Improper, blue-shifting hydrogen bond between fluorobenzene and fluorobenzene

Weakly-bonded 1:1 complexes between fluorobenzene (Fb)/fluorobenzene-d(5) (Fb-d(5)) and fluoroform (Ff) were investigated spectroscopically by infrared ion-depletion spectroscopy (IR/R2PI) and theoretically by correlated ab initio methods. Their predissociation spectra exhibit an absorption comprised of two superimposed bands. These are blue-shifted by 12 and 21 cm(-1), respectively, relative to the CH stretch of isolated fluoroform. Each IR band is assigned to a different hydrogen-bonded fluorobenzene fluoroform isomer; The isomer with the most blue-shifted CH stretching vibration (21 cm(-1)) is assigned to a sandwich type structure, exhibiting a CH rt hydrogen bond. The cluster structures have been calculated by counterpoise- (CP-) corrected gradient optimization combined with anharmonic vibrational analysis using the CP-corrected Hessians. The predicted blue-shifts are 21 and 20.5 cm(-1) for the CH stretching frequencies of fluoroform upon formation of a sandwich and a planar structure, respectively. The theoretical and experimental shifts are thus well comparable. Natural bond orbital (NBO) analysis of the sandwich complex as well as analysis of the type and shape of the occupied molecular orbitals revealed the nature of the blue-shift. It is shown that the nature of the improper, blue-shifting H-bond in this complex differs from that in a common I-I-bond. While in the common XH . . .Y hydrogen bond the primary interaction is caused by an electron density transfer (EDT) from the electron donor Y to the antibonding orbitals of XH, leading to the red-shift and bond elongation in XH, the features of the improper, blue-shifting H-bond are due to secondary effects. In the sandwich complex the EDT takes place between the electron donor (pi electron clouds of fluorobenzene) and the lone pairs of the fluorine atoms of fluoroform, leading to a structural reorganization of the fluoroform, including the contraction of the CK,bond and a corresponding blue-shift of its CH stretching frequency. The NBO analysis as well as the analysis of the type and shape of the HOMO and HOMO-1 orbitals both elucidate the larger blue-shift for the sandwich-type isomer of the fluorobenzene fluoroform cluster compared to the equivalent chloroform complex.