Room-temperature H-NMR spectra

Figure 4.4 (a) Room temperature H NMR spectra of (I) silica-supported l-ethyl-3-methylimidazolium trifluoromethane sulfonate ([EMIM][OTf]) with an immobilized Pd complex ([Pd(DPPF)(OTf)2]) in comparison to (ii) silica-supported [EMIM][OTf] with a dissolved Brpnsted acid (CFjSOjH) and... 

The compound (LXIV) (entry 14) provided an early example of a stereochemically nonrigid bridged system. Variable-temperature H-NMR spectra showed it to be fluxional at room temperature. A number of related systems have now been studied, including (OC)3Co(M-GeMe2)2Co(CO)3 (5), (CpXCO)Fe(M-COXp.- GeMe2)Fe(CO) (Cp) (4), and (OC)3Fe(/i-CO)(/i-SnR2)2Fe(CO)3 (213). All are fluxional, although the second becomes so only above about 90°C. 

The unusual conformation is also in agreement with the dynamic NMR behavior exhibited by compounds 16 and 17. For 16, the geminal dimethyl singlet (0.98 ppm) in its room temperature H NMR spectrum (in toluene-dg, 500 MHz spectra) splits into two singlets (A = 68.8 Hz) upon cooling indicating that the two methyl groups are now nonequivalent, as indicated in structures 16 and 16". The coalescence temperature (Tc = —69 °C), and the calculated AG values (9.9 kcalmol ) based on the etiuation ... 

The butterfly clusters 3 and 5 show a broad averaged signal for the olefinic protons of the Rh(CO)(diolefln) vertex and equivalent para-to y rings at room temperature, which split into two well resolved multiplets and inequivalent para-. o y rings, respectively, on cooling the sample. The low-temperature H NMR spectra are consistent with the structure found in the solid state for 3. For example, one mul-tiplet comes for two olefinic protons trans to the carbonyl group and the other for... 

Electron diffraction and infra-red spectra indicate a trigonal bipyramidal configuration for RPF4 type molecules (R = H, Me, Et, Ph, NH) with the group R lying in an equatorial position. At room temperature their NMR spectra indicate F atoms with only one kind of environment. This can be explained on the basis of a pseudorotation process (6.534). The process is inhibited at low temperature when the presence of two kinds of fluorine atoms F and F are indicated by NMR spectra. 

Figure 2.13 Room-temperature MAS NMR spectra of TMP adsorbed on H-MORand H-beta acquired (A, C) with and (B, D) without proton decoupiing (spinning frequency =10 kHz) [43].

At room temperature the H NMR spectrum of [Th(OBu )4(py)2] reveals that the molecule is fluxional" and low-temperature (—75°C) spectra in toluene-dg failed to freeze out a limiting structure. In benzene-de the room temperature H NMR spectrum" of [Th2(OBu )g(HOBu )] shows only one broad signal due to OBu groups at <5 1.54 and a broad peak at S 3.22 for alcoholic OH protons in the intensity ratio 80 1, consistent with these assigmnents. 

The H NMR spectra of acetic acid and acetamide are quite different. The OH proton generates a single sharp peak at room temperature, while the NHi protons generate a broad, double-humped peak that turns into two sharp peaks at lower temperatures. This suggests that the NH, protons occupy different chemical environments, while the OH proton occupies a single environment. 

Fig. 7. H NMR spectra for ASt-x (7) wjtj, different St content in DMSO-d6 and in D20 at room temperature. The concentrations of the sample solutions were adjusted to 7 wt% [29]...

In another study (Ji8), it was found that graphite does not intercalate with neat XeF2 or with solutions of XeFa in acetonitrile. However, reaction with solutions of XeF2 in AHF led to copious xenon evolution, indicating that oxidation does take place, even at room temperature. Broad-line, F- and H-NMR spectra (Ell) showed the presence of both XeF2 and HF in the product, but no definite stoichiometry could be as-... 

X-ray crystallography and variable temperature H NMR studies show that the conformation of the coordinated imidazolidin-2-ylidene, in both the neutral and cationic complexes 70, is anti, anti with respect to the Ph of the backbone of the NHC, exclusively in the solid state and predominantly in solution at lower temperatures (-75°C). At room temperature in solution, possible conformer interconversion by the rotation around the phenyl-N bond of the NHC substituent is apparent from the broadness of the peaks in the NMR spectra. Hydrosilylation of acetophenone by Ph SiH catalysed by 70 at room temperature or at -20°C results in maximum ee of 58%. However, at lower temperatures the reaction rates are much slower [55]. 

Fig. 10. High-pressure ID H-NMR spectra of sonicated POPC—TTC vesicles (25 mol% TTC) at room temperature (T=22°C) and pD = 5.5 (after Ref 49).

Only with less efficient catalysts and at low temperature, have p-chelate intermediates been intercepted by P H HP NMR spectroscopy in the course of copolymerisations in MeOH-d4 [5g]. The unambiguous detection of p-chelates has been observed in a reaction catalysed by the l,r-bis(diphenylphosphino)ferro-cene complex [Pd(H20)2(dppf)](0Ts)2 (3) at room temperature (Scheme 7.7) [5g]. As shown in the sequence of P H NMR spectra reported in Figure 7.8, the P-chelate intermediates 4- disappeared already at 50 °C. A parallel model study confirmed the formation and the structure of the dppf P-chelates and also provided information of more elusive intermediates (see Section 7.2.1.8) [19]. 

In anhydrous organic solvents, ethene/CO copolymerisation termination occurs exclusively by P-H transfer to give vinyl terminated polyketone and Pd-H (Scheme 7.15c). On the other hand, traces of water are very difficult to eliminate and consequently chain transfer by protonolysis is often observed, together with p-H transfer. Experimental evidence in this sense has been straightforwardly obtained by an in situ NMR study of the chemical stability of the p-chelate [Pd(CH7CH7C(0)-Me)(dppe)]PF5 (7) in wet and anhydrous CD2CI2 [5ej. Figure 7.13 reports a sequence of P H NMR spectra taken after dissolution of the p-chelate in the wet solvent already the first spectrum at room temperature showed the formation of the p-hydroxo binuclear complex [Pd(OH)(dppe)]2(PF )2 (8), that was the only detectable species after 15 h. 

Alkoxyl and acetoxyl protons in A-acetoxy-A-alkoxybenzamides give rise to sharp signals well below room temperature. In contrast, hydroxamic esters usually exhibit line broadened alkoxyl group resonances in their H NMR spectra at or even signihcantly above room temperature" . In toluene-rfg, the benzylic and acetoxyl methyl resonances of A-acetoxy-A-benzyloxybenzamide (100) showed signihcant line broadening below 250 K but remained isochronous down to 190 K. 

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

The nB NMR spectrum of B6Hro presents a simple positive means of identifying this compound. At room temperature it exhibits doublets at -14.1 (basal boron atoms) and 51.8 ppm (apical boron atom) relative to BF3 0(C2H5)2 in the area ratio 5 1. The H NMR spectrum contains basal terminal, bridging, and apical resonances at t 5.82, 11.10, and 11.22, respectively. The low-temperature nB and H NMR spectra are discussed in the literature16 with emphasis on the fluxional character of the bridginghydrogen atoms. 

---