Electronic Spectra of Polyatomic Molecules. I. Saturated Aldehydes

Of the two methods which are now available to describe the electronic structures of polyatomic molecules, only the more recent—the orbital method—can at present be applied to the excited levels of molecules, and so any interpretation of the spectra of polyatomic molecules must be done in its terms. This method, which was suggested by Lennard-Jones for diatomic molecules and applied to polyatomic molecules by Hund, Mulliken, and Lennard-Jones, starts on the assumption of the symmetry of the nuclear framework; the electrons are then added one by one to orbitals characteristic of the molecule as a whole and not of any particular atom-pair. In this last feature, the method differs from the earlier one developed by Pauling and Slater on the lines of the Heitler-London treatment of the hydrogen molecule, where the electron-pair bond between two atoms is generally assumed. For some purposes the electron-pair method has advantages over the orbital method (an example of the use of the electron-pair method where the other would be inapplicable is given by fennel's recent work on benzene); the advantages and disadvantages of each have been discussed in various papers, and it is probable that they will both be necessary complement of each other for some time in the elucidation of any particular problem.