Trends in Bond Dissociation Energies for the Homolytic Cleavage of Successive Molecular Bonds

Abstract

The students in our physical chemistry course pointed out an interesting trend in the bond dissociation energies (BDEs) of successive homolytic cleavages of hydrogens from methane and water. Namely, while there is an increase in BDE from CH4 to CH3, there is a decrease in BDE from H2O to OH. In order to explain this trend, we employed a theoretical approach that could be used in an undergraduate setting. We compare our theoretical results with experimental and highly accurate theoretical values readily available in the literature. Having validated our theoretical approach, we use equilibrium molecular structure as evidence of the change in dominating attraction and repulsion effects of the central atom nucleus and nonbonding electrons and explain the anomaly in BDEs in undergraduate chemistry terms.

VSEPR-Plus: Correct Molecular and Electronic Structures Can Lead to Better Student Conceptual Models

“…Replacing a VSEPR-only model with a VSEPR-plus model leads to a more straightforward conceptual approach. Several previously described examples are highlighted here. − In each of these cases, a correct understanding of atom and lone pair hybridizations makes understanding the chemical concept much more straightforward. In each case, the electron-geometry predicted by a VSEPR-only approach makes the chemical concept more intellectually awkward to understand.…”

“…A good scientific model, and thus a good model for science students to learn, explains the available scientific data and allows the scientist (or student) to extrapolate to related cases and examine those cases by experimentation and observation. Valence shell electron pair repulsion (VSEPR) has long served as the go-to model for students to understand the molecular and electronic structures of simple molecules. − Well before its adoption into the undergraduate curriculum, the photoelectron measurements of many small molecules provided experimental data that VSEPR was unable to rationalize − and are described in several previous works in the chemical education literature. − Shown in Figure are several known examples (not an all-inclusive list) that highlight the shortcomings of the VSEPR model. VSEPR fails to correctly describe the electron-geometry of water (explored further below); − ,, the electron and molecular geometry of aldehydes, ketones, and carboxylic acid derivatives; ,,, the structure of metal complexes; the molecular geometry of thiols, disulfides, and peroxides; ,, and the molecular and electron-geometry of organic halides. ,, While VSEPR may often predict the correct molecular geometry, it does not always do so and it predicts the geometry from an overly simplistic basis, focusing solely on steric repulsion and ignoring orbital mixing, π-conjugation, dipole–dipole interactions, and hydrogen-bonding.…”

“…8−18 Shown in Figure 1 are several known examples (not an all-inclusive list) that highlight the shortcomings of the VSEPR model. VSEPR fails to correctly describe the electron-geometry of water (explored further below); [9][10][11][12][13]15,17 the electron and molecular geometry of aldehydes, ketones, and carboxylic acid derivatives; 4,9,15,16 the structure of metal complexes; 14 the molecular geometry of thiols, disulfides, and peroxides; 9,13,18 and the molecular and electron-geometry of organic halides. 13,16,19 While VSEPR may often predict the correct molecular geometry, it does not always do so and it predicts the geometry from an overly simplistic basis, focusing solely on steric repulsion and ignoring orbital mixing, π-conjugation, dipole−dipole interactions, and hydrogen-bonding.…”

The Computational Design of Two-Dimensional Materials

“…In addition to the mandatory experiments, students are required to design an independent computational project in consultation with the instructor. They may choose an experiment that has already been published in this journal (e.g., refs − ), design a project that is relevant to research projects they have carried out in experimental groups, or explore technical aspects of first-principles calculations. This allows students to focus on topics that are interesting to them and is relevant to their diverse backgrounds.…”

Determination of Energy Content of “Zero-Sugar” Artificial Sweeteners via Bomb Calorimetry and Theoretical Calculations

“…Another method that can be used to determine the energy content of molecules and chemical reactions is tabulated average bond energies. − This method is often taught during the first semester of an introductory/general chemistry course. After this topic is covered in general chemistry, it is often not tied to laboratory activities for the remainder of the chemistry curriculum.…”