectra: A Case Study of Allyl Isothiocyanate," Journal of Organic Chemistry, vol. 80, no. 9, pp. 4360-4369, American Chemical Society (ACS), Apr 2015.
The definitive version is available at https://doi.org/10.1021/acs.joc.5b00080
Keywords and Phrases
Carbon; Chemical shift; Dihedral angle; Molecular dynamics; Nuclear magnetic resonance spectroscopy; Potential energy; Quantum chemistry; Allyl isothiocyanate; Conformation change; Conformational dynamics; Conformational effect; Exchange dynamics; Isothiocyanates; Isotropic chemical shifts; Structural flexibilities; Conformations; allyl isothiocyanate; carbon; allyl compound; isothiocyanic acid; isothiocyanic acid derivative; atom; carbon nuclear magnetic resonance; chemical bond; chemical structure; conformational transition; density; chemistry; conformation; nuclear magnetic resonance spectroscopy; quantum theory; Allyl Compounds; Carbon; Carbon Isotopes; Isothiocyanates; Magnetic Resonance Spectroscopy; Models; Molecular; Molecular Conformation; Quantum Theory
International Standard Serial Number (ISSN)
Article - Journal
© 2015 American Chemical Society (ACS), All rights reserved.
01 Apr 2015