Deprotonation enthalpy

Tab. 13.4 Thermochemistry of Selected Acid/Base Reactions Deprotonation Enthalpies (kcal/mol) for Deprotonations of /PrOH with Various Organolithium Compounds and Lithium Amides...

The relationships between the deprotonation energy of proton donor and complex stability as well as its VDE were characterized in our work devoted to complexes between uracil and a series of alcohols with deprotonation enthalpy (HDP) varied in a systematic manner [48], We found out that a H p smaller than 14.3 eV is required for BFPT with the product being UH OR. Two minima coexist on the anionic energy surface for 14.8 eV < HDP < 14.3 eV. These minima correspond to the UH OR and 1 HOR structures. For ROH s with deprotonation enthalpies above 14.8 eV only the U HOR minimum exists on the potential energy surface. 

TABLE 3.4 Deprotonation Enthalpies" for Acetic Acid, Acetone, and Propene... 

The agreement between the experimental and computational deprotonation enthalpies is quite good. The correlation coefficient of a plot of the two sets of values is 0.9799, but all the computed values lie within the error bars of each experiment. While an ordered list by DPE of the 20 amino acids are not quite the same using the experimental and computed values, there are really very small disagreements. 

Another relevant phenomenon is the reaction of carbon monoxide with the lithium trimethylsilyldiazomethanide (MesSiCNaLi) to form either an ynolate (MesSiC COLi) or a ketenide (Me3SiC(Li)=C=0) derivative. The corresponding neutral species Mes SiCHN2 reacts with CO, in the presence of Co2(CO)s, to form Me3SiCH=C=0 . Neither reaction was studied calorimetrically. Accordingly, they cannot be compared with the energetics of the reaction of HCNi with CO to form HCCO and N2, a quantity indirectly obtainable from the deprotonation enthalpies of diazomethane and ketene. ... 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

There are several relevant aspects of rhodium enolate chemistry. Rh(I) catalyzes the isomerization of allylic aUtoxides to enolates. We welcome this reaction done in a direct thermochemical context analogous to the related isomerizations of allyl halides , ethers and allyl amines . From the enthalpies of formation of allyl alcohol and propanal (—185.6 and —124.5 kJmol ), and their respective gas phase deprotonation enthalpies (1564 and 1530 kJ mol 252,253 jj. jjg concluded that the rearrangement of the allyloxide to propen-1-olate is exothermic by 95 kJmoR. ... 

One might also have thought that ion chemistry would have provided the enthalpy of formation of an AT,A -dialkylated enamine. However, we know of no measured appearance potential of any immonium ion [R R C=NR R ] for which there is also a measured deprotonation enthalpy that must result in an enamine. That is, R and R cannot be H, and at most one of R and R may be. The simplest cation that could qualify is [MeCH=NMe2] for which a deprotonation enthalpy is known , but we know of no appearance potential of this ion from Mc2CHNMe2 or from any other source. That is, what is the energy of the simple fragmentation process... 

Calculation can give some idea of the relative acidities of sugar hydroxyls. The deprotonation enthalpies (kJ oC ) in vacuo of the various OH groups of a-D-glucopyranose, relative to that for the anomeric hydroxyl, are 2-OH, 35.5 ... 

The present work discusses some properties of thiouracil nucleobases calculated at the B3LYP/6-31+G(d,p) computational level, namely, their structures, most stable tautomers, proton affinities and deprotonation enthalpies, hydrogenation, interaction with water, and base pairing, in order to shed a certain light on the problem of why their functioning in RNA and modified DNA base pairs is so unusual. 

Table 4 presents the proton affinities PA and deprotonation enthalpies DPE of thiouracils calculated at the B3LYP/6-31+G(d,p) computational level. Inspecting this Table, we find that first, the thio substitution of uracil systematically increases its PAs by 3-6 kcal/mol and second, it decreases the DPEs of uracil by 6-13 kcal/mol. As far as the base pair A thioU is considered, it particularly implies a lowering, on the one hand, of the potential well at the Sio atom of thiouracil corresponding to the proton transfer from the N8 atom of adenine to the Sio of thiouracil and, on the other one, a raising of the potential well at the N3 atom of thiouracil involved in the proton transfer from the N3-H bond to the Ni atom of adenine. Therefore, summarizing, the thio substitution of uracil is in a favor to the double proton transfer mechanism of the occurrence of the spontaneous point mutations proposed by Lowdin [19]. Table 4 also includes the PAs of the T2 tautomer of uracil and thiouracils. A comparison with the corresponding PAs of the parental normal nucleobases shows that their tautomerization to the T2 form is accompanied by an increase of the proton affinity by 20-23 kcal/mol. 

Table 4. Proton affinities (PA) and deprotonation enthalpies (DPE) of uracil and thiouracils in kcal/mol.

ArG298 = -RT InK. This experimental scale of relative acidities was converted to a scale of absolute acidities by including certain compounds as anchor points. Thus, the gas-phase acidity of PH3 was determined to be ArG29s = 363 2 kcal/mol. The entropy change for the deprotonation process was evaluated by procedures using statistical mechanics as ArS = 24.9 2 cal - mol" K From these data the deprotonation enthalpy of PH3 at 298 K was calculated to be ArH298=PA(PHi) = 370.4 2 kcal/mol [1, 2]. 

Recently, Koppel and co-workers have calculated intrinsic acidities and gas-phase deprotonation enthalpies for a range of superacids using the DFT method (15). Acidity values (AGadd) for Br0nsted-Lewis conjugate superacids (eg, HF-SbFs) were found to be as low as about 1040 kJ/mol. 

Alternatively, deprotonation enthalpies can be evaluated from probe adsorption calorimetric data or from temperature programmed desorption (TPD) measurements. The strengths of surface Lewis acid sites and of surface basic sites can also be evaluated, in principle, by the heat of probes adsorption or desorption. In all cases, however, probes adsorption on solids can result in multiple interactions for example, van der Waals interactions can be superimposed to true acid-base interactions, which can also be multiple and finally give rise to some kind of solvation effects, in particular, in the zeolite cavities [23-25]. Thus, the pure acidity/basicity... 

The hydroxyl acidities of sucrose, assessed through semiempirical calculations of the deprotonation enthalpies, followed the order OH-2g OH-3g > OH-3f > OH-lf = OH-4g > OH-4f OH-6g > OH-6f. The molecular electrostatic potential profile of sucrose in polar, aprotic solvents indicated likewise that in the main conformation OH-2g is the most electropositive hydroxyl group the preparations of selectively benzylated and acetylated sucrose derivatives on the basis of these findings are referred to in Chapters 5 and 7, respectively. ... 

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