Computational Infrared Spectroscopy of 958 Phosphorus-Bearing Molecules

Abstract

Phosphine is now well-established as a biosignature, which has risen to prominence with its recent tentative detection on Venus. To follow up this discovery and related future exoplanet biosignature detections, it is important to spectroscopically detect the presence of phosphorus-bearing atmospheric molecules that could be involved in the chemical networks producing, destroying or reacting with phosphine. We start by enumerating phosphorus-bearing molecules (P-molecules) that could potentially be detected spectroscopically in planetary atmospheres and collecting all available spectral data. Gaseous P-molecules are rare, with speciation information scarce. Very few molecules have high accuracy spectral data from experiment or theory; instead, the best current spectral data was obtained using a high-throughput computational algorithm, RASCALL, relying on functional group theory to efficiently produce approximate spectral data for arbitrary molecules based on their component functional groups. Here, we present a high-throughput approach utilizing established computational quantum chemistry methods (CQC) to produce a database of approximate infrared spectra for 958 P-molecules. These data are of interest for astronomy and astrochemistry (importantly identifying potential ambiguities in molecular assignments), improving RASCALL's underlying data, big data spectral analysis and future machine learning applications. However, this data will probably not be sufficiently accurate for secure experimental detections of specific molecules within complex gaseous mixtures in laboratory or astronomy settings. We chose the strongly performing harmonic ωB97X-D/def2-SVPD model chemistry for all molecules and test the more sophisticated and time-consuming GVPT2 anharmonic model chemistry for 250 smaller molecules. Limitations to our automated approach, particularly for the less robust GVPT2 method, are considered along with pathways to future improvements. Our CQC calculations significantly improve on existing RASCALL data by providing quantitative intensities, new data in the fingerprint region (crucial for molecular identification) and higher frequency regions (overtones, combination bands), and improved data for fundamental transitions based on the specific chemical environment. As the spectroscopy of most P-molecules have never been studied outside RASCALL and this approach, the new data in this paper is the most accurate spectral data available for most P-molecules and represent a significant advance in the understanding of the spectroscopic behavior of these molecules.

VIBFREQ1295: A New Database for Vibrational Frequency Calculations

“…Routine quantum chemistry calculations, e.g., harmonic and anharmonic frequency calculations, have the potential to enable high-throughput approaches for producing approximate spectral data for thousands of molecules of astrochemistry interest . However, it is currently unclear what model chemistry should be used and what errors in frequency predictions should be expected; this is essential information to help assess the usefulness of these high-throughput calculations to astrochemistry.…”

“…Definitive remote detection of molecular species in different astronomical settings, e.g., (exo)­planetary atmospheres, relies on the availability of high-resolution spectroscopic data, − which is very time-intensive to produce. As a complementary approach, it is likely to be useful to obtain approximate data for the thousands of molecular species of interest astrophysically, e.g., as discussed by Seager et al Though this novel approach , cannot enable definitive molecular detections, there are nevertheless many anticipated applications of this big data including identification of molecules with strong absorption (that will be easier to detect), recognizing potential ambiguities in molecular detections over some spectral windows, understanding the relative congestion of signals at different frequencies, and providing data for machine learning of vibrational frequencies.…”

“…An obvious way to produce this approximate vibrational spectral data for thousands of molecules is through harmonic or anharmonic quantum chemistry vibrational frequency calculations, as piloted by Zapata Trujillo et al However, existing literature, as recently reviewed by Zapata Trujillo and McKemmish, does not provide a definitive recommendation for a quantum chemistry methodology, even within the extremely popular harmonic approximation, nor does it explore the likely errors. These recommendations and error analysis are of interest far beyond the astrochemistry community (as evidenced by high citation numbers of, for example, Scott and Radom).…”

Meta‐analysis of uniform scaling factors for harmonic frequency calculations

“…Today, vibrational frequency calculations are one of the most routinely performed quantum chemistry calculations 3–11 . Arguably, their most common application is aiding in the interpretation of experimental vibrational spectra, where the computational predictions support accurate band assignments, especially for larger molecules with complex vibrational spectral patterns 12–17 .…”

A large-scale approach to modelling Biosignatures: First Steps

“…Following this discovery, (Zapata Trujillo et al 2021) noted the importance of being able to detect spectroscopically the presence of phosphorous bearing species. They enumerated a list of phosphorous bearing species, which could potentially be detected in planetary atmospheres, and compiled all available spectral data.…”