Knowledge Bank

The structure of the fluorobenzene-HCl complex has been studied by Fourier-transform microwave spectroscopy and ab initio calculations. Rotational constants for three isotopomers ($C6H5F-H35Cl,C6H5F-H37Cl$ and $C6D5F-H35Cl$) are consistent with a $\pi$-hydrogen bonded structure with the HCl located above the fluorobenzene ring, near the ring center, and the H atom of HCl pointing towards the $\pi$-cloud of the ring. Two structures are consistent with the inertial data, one with the HCl almost perpendicular to the ring and the other with the H pointing toward the fluorine end of the ring. Ab initio calculations at the $MP2/6-311++G(2df,2pd)$ level (+ BSSE corrections) were carried out to obtain difference electron density plots and indicate that the HCl is tilted by about $14\circ$; from perpendicular, with the H atom toward the para carbon atom (where the $\pi$-density is significantly higher). An internal semi-circular libration of the H atom is possible, leading to an average inclination angle of about $0.7\circ$, in reasonable agreement with the first of the two possible experimental structures. The calculations indicate that the bonding in the fluorobenzene-HCl complex is primarily electrostatic in nature with about 5.5 melectron transferred from the benzene ring to the HCl. Calculation of the complex binding energy at the $CCSD(T)/6-311++G(2df,2pd)+CP(BSSE)$ level gives a value of 2.8 kcal/mole.