The microwave spectrum, ab initio analysis, and structure of the fluorobenzene–hydrogen chloride complex

Abstract

The fluorobenzene-hydrogen chloride-hydrogen-bonded complex has been studied by high resolution microwave spectroscopy and ab initio calculations. Rotational spectra of the C 6 H 5 F-H 35 Cl, C 6 H 5 F-H 37 Cl, and C 6 D 5 F-H 35 Cl isotopomers were assigned using pulsed molecular beam techniques in a Fourier-transform microwave spectrometer. The spectra are consistent with a structure of the complex in which the HCl is above the fluorobenzene ring near the ring center, similar to the benzene-HCl prototype dimer. An analysis of the inertial data and the chlorine quadrupole coupling tensor results in two mathematically possible locations for the HCl subunit with respect to the fluorobenzene arising from sign ambiguities in interpreting the spectral constants. One structure has the HCl nearly perpendicular to the aromatic ring; the other has the HCl pointing toward the fluorine end of the ring. Spectral intensities for the a and b transitions favor the former configuration. Ab initio calculations ͑MP2/6-311ϩϩG͑2df,2pd͒ϩBSSE corrections͒ indicate that the position of the HCl is driven by electrostatic interactions with the electrons of the benzene ring. HCl is shifted by 0.16 Å from the center of the ring toward the para-C atom, where the density is significantly higher. In the equilibrium form, HCl is tilted by ␦ϭ14°f rom perpendicular to the ring with the hydrogen end toward the para-C atom. The H atom can perform an internal rotation or at least a half-circular libration ͑barriers smaller than 100 cm Ϫ1 ͒. An average ␦ value of 0.7°is estimated in reasonable agreement with the derived vibrationally averaged value of 3.8°. The complex binding energy ⌬E calculated at the CCSD͑T͒/ 6-311ϩϩG͑2df,2pd͒ϩCP͑BSSE͒ level of theory is 2.8 kcal/mol, suggesting a lower ⌬E value for benzene-HCl than previously reported. Fluorobenzene-HCl possesses some charge transfer character; however, just 5.5 melectron are transferred from the benzene ring to HCl. In view of this,-H bonding in fluorobenzene-HCl is predominantly electrostatic rather than covalent in character contrary to claims made in connection with benzene-HCl.

Effect of aromatic ring fluorination on CH⋯π interactions: rotational spectrum and structure of the fluorobenzene⋯acetylene weakly bound dimer

“…39 The B1.5% reduction of this planar moment upon complexation is fairly typical of weakly bound complexes (with, for example, reductions of B1.2% 36 and 1.9%, 37 respectively, in the complexes of CHClF 2 and CHBrF 2 with acetylene), and a little higher than the B1% reduction in fluorobenzeneÁ Á ÁHCl. 27 This is likely due to motions of the monomers (in this case acetylene) within the complex, leading to Coriolis perturbation effects similar to those described for argon vibration in pyrimidineÁ Á ÁAr. 38 The planar moment values for 13 C substitutions at the ortho and meta carbon atoms are slightly larger than for the normal species, indicating that these atoms lie out of the dimer symmetry plane.…”

“…The relative energies are given using the uncorrected electronic energy (DE), electronic energy with zero-point energy correction (DE(+ZPE)), and zero-point energy corrected electronic energy with basis set superposition error (BSSE) correction (DE(+ZPE+BSSE)) proposed in the FBZÁ Á ÁHCl 27 or BZÁ Á ÁHF 19 cases; however, there is no strong evidence either for or against this motion in the spectrum and a detailed theoretical analysis similar to that in ref. 27 is beyond the scope of the current work.…”

Radicals formed in cytosine–hydrochloride(thiocytosine) single crystals: Insights from a DFT study

“…In many instances the charge transfer properties between the chlorine-containing moiety and the biomolecule are of special interest [11,12]. Following the results obtained from this work it will be possible to give comments on this kind of interactions but, as is well known and to be explained in detail later, special care has to be taken of the effects from the radical environment.…”

Pulsed Nozzle Fourier Transform Microwave Spectrometer: Advances and Applications

“…Both of them have similar structure with H 2 X lying along the C 6 axis resulting in a symmetric top spectrum for the ground state. [122] Rotational spectrum of fluorobenzene -HCl [123] and fluorobenzene -H 2 O [124] have been determined quite recently. Experimental results on hydrogen bonded complexes with first and second group hydrides will be useful in bringing out the similarities and differences in bonding.…”