Identification and characterization of a gas-phase complex of methylenecyclopropane and chlorine monofluoride by rotational spectroscopy

The rotational spectrum of a complex formed between methylenecyclopropane (mecp) and chlorine monofluoride was observed by using a fast-mixing nozzle in combination with a Balle-Flygare microwave spectrometer. Rotational constants A(0), B-0 and C-0, quartic centrifugal distortion constants Delta(J), Delta(JK), delta(J) and delta(K), and the components chi(aa), chi(bb)-chi(cc) and chi(ab) of the Cl nuclear quadrupole coupling tensor were evaluated for each of the isotopomers mecp ...(ClF)-Cl-35 and mecp ...(ClF)-Cl-37. Interpretations of the various spectroscopic constants demonstrated that the observed species is a Mulliken outer complex which may be described formally as of the b pi.a sigma type, i.e. with the electrophilic end Cldelta+ of ClF interacting weakly with the pi bond of mecp and with only minor electric charge redistribution within the ClF subunit. The geometry of the complex was established to be of C-s symmetry. The ClF molecule lies in the principal inertial plane ab, which is the molecular symmetry plane and is perpendicular to the plane of the four carbon atoms of mecp. The angle phi, subtended at the centre (*) of the pi bond by the unique carbon atom of the mecp ring and the Cl atom, is 92.5(5)degrees and the distance r(*... Cl)=2.675(10) Angstrom. The system *... Cl-F deviates from collinearity by 4.9(1)degrees. These properties of mecp ... ClF are discussed in the context of two different series of B ... ClF complexes. In the first, the molecules B are the prototype pi or pseudo-pi electron donors ethene, cyclopropane, allene, mecp and buta-1,3-diene. The molecules B in the second series are the three-membered rings (CH2)(2)Y, where Y is either one of the n-pair donors O or S or the pi donor C=CH2.