Photodissociation Of Nitromethane Research Articles

Multiple product pathways in photodissociation of nitromethane at 213 nm.

In this paper, we present a photodissociation dynamics study of nitromethane at 213 nm in the π → π(*) transition. Resonantly enhanced multiphoton ionization spectroscopy and ion-imaging were applied to measure the internal state distributions and state-resolved scattering distributions of the CH3, NO(X (2)Π, A (2)Σ(+)), and O((3)PJ) photofragments. The rotationally state-resolved scattering distribution of the CH3 fragment showed two velocity components, of which the slower one decreased the relative intensity as the rotational and vibrational excitations. The translational energy distribution of the faster CH3 fragment indicated the production of the NO2 counter-product in the electronic excited state, wherein 1 (2)B2 was the most probable. The NO(v = 0) fragment exhibited a bimodal translational energy distribution, whereas the NO(v = 1 and 2) fragment exhibited a single translational energy component with a relatively larger internal energy. The translational energy of a portion of the O((3)PJ) photofragment was found to be higher than the one-photon dissociation threshold, indicating the two-photon process involved. The NO(A (2)Σ(+)) fragment, which was detected by ionization spectroscopy via the Rydberg ← A (2)Σ(+) transition, also required two-photon energy. These experimental data corroborate the existence of competing photodissociation product pathways, CH3 + NO2,CH3 + NO + O,CH3O + NO, and CH3NO + O, following the π → π(*) transition. The origins of the observed photofragments are discussed in this report along with recent theoretical studies and previous dynamics experiments performed at 193 nm.

Study of acoustic fingerprinting of nitromethane and some triazole derivatives using UV 266 nm pulsed photoacoustic pyrolysis technique

We report a comparative study of acoustic fingerprints of nitromethane, nitrobenzene and some nitro rich triazole derivatives using pulsed photoacoustic technique. UV 266nm wavelength i.e. Fourth harmonic of Q-switched Nd: YAG laser having pulse duration 7ns and 10Hz repetition rate is employed to record the time resolved PA spectrum. The PA fingerprint is produced due to absorption of incident UV light by molecule itself and photo dissociation of nitromethane and nitrobenzene at room temperature while in case of triazole it is attributed to the combination of thermal and photo-dissociation process. The entire dissociation process follows the root of cleavage of C-NO2 bond to produce free NO, NO2 and other by product gases due to π∗←n excitation. In addition, we have studied the thermal stability criteria of nitro rich triazoles based on the quality factor of acoustic resonance frequencies of the PA cavity. We have also studied the effect of data acquisition time to ascertain the decay behavior of HEMs samples.

Stereodynamics of the Photodissociation of Nitromethane at 193 nm: Unravelling the Dissociation Mechanism

The photodissociation of nitromethane at 193 nm is reviewed in terms of new stereodynamical information provided by the measurement of the first four Dixon's bipolar moments, β0(2)(20), β0(0)(22), β0(2)(02), and β0(2)(22), using slice imaging. The measured speed-dependent β0(2)(20) (directly related with the spatial anisotropy parameter β) indicates that after one-photon absorption to the S3(2 (1)A″) state by an allowed perpendicular transition, two reaction pathways can compete with similar probability, a direct dissociation process yielding ground-state CH3 and NO2(1 (2)A2) radicals and a indirect dissociation through conical intersections in which NO2 radicals are formed in lower-lying electronic states. A particularly important result from our measurements is that the low recoil energy part of the methyl fragment translational energy distribution presents a contribution with parallel character, irrespective of the experimental conditions employed, that we attribute to parent cluster dissociation. Moreover, the positive values found for the β0(0)(22) bipolar moment indicates some propensity for the fragment's recoil velocity and angular momentum vectors to be parallel.

Photodissociation Dynamics of Nitromethane at 226 and 271 nm at Both Nanosecond and Femtosecond Time Scales

Photodissociation of nitromethane has been investigated for decades both theoretically and experimentally; however, as a whole picture, the dissociation dynamics for nitromethane are still not clear, although many different mechanisms have been proposed. To make a complete interpretation of these different mechanisms, photolysis of nitromethane at 226 and 271 nm under both collisional and collisionless conditions is investigated at nanosecond and femtosecond time scales. These two laser wavelengths correspond to the pi* <-- pi and pi* <-- n excitations of nitromethane, respectively. In nanosecond 226 nm (pi* <-- pi) photolysis experiments, CH(3) and NO radicals are observed as major products employing resonance enhanced multiphoton ionization techniques and time-of-flight mass spectrometry. Additionally, OH and CH(3)O radicals are weakly observed as dissociation products employing laser induced fluorescence spectroscopy; the CH(3)O product is only observed under collisional conditions. In femtosecond 226 nm experiments, CH(3), NO(2), and NO products are observed. These results confirm that rupture of C-N bond should be the main primary process for the photolysis of nitromethane after the pi* <-- pi excitation at 226 nm, and the NO(2) molecule should be the precursor of the observed NO product. Formation of the CH(3)O radical after the recombination of CH(3) and NO(2) species under collisional conditions rules out a nitro-nitrite isomerization mechanism for the generation of CH(3)O and NO from pi pi* CH(3)NO(2). The OH radical formation for pi pi* CH(3)NO(2) should be a minor dissociation channel because of the weak OH signal in both nanosecond and femtosecond (nonobservable) experiments. Single color femtosecond pump-probe experiments at 226 nm are also employed to monitor the dynamics of the dissociation of nitromethane after the pi* <-- pi excitation. Because of the ultrafast dynamics of product formation at 226 nm, the pump-probe transients for the three dissociation products are measured as an autocorrelation of the laser pulse, indicating the dissociation of nitromethane in the pi pi* excited state is faster than the laser pulse duration (180 fs). In nanosecond 271 nm (pi* <-- n) photolysis experiments, pump-probe experiments are performed to detect potential dissociation products, such as CH(3), NO(2), CH(3)O, and OH; however, none of them is observed. In femtosecond 271 nm laser experiments, the nitromethane parent ion is observed with major intensity, together with CH(3), NO(2), and NO fragment ions with only minor intensities. Pump-probe transients for both nitromethane parent and fragment ions at 271 nm excitation and 406.5 nm ionization display a fast exponential decay with a constant time of 36 fs, which we suggest to be the lifetime of the excited n pi* state of nitromethane. Combined with the 271 nm nanosecond pump-probe experiments, in which none of the CH(3), NO(2), CH(3)O, or OH fragment is observed, we suggest that all the fragment ions generated in 271 nm femtosecond laser experiments are derived from the parent ion, and dissociation of nitromethane from the n pi* excited electronic state does not occur in a supersonic molecular beam under collisionless conditions.

