Photofragmentation Translational Spectroscopy of Methyl Azide (CH3N3) Photolysis at 193 nm: Molecular and Radical Channel Product Branching Ratio

Electric field induced intra-molecular self-redox: superalkali Li3N3Mg as a candidate for NLO molecular switches

A novel intra-molecular self-redox switch, Li₃N₃Mg, is constructed theoretically. Our investigation showed that a suitably oriented external electric field (OEEF) can drive a long-range excess electron transfer from Mg atoms to Li₃ rings. And subsequently, an interesting intra-molecular self-redox from Li₃²⁺N₃^3-Mg⁺ to Li₃⁺N₃^3-Mg²⁺ accompanying the large different electronic static first hyperpolarizability (β) is exhibited. The increase of the β value constitutes an order of magnitude improvement from Li₃²⁺N₃^3-Mg⁺ (34 986 a.u.) to Li₃⁺N₃^3-Mg²⁺ (101 225 a.u.), which indicates that Li₃N₃Mg is a good candidate for a self-redox NLO molecular switch.

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

The collision-free photochemistry of methyl azide at 157 nm: Mechanism and energy release

Synchrotron radiation VUV-photoionization based photofragment translational spectroscopy was used to identify the primary and secondary photodissociation reactions of methyl azide (CH₃N₃) at 157 nm under collision-free conditions. Two primary dissociation channels are identified, leading to CH₃ + N₃ (the radical channel) and CH₃N + N₂ (the molecular elimination channel). The last channel is the major dissociation pathway, but unlike work at longer photolysis wavelengths, here, the radical channel exclusively produces the higher energy isomer cyclic-N₃. Product time-of-flight data for both channels were obtained and compared with earlier work on methyl azide photochemistry at 193 nm based on electron impact ionization, allowing us to estimate a product branching ratio Φ_CH3-N3 Φ_CH3N-N2 =2.3%±0.6%97.7%±0.6%.

A DFT study of the ionization and electron attachment of 2-azido pyridine

A DFT study using B3LYP/6-31G(d,p) level of theory is pursued to investigate the energy and geometry changes of 2-azido pyridine isomers in the S0, D0(+), and D0(-). In S0, cis-2 azidopyridine is the most stable, followed by cyclic tetrazole and trans isomer is the least stable. In D0(+), the cis isomer is the most stable, followed by trans isomer, and tetrazole isomer is the least stable. In the D0(-) state, the trans form is the most stable, followed by the cis and the tetrazolo form is the least stable. Single point vertical excitation energy calculations of the cis form gave four excited states at 266.4, 239.4, 199.4, and 196.1nm, all having π-π(∗) character. For trans isomer, five excited states at, 256.7, 250.3, 229.7, 199.0, 197.0nm all resulting from π-π(∗) transitions. For the tetrazole isomer, seven excited states at 269.3, 242.7, 198.0, 187.2, 183.9, 182.1, 178.7nm all resulting from π-π(∗) transitions. The geometries of the 3 isomers show noticeable changes in bond lengths, bond angles and dihedral angles, upon ionization and electron attachment. The ionization results in a remarkable variation of the NBO atomic charges as a result of ionization and electron attachment. where electron density of HOMO of the D0(+) state is found to be much lower than that of the HOMO of the S0 in the case of ionization. The dipole moment calculations show that tetrazole isomer has the largest polarization in the S0 state, followed by cis isomer and trans isomer is the least polarized. The same pattern is found in the D0(+) state. In the D0(-) state, trans isomer is the highest polarized, followed by the cis, and tetrazole isomer is the least polarized. The rotational constants calculations of cis- isomer show that the cation is elongated along the long in-plane axis and compressed along the two short out-of-plane axes while the anion is compressed along the long in-plane axis, and elongated along the two short out-of plane axes. The cation and anion of trans isomer show similar behavior to the cis isomer. The cation of cyclic tetrazole, is compressed along the long in-plane axis and elongated along the two short out-of-plane axes, while the anion elongates along the 3 mutually perpendicular axes.