¹J_CH couplings in Group 14/IVA tetramethyls from the gas-phase NMR and DFT structural study: a search for the best computational protocol

Abstract

Four tetramethyl compounds EMe4 (E = C, Si, Ge, and Pb) were studied by high-resolution NMR spectroscopy in gaseous and liquid states at 300 K. Extrapolation of experimental vapor-phase C-H J-couplings to a zero-pressure limit permitted determining the (1)J(0,CH)s in methyl groups of their nearly isolated molecules. Theoretical predictions of the latter NMR parameters were also performed in a locally dense basis sets/pseudopotential (Sn, Pb) approach, by applying a few DFT methods pre-selected in calculations of other gas-phase molecular properties of all these species and SnMe4 (bond lengths, C-H stretching IR vibrations). A very good agreement theory vs. experiment was achieved with some computational protocols for all five systems. The trends observed in their geometry and associated coupling constants ((1)J(CH)s, (2)J(HH)s) are discussed and rationalized in terms of the substituent-induced rehybridization of the methyl group (treated as a ligand) carbon, by using Bent's rule and the newly proposed, theoretically derived values of the Mulliken electronegativity (χ) of related atoms and groups. All these χ data for the Group-14/IVA entities were under a lot of controversy for a very long time. As a result, the recommended χ values are semi-experimentally confirmed for the first time and only a small correction is suggested for χ(Ge) and χ(GeMe3).

Improvements in DFT Calculations of Spin–Spin Coupling Constants

“…Some authors have shown that the most used functional, the hybrid B3LYP, , is a good option to predict accurate SSCCs. − However, our studies of 1 J CC and 1 J CH SSCCs , suggest that this functional overestimates the calculated values. This overestimation can be ameliorated using relatively small basis sets as TZVP. , Several other density functionals have been used to predict SSCCs.…”

Shortfall of B3LYP in Reproducing NMR JCH Couplings in Some Isomeric Epoxy Structures with Strong Stereoelectronic Effects: A Benchmark Study on DFT Functionals

“…Equilibrium structures optimized at this level of theory were also used in a critical test of some computational methods for prediction of δ H and δ C values . The “Tight” SCF and Opt convergence criteria were applied to ascertain accuracy ,. A fine‐pruned (150,590) numerical integration grid was used, owing to soft modes arising from the dynamic phenomena of methyl‐group rotations .…”

“…The computed versus experimental J CH correlations were analyzed by a linear regression model, and predicted J CH values were plotted against the observed values in the usual way ,,. The greater value of the Pearson correlation coefficient r or its square r 2 was considered as a very simple indication of better adjustment of the compared J data (vide infra).…”

Stereoelectronic Evaluation of Pyrazole- and Indazole-Derived N-Heterocyclic Carbenes

“…From a synthetic point of view, the use of 1 J (C–H) values determined for azolium salts seems particularly attractive, since these compounds are the most common precursors to NHCs . In line with Bent’s rule, high 1 J (C–H) values are correlated with a high s character of the respective carbene’s lone pair, ultimately indicating its inferior σ-donating strength . Despite readily accessible azolium salts, only a few examples mostly based on (benz)­imidazole and imidazoline with various counterions have been reported so far. ,− Furthermore, measurement of the coupling constants was often conducted in different deuterated solvents with inconsistent accuracy, indicating the potential uncertainty when structurally similar or distinctively different azolium salts were compared .…”