Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene.

The reaction of the OH radical with cyclopentadiene (C5H6) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H2O2 or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C5H6OH) is the main observed reaction product. Abstraction products (C5H5) were not detected. The C5H6OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene.

The reaction of the methylidyne radical (CH(X2Π)) with cyclopentadiene (c-C5H6) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C6H6 + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C6H6 isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C6H6 isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C6H7 potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Kinetic Investigations of the CH (X2Π) Radical Reaction with Cyclopentadiene.

The reaction of the ground state methylidyne radical (CH (X2Π)) with cyclopentadiene (C5H6) is studied in a quasi-static reaction cell at pressures ranging from 2.7 to 9.7 Torr and temperatures ranging from 298 to 450 K. The CH radical is generated in the reaction cell by pulsed-laser photolysis (PLP) of gaseous bromoform at 266 nm, and its concentration monitored using laser-induced fluorescence (LIF) with an excitation wavelength of 430.8 nm. The reaction rate coefficient is measured to be 2.70(±1.34) × 10-10 cm3 molecule-1 s-1 at room temperature and 5.3 Torr and found to be independent of pressure or temperature over the studied experimental ranges. DFT and CBS-QB3 methods are used to calculate the reaction potential energy surface (PES) and to provide insight into the entrance channel of the reaction. The combination of the experimentally determined rate constants and computed PES supports a fast, barrierless entrance channel that is characteristic of CH radical reactions and could potentially lead to the formation of benzene isomers.

Product Detection of the CH Radical Reactions with Ammonia and Methyl-Substituted Amines.

Reactions of the methylidyne (CH) radical with ammonia (NH3), methylamine (CH3NH2), dimethylamine ((CH3)2NH), and trimethylamine ((CH3)3N) have been investigated under multiple collision conditions at 373 K and 4 Torr. The reaction products are detected by using soft photoionization coupled to orthogonal acceleration time-of-flight mass spectrometry at the Advanced Light Source (ALS) synchrotron. Kinetic traces are employed to discriminate between CH reaction products and products from secondary or slower reactions. Branching ratios for isomers produced at a given mass and formed by a single reaction are obtained by fitting the observed photoionization spectra to linear combinations of pure compound spectra. The reaction of the CH radical with ammonia is found to form mainly imine, HN═CH2, in line with an addition-elimination mechanism. The singly methyl-substituted imine is detected for the CH reactions with methylamine, dimethylamine, and trimethylamine. Dimethylimine isomers are formed by the reaction of CH with dimethylamine, while trimethylimine is formed by the CH reaction with trimethylamine. Overall, the temporal profiles of the products are not consistent with the formation of aminocarbene products in the reaction flow tube. In the case of the reactions with methylamine and dimethylamine, product formation is assigned to an addition-elimination mechanism similar to that proposed for the CH reaction with ammonia. However, this mechanism cannot explain the products detected by the reaction with trimethylamine. A C-H insertion pathway may become more probable as the number of methyl groups increases.