Pathways, energetics, and mechanisms of the 193 nm photodissociation of acetyl cyanide (CH 3 COCN) have been investigated using high-resolution transient frequency modulation (FM) spectroscopy and photofragment translational spectroscopy. Vibrational and rotational state distributions of CN fragments measured by FM spectroscopy are in good agreement with previous work. The quantum yield of CN is found to be 0.9 ( 0.2. State-dependent translational energy distributions are nonstatistical. The laboratory-frame anisotropy of both velocity and angular momentum of CN fragments is vanishingly small, yet significant v‚j correlations are observed, indicating a preference for the CN angular momentum to be perpendicular to the recoil velocity. Photofragment translational spectroscopy provided independent confirmation of kinetic energy distributions and fragmentation pathways. The combined measurements are consistent with the strong preference for primary CN elimination over CH 3 elimination, despite a large difference in C-C bond strengths. The primary acetyl radicals undergo virtually complete secondary dissociation.
The photodissociation of a series of four cyano carbonyl compounds NCC(O)X with X = methyl, isopropyl, tert-butyl, and methoxy was studied after excitation at 193 nm using photofragment translational energy spectroscopy. For all the fragments generated (OCCN, XCO, CO, CN, X) the kinetic energy distributions were measured and the two radical decay channels, NCC(O)X → CN + OCX and NCC(O)X → OCCN + X, were identified. Dissociation leading to CN + OCX is the main decay path (∼85%) for acetyl cyanide (X = methyl), but is the minor pathway for X = isopropyl (30%), X = tert-butyl (17%), and X = methoxy (<5%). The primary fragments CN + OCX were found to be stable with respect to secondary dissociation in all cases, except for acetyl cyanide which exhibits spontaneous fragmentation of the acetyl fragments to CH3 + CO with a yield of ∼9%. The stability of the remaining acetyl + CN fragment pairs is probably due to electronic excitation of one of the fragments. Elimination of X is the major decay path for X = methoxy, and the anisotropic recoil distribution of the fragments suggests the decay to be fast on the time scale of a parent rotation. Within the homologous series X = methyl, isopropyl, and tert-butyl the propensity for X elimination, and thus OCCN production, increases with the size of the alkyl moiety. The observed trend toward increasing fragment internal energy with increasing size of the alkyl fragment indicates a considerable amount of randomization of the excess energy prior to bond scission. The investigation of the four compounds proved methyl cyanoformate to be the most favorable species for an efficient photolytical production of stable OCCN radicals, whereas acetyl cyanide is the most efficient source of CN radicals within this series.
The photodissociation of nitryl chloride (ClNO 2 ), a potentially active species in atmospheric chemistry, has been studied following excitation into the weakly structured band at 248 nm using photofragment translational energy spectroscopy. Among the energetically accessible decay channels, only the formation of the primary fragments Cl + NO 2 has been found to be active involving the fission of the weakest bond (D 0 (Cl-NO 2 ) ) 138 kJ/mol). The two fragments exhibit a well-structured translational energy distribution. The structure is attributed to different decay routes which include the formation of NO 2 fragments in different electronic states. Thus, about 30% are produced in the ground state while the rest, in accordance with the kinetic energy structure and the available fragment energy, is consistent with production in the A 2 B 2 and B 2 B 1 excited electronic states. The similar rovibrational energy channeled into NO 2 along the three decay routes suggests an indirect decay with an exit barrier. In addition, at high laser fluences secondary photodissociation of the hot or electronically excited NO 2 products to NO + O was observed.
The photodissociation OClO(à 2A2)→ClO(X̃ 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the à 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ<325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the à 2A2 potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol.
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