Magnetic resonance tensors in Uracil: Calculation of 13C, 15N, 17O NMR chemical shifts, 17O and 14N electric field gradients and measurement of 13C and 15N chemical shifts

Influence of the basis set on the calculation of the absolute 13C shieldings of methyl derivatives (CH3X with X = CH3, CN, NH2, NO2, OH, F) with special emphasis in the cases of X = Cl, Br, SH, SeH, and PH2

“…Three experimental absolute shieldings of CH 3 X compounds were summarized previously [31]. CH 4 195.1 ppm [58,63] (this value has been often used as reference in theoretical papers [64][65][66][67]), CH 3 NO 2 126.9 ppm [59] (see also [68]), and TMS 188.1 ppm [58] (see also [64,[69][70][71][72][73][74][75][76]), the method that best reproduce these three values is MPW1PW91/6-311? ?G(d,p): 193.64, 123.68, and 188.33 ppm, respectively (see Table 1).…”

Layer selection effect on solid state 13C and 15N chemical shifts calculation using ONIOM approach

“…A heptamer and octamer cluster of crystal uracil and imidazole molecules was respectively used to calculate NMR shielding [18,19]. These clusters not only take in account strong hydrogen bond interactions, but also contain weak van der Waals contacts of surrounding molecules with central one.…”

“…Based on preliminary experiences [18,19,[50][51][52][53][54][55][56], three factors of electron correlation (A), triple-x valance shell (B), and diffuse function (C) were selected for a high layer of 2-layer ONIOM approach. Each factor was considered at two levels, with ( þ) and ( À ) for high and low level, respectively.…”

“…Experimental values of chemical shifts were taken from the Refs. [18,60,61]. In these designs, RMS was considered as a response.…”

Applications of nuclear shielding

“…These experimental values are supported by extensive calculated data of the 13 C, 15 N and 17 O chemical shielding and 17 O and 14 N electrical field gradient (EFG) tensors. 421 The relative acidities of the cis and trans isomers of 1,5-oxazaspiro [5.5]undecane derivatives were determined by measuring DpK in acid-base titrations followed by 1 H NMR. Relative structural stabilities were determined by measuring substituent chemical shift and g-gauche effects in 13 C, 15 N and 17 O NMR.…”