2- relative stability

The experimental and theoretical investigations of the relative stability of the tautomers of the DNA bases reveal that several tautomers of guanine and cytosine have (in gas phase) the relative stability which is only slightly different (1-2 Kcal-mof ) from the stability of their canonic forms [61, 125]. Such studies qualitatively support the theory of spontaneous mutations in the DNA which considers the appearance of the rare DNA bases tautomers as the natural source of the mutations [130]. On the other hand, the surrounding of the DNA bases in the living cell is definitely different from the gas phase, so the investigations of the role of the polar environment on the stability of the different tautomers are extremely important. 

In principal, there are several models which could shed light on this phenomenon. Among them, probably the simplest ones are the investigation of the relative stability of monohydrated models (because a limited number of water molecules is included in the backbone of the DNA [1-19]) and the model of the total hydration which could be associated with the existence of the DNA bases in the cell before they are incorporated in the DNA backbone. Such studies have been reported for the guanine [119-122, 131], cytosine [123, 124, 132, 133] and adenine [134], The relevant results for these systems are collected in the Tables 9-11. 

The following relative stability orders can be derived from the data presented in the Tables 9-11. 

Relative energies, free energy, and dipole moments of the tautomers of adenine (A) at the 

The predicted patterns of relative stability in the gas phase and in solutions are in exact correspondence with the experimental data [135,136]. According to the experiments 7A, both CYT and CYT, 7GUA, 9GUA and 9GUA are the observable species in the gas phase. Only normal (canonical) forms of these bases are detectable in the polar media [13, 87, 137]. 

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MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

For the transition metals it is often impossible to reach a noble gas structure except in covalent compounds (see effective atomic number rule) and it is found that relative stability is given by having the sub-shells (d or f) filled, half-filled or empty. 

Chemists are interested not only in the thermodynamics of a process (the relative stability o the various species) but also in its kinetics (the rate of conversion from one structure tc another). Knowledge of the minimum points on an energy surface enables thermodynamic data to be interpreted, but for the kinetics it is necessary to investigate the nature of the... 

Barrows S E, J W Storer, C J Cramer, A D French and D G Truhlar 1998. Factors Controlli Relative Stability of Anomers and Hydroxymethyl Conformers of Glucopyranose. Journal Computational Chemistry 19 1111-1129. 

Sometimes we can be guided in our disconnection by the relative stabilities of the possible anionic fragments. How would you make TM 106 ... 

The three isomers of thiazoleacetic acid can be decarboxylated, the order of facility being 2>5>4, though the relative stability depends on each particular compound and the reaction conditions (72-75). This reaction may be used to obtain certain alkylthiazoles (73). Malonic derivatives can also be decarboxylated to give aliphatic thiazole acids (49, 51)... 

It IS good chemical practice to represent molecules by their most stable Lewis structure The ability to write alternative resonance forms and to compare their relative stabilities however can provide insight into both molecular structure and chemical behavior This will become particularly apparent m the last two thirds of this text where the resonance concept will be used regularly... 

Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons They tell us not only which isomer is more stable than another but by how much Consider a group of C His alkanes... 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

In each of the following groups of compounds identify the one with the largest heat of combustion and the one with the smallest In which cases can a comparison of heats of combus tion be used to assess relative stability" ... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

Could the fact that hydrogenation of 2 methyl(methylene)cyclo hexane gives more as 1 2 dimethylcyclohexane than trans be explained on the basis of the relative stabilities of the two stereoisomeric products ... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

Conformational analysis (Section 3 1) Study of the conforma tions available to a molecule their relative stability and the role they play in defining the properties of the molecule Conformations (Section 3 1) Nonidentical representations of a molecule generated by rotation about single bonds Conformers (Section 3 1) Different conformations of a single molecule... 

NR qq g relative stability of iminoboranes and amino iminoboranes shows strong dependence on the stetic bulk offered by R and R7 This is illustrated by the stabiUty of at 0°C whereas the permethyl analogue, (CH3) S[(CH3), decomposes above —110°C (72). The... 

Electric Power Generation. Coal is the primary fuel for thermal electric power generation. Since 1940 the quantity of bituminous coal consumed by electric utilities has grown substantially in each successive decade, and this growth is expected to continue for many years. Coal consumed by electric utilities increased from about 536 x 10 t in 1981 to 689 x 10 t in 1989 (2). The reasons for increased coal demand include availability, relative stability of decreasing coal prices, and lack of problems with spent fuel disposal as experienced in nuclear power plants (see Nuclearreactors). 

The tlrree impurities, iron, silicon and aluminium are present in the metal produced by the Kroll reduction of zirconium tetrachloride by magnesium to the extent of about 1100 ppm. After dre iodide refining process tire levels of these impurities are 350, 130 aird 700ppm respectively. The relative stabilities of the iodides of these metals compared to that of zirconium can be calculated from the exchange reactions... 

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