Formation of semi-covalent bond in [(N(2)O)(n)H(2)O](+) (n = 2-7) cluster ions studied by IR spectroscopy

J Phys Chem A. 2010 Oct 28;114(42):11037-42. doi: 10.1021/jp100336h.


IR photodissociation (IRPD) spectra of [(N(2)O)(n)H(2)O](+) with n = 2-7 are measured in the 1100-3800 cm(-1) region. The IRPD spectra show the ν(1) and ν(3) vibrations of the N(2)O components at around 1250 and 2200 cm(-1) and the OH stretching vibrations of the H(2)O part in the 2400-3800 cm(-1) region. In the OH stretching region, the IRPD spectrum of the [(N(2)O)(2)H(2)O](+) ion shows a sharp band at 3452 cm(-1) and a broad one at ∼2700 cm(-1), which are assignable to the stretching vibrations of the free and hydrogen-bonded OH groups, respectively. The IRPD spectrum of the [(N(2)O)(3)H(2)O](+) ion displays no band of the free OH stretching vibration; the solvent N(2)O molecules are preferentially hydrogen-bonded to the OH groups. In parallel, the geometry optimization and the vibrational analysis are carried out at the B3LYP/6-311++G(d,p) level of theory. Comparison of the IRPD spectra with the calculated IR spectra suggests that the [(N(2)O)(n)H(2)O](+) cluster ions have an (N(2)O·H(2)O)(+) ion core, in which the positive charge is delocalized over the H(2)O and N(2)O components and that an intermolecular semicovalent bond is formed between the oxygen atoms of H(2)O and N(2)O through the charge resonance interaction. In the clusters larger than n = 3, two solvent N(2)O molecules are strongly hydrogen-bonded to the OH groups, and the other ones are weakly bound to the ion core. The band position of the ν(1) vibration of the solvent N(2)O molecules suggests that the oxygen end of the solvent molecules is bonded to the ion core.