Search for: All records

Although eight species names are available for the Southeast Asian genus Paralaubuca, and four names currently are in use, morphological and molecular data support recognition of only three names as valid, P. typus Bleeker, 1864, P. harmandi Sauvage, 1883, and P. barroni Fowler, 1934. Paralaubuca harmandi is easily distinguished from P. typus and P. barroni by having more lateral-line scales (70–87 vs. 51–68), fewer branched anal-fin rays (usually 22–24 vs. usually 24–28), and more scales around the caudal peduncle (usually 26–33 vs. usually 20–26). Paralaubuca typus and P. barroni are morphologically similar but can be separated consistently by number of rakers on the first gill arch, with P. typus having 31–51, and P. barroni having 22–29. Paralaubuca harmandi and P. typus are also separated by a COI genetic p-distance of 6.86–8.79%; molecular data are not available for P. barroni. Paralaubuca typus and P. harmandi have been recorded from the Chao Phraya, Mekong, and Mae Klong River basins, but both species appear now to be absent from the Mae Klong basin, not having been recorded there since 1966. Paralaubuca harmandi appears to be extremely rare in the Mekong River basin, where it has been documented at only two localities and not recorded since 1968. Paralaubuca barroni is found only in the Mekong River basin and documented at only three localities.  

The yields of stabilized Criegee intermediates (sCIs), CH2OO and RCHOO (C2H5CHOO, C3H7CHOO, C4H9CHOO, and C5H11CHOO), produced from ozonolysis of asymmetrical 1-alkenes (1-butene, 1-pentene, 1-hexene, and 1-heptene) were investigated at low pressures (5-16 Torr) using cavity ring-down spectroscopy (CRDS) and chemical titration with sulfur dioxide (SO2). By extrapolating the low-pressure measurements to zero-pressure limit, nascent sCI yields were obtained. Combined with our previous studies on ethene and propene ozonolysis, the nascent sCI yields demonstrated an intriguing trend of increasing with the addition of CH2 groups and eventually reached a plateau at around 31% for longer chain 1-alkenes. In particular, the fraction of stabilized CH2OO reached the plateau from 1-butene, indicating that CH2OO was produced with nearly the same internal energy distribution from 1-butene to 1-heptene. The comparison between the experiments and RRKM calculations suggests that the dissociation of primary ozonide (POZ) of O3 + ethene and propene can be treated by statistical theory, while that of O3 + 1-butene to 1-heptene is non-statistical and intramolecular vibrational redistribution (IVR) of the initial energy on the 1,2,3-trioxolane of POZ throughout the entire molecule was incomplete on the dissociation time scale. 

The ultraviolet (UV) photodissociation dynamics of the 1-methylallyl (1-MA) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the wavelength region of 226–244 nm. The 1-MA radicals were produced by 193 nm photodissociation of the 3-chloro-1-butene and 1-chloro-2-butene precursor. The 1 + 1 REMPI spectrum of 1-MA agrees with the previous UV absorption spectrum in this wavelength region. Quantum chemistry calculations show that the UV absorption is mainly attributed to the 3pz Rydberg state (perpendicular to the allyl plane). The H atom photofragment yield (PFY) spectrum of 1-MA from 3-chloro-1-butene displays a broad peak around 230 nm, while that from 1-chloro-2-butene peaks at ∼236 nm. The translational energy distributions of the H atom loss product channel, P (ET)’s, show a bimodal distribution indicating two dissociation pathways in 1-MA. The major pathway is isotropic in product angular distribution with β ∼ 0 and has a low fraction of average translational energy in the total excess energy, ⟨fT⟩, in the range of 0.13–0.17; this pathway corresponds to unimolecular dissociation of 1-MA after internal conversion to form 1,3-butadiene + H. The minor pathway is anisotropic with β ∼ −0.23 and has a large ⟨fT⟩ of ∼0.62–0.72. This fast pathway suggests a direct dissociation of the methyl H atom on a repulsive excited state surface or the repulsive part of the ground state surface to form 1,3-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03–0.08.