Modulating the Roaming Dynamics for the NO Release in ortho-Nitrobenzenes

Abstract

The dynamics of NO release upon photodissociation of nitroaromatic compounds is dependent on the nature of the interaction between the NO2 group and substituent in the ortho position. A bimodal (slow and fast) translational energy distribution of the NO photofragment indicates the presence of two distinct NO elimination channels. The slow-to-fast branching ratio for the NO release is regulated by the hydrogen bonding ability of the ortho substituent and follows the order [OH > NH2 > CH3 > OCH3], indicating that the intramolecular hydrogen bonding plays a pivotal role in NO release dynamics. Further, the topology of the triplet state potential energy surface acts as a doorway to the dissociation pathway switching between the roaming and nonroaming mechanisms, with hydrogen bonding substituents (OH and NH2) favoring the roaming mechanism.

Photodegradation of Flutamide and Halogen Derivatives of Nitrobenzotrifluoride: The NO Release Channel

“…It must be noted that even though in the case of NB the s/f branching ratio depends on the NO product state, , it has been shown that for NBs and nitrotoluenes, a strong correlation exists between the NO rotational and translational excitation . Therefore, in the present work, even though a specific product state is being probed for all the molecules, the trends in the s/f branching ratios are expected to follow the same order …”

“…The experimental data were fitted with an empirical function shown in eq and the fitting parameters are listed in Tables S1 and S2. The P ( E T ) distributions in Figure , in all five cases, show that the ratio of slow-to-fast (s/f) components is less than one, similar to NB, o -nitrotoluene, and o -nitroanisole . For FT, the character of P ( E T ) is very similar to NB with a comparable s/f branching ratio (0.65 and 0.61 for FT and NB, respectively).…”

“…The laser flux was optimized such that signal intensity for the NO fragment was observed only in the presence of both the pump and probe lasers and was absent with either the probe or pump lasers (Figure S1, see the Supporting Information). The NO radicals thus formed (IP = 9.26 eV) were resonantly probed using a 213 nm (5.82 eV) laser utilizing the (X 2 Π) → (A 2 Σ + ) [(1,0); P 1′ ( J = 50.5)] transition region of the NO radical. ,− The velocity mapped images of the NO photofragment were collected in the pump–probe configuration by appropriately time gating the front plate of the detector. The images were acquired by raw imaging mode (up to 100,000 laser shots) with NuAcq software and the images were quadrant-symmetrized using ImageJ .…”

“…Experimental investigations on the NO elimination reaction in polycyclic nitroaromatic hydrocarbons , and ortho -NBs in particular have shown that appropriate substitution in the ortho position can modulate the dynamics of NO release. ,,, Photolysis of several nitroaromatic compounds yields bimodal (slow and fast) translational energy distribution of the NO product fragment, , which suggests the presence of two distinct pathways. As described in detail in our previous work, the formation of an oxaziridine ring-type transition state in the triplet state prior to the dissociation results in NO photoproducts with fast translational energy, whereas the slow translational energy NO photoproducts are attributed to nitro to nitrite photoisomerization in the ground state either arising from a tight (oxaziridine ring type) and/or a roaming transition state. , Alternatively, based on ab initio calculations, it has been suggested that the fast component could also originate from the oxaziridine ring-type transition state in the ground state .…”

“…Experimental investigations on the NO elimination reaction in polycyclic nitroaromatic hydrocarbons , and ortho -NBs in particular have shown that appropriate substitution in the ortho position can modulate the dynamics of NO release. ,,, Photolysis of several nitroaromatic compounds yields bimodal (slow and fast) translational energy distribution of the NO product fragment, , which suggests the presence of two distinct pathways. As described in detail in our previous work, the formation of an oxaziridine ring-type transition state in the triplet state prior to the dissociation results in NO photoproducts with fast translational energy, whereas the slow translational energy NO photoproducts are attributed to nitro to nitrite photoisomerization in the ground state either arising from a tight (oxaziridine ring type) and/or a roaming transition state. , Alternatively, based on ab initio calculations, it has been suggested that the fast component could also originate from the oxaziridine ring-type transition state in the ground state . However, in the case of FT, it has been proposed that the twist in the geometry of the nitro group with respect to the aromatic plane induced by the CF 3 group at the ortho position plays a key role in triggering photoisomerization prior to dissociation. , Furthermore, based on high-level electronic structure calculations, in addition to extending the π-conjugation, it has been shown that the addition of methyl groups in the ortho position forces the nitro group to adopt a perpendicular orientation with respect to the plane of the aromatic ring, biasing toward the formation of an oxaziridine ring that facilitates the photorelease of NO .…”