Typical Low-Z Small Organic Molecules Under High Pressure

Low-Z elements and compounds are the predominant components of theuniverse. At high pressure (sometimes coupled with high temperature), severallight elements or compounds have been shown to transform to new phases, andeven to exhibit unexpected properties. For example,“Metallic hydrogen” and“polymeric nitrogen” have been hot issues in the high-pressure science. Thisthesis has typically investigated several organic small molecules with cyclicpattern containing hydrogen (H), nitrogen (N) and carbon (C) elements at highpressuresFor light small organic molecules, the inner of crystals involves thecombination of strong intramolecular covalent bonds with weak intermolecularforces (van der Waals forces, charge transfer and hydrogen bonding). Smallvariations of applied forces would result in large changes in intermolecularseparations, which often trigger dramatic reorganizations of crystal packing.Consequently, the properties influenced by electronic structure will also bechanged. Furthermore, with pressure increase, the repulsive intermolecular and intramolecular interactions will increase, then the unsaturated covalent bondsbecome instability. In order to reach low inner energy and to stabilize thesystem, high pressure chemical reaction may take place in solid phase withoutcatalyst and solvent. Thus, monomers containing unsaturated bonds link andtransform into more saturated networks. In this thesis, typical hydrogen-rich(pyrrole and azetidine) and nitrogen-rich (tetrazole) cyclic compounds as wellas the related system (TCNQ) are investigated under high pressure. Theoriginal and innovational research results as follows:1. High pressure study on pyrrole (C4H5N) and azetidine (C3H7N): Pyrroleas a five-membered aromatic compound, undergoes a series of structuralchanges at high pressures. A crystalline-to-crystalline transition has been foundat about6.2GPa, with a large collapse of volume (40%at6GPa) from Pnmato P21/c phase after a liquid-to-solid transition at0.6GPa, which is caused bythe molecular rotational repacking of π-stacking. The new phase P21/c plays acentral role for the following pressure-induced polymerization due to theformation of a closed dimer. The threshold of C···C distance with sterichindrance in dimer is about1.62at~10.2GPa. After this steric hindrance isovercome, a crystal-amorphous transformation starts at~14.3GPa. Whencompletely released from34GPa, the recovered solid product with single bondis identified by in situ Raman measurement and keeps amorphous state asprevious study of benzene. Comparing with pyrrole, azetidine is afour-membered cyclic molecule with saturated covalent bonds. Whencompression to1.4GPa, a liquid-to-crystalline phase transition takes place.Then at15.4GPa, the crystalline sample transforms into amorphous state. Forazetidine, the compression-decompression cycle is a reversible process under43GPa. In all, comparison of the two experiments shows pressure favors thelinkage of molecules and solids containing unsaturated bonds into moresaturated networks. And the new phase P21/c of pyrrole with huge volume shrinkage during phase transition might be more favourable for designinghydrogen-storage materials.2. High pressure study on tetrazole (1H-tetrazole, CH2N4): Polymericnitrogen has been successfully synthesized by Eremets et al. However, itspractical application has come into question, because it is not stable at ambientcondition. A good way to form and stabilize the polymeric nitrogen might be byintroducing additional elements. Nitrogen-rich tetrazole (CH2N4) involvingunsaturated bonds (N═N, C═N) attracts our sight. Our experimental resultsshow that: a crystalline-to-crystalline transition starts at3.1GPa, and ends up at7.8GPa with a large volume collapse (18%at4.4GPa) from phase I to phaseII. Phase II is a high-density stacking with P1symmetry. This discovery iscaused by structural repacking from layered packing to closed dimers. Then, acrystalline-to-amorphous transition takes place over a large pressure range from13.8to50GPa, which is accompanied by an interphase region approachingparacrystalline state. The amorphization is considered a gradual lattice andmolecular distortion between π-π stacking on compression. Up to60GPa,sample shows a distinguished red color, which may represent the increasingnumber of conjugated double bonds (C=N–N=N) in system. When pressure iscompletely released to ambient conditions, the product shows an amorphousstate. UV absorption spectrum suggests the product exhibits an increase inmolecular conjugation, possibly occurring polymerization. This study providesa new idea for synthesis of energetic material with nitrogen-richheterocycliccompounds by high-pressure techniques.3. High pressure study on TCNQ7,7,8,8-Tetracyanoquinodimethane(TCNQ): TCNQ is organic semiconductor. The molecules in crystal are stackedface-to-face forming molecular column, which makes the π electron cloudshave significant overlap. High pressure can induce variations of overlap degreeof the electronic cloud, which may affect the optical and electronic properties of material. We perform in situ high-pressure synchrotron X-ray diffraction(XRD) and Raman scattering spectra up to18GPa to investigate the behaviorsof TCNQ. At ambient conditions, the crystal structure belongs to C2/csymmetry. The band gap in theoretical calculation is1.477eV. Uponcompression, phase transition takes place at0.9GPa, and completes at3.7GPa.The new phase belongs to triclinic symmetry. Further compression to10GPa,crystalline to amorphous transition occurs. At~18GPa, sample completelytransforms into amorphous state. Then when decompression to ambientconditions, product keeps amorphous state with fluorescence.