Linear free energy relationships in C-N bond dissociations in molecular ions of 4-substituted N-(2-furylmethyl)anilines in the gas phase

Relationship Investigation between C(sp2)-X and C(sp3)-X Bond Energies Based on Substituted Benzene and Methane

The C-X bonds of organic compounds between group X and a saturated or unsaturated carbon atom differ in bond energy. To identify the causes of variation is of great significance in terms of bond nature understanding and bond energy estimation. In this paper, the electronegativity χ[X] of group X was calculated by the "valence electron equalized electronegativity" method. Then, χ[X] and the electronic effect constant of the substituent were taken as variables to establish equations for quantitative correlation between C(sp³)-X and C(sp²)-X for the calculation of C-X bond energies. The aim is make comparison between substituted methane, Me-X, and substituted benzene, Ph-X, as well as that between Me-X and substituted ethylene, C₂H₃-X. We conducted calculation over 40 compounds that contain different X groups, and the results reveal that the C(sp³)-X and C(sp²)-X bond energies are under the influence of a number of factors. In addition to the covalent properties of C and X atoms and χ[X], the bond energies of C(sp²)-X (i.e., D[C(sp²)-X]) are under the influence of the field/inductive effect (σ_F[X]) and conjugated effect (σ_R[X]) of group X, with the former causing a decrease while the latter an increase of D[C(sp²)-X]. Using the acquired quantitative correlation equations and on the basis of a relatively rich set of measured D[Me-X] data, we estimated D[Ph-X] of Ph-X and D[C₂H₃-X] of C₂H₃-X, and the estimation accuracy is within experimental uncertainty. Employing the above method, the D[C(sp²)-X] of 33 substituted benzenes, 53 substituted ethenes, and 82 α-substituted naphthalenes was estimated with satisfactory outcomes.

Density functional theory and RRKM calculations of the gas-phase unimolecular rearrangements of methylfuran and pyran ions before fragmentations

The potential energy profiles for the mutual conversion of the isomeric molecular ions [C5H6O]+* of 2-methylfuran, 3-methylfuran and 4H-pyran and the fragmentations that lead to [C(5)H(5)O](+) ions were obtained from calculations at the B3LYP/6-311G++(3df,3pd)//B3LYP/6-31G(d,p) level of theory. The various competing unimolecular processes were characterized by their RRKM microcanonical rate coefficients, k(E), using the sets of reactant and transition state frequencies and the kinetic barriers obtained from the density functional method. In either a high- or a low-energy regime, the pyrylium ion [C5H5O]+ is generated directly from the 4H-pyran molecular ion by a simple cleavage. In contrast, in the metastable kinetic window, the molecular ions of methylfurans irreversibly isomerize to a mixture of interconverting structures before dissociation, which includes the 2H- and 3H-pyran ions. The hydrogen atoms attached to saturated carbons of the pyran rings are very stabilizing at the position 2, but they are very labile at position 3 and can be shifted to adjacent positions. Once 4H-pyran ion has been formed, the C-H bond cleavage begins before any hydrogen shift occurs. According to our calculation, there would not be complete H scrambling preceding the dissociation of the molecular ions [C5H6O]+*. On the other hand, as the internal energy of the 2-methylfuran molecular ion increases, H* loss can become more important. These results agree with the available experimental data.