High-pressure Studies Of Typical Organic Azides

Energy is not only the material basis of human life,but also the permanent subject of scientific research.The synthesis of high-energy-density-materials(HEDMs)is always the hot topic in the scientific research of Condensed Matter Physics and Material Science.Pressure is a crucial thermodynamic parameter,which is independent of temperature and component.It is an important method of atomic arrangement and electronic distribution that changes the structure of material,which make it possible to synthesize materials that is hard to be synthesized under ambient pressure.The polymeric nitrogen has attracted considerable attention among scientists due to the high energy density(38.4 k J/cm3)and the environmental friendly products(N2).Pressure is an effective method to synthesize polymeric nitrogen.Azides have become the ideal precursor in the synthesis of HEDMs under high pressure since Na N3 successfully transformed into polymeric nitrogen under 3300 K and 120 GPa.While the theoretical calculations and the experimental results of inorganic azides show that the azide ion is difficult to bend,the hybridization of electron orbit is hard to occur,and the polymerization pressure is extremely high,which limits the practical applications and the scientific researches.As another important branch of azides,organic azide is comprised of azide group which has a different electronic structure from azide ion of inorganic azide.Azide group can exhibit different behavior from azide ion under high pressure,which is probably easier to form polymeric nitrogen,and lower the pressure of nitrogen polymerization.In this work,high pressure studies of typical organic azides have been performed in diamond anvil cells(DACs)combined with high-pressure Raman scattering,high-pressure infrared absorption,and high-pressure synchrotron X-ray diffration(XRD)techniques.The high-pressure phase transitions,the behavior of azide group,and the possible reasons for the change of azide group are discussed.We present the high pressure studies of alkyl azide.Benzyl azide has been studied by high-pressure Raman scattering and high-pressure synchrotron XRD measurements in DACs up to 30.8 GPa.A complete vibrational analysis of benzyl azide was performed by combining the experimental measurements and theoretical calculations.Benzyl azide underwent a conformational change at 0.67 GPa and a phase transition at 2.7 GPa.The conformational change is caused by the rotation of methylene group.The phase transition is a liquid to solid transformation,which is confirmed by the red shifts of all the vibration modes,the appearance of diffraction spots,and the disappearance of diffration ring.The azide group rotates under high pressure,which is affected by the methylene group.As the pressure reaches to 25.6 GPa,the azide group decomposed.The decomposition pressure of azide groups is lower than that of the azide ions,which is beneficial to the nitrogen polymerization at a relatively low pressure.Up to the highest pressure of 30.8 GPa,benzyl azide completely transformed into amorphous state.We report the high pressure studies of sulfonyl azides.4-Acetamidobenzenesulfonyl azide(4-ABSA),4-carboxybenzenesulfonyl azide(4-CBSA),and 4-toluenesulfonyl azide(4-Ts N3)have been studied by high-pressure Raman scattering,high-pressure infrared absorption,and high-pressure synchrotron XRD measurements in DACs,respectively.4-ABSA underwent two phase transitions at 0.8 ~ 2 GPa and 4.2 GPa,respectively,and transformed into amorphous state at 13.0 GPa.The first phase transition is attributed to the ring distortion and the rotation of methyl group.The second phase transition is induced by the distortion of methyl group and the change of hydrogen bonds.The azide group gradually rotates under high pressure.4-CBSA underwent two phase transitions at 0.5 GPa and 2.5 ~ 5.5 GPa,respectively,and transformed into amorphous state at 14.6 GPa.The first phase transition of 4-CBSA is induced by the change of molecular conformation,and the second phase transition is attributed to the distortion of benzene ring and the change of intermolecular hydrogen bonds.The azide groups bend first and then rotate with increasing pressure,and start to decompose at 10.5 GPa.4-Ts N3 underwent three phase transitions at 0.7 GPa,2.7 GPa,and 6.3 GPa,respectively,and transformed into amorphous state at 15.6 GPa.The first phase transition is triggered by the rearrangement of C-H…? interactions,leading to a decrease of the symmetry of the crystal structure.The second phase transition is attributed to the conformational change,then the rotation of sulfonyl leads to the third phase transition.The variation of sulfonyl has an influence on the behavior of azide group which will bend upon compression.We present the high-pressure studies of trimethyltin azide(TMSn A)by high-pressure Raman scattering,high-pressure infrared absorption,and high-pressure synchrotron XRD measurements in DACs up to 35.2 GPa.At ambient pressure,the azide group of TMSn A is linear and asymmetric that presents unique properties of partly ionic and partly covalent characters.At 1.4 GPa,the rotation of CH3 group causes the first phase transition,leading to the slightly change of relative position of organic groups.The second phase transition at 6.6 GPa is induced by the modification of CH3 group,resulting in the distortion of the unit cell.The azide group becomes increasingly asymmetric with increasing pressure,indicating a decrease in ionic character and an increase in covalent character.It can be predicted that the unique characteristic of the azide groups in TMSn A might probably bonding between adjacent azide groups at sufficiently high pressure,which give rise to the polymerization reaction in the azide groups.