Charge distributions and chemical effects. XXIV. On the role of vibrational energies in determining molecular stabilities

Abstract

A simple equation has been derived for calculating accurate ZPE + HT -Ho (zero-point and heat content) energies of saturated hydrocarbons from their enthalpies of formation and carbon-13 nuclear magnetic resonance spectra. Applications to conformational analysis indicate a near invariance of vibrational energies with respect to chair-boat conformational changes of the cyclohexane ring, the loss in molecular stability arising then from a weakening of the chemical binding due to a reorganization of the electronic charges. The origin of the destabilizing effect of butanegauche interactions is found, in the cyclohexane series, in a weakening of the chemical binding (-1.85 kcallmol) which is partially compensated by a lowering (-0.85 kcal/mol) vibrational energy, thus offering an explanation for the loss in molecular stability of -1.00 kcallmol for one gauche interaction without invoking Coulomb-type repulsions between non-bonded atoms. Calculated enthalpies of formation are presentd for a number of cycloalkanes.On a deduit une equation simple pour calculer avec precision les energies ZPE + HT -Ho (point zero et contenu de chaleur) des hydrocarbures satures a partir de leurs enthalpies de formation et de leurs spectres rmn du I3C. Les applications a I'analyse conformationnelle indiquent une quasi invariance des energies vibrationnelles par rapport aux changements de conformation chaise-bateau du cycle cyclohexane, la perte de la stabilitC moleculaire provient alors de I'affaiblissement des liaisons chimiques provoqukes par une reorganisation des charges Clectroniques. En s6rie cyclohexanique I'origine de I'effet destabilisant des interactions butane gauche reside dans un affaiblissement des liaisons chimiques (-1.85 kcallmol) qui est partiellement compense par un abaissement (-0,85 kcallmol) de l'energie vibrationnelle, apportant ainsi une explication de la perte de stabilite molCculaire de -1,00 kcallmol pour une interaction gauche sans invoquer des repulsions de type Coulombien entre les atomes non lies. On presente des enthalpies de formation calculCes pour un certain nombre de cycloalcanes.[Traduit par le journal] IThe atomizationof a molecule into its constituent n, atoms A,, n2 atoms A2, ... provides a direct insight into fundamental aspects of molecular energies. The relevant thermodynamic information is usually expressed in terms of enthalpy of formation, AHf, or enthalpy of atomization, AH,, of the molecule under consideration, i.e.[I] AH, = CniAHXAi) -AHf where the AHXAi)'s are the enthalpies of formation of the gaseous atoms Ai. A number of schemes viewing the molecule as an assembly of chemical bonds or groups and the " molecular energy" as a sum of appropriately defined contributions has been proposed for calculating AH, (1). In the study of isolated molecules, however, we refer more conveniently (2) to the energy of atomization Ma, i.e.[2] AE, = AH, -(Ijzi -l)RT Both AE, and AH, are considered at some temperature, say 25"C, i.e. under working conditions which are of real practical interest. This situati...

Charge distributions and chemical effects. XXXI. Molecular energies of ethylenic compounds

“…The appropriate expressions were successfully tested (1)(2)(3) in comparisons with experimental atomization energies, AE:, given by eq. [l] (see Appendix I: glossary).…”

“…These verifications were made for linear and branched paraffins, including saturated hydrocarbons constructed from chair and/or boat cyclohexane rings (1,2), as well as for ethers, aldehydes, and ketones (3). The present study offers a test for simple alkenes C,,H2,.…”

Charge distributions and chemical effects. XXVII. Molecular energies of carbonyl compounds and ethers, a calculation involving the charges of the bond-forming atoms

“…With these reservations in mind, our predicted BNP values should represent reasonably good estimates. 4Similar situations are described for saturated hydrocarbons (22 (This conclusion holds whenever the sum Cjaii, measuring the stabilization of all the bonds formed by atom i by an electronic charge added to it, is more negative than a,,). So, for example, if we are asked why adamantane is more stable than twistane (by 9.1 kcallmol in AE,* energy), the answer is that the largest part by far of this energy difference is due to the fact that adamantane has less electrons on the H atoms and, consequently, more on the C atoms, than twistane.…”