Tuesday, August 08, 2006

MRC: 33S NMR of ethanesulfonates, plus DFT and NBO methods

Magnetic Resonance in Chemistry
Vol: 44, Issue: 8, August 2006
pp. 753 - 760

Title: 33S NMR spectroscopy 3. substituent effects on 33S NMR parameters in 2-substituted ethanesulfonates
Authors: Musio, Robertaa; Sciacovelli, Oronzoa
Affiliations: a. Istituto C.N.R. di Chimica dei Composti Organo‐Metallici—Sezione di Bari and Dipartimento di Chimica, Universit√† degli Studi di Bari, Via E. Orabona 4, 70126 Bari, Italy
Keywords: NMR; 33S NMR; 33S chemical shift; 33S nuclear shielding calculations; 33S nuclear quadrupole coupling constant; substituent effect; 2-substituted ethanesulfonates
Abstract (English):

33S NMR parameters (chemical shifts and linewidths) in 2-substituted sodium ethanesulfonates, XCH2CH2SO3Na (X = H, CH3, OH, SH, NH2, Cl, Br, NH3+) depend upon the electronic properties of substituents. To explain experimental results and obtain additional information on the origin of the observed substituent effect (SE), sulfur isotropic absolute shielding constants have been calculated at DFT level of theory (B3LYP/6-311 + + G(2d,p)) by gauge-including atomic orbitals (GIAO) method. Data have been interpreted with the aid of natural bond orbital (NBO) method and natural chemical shielding (NCS) analysis. It has been demonstrated that in the class of compounds considered the diamagnetic contribution to sulfur-shielding constant is constant and the observed upfield shift of 33S resonance induced by electron-withdrawing substituents (reverse chemical shift effect) can be related to variations of the paramagnetic contribution. Substituents with different electronic properties cause variations in the polarization of S—C and S—O bonds of the —C—SO3- moiety thus determining changes of the electron density at sulfur nucleus and consequently the expansion or contraction of 3p sulfur orbitals. Also oxygen lone-pairs and sulfur core 2p electrons can play an active role in determining the paramagnetic contribution to sulfur shielding. With regard to linewidth variations, they can be ascribed primarily to changes in the nuclear quadrupole coupling constant values. B3LYP/6-311 + + G(2d,p) method allows obtaining a good reproducibility of SE on the electric field gradient (EFG) at sulfur, although its values tend to be underestimated significantly. Moreover, 17O shielding constants have been calculated. Copyright © 2006 John Wiley & Sons, Ltd.

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