Photodissociation Dynamics of 2-Nitropropane and 2-Methyl-2-nitropropane at 248 and 193 nm

Dynamics of formation of electronically excited NO2 and formation of OH fragment, during photo dissociation of 2-nitropropane (NP) and 2-methyl-2-nitropropane (MNP), were investigated at 193 and 248 nm. The radiative lifetime of the electronically excited NO2 fragment, observed at 193 nm, was measured to be 1.2 ( 0.1 micros and the rate coefficient of quenching of its emission by MNP was measured as (2.7 ( 0.1) x 10(-10) molecule(-1) cm3 s(-1). Formation of the ground electronic state of OH was confirmed in both molecules. State selective laser induced fluorescence technique was used to detect the nascent OH (X 2Pi, v'', J'') fragments in different ro-vibrational states, and to obtain information on energy partitioning. Though MNP and NP differ in the types of the available H atoms, the dynamics of OH formation is found to be the same in both. The relative population in different rotational states does not follow Boltzmann equilibrium distribution in both the molecules at 193 and 248 nm. The translational energies of the OH fragments, calculated from the Doppler width, are 21.2 ( 7.2 and 25.0 ( 2.5 kcal mol-1 for NP at 248 and 193 nm, respectively. The translational energies of the OH fragments, in the case of MNP, are found to be lower, 17.5 ( 4.1 and 22.0 ( 3.2 kcal mol-1,respectively, at 248 nm 193 nm. These results are compared with the earlier reports on photodissociation of nitromethane (NM), nitroethane (NE), and other nitroalkanes. All possible dissociation pathways of these molecules--NM, NE, NP, and MNPs leading to the formation of the OH fragment were investigated computationally, with geometry optimization at the B3LYP/6-311+G(d,p) level and energy calculation at the MP4(SDQ)/6-311+G (d,p) level. The results suggest that in NM, OH is formed after isomerization to CH2N(OH)O, whereas in all other cases OH is formed from HONO, a primary product of molecular elimination of nitroalkanes, formed with sufficient internal energy.

Photodissociation Dynamics of Nitromethane and Nitroethane at 266 nm

Measurements of the nascent OH product from photodissociation of gaseous nitromethane and nitroethane at 266 nm were performed using the single-photon laser induced fluorescence technique. The OH fragment is found to be vibrationally cold for both systems. The rotational state distribution of nitromethane are Boltzmann, with rotational temperature of Trot=2045150 and 1923150 K for both 23/2 and 21/2 states, respectively. For nitroethane, the rotational state distribution shows none Boltzmann and cannot be well characterized by a rotational temperature, which indicates the different mechanisms in producing OH radicals from photodissociation of nitromethane and nitroethane. The rotational energy is calculated as 14.360.8 and 4.980.8 kJ/mol for nitromethane and nitroethane, respectively. A preferential population of the low spin-orbit component (23/2) is observed for both nitromethane and nitroethane. The dominant population of + state in two -doublet states is also observed for both nitromethane and nitroethane, which indicates that the unpaired lobe of the OH fragment is parallel to the plane of rotation.

Time-Dependent Infrared Emission Following Photodissociation of Nitromethane and Chloropicrin

Nitromethane (CH(3)NO(2)) and its chlorinated analogue, chloropicrin (CCl(3)NO(2)), were photolyzed at 193, 248, and 266 nm, and the products were observed by time-dependent Fourier transform infrared emission spectroscopy. At 193 and 248 nm, the primary photodissociation pathway for nitromethane was cleavage of the C-N bond to produce CH(3) + NO(2)(A (2)B(2)). At 266 nm, weak emission was observed following photodissociation of nitromethane, but an infrared spectrum could not be obtained. The photodissociation of chloropicrin at 193 nm produced the analogous product channel CCl(3) + NO(2)(A (2)B(2)) in addition to several other product channels. At 248 and 266 nm, only CCl(3) + NO(2)(A (2)B(2)) was observed. The production of phosgene (CCl(2)O) from chloropicrin photodissociation was not observed in this study.

The photodissociation of nitromethane at 193 nm

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular dissociation to NO+O, which will occur for NO2 with 72 kcal/mol or more internal energy. Evidence is presented which suggests that the NO2 produced via the minor mechanism, which releases a smaller fraction of the available energy to translation, has a large cross section for absorbing an additional photon via a parallel transition and dissociating to NO+O.

On the photodissociation of nitromethane at 266 nm

AbstractIn a crossed laser–molecular beam study of nitromethane it was found that the excitation of nitromethane at 266 nm did not yield dissociation products under collision‐free conditions. When a small cluster of nitromethane was excited at the same frequency, the product was seen only at energies and masses consistent with the rupture of the van der Waals bond by vibrational predissociation of the excited state.