Improper, Blue-Shifting Hydrogen Bond between Fluorobenzene and Fluoroform

Abstract

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···π 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 H-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 (π 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 CH 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.

On the Problem of He–He Bond in the Endohedral Fullerene He2@C60

“…C 60 is a polyhedron with 20 hexagonal and 12 pentagonal faces follows from a tendency to hold the "foreign" atom or molecule in the fullerene cavity as in a "cage". This ability was confirmed within a week after the discovery of C 60 when synthesizing the so-called lanthanum endofullerene 6 La@C 60 , where the symbol "@" means that the La atom is located inside C 60 [43,53]. Later, a lot of other endofullrenes were discovered.…”

“…It is known that the characteristic feature of the H · · · Y hydrogen bond formation in the complex X-H · · · Y shorter, and the frequency of the valence X-H vibration shifts toward the blue spectral section, i.e. for which Δ (X-H) > 0 (see works [5][6][7] and references therein).…”

“…Saunders, Schwartz et al (later, Thiel et al) [54][55][56][57][58] demonstrated that all atoms of noble gases up to Xe can exist in the C 60 cavity. Furthermore, it was recently shown that more atoms of 6 The etymology of the word "endo" means "inside, in, inner". It comes from the Greek words [endon], which means "in, inside", and…”

Análisis AIM y NBO del enlace de hidrógeno aromático C-H...π en el complejo C2H2/C6H6

“…Se ha informado de su participación en la interacción proteínas-ligando o cofactor como el grupo hemo (Backer et al, 1984) y en la unión de carbohidratos en sitios lectinas (Fernández-Alonso et al, 2005;Sujatha et al, 2005;Kerzmann et al, 2006). En los últimos años, se han realizado estudios teóricos sobre complejos con benceno para evaluar la energía de interacción y para profundizar en el entendimiento de su naturaleza (Hobza et al 1998;Cubero et al, 1999b;Samata et al, 1998;Novoa et al 2000;Reimann et al, 2001;Tsuzuki et al, 1999Tsuzuki et al, ,2000Tsuzuki et al, ,2002Tarakeshwar et al, 2001;Lee et al, 2005;Sinnokrot et al, 2002Sinnokrot et al, ,2004, encontrándose que la interacción C-H…C 6 H 6 , donde C-H corresponde a fuertes donores de protón, como por ejemplo HCN, presentan características similares a la de los EHs clásicos, como son el alargamiento de la distancia del enlace covalente C-H, observación obtenida de los espectro vibracionales Raman e IR muestran un desplazamiento de la frecuencia de estiramiento de los modos vibracionales simétricos, ν s , hacia menores frecuencias (desplazamiento "hacia el rojo") acompañados de un aumento de la intensidad de ν s . Sin embargo existen otros casos donde se produce una contracción del enlace covalente C-H, observados por ejemplo en el dímero C 6 H 6 /C 6 H 6 o el CH 4 /C 6 H 6 con estructura T-shape.…”

“…Sin embargo existen otros casos donde se produce una contracción del enlace covalente C-H, observados por ejemplo en el dímero C 6 H 6 /C 6 H 6 o el CH 4 /C 6 H 6 con estructura T-shape. En estos casos el espectro vibracional exhibe un desplazamiento de la frecuencia ν s hacia mayores frecuencias (desplazamiento hacia el azul), frecuentemente acompañado por una disminución de la intensidad ν s (Hobza et al, 1998(Hobza et al, , 2000Novoa et al, 2000;Reimann et al, 2001). Novoa y colaboradores (2000) han comparado la fuerza de la interacción C-H… con la O-H… y Cubero ha analizando la distribución de carga electrónica, en base a los criterios establecidos por Popelier (Koch et al, 1995;Popelier, 1998), intentando clasificar a las diferentes interacciones, en enlace de hidró-geno propio e impropio, siendo un representante del primero la interacción C-H… encontrada en el complejo C 2 H 2 /C 6 H 6 .…”