Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere

Abstract

A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth’s upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A′) excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A′) excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al–N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.

Spectroscopy and Photochemistry of Aluminum-Bearing Species in the Universe

“…The studies of ABMs ,,− may be regarded as exploring novel aluminum compounds which are not accessible to experimental studies. In this Account, the ground-state electronic structure and spectroscopic properties are computed for a large number of aluminum-bearing molecules.…”

“…Consequently, the collision between an atom and triatomic species such as Al + NCO or AlNC + O or the interaction between two diatomic molecules such as AlO + CN could lead to the formation of stable isomers such as linear-AlNCO and linear-AlOCN. Similarly, other triatomic species were suggested such as AlNX (X = S, O, and O 2 ). , By studying the stability and spectroscopy of these systems, we aimed to gather insightful information on the nature of the AlN bond. In the linear-AlNO isomer, the AlN bond is predicted to be 1.816 Å at the CCSD­(T)/aug-cc-PV­(T+d)­Z level, which is 0.0052 Å greater than for free AlN­(X 3 Π) .…”

“…Uncommon in most chemical systems, many small aluminum-bearing species have a triplet ground state (Table ). This includes triatomic molecules such as AlOP (X 3 Σ – ) and AlNO (X 3 Σ – ) as well as larger molecules such as OAlNO (X 3 A′′) . To investigate why some aluminum-bearing molecules exhibit this strange ground-state behavior, the model system OAlNO was chosen for study via NBO analysis.…”

“…Frontier molecular orbitals of the 1 3 A′′ and 1 1 A′ states of OAlNO from NBO analysis. Reproduced with permission from ref . Copyright 2023 American Chemical Society.…”