Charge distributions and chemical effects. XLIII. Bond dissociation energies and radical formation

Abstract

SANDOR FLISZAR and CAMILLA MINICHINO. Can. J. Chem. 65, 2495 (1987).The problem of bond dissociation, RlR2 4 R I . + R2., is addressed from the viewpoint that the fragments, R1 and R2, may not be individually electroneutral in the host molecule, whereas the corresponding radicals certainly are. The mutual charge neutralization of R, by R2 during the cleavage of the bond linking R1 to R2 is described by an expression featuring only molecular ground-state properties. This expression translates directly into a new energy formula for the dissociation energy, D *(RIR2) = e(R1R2) + CNE -E*nb + RE(RI) + RE(R2), where both the molecule and the radicals are taken at their potential minimum.The charge neutralization energy, CNE, profoundly affects the relationship between the dissociation (D*) and contributing bond energy (E), i.e., the energy in the unperturbed molecule. Nonbonded interactions between R l and R2, E*nb, are almost negligible. The reorganizational energy, RE, measures the energy difference between R . and the corresponding electroneutral group found in the symmetric molecule RR. Numerical applications to alkanes reveal an important cancellation of individual CNE terms accompanying the mutual charge neutralization of alkyl groups during the cleavage of CC bonds, i.e., CNE -E*,, = 0. Theoretical eCC's lead to valid CC bond dissociation energies. In CH bond dissociations, on the other hand, the sum ECH + CNE remains nearly constant although individual eCH's may differ from one another by as much as 6 kcal mol-'. The concept of chemical bond is fundamental to chemistry. It turns out that nonbonded interactions are minor (1, 2), Its energetic aspects are at the center of this study, both for -0.02-0.05% of AE*,, so that the study of AE*, essentially non-reacting molecules and for molecules undergoing bond reduces to a consideration of bond energies, AE", -2 E,;.cleavage.Under laboratory conditions, molecules contain vibrational, translational, and rotational energies which are not fairly partitionable among chemical bonds. For convenience, our analysis considers the molecules at their potential minimum, i.e., with reference to their hypothetical vibratiocless state at 0 K; the role of vibrational energies is treated as a separate problem. The measure for what holds the atoms together in a molecule is thus offered by the difference AE*, = energy of all the isolated ground-state atoms minus the ground-state energy of the molecule which is the energy required for breaking up that molecule into its constituent atoms. The atomization energy, AE*,, includes the annihilation of all interactions between atom pairs which are not chemically bonded to one another in the conventional sense, AE*nb, and the destruction of all chemical bonds ij, represented by their energies eij, i.e.,'Visiting Scientist on leave from: Dipartimento di Chimica, Facolta di Scienze, Via Mezzocannone 4, Napoli, 1-80134, Italia.We must now become familiar with the precise meaning of eij. It represents the part of the total atomization energy which...

An Electronegativity Model for Polar Ground-State Effects on Bond Dissociation Energies

“…[71][72][73] Second, while the rationalization by means of electronegativity arguments allows the formulation of relatively simple and intuitive relationships (see below), the quantitative description in terms of variations of atomic charges requires generally the use of calculations 74 or empirical charge-13 C NMR shift relationships. 71,73,75 Not surprisingly, the consequent analysis in terms of atomic charges becomes complicated even for simple alkanes, 47,76 and has not been rigorously achieved for benzylic and related systems. 29 A number of alternative qualitative explanations for polar ground-state substituent effects have been proposed.…”

Charge distributions and chemical effects. XLVII. Density matrix contribution to the charge distribution in hydrocarbons, arising from singly excited configurations in CI calculations

“…[9], of each single excitation to the second-order correction pf'(r) of the density matrix, it is clear that the singly excited configurations do not mix among themselves at this order of the perturbation expansion. It is thus straightforward to estimate the individual contributions of the various single excitations that are independently involved in the CI wave function.…”

On the calculation of atomization energies of organic molecules in the Xα local spin density approximation

“…The appropriate AE,* energies for the other alkanes (7) and selected alkyl radical (9) were given earlier. In addition, we have used AH: = 51.1 * 1.6 (cyclopropylcarbinyl radical) (22) hydrocarbons, following an empirical rule described elsewhere (9). For the nitrogen compounds we have used standard AH: results (21) and reported values for the ZPE + (H, -H,) energies (10).…”