Precise Equilibrium Structure of Benzene

Recent advances in gas-phase structure determination afford outstanding agreement between the CCSD­(T)/cc-pCVTZ-corrected semi-experimental (<i>r_e</i>^SE) equilibrium structures and their corresponding best theoretical estimates (BTEs) of the equilibrium structures (<i>r_e</i>) based upon corrections to the CCSD­(T)/cc-pCV5Z geometries for the aromatic heterocycles pyrimidine and pyridazine. Herein, that same analysis is extended to the fundamental aromatic molecule benzene, using published experimental spectroscopic data for a total of 11 available isotopologues. The incorporation of rotational constants from all of these isotopologues and CCSD­(T) corrections to address the impacts of both the vibration-rotation interaction and electron-mass distribution results in a highly precise and accurate <i>r_e</i>^SE structure. The CCSD­(T)/cc-pCV5Z optimized geometry has been further corrected to address a finite basis set, untreated electron correlation, relativistic effects, and a breakdown of the Born–Oppenheimer approximation. This analysis achieves outstanding agreement between the <i>r_e</i> (BTE) and <i>r_e</i>^SE structural parameters of benzene to a highly satisfying level (0.0001 Å), an agreement that surpasses our recently published structures of the aforementioned nitrogen-substituted benzene analogues. The <i>D</i>_6<i>h</i> geometry of benzene is now known to an unprecedented precision: <i>R</i>_C–C = 1.3913 (1) Å and <i>R</i>_C–H = 1.0809 (1) Å. The mutual agreement between theory and experiment presented in this work validates both, substantially resolving all discrepancies between the <i>r_e</i>^SE and theoretical <i>r_e</i> structures available in the literature.