Periodic Reporting for period 1 - LACRIDO (Laser Activated Chemistry of Reactive Intermediates: Direct Observation in the jet)
Reporting period: 2022-10-01 to 2025-03-31
The goal of LACRIDO is to lock reactive intermediates in fast chemical reactions by freezing them through Joule-Thompson expansion in a molecular jet, then precisely determine their three-dimensional (3D) structures using chirped-pulse microwave spectroscopy to capture all species marking the reaction mechanism.
LACRIDO comprises two stages. The first stage focuses on detecting van der Waals complexes formed by reactants. These complexes provide crucial insights into the initial interactions and 3D arrangements of the molecules before a reaction occurs. This stage lays the groundwork for the more ambitious second stage where reactive intermediates are (1) formed by laser activation; (2) adiabatically frozen and locked in the collision-free zone of the molecular jet where the rotational temperature is close to the absolute zero-point; then (3) captured exploiting the strength of microwave spectroscopy in its specificity of conformation-sensitive detection unrivalled by any other type of spectroscopy and chemical methods.
The first reaction chosen as key target is the Diels-Alder reaction, one of the most classic and impactful transformations in chemistry. Backed up by the Hückel theory, Diels-Alder is theoretically described to be a single-step reaction, but details on the reaction course through the van der Waals complex of the reactants have never been demonstrated experimentally and will be explored by LACRIDO, giving the final answer for an old, traditional question. The second experiment targets the atmospherically important reaction of isoprene with the hydroxyl (OH) radical, a reaction critical to understanding atmospheric chemistry. Its reaction mechanism is extremely diverse with many steps and possible pathways to different stable products. Though lacking experimental evidence, this reaction is believed to proceed through two radical intermediates, each existing in a cis and a trans configuration. By capturing these short-lived intermediates, determining their branching ratios and their 3D structures, LACRIDO will deliver direct experimental proof of the reaction pathways taken, addressing a major gap in atmospheric science.
LACRIDO comprises two stages. The first stage focuses on detecting van der Waals complexes formed by reactants. These complexes provide crucial insights into the initial interactions and 3D arrangements of the molecules before a reaction occurs. This stage lays the groundwork for the more ambitious second stage where reactive intermediates are (1) formed by laser activation; (2) adiabatically frozen and locked in the collision-free zone of the molecular jet where the rotational temperature is close to the absolute zero-point; then (3) captured exploiting the strength of microwave spectroscopy in its specificity of conformation-sensitive detection unrivalled by any other type of spectroscopy and chemical methods.
The first reaction chosen as key target is the Diels-Alder reaction, one of the most classic and impactful transformations in chemistry. Backed up by the Hückel theory, Diels-Alder is theoretically described to be a single-step reaction, but details on the reaction course through the van der Waals complex of the reactants have never been demonstrated experimentally and will be explored by LACRIDO, giving the final answer for an old, traditional question. The second experiment targets the atmospherically important reaction of isoprene with the hydroxyl (OH) radical, a reaction critical to understanding atmospheric chemistry. Its reaction mechanism is extremely diverse with many steps and possible pathways to different stable products. Though lacking experimental evidence, this reaction is believed to proceed through two radical intermediates, each existing in a cis and a trans configuration. By capturing these short-lived intermediates, determining their branching ratios and their 3D structures, LACRIDO will deliver direct experimental proof of the reaction pathways taken, addressing a major gap in atmospheric science.
The goal of LACRIDO is to capture and analyze reactive intermediates from fast reactions by freezing them through Joule-Thomson expansion in a molecular jet. Using chirped-pulse microwave spectroscopy, we precisely determine their three-dimensional (3D) structures, enabling comprehensive identification of species involved in reaction mechanisms. This is achieved using the “Chirped Pulse And Resonator In one Spectrometer” (PARIS), an advanced instrument uniquely designed for high-sensitivity and high-resolution pure rotational spectroscopy.
PARIS is the first and only spectrometer worldwide that integrates a pulsed-jet resonator-type spectrometer (narrowband) and a chirped-pulse Fourier transform microwave spectrometer (broadband) with design allowing for interchangeable operation without turnaround time. Both share electronic components in a dual-purpose configuration while utilizing identical vacuum parts, offering rapid broadband capabilities alongside unmatched resolving power and sensitivity.
Stage 1 of LACRIDO has been successfully completed, with following main technical and scientific achievements:
1) Technical achievements in infrastructure and equipment development include:
• Acquisition of the tunable pulsed laser NT342E-OPO (Ekspla) with deep UV-extension and additional harmonic outputs with high power.
• Redesign of laboratory space, installation of laser protection, and redesign of the PARIS spectrometer to enhance sensitivity and resolution as well as making it compatible with the newly purchased UV laser. This achievement has resulted in a performance level ahead of any other instrument worldwide, making a significant milestone toward the project goal of capturing reactive intermediates from laser-initiated bimolecular reactions produced in trace concentrations.
• Installation, setup, and testing of the laser system, which is now ready to be coupled and synchronized with PARIS.
2) Scientific achievements include:
• Study of acrolein dimer: We investigated the product of the acrolein Diels-Alder self-reaction, 3,4-dihydro-2H-pyran-2-carboxaldehyde, commonly known as the acrolein dimer. Quantum chemical calculations, combined with chirped-pulse and resonator-enhanced microwave spectroscopy, identified two conformers. Their 3D structures were determined through the detection of their 13C and 18O isotopologues, the latter present at only 0.2% natural abundance.
