A study of the ozonolysis of isoprene in a cryogenic buffer gas cell by high resolution microwave spectroscopy

We have developed a method to quantify reaction product ratios using high resolution microwave spectroscopy in a cryogenic buffer gas cell. We demonstrate the power of this method with the study of the ozonolysis of isoprene, CH2=C(CH3)-CH=CH2, the most abundant, non-methane hydrocarbon emitted into the atmosphere by vegetation. Isoprene is an asymmetric diene, and reacts with O3 at the 1,2 position to produce methyl vinyl ketone (MVK), formaldehyde, and a pair of carbonyl oxides: [CH3CO-CH=CH2 + CH2=OO] + [CH2=O + CH3COO-CH=CH2]. Alternatively, O3 could attack at the 3,4 position to produce methacrolein (MACR), formaldehyde, and two carbonyl oxides [CH2=C(CH3)-CHO + CH2=OO] + [CH2=O + CH2=C(CH3)-CHOO]. Purified O3 and isoprene were mixed for approximately 10 seconds under dilute (1.5-4% in argon) continuous flow conditions in an alumina tube held at 298 K and 5 Torr. Products exiting the tube were rapidly slowed and cooled within the buffer gas cell by collisions with cryogenic (4-7 K) He. High resolution chirped pulse microwave detection between 12 and 26 GHz was used to achieve highly sensitive (ppb scale), isomer-specific product quantification. We observed a ratio of MACR to MVK of 2.1 +/- 0.4 under 1:1 ozone to isoprene conditions and 2.1 +/- 0.2 under 2:1 ozone to isoprene conditions, a finding which is consistent with previous experimental results. Additionally, we discuss relative quantities of formic acid (HCOOH), an isomer of CH2=OO, and formaldehyde (CH2=O) under varying experimental conditions, and characterize the spectroscopic parameters of the singly-substituted 13C trans-isoprene and 13C anti-periplanar-methacrolein species. This work has the potential to be extended towards a complete branching ratio analysis, as well towards the ability to isolate, identify, and quantify new reactive intermediates in the ozonolysis of alkenes

HSCO+^+ and DSCO+^+: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions

Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the ISM evolution (e.g. gas ionisation fraction) or as tracers of non-polar, hence not directly observable, species. The identification of these molecular species through radioastronomical observations is directly linked to a precise laboratory spectral characterisation. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO$^+$) and its deuterium substituted isotopomer (DSCO$^+$). At the same time, we show how implementing different laboratory techniques allows the determination of different spectroscopical properties of asymmetric-top protonated species. Three different high-resolution experiments were involved to detected for the first time the $b-$type rotational spectrum of HSCO$^+$, and to extend, well into the sub-millimeter region, the $a-$type spectrum of the same molecular species and DSCO$^+$. The electronic ground-state of both ions have been investigated in the 273-405 GHz frequency range, allowing the detection of 60 and 50 new rotational transitions for HSCO$^+$ and DSCO$^+$, respectively. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the astronomically most relevant transitions to be predicted to better than 100 kHz for both HSCO$^+$ and DSCO$^+$ up to 500 GHz, equivalent to better than 60 m/s in terms of equivalent radial velocity. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency, and how a similar approach can be adopted when dealing with reactive species.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy and Astrophysic

The Rotational Spectrum and Potential Energy Surface of the Ar-SiO Complex

The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch, in which lines up to v = 12 (~14 500 cm-1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar-Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar-SiO is predicted to be T-shaped with a well depth of 152 cm-1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm-1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar-SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar-SiO, and the putative Ka = ±1 levels for Ar-28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential

Probing the CH3SH + N2O3 reaction by automated microwave double resonance spectroscopy

Because HSNO is formed abundantly and selectively from H$_2$S and N$_2$O$_3$ in the presence of metallic surfaces, it may be feasible to synthesize larger RSNOs in analogous reactions using RSH precursors. To critically explore this possibility, products of the CH$_3$SH + N$_2$O$_3$ reaction have been studied using a combination of chirped-pulse microwave spectroscopy and automated double resonance techniques. As with HSNO, we find that \textit{anti}-CH$_3$SNO is formed in high abundance under similar experimental conditions, suggesting that this production method might be extended to study still larger $S$-nitrothiols in the gas-phase. This talk will provide a status report of our analysis, high-level quantum chemical calculations of minima on the CH$_3$SNO potential energy surface, and searches for secondary products

To kink or not: the search for long chain cumulenones using microwave spectral taxonomy

Although cumulene carbenes terminated with sulfur up to H$_2$C$_7$S are known to possess $C_{2v}$ geometries, the analogous oxygen species have only been characterized in the gas-phase up to H$_2$C$_4$O, and propadienone (H$_2$C$_3$O) and butatrienone (H$_2$C$_4$O) exhibit kinked heavy atom backbones. Using microwave spectral taxonomy, searches have been undertaken for pentatetrenone (H$_2$C$_5$O) and its isomers. Surprisingly, no evidence has been found for the cumulenone, but rotational lines of a bent-chain isomer, HC(O)C$_4$H, analogous in structure to propynal, HC(O)CCH, have been detected instead. In closely-related work, the sulfur analog HC(S)C$_4$H has also been identified for the first time. This talk will provide a summary of our search procedure and experimental findings, quantum chemical calculations of isomeric stability and dipole moments, and prospects for detecting these longer chains in astronomical sources where $c$-C$_3$H$_2$O and HC(O)CCH are known