Red and blue shifted hydridic bonds

By performing MP2/aug-cc-pVTZ ab initio calculations for a large set of dimer systems possessing a R-H hydridic bond involved in diverse types of intermolecular interactions (dihydrogen bonds, hydride halogen bonds, hydride hydrogen bonds, and charge-assisted hydride hydrogen bonds), we show that this is rather an elongation than a shortening that a hydride bond undergoes on interaction. Contrary to what might have been expected on the basis of studies in uniform electric field, this elongation is accompanied by a blue instead of red shift of the R-H stretching vibration frequency. We propose that the "additional" elongation of the R-H hydridic bond results from the significant charge outflow from the sigma bonding orbital of R-H that weakens this bond. The more standard red shift obtained for stronger complexes is explained by means of the Hermansson's formula and the particularly strong electric field produced by the H-acceptor molecule.