A Theoretical Computation of the Aromaticity of (Benzene)Cr(CO)₃ Compared to Benzene Using the Exaltation of Magnetic Susceptibility Criterion and a Comparison of Calculated and Experimental NMR Chemical Shifts in These Compounds

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Abstract

A theoretical calculation of the aromaticity of benzene relative to (benzene)Cr(CO)3 (1) based on the exaltation of magnetic susceptibility criterion was carried out using ab initio MO theory. As others have also found, benzene exhibits a diamagnetic susceptibility exaltation, Λcalc = −15.1 ppm cgs, Λexp = −13.7 ppm cgs, and is aromatic. In contrast, (benzene)Cr(CO)3 (1) has a positive susceptibility exaltation, Λcalc = 12.3 ppm cgs, characteristic of an antiaromatic compound. The validity of susceptibility exaltation as an aromaticity indicator for organometallic compounds was also tested for (cyclobutadiene)Fe(CO)3 (16), which proved to be aromatic (Λcalc = −6.10 ppm cgs). The validity of the calculations was further supported by a comparison of the calculated isotropic susceptibility χav of 1 (−109.3 ppm cgs) with an experimental result (−113 ± 22 ppm cgs). The related NMR calculations for 1 reproduce very well the 13C solid state results of Waugh and also the experimental isotropic upfield shift of ca. 2 ppm seen in the 1H NMR spectra of complex 1 relative to benzene. Contrary to the usual assumptions, the in-plane shieldings of the complexed benzene ring are more important than the perpendicular (ring current) counterparts. As expected, the present theoretical study reproduces very well the experimental geometries, energies, and harmonic frequencies of the purely organic compounds, but there is also very good agreement in the calculated properties of the organometallic compounds, where such data are available for comparison. The present study is based on GIAO, CSGT, and IGAIM NMR calculations performed on the optimized geometry of the most stable conformation at the B3LYP/6-311+G** level for the 12 organic and organometallic compounds needed directly or indirectly for the “group increment” magnetic susceptibility exaltation determinations. The organometallic structures include Cr(CO)6 (10), (ethylene)Cr(CO)5 (11), (1,3-butadiene)Cr(CO)4 (12), (benzene)Cr(CO)3 (1), Fe(CO)5 (13), (ethylene)Fe(CO)4 (14), (1,3-butadiene)Fe(CO)3 (15), and (cyclobutadiene)Fe(CO)3 (16).

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