Density Functional Theory/GIAO Studies of the ¹³C, ¹⁵N, and ¹H NMR Chemical Shifts in Aminopyrimidines and Aminobenzenes:  Relationships to Electron Densities and Amine Group Orientations

Abstract

The dependence of the 13C, 15N, and 1H isotropic NMR chemical shifts on amine substitution of aromatic ring systems are examined both experimentally and by DFT/GIAO (density functional theory/gauge including atomic orbitals) methods. There are large, monotonic decreases in the chemical shifts at odd-numbered (ortho and para) pyrimidine ring positions which do not occur at the even-numbered (ipso and meta) atoms as amine groups progressively replace hydrogens at the latter positions. This behavior parallels the computed 2p z electron densities which for the pyrimidine series increase monotonically at N1, N3, and C5 but exhibit small changes at the C2, C4, and C6 positions. Identical trends are noted for the aminobenzenes. The ring atom chemical shifts and 2p z electron densities at ortho and para (but not meta) positions are quite sensitive to the orientations of the amine groups which are pyramidalized as the result of balance between delocalization with the ring and the use of strongly directed sp3 orbitals at the nitrogen. The calculated results show that the barriers to amine group torsional and inversion motions are low, but averaging the chemical shifts over these appears to be relatively unimportant. Differences between the DFT and Hartree−Fock-based chemical shifts show that electron correlation effects monotonically increase with the number of NH2 substituents.

Nuclear magnetic resonance spectroscopic studies of the trihexyl (tetradecyl) phosphonium chloride ionic liquid mixtures with water

“…This large chemical shift is due to the hydrogen bonding of the C-H proton to water molecules or Cl -. The significant shift of C-H chemical shifts to larger values is also in accordance with computational studies of NMR for imidazolium in the presence of interaction with water [22], where the chemical shift of the hydrogen atom in Position 2 of the imidazolium ring is shifted to high values through specific interactions with water molecules. Our water-cation-anion calculation result is similar to the NMR results of Mele et al [11], which suggests that the presence of small amounts of water could replace the C-H-anion interactions in imidazolium based ionic liquids.…”

“…The gauge including atomic orbitals (GIAO) method [21] is documented to provide acceptable chemical shifts for different nuclei [21][22][23] in relatively large molecules. The success of GIAO even extends to molecular systems that include hydrogen bonding interactions [24][25][26] and ionic liquids [27].…”

Synthesis and GIAO NMR Calculations for Some Novel 4-Heteroarylidenamino-4,5-dihydro-1H-1,2,4-triazol-5-one Derivatives: Comparison of Theoretical and Experimental 1Hand 13C- Chemical Shifts

“…The widely used methods to calculate chemical shifts are as follows: IGLO (individual gauge localized orbital), LORG (localized or Loacaorbital origin) and GIAO (gauge independent or invariant or including atomic orbital). The GIAO approach [14] is known to give satisfactory chemical shifts for different nuclei [14][15][16] with larger molecules, yet these quantum chemical calculations often have to be limited to isolated (gas-phase) molecules in some preferred (optimized) structures, while experimental NMR spectra are commonly statically averages affected by dynamic process such as conformational equilibria as well as intra and/or intramolecular interactions.…”

Analyses of Quantum Chemical Parameters, Fukui Functions, Magnetic Susceptibility, Hyperpolarizability, Frontier Molecular Orbitals, NBO, Vibrational and NMR Studies of 1(4-Aminophenyl) Ethanone

“…Density functional theory (DFT) shielding calculations are rapid and applicable to large systems. The ''gauge independent atomic orbital'' (GIAO) method [40][41][42][43] has proven to be quite accepted and accurate. To provide an explicit assignment and analysis of 13 C and 1 H NMR spectra, theoretical calculations on chemical shift of the title compound are carried out by GIAO method at B3LYP/ cc-pVDZ and ccpVTZ level [44] …”