Computed¹³C NMR Chemical Shifts via Empirically Scaled GIAO Shieldings and Molecular Mechanics Geometries. Conformation and Configuration from¹³C Shifts

Abstract

Accurate (rms error ∼3 ppm) predictions of 13C chemical shifts are achieved for many of the common structural types of organic molecules through empirical scaling of shieldings calculated from gauge including atomic orbitals (GIAO) theory with a small basis set and with geometries obtained from computationally inexpensive molecular mechanics methods. Earlier GIAO calculations are shown to be much better at predicting relative chemical shifts when density functional theory with the B3LYP hybrid functional is used to account for electron correlation, in comparison with Hartree−Fock calculations. The GIAO isotropic shieldings need to be empirically scaled to achieve good numerical agreement with experimental δC. GIAO calculations with different small basis sets are compared for a set of 38 model compounds containing C, H, O, and N with MMX and MM3 force fields and B3LYP/6-31G* optimizations providing the geometries. The best MM3-based results are obtained with B3LYP/3-21G(X,6-31+G*)//MM3 calculations in which the 3-21G basis set is augmented for heteroatoms with polarization and diffuse functions. The examples of the (E)- and (Z)-2-butenes, axial and equatorial methylcyclohexanes, exo- and endo-2-norbornanols, vulgarin and epivulgarin, and chair and twist-boat forms of 3α-hydroxy-2β-(4-morpholinyl)-5αH-androstan-17-one are examined to establish whether δpred values could determine the structure if only one of each pair of structures were available to provide experimental δC values. The δpred from B3LYP/3-21G(X,6-31+G*)//MM3 calculations are adequate for addressing questions of conformation and relative stereochemistry.

Synthesis and GIAO NMR Calculations for Some Novel 4-Heteroarylidenamino-4,5-dihydro-1H-1,2,4-triazol-5-one Derivatives: Comparison of Theoretical and Experimental 1Hand 13C- Chemical Shifts

“…The overall standard error of estimate is 3.43 and 2.78 at B3LYP/6-311G and HF/6-311G basis sets, respectively. The approach of Forsyth and Sebag [23] for emprically scaling theoretical data linear regressions, δ calc = a + bδ exp was made and details are summarized in Table 5. Table 5.…”

Structural Properties and Biological Prediction of ({[(1E)-3-(1H-Imidazol-1-yl)-1-phenylpropylidene] amino}oxy)(4-methylphenyl)methanone: An In Silico Approach

“…The computed chemical shifts were scaled down by the linear regression method for neglecting the systematic errors [20][21][22][23][24][25]. The empirically scaling was carried out by following equation…”

Spectroscopic methods and theoretical studies of bromoacetic substituted derivatives of bile acids

“…The characteristic protons shifts for compounds 4-10 are collected in Table 3. ppm assigned to C(7)OH and C (7) 45,46,[53][54][55] are usually linear and described by the equation δ exp = a + b σ calc . The slope and intercept of the least-square correlation lines are used to scale the GIAO isotropic absolute shielding constants σ, and to predict chemical shifts in CDCl 3 for 10 ( Figure 5, Table 4).…”