A Study of Chemical Exchange in Unequally Populated Systems by Novel NMR Methodologies. Application to the Cis-Trans Isomerization in Furfural

Abstract

The internal rotation of the aldehyde group with respect to the ring in 2-furanaldehyde (furfural) provides an example of chemical exchange in a two-site system with unequal populations. In a polar solvent, the two isomers are present in unequal amounts. In the present study, rates of exchange for the cis-trans isomerization process of furfural in acetone are measured using a combination of the offset-saturation method for determination of fast rates of exchange, time-domain line shape analysis for measurement of intermediate rates of exchange, and selective-inversion measurements of slow exchange rates. This combination of NMR techniques permits the determination of rates of exchange over 5 orders of magnitude, covering a temperature range of 177-318 K. The utility of the offset-saturation method for fast rate measurement is highlighted in the present system, where the exchange rapidly enters the fast exchange regime. The corresponding values of the activation parameters are the energy of activation E, = 41.9 & 0.9 kJ mol-', the enthalpy of activation AH* = 40.0 i 1 kJ mol-', and the entropy of activation AS* = -26.8 k 5.2 J K-' mol-'.

MF Interactions and Heterobimetallics: Furthering the Understanding of Heterobimetallic Stabilization

“…This result alone indicates that these interactions are maintained in solution at room temperature and have a significant impact on the structure. Line-shape analysis methods were utilized to determine the energy of rotation for the À CF 3 group, [56][57][58][59][60] giving values of 18.75 and 19.08 kcal mol À1 for 6 and 7, respectively. These interaction energies are rather significant, because they rank in the same energy range as strong hydrogen bonds (ca.…”

Theoretical model of internal rotation in monosubstituted derivatives of furfural

“…The conformational equilibrium between planar forms of furfural (Fig. 1) has received special attention in both experimental and theoretical articles,1–14 including the effect of solvent on the OO‐cis/OO‐trans population ratio and on rotational barriers. The relative facility for the industrial production of this compound from pentose‐rich biomass and its two well‐defined, self‐exchangeable and stable isomers have sparkled imagination as a prospect for a single molecule bit of information storage.…”

Nickel as a Lewis Base in a T‐Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand

“…A lineshape analysis yielded the activation parameters ΔH ≠ = 32.7 kJ·mol -1 and ΔS ≠ = -96 J·mol -1 ·K -1 , suggesting an associative chloride exchange rather than ring flipping dynamics. 23 Complex 5 crystallizes as an ion pair, [(2)GeCl]Cl ( Figure S4), while 6 has a disphenoidal (2)SnCl2 structure ( Figure S5). Reaction of Ni(cod)2 with 5 led to transmetalation where Ni(0) replaced GeCl2, followed by coordination of the extruded metalloid fragment to Ni(0) to form complex 4 (Scheme 2).…”