Resonance assignment alcohol

Despite the intense investigations, data on H H bond formation rates are rather limited since the process is usually too fast for studies by standard kinetic techniqnes. Nevertheless, some estimations are possible from low-temperatnre NMR spectra, nsually recorded in Freon solutions. These Freon solvents stop MH HX bond formation even on the H NMR time scale. For example, the ReH resonance of [Cp ReH(CO)(NO)] decoalesces in the presence of acidic alcohols at 96 K, giving two resolved lines at —7.54 and —8.87 ppm, assignable to the free and dihydrogen-bonded hydride, respectively [33]. Under these conditions, the lifetime of the MH- HX complexes, x, can be calculated via... 

The principles and techniques of nuclear resonance measurements are fairly well known and will not be described in detail here. It may be worthwhile, however, to review briefly what we mean by the chemical shift. For this we turn to the now classical example of ethyl alcohol. In this molecule we have three sets of non-equivalent protons. Under moderate resolution three resonance signals are obtained having intensity ratios 1 2 3. The assignment of the signals is therefore readily made as shown in Fig. 1. The separation between these different signals is referred to as the chemical shift. The three separate... 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

Fig. 13. 90.5-MHz 13C CP/MAS spectra of allyl-/-l3C alcohol (spectra a and b) and propanal-I-13C (spectra c and d) on zeolite FIZSM-5. All the spectra were acquired at ambient temperature (a) after heating for 5 h at 323 K (b) after heating at 393 K for 0.5 h (c) prior to heating (d) after heating at 353 K for 0.5 h. The downfield spectral features in spectra c and d, e.g., the isotropic shifts and the number of resonances, are consistent with those derived from allyl-/-l3C alcohol on HZSM-5 (spectra a and b), thus providing unambiguous evidence that the disputed resonance at 216 ppm is propanal-7-l3C. See Xu et at. (Ill) for a more detailed assignment of the resonances.

The full details of one synthesis of presqualene alcohol have been published.7 The 13C resonances of a series of presqualene esters, related cyclopropanes, and acyclic precursors have been assigned.8 The stereochemistry of the biosynthesis of presqualene alcohol has been investigated.9... 

Chugaev obtained carbene complexes like these as early as 1915, but the right structure was only assigned much later. Acetylides L M-C=CR are unexpectedly good bases via their resonance form L M+=C=C -R. They can react with acid in alcohol solution to give the carbenes shown in equations (7) and (11). An intermediate vinylidene cation probably undergoes nucleophilic attack by the alcohol. In this case, the usual order of attack of equations (1) and (2) - nucleophile, then electrophile - is inverted. 

The weak resonance at 864.2 is more difficult to assign. This resonance is compatible with the signal expected for butyl esters and the butyl ethers of tertiary alcohols, for example, trityl butyl ether and t-butyl n-butyl ether. Progress toward a definite assignment can be made by the consideration of other chemical and spectroscopic information. 

Structural assignments have been made for each peak in the proton magnetic resonance spectrum of sodium phenoxymethyl penicillin (fourth line of Table 5 in reference 5) by Green and co-workers. Proton resonance lines were measured in D2O solution at 38°,with either t-butyl alcohol or sodium 3-(trimethylsilyl) propane-l-sulfonate (assumed to be 2 cycles per second below Me Si) as the internal standard, with the results referred to tetramethylsilane (Me4Si=10.Ot). Cohen and Puar made structural assignments for the peaks of potassium phenoxymethyl penicillin measured in D2O solution at 33° with the results referred to tetramethylsilane as the external standard (Me Si O. Ot). Figure 2 is the spectrum of the Squibb Primary Reference Substance from which proton assignments were made . Spectra in both laboratories were measured on Varian A-60 spectrophotometers. 

Fig. 21. Top Experimental ( H)- C cross-polarization spectrum of freshly prepared poly-e-caprolactone nanospheres in an aqueous dispersion (wc—100 MHz, full proton decoupling). Five polymer signals ( P ) are detected within the given spectral range, other resonances can be assigned to the surfactant polyvinyl alcohol and small amounts of polyvinyl acetate. Bottom Corresponding ( H)- C cross-polarization spectrum of a solid bulk sample of poly-f-caprolactone under static conditions.

Additional chemical evidence for the assignment of the 58 ppm resonance to the methoxy species III was the observation that it also formed from methyl bromide and methyl chloride in relative yields consistent with the leaving group stability T > Br > Cl. Methyl iodide was adsorbed on several zeolites with different Si/Al ratios, and the intensity of the 58 ppm resonance correlated with the A1 content, as it must for a framework-bound alkoxy. The final example of chemical evidence for the assignment regards the expected chemistry of species such as III and VI upon exposure to moisture. The Si-O-C linkage is easily hydrolyzed on a silica gel surface to form alcohols and/or ether. As demonstrated in Fig. 16, the species assigned to III readily hydrolyzes to methanol and dimethyl ether, whereas the proposed ethoxy species formed from ethyl iodide- C hydrolyzed to ethanol upon exposure to atmospheric moisture. 

The assignment of these isomers was confirmed by nuclear magnetic resonance spectroscopy on a Perkin-Elmer 60 Mc./sec. NMR spectrometer, after removal of -OH by exchange with deuterium oxide. The spectrum of the cw-alcohol showed a single broad peak, while that of the trans-alcohol showed two peaks they thus resembled the spectra of the corresponding hydrocarbons (8). 

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