Evidence concerning the relative nucleophilicities of non-bonding and π-bonding electrons in furan from the rotational spectrum of furan···ClF

Abstract

The rotational spectra of the three isotopomers C₄H₄O···³⁵ClF, C₄H₄O···³⁷ClF and C₄D₄O···³⁵ClF of a complex formed between furan and chlorine monofluoride have been observed by means of a pulsed-nozzle, Fourier-transform microwave spectrometer fitted with a fast-mixing nozzle to preclude any chemical reaction among the component gases. Transitions allowed by all three components µ_a, µ_b and µ_c of the electric dipole moment were observed, those associated with µ_c exhibiting a small doubling (ca. 20 kHz). Spectral analysis yielded rotational constants, centrifugal distortion constants and (for the isotopomers C₄H₄O···³⁵ClF and C₄H₄O···³⁷ClF) all five independent components of the Cl nuclear quadrupole coupling tensor χ_αβ (α, β = a, b, c). Diagonalisation of the tensor χ_αβ led to the values of the components χ_ii (i = x, y, z) in the ClF principal axis system and the direction cosines θ_αz (α = a, b, c) relating the ClF axis (z) to the principal inertial axis system. The geometry obtained by fitting the principal moments of inertia of the three observed isotopomers is consistent with the θ_αz values and has the ClF subunit approximately perpendicular to one of the C(2)–C(3) bonds of furan. The end ^δ+Cl of ClF thus appears to interact with the π electron density between these two atoms. The distance of the Cl atom from the point of intersection of the extrapolated ClF axis with the ring is r(···Cl) = 2.726 Å. The doubling of the c-type transitions is interpreted in terms of a motion in which the interaction switches from C(2)–C(3) to C(2′)–C(3′). The contrast between the angular geometries of furan···HCl and furan···ClF is discussed in the context of the relative nucleophilicities of the n-pair and the aromatic π-electrons of furan.