Rotational spectrum, nuclear quadrupole coupling constants, and structure of six isotopomers of the argon-chlorocyclobutane van der Waals complex

The Fourier Transform Pulsed Jet Microwave spectra of the argon-chlorocyclobutane van der Waals complex have been assigned for six isotopomers. These include the 35Cl and 37Cl each with four 12Cs in the ring plus the four isotopomers of the 35Cl with one 13C at each position in the ring. The spectroscopic constants for the Ar-12C4H7 35Cl complex are AZ2675.2641(6) MHz, BZ977. 8306(3) MHz, CZ764.9382(2) MHz; DJZ1.737(1), DJKZ7.176(6), DKZ15.28(2), dJZ0.4148(6), dKZ6.61(3) kHz; and caaZ28.011(6) MHz, (cbb–ccc)ZK80.378(6) MHz, cabZK22.8(1) MHz, cacZK5.4(7) MHz and cbcZK26.47(9) MHz. The structure of the ground vibrational state of the complex has an equatorial chlorine as in the non-complexed ring and the argon coordinates are aZ1.267 A ° , bZ2.824 A ° and cZ2.517 A ° in the principal axis system of the monomer ring. The chlorine coordinate in this axis system is aZ1.66, bZ0.0 and cZ0.08 A ° . The argon is exo to the ring 2.4 A ° above the C–Cl bond but 2.8 A ° to the side of the plane of symmetry of the chlorocyclobutane monomer. Argon is within the van der Waals radii of the nearest cross-ring methylene hydrogen and outside the van der Waals radii of chlorine. It is not directly above the ring as has been observed in other four-membered ring complexes. The off-diagonal quadrupole moment of the monomer, chlorocyclobutane, has been measured, cacZ32.0(6) MHz. The quadrupole tensor for the chlorine in the complex was fit including all three off diagonal elements. The van der Waals bond does not significantly perturb the electronic gradients at the chlorine nucleus of the monomer, since each element of the quadrupole tensor measured for the complex, except one small off-diagonal constant, is within 3% of that projected onto the complex from chlorocyclobutane. The argon position is also consistent with the observation and intensities of a, b, and c-type rotational transitions from the projection of the monomer dipole moment onto the complex principal axis system. q 2005 Elsevier B.V. All rights reserved.