Hydrogen bonds proton donors

When the intermolecular forces are relatively strong and directional, crystalline molecular compounds (crystalline molecular complexes) are formed. They have fixed stoichiometries and ordered structures. These two-component molecular crystals are also called cocrystals or maybe adduct crystals. Hydrogen-bonded cocrystals, donor-acceptor crystals (charge transfer crystals), and inclusion crystals (host-guest crystals) are examples of crystalline molecular complexes. Crystalline organic salt is a special case of hydrogen-bonded cocrystal or donor-acceptor crystal, i.e., proton (or electron) transfer from the acid (or donor) to base (or acceptor) occurred. 

IR spectra provide the possibility to observe the separate bands of free and bonded proton donor groups, molecular and ionic complexes formed after proton transfer, and importantly to access the thermodynamic parameters of hydrogen bond and proton transfer equilibria. The combination of several spectroscopic techniques (IR, multinuclear NMR, UV-VIS, all at variable temperatures) with computational analysis is very useful in the band assignment, determination of structure and the thermodynamic parameters and, ultimately, of reaction mechanisms. 

The usual observation is that the NMR spectrum of a hydrogen-bonded proton is shifted downfield with respect to that of a nonhydrogen-bonded proton. Berkeley and Hanna (1963,1964) refer to this as a hydrogen-bond shift. They use the terms proton donor, proton acceptor, and the unqualified terms donor and acceptor and so unnecessarily introduce confusion with the terms... 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

But ethers lack OH groups and cannot act as proton donors in forming hydrogen bonds... 

Hydrogen bonding to the nitrogen lone pair leads to an upheld shift, the extent of which depends on the proton-donor ability of the solvent, and the acceptor ability of the base shifts of some 20 p.p.m. are commonly found. 

Several situations can lead to the observation of general acid catalysis. General acid catalysis can occur as a result of hydrogen bonding between the reactant R and a proton donor D—H to form a reactive complex D—H—R which then reacts with a substance Z ... 

Under these circumstances, a distinct contribution to the overall rate will be seen for each potential hydrogen-bond donor D—H. General acid catalysis is also observed when a ratedetermining proton transfer occurs fiom acids other than the solvated proton ... 

With very electrophilic olefins, an alternative hydrogen fluoride addition process is often preferred This process, involving reaction of the olefin with fluoride ion in the presence of a proton donor, is applicable to certain perhalogen ated alkenes [/] and substrates with other electron attracting groups attached to the double bond [i5, 36] (equations 4 and 5)... 

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