Weak intermolecular interactions in the gas phase

The shapes and sizes of molecules are closely related to their properties; scientists of all kinds use information about molecular structure to interpret their findings and guide new experiments. We can find out about structures either by measuring them experimentally, or by calculating them from scratch (ab initio), using a few fundamental constants and powerful computers. Both ways are important, but neither is perfect. We need both of them.We will use gas electron diffraction (GED) to determine structures of molecules. In conventional GED experiments the sample is at room temperature or hotter to get it into the gas phase. In contrast, this proposal is concerned with molecules that will be very cold, to within a few degrees of absolute zero, while remaining in the gas phase. We will do this by diluting the sample with a lot of a carrier gas, helium or argon, and expanding it into a vacuum from a high pressure.As the molecules cool, two things happen. First, the relative movements of the atoms decrease, dramatically so for very flexible molecules. Secondly, some molecules may stick to one another in pairs to give dimers (or larger clusters), with their own distinctive structures.The apparatus required is complete, with a sophisticated vacuum system, high-intensity telefocus electron gun, and CCD detector for recording diffraction patterns in real time. We need to build an inlet system for molecules at high temperatures, and also cold traps, to collect compounds after they have been used. Then we will use the equipment to look at several types of species, about which there is almost no experimental information. (a) First we will look at very flexible molecules, such as carbon suboxide (O=C=C=C=O), which bends enormously in the middle, and S(PF2)2, the simplest molecule available with two very large twisting motions. These will be cooled to minimise the extent of these vibrations.(b) Then we will study dimers (two identical molecules joined together) and adducts (linking two different molecules). We will start with carboxylic acids, with two O-H...O hydrogen bonds in the dimer. Then we will compare other dimers, including that of pyrazole, with N-H...N links; then 2-hydroxypyridine, which calculations suggest will change to its pyrid-2-one form in the dimer, rearranging double and single bonds and shifting hydrogen in the hydrogen bond from oxygen to nitrogen (OH...N to O...H-N). Such shifts are extremely important in many reactions, particularly in biological systems, and it is very difficult to study them in model systems experimentally.These dimers are formed from two molecules, each with one donating and one accepting group. We will study cold gases including one molecule that can only donate, and one that can only accept. They should join to form adducts. If there are two donor or two acceptor groups on each molecule, the adducts should hold together much more strongly. Finally, we will look at dimers of chiral molecules - those with left- and right-handed forms. Touch your two thumbs together and your two forefingers; the palms of your hands are on the same side of the square that you make. Do the same with the right hands of two people, and the palms are on opposite sides of the square, one above and one below. In the same way adducts of chiral molecules will have different structures, depending on whether the molecules are a mixture of left-and right-handed forms, or all the same. We will thus find out about how molecules can recognise one another. The information will help to explain the properties and reactions of the compounds, and will be used by chemists, academic and industrial, especially those studying materials in which links between molecules affect their properties. Experimental data will be of vital importance to people developing ways of computing chemical properties from scratch, as accurate structures of unusual molecules help to define the goals for such research.