Bond energies and reactivity

Protic solvents, such as methanol and ethanol, slow down SN2 reactions by solvation of the reactant nucleophile. The solvent molecules hydrogen bond to the nucleophile and form a "cage" around it, thereby lowering its energy and reactivity. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

The ability of various molecules to act as H atom donors in photoreduction varies greatly with the Z-H bond energy and with the electron density on the abstractable H atom. Molecules with electron donor groups (amino, hydroxy, etc.) near the H-donor GH group are very efficient. These include some amines, amides, alcohols and ethers hydrocarbons are substantially poorer H-donors and pure aromatics like benzene show very little reactivity. Acetonitrile H3C-CN is a very poor H-atom donor because its C-H bonds are made dipolar by the action of the strong electron acceptor CN so that the electron density on H is low. Water itself is an extremely bad H-atom donor in view of both the high OH bond energy and the acidic character of these bonds. 

This similarity in reactivities probably derives from a fortuitous cancellation of substituent effects in 11. Fluorination increases chain stiffness and creates an unfavorable polarity mismatch between an electrophilic radical and an electron-poor double bond, but this is offset by the significant decrease of 7r-bond energy in 11. The vinyl ether 12 analog cyclizes about seven times faster than 11, which is consistent with the known lower 7r-bond energy and higher free-radical reactivity of perfluorovinyl ethers vs perfluoroalkenes [142]. 

The high reactivity of fluorine results from a combination of the low F—F bond energy and the high strength of bonds from fluorine to other atoms. The small size and high electronegativity of the F atom account for many of the other differences between fluorine and the other halogens. 

In particular, more structural data, more kinetic studies, a more thorough investigation of the platinum and palladium chemistry, the use of new techniques, such as NMR spectroscopy, and the determination of bond energies could increase our understanding of the bond nature and reactivity and make the interpretation of important catalytic mechanisms of these compounds easier. 

The other molecular properties to be considered include bond energies, electron distributions, the vibrational spectrum and the energies of various excited states. These properties are by no means an exhaustive list but they do represent some of the most frequently reported ones. They are subject to experimental verification and can therefore be used as yardsticks by which the quality of a given theoretical method can be judged. Furthermore, bond energies and electron distributions (along with the Molecular Orbitals (MOs) implied by the latter) provide the foundations for a discussion of chemical reactivity. 

Dq and ro were taken to be the free energy of activation for a parent tetrahydropyranyl alkyl acetal, and its exocyclic C-0 bond length, respectively. The plot of ln[( )/Do) -1] vs. r-rQ gives then the slope, 13.9 A and intercept 0.436. The Morse curve defined by the selected value of Dq and obtained value of / is shown in Figure 6.44. The experimental data map out a line parallel to this curve but displaced from it by 0.031 A. The authors concluded that the origin of the observed linear relationship between bond length and reactivity is to be found in the approximate linearity of the Morse function at small-to-moderate bond extensions. Since it is an intrinsic property of this function, such linear relationships may, according to Jones and Kirby, turn out to be the rule. 

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