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

Abstract

Theory. The approximate construction, for shared electrons in molecules RX_n, 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, RX_n 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.