JET-COOLED ROTATIONAL STUDIES AT THE VALLADOLID LABORATORY
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Date
2001
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
The low-temperature environment of a free jet expansion provides an excellent laboratory for probing the conformational behaviour of hydrogen bonding. The first observation of axial and equatorial hydrogen-bonded complexes has been reported for tetrahydropyran and pentamethylene sulphide complexes with HCl and $HF.^{a}$ Both six-membered rings present two nonequivalent lone pairs at the O or S atoms and the formation of hydrogen bonds gives rise to two different axial and equatorial conformers. Recently, we have also reported axial and equatorial hydrogen bonds for trimethylene sulphide$\cdots HCl.^{b}$ For isolated trimethylene sulphide the ring-puckering tunnellig motion makes equivalent the nonbonding electron pairs at the S atom. This equivalence is broken by effect of complexation, giving rise to axial and equatorial conformers. In this contribution we present the results on the related pentamethylene and trimethylene sulphide$\cdots$ HF complexes. An unsubstituted alkane such as methane might act as a proton donor only in certain extraordinary circunstances. However 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 alkane trifluoromethane and oxirane has been analyzed by MB-FTMW spectroscopy for the parent and $^{13}C$ isotopomers. A $r_{0}$-like structure has been derived and the barrier to internal rotation of the $CF_{3}$ group has been determined from the observed A-E splittings. A MB-FTMW Spectrometer incorporating a laser ablation source has been constructed in our lab which avoids the use of high temperature techniques commonly used for vaporising low-volatility compounds. As a preliminary test, the rotational spectrum of 1,3,5-trithiane (m.p. $216 ^{\circ}C$) has been observed. The sensitivity of the instrument proved to be high enough to observe $^{33}S$ isotopic species in natural abundance.
Description
$^{a}$S. Antol\'inez, J.C. L\'opez and J.L. Alonso, Angew. Chem. Int. Ed., 1999, 38,1772-1774. M.E. Sanz, J. C. L\'opez and J.L. Alonso, Chem. Eur. J., 1999, 5, 3293-3298. S. Antol\'inez, J.C. L\'opez and J.L. Alonso, ChemPhysChem, 2001, 2, 114-117. $^{b}$M.E. Sanz, A. Lesarri, J.C. L\'opez and J.L. Alonso, Angew. Chem. Int. Ed., 2001,(in press).
Author Institution: Facultad de Ciencias, Universidad de Valladolid; Departamento de Qu\'imica F\'isica, Facultad de Ciencias, Universidad de Valladolid
Author Institution: Facultad de Ciencias, Universidad de Valladolid; Departamento de Qu\'imica F\'isica, Facultad de Ciencias, Universidad de Valladolid