To calculate NMR shift using QM methods, a standard must first be determined

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However, in reality the shielding appears to vary somewhat with magnitude, leading to a relationship δ = k*σcalc + m, where k and m are determined by fitting the calculated shielding values for several known standards to their experimental shifts. For example, TMS has a δ=0 ppm and benzene has a δ=7.27 ppm. Using B3LYP/LACV3P**, the calculated averaged shielding for the TMS hydrogens is 31.929633, and for benzene 24.350454. A linear fit between these two points show that δ = -0.957887412 * σcalc + 30.58499337.

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تاریخ انتشار 2005