Rotational alignment and non-statistical a doublet population in no following (CH3)3CONO photodissociation

The photodissociation of N,N-dimethylnitrosamine at 355 nm: The effect of excited-state conformational changes on product vector correlations

In a photodissociation experiment, the dynamics associated with creating reaction products with specific energies can be understood by a study of the product vector correlations. Upon excitation to the S₁ state, N,N-dimethylnitrosamine (DMN) undergoes an excited-state geometry change from planar to pyramidal around the central N. The significant geometry change affects the vector correlations in the photoproducts. Using polarized lasers for 355 nm photodissociation of DMN and for NO photoproduct excitation in a velocity-mapped ion imaging apparatus reveals new vector correlation details among the parent transition dipole (μ), photofragment velocity (v), and photofragment angular momentum (j). The dissociation of DMN displays some μ-v correlation [β₀²(20)=-0.2], little μ-j correlation [β₀²(02)∼0], and, surprisingly, a v-j [β₀⁰(22)] correlation that depends on the NO lambda doublet probed. The results point to the importance of the initial excited-state conformational change and uncover the presence of two photolysis channels.

Dissociation Pathways of the CH2CH2ONO Radical: NO2 + Ethene, NO + Oxirane, and a Non-Intrinsic Reaction Coordinate HNO + Vinoxy Pathway

We first characterize the dissociation pathways of BrCH2CH2ONO, a substituted alkyl nitrite, upon photoexcitation at 193 nm under collision-free conditions, in a crossed laser-molecular beam scattering apparatus using vacuum ultraviolet photoionization detection. Three primary photodissociation pathways occur: photoelimination of HNO, leading to the products HNO + BrCH2CHO; C-Br bond photofission, leading to Br + CH2CH2ONO; and O-NO bond photofission, leading to NO + BrCH2CH2O. The data show that alkyl nitrites can eliminate HNO via a unimolecular mechanism in addition to the commonly accepted bulk disproportionation mechanism. Some of the products from the primary photodissociation pathways are highly vibrationally excited, so we then probe the product branching from the unimolecular dissociation of these unstable intermediates. Notably, the vibrationally excited CH2CH2ONO radicals undergo two channels predicted by statistical transition-state theory, and an additional non-intrinsic reaction coordinate channel, HNO elimination. CH2CH2ONO is formed with high rotational energy; by employing rotational models based on conservation of angular momentum, we predict, and verify experimentally, the kinetic energies of stable CH2CH2ONO radicals and the angular distribution of dissociation products. The major dissociation pathway of CH2CH2ONO is NO2 + ethene, and some of the NO2 is formed with sufficient internal energy to undergo further photodissociation. Nascent BrCH2CHO and CH2Br are also photodissociated upon absorption of a second 193 nm photon; we derive the kinetic energy release of these dissociations based on our data, noting similarities to the analogous photodissociation of ClCH2CHO and CH2Cl.

Photolysis of n-butyl nitrite and isoamyl nitrite at 355 nm: a time-resolved Fourier transform infrared emission spectroscopy and ab initio study

We report on the photodissociation dynamics study of n-butyl nitrite and isoamyl nitrite by means of time-resolved Fourier transform infrared (TR-FTIR) emission spectroscopy. The obtained TR-FTIR emission spectra of the nascent NO fragments produced in the 355 nm laser photolysis of the two alkyl nitrite species showed an almost identical rotational temperature and vibrational distributions of NO. In addition, a close resemblance between the two species was also found in the measured temporal profiles of the IR emission of NO and the recorded UV absorption spectra. The experimental results are consistent with our ab initio calculations using the time-dependent density functional theory at the B3LYP/6-311G(d,p) level, which indicate that the substitution of one of the two gamma-H atoms in n-C(4)H(9)ONO with a methyl group to form (CH(3))(2)C(3)H(5)ONO has only a minor effect on the photodissociation dynamics of the two molecules.