Calculation of 15 N NMR chemical shifts: Recent advances and perspectives

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

“…Many chemical transformations and rearrangements of diverse imines (in particular, oximes) utilized in the synthesis of azoles and larger nitrogen‐containing heterocycles (see classical paper by Trofimov and Mikhaleva together with some more recent contributions from this team) occur stereoselectively, which makes the problem of stereoelectronic effects at the C═N bond to be of crucial importance. In this connection, our earlier and later experimental and theoretical studies of imines received a new impact, especially in line with the recent breakthrough in the NMR‐oriented theory and computation including, in particular, general computational approaches for 15 N NMR chemical shifts …”

“…In this connection, our earlier [20,21] and later [7,[22][23][24] experimental and theoretical studies of imines received a new impact, especially in line with the recent breakthrough in the NMR-oriented theory and computation [25][26][27][28][29][30][31][32][33][34][35][36][37] including, in particular, general computational approaches for 15 N NMR chemical shifts. [38,39] 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 [40][41][42][43][44] ), in particular, those dealing with protonation effects. In this connection, in continuation of our earlier [45][46][47][48] and most recent [49][50][51][52] 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).…”

Substitution effects in the 15N NMR chemical shifts of heterocyclic azines evaluated at the GIAO‐DFT level

“…For more details on the functionals, basis sets, and solvation models used for the computation of 15 N NMR chemical shifts, see our recent review. [6] Calculated 15 N absolute shielding constants were converted into 15 N NMR chemical shifts scale as recommended by International Union of Pure and Applied Chemistry (IUPAC). [35] For a nitromethane standard, we used the values of −139.2 ppm for IEF-PCM model and −140.9 ppm for supermolecules in IEF-PCM media calculated at the KT3/pcS-3 level for the optimized cluster of the mostly computationally achievable five molecules of nitromethane in the IEF-PCM continuum.…”

“…In line with the recent breakthrough in the NMR-oriented theory and computation, [1][2][3][4][5] much interest has been focused recently on the calculation of 15 N NMR chemical shifts in the chemical diversity of the nitrogen-containing compounds including classical neutral and protonated nitrogen heterocycles, open-chain nitrogen-containing aliphatic compounds, coordination complexes with nitrogen-containing ligands, intermolecular complexes including those with nitrogen hydrogen bonding, and, obviously to a lesser extent, biological species including nucleotides, nucleosides, peptides, and even small proteins, as reviewed very recently by Krivdin. [6] For the more specialized analysis of the calculation of 15 N NMR chemical shifts in biological molecules, see most substantial reviews by Lodewyk et al [7] and, on the other hand, by Mulder and Filatov. [8] Indeed, computation of 15 N NMR chemical shifts provides a powerful tool in the structural elucidation of the 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 earlier [9][10][11][12] and most recent [13][14][15][16] interest in the calculation of nitrogen chemical shifts, in this communication, we have performed calculation of 15 N NMR chemical shifts in the representative series of 72 monosubstituted heterocyclic azines (see Scheme 1) including pyridines (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), pyrimidines (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24), pyrazines (25-36), 1,3,5-triazines (37-48), 1,2,4,5-tetrazines (49-60), and pentazines (61-72) with a classical series of substituents traditionally used in the studies of substitution effects, X = H, CH 3 , F, Cl, Br, NH 2 , OCH 3 , SCH 3 , COCH 3 , CONH 2 , COOH, and CN. Among those, are classical σ-donors (CH 3 ), σ-acceptors (F, Cl, and Br), π-donors (NH 2 , OCH 3 , and SCH 3 ), and π-acceptors (COCH 3 , CONH 2 , COOH, and CN), as compared with the reference hydrogen (H).…”