Gas-Phase Structure Determination of Dihydroxycarbene, One of the Smallest Stable Singlet Carbenes

Abstract

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 ...

Stabilization of the Simplest Criegee Intermediate from the Reaction between Ozone and Ethylene: A High-Level Quantum Chemical and Kinetic Analysis of Ozonolysis

“…Finally, hot formic acid can isomerize (by 1,2 H-shift via TS8 or TS9) to dihydroxy-carbene, followed by rapid decomposition of dihydroxy-carbene to various products. Recently, three conformers of dihydroxy-carbene have been determined experimentally, , but they will not be formed from the title reaction.…”

Infrared laser Stark spectroscopy of hydroxymethoxycarbene in 4He nanodroplets

“…Isomerization rates for this class of reactive intermediates are largely dictated by the R and R substituent groups [17]. For example,dihydroxycarbene (HOC OH) shows no propensity for tunneling on any practical experimental timescale, as recently demonstrated in Ar matrix isolation studies [18].Recent gas-phase and He nanodroplet spectroscopic studies with theoretical calculations have structurally characterized HOC OH [19,20]. The back-donated electron density from the second O atom into the empty C atomp-orbital of HOC OH plays a significant role in its stability towards tunneling, in comparison to the analogous HC OH and CH 3 C OH systems, which undergorelatively prompt isomerization [15,16,21].…”

“…The dynamics are heavily influenced by the level of deuteration of the hydroxymethylene photoproduct, which they conclude is a result of passage through different conical intersections. [29]Using a combination of microwave rotational spectroscopy and highlevel calculations, McCarthy and coworkers reported a detailed structural characterization of the trans,cis-HOC OH rotamer [19].Most recently, HOC OH was isolated and characterized in He nanodroplets using infrared (IR) laser Stark spectroscopy [20]. This technique directly probesthe permanent electric dipole moment of themolecule, which provides insight into the ground vibronic wavefunction, which is related to the structural rigidity of the system, and its stability, or lack thereof, towards tunneling.…”