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In form II, each aspirin molecule forms the same hydrogen bonds, but with two neighbouring molecules instead of one. With respect to the hydrogen bonds formed by the carboxylic acid groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described ...
An ubiquitous example of a hydrogen bond is found between water molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. The simplest case is a pair of water molecules with one hydrogen bond between them, which is called the water dimer and is often used as a model system. When more molecules are present, as is ...
With respect to the hydrogen bonds formed by the carboxylic acid groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described as polytypes. [49] Pure Form II aspirin could be prepared by seeding the batch with aspirin anhydrate in 15% weight. [11]
The first theoretical study of the water dimer was an ab initio calculation published in 1968 by Morokuma and Pedersen. [10] Since then, the water dimer has been the focus of sustained interest by theoretical chemists concerned with hydrogen bonding—a search of the CAS database up to 2006 returns over 1100 related references (73 of them in 2005).
Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself, it is responsible for several of the water's physical properties. These properties include its relatively high melting and boiling point temperatures: more energy is required to break the hydrogen bonds between water molecules.
Salicylic acid has long been a key starting material for making acetylsalicylic acid (ASA or aspirin). [8] ASA is prepared by the acetylation of salicylic acid with the acetyl group from acetic anhydride or acetyl chloride. [17] ASA is the standard to which all the other non-steroidal anti-inflammatory drugs are compared. In veterinary medicine ...
No more than 5 hydrogen bond donors (the total number of nitrogen–hydrogen and oxygen–hydrogen bonds) No more than 10 hydrogen bond acceptors (all nitrogen or oxygen atoms) A molecular mass less than 500 daltons; A calculated octanol-water partition coefficient (Clog P) that does not exceed 5
Researchers have since conducted increasingly detailed investigations of the triad's exact catalytic mechanism. Of particular contention in the 1990s and 2000s was whether low-barrier hydrogen bonding contributed to catalysis, [18] [19] [20] or whether ordinary hydrogen bonding is sufficient to explain the mechanism.