Molecular hydrogen, carbon-oxygen

Molecular sizes and shapes play key roles in determining chemical and physical properties. The immense variety of chemical and physical properties displayed by substances in the natural world mirrors an equally immense variety of different types of molecules. However, variety need not come from a large number of different elements. The molecules that make up a cup of coffee are made up almost entirely of atoms of just five elements hydrogen, carbon, oxygen, nitrogen, and sulfur. Carbon, in particular, is capable of combining in many different ways, generating molecules with elaborate stractures. 

An alternative to the atomization method is what has been called the formation method, which is illustrated for methanol in Fig. 5.27. This method utilizes a kind of pseudo heat of formation , AH f0, of the compound from atomic carbon and molecular hydrogen and oxygen (the conventional heat of formation is relative to graphite and molecular hydrogen and oxygen). 

Fig. 5.27 The principle behind the ab initio calculation of heat of formation (enthalpy of formation) by the formation method. Methanol is (conceptually) formed from atomic carbon and molecular hydrogen and oxygen the enthalpy input for this resembles that for the heat of formation of methanol (hence the name) except that atomic carbon rather than graphite is used. Graphite is converted to atomic carbon, and the elements in their normal states are also used to make methanol. The heat of formation of methanol at 0 K follows from equating this quantity to the heat of atomization of graphite plus the energy needed to make methanol from atomic carbon and molecular hydrogen and oxygen. The diagram is not meant to imply that methanol necessarily lies above its elements in enthalpy...

Russell, R.L., and F.S. Rowland (1968), Reactions of triplet methylene with oxygen. Formation of molecular hydrogen, carbon monoxide and carbon dioxide, J. Am. Chem. Soc., 90, 1671-1677. 

First, the peak for the molecular- ion for all compounds that contain only carbon, hydrogen, and oxygen has an m/z value that is an even number. The presence of a nitrogen atom in the molecule requires that the m/z value for the molecular- ion be odd. An odd number of nitrogens conesponds to an odd value of the molecular- weight an even number of nitrogens conesponds to an even molecular weight. 

Example the molecular ions of nitrogen, N2, carbon monoxide, CO, and ethene, C2H4, have the same nominal mass of 28 u, i.e., they are so-called iso-baric ions. The isotopic masses of the most abundant isotopes of hydrogen, carbon, nitrogen and oxygen are 1.007825 u, 12.000000 u, 14.003070 u and 15.994915 u, respectively. Using these values, the calculated ionic masses are 28.00559 u for Nz"" , 27.99437 u for CQ-", and 28.03075 u for CjH/. This means they differ by some millimass units" (mmu) from each other, and none of these isobaric ions has precisely 28.00000 u (Chap. 3.3.4 and Chap. 6.9.6). 

It turned out that aflatoxin was actually a mixture of four different but closely related chemicals. All possessed the same molecular backbone of carbon, hydrogen, and oxygen atoms (which backbone was quite complex and not known to be present in any other natural or synthetic chemicals), but differed from one another in some minor details. Two of the aflatoxins emitted a blue fluorescence when they were irradiated with ultraviolet light, and so were named aflatoxin Bi and B2 the names aflatoxin Gi and G2 were assigned to the green-fluorescing compounds. The intense fluorescent properties of the aflatoxins would later prove an invaluable aid to chemists interested in measuring the amount of these substances present in various foods, because the intensity of the fluorescence was related to the amount of chemical present. 

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