Directional properties in photodissociation: a probe for the symmetry and geometry of excited states of dimethylnitrosamine and tert-butyl nitrite

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.

Cold-surface photochemistry of primary and tertiary alkyl nitrites

Reflection-absorption infrared spectroscopy (RAIRS) is used to explore the photochemistry of primary and tertiary alkyl nitrites deposited on a gold surface. The primary alkyl nitrites examined for this study were n-butyl, isobutyl, and isopentyl nitrite. These compounds showed qualitatively similar spectra to those observed in previous condensed-phase measurements. The photolysis of the primary nitrites involved the initial formation of an alkoxy radical and NO, followed by production of nitroxyl (HNO) and an aldehydic species. In addition, the formation of nitrous oxide, identified from its distinctive transition near 2230 cm(-1), was observed to form from the self-reaction of nitroxyl. The reaction rates for cis and trans conformer decay, as tracked through their intense N═O stretching modes, were found to be significantly different, potentially due to a structural bias that favors HNO formation for the initial trans conformer photoproducts over recombination. Tert-butyl nitrite demonstrates only the trans conformer in the RAIRS spectra prior to photolysis; however, recombination of the initial NO and RO(•) photoproducts was observed to produce the cis conformer in the photolyzed samples. The primary photoproducts from tert-butyl nitrite can also react to form acetone and nitrosomethane, but the absence of HNO prohibits the formation of N(2)O that was observed for the primary alkyl nitrites. Additionally, the RAIRS spectrum of isobutyl nitrite co-deposited with water was measured to examine the photolysis of this species on a water-ice surface. No change in the identity of the photoproducts was observed in this experiment, and minimal frequency shifting (1-3 cm(-1)) of the vibrational modes occurred. In addition to being a known atmospheric source of NO and various aldehydes, our results point to cold surface processing of alkyl nitrites as a potential environmental source of nitrous oxide.

Dependence of N-nitrosodimethylamine photodecomposition on the irradiation wavelength: excitation to the s(2) state as a doorway to the dimethylamine radical ground-state chemistry

The photochemistry of N-nitrosodimethylamine after excitation to the S(1) and S(2) states has been studied with the complete active space self-consistent field method (CASSCF) and the second-order multiconfigurational perturbation theory (CASPT2). The calculated vertical transitions agree with the experiment: the S(0) --> S(1) transition occurs at 3.29 eV (f = 0.003 au), the S(0) --> S(2) transition at 5.30 eV (f = 0.17 au) and the first excited triplet state is computed at 2.48 eV. Solvent effects have been reproduced by means of PCM calculations. It is shown that excitation to S(1) and S(2) yields the same photoproducts: (CH(3))(2)N (1(2)B(1)) and NO (X(2)Pi). However, while on S1 the process is adiabatic, the process on S(2) implies an ultrafast decay through a planar S(2)/S(1) conical intersection. Our calculations are consistent with the reversibility of the N-N dissociation after irradiation at 363.5 nm and the observed dimethylamine radical reactions when irradiated at 248 nm, namely, H extrusion, a one-step process (41.3 kcal/mol), and methyl radical extrusion, a two-step process (44.0 kcal/mol and 31.5 kcal/mol). Finally, two more aspects are considered: (i) the topology of the T(1) surface where two minima have been found to correlate with the phosphorescence emission band and (ii) the influence of tautomerizations which is shown to be neglectable.

Vibrational analysis of n-butyl, isobutyl, sec-butyl and tert-butyl nitrite

Raman and infrared spectra of n-butyl, isobutyl, sec-butyl and tert-butyl nitrite are reported. Density functional theory and Møeller-Plesset calculations with 6-31G* and 6-311G* basis sets were used to determine ground state molecular geometries and vibrational frequencies of these compounds. Calculations and spectral data of these molecules were used to perform partial vibrational mode assignments for the observed transitions. In agreement with previous investigations of alkyl nitrites, cis and trans rotational conformers of n-butyl, isobutyl and sec-butyl nitrite were observed in the gas phase infrared spectra and the condensed phase Raman and infrared spectra. Among the several predicted geometries of these compounds, the cis-trans geometry (cis with respect to the C-O-N=O dihedral angle and trans with respect to the C-C-O-N dihedral) was calculated to be the most stable conformer of n-butyl and isobutyl nitrite, while the cis-gauche conformer was found to be the most stable geometry of sec-butyl nitrite. The cis-type structures of these three molecules are favored due to formation of a pseudo hydrogen bond between the nitrite group and the alpha-carbon hydrogen atoms. Hindrance with the alkyl moiety, however, causes the trans conformer (trans with respect to the C-O-N=O dihedral) of tert-butyl nitrite to lie lower than its cis conformer, a result that was supported by our spectroscopic measurements. The characteristic N=O stretch frequency for the trans conformers of all the compounds examined was observed to decrease with increasing branching at the alpha-carbon, while the same mode for the cis conformers shows no change among the primary and secondary nitrites. Evidence is also provided that suggests that the relative number of cis conformers to trans conformers decreases as the alpha-carbon branching increases.

Dissociation dynamics of thiolactic acid at 193 nm: Detection of the nascent OH product by laser-induced fluorescence

Electronically excited thiolactic acid (2-mercaptopropionic acid), H(3)C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S(2) state at 193 nm, generating the primary product OH (v,J), which is detected by laser-induced fluorescence technique in a collisionless condition of flow system. The partitioning of the available energy between vibrational, rotational, and translational degrees of freedom of nascent photofragments is obtained from relative intensities of ro-vibronic lines in laser-induced fluorescence spectrum of OH, and their Doppler profiles. The rotational population of OH (v(")=0) is characterized by rotational temperature of 408+/-25 K. OH is produced in a vibrationally cold state, i.e., mostly in v(")=0. The average translational energy of OH (v(")=0,J(")) is found to be 21.5+/-2.0 kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f(t) value of approximately 0.6. The observed high translational energy is due to the presence of a barrier in the exit channel, implying that the C-OH bond scission takes place on an electronically excited potential energy surface. The observed partitioning of the available energy between various degrees of the photofragments is theoretically modeled, and the hybrid model, with 26.0 kcal/mol of barrier in the exit channel, is found to explain the measured data quite well. The experimental results are also supported with ab initio molecular orbital calculations for both the ground and the excited electronic states. Time-dependent density functional theory is used to understand the nature of various electronic transitions connecting the lower excited states. Potential energy curves as a function of the C-OH bond length of thiolactic acid suggest distinct exit barriers in the S(1), T(1), and T(2) states. But, we could locate the transition state structure for OH formation in the S(1) state alone. Thus, although thiolactic acid is excited to the S(2) state at 193 nm, it undergoes internal conversion to S(1) where it dissociates to yield OH. In addition to the OH channel from excited electronic states, we studied theoretically all probable dissociation channels occurring on the ground electronic state of thiolactic acid.