Supramolecular silanol chemistry in the gas phase.: Topological (AIM) and population (NBO) analyses of hydrogen-bonded complexes between H3SiOH and selected O- and N-acceptor molecules

Hydrogen bonding of the type SiO-H center dot center dot center dot A (A = O, N) has been studied in the gas phase for simple H3SiOH center dot acceptor complexes with the acceptor molecules being O(H)SiH3, OH2, O(H)CH3, O(CH3)(2), O(CH3)SiH3, O(SiH3)(2), NH3, N(CH3)H-2, N(CH3)(2)H, N(CH3)(3), N(CH3)(2)C6H5, and NC5H5, respectively, at the B3LYP/6-311+(2d,p) level of theory, using Bader's atoms in molecules (AIM) and Weinhold's natural bond orbital (NBO) methodology. For all complexes (except H3SiOH center dot N(CH3)(2)C6H5) the complex energy E-add. is a good estimate for the hydrogen bond energy E-HB, which is generally higher in N-acceptor complexes (-5.52 to -7.17 kcal mol(-1)) than in O-acceptor complexes (-2.09 to -5.06 kcal mol(-1)). In case of H3SiOH center dot N(CH3)(2)C6H5, E-HB and E-add. differ by the energy associated with the loss of n(N)->pi conjugation in N(CH3)(2)C6H5 upon complex formation. E-HB shows no correlation with O center dot center dot center dot A distances and the red shifts Delta nu(OH) of the OH-stretching vibrations when different acceptors are compared, although both parameters are commonly used to estimate the strength of the hydrogen bond from spectroscopic and diffraction data. A good linear correlation of the hydrogen bond energy E-HB has been established with parameters derived from the AIM and NBO analyses, namely, the electron densities rho(HA) and rho(OH) at the H center dot center dot center dot A and O-H bond critical points (BCPs) and the NLMO bond orders BONLMO(HA) of the H center dot center dot center dot A bonds of the H3SiOH center dot acceptor complexes as well as the change of natural charges Delta q(NPA)(O) at the O-donor atom upon H3SiOH center dot acceptor complex formation. Hydrogen bonding of the type SiO-H center dot center dot center dot A (A = O, N) has been also studied in the related cyclic multiple H3SiOH center dot acceptor complexes (H3SiOH)(3), (H3SiOH)(2)center dot NC5H5, and (H3SiOH)(4), respectively, at the same level of theory. Cooperative hydrogen bonding is evident for all cyclic multiple H3SiOH center dot acceptor complexes, whereby the strongest concomitant strengthening of the hydrogen bonds is observed for (H3SiOH)(4) and (H3SiOH)(2)center dot NC5H5.