Intermolecular Complexes and Molecular Conformations Directed by Hydrogen Bonds: Matrix Isolation and Ab Initio Studies

Abstract

Studies on hydrogen bonding interaction in various systems,
involving phenylacetylene (PhAc), propargyl alcohol (PA), borazine
(BNH), propargyl amine (PAm) were performed using matrix isolation
infrared spectroscopy and supported by ab initio computations. Weak
intermolecular interactions of the above mentioned precursors with
water, methanol, ether, acetylene and benzene were studied. These
systems manifested O–H···π and n–*
interactions, such as C–H···O, N–H···O, O–H···O and O–H···N. In several cases the complexes were multiply tethered involving two or more of the above mentioned contacts.
Many of the weak complexes exhibited a number of isomers, and the
relative importance of the multiple non-covalent contacts resulted in a
competition between the various isomers for the global minimum. It was
found that subtle changes in the structures of the precursors tilted the
balance towards one isomer or the other. Our studies also threw up a
systematic method of building possible structures for complex systems
starting from the known structures of related simple systems. We also
studied the homodimers of PA and BNH. The BNH dimer was particularly
interesting as one of its isomers was characterized by a bis-dihydrogen
bond. We also studied the influence of hydrogen bonding interactions
in determining the conformational landscape and preference in amino
acids. Here again we were able to draw some generalizations regarding
the conformational stability of amino acids. The combination of matrix
isolation and ab initio computation is a powerful tool for studies on weak
intermolecular interactions and conformations.