On the HALA effect in the NMR carbon shielding constants of the compounds containing heavy p-elements

摘要

The heavy atom (HA) effect on the NMR isotropic carbon shielding constants is computationally investigated in the series of model ethanes, ethylenes, and acetylenes, CH3CH2XHn, CH2CHXHn, CHCXHn (n=0, 1, 2, or 3 depending on X), where X covers p-elements in the 13-17 groups of the 3-6 periods in as many as 60 compounds. Compounds under study provide diverse bonding situations for the - and -carbons, which are characterized by the consecutive increase of the s-character of the CC and CX bonds, being one of the factors influencing spin-orbit part of the HA on light atom effect (SO-HALA). The chalcogen dependence, pnictogen dependence, tetrel dependence, and triel dependence are established for the 16th, 15th, 14th, and 13th groups, respectively. A well-known normal halogen dependence for the C-13 NMR chemical shifts, established much earlier for the compounds containing 17th group elements, also revealed itself in all three series under investigation. The dependence of the spin-orbit effects size depending on the number of the lone electron pairs (LEPs) on HA X has also been investigated. The comparison of theoretical C-13 NMR chemical shifts with experiment is performed for three representative tellurides. The HALA effect in this series has been shown to be strongly dependent on the number of tellurium LEPs.