On the long‐range relativistic effects in the ¹⁵N NMR chemical shifts of halogenated azines

Abstract

Long-range β- and γ-relativistic effects of halogens in N NMR chemical shifts of 20 halogenated azines (pyridines, pyrimidines, pyrazines, and 1,3,5-triazines) are shown to be unessential for fluoro-, chloro-, and bromo-derivatives (1-2 ppm in average). However, for iodocontaining compounds, β- and γ-relativistic effects are important contributors to the accuracy of the N calculation. Taking into account long-range relativistic effects slightly improves the agreement of calculation with experiment. Thus, mean average errors (MAE) of N NMR chemical shifts of the title compounds calculated at the non-relativistic and full 4-component relativistic levels in gas phase are accordingly 7.8 and 5.5 ppm for the range of about 150 ppm. Taking into account solvent effects within the polarizable continuum model scheme marginally improves agreement of computational results with experiment decreasing MAEs from 7.8 to 7.4 ppm and from 5.5 to 5.3 ppm at the non-relativistic and relativistic levels, respectively. The best result (MAE: 5.3 ppm) is achieved at the 4-component relativistic level using Keal and Tozer's KT3 functional used in combination with Dyall's relativistic basis set dyall.av3z with taking into account solvent effects within the polarizable continuum solvation model. The long-range relativistic effects play a major role (of up to dozen of parts per million) in N NMR chemical shifts of halogenated nitrogen-containing heterocycles, which is especially crucial for iodine derivatives. This effect should apparently be taken into account for practical purposes.

Calculation of 15N NMR Chemical Shifts in a Diversity of Nitrogen-Containing Compounds Using Composite Method Approximation at the DFT, MP2, and CCSD Levels

“…Indeed, calculation of 15 N NMR chemical shifts provides a powerful tool in the structural elucidation of nitrogen-containing organic and biological molecules and gives a deeper insight into vitally important biochemical phenomena such as self-association, molecular recognition, and base-pairing. In continuation of our previous 15 N NMR computational studies, − recently reviewed by one of the authors, in this article, we performed a high-level computational study of 15 N NMR chemical shifts at the density functional theory (DFT), second-order Møller–Plesset perturbation theory (MP2), and coupled cluster singles and doubles (CCSD) levels in a broad series of 93 nitrogen-containing compounds representing about 50 different types, namely, amines ( 1 , 2 ), hydrazines ( 3 , 4 ), imines ( 5 , 6 ), hydrazones ( 7 , 8 ), guanidines ( 9 , 10 ), diazirines ( 11 ), azo compounds ( 12 ), carbodiimides ( 13 , 14 ), triazenes ( 15 , 16 ), nitriles ( 17 , 18 ), cyanamides ( 19 , 20 ), diazo compounds ( 21 ), azides ( 22–24 ), isonitriles ( 25 , 26 ), hydroxylamines ( 27 , 28 ), amides ( 29 , 30 ), oximes ( 31 , 32 ), ureates ( 33 , 34 ), amino acids ( 35 , 36 ), carbamates ( 37 , 38 ), lactams ( 39 , 40 ), nitroso compounds ( 41 ), nitrites ( 42 , 43 ), isocyanates ( 44 , 45 ), nitrosamines ( 46 ), esters of cyanic acid ( 47 , 48 ), nitrones ( 49 , 50 ), nitroalkanes ( 51 , 52 ), nitramines ( 53 , 54 ), nitrates ( 55 , 56 ), thioamides ( 57 , 58 ), isothiocyanates ( 59 , 60 ), sulfinylamines ( 61 , 62 ), thiocyanides ( 63 , 64 ), sulphonamides ( 65 , 66 ), pyrroles ( 67 , 68 ), pirazoles ( 69 , 70 ), imidazoles ( 71 , 72 ), triazoles ( 73 , 74 ), tetrazoles ( 75 , 76 ), oxazoles ( 77 , 78 ), furoxanes ( 79 , 80 ), thiazoles ( 81 , 82 ), heterocyclic azines ( 83 , 84 ), azinoxides ( 85 , 86 ), diazines ( 87 , 88 ), triazines ( 89 , 90 ), and azoloazines (indolizines) ( 91–93 ); see Scheme for chemical structures.…”

DFT computational schemes for 15N NMR chemical shifts of the condensed nitrogen‐containing heterocycles

“…In particular, due to the recent breakthrough in this area, much interest has been focused on the calculation of 15 N NMR chemical shifts; see comprehensive review on this topic. In the present communication, in line with our recent interest in structural applications of computational 15 N NMR, we performed a thorough DFT study of 15 N NMR chemical shifts in a wide series of the condensed nitrogen‐containing heterocycles. In this communication, on one hand, much effort has been traditionally focused on the accuracy factors (functionals and basis sets, locally dense basis set (LDBS) schemes, and solvent effects) for this particular class of compounds with experimentally measured 15 N NMR chemical shifts.…”

GIAO‐DFT calculation of 15N NMR chemical shifts of Schiff bases: Accuracy factors and protonation effects

“…An interest in 15 N NMR chemical shifts stemmed mainly from the early experimental papers by Witanowski and coworkers (for the most comprehensive compilations, see reviews), in particular, those dealing with protonation effects. In this connection, in continuation of our earlier and most recent interest in the calculation of nitrogen chemical shifts of diverse open‐chain and heterocyclic nitrogen‐containing compounds, in this communication, we report on the calculation of 15 N NMR chemical shifts in the representative series of Schiff bases—a specific group of imines with a functional group that contains a carbon–nitrogen double bond with the nitrogen atom connected to an alkyl or aryl group and having a general formula R 1 R 2 C═NR 3 , where R 3 = Alk or Ar (but not H).…”