Proton transfers are amongst the commonest of all chemical reactions. They’re typically considered “trivial” and even could not function in lots of mechanistic schemes, apart from maybe the notation “PT”. The categories with the bottom power limitations for switch typically contain heteroatoms equivalent to N and O, and the standard transition state may be presupposed to be when the proton is situated at in regards to the half approach distance between the 2 heteroatoms. This needs to be the power excessive level between the 2 positions for the proton. However what if a crystal construction is set with the proton in precisely this place? Effectively, the primary speculation is that utilizing X-rays because the diffracting radiation is unreliable, as a result of protons scatter x-rays very poorly. Then a extra arduous neutron diffraction examine is usually undertaken, which is usually assumed to be extra dependable in figuring out the place of the proton. Simply such a examine was undertaken for the construction proven under (RAKQOJ), dataDOI: 10.5517/cc57db3 for the 80K dedication. The substituents had been chosen to attempt to maximise the symmetry of the O…H…N motif by way of pKa tuning (for one more tuning try, see this weblog). The extra basic panorama this molecule suits into is proven under:
The outcomes obtained for the place of the proton for RAKQOJ had been fascinating. They had been very depending on the temperature of the crystal! At room temperatures (utilizing X-rays), the proton was measured as 1.09Å from the oxygen and 1.47Å from the nitrogen (impartial kind above). At 20K, the OH distance was 1.309Å and the HN 1.206Å (~ionic kind above). Certainly, the very title of this text is First O-H-N Hydrogen Bond with a Centered Proton Obtained by Thermally Induced Proton Migration. The authors give a lot of causes for this behaviour (their ref 17 and likewise), however one they don’t point out is thermally induced adjustments within the dielectric fixed of the crystal with temperature, on condition that in a single place for the proton the molecule is ionic and within the different impartial. So I made a decision to mannequin the system as a perform of solvent. On this mannequin, the solvent dielectric is used to approximate the crystal dielectric. My first selection of power perform is to compute geometries utilizing the B3LYP+GD3BJ/Def2=TZVPP/SCRF=solvent technique to see what would possibly emerge and as a potential prelude to making an attempt different functionals. FAIR knowledge for these calculations are collected at DOI: 10.14469/hpc/10368.
|Solvent||ε||ΔG298 for O…HN||rO…H||rHN||ΔG298 for OH…N||rOH||rH…N||ΔG298
♠ At 20K
- The geometries for every mannequin are obtained by minimising the whole power of the system as a perform of the 3N-6 geometric variables (coordinates).
- The geometries present that for all solvents, TWO minima within the whole power are obtained, one for the ionic and one for the impartial kind. That is referred to as a double-well power potential. Even a non-polar solvent equivalent to toluene produces a solvation power of ~3.1 kcal/mol in comparison with the gasoline section, which is adequate to induce a double-well potential.
- With out solvent (gasoline section), solely the impartial geometry is obtained.
- In probably the most polar solvent water, the double nicely potential appears like this:
The ionic nicely is about 0.4 kcal/mol decrease in whole power (and ~0.3 kcal/mol in free power, see desk above) than the impartial kind, with a barrier connecting impartial to ionic only one.0 kcal/mol. A transition state + intrinsic response coordinate (IRC) will be simply situated on this whole power potential, confirming the double-well kind.
- When free energies ΔG are computed, which embrace thermal results equivalent to entropy and zero-point power, the transition state emerges as 0.3 kcal/mol lower than the whole power of the ionic kind (pink entries, Desk). In impact, the free power potential floor is INVERTED in comparison with the whole power floor and the “transition state” turns into the bottom level on the power floor. So this level is a minimal within the free power however a most within the whole power, the results of including thermal results to the whole power.
- In dichloromethane, the free power of the impartial kind is now decrease by 0.3 kcal/mol than the ionic kind. The OH bond is beginning to get shorter and the NH one longer. The transition state is now 0.22 kcal/mol decrease than the impartial kind. With chloroform, the OH and HN bonds have develop into ~equal in size, the proton is symmetrically disposed.
- By the point dibutyl ether as solvent is reached, the transition state is now not decrease in ΔG than the impartial kind, transferring on to being 2.0 kcal/mol larger for toluene. In order the solvent polarity decreases, we see a change within the potential from a single nicely in ΔG, during which the proton is centred, to a really uneven nicely during which the proton is connected to the oxygen.
- Can we match the noticed neutron diffraction outcomes to the calculations? Because the temperature decreases, the neutron diffraction exhibits the beginning of proton switch from oxygen to nitrogen to kind an ionic species. The calculations present that this may be modelled by a rise within the efficient dielectric fixed of the medium. The computed “transition state” for proton switch someplace between dibutyl ether and toluene (as a dielectric media) emerges as roughly one of the best mannequin for the construction of this species. At this dielectric, the calculated ΔG is now not fairly the bottom free power level within the potential. This may be because of the many approximations used on this mannequin equivalent to minimisation of whole power, the partition perform technique used to calculate entropy, the character of the DFT purposeful, the continuum solvation mannequin, the idea set, and so forth.
These outcomes had been obtained with the approximation that minimising the whole molecular power produces a computed geometry that may be in comparison with the experimental neutron diffraction buildings. However can one do higher? Acquiring molecular geometries by minimising the computed free energies could be non-trivial. Firstly, minimisation would rely upon availability of first derivatives of the power perform with respect to coordinates, on this case ΔG. These should not out there for any DFT codes. The outcome would itself be temperature dependent (as certainly are the experimental outcomes proven above). Moreover, ΔG is computed from regular vibrational modes and these are solely applicable when the primary derivatives of the perform are zero, at which level the so-called six rotations and translations of the molecule in free house even have zero power. So we want vibrations to compute derivatives, however we want derivatives to compute vibrations on this classical method.
It will be nice for instance if the approximate mannequin of the potential for a hydrogen switch used above as primarily based on minimising whole energies for derivatives may very well be checked towards a mannequin primarily based on geometries optimised utilizing free energies as an alternative. Such procedures do exist, utilizing molecular dynamics trajectory strategies.
This publish has DOI: 10.14469/hpc/10382