Structure and molecular dimensions

The hitherto unknown 3-thioallophanate anion has been trapped in the host lattice of [( -C4H9)4N]+[H2NCSNHC02] -(NH2)2CS. The cyclic structure and molecular dimensions of the allophanate and 3-thioallophanate ions are compared in Fig. 20.4.18. As expected, the C-Obond involved in intramolecular hydrogen bonding in the 3-thioallophanate anion is longer than that in the allophanate anion. 

Figure 1. Chemical structure and molecular dimensions of a-, 6-and Y-cyclodextrins.

A time span of 33 years has elapsed since the original formulation of the antimetabolite theory by Fildes1 based upon the discovery by Woods2 of the biological antagonism between sulfanilamide (1 b) and p-aminobenzoic acid (la), two compounds strikingly similar to each other in chemical structure and molecular dimensions (see illustration). Up to the present day, this theory... 

Figure 11 Chemical structures and molecular dimensions of the most common cyclodextrins, abbreviated oc, P, and y-CDX.

The chemical structures and molecular dimensions of deoxyribose, the nitrogenous bases, and phosphate. 

By far the greatest number of literature citations are devoted to the pyrrolotriazines (1-6). Compounds of this type are the only representatives in this section which can form electronically neutral fully delocalized lOTt-electron systems. Most of the parent systems are known, but surprisingly, these have received scant attention in the literature, the greatest number of citations being devoted to the nonconjugated systems. There has been little systematic study of spectral properties, x-ray structure, and molecular dimensions determination, and the use of theoretical methods to predict reactivity at ring atoms. Thus, owing to the sparsity of information, little space is devoted here to these subjects unless some feature warrants special note. 

At these nano scales, matter behaves quite differently than it would at the macro level to which we are accustomed. Properties such as conductivity, magnetism, melting and boiling points, and reactivity may be dissimilar at the nano and macro scales due to the quantum mechanical behavior of small structures at molecular dimensions. 

The book concludes with a Case Study on zeolites, examining their structure and internal dimensions in relation to their behaviour as molecular sieves and catalysts. Examples are discussed, and also the later development of mesoporous materials. 

Some sample properties may be obvious to the analyst, such as colour, shape and dimensions or may be measured easily, such as mass, density and mechanical strength. There are also properties which depend on the bonding, molecular structure and nature of the material. These include the thermodynamic properties such as heat capacity, enthalpy and entropy and also the structural and molecular properties which determine the X-ray diffraction and spectrometric behaviour. 

Physical activation converts the carbonized raw material into a product that contains an extremely high surface area and a porous structure of molecular dimensions. The aim of this process is to enhance the volume and to enlarge the diameters of the pores formed during carbonization and to create some new porosity. Physical activation is usually carried out at temperatures between 800 and 1000°C in the presence of suitable oxidizing gases such as steam, CO2 or air, or any mixture of these gases. 

Fig. 21 Force histogram of the first four layers (a), layer thickness histogram at —0.8 V (b) and molecular structures and approximate dimensions in nm of the ions used in this study (BMIPF6) (c). Reproduced with permission from ref. 95. Copyright 2012 Royal Society of...

Martens JA, Tielen M, Jacobs PA, Weitkamp J. Estimation of the void structure and pore dimensions of molecular sieve zeolites using the hydroconversion of n-decane. Zeolites 1984 4 98-107. 

---