Hydroxyl protons, coupling

Anomeric pairs of pyranoses show characteristically diflFerent OH-1 —H-1 coupling constants. Thus, the signals from equatorial protons have relatively large spacings of 6.5-8 Hz, whereas the signals for axial protons show spacings of 4.5—5 Hz. The size of the hydroxyl proton coupling-constant appears to be indicative of the orientation at C-1. 

Similar to lifetime effects based on chemical shift differences, the J coupling can be exploited to study changes in reaction rates as well (49). Using this approach, the collapse of the J-coupling multiplets in ethanol (from the hydroxyl proton coupling with the CH2 protons) was monitored as a function of pressure... 

The phenomenon of intemiolecular exchange is very common. The loss of couplings to hydroxyl protons in all but the very purest etiianol samples was observed at a very early stage. Proton transfer reactions are still probably the most carellilly studied [14] class of intemiolecular exchange. 

Section 13 12 Splitting resulting from coupling to the O—H proton of alcohols is not normally observed because the hydroxyl proton undergoes rapid inter molecular exchange with other alcohol molecules which decouples it from other protons in the molecule... 

Potassium borohydride reduction of runanine (17) yielded dihydro-runanine (24), the H-NMR spectrum of which (Table II) exhibited a triplet (64.25), the proton bearing the hydroxyl group coupling with those of C-5 (35). The optical activity of runanine (17), [a]D —400°, was similar to that of hasubanonine (5), [a]D —214° (3) therefore, it was concluded that the ethylamine linkage must have the same configuration as hasubanonine [C-13 (R) and C-14 (S)]. From these results, structure 17 was proposed for runanine (35) however, no application of mass spectral data to the structure elucidation was presented (35). 

If you take a pure sample of ethanol, and run its NMR spectrum in dry CDCI3, the hydroxyl proton will appear as a well-defined triplet, which couples to the adjacent -CH2-, rendering it a multiplet. This is because the hydroxyl proton remains on the oxygen for relatively long periods of time, as there is nothing in the solution to entice it off, i.e., exchange (if any) is said to be very slow on the NMR timescale (less than about 1 s). 

The 3H chemical shifts, coupling constants, temperature coefficients, exchange rates and inter-residual ROEs of the hydroxyl protons of various synthetic type II trisaccharides, analogues of (3-D-Galp-(l - 4)-p-D-GlcpNAc-(l - 2)-a-D-Man-(l - 0)(CH2)7CH3, were reported and interpreted,8 assisted with molecular dynamic simulations, to deduce key information on the conformational behavior of these important molecules in aqueous solution. 

When a proton is coupled to two non-equivalent sets of neighbouring protons more complex multiplets result this is illustrated by the spectrum of pure ethanol (Fig. 3.67). Thus the methylene protons are coupled to the three methyl protons, giving rise to a quartet, and are further coupled to the hydroxyl proton which therefore causes each of the peaks of the quartet to appear as a doublet. The multiplet therefore consists of eight peaks due to the two overlapping quartets i.e. (N -I- 1)(M + 1). On occasions there may be difficulty in recognising the components of these more complex multiplets, as some peaks may be superimposed. 

Morgan, Rennick, and Soong (41) used coupling between platinum-195 and the hydroxylic protons of [ (CH3) 3PtOH] 4, XII, to show that the... 

Resonances A and B are slowly-exchanging hydroxyl protons they exchange on addition of acid, and this decouples them from C and G. C, the carbon-bound proton with the largest chemical shift, is likely to be on the anomeric carbon with two attached oxygens. D and E are distinguished by the number of couplings. F and H may be reversed. 

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