Ozonolysis is one of the dominant oxidation pathways for tropospheric alkenes. Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R 1 R 2 COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity. We present the first experimental detection of CH 2 OO in the gas-phase ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle, which combines high reactant concentrations with rapid sampling and sensitive detection. Nine other product species of the O 3 + C 2 H 4 reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide. The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH 2 OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.
Carbenes are reactive molecules of the form R 1 À C À R 2 that play a role in topics ranging from organic synthesis to gas-phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HOÀC ÀOH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high-level coupledcluster calculations. The semi-experimental equilibrium structure derived from five isotopic variants of HOÀC ÀOH contains two very short CO single bonds (ca. 1.32 ). Detection of HOÀC ÀOH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH 2 O 2 chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH 2 OO.Carbenes (R 1 ÀC ÀR 2 ) comprise an important class of molecules in chemistry owing to a highly reactive electrondeficient carbon atom. Relatively stable singlet carbenes with bulky electron-donating substituents, such as N-heterocyclic carbenes, have long been used in organic synthesis, [1,2] but smaller carbenes are more difficult to isolate and characterize. Numerous studies [3][4][5][6][7] implicate small organic carbenes as intermediates in chemical reactions such as the thermal decomposition of dicarboxylic acids (which are a key component of secondary organic aerosols, [8][9][10] photochemical smog, [11] and interstellar clouds [12] ) and the reactions of carbon dioxide and carbon monoxide with H 2 O/H 2 , thought to be important in prebiotic chemistry, [12][13][14] and in reversible hydrogen storage. [15,16] Although triatomic carbenes (DCX 2 where X = H, F, Cl, Br, or I) and vinylidene (DC=CH 2 ) have been studied extensively, [17][18][19][20][21][22] fewer experimental studies have examined the properties of small carbenes with substituent groups containting two to three atoms, which have a larger range of possible reaction pathways. [23][24][25][26][27][28][29] It is understood that carbenes are stabilized by nearby electron-donating groups. [1] Several studies have reported the potential energy surface (PES) of one of the smallest carbenes with two electron-donating groups: the singlet ground state of dihydroxycarbene (HO À C À OH), [3,4,23] which has been suggested to be an intermediate in the photolysis of oxalic acid. [30,31] The three local minima of the singlet HO À C À OH PES, shown in Figure 1, are some of the most thermodynamically stable isomers of the CH 2 O 2 family, lying approximately 167 kJ mol À1 higher than formic acid, but roughly 335 kJ mol À1 lower than the recently detected Criegee intermediate, [32][33][34][35] CH 2 OO, and 250 kJ mol À1 lower than dioxirane, [4] yet are rarely considered in the literature. The stability of singlet HO À C À OH is provided by the overlap between the electrons in the oxygen p orbitals and the empty p orbital on the carbon, leading to much shorter CO bond lengths (1.32 ) [4,23] than those found in alcohols (such as the 1.42 CO bond length in methanol [36] ). The triplet ...
Little is known on sulfur analogs of nitrous acid Investigation of the microwave spectrum of HSNO Accurate determination of its geometry
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