Hi solid-state NMR experts,
What happens to the 1H resonances when a proton exchange starts to take place at the surface of metal oxides? Let's say we have multiple highly resolved 1H resonances in the MAS spectrum of an evacuated metal oxide material. There are basic and acidic hydroxyl functions (terminal, bridging and triply bridging).
If this material is exposed to humidity and rehydroxylation starts to occur, a proton exchange with the acidic OH groups (?) is expected to occur under ambient conditions, isn't it? If yes, what would be the possible effect(s) on the 1H MAS spectrum?
I have no idea about how to predict if exchange will, in fact occur, in your system.
I don’t think H exchange on metal-oxide surfaces is different from any other type of chemical exchange.
In the lest complicated case (mutual, two-site exchange with equal populations) the exchange rate and peak separation frequency determines the exchange peak(s) line shape. When intermediate exchange occurs the observed line width can be very broad and difficult to observe.
Your case is seems more complicated (terminal, bridging and triply bridging sites).
Given the information available in your question I would attempt to change the exchange rate by changing the temperature.
Another potentially interesting experiment would be to expose the system to D2O and see if any of the three classes of peaks disappears.
Hi William C Hutton ,
Thank you for your answer. Unfortunately, due to circumstance I cannot do any more experiments now (otherwise I was thinking of using D2O already).
What I have observed is that I have got splitting of some of the acidic sites into a 2:1 doublet or an asymmetric but very well-resolved triplet.
I have re-admitted air into the sample rotor step-by-step and measured a few times. The (sharpening and) splitting has come progressively with more exposure to air and has passed through a max. splitting. The magnitude of splitting is 60 to 70 Hz. We're talking about isolated OH groups (no or little H bonding involved), I've very sharp lines for solid-state NMR.
For instance, an initially 2.73 ppm resonances gradually splits into 2.85, 2.72 and 2.60 ppm. There is slight shift of position.
The basic OH group at very high-field does not undergo splitting, supporting the assumption of proton exchange at the acidic OH sites.
But I cannot understand why such a splitting occurs if it is simply the protons being exchange between H2O in the immediate surrounding and the OH at the surface.
The sample is gamma-alumina. My first theory was actually something completely different: namely, the asymmetric triplet is caused by a residual dipole-dipole interaction between 1H and 27Al (1/2 vs 1/2, 1/2 vs 3/2, 1/2 vs 5/2). But the way the splitting occurs (i.e. increases by increasing humidity exposure) makes me a bit skeptical about this theory.
So based on the above details, what would you assume?
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