An improved statistical approach for conformational analysis from 13C NMR chemical shifts

https://doi.org/10.1016/j.chemolab.2021.104416Get rights and content
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Highlights

  • Study of conformational equilibria of several aromatic and aliphatic compounds.

  • Calculation of shielding constants for conformers with functionals B3LYP, M06, PW91, TPSS.

  • Regressions between experimental 13C NMR chemical shifts and shielding constants.

  • Iterative fitting with two estimates for iteration (ridge regression).

  • Estimation of conformational equilibrium populations with high accuracy.

Abstract

In this article we propose an improved statistical approach for the estimation of the mole fraction of each conformer in a given conformational equilibrium from the measured averaged 13C NMR chemical shifts. As each compound has a different number of conformers, an iterative process of fitting (backfitting type) is necessary. In each iteration, two estimates are made, i) fitting to a ridge regression model with equality and inequality constraints and selection of variables for each compound and ii) re-fitting for all compounds. This new procedure leads to consistent results, with low standard error and good p-values, allowing the estimation of the conformational composition for small molecules with a reduced number of conformers.

Keywords

13C NMR chemical shift
Isotropic shielding constant
Conformational analysis
Two–stage least squares iterative algorithm
Ridge regression
Bootstrap

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