Thermodynamic destabilization

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

Perfluocoalkyl groups thermodynamically destabilize double bonds and small rings, but they can kineiically stabilize highly stramed molecules [75]. This remarkable perfluoroalkyl effect has made possible the isolation of stmctures that are uncommon m hydrocarbon chemistry, especially valence-bond isomers of aromatics and heteroaromatics such as 1, 2, and 3 [108],... 

Further experiments focused therefore on [RuCl(en)(r 6-tha)]+ (12) and [RuCl(rj6-p-cym)(en)]+ (22), which represent the two different classes, and their conformational distortion of short oligonucleotide duplexes. Chemical probes demonstrated that the induced distortion extended over at least seven base pairs for [RuCl(rj6-p-cym)(en)]+ (22), whereas the distortion was less extensive for [RuCl(en)(rj6-tha)]+ (12). Isothermal titration calorimetry also showed that the thermodynamic destabilization of the duplex was more pronounced for [RuCl(r 6-p-cym)(en)]+ (22) (89). DNA polymerization was markedly more strongly inhibited by the monofunctional Ru(II) adducts than by monofunctional Pt(II) compounds. The lack of recognition of the DNA monofunctional adducts by HMGB1, an interaction that shields cisplatin-DNA adducts from repair, points to a different mechanism of antitumor activity for the ruthenium-arenes. DNA repair activity by a repair-proficient HeLa cell-free extract (CFE) showed a considerably lower level of damage-induced DNA repair synthesis (about six times) for [RuCl(en)(rj6-tha)] + compared to cisplatin. This enhanced persistence of the adduct is consistent with the higher cytotoxicity of this compound (89). 

Recent ab initio calculations delineate the remarkable thermodynamic destabilization of lead(IV) compounds by electronegative substituents182,183. Based on population analyses of the molecular wave functions it was proposed that electronegative substituents increase the charge of the metal and increase the difference in the radial extensions of the 6s and 6p orbitals. By increasing the differences in the radial extensions of the s and p orbitals, 6th-row relativistic effects also contribute to a destabilization of the higher valence state. 

Vajo, J. J. Salguero, T. T. Gross, A. F. Skeith, S. L. Olson, G. L., Thermodynamic destabilization and reaction kinetics in light metal hydride systems. Journal of Alloys and Compounds 2007,446-447,409-414. 

Furthermore, Fig. 3.35 shows the desorption cnrves of the ball-milled (MgH h-50 wt%LiAlH ) composite dnring continuous heating np to 250,260,275 and 300°C under 0.1 MPa hydrogen atmosphere. Under these conditions the composite desorbs 4.3 and 4.9 wt%H2 at 250 and 260°C, respectively. The pnrity-corrected amonnt of hydrogen which conld be desorbed from the 50 wt%LiAlH constitnent in a composite at these temperatnres which are higher than the temperatnres of the solid state reaction (Rib) of (3.12) and (R2) of (3.13) in Fig. 3.30b, is 3.8 wt%. Experimentally observed values are larger by about 0.5-1.0 wt% than the theoretical one. This excess could only be desorbed from MgH. That means that MgH is able to desorb at temperatnres 250 and 260°C, which are lower than its eqnilibrinm temperature of desorption nnder 0.1 MPa equal to 275°C. Apparently, MgH is thermodynamically destabilized by the second composite constituent LiAlH. Additional evidence that MgH is, indeed, destabilized is provided by the shape of the desorption cnrves at 250 and 260°C in Fig. 3.35 in which one can see a clearly discernible third... 

Since the ROM of (3.44b) is just a mathematical expression, of course, a question arises as to what the underlying physical mechanism is that could be responsible for the ROM dependence of decomposition temperature of the NaBH constituent on the content of MgH A similar question was raised for the case of the (MgH + LiAIH ) composite. The first physical model of interest is the one proposed by Vajo et al. [196-198]. According to this model the enthalpy change of LiBH during decomposition is reduced by the formation of an intermediate compound MgB from the free Mg obtained due to the decomposition of the MgH constituent. This model is presented graphically for the thermodynamic destabilization of LiBH by MgH in Fig. 3.25b. By analogy, the reaction of (3.43) can be adapted to the NaBH and MgH system in the following form... 

J.J. Vajo, T.T. Salguero, A.F. Gross, S.L. Skeith, G.L. Olsen, Thermodynamic destabilization and reaction kinetics in light metal hydride systems , J. Alloys Compd. 446-447 (2007) 409-414. 

Spontaneous DNA Damage Hydrolysis of the A-glycosyl bond between deoxyribose and a purine in DNA creates an AP site. An AP site generates a thermodynamic destabilization greater than that created by any DNA mismatched base pair. This effect is not completely understood. Examine the structure of an AP site (see Fig. 8-33b) and describe some chemical consequences of base loss. 

STRESSES DURING FREEZE-DRYING THERMODYNAMIC DESTABILIZATION FACTORS... 

The differences in the thermodynamic destabilization intfoduced by various mismatches did not dictate the electrochemical response obtained at the corresponding films. For example, a GT mispair that caused a 6° decrease in the T of a 15-mer duplex gave the same attenuation in integrated current for the reduction of DM as a CA mispair that caused a 12° decrease in the T of the same duplex. Both perturbations in the orientations of bases involved in mispairs and increased base dynamics for wobble base pairs may disrupt stacking and attenuate rates of electron transfer. This attenuation may allow for the detection of mismatches with varying thermodynamic stabilities. 

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