Infrared matrix isolation studies of molecular interactions: Complexes of alkynes, alkenes, haloalkanes, and trichlorosilane with selected bases

The matrix isolation technique combined with infrared spectroscopy has been employed to isolate and characterize hydrogen-bonded complexes. A series of molecules containing an acidic C-H group, as proton donors, were codeposited with lone pair electron donors containing N, P, As, O, S, Se atoms and halide anions into argon matrix. Hydrogen bond formation was evidenced by a distinct red shift of C-H stretching mode, ranging from 10 to 550 cm{dollar}sp{lcub}-1{rcub}{dollar}. Perturbation of C-H bending modes was also observed as well as the C{dollar}equiv{dollar}C stretch of the alkynes and/or the carbon-halogen stretching and bending modes of the halogen compounds. For a given base, the observed shifts were in the order alkyne {dollar}>{dollar} alkene {dollar}>{dollar} alkane. For a given acid, the magnitude of shifts, and hence the strength of interaction, correlated well with the proton affinities of bases and the Hammett substituent constants for the alkyne substitutents. For some systems, the trend of magnitude of shifts was counter to the proton affinity of the bases and was rationalized in terms of Hard/Soft Acid/Base theory. When halide anions were codeposited with alkyne containing electron withdrawing groups, such as CF{dollar}sb3{dollar}CCH and CH{dollar}sb2{dollar}ClCCH, complexes involving nucleophilic attack rather than hydrogen bonding were observed, although for the CsF/CF3CCH system evidence was obtained for both types of complexes. Acetylene was also observed to be a proton acceptor in the complexes with H{dollar}sb2{dollar}S and AsH{dollar}sb3{dollar}.; Trichlorosilane was codeposited with amines and led to coordination complexes in which the donor atom of the base interacted with the silicon center on HSiCl{dollar}sb3{dollar}. The complexes were characterized by a blue shift of the Si-H stretch and red shifts of the Cl-Si-H bending and Si-Cl stretching modes. The Si-N stretching mode in the complex was identified near 700 cm{dollar}sp{lcub}-1{rcub}{dollar} for the amine complexes. Also, a red-shifted and broadened Si-H stretching mode was detected, providing tentative evidence for hydrogen-bonded complex formation.