Browsing by Author "Akant Vats"

Now showing 1 - 13 of 13

A comprehensive rotational study of astronomical iso-pentane within 84 to 111 GHz
(Elsevier B.V., 2023) Anshika Pandey; Satyam Srivastav; Akant Vats; Amit Pathak; K.A.P. Singh
The rotational line survey by ALMA (Atacama Large Millimeter/submillimeter Array) recently revealed the presence of i-C3H7CN (i-PrCN) and n-C3H7CN (n-PrCN) in 3-mm atmospheric window between 84 to 111 GHz towards the hot core region Sagittarius B2(N) (Sgr B2(N)). This was the first interstellar detection of a linear straight chain molecule. In this light, we report the rotational spectra of C5H12 isomeric group in the same frequency range. We performed quantum chemical calculations for spectroscopic parameters. The pure rotational spectrum of the species has been simulated using the PGOPHER program. The rotational spectrum of this molecule makes it a good candidate for future astronomical detections since the radio lines can be calculated to very high accuracy in mm/sub-mm wave region. © 2022 Elsevier B.V.
C-H Stretch Vibrational Modes: Tracers of Interstellar PAH Geometries?
(American Chemical Society, 2023) Akant Vats; Amit Pathak; Takashi Onaka; Itsuki Sakon; Izumi Endo
Polycyclic aromatic hydrocarbon (PAH) molecules have long been adjudged as carriers of the frequently detected interstellar emission features in the 3-20 μm region. In the present work, PAHs with straight edges having solo-duo (PAHD) and solo-duo-trio (PAHT) C-H modes along with PAHs with irregular edges (PAHI) have been studied theoretically to understand the effect of molecular geometry on the interstellar C-H stretch vibrations at 3.3 μm. The C-H out-of-plane bending vibrations at 11.2 and 12.7 μm are also included for completeness. Using the NASA Ames PAH IR Spectroscopic Database, the mid-infrared spectra have been studied for 125 PAH molecules of varying molecular geometries, sizes, charge states, and symmetries. Results show that the individual solo, duo, and trio C-H stretches follow an order in the peak wavelength (λ3.3 (solo) > λ3.3 (duo) > λ3.3 (trio)) and intensity (I3.3 (solo) < I3.3 (duo) < I3.3 (trio)). If only PAHD’s are considered, the contribution of each charge state is required to account for the observed peak wavelength of the 3.3 μm band, or if only neutrals are contributors, PAHD and PAHT neutrals can explain the 3.3 μm band variations. The observed emission at 11.2 and 12.7 μm is found to match effectively with PAHD with increasing size, and the 11.2 μm band is present at longer wavelengths for PAHT contributing to the red wing. When the solo to duo hydrogen ratio is nearly equal to or greater than 1.0, PAHD neutrals yield better 3.3 μm peak positions. The ratio has a lower limit of 0.8 for the 11.2 μm band and converges at 1.5, indicating a size range of PAHD neutrals with 80 to larger numbers of carbon atoms. The present work examines the presence of solo, duo, and trio modes in the C-H stretching band, which must be taken into consideration when interpreting accurate data from James Webb Space Telescope (JWST) to further explain the observed variations in the interstellar 3.3 μm. © 2023 American Chemical Society.
DFT study of Interstellar PANH: Vibrational spectra of anionic and cationic variants
(Elsevier Ltd, 2022) Gauri Devi; Amit Pathak; Akant Vats
Polycyclic aromatic Hydrocarbons are present in the universe in varied forms and are extensively studied over the years to account for the complex natured Aromatic Infrared Bands (AIBs). As an abundant element in the interstellar medium, nitrogen may substitute carbon atoms in the aromatic rings of PAHs to form Polycyclic Aromatic Nitrogen Heterocyclic (PANH) molecules. These PANH molecules can have new emission features and may contribute to some of the AIBs. Extreme conditions of the ISM may support PAH and/or PANH molecules in the formation of their cationic or anionic forms to further explain the AIBs. The present work reports DFT study of PANH molecules and their ionized variants and discusses their IR spectroscopic features in the 6–10 μm region. © 2022
Formation routes, structures, and spectroscopic parameters of butyl isocyanides: The case of t-C4H9NC isomer in TMC-1
(Oxford University Press, 2025) Satyam Srivastav; Vincenzo Barone; Akant Vats; Anshika Pandey; Luigi Crisci; Shivani Mishra; Amit Pathak; Liton Majumdar
