Electronic Structures of Polyatomic Molecules and Valence. V. Molecules RXn

Theory. The approximate construction, for shared electrons in molecules RXn, of molecular orbitals (``orbital'' means one-electron orbital wave function) as linear combinations of atomic orbitals is discussed and illustrated by equations for RX2, RX3, RX4 types. Properties of bonding, nonbonding, antibonding, also excited, molecular orbitals are described. Valence orbitals include both bonding and nonbonding types. RH3 and H3 orbitals are given as examples. Molecular orbitals constructed from atomic orbitals contain undetermined coefficients and implicit parameters (effective Z's of the atomic orbitals) which make them very flexible. They are useful for a qualitative theory which can be compared with empirical, especially chemical and spectroscopic, data. They also have value in semi-quantitative calculations (Van Vleck). Applications. Electronic structures, ionization potentials, form and stability (at least some of these properties in each case) of the molecules CH4, NH3, H2O, NH4, CH3, NH2, BeH2, RXn are described and interpreted in terms of molecular orbitals. It is shown that an electronic structure 1s22[s]22[p]6, closely related to that of the Ne atom, can be assigned to CH4 without the least necessity or justification for departing from the idea of the equivalence of the four C–H bonding directions. This seems in some respects more natural than the Pauling-Slater method in which four equivalent bonds are obtainable only with mixtures of 2s and 2p carbon orbitals. The structures assigned for NH3 and H2O are also related to that of Ne, and that for NH4 to that of Na. The ionization potential of NH4 is estimated as 4.5 volts, slightly greater than for the K atom. The proton affinity of NH3 (energy change for NH3+H+→NH4+) is estimated to be 9 volts. The ionization potential of CH3 is roughly estimated as 8.5 volts, and the energy of formation of CH4 from CH3+H is discussed. Whether molecules RX3 are plane or pyramidal can be considered to depend on the quantitative outcome of a conflict of different factors; three such factors are noted. The first of these gives an explanation of Zachariasen's rule. Empirical evidence is cited showing that the geometrical possibilities of forms RX4 tend to produce stabler electron configurations than are permitted by the possible geometry of RX3. The ultraviolet absorption spectra of CH4, NH3, and H2O are discussed in relation to the ionization potentials of these molecules.