Intermolecular interactions in hydrogen bonded 1:1 molecular complexes. A matrix isolation study.

A number of matrix isolated binary hydrogen bonded complexes with water as one of the components have been investigated with FTIR in the mid and far infrared regions. Water has been found to be the hydrogen bond donor in complexes with water, ammonia, dimethyl ether and acetone. With formamide two equally strong hydrogen bonds are formed one from water to the carbonyl oxygen and one from an amide hydrogen to the water oxygen. In the formic acid water complex the strongest bond is from formic acid to water but there is also a second bond where the water molecule acts as the proton donor. HDO prefers as usual to be D-bonded in the studied complexes. A model developed in cooperation with the Theoretical Chemistry Department has been used to make ab initio calculations on the observed complexes. The six intermolecular fundamentals have been characterised with this model. It has been found that it is possible to make a quite simple description of the vibrations that holds for all complexes where water is one of the complex partners. Even for the formamide water complex with its cyclic structure the description is valid, but for the formic acid water complex the description fails. In this complex all the intermolecular vibrations, except for the hydrogen bond stretch that is quite pure, are heavily mixed. A new method for infrared laser irradiation has been tested on water dimers and on the formaldehyde water complex. With this tool it was possible to see an intermolecular fundamental of the water dimer. (Less)