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Carbon‐13 NMR spectra of a series of para ‐substituted phenyl isothiocyanates. Linear free energy relationships for carbon‐13 substituent chemical shifts
Author(s) -
Jones Ronald G.,
Allen Grace
Publication year - 1982
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270190408
Subject(s) - substituent , moiety , chemistry , chemical shift , hammett equation , carbon fibers , carbon 13 nmr , chloroform , stereochemistry , analytical chemistry (journal) , medicinal chemistry , organic chemistry , reaction rate constant , kinetics , materials science , physics , quantum mechanics , composite number , composite material
Carbon‐13 chemical shifts are reported for 16 para ‐substituted phenyl isothiocyanates measured at 1 and 10 mol % in chloroform‐ d solution. Data for the NCS group were not obtained at 1 mol %, but concentration effects for the other resonances were negligible. Hammett, dual substituent parameter (DSP) and DSP‐nonlinear resonance (DSP‐NLR) analyses were used to evaluate substituent effects on the substituent chemical shifts (SCS) for the ipso ‐carbon (C‐1), C‐2, and the NCS carbon atoms. A good Hammett correlation was observed for C‐1 (ν p + = 8.1 ppm, r = 0.98 at 1 mol %) but was improved for the higher order correlations with the following results, DSP:ρ I = 5.4, ρ R ° = 22.2, r = 0.998; DSP‐NLR: ρ I = 5.6, ρ R ° = 20.5, ϵ = −0.22, r = 0.999. The 10 mol % data were very similar except the value of ϵ was −0.26 and confirms the phenyl‐bonded NCS moiety as a mild electron acceptor substituent. Hammett correlations were unsuccessful for the C‐2 data, but fairly good results were obtained from the higher order analyses. For the 1 mol % data, DSP: ν I = 1.6, ν R ° = −2.0, r = 0.976; DSP‐NLR: ν I = 1.8, ν R ° = −2.6, ϵ = 1.1, r = 0.982. Excellent correlations were obtained for the 10 mol % NCS carbon data. Hammett: ν p ° = 6.2, r = 0.997; DSP: ν I = 5.9, ν R ° = 7.0, r = 0.997; DSP‐NLR: ν I = 5.8, ν R ° = 7.6, ϵ = 0.25, r = 0.997. The positive ν values in these three correlations contrast the negative values usually observed for carbonyl and thiocarbonyl carbons, and more closely parallel results previously reported for the β‐carbon of styrenes and benzylidene anilines with para ‐substituents in the aniline ring.