Theoretical and Computational Chemistry by Iwona Gulaczyk Bartosz Tylkowski

Author:Iwona Gulaczyk, Bartosz Tylkowski
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2021-04-28T18:45:14.910000+00:00

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Floppy molecules applications

Hydrazine is mainly used as a foaming agent in the preparation of polymer foams, but is also applied as a precursor to polymerization catalysts, pharmaceuticals and agrochemicals. Besides, it is used in various rocket fuels as well as to prepare gas precursors for air-bags. Hydrazine is also used as a long-term storable propellant on board space vehicles, such as the NASA Dawn probe to Ceres and Vesta. The F-16 fighter jet, NASA Space Shuttle and U-2 spy plane use hydrazine to fuel their emergency power units [130]. Hydrazine’s role in pharmacy is a precursor to many pharmaceutical compounds. It involves conversion of hydrazine to heterocyclic rings such as pyrazoles or pyridazines. Hydrazine compounds combined with other agricultural chemicals such as insecticides, fungicides or herbicides can be effective in agriculture.

Many floppy molecules have astrophysical importance. A list of such molecules is very long, thus we would like to mention just some of them like ammonia, methanol, ethylene, formamide, acetylaldehyde etc. One of such molecules is methylamine, CH3NH2, which is the simplest primary alkylamine. It is considered as a precursor of the simplest amino acid glycine. It has been detected as a constituent in the interstellar medium for the first time in 1974 at 3.5 cm [131] and at 3 cm [132] bands. Interstellar methylamine was also detected in a spiral galaxy with a high redshift of 0.89 [133] and in cometary samples of the Stardust mission [134]. The analysis of the molecular dynamics of the interstellar molecules appeared to be necessary for understanding the frequencies and intensities of the observed spectra in the laboratory and in interstellar space.

Apart from molecules described above, there is a group of large molecules with conjugated π-systems that play an important role in natural processes and can be used in different materials of technological usage [135], [136], [137], [138], [139], [140]. The structure of these molecules is determined mainly by a balance between the electronic forces governing the π-system and the steric forces between nonbonded atoms in the molecule. To such molecules belong, for instance, biphenyl, benzophenone or cis-stilbene. If the molecules are electronically excited, the forces responsible for π-system change, and one can notice the difference between the structures of the ground state and the excited state. The vibrations caused by electronic excitation are usually considered to be slow and can lead to structural isomerization. For instance, once cis-stilbene molecule gets relaxed to the ground electronic state, two parallel photoisomerization reactions take place in its lowest excited singlet state [141], [142], [143].

Most recent and spectacular application of floppy molecules considers them as candidates for achieving optoelectronic molecular devices without skeletal rearrangement or bond breaking [144]. For this purpose, specific floppy molecules are used, usually the molecules that consist of two benzene or benzene-like rings, and which rotate relative to each other. In the paper by Baldea [144], a mechanism is proposed for the photoinduced switching of such molecules from a nonplanar conformation to the planar one with neither skeletal rearrangement nor bond breaking or significant molecular length changes.

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