• Measurements and spectroscopic characterization of a large number of astrophysical and atmospherically relevant molecules, such as isoprene, methylnitrophenols, lutidines, dimethylfluorobenzenes, dimethylanisoles, as well as thiophene, pyrrole, and 1-propene derivatives. In parallel, computational codes were developed to address diverse quantum chemical effects observed in the experimental spectra.
• Advances in ethylene-ozone studies: Building on prior research (J. Z. Gillies et al., J. Am. Chem. Soc. 110, (1988) 7991), we used the PARIS spectrometer and quantum chemistry to study the ethylene primary ozonide and the ozone-ethylene complex, with ozone generated in situ using an ozone generator.
These achievements represent essential groundwork for the subsequent Stage 2 of LACRIDO, particularly the integration of laser activation with the PARIS spectrometer and the study of more complex reactive intermediates.
PARIS is the first and only spectrometer worldwide that integrates a pulsed-jet resonator-type spectrometer (narrowband) and a chirped-pulse Fourier transform microwave spectrometer (broadband) with design allowing for interchangeable operation without turnaround time. Both share electronic components in a dual-purpose configuration while utilizing identical vacuum parts, offering rapid broadband capabilities alongside unmatched resolving power and sensitivity.
Stage 1 of LACRIDO has been successfully completed, with following main technical and scientific achievements:
1) Technical achievements in infrastructure and equipment development include:
• Acquisition of the tunable pulsed laser NT342E-OPO (Ekspla) with deep UV-extension and additional harmonic outputs with high power.
• Redesign of laboratory space, installation of laser protection, and redesign of the PARIS spectrometer to enhance sensitivity and resolution as well as making it compatible with the newly purchased UV laser. This achievement has resulted in a performance level ahead of any other instrument worldwide, making a significant milestone toward the project goal of capturing reactive intermediates from laser-initiated bimolecular reactions produced in trace concentrations.
• Installation, setup, and testing of the laser system, which is now ready to be coupled and synchronized with PARIS.
2) Scientific achievements include:
• Study of acrolein dimer: We investigated the product of the acrolein Diels-Alder self-reaction, 3,4-dihydro-2H-pyran-2-carboxaldehyde, commonly known as the acrolein dimer. Quantum chemical calculations, combined with chirped-pulse and resonator-enhanced microwave spectroscopy, identified two conformers. Their 3D structures were determined through the detection of their 13C and 18O isotopologues, the latter present at only 0.2% natural abundance.
• Measurements and spectroscopic characterization of a large number of astrophysical and atmospherically relevant molecules, such as isoprene, methylnitrophenols, lutidines, dimethylfluorobenzenes, dimethylanisoles, as well as thiophene, pyrrole, and 1-propene derivatives. In parallel, computational codes were developed to address diverse quantum chemical effects observed in the experimental spectra.
• Advances in ethylene-ozone studies: Building on prior research (J. Z. Gillies et al., J. Am. Chem. Soc. 110, (1988) 7991), we used the PARIS spectrometer and quantum chemistry to study the ethylene primary ozonide and the ozone-ethylene complex, with ozone generated in situ using an ozone generator.
These achievements represent essential groundwork for the subsequent Stage 2 of LACRIDO, particularly the integration of laser activation with the PARIS spectrometer and the study of more complex reactive intermediates.
The optimization of the PARIS spectrometer during LACRIDO Stage 1 has brought its sensitivity and resolution significantly beyond the state-of-the-art. One unexpected breakthrough was achieving a record resolution of 2 kHz in the chirp-excitation mode, surpassing the initial expectation of 5 kHz, already five times better than the 25 kHz resolution of common chirp-excitation spectrometers. This new resolution matches that of the resonator-enhanced setup, redefining the standard applications of Fourier transform microwave spectroscopy. In the state-of-the-art, chirp excitation has been used for rapid survey scans, while resonator-enhance was reserved for high-resolution and high-sensitivity measurements. With PARIS, the chirp-excitation mode now provides both high resolution and rapid survey scans with sufficient sensitivity for many cases, while the resonator-enhanced mode remains necessary only for detecting weaker lines, such as those from minor isotopologues.
Concerning sensitivity, the PARIS spectrometer has reached a new benchmark in the ppb range. A demonstration is the detection of the s-gauche conformer of isoprene. Isoprene exists as the s-trans conformer (98.5% abundance at room temperature), with the s-gauche conformer comprising only 1.5% of the population. The detection is further challenged by its low dipole moment of 0.36 D. Previously, this conformer required a DC-discharge to enhance its population for detection (Porterfield et al., J. Phys. Chem. Lett., 2019). Remarkably, the PARIS spectrometer achieved detection under standard jet-cooled conditions without requiring additional enhancements, underscoring its exceptional sensitivity.
Concerning sensitivity, the PARIS spectrometer has reached a new benchmark in the ppb range. A demonstration is the detection of the s-gauche conformer of isoprene. Isoprene exists as the s-trans conformer (98.5% abundance at room temperature), with the s-gauche conformer comprising only 1.5% of the population. The detection is further challenged by its low dipole moment of 0.36 D. Previously, this conformer required a DC-discharge to enhance its population for detection (Porterfield et al., J. Phys. Chem. Lett., 2019). Remarkably, the PARIS spectrometer achieved detection under standard jet-cooled conditions without requiring additional enhancements, underscoring its exceptional sensitivity.