The C−F···H−O Hydrogen Bond in the Gas Phase. Rotational Spectrum and ab Initio Calculations of Difluoromethane-Water

Abstract

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.

The C-F⋅⋅⋅H-C “Anti-Hydrogen Bond” in the Gas Phase: Microwave Structure of the Difluoromethane Dimer

“…[15] E D 1/72 k s R 2 CM (2) Such a dissociation energy corresponds to a binding energy of 2.2 kJ mol À1 for each C-H´´´F-C linkage. This value is much smaller than the bond energy (8.7 kJ mol À1 ) for the O-H´´´F-C bond in DFM ± H 2 O, [8] but the ªcooperative effectº of the three weak C-H´´´F-C linkages makes the complex sufficiently stable for its detection with our technique. This is, to our knowledge, the first report on such a small triply hydrogen bonded dimer and on the C-H´´´F-C hydrogen (or anti-hydrogen?)…”

“…[3] A few years ago we introduced free jet millimeter wave absorption spectroscopy for the study of molecular complexes, [4] and obtained the first data from high-resolution spectroscopy on the solvation of ethers, [5] cyclic amines, [6] and diazines [7] through the study of the 1/1 complexes with water. Recently, while investigating the complex of difluoromethane (DFM) with water, [8] we noticed, besides the spectrum of DFM ± H 2 O, a set of evenly spaced m a -R-branch lines. These lines were observed when either argon or neon was used as carrier gas, and persisted when water was removed from the system.…”

On the weak O–H⋯halogen hydrogen bond: a rotational study of CH3CHClF⋯H2O

“…This is confirmed by the investigations of adducts of water with freons. While in CH 2 F 2 -H 2 O the two constituent molecules are held together by a weak (bonding energy, E B = 7.5 kJ mol À1 ) O-HÁ Á ÁF bond, 15 in CF 4 -H 2 O a halogen bond FÁ Á ÁO is observed. 16 Naturally the interesting question arises 1-Chloro-1-fluoroethane (Freon 151, from now CFE), has a structure similar as chlorofluoromethane, but with a hydrogen atom substituted by a methyl group.…”

Noncovalent Interactions and Internal Dynamics in Dimethoxymethane–Water

“…As a consequence, splittings are observable in the pure rotational spectra, [1,3] from which information on the potential-energy surface is obtained. When water acts as a proton donor towards weak proton acceptors, such as a fluorine or chlorine atom, or a p system, the water moiety has a considerable dynamic freedom, which still produces features in the microwave (MW) spectrum, as in the cases of difluoromethane-W, [6] chlorofluoromethane-W, [7] and benzonitrile-W. [8] Finally, when water acts as a proton donor and forms strong hydrogen bonds, such as OÀH···O, [9][10][11][12] OÀH···N, [5,[13][14][15][16] or O À H···S, [17] the water moiety is blocked and no splitting due to its internal dynamics is observed. Some exceptions have been observed, however: 1) in the case of dimethylether-W, [18] the W molecule tunnels between the two lone pairs of the ether oxygen, generating large tunnelling splittings; 2) in anisole-W, [19] the W moiety is delocalized while forming secondary interactions with the adjacent methyl or phenyl hydrogen atoms, allowing tunnelling of the water hydrogen atoms.…”