Michael D. Mosher and S. Ojha describe an experiment that helps students to become familiar with NMR spectroscopy and deals with orbital hybridization (J. Chem. Educ. 1998, 75, 888). Hybridization is an important concept, especially in physical organic chemistry, and has had different interpretations and been at the center of some controversy. For some authors, hybridization appears as a real change that atoms can undergo and is considered capable of explaining structural properties such as molecular geometry. Others, recognizing the nonphysical nature of the concept, choose to ignore it entirely in their discussion of molecular structure and properties. A third group seems to do both. That is, they discuss the "effects" of orbital hybridization at the same time that they warn that this is only a mathematical artifact or, more vaguely, an approximation.

Orbital hybridization is invoked when a localized description is made of chemical bonds. It is only when the residual delocalization interactions are neglected that hybridization seems to have an effect. Indeed, when a full delocalized (MO or VB) treatment is performed, the calculated properties (with a given orbital basis set) and the molecular electronic wave function are oblivious of the type of hybridization chosen, if any.

Hybrid orbitals can, of course, be chosen so that those residual interactions are minimized, keeping identical descriptions for equivalent bonds. This is why it is more appropriate, for example, to choose approximate sp³ carbon hybrids for C₂H₆ and approximate sp hybrids for C₂H₂, and not the other way round, as a consequence of the differences of molecular geometry. Residual delocalization is unimportant for some properties (e.g., total electron density, dipole moments, bond energies, pK_a values) so that direct correlations can be established with the hybridization. However, these are not cause-effect relationships because those properties and the hybridization corresponding to minimized delocalization are different manifestations (in the former case physical, in the latter, mathematical) of the same real feature--that is, the molecular geometry. The same residual delocalization can, however, be of paramount importance in the interpretation of other properties, in particular those that depend directly on the energy levels of the electrons. A clear example is the photoelectron spectrum (two valence ionization energies) of CH₄, for which a localized bond description would directly predict just one ionization energy. Another example is provided by nuclear spin coupling in NMR spectroscopy, which is also related to individual electronic energy levels.

The article by Mosher and Ojhan rightly aims at correlating several properties--especially, directly bonded ¹³C-¹H coupling constants (J) with carbon hybrid orbitals. However, in places the authors seem to hesitate between explanation and correlation and seem to regard orbital hybridization as something unique, although no reference to residual delocalization is made. We also note that, after the relationship ¹J_CH = 500 x (s character) was established, residual delocalization effects were shown to be non-negligible (1) and the above simplistic relationship was shown to be the result of a fortuitous cancellation of effects: (i) the use of sp^n-1 orbitals for hydrocarbons (with n the carbon coordination number) and (ii) the neglect of changes in electronic excitation energies with n. Should hybrid orbitals appropriate to minimized residual delocalization be used, the ¹J_CH values are no longer proportional to the s character of the carbon hybrid but are given by

¹J_CH = 769 (s character)^3/2 + 6.5 Hz (SD = 5.9 Hz)

for hydrocarbons and derivatives (2).

Literature Cited

1. van Duijneveldt, F. B.; Gil, V. M. S.; Murrell, J. N. Theor. Chim. Acta 1966, 4, 85.
2. Gil, V. M. S. Theor. Chim. Acta 1989, 76, 291.