A shortening of the C-H bond lengths and a blue shift of the C-H stretching frequencies for the C-F small middle dot small middle dot small middle dotH-C groups indicates that anti-hydrogen bonds are present the difluoromethane dimer. The most stable conformer has three such interactions (shown schematically).

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The free jet millimeter wave spectra of four isotopomers of the 1:1 complex between difluoromethane and water have been observed and assigned. The water molecule lies in the FCF plane of difluoromethane, linked through an O−H···F hydrogen bond to one fluorine atom, and entgegen to the second one. The H bond parameters are r(F···H) = 2.20 Å, ∠(F···H−O) ≅ 135°, and ∠(C−F···H) ≅ 93°, respectively. The oxygen is zusammen to the C atom, and the “free” water hydrogen is entgegen to the difluoromethane moiety. The value of 90° of the C−F···H angle is indicative of the preservation of the p character of the lone pairs of fluorine in the adduct.

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Articles you may be interested inRotational spectra of rare isotopic species of fluoroiodomethane: Determination of the equilibrium structure from rotational spectroscopy and quantum-chemical calculations Internal dynamics features in the free jet rotational spectrum of the acetaldehyde-Kr molecular complexThe rotational spectrum of jet-cooled tetrahydrofuran has been investigated in the 8-18 and 60-78 GHz frequency ranges. Only the four lowest vibrational levels remain populated in the jet conditions. Two pseudorotation vibrational spacings have been measured directly to be ⌬E 01 ϭ21 307.71(3) MHz and ⌬E 23 ϭ61 205.69(3) MHz. They have been used, together with the lowest 10 measured far infrared transition frequencies and with the effective moments of inertia of the ground state and their shifts upon excitation observed in this work, to determine the potential energy function for pseudorotation and the associated structural relaxations. The potential energy barriers are 45 cm Ϫ1 at the envelope conformation of the oxygen and 16 cm Ϫ1 at the twisted conformation of the C ␤ -C ␤ Ј bond relative to the C ␣ OC ␣ Ј plane. The four symmetrically equivalent equilibrium structures are close to the envelope conformations of the CH 2 groups next to the oxygen atom. While the C-O bond lengths and the local CH 2 angles appeared to remain unaffected by pseudorotation, significant changes found for the diagonal O•••C distances as well as for the C-C and C-H bond lengths are of interest as possible effects of hyperconjugation.

Completely planar is the hydrogen-bonded complex of pyrazine and water (see sketch), which was obtained by supersonic expansion and investigated by rotational spectroscopy. The water molecule lies in the plane of the aromatic ring, and the lone pair of electrons on the nitrogen atom functions as the acceptor in the N⋅⋅⋅H-O hydrogen bond, not-as in the corresponding benzene complex-the π electrons.

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