NH₂ and NO₂ group effects on ¹³C NMR chemical shifts in substituted benzenes are explained by σ- instead of π-orbitals.

X (F, Cl, Br, I) and R (NH₂, NO₂) group effects on ¹³C NMR chemical shifts are explained by π and σ orbitals, respectively.

Intramolecular hydrogen bonding (IAHB) is one of the most important intramolecular interactions and a critical element in adopted molecular arrangements. Moreover, slight substitution in a molecule can affect its strength to a great extent. It is well established that alkyl groups play a positive role in IAHB strength. However, the effects that drive it are specific to each system. To investigate the influence of IAHB and its strength dependency on different acyclic compounds, the conformational preferences of propane-1,3-diol, 3-methoxypropan-1-ol, 3-ethoxypropan-1-ol, 3-isopropoxypropan-1-ol, 3-(tert-butoxy)propan-1-ol, butane-1,3-diol, 3-methoxybutan-1-ol, 3-methylbutane-1-diol, and 3-methoxy-3-methylbutan-1-ol were evaluated experimentally using infrared spectroscopy theoretically supported by topological and natural bond orbital analyses. The most stable conformation of each compound exhibited IAHB and these conformers are more populated in the equilibrium for all studied compounds. Experimental infrared and topological data suggest that the strength of IAHB increases for each methyl group addition. NBO analyses indicate that methyl groups in different positions related to an OH moiety affect the charge transfer energy involved in intramolecular hydrogen bonding. This occurs mostly due to an increase in the spⁿ-hybridized lone pair (LP₁O) contribution to the charge transfer , which is a result of changes in s-character and orbital energy caused by geometrical rearrangements, rehybridization, and/or electronic effects.

In this study, stereoelectronic interactions were considered to explain the experimental difference in the magnitude of the known heavy-atom effect on the ¹³C NMR chemical shifts in cis- and trans-1,2-dihaloethene isomers (halo = F, Cl, Br or I). The experimental values were compared to the calculated values with various DFT functionals using both the nonrelativistic approach (NR) and the relativistic approximations SR-ZORA (SR) and SO-ZORA (SO). NBO and NLMO contributions to the ¹³C NMR shielding tensors were determined to assess which stereoelectronic interactions have a more important effect on the shielding tensor in each principal axis system (PAS) coordinate. These analyses associated with the orbital rotation model and the HOMO–LUMO energy gap enable rationalization of trends between cis and trans isomers from fluorine to iodine derivatives. Both paramagnetic and SO shielding terms were responsible for the observed trends. It was possible to conclude that the steric interactions between the two iodine atoms and the hyperconjugative interactions involving the halogen lone pairs (LP(X)) and π_CC*, σ_CC* and σ_C–X* antibonding orbitals are responsible for the lower ¹³C NMR shielding for the cis isomers of the bromine and the iodine compounds than that of the trans isomers.