No abstract

Ammonia borane (AB, NH3BH3), as a hydrogen-rich material, has been attracting great interest in the field of hydrogen storage. However, the mechanism of its dehydrogenation and rehydrogenation is not sufficiently understood yet. In this work, the initial decomposition process of AB under shock loading is investigated using the ab initio molecular dynamics method. The results show that the B–H bond breaking plays a more important role in the reaction initiation. Three main reaction pathways for H2 release are revealed. (I) Heteropolar dihydrogen interaction (N-Hδ+···δ−H–B) and (II) homopolar dihydrogen interaction (B–Hδ−···δ−H–B) are still the most popular reaction mechanisms. However, a direct hydrogen adsorption and H2 liberation mechanism (III) is uncovered. The H radical readily adsorbs a B atom to form the pentacoordinate boron-containing species and efficiently activates the adjacent B–H bond, which further ruptures to form H2 molecules. What is more, we discover the unexpected hydrogenation behaviors between new-formed H2 and BNH compounds. In addition, the similar H exchange reactions assisted by H2 are also explored. The probable rehydrogenation mechanisms are proposed based on our results. These theoretical calculations not only give a comprehensive understanding about AB decomposition under shock loading but also shed light on its rehydrogenation for hydrogen storage.

It is reported that the cis/trans conformation change of the peptide hormone oxytocin plays an important role in its receptors and activation and the cis conformation does not lead to antagonistic activity. Motivated by recent experiments and theories, the quasi-static amide-I 2D IR spectra of oxytocin are investigated using DFT/B3LYP (D3)/6-31G (d, p) in combination with the isotope labeling method under different electric fields. The theoretical amide-I IR spectra and bond length of the disulfide bond are consistent with the experimental values, which indicates that the theoretical modes are reasonable. Our theoretical results demonstrate that the oxytocin conformation is transformed from the cis conformation to the trans conformation with the change of the direction of the electric field, which is confirmed by the distance of the backbone carbonyl oxygen of Cys6 and Pro7, the Ramachandran plot of Cys6 and Pro7, the dihedral angle of C β -S-S-C β , and the rmsd of the oxytocin backbone. Moreover, the trans conformation as the result of the turn in the vicinity of Pro7 has a tighter secondary spatial structure than the cis conformation, including stronger hydrogen bonds, longer γ-turn geometry involving five amino acids, and a more stable disulfide bond. Our work provides new insights into the relationship between the conformation, the activation of the peptide hormone oxytocin, and the electric fields.

Inspired by the recent cocrystallization and theory of energetic materials, we theoretically investigated the intermolecular vibrational energy transfer process and the non-covalent intermolecular interactions between explosive compounds. The intermolecular interactions between 2,4,6-trinitrotoluene (TNT) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and between 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and CL-20 were studied using calculated two-dimensional infrared (2D IR) spectra and the independent gradient model based on the Hirshfeld partition (IGMH) method, respectively. Based on the comparison of the theoretical infrared spectra and optimized geometries with experimental results, the theoretical models can effectively reproduce the experimental geometries. By analyzing cross-peaks in the 2D IR spectra of TNT/CL-20, the intermolecular vibrational energy transfer process between TNT and CL-20 was calculated, and the conclusion was made that the vibrational energy transfer process between CL-20 and TNTII (TNTIII) is relatively slower than between CL-20 and TNTI. As the vibration energy transfer is the bridge of the intermolecular interactions, the weak intermolecular interactions were visualized using the IGMH method, and the results demonstrate that the intermolecular non-covalent interactions of TNT/CL-20 include van der Waals (vdW) interactions and hydrogen bonds, while the intermolecular non-covalent interactions of HMX/CL-20 are mainly comprised of vdW interactions. Further, we determined that the intermolecular interaction can stabilize the trigger bond in TNT/CL-20 and HMX/CL-20 based on Mayer bond order density, and stronger intermolecular interactions generally indicate lower impact sensitivity of energetic materials. We believe that the results obtained in this work are important for a better understanding of the cocrystal mechanism and its application in the field of energetic materials.

The external electric field (E-field), which is an important stimulus, can change the decomposition mechanism and sensitivity of energetic materials. As a result, understanding the response of energetic materials to external E-fields is critical for their safe use. Motivated by recent experiments and theories, the two-dimensional infrared (2D IR) spectra of 3,4-bis (3-nitrofurazan-4-yl) furoxan (DNTF), which has a high energy, a low melting point, and comprehensive properties, were theoretically investigated. Cross-peaks were observed in 2D IR spectra under different E-fields, which demonstrated an intermolecular vibration energy transfer; the furazan ring vibration was found to play an important role in the analysis of vibration energy distribution and was extended over several DNTF molecules. Measurements of the non-covalent interactions, with the support of the 2D IR spectra, indicated that there were obvious non-covalent interactions among different DNTF molecules, which resulted from the conjugation of the furoxan ring and the furazan ring; the direction of the E-field also had a significant influence on the strength of the weak interactions. Furthermore, the calculation of the Laplacian bond order, which characterized the C-NO2 bonds as trigger bonds, predicted that the E-fields could change the thermal decomposition process of DNTF while the positive E-field facilitates the breakdown of the C-NO2 in DNTFⅣ molecules. Our work provides new insights into the relationship between the E-field and the intermolecular vibration energy transfer and decomposition mechanism of the DNTF system.