Dynamic NMR of low‐sensitivity fast‐relaxing nuclei: ¹⁷O NMR and DFT study of acetoxysilanes

Abstract

(17)O NMR is not routinely used for structure characterization, and kinetic studies of fluxional organic compounds are seldom undertaken because poor sensitivity and fast quadrupole relaxation are frequently regarded as intractable issues. This work shows how, nowadays, quantitative (17)O dynamic NMR studies on small organic molecules are feasible without enrichment being needed. It reports on acetoxysilanes, a class of fluxional compounds whose structure and dynamics were to be clarified. Natural abundance (17)O NMR spectra were recorded over a wide range of temperatures using standard instrumentation. The analysis relies on simple linewidth measurements and directly provides the activation parameters. The activation enthalpy is found to decrease with increasing number of acetoxy groups bound to silicon. Density functional theory calculations properly predict this trend and show that a single oxygen atom of the acetoxy group is bound to silicon, excluding chelation as binding mode, and that the dynamic process involves the shift of the silicon atom between the two oxygen atoms of the acetoxy group.

17O-Dynamic NMR and DFT Investigation of Bis(acyloxy)iodoarenes

“…Two signals of similar integrated intensity are observed at low temperature (see Supporting Information ). On the basis of chemical shift data reported for similar compounds [ 8 , 9 , 10 , 11 , 12 ], the signal at about 350 ppm is assigned to the carbonylic O 1 oxygen atoms and, conversely, the signal at about 250 ppm is assigned to the ester-like O 2 oxygen atoms. At −30 °C, the chemical shift (δ) and the full linewidth at half height (LW), as determined by fitting Lorentzian lines, are respectively 349 ppm and 3.28 kHz for the O 1 signal and 246 ppm and 1.49 kHz for the O 2 signal.…”

“…Further information can be gained from the temperature dependence of the LW, which is shown in Figure 5 and was analyzed according to Equations (1–4), as in our previous 17 O-DNMR investigation [ 10 ].…”

“…Only rough estimations of the activation free energy (ΔG # ) could therefore be provided. Recently, using acetoxysilanes as model compounds, we showed that currently available NMR instrumentations and pulse sequences allow for quantitative 17 O-DNMR studies to be carried out without enrichment being needed [ 10 ]. This opened the perspective to precisely determine ΔG # values for the [1,3]-sigmatropic shift of iodine in I(III) acyloxy derivatives and, also, to determine the activation enthalpy (ΔH # ) and the activation entropy (ΔS # ) of this dynamic process.…”

“…This opened the perspective to precisely determine ΔG # values for the [1,3]-sigmatropic shift of iodine in I(III) acyloxy derivatives and, also, to determine the activation enthalpy (ΔH # ) and the activation entropy (ΔS # ) of this dynamic process. It is noteworthy that, in acetoxysilanes of the general formula (CH 3 COO) n Si(CH 3 ) 4-n with n = 1–4, the [1,3]-sigmatropic shift of the silicon atom is characterized by a significant activation entropy [ 10 ].…”

An 17O NMR study of diamagnetic and paramagnetic lanthanide‐tris(oxydiacetate) complexes in aqueous solution

“…[25] Our investigations by 17 O NMR began with a study of the dynamic behaviour of a series of organic diamagnetic molecules characterised by carboxylate shifts, which might be expected to dramatically affect the line width of the 17 O resonance lines. [26][27][28] Diamagnetic and paramagnetic Ln complexes of DTPA and DOTA ligands were investigated next, obtaining dynamic information about some complexes with light elements of the Ln series. [29,30] Then, DOTA-based complexes with all stable lanthanides were studied, and the bound-state signals of the carboxylate groups were observed.…”