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

• Published in: Magnetic resonance in chemistry 2017-11, Vol.55 (11), p.990-995
• Main Authors: Samultsev, Dmitry O., Rusakov, Yury Yu, Krivdin, Leonid B.
• Format: Article
• Language: English
• Subjects: 15N NMR; Azines; chemical shift; Computational chemistry; Continuum modeling; Derivatives; GIAO‐DFT; halogenated azines; Halogenation; Halogens; Iodine; magnetic shielding constant; Mathematical models; NMR; Nuclear magnetic resonance; Pyrazines; Pyridines; Pyrimidines; Relativism; Relativistic effects; Solvation
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.4618

Long‐range β‐ and γ‐relativistic effects of halogens in 15N 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 15N calculation. Taking into account long‐range relativistic effects slightly improves the agreement of calculation with experiment. Thus, mean average errors (MAE) of 15N 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 15N 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. Long‐range β‐ and γ‐relativistic effects of halogens in 15N NMR chemical shifts of 20 halogenated azines (pyridines, pyrimidines, pyrazines, and 1,3,5‐triazines) are evaluated and incorporated into proposed computational scheme. The best result (mean absolute error: 5.3 ppm for the range of 150 ppm) is achieved at the 4‐component relativistic level using Keal and Tozer's KT3 functional used in combination with Dyall's dyall.av3z relativistic basis set with taking into account solvent effects within the polarizable continuum model solvation model.