Natural bond orbitals analysis of C-H ··· O interactions in NCH/H₂O and NCH/OCH₂, and their effect on nuclear magnetic shielding constants

C-H ··· O interactions and their effect on nuclear magnetic shielding constants for differently coordinated oxygen atoms in NCH/H₂O, I, and NCH/O = CH₂, II, complexes are analyzed for different intermolecular distances. The effects of electrostatic and charge transfer interactions on the C-H bond lengths and on the nuclear magnetic shielding constants of the nuclei directly involved in the C-H ··· O hydrogen bond, are discussed. Charge transfer effects are analyzed employing Natural Bond Orbitals. The intermolecular n → (C-H)* charge transfer interaction causes a slight lengthening of the corresponding C-H bond. The intermolecular charge transfer interaction in II yields an inhibition of the intramolecular n(p) → (C-H)* charge transfer in O = CH₂. A conspicuous difference between the electrostatic and total effects on the ¹⁷O magnetic shielding constant upon formation of the complex is found in I. Experimental data from the literature, supporting the obtained results, are discussed.