Hydride anion affinities

In fast reactions, ionized amines show an even greater propensity to undergo the ubiquitous a-cleavage than do ionized alcohols and ethCTs. This distinction may reflect the superior stability of the immonium ions compared to their oxonium ion counterparts . Thus, the hydride anion affinities [D(R+ — H ), corresponding to the heterolytic bond dissociation energy of the conjugate compound] of immonium ions are ca 100 kJmol lower than those of the analogous oxonium ions [e.g. D(R+ — H ) of CH2=NH2" and CH2=0H+ are 912 and 1054 kJmol, respectively] . 

The determination of electron affinities (EAs) is one of the most serious problems in quantum chemistry. While the Hartree-Fock electron affinity can be easily evaluated, most anions turn out to be unbound at this level of theory. Thus, the correlation effects are extremely crucial in evaluating EAs. At this point, lithium hydride and lithium hydride anion make up a very good benchmark system because they are still small enough yet exhibit features of more complicated systems. Four and five electrons, respectively, give rise to higher-order correlation effects that are not possible in H2. 

Addition of an electron yields the hydride anion, H, the electron affinity (energy drop) being 0.7 eV. The H ion is large (radius 1.45 A) because of the mutual repulsion of the electrons which offsets the nuclear attraction. The relatively diffuse charge cloud of the ion is then very easily polarised. However, hydride ions do appear to exist in alkali metal hydrides. The H ion is an extremely strong base the reaction... 

Use of the AMI method to calculate proton and hydride ion affinities for 4- and 5-substituted 2-methylimidazoles (9) and attempting to relate these to model benzyl systems has demonstrated excellent correlations between 4-substituted compounds and w-toluenes and the 5-isomers and p-toluenes. The substituents appear to exert only minimal effects on the change in charges on the annular nitrogens as the anionic or cationic species are formed (Scheme 1) <93JOC7336>. 

Unfortunately, since a true experimental value for the heat of formation of thiazole is not known, it is not possible to evaluate the accuracy of the calculated values. However a close practical estimation of an experimental AHf° for thiazole can be obtained (see Section 3.06.4). Dewar s AMI semiempirical MO method has also been used by Bean <93JOC7336> to calculate the heats of formation and charges in the 4- and 5-substituted-2-methyl thiazoles and their benzyl-like anions. This study afforded also proton and hydride ion affinities of the benzyl-like anion and cation of 2-methylthiazoles (see Section 3.06.7.3.2). 

TEMPO+ abstracted hydride anions from the hydrides of aldehydes and ketones in acetonitrile without any side products. The hydride affinity of aldehydes and ketones in acetonitrile was defined as fhe enthalpy change of the aldehydes and ketones. Several conclusions regarding the hydride-accepting abilities of aldehydes and ketones, based on thermodynamic data, were listed." " ... 

Other Ion Affinities Binding affinities for many different types of ions to neutrals are defined analogously to hydride affinity, as the 298 K enthalpy required to dissociate the complexed species. The ion can be a cation or an anion. Conversely, ion affinities can be described in terms of the dissociation. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

A novel form of Y HX hydrogen bonding49 results when the Lewis base Y is itself a hydride ion (H-). Because the electron affinity of a hydrogen atom is extremely weak (21 kcal mol-1), the H- ion is among the most weakly bound and diffuse anionic species known, and hence a powerful Lewis base. In this case, the H - -HX complex can be referred to as a dihydrogen bond 50 to denote the unusual H-bonding between hydrogen atoms. A water complex of this type was... 

In die Pople family of basis sets, the presence of diffuse functions is indicated by a + in die basis set name. Thus, 6-31- -G(d) indicates that heavy atoms have been augmented with an additional one s and one set of p functions having small exponents. A second plus indicates the presence of diffuse s functions on H, e.g., 6-311- -- -G(3df,2pd). For the Pople basis sets, die exponents for the diffuse functions were variationally optimized on the anionic one-heavy-atom hydrides, e.g., BH2 , and are die same for 3-21G, 6-3IG, and 6-3IIG. In the general case, a rough rule of thumb is diat diffuse functions should have an exponent about a factor of four smaller than the smallest valence exponent. Diffuse sp sets have also been defined for use in conjunction widi die MIDI and MIDIY basis sets, generating MIDIX+ and MIDIY-I-, respectively (Lynch and Truhlar 2004) the former basis set appears pardcularly efficient for the computation of accurate electron affinities. 

Suggest experimental techniques for assessing the affinity of a host for a particular anion in solution. Which of these techniques might be appropriate for (a) podand (4.40), (b) diboryl hosts such as hydride sponge (4.69) and (c) zwitterions such as (4.35). 

Cubane has had an interesting place in the discussion of the correlation between C-H acidity and carbon hybridization. Its acidity was measured by the H exchange NMR technique and found to be about 6.6 x 10 as reactive as benzene. An experimental gas phase measurement of the proton affinity (PA) as 404kcal/mol is available. (See Tables 3.14 and 3.38 for comparable data on other hydrocarbons.) Both of these values indicate that cubane is somewhat more acidic than expected on the basis of the carbon hydridization. There appears to be unusual hybridization of the anion in this case. An AIM analysis suggests that the C—C bond paths in the anion are less than 90°, suggesting that the bonds bend inward toward the center of the ring. Sauers also noted an increase in s character on going from the hydrocarbon to the anion. Of the 28 deprotonations he examined, only cyclopropane and bicyclo[l.l.l]pentane also showed increased s character in the anion. 

Finally, the proton affinities of several atomic metal anions, M" (M -V, Cr, Fe, Co, Mo, and W), have been determined by bracketing methods (47). Combining these data with measured electron affinities of the metals yielded homolytic bond energies for the neutral hydrides, D (M-H). The monohydride bond energies compare favorably with other experimental and theoretical data in the literature and were used to derive additional thermodynamic properties for metal hydride ions and neutrals. 

In the case of the main-group hydrides, the basicity of the anions decreases in each group of the periodic system from lighter to heavier elements, whereas with the proton affinities of the neutrals a reversal is observed in the group of the periodic system. 

An ab initio study of the addition of lithium aluminium hydride (LAH) to acetonitrile and malononitrile is reported the free anions generated by hydride addition show clear preferences for the enamide (RCH CH=NH RCH=CHN H) over the imide (RCH2CH=N ). Lithium ion pair formation stabilizes both tautomers, the localized imide is stabilized slightly more than the enamide, and the enamide preference is somewhat reduced but persists. The alane-complexed lithium ion pairs result in a small imide preference for the LAH adduct of acetonitrile and a dramatically reduced enamide preference for the LAH adduct of malononitrile. Alane affinities were determined for the lithium ion pairs formed by LiH addition to the nitriles. The alane binding greatly affects the imide-enamide equilibria such that alane complexation might even provide... 

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