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NMR of Terminal Oxygen—17— Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π‐Systems
Author(s) -
Dahn Hans,
Péchy Peter,
Carrupt PierreAlain
Publication year - 1996
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199604)34:4<283::aid-omr876>3.0.co;2-4
Subject(s) - chemistry , heteronuclear molecule , ab initio , linear molecular geometry , electromagnetic shielding , molecule , diamagnetism , anisotropy , ab initio quantum chemistry methods , acetylene , oxygen 17 , isotropy , atom (system on chip) , molecular physics , computational chemistry , crystallography , atomic physics , magnetic field , physics , organic chemistry , quantum mechanics , computer science , embedded system
Linear heteronuclear π‐systems of sp character, when compared with corresponding sp 2 systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In 17 O NMR, the molecules NO + , PhCO + , RCNO, NξCξO − , RNξCξO and NO 2 + were examined (in addition to known CO, CO 2 , COS and N 2 O). For these molecules, 13 C and 15 N shieldings were also analysed. As had been shown before for 13 C and 15 N, the increase in shielding of 17 O in linear molecules is mainly attributable to the vanishing of the deshielding component along the linear axis of the molecule (manifest also in the increase in the anisotropy of shielding Δσ). Ab initio IGLO calculations were executed to evaluate Δσ ( = σ ‖ ‐σ ⊥ ). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, σ iso . As in the case of C and N, the values of σ ‖ of 17 O in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, σ d (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed.