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 infrare d ion-depletion spectroscopy (IR/R2PI) and theoretically by correlated ab i nitio methods. Their predissociation spectra exhibit an absorption comprise d of two superimposed bands. These are blue-shifted by 12 and 21 cm(-1), re spectively, 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-) correc ted gradient optimization combined with anharmonic vibrational analysis usi ng 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 san dwich and a planar structure, respectively. The theoretical and experimenta l shifts are thus well comparable. Natural bond orbital (NBO) analysis of t he sandwich complex as well as analysis of the type and shape of the occupi ed molecular orbitals revealed the nature of the blue-shift. It is shown th at 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 bon d the primary interaction is caused by an electron density transfer (EDT) f rom 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-shi fting 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 stru ctural reorganization of the fluoroform, including the contraction of the C K,bond and a corresponding blue-shift of its CH stretching frequency. The N BO analysis as well as the analysis of the type and shape of the HOMO and H OMO-1 orbitals both elucidate the larger blue-shift for the sandwich-type i somer of the fluorobenzene fluoroform cluster compared to the equivalent ch loroform complex.