Also Formula chemical features

As the sample was warmed past the melting point of the film (165 K), clear Al(2p) XPS features could be identified in the spectrum consistent with the sublimation of the THF film. Furthermore, the binding energies of the C(ls) and O(ls) peaks shifted uniformly by ca. 0.6 eV toward lower values, an effect attributed by the investigators to the charging of the rather insulating character of the thick THF film, which disappears as the material evaporates. Also noted was a decrease in the C/O integrated peak areas from 4.2, as predicted from the chemical formula of THF, to 3.3, which remained constant up to 470 K. Despite the... 

Whereas strained ring systems are usually reactive and often unstable, molecules which satisfy the criteria for aromaticity exhibit enhanced stability. As is evident from the structural formula of 1, the cycloproparenes set these features in juxtaposition as they are strained molecules in which a single carbon atom is fused across adjacent centres of an aromatic system. The interest of the experimentalist in strained molecules has been matched by the theoretician in the search for suitable models for developing the concepts of chemical bonding and aromaticity. The cycloproparenes have been particularly important in this regard as they meet the criterion for partial aromatic bond localization and consequent bond length alternation in the aromatic ring as proposed by Mills and Nixon in 1930, viz. la vs lb. The cation 5, anion 6 and radical 7 derived from 1, and also the ketone 8 and exocyclic methylene derivative 9, are of interest in this respect. 

Once the atomic coordinates have been determined, often before any refinement, it is usual to calculate interatomic distances to 3.5 or 4 A, in order to check the connectivity of atoms, that is, to determine which atoms are bonded to which. This will show whether the chemical formula is correct. The connectivity also demonstrates whether or not the experimental atomic positions are for atoms connected in one molecule, or for atoms which are in different molecules (related by space-group symmetry). Finally, if the connectivity calculation includes longer distances, it will show how the molecules or ions pack with respect to each other in the crystal. Information on the different intermolecular interactions, such as hydrogen bonds present in the crystal, are found in this way. If any intermolecular distance is substantially less than the expected value (see Table 11.4, for example), implying that molecules approach each other too closely, the reported crystal structure may not be correct. The derived set of relative phases (see Chapter 8) should be scrutinized, as there may be another more suitable set. Any truly unusual geometrical features in a structure determination should be carefully analyzed before being accepted as experimental evidence. 

So far, the occurrence of seven polycyclic musks has been reported. Their chemical names, chemical structures and molecular formulae are described elsewhere (Rimkus, 1999). The structural feature of all polycyclic musks is an indane or tetraline skeleton, which is highly substituted mainly by methyl groups (Fig. 1). 7-Acetyl-l,l,3,4,4,6-hexamethyl-l,2,3,4-tetrahydronaphthalene (AHTN, trade name Tonalide) and 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta[g]-2-benzo-pyrane (HHCB, trade name Galaxolide) are the most abundant ones. In this study, also 4-acetyl-1,1 -dimethyl-6-fert-butylindane (ADBI, trade name Celestolide) and 6-acetyl-l,l,2,3,3,5-hexamethylindane (AHMI, AHDI, trade name Phantolide) are investigated (Fig. 1). Important physicochemical properties of these compounds, which determine their environmental distribution and transport (e.g. solubility), are presented in chapter 4.1.1, in Ricking et al. (2003) as well as in Simonich et al. (2000). 

Several companies offer instruments capable of determining carbon, hydrogen, nitrogen and some other elements [72]. Such instruments feature an automatic sample injector. Samples are normally encased in sealed capsules. There is also a small computer which integrates peak areas and performs all the necessary calculations, which in some instances include the calculations of chemical formulae the computer also prints out the results. Finally, the new instruments feature advanced combustion and chromatographic separation systems. 

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