Structure of ClF₃ and Exchange Studies on Some Halogen Fluorides by Nuclear Magnetic Resonance¹

Abstract

of the two 1-fluorine atoms are more nearly the same than if only one of the two rotational configurations alone were populated. The environments averaged over the oscillation, however, arc not identical. Increased frequency of oscillation with temperature elevation would have no effect on the chemical shift since presumably the frequency is sufficiently high even a t the lowest temperature studied to prohibit observation of fluorine resonances characteristic of the two configurations at frequencies of 30 MC. or less.Of the four substituted gem-difluoroethanes studied, only I ,2-dichloro-l, 1-difluoro-2-phenylethane exhibited the temperature dependent spectra discussed above. Spectra for the other ethanes a p peared to be teniperature independent over their liquid ranges. Of the lour substituted ethanes eamined, l,'-dichloro-l, l-difluoro-"-I'heiiylethane with two small chloro substituents rather than with two large bromo substituents would be expected to exhibit the lowest barrier to the 1-30' torsional oscillation postulated in Fig. 4, and consecluently to be most likely to exhibit a temperature dcpendcnt spectrum over the nccessiblc tcinperature range.U'rr.~rvc,rov, DEL The high resolution F' 9 magnetic resonance spectra of CIFa at 10, 30 and 40 Mc. are presented and interpreted in terms of Observed temperature dependencies of the spectri of CIFB and IF:, are .Activation energies for exchange of 4.8 kcal. for C1Fs and 13 kcal. for IF6 arc I t is concluded that fluorine exchange occurs in these halogen the structure of C,, symmetry for the molecule. interpreted in terms of fluorine exchange. estimated from the n-m-r temperature-dependence results.fluorides through a dimer intermediate.

Multi-State VALBOND for Atomistic Simulations of Hypervalent Molecules, Metal Complexes, and Reactions

“…Hence, the MS-VALBOND force field allows us to estimate the barrier involved for the transitions among different T-shape arrangements. By considering the F B –Cl A –F C angle, the free energy for the pseudorotation in ClF 3 was computed and found to be 4.3 kcal/mol (see Figure ), which agrees very favorably with a value of 4.8 kcal/mol from NMR experiments without adjusting the parametrization of the MS-VALBOND force field.…”

“…Comparison of selected geometrical parameters for ClF 3 with their corresponding NMR data 71. The bond distances are inÅ and angles are in degree.…”

Describing Polytopal Rearrangements of Fluxional Molecules with Curvilinear Coordinates Derived from Normal Vibrational Modes: A Conceptual Extension of Cremer–Pople Puckering Coordinates

“…Several years later in 1957, Muetterties and Phillips carried out 19 F NMR studies on IF 5 , and they observed IF 5 being stable at room temperature. However, the absence of F–F coupling pattern in NMR spectra at elevated temperatures indicated apical-basal fluorine exchange . Therefore, they concluded that such apical-basal exchange has a relatively high energy barrier over 25 kcal/mol and they hypothesized that this process would proceed via a dimer structure.…”

“…Sulfur tetrafluoride (SF 4 ), first reported in 1911, has a seesaw ( C 2 v ) geometry. On the basis of 19 F NMR data recorded at different temperatures, SF 4 is considered fluxional as it undergoes fast exchange of ligand positions at room temperature. − Klemperer and co-workers concluded in 1975 that the nonrigidity of SF 4 arises from Berry pseudorotation, , where the axial F–S–F angle closes accompanied by the opening of the equatorial F–S–F angle, passing through a TS with C 4 v symmetry.…”

“…However, the absence of F−F coupling pattern in NMR spectra at elevated temperatures indicated apical-basal fluorine exchange. 121 Therefore, they concluded that such apical-basal exchange has a relatively high energy barrier over 25 kcal/mol and they hypothesized that this process would proceed via a dimer structure. During the following years, little progress was made with regard to a more detailed understanding of the fluxional mechanism of IF 5 until in 2006 when Rzepa and Cass suggested a nondissociative (i.e., intramolecular) mechanism based on quantum chemical calculations.…”