Superimposable molecules

Figure 8. Model of the dopamine receptor, with the (- -)-isobutactamol molecule superimposed, generated by the MMS-X computer-based molecular modeling and graphic display system. (Courtesy of Dr. G. Marshall, Washington University, St. Louis).

It is easy to tell whether two molecules are enantiomers (nonsuperimposable) or identical molecules (superimposable) if you have molecular models of the molecules— just see whether the models superimpose. If, however, you are working with structures on a two-dimensional piece of paper, the easiest way to determine whether two molecules are enantiomers or identical molecules is by determining their configurations. If one has the R configuration and the other has the S configuration, they are enantiomers. If they both have the R configuration or both have the S configuration, they are identical molecules. 

The spectra consist of a series of sharp lines of the excited vibrational modes of the adsorbed molecules superimposed on a broad, enhanced background. Ethylene has been used to study the formation of intermediates on catalytic surfaces. Ethylene is chemisorbed dissociatively as acetylene at room temperature. This is revealed by the appearance of the C=C stretching vibration at 1204 cm and was confirmed by inelastic electron loss spectroscopy applied to acetylene chemisorbed on Ni(lll) surfaces. The strongest line in the spectmm of benzene chemisorbed at room temperature is the totally symmetric ring-breathing mode at 990 cm . All molecules with ring systems exhibit this characteristic band, it is the most strongly enhanced mode. 

Figure 3 Angular variation of electron densities calculated with Ligfield and rendered in POV-ray 3.0. The ( -electron densities (a) of the ground state of an octahedral (or system, and (b) of the ground state for the all-horizontal configuration of [Ti(H20)6] with the ligating water molecules superimposed (Z is...

The structure of the 4-methyl benzoic acid molecule superimposed on its electron density map... 

It is this dispersion effect which explains the general additive cohesion between two particles. The orientation effect and the induction effect require an alignment of permanent dipoles along the vector tij joining the interacting molecules. The orientation effect is not necessarily additive between three molecules and in many cases repulsion of the third molecule rather than attraction is to be expected. Similarly, the induction effect is greatly reduced if many molecules superimpose their... 

Fig. 1. The first crystalline molecule superimposed with the mirrored second one. The root mean square value between the two geometries is 0.04 A.

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