Experimental Determinations of Molecular Structures

Another factor that affects bond length is electronegativity. Bonds tend to be shortened, relative to the expectations for nonpolar bonds, in proportion to the electronegativity difference of the component atoms. Thus the experimental bond length in HF is 91.8 pm versus an expected value of 108 pm. The quantitative shortening of bonds because of electronegativity differences and multiple bonding in elements other than carbon will be discussed in Chapter 8. 

X-ray diffraction (Chapter 3) has provided more structural information for the inorganic chemist than any other technique. It allows the precise measurement of bond angles and bond (engths. Unfortunately, in the past it was a time-consuming and difficult process, and molecular structures were solved only when there was reason to believe they would be worth the considerable effort involved. The advent of more efficient methods of gathering data and doing the computations has made it relatively easy to solve most structures. 

43 Drago, R. S. Physical Methods for Chemists, 2nd ed. Saunders Philadelphia, 1992. Ebsworth, E. A. V. Rankin, D. W. H. Cradock. S. Structural Methods in Inorganic Chemistry, 2nd ed. Blackwell Oxford, 1991. 

46 ORTEP is an acronym for Oak Ridge Thermal Ellipsoid Program, a computer program frequently used in structural analysis. The acronym is often used as a short label to indicate a drawing in which ellipsoids indicate the extern of thermal motions of Ihe atoms. 

Neutron diffraction is very similar in principle to X-ray diffraction. However, it differs in two important characteristics (I) Since neutrons are diffracted by the nuclei (rather than the electrons), one indeed locates the nuclei directly. (2) Furthermore, the hydrogen nucleus is a good scalterer, thus the hydrogen atoms can be located easily and precisely. The chief drawback of neutron diffraction is that one must have a source of neutrons, and so the method is expensive and not readily available. X-ray diffraction and neutron diffraction may be used to complement each other to obtain extremely useful results (cf. Fig. 12.24). 

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The question of molecular structure and shape is considered in the next chapter. It will be shown that the familiar molecular structure is a function of chemical history and the thermodynamic environment. It is emphasized, in particular, that experimental determination of molecular structure is strictly confined to the solid crystalline state. 

Information obtainable from experimental determination of molecular structure in other phases than those mentioned above is severely limited for two reasons the scarcity of available means except for the NMR technique, and the complications arising from heterogeneous environments. Recent intensive interest in the structure of molecules forming monolayers or other types of aggregates absorbed on the surface spur exploration of new methods for observing surface structures such as scanning tunneling microscope (STM). Dynamic structural aspects obtainable from these studies are, however, outside of this review. ... 

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