X-H bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data reference in parentheses) Methods (reference in parentheses) References 

Note The data on proton affinities (PA), electro affinities (EA) and ionization energies (IE) are available in 1988LIA/BAR, 1996NIST, and 1998HUN/LIA. 

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The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

Hydrogen bonds require electron-deficient hydrogen atoms in polar H—X bonds and highly electronegative atoms with nonbonding pairs of electrons are present. Use Lewis structures to determine whether these requirements are met. 

Acetone and CH3F contain electronegative atoms with nonbonding pairs, but neither has any highly polar H—X bonds. Thus, there is no hydrogen bonding between molecules of these substances. 

A proton transfer reaction involves breaking a covalent bond. For an acid, an H — X bond breaks as the acid transfers a proton to the base, and the bonding electrons are converted to a lone pair on X. Breaking the H — X bond becomes easier to accomplish as the bond energy becomes weaker and as the bonding electrons become more polarized toward X. Bond strengths and bond polarities help explain trends in acidity among neutral molecules. 

The lengths of the H-X bond and, to a lesser extent, the bonds involved in B deviate from their equilibrium values... 

A topological analysis of the total static density has been carried out. The analysis is not complete, and will not be discussed in any great detail in the present context. It is worth mentioning however that similar results as found for benzoylacetone were obtained. The values of pb and V2pbat the 0(1)-H(X) and 0(3)-H(X) bond critical... 

The palladium may then be considered as either Pd° ligated by a proton or as Pd11 ligated by a hydride,376 albeit that the very negative shift of the H-NMR signal is more indicative of a hydride linked to a palladium(II) center. This is why this reaction is also considered as an oxidative addition.376 When weak carboxylic acids are involved (e.g., acetic acid, formic acid), the reaction is regarded as an oxidative addition, i.e., insertion of palladium(0) into the H X bond (see Equation (2)) 367,376... 

In contrast to the situation on flash pyrolysis, methyleneoxophosphoranes generated by thermolysis or photolysis in the presence of protic nucleophiles can be directly trapped to form corresponding derivatives of phosphinic acid (17- 19) however, the possibility of competing insertion of carbenes into the H/X bond of the additives is always present, giving phosphine oxides with X in the a-position (16- 18). Reaction branching at the carbene 16 was first observed on photolysis of 7 in water 13) and prompted detailed investigations on the phosphorylcarbene/ methyleneoxophosphorane rearrangement. 

The most important heuristics relate to reaction conditions, in particular, to acid-base catalysis. Depending on whether acidic or basic conditions are specified, the reactivity of certain bonds is changed. As an example, under basic conditions the breakability of H-X bonds is increased in comparison with other bonds. In fact, the relative acidities of all H-X bonds (X = any other element) can be rapidly calculated in EROS, and this allows further distinction within this class of bonds. 

What Pauling electronegativity is predicted for an element X if the ff-Xbond energy is 402 kj mol-1 The ff-ff bond energy is 432 kj mol-1 and the X-X bond energy is 335 kj mol-1. What would be the percent ionic character of the H-X bond If the molecular wave function is written as... 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

The facility for hydrogen incorporation into metal carbonyls appears to be general, and a wide range of H-X bonds oxidatively add to the... 

Table 5. Bond ionization potentials and computed orbital energies of H-X bonds...

Here, shb is the hydrogen bonding energy parameter, r is the natural hydrogen bond distance, rYh is the actual hydrogen bond distance Y... H, ft is the H—X. .. Y angle, rxH and 4h are the actual and natural H—X bond lengths, respectively, and e is the dielectric constant. 

The geometry of M- H-X bonds is again close to linearity according to theoretical studies of the Cp20s- HOH system. Neutron diffraction data collected for the Et3NH+- Co(CO)4 complex have determined the Co- H-N angle to be 180°. 

H-X bonds generally show linear H- H-X fragments (or close to linearity), whereas the Y-H- H fragments are bent. 

Initially, the topological approach has been widely used only for the analysis of theoretical charge densities. Theoretical analysis allows us to consider two atoms as bonded, whereas experiments show only their spatial proximity (x-ray or neutron diffraction), changes in the vibrational behavior of an H-X bond involved in interaction, changes in magnetic shielding of target nuclei (NMR), and so on. 

The energies of the covalent homoatomic X—X bonds are shown in figure 4.5, where the heavier atoms make weaker bonds. The energies of the covalent het-eroatomic C—X and H—X bonds also decrease with the period, as shown in figure 4.6. 

Large uncertainties in our knowledge of the lengths of H-X bonds means that there are bound to be uncertainties in the determination of experimental bond valences, a problem that is made worse by the inability of eqns (3.1) and (3.2) to give a good description of the bond valence-bond length correlation over the... 

On other occasions, both types of secondary interactions are present in the same compound and can afford supramolecular entities via cooperative interactions. Table 5.3 shows examples of gold complexes containing N—H X bonds in addition to or instead of Au- Au contacts. 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

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