Assessment of Quantum Mechanical Models Based on Resolved Orbital Momentum Distributions of n-Butane in the Outer Valence Shell.

Fully resolved outer valence orbital momentum distributions (MDs) of n-butane (C4H10) in the ground electronic state (X1Ag) are studied quantum mechanically using RHF/TZVP, density functional theory (DFT) DFT-BP/TZVP, and B3LYP/TZVP methods. The orbital MDs are simulated to reflect the recent experimental conditions with the plane wave impulse approximation (PWIA) and are compared favorably with the available experimental orbital cross sections. However, the majority of the outer valence molecular orbitals (MOs) of n-butane has been only partially resolved experimentally, forming into three clustered MOs of 7ag + 2bg + 6ag, 2au + 6bu and 1bg + 5bu + 5ag. Deconvolution of the clustered MOs is a challenge experimentally but rather straightforward theoretically, as the inversion is a multiple channel process. The outer valence MOs are crucial to understanding the chemical bonding mechanism and the unresolved outer valence orbitals cause significant bonding information loss. This work provides an orbital based assessment to the quality of the RHF/TZVP, DFT-BP/TZVP, and B3LYP/TZVP models using orbital MD information, by decomposing the clustered outer valence MOs of n-butane, which also reveals the bonding mechanism of the species.