It seems like sometimes the electron volt value of the molecular orbital is given but most of the time it just shows relative values of s p d orbitals. Is there some lookup table for such values?

  • $\begingroup$ It can be either experimentally determined (look up photoelectron spectroscopy) or just calculated with a computer (lots of QM if you want to find out more - start with the secular equations perhaps). $\endgroup$ – orthocresol Nov 13 '15 at 17:01

Exact values of MO energy are rarely given as they are difficult to obtain in anything other than relatively trivial systems. When they are given, it is often not even clear whether they are experimentally derived (Photoelectron spectroscopy, as mentioned by orthocresol), or computationally derived (by calculations to approximate the wave functions and their properties). For this reason, most MO diagrams presented are qualitative, derived from our knowledge of symmetry, orbital energy etc.

There is a book, The Organic Chemist's Book of Orbitals (Salem and Jorgensen) which gives the MO's of many simple, and slightly more complex molecules along with computationally derived energies (in eV).

For anything more complicated, you could also try searching the literature, however this would probably just turn up very specific examples that people have calculated to support their research.

If you need specific energies for molecules, you might look into computing them yourself. Simple software such as Spartan does an acceptable job at computing MO energy, with a variety of computational methods available depending on how much computing power you have.

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