Photodissociation Dynamics of the Highly Stable ortho-Nitroaniline Cation

0rtho-Nitroaniline (ONA) is a model for the insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) that shares strong hydrogen bonding character between adjacent nitro and amino groups. This work reports femtosecond time-resolved mass spectrometry (FTRMS) measurements and theoretical calculations that explain the high stability of the ONA cation compared with related nitroaromatic molecules. Ab initio calculations found that the lowest-lying electronic excited state of the ONA cation, D1, lies more than 2 eV above the ground state, and the energetic barriers to rearrangement and dissociation reactions exceed this D1 energy. These theoretical results were confirmed by FTRMS pump–probe measurements showing that (1) fragment ions represented less than 30% of the total ion yield when a 1014 W cm–2, 1300 nm, 20 fs pump pulse was used to ionize ONA; and (2) 3.1 eV (400 nm) photons were required to induce dissociation of the ONA cation. Stronger coupling between the ground D0 and excited D4 states of the ONA cation at the geometry of neutral ONA resulted in a transient enhancement of fragment ion yields at <300 fs pump–probe delay times, prior to relaxation of the ONA cation to its optimal geometry.


SII Fit Coefficients
Reproduce the equation.

SVII Alternative method for the estimation of relaxation times
Figure S5a shows the atom labels in ONA cation.Figure S5b shows the evolution of the C2-C5 bond length along one representative AIMD trajectory out of the 105 trajectories that were calculated.The black line represents the recorded bond length; the red and blue lines represent the upper and lower envelopes of the trajectory amplitude as the magnitude of its analytic signal.The analytic signal is found using the Hilbert transform as implemented in the hilbert function of MATLAB.The trajectory shows an oscillatory behaviour that is strongly damped in regions around the "equilibrium value", the C2-C5 equilibrium bond length in the case of Figure S5b (magenta dotted line).We use the broad term "equilibrium value" because this kind of oscillatory behavior is present in all the degrees of freedom no matter if they were bond lengths, angles, or dihedrals.Considering this behavior, we estimate the relaxation time as the time corresponding to the minimum of the upper envelope for each trajectory (identical results are obtained if the maximum of the lower envelope is considered instead).It can be seen that the relaxation time for the trajectory shown in Figure S5b is 315 fs.By performing this procedure for the C2-C5 bond length along the 105 trajectories, we obtained an average of 233±131 fs to reach the equilibrium bond length.Figure S5c shows a histogram of the relaxation times obtained for this bond length.Additionally, we S16 performed the same analysis considering the C1-N7 bond length (Figure S6), C1-N7-H8 angle (Figure S7), and H9-O11 bond length (Figure S8), and the results are summarized in Table S19.

Figure
FigureS1: SFI-MS of ONA taken at the threshold intensity of 1.6 × 10 13 W cm −2 for ionization.In the FTRMS measurements, the intensity of the 400 nm probe was kept at half of this value (8 × 10 12 W cm −2 ) by lowering the measured average power by a factor of 2.
Figure S5: (a) ONA cation (S 0 geometry) with the corresponding atom labels.(b) C2-C5 bond length along a sample AIMD trajectory (black line), upper and lower amplitude envelopes (red and blue lines respectively), and C2-C5 equilibrium bond length for ONA cation (magenta dotted line).(c) Histogram for the C2-C5 bond relaxation times obtained from 105 AIMD trajectories (average and standard deviation shown).Results were obtained at the CAM-B3LYP/6-311G(d) level.

Table S3 :
Excited state energies and oscillator strengths for neutral ONA.

Table S4 :
Excited state energies and oscillator strengths for ONA cation at the S 0 geometry.

Table S5 :
Excited state energies and oscillator strengths for ONA cation at the D 0 geometry.

Table S6 :
Coefficients extracted from curve fitting for direct dissociation pathways.

Table S7 :
Coefficients extracted from curve fitting for NNR and H transfer pathways.
Figure S3: FTRMS measurements of ONA using a pump intensity of 5 × 10 13 W cm −2 .Similar fragment ion dynamics are observed as those in Figure4, where 10 14 W cm −2 pump intensity was used.S15

Table S19 :
Mean relaxation time and the corresponding standar deviation (SD) over 105 AIMD trajectories for different degrees of freedom (DOF).