EVIDENCE CONCERNING THE RELATIVE NUCLEOPHILICITIES OF NONBONDING AND PI-BONDING ELECTRONS IN FURAN FROM THE ROTATIONAL SPECTRUM OF FURAN-CLF

The rotational spectra of the three isotopomers C4H4O...(35)C1F, C4H4O ...(37)C1F and C4D4O...(35)C1F of a complex formed between furan and c hlorine monofluoride have been observed by means of a pulsed-nozzle, F ourier-transform microwave spectrometer fitted with a fast-mixing nozz le to preclude any chemical reaction among the component gases. Transi tions allowed by all three components mu(a), mu(b) and mu(c) of the el ectric dipole moment were observed, those associated with mu(c) exhibi ting a small doubling (ca. 20 kHz). Spectral analysis yielded rotation al constants, centrifugal distortion constants and (for the isotopomer s C4H4O...(35)C1F and C4H4O...(37)C1F) all five independent components of the C1 nuclear quadrupole coupling tensor chi(alpha beta) (alpha, beta = a, b, c). Diagonalisation of the tensor chi(alpha beta) led to the values of the components chi(ii) (i = x, y, z) in the C1F principa l axis system and the direction cosines theta(alpha z) (alpha = a, b, c) relating the C1F axis (z) to the principal inertial axis system. Th e geometry obtained by fitting the principal moments of inertia of the three observed isotopomers is consistent with the theta(alpha z) valu es and has the C1F subunit approximately perpendicular to one of the C (2)-C(3) bonds of furan. The end (delta+)C1 of C1F thus appears to int eract with the pi electron density between these two atoms. The distan ce of the C1 atom from the point of intersection of the extrapolated C 1F axis with the ring is r(o...C1) = 2.726 Angstrom. The doubling of t he 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 bet ween the angular geometries of furan...HCl and furan...C1F is discusse d in the context of the relative nucleophilicities of the n-pair and t he aromatic pi-electrons of furan.