Ab initio calculation of isotope effects in IR and NMR spectroscopy of H-bonded systems

Abstract

Abstract In this report recent computational developments are reviewed rather than novel H-bonded systems being presented. The well examined prototypes for neutral and ionic H-bonds, (H2O)2 and [H5O2]+, respectively, have been investigated anew by means of the MP2/6-311G(2df,p) level of ab initio theory, employing the program gaussian 92 . The complete sets of harmonic vibrational frequencies and the corresponding absolute IR intensities are presented. Most surprisingly, the main spectroscopic features of H-bonds, frequency decreases and IR intensity increases, can be understood quite well without consideration of anharmonicity at this level of theory. Frequency shifts and IR intensity decreases upon deuteration are also presented and discussed. Thermochemical relations are applied to obtain the enthalpies of formation of H-bonded complexes. NMR chemical shifts for 1H and 17O in the above H-bonded systems have been calculated ab initio by means of the Individual Gauche for Localized Orbitals (IGLO) method. The compositions of the chemical shift parameters are discussed in terms of dia- and paramagnetic contributions.