Weak intermolecular interactions in the gas phase

分子的形状和大小与其性质密切相关；各种各样的科学家都利用分子结构的信息来解释他们的发现并指导新的实验。我们可以通过实验测量来发现结构，也可以使用一些基本常数和强大的计算机从头开始计算。这两种方式都很重要，但都不是完美的。我们两个都需要。我们将使用气体电子衍射（GED）来确定分子的结构。在传统的GED实验中，样品在室温或更高的温度下进入气相。相比之下，这一提议涉及的分子将非常冷，在绝对零度几度以内，同时保持在气相。我们将用大量的载气，氦气或氩气来稀释样品，并在高压下将其膨胀到真空中。当分子冷却时，会发生两件事。首先，原子的相对运动减少，对于非常柔韧的分子来说尤其如此。其次，一些分子可能成对地粘在一起，形成二聚体（或更大的簇），它们有自己独特的结构。所需要的设备是完整的，有一个复杂的真空系统，高强度远焦电子枪，和CCD探测器，用于实时记录衍射图样。我们需要为高温下的分子建立一个入口系统，以及冷阱，以便在化合物使用后收集它们。然后，我们将使用该设备来观察几种几乎没有实验信息的物种。首先，我们将看看非常灵活的分子，如一氧化碳（O=C=C=C= C=O），它在中间弯曲很大，S(PF2)2，最简单的分子，有两个非常大的扭曲运动。这些将被冷却，以尽量减少这些振动的程度。然后我们将研究二聚体（两个相同的分子连接在一起）和加合物（连接两个不同的分子）。我们从羧酸开始，有两个O-H。二聚体中有O个氢键。然后我们将比较其他二聚体，包括吡唑二聚体与N-H…N链接;然后是2-羟基吡啶，计算表明它会在二聚体中变成它的pyrid-2- 1形式，重新排列双键和单键，并将氢键中的氢从氧转移到氮(OH…N到O…H-N)。这种转变在许多反应中，特别是在生物系统中是极其重要的，并且在模型系统中进行实验研究是非常困难的。这些二聚体是由两个分子组成的，每个分子都有一个供基和一个接受基。我们将研究冷气体，包括一种只能捐赠的分子和一种只能接受的分子。它们应该结合形成加合物。如果每个分子上有两个给体基团或两个受体基团，加合物就会更牢固地结合在一起。最后，我们将研究手性分子的二聚体，即左手型和右手型的二聚体。用你的两个拇指和两个食指碰在一起；你的手掌在你做的正方形的同一侧。两个人的右手也这样做，手掌在正方形的两边，一个在上面，一个在下面。同样，手性分子的加合物也会有不同的结构，这取决于分子是左旋和右旋的混合物，还是完全相同。因此，我们将发现分子是如何识别彼此的。这些信息将有助于解释化合物的性质和反应，并将被化学家、学术界和工业界使用，特别是那些研究分子之间的联系影响其性质的材料的化学家。实验数据对于开发从零开始计算化学性质的方法至关重要，因为不寻常分子的精确结构有助于确定此类研究的目标。

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.