Structural characterization of anti- and syn-allyltricarbonyliron bromide: Rotational spectra, quadrupole coupling, and density functional calculations

Rotational transitions for two distinct structural isomers of allyltricarbonyliron bromide have been clearly observed in the cold molecular beam of a pulsed-beam Fourier transform microwave spectrometer. Rotational transitions exhibiting quadrupole splitting patterns for each isomer were measured for the ⁷⁹Br and ⁸¹Br isotopomers. Both isomers are accidental near-prolate symmetric tops. The measured rotational constants for the ⁷⁹Br isotopomer are A(anti) = 920.6148(2) MHz, B(anti) = 582.8866(12) MHz, C(anti) = 581.3027(12) MHz, A(syn) = 919.5055(1) MHz, B(syn) = 584.1865(1) MHz, and C(syn) = 581.6392(1) MHz. Analysis of the isotopic substitution data and possible transition assignments indicates that these molecules have C(s) symmetry. Both isomers are found to have a dipole component along the a axis. However, the anti isomer has a 'c' type dipole component, whereas a 'b' dipole component is found for the syn isomer. It was found necessary to carefully analyze both rotational constants and the quadrupole coupling data in order to determine the correct assignment of dipole moment components for each isomer. This change in dipole assignments implies that there is a switch of inertial axes upon isomerization resulting from a subtle shift of the allyl center of mass coordinates, upon reorientation of the allyl ligand. The X-ray and DFT calculated structures for the anti isomer are in excellent agreement with the present data. No previous structural data for the syn isomer were available, and the present analysis strongly supports the expected conformation.