Natural bond orbitals analysis of C-H center dot center dot center dot 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 const ants for differently coordinated oxygen atoms in NCH/H2O, I, and NCH/O=CH2, II, complexes are analyzed for different intermolecular distances. The eff ects of electrostatic and charge transfer interactions on the C-H bond leng ths and on the nuclear magnetic shielding constants of the nuclei directly involved in the C-H ... O hydrogen bond, are discussed. Charge transfer eff ects are analyzed employing Natural Bond Orbitals. The intermolecular n --> (C-H)* charge transfer interaction causes a slight lengthening of the corr esponding C-H bond. The intermolecular charge transfer interaction in II yi elds an inhibition of the intramolecular n(p) --> (C-H)* charge transfer in O=CH2. A conspicuous difference between the electrostatic and total effect s on the O-17 magnetic shielding constant upon formation of the complex is found in I. Experimental data from the literature, supporting the obtained results, are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.