WEAK $C-H\cdots O$ AND $C-H\cdots F$ HYDROGEN BONDS IN THE TRIFLUOROMETHANE$\cdots$OXIRANE AND DIFLUOROMETHANE$\cdots$OXIRANE COMPLEXES
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Date
2002
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Publisher
Ohio State University
Abstract
Alkanes appear capable of forming hydrogen bonds when sufficiently activated by neighboring electronegative substituents. In this context, the $C-H\cdots O$ interaction between the substituted alkanes trifluoromethane and difluoromethane with oxirane has been analyzed by molecular beam Fourier transform microwave spectroscopy. The ground state rotational spectra of $C_{2}H_{4}O\cdots HCF_{3}$, $^{13} CCH_{4}O\cdots HCF_{3}$, $C_{2}H_{4}O\cdots H^{13}CF_{3}$, $C_{2}H_{4}O\cdots H_{2}CF_{2}$, $C_{2}H_{4}O\cdots H^{13}_{2}CF_{2}$ and $^{13}CCH_{4}O\cdots H_{2}CF_{2}$ isotopomers have been studied, in their natural abundances, in the frequency range $6-18$ GHz. A $C_{s}$ symmetry has been established for the oxirane$\cdots$trifluoromethane complex with the C-H bond of trifluoromethane pointing to the domain of the nonbonding electron pairs of the O atom. The cooperative effect of two $C-F\cdots H-C$ interactions increases the stability of the complex. The barrier to internal rotation of the $CF_{3}$ group has been determined from the observed A-E splittings. In the oxirane$\cdots$difluoromethane complex the determined structural data reveals the existence of two bifurcated hydrogen bonds: one between the $CH_{2}$ group of difluoromethane with the O atom of oxirane and the other between the methylenic C-H groups of oxirane with the closest fluorine atom of difluoromethane. To our knowledge this is the first experimental evidence of this behaviour. The $C-H\cdots S$ interaction has been also analyzed in the trifluoromethane$\cdots$tiirane complex. The rotational spectra of $C_{2}H_{4}S\cdots HCF_{3}, C_{2}H^{34}_{4}S\cdots HCF_{3}, {^{13}}CCH_{4}S\cdots HCF_{3}$ and $C_{2}H_{4}S\cdots H^{13}CF_{3}$ isotopic species have been studied in their natural abundance.
Description
Author Institution: Grupo de Espectroscopia Molecular, Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de Valladolid; Dipartimento di Chimica, G.Ciamician Universita' degli Studi di Bologna