Benchmarking DFT Calculations of ¹H and ¹³C Chemical Shifts in Monosubstituted Ferrocenes

Abstract

Different density functional theory (DFT) approaches were tested for the computation of 1H and 13C nuclear magnetic resonance (NMR) chemical shifts in monosubstituted ferrocenes. The results were evaluated against experimental values. Generally, the conductor‐like polarizable continuum model and cc‐pVTZ basis set are recommended. The geometries providing the best accuracies are B3LYP‐optimized for 1H and M06‐L‐optimized for 13C. Functional rankings at these geometries are: TPSSh > M06‐L > CAM‐B3LYP > B3LYP > PBE0 > M06 (the first one is the most accurate) for 1H NMR computations and M06 > M06‐L > PBE0 > TPSSh > B3LYP > CAM‐B3LYP for 13C. The most accurate functionals have root‐mean‐square deviations of 0.08 ppm (1H, TPSSh) and 3.97 ppm (13C, M06) and showed similar accuracy for a set of disubstituted ferrocenes and decamethylferrocene. The utilization of Jensen's pcSseg‐2 basis set improves the results for 1H but worsens the results for 13C. The linear scaling is generally not recommended. The errors can be minimized using an appropriate method for a given nucleus, so the DFT‐assisted signal assignment is possible for substituted ferrocenes.

Heteroatom-Bridged Carba[2]ferrocenophanes with a Surprising Chemical Shift Pattern of Cp Protons

“…The experimentally determined assignments were further supported by calculated chemical shifts using a DFT method that was recently recommended in a benchmark study for monosubstituted ferrocenes. 41 The match between experimental and calculated chemical shifts of Cp protons is illustrated in Figure 5. It should be noted that for peaks with very similar chemical shifts, like those for α protons in 13 BMes (Figures 4D and 5) or those at 4.24 (β proton) and 4.28 ppm (α proton) of 13 SiMe2 (Figures 4C and 5), calculated chemical shifts are not reliable enough to provide a confident assignment.…”

“…From the set of two-dimensional NMR experiments (see Experimental Section and the Supporting Information), the NOESY spectra were particularly insightful as the proximity of groups on the bridging elements resulted in cross peaks revealing peaks of the α-Cp protons. The experimentally determined assignments were further supported by calculated chemical shifts using a DFT method that was recently recommended in a benchmark study for monosubstituted ferrocenes . The match between experimental and calculated chemical shifts of Cp protons is illustrated in Figure .…”

“…For comparing molecular structures (species in Table ) and obtaining strain enthalpies (Scheme ), molecular geometries were optimized at the B3PW91 level − with the 6-311+G­(d,p) basis set. Geometries used for calculations of chemical shifts were also optimized at the B3PW91/6-311+G­(d,p) level, but effects of the solvent benzene were included by the polarizable conductor calculation model (CPCM). − Guided by a recent benchmark study, NMR shielding tensors were computed using gauge including atomic orbitals (GIAO) at the TPSSh level − with the cc-pVTZ basis set. All calculated δ­( 1 H) values refer to Me 4 Si as the standard that gave a shielding tensor of 31.742 ppm on the same level of theory.…”

“…Geometries used for calculations of chemical shifts were also optimized at the B3PW91/6-311+G(d,p) level, but effects of the solvent benzene were included by the polarizable conductor calculation model (CPCM). 60−62 Guided by a recent benchmark study, 41 NMR shielding tensors were computed using gauge including atomic orbitals (GIAO) 63 at the TPSSh level 64−66 with the cc-pVTZ 67 values refer to Me 4 Si as the standard that gave a shielding tensor of 31.742 ppm on the same level of theory. For all species, frequency calculations were used to confirm minima and to obtain thermochemical data.…”