Difference between 2JC2H3 and 2JC3H2 spin-spin couplings in heterocyclic five- and six-membered rings as a probe for studying σ-ring currents: a quantum chemical analysis

Abstract

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ-Fermi contact (FC) coupling pathways (FC term transmitted through the σ-skeleton). Knowledge of the spatial distribution of CMOs is obtained by expanding them in terms of natural bond orbitals (NBOs). Their relative importance for transmitting the σ-FC contribution to a given spin-spin coupling constants (SSCCs) is estimated by resorting to the expression of the FC term given by the polarisation propagator formalism. In this way, it is possible to classify the effects affecting such couplings in two different ways: delocalisation interactions taking place in the neighbourhood of the coupling nuclei and 'round the ring' effects. The latter, associated with σ-ring currents, are observed to yield significant differences between the FC terms of (2)J(C2H3) and (2)J(C3H2) SSCCs which, consequently, are taken as probes to gauge the differences in σ-ring currents for the five-membered rings (furan, thiophene, selenophene and pyrrol) and also for the six-membered rings (benzene, pyridine, protonated pyridine and N-oxide pyridine) used in the present study.

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

“…In the context of this work, most important are more recent calculation results on the molecular fragments C−O−C−H and C−C−C(−O)−H . In turn, the 2 J CH values in a few heteroaromatic systems were subjected to a natural bond orbital (NBO) analysis . The NBO approach to n J CH values was also recently applied with the DU8c method, in which empirical scaling of the dominant FC terms in the formulated three multiterm predictive equations was performed by multivariate regression analysis.…”

“…Hence, the two combined ranges of J CH values mentioned above could be used in the search for such an improvement. 5) Our best results for J CH values were found with the DFs rankedr ecently by Medvedev et al [66] as yieldingt he best electron-density distributions (APF-D (2), PBE0 (4) and mPW1PW91 (12), where the numbers are ranking positions of the 128 considered DFs) or affording small errors in chemically relevant density differences (insights from the Fukuif unction); [67] unfortunately,m PW1PW91 was not analyzed in ref. [67].I nt urn, M06-L,f ound here to be inappropriatef or J CH predictions, was ranked among the three DFs yielding the worst densities [66] or larger errors in the Fukui function.…”

“…The analysis of such relatively large databases of J CH values collected exclusively in one NMR laboratory is noteworthy in itself. On the contrary, discrepancies in J CH values are observed frequently in measurements from different laboratories …”

Stereochemical Dependence of 3JCH Coupling Constants in 2-Substituted 4-t-Butyl-cyclohexanone and Their Alcohol Derivatives

“…This supports the assumption made above that a decrease in θ activates additional coupling pathways. By recalling that hyperconjugative interactions behave like carriers of the spin information associated to the FC interaction, , it is easy to observe the following additional coupling pathway σ C6–H6eq → π* C1O /σ C2–Cl2 → π* C1O ; see Table . This is similar to that of homoallylic 4 J CH SSCCs, and therefore, it should correspond to a negative contribution.…”

The Antagonist Effect of Nitrogen Lone Pair: 3JHFversus5JHF

“…However, distance and geometry are not the only parameters that have influence over the J values. It was previously shown that the presence of heteroatoms in aromatic and non‐aromatic rings has a great influence on 2 J CH values. Nowadays, the application of Computer‐Assisted Structure Elucidation (CASE), algorithms to determine the 3D structures, is increasing, and according to authors it is not subject to the interpretational biases of human investigators .…”