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With the recent radioastronomical detection ofcis-trans-carbonic acid (H₂CO₃) in a molecular cloud toward the Galactic center, the more stable but currently unobservedcis-cisconformer is shown here to have strong IR features. While the higher-energycis-trans-carbonic acid was detected at millimeter and centimeter wavelengths, owing to its larger dipole moment, the vibrational structure ofcis-cis-carbonic acid is more amenable to its observation at micron wavelengths. Even so, both conformers have relatively large IR intensities, and some of these fall in regions not dominated by polycyclic aromatic hydrocarbons. Water features may inhibit observation near the 2.75μm hydride stretches, but other vibrational fundamentals and even overtones in the 5.5–6.0μm range may be discernible with JWST data. This work has employed high-level, accurately benchmarked quantum chemical anharmonic procedures to compute exceptionally accurate rotational spectroscopic data compared to experiment. Such performance implies that the IR absorption and even cascade emission spectral features computed in this work should be accurate and will provide the needed reference for observation of either carbonic acid conformer in various astronomical environments.

We obtained accurate vibrational frequencies, rotational constants, and vertical transition energy for AlNH₂(X¹A₁) and HAlNH(X¹A′) isomers using ab initio calculations at various levels of theory. These two isomers are potential candidates for astronomical observation. AlNH₂and HAlNH are thermodynamically stable, with Al-NH₂and HAl-NH bond dissociation energies predicted to be 4.39 and 3.60 eV, respectively. The two isomers are characterized by sizable dipole moments of 1.211 and 3.64 D, respectively. The anharmonic frequencies and spectroscopic constants reported for the two isomers should facilitate their experimental differentiation. In addition, we evaluated the evolution of the low-lying electronic states along the stretching coordinates, as well as the absorption cross sections. AlNH₂absorbs strongly around 287, 249, and 200 nm, whereas the HAlNH absorption is centered around 370 and 233 nm.

Aluminum and silicon are present in large quantities in the interstellar medium, making the triatomic species consisting of both elements intriguing with regard to the foundations of astrochemistry. Spectroscopic parameters have been calculated via high-level ab initio methods to assist with laboratory and observational detection of [Al, O, Si]^x(x= 0,+1). All [Al, O, Si]^x(x= 0,+1) isomers exist in the linear geometry, with linear AlOSi (X²Π) and linear AlOSi⁺(X¹Δ) being the most stable neutral and cationic species, respectively. Formation of the neutral species most likely occurs via reaction of AlO/SiO on an Si/Al dust grain surface, respectively. The cation molecules may form via ion–neutral reaction or as a consequence of photoionization. The rotational frequencies of linear AlOSi (X²Π) have been calculated using vibrationally corrected rotational constants and centrifugal distortion to lead experimental and observational radio detection. The rotational frequencies are discussed for three temperatures indicative of various astronomical environments: the central circumstellar envelope (CSE) (100 K), outer CSE (30 K), and the interstellar medium (3 K). At 100 K, the lines originating fromJ′ > 30 are the best candidates for detection via ground-based telescope. Anharmonic vibrational analysis revealed various Fermi resonances that may complicate the vibrational spectrum of linear AlOSi (X²Π). Finally, electronic spectroscopy may be the best means for laboratory detection of linear AlOSi (X²Π), due to the presence of two overlapping electronic transitions with large oscillator strengths occurring at approximately 250 nm.

The high cosmic abundance of carbon monoxide (CO) and the ubiquitous nature of aluminum-coated dust grains sets the stage for the production of weakly bound triatomic molecules AlCO (X²Π) and AlOC (X²Π) in circumstellar envelopes of evolved stars. Following desorption of cold AlCO and AlOC from the dust grain surface, incoming stellar radiation in the 2–9 eV wavelength range (visible to vacuum ultraviolet) will drive various photochemical processes. Ionization to the singlet cation state will cause an immediate Al–X (X = C, O) bond dissociation to form Al⁺(¹S) and CO (X¹Σ⁺) coproducts, whereas ionization to the higher-lying triplet states will lead to stabilization of AlCO⁺(X³Π) and AlOC⁺(X³Π) in deep potential wells. In competition with ionization is electronic excitation. Excitation to the spectroscopically bright 1²Π and 2²Σ⁺states will lead to either highly Stokes-shifted fluorescence, or photodissociation to yield Al (²D) + CO (X¹Σ⁺) products via nonadiabatic pathways, making AlCO and AlOC good candidates for electronic experimental studies. These many photoinduced pathways spanning orders of magnitude of the electromagnetic spectrum will lead to the depletion of AlCO and AlOC in astronomical environments, potentially explaining the lack of observational detection of these molecules. Furthermore, these results indicate new catalytic pathways to the freeing of aluminum atoms trapped in solid aluminum dust grains. Additionally, the results herein implicate an ion–neutral reaction as a possible important pathway in [Al, C, O] cation formation.

Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates. In this study, the excited state chemistry of two small Criegee intermediates, formaldehyde oxide (CH₂OO) and acetaldehyde oxide (CH₃CHOO), was modeled to compare their specific dynamics and mechanisms following excitation to the bright ππ* state and to assess the involvement of triplet states to the excited state decay process. Following excitation to the bright ππ* state, the photoexcited population exclusively evolves to form oxygen plus aldehyde products without the involvement of triplet states. This occurs despite the presence of a more thermodynamically stable triplet path and several singlet/triplet energy crossings at the Franck‐Condon geometry and contrasts with the photodynamics of related systems such as acetaldehyde and acetone. This work sets the foundations to study Criegee intermediates with greater molecular complexity, wherein a bathochromic shift in the electron absorption profiles may ensure greater removalviasolar photolysis.