Sometimes you come across a bond in chemistry that just shouts at you. This happened to me in 1989[1] with the molecule shown below. Here is its story and, 26 years later, how I responded.
To start at the beginning, there was a problem with the measured 1H NMR spectrum; specifically (Y=H, Z=O) there are supposedly 16 protons, but only 15 could be located. What had happened to the 16th? To understand how one proton had been “lost”, you should appreciate that on most FT-NMR instruments, one has to specify a spectral window to collect data, and normally for protons, that window ranges from ~14 to -2 ppm. So the standard response to lost signals is to expand the window. When that was done, the offending proton appeared at 19 ppm! You should understand that this is an unusual chemical shift for a proton, and is normally taken as indicating very high acidity. But carboxylic acid protons are not regarded as particularly acidic? The mystery was resolved by recording the crystal structure at low temperatures, and this revealed that this hydrogen was (almost) symmetrically disposed between the oxygen and the nitrogen. The N-H distance was 1.32Å and the OH 1.17Å. Whilst such symmetric disposition is not that unusual between two atoms of the same type (O-H-O or N-H-N) it was quite unexpected between two different heteroatoms. And such symmetry alone is sufficient to induce very high chemical shifts; acidity per se does not come into it.
That bond clearly shouted at me; so much so that in the text of the original article, we wrote “it is interesting to speculate whether these characteristics could be fine tuned by modification of the pKa values with suitable ring substitution“. What I had in mind was whether the position of the H could be made perfectly symmetric by adjusting the substituents. But for 26 years this idea lay dormant. Until this post! Rather than make lot of compounds (1-3 years!) I will do it with (lots of) computation (2 days!!).
So to start we need a reality check. I am using the pbe1pbe/tzvp/scrf=chloroform method (this functional is often used for hydrogen bonds) and the collected results are shown in the table below.
I will stop at that point. Unfortunately of course the Y=BeF derivative is unfeasible synthetically and hence unlikely to be tested.
Y | N-H, Å | O-H, Å | δ, ppm | FAIR Data Citation |
---|---|---|---|---|
H (expt) | 1.32 | 1.17 | 19.0 | [1] |
H (calc) | 1.48 | 1.04 | 18.6 | [2] |
Li | 1.06 | 1.52 | 16.5 | [2] |
Na | 1.05 | 1.55 | 15.6 | [3] |
Li.H2O | 1.06 | 1.52 | 16.6 | [4] |
Li.2H2O | 1.06 | 1.52 | 16.6 | [5] |
BeH | 1.11 | 1.39 | 20.6 | [6] |
BeH | 1.49 | 1.04 | 18.7 | [7] |
BH2 | 1.06 | 1.56 | 16.6 | [8] |
BH2 | 1.53 | 1.03 | 17.6 | [7] |
SiH3 | 1.48 | 1.04 | 18.8 | [9] |
Z=S | 1.50 | 1.03 | 18.8 | [10] |
BeF | 1.12 | 1.38 | 20.9 | [11] |
BeF (TS) | 1.15 | 1.32 | 22.5 | [12] |
BeF | 1.48 | 1.04 | 18.7 | [13] |
Another reality check, a search of crystal structures. DIST2 = OH, DIST1 = NH, for structures recorded below 140K, R < 0.05%, no errors, no disorder. The structure above is shown as a blue dot. They do tend to show asymmetry, but it is interesting how many such structures have emerged since our own 1989 report; the effect is not that rare any more.
The above plot shows lots more systems that might be subjected to the sort of tuning above, and who knows one of them may even yield to experimental validation.
DOI: 10.14469/hpc/10731
This post has been cross-posted in PDF format at Authorea.
In the mid to late 1990s as the Web developed, it was becoming more obvious…
I have written a few times about the so-called "anomeric effect", which relates to stereoelectronic…
The recent release of the DataCite Data Citation corpus, which has the stated aim of…
Following on from my template exploration of the Wilkinson hydrogenation catalyst, I now repeat this…
In the late 1980s, as I recollected here the equipment needed for real time molecular…
On 24th January 1984, the Macintosh computer was released, as all the media are informing…