Cyanides and isocyanides are key prebiotic molecules in the interstellar medium, although the latter have been rarely detected. The observed variations in the isocyanide-to-cyanide ratio among different sources indicate incomplete understanding of their formation chemistry. We report a comprehensive computational study of the conformational landscape of butyl isocyanides (n-, i-, s-, and t-CHNC), including structural and spectroscopic characterization of the low-energy conformers. Equilibrium geometries and rotational constants were obtained using the Pisa Composite Schemes combined with second-order vibrational perturbation theory, yielding mean deviations from experimental data of 0.1 per cent, equivalent to 1 mÅ in bond lengths and in bond angles. Simulated rotational spectra in the 20-200 GHz range show that the most intense transitions of all isomers occur mainly between 75 and 160 GHz at 100-300 K, shifting to 50-110 GHz at 10 K. Formation and destruction pathways of t-CHNC and its precursors were analysed under cold interstellar conditions, and global rate coefficients were derived for chemical modelling. The predicted abundance of t-CHNC is very low (1.45 10), consistent with its current non-detection in astronomical surveys. © 2025 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
Infrared spectra of protonated and deuteronated C 60 in interstellar environments
(Springer, 2023) Akant Vats; Amit Pathak
With the credible detection of C60, C70 and C70+ in the interstellar medium (ISM), new prospects have opened up for the search of other fullerenes and their derivatives. Since fullerenes show high proton affinities, their protonated forms should predominate in the ISM, which can easily go through deuterium enrichment. Here, we present the infrared (IR) spectra and standard enthalpy of formation of C60H+, C60D+, C60H18+ and C60D18+ using Density Functional Theory (DFT) in singly ionized forms. The obtained computed IR spectra are compared with the observations. The results show that the four mid-infrared bands of neutral C60 are still visible in C60H+ and C60D+, but their strength diminishes in C60H18+ and C60D18+. As a conclusion, it is possible that the IR bands ascribed to C60 are a mixture of pure and slightly protonated and deuteronated fullerenes. In this way, the observed scattering of the C60 band ratios could be explained. © 2023, Indian Academy of Sciences.
Interstellar branched chain molecules: A theoretical-rotational study
(Springer, 2023) Satyam Srivastav; Akant Vats; Anshika Pandey; Amit Pathak
Interstellar detection of the straight-chain (n-propyl cyanide, n-C3H7CN) and branched-chain (i-propyl cyanide, i-C3H7CN) molecules toward the star-forming region, Sagittarius B2(N2) (Sgr B2(N2)) has attracted attention to study the formation mechanism and chemical evolution of branched carbon-chain molecules. These molecules are the precursors of biologically relevant prebiotic molecules, i.e., amino acids. In this light, we consider n-butyl cyanide and higher-order branched chain molecule, t-butyl cyanide from the C5H9N isomeric group. Quantum chemical calculations, such as rotational constants, dipole moments and other spectroscopic information will assist to study the chemical evolution and examine the possibility of detecting higher-order branched-chain molecules in high-mass star-forming regions. © 2023, Indian Academy of Sciences.
Investigating C−D out-of-plane vibrational modes in PAHs as a tool to study interstellar deuterium-containing PAHs
(Oxford University Press, 2025) Mridusmita Buragohain; Takashi Onaka; Amit Pathak; Akant Vats; Itsuki Sakon
Previous as well as recent observations by ISO, Spitzer, AKARI, SOFIA, JWST etc. have revealed various characteristics of midinfrared emission bands between 3 and 20 μm. Subsequently,severalforms of organicsincluding polycylic aromatic hydrocarbons (PAHs)/PAH-like molecules are proposed as carriers for these bands. Deuterated PAH (PAD) is one such substituted PAH, which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 μm, detected towards few astronomical targets and are characteristics of aromatic and aliphatic C−D stretching modes in a PAD molecule, respectively. However, the 4.4 μm band is not widely detected. In order to validate PADs as carriers for mid-infrared emission bands, an additional alternative tool is crucial. If PAHs are deuterated, they should also possess an inherent signature from the C−D out-of-plane (C−Doop) vibrations, which are at the longer wavelength side. In this report, features due to C−Doop modes in PAHs bearing a single to multiple deuterium atoms are reported by performing quantum-chemical calculations. This paper reports that some of the C−Doop vibrations appear at the 14–19 μm range. Also, the strength of C−Doop modes is not proportional to the D/H ratio in PAHs. In addition, a moderate change in the spectra of deuterated PAHs is observed from that of the undeuterated counterparts, as deuteration would alternate the adjacency class of the C-H bonds and the mass distribution of the molecule. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting mid-infrared emission bands. © 2025 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
Rotational spectra of interstellar N- and CN-PAHs: pyrene and coronene
(Oxford University Press, 2022) Akant Vats; Amit Pathak
The detection of benzonitrile (C6H5CN), 1- and 2-cyano-naphthalene (C10H7CN) in the cold, dark molecular cloud TMC-1 at centimetre (cm) wavelengths has opened up prospects for the detection of other N- and CN-containing polycyclic aromatic hydrocarbons (PAHs). In this light, the pure rotational spectra of N-pyrene (C15H9N), CN-pyrene (C15H9CN), N-coronene (C23H11N), and CN-coronene (C23H11CN) are reported here for the first time. The B3LYP/6-311 + G (d, p) level of theory, in the density functional theory (DFT) calculations, achieves the best performance for calculating the spectroscopic parameters and simulating the rotational spectra. The large permanent dipole moment of CN-PAHs makes them the most suitable PAH species for detection in the interstellar medium. Additionally, pyrene's smaller partition function makes CN-pyrene a prime candidate to be discovered in cold, dark molecular clouds such as the TMC-1. The present work sets a benchmark for theoretical rotational spectra of N- and CN-containing PAHs and may act as a guide for laboratory experiments and observational searches. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
Theoretical investigation of interstellar 3-pyrroline: formation and, rotational and vibrational spectroscopy
(Oxford University Press, 2024) Anshika Pandey; Akant Vats; Satyam Srivastav; Amit Pathak; K.A.P. Singh
The recent detection of CN-functionalized aromatics partly addresses the long-standing mystery of the apparent absence of five- and six-membered rings in interstellar environments. Nitrogen (N)-heterocycles, which are crucial as the fundamental structures of nucleobases, have been a focus of these aromatic searches due to their biological significance. Although N-heterocycles have not been conclusively detected in astrophysical environments, their presence in chondrites and meteorites signifies their interstellar and circumstellar connection. Precise spectral data identify the unique signatures of molecules, confirming their presence in space. In this light, this work reports an extensive computational investigation on interstellar 3-pyrroline, a five-membered ring N-heterocycle. This includes an alternative formation route in cold interstellar environments and highly accurate rotational and vibrational spectroscopy. The results indicate that 3-pyrroline can form on dust grain surfaces from vinyl cyanide, as its formation from pyrrole through double hydrogenation may lead to the formation of pyrrole itself via an H2-abstraction process. 3-Pyrroline's rotational transition at 52.3 GHz offers a potential tool for its detection in cold interstellar regions. Additionally, the strongest infrared (IR) features of 3-pyrroline at 16.09 and 3.50 m are observable with JWST. The provided data are crucial for laboratory identification and future interstellar observations of 3-pyrroline at both radio and IR wavelengths. © 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
Theoretical microwave spectra of interstellar nitrogen-containing PAHs
(Royal Society of Chemistry, 2023) Akant Vats; Satyam Srivastav; Anshika Pandey; Amit Pathak
The recent discovery of naphthalene (C10H8) in cyano-substituted polycyclic aromatic hydrocarbon (CN-PAH) form in the Taurus molecular cloud (TMC-1) has sparked curiosity regarding the search for other nitrogen-containing naphthalenes in similar interstellar environments. In this light, naphthalenes having N atoms in the structure are promising candidates to be searched for in cold, dark molecular clouds such as TMC-1. Since obtaining data on such samples in the laboratory is complicated, the present work reports theoretical microwave spectra of naphthalene in all N-substituted forms. Density functional theory (DFT) calculations are employed to calculate the spectroscopic constants and simulate the rotational spectra with hyperfine splitting. For cold temperature regions such as TMC-1 (about 5 K), the considered N-naphthalene species show the strongest transition around centimetre wavelengths, a typical range for PAH-related species in dark molecular clouds. Accurate rotational data provided here may act as a guide for laboratory experiments and astronomical searches. © 2023 The Royal Society of Chemistry.
Theoretical Rotational and Vibrational Investigation of Oxygen-Functionalized Interstellar PAHs
(Oxford University Press, 2025) Shivani Mishra; Akant Vats; Satyam Srivastav; Amit Pathak; Peter J. Sarre; Takashi Onaka; Itsuki Sakon
Oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs) with OH, CHO, and CO side groups can form in the interstellar medium (ISM) mainly through the UV irradiation of PAHs in water ice. Inspired by the detection of nitrogen-containing PAHs in the ISM, this study uses highly accurate computational techniques to investigate the rotational and vibrational spectra of oxygen-functionalized pyrene derivatives (Py-OH, Py-CHO, Py-HO, and Py-O2) for comparison with experiments that could aid in their future detection. All four OPAHs exhibit strong dipole moments and rotational lines, observable in denser ISM regions near 12 GHz, 10.2 GHz, 12.6 GHz, and 9.6 GHz, respectively. The strongest IR absorption features are identified at 1179.9 cm−1 (8.47 μm) and 1385.0 cm−1 (7.22 μm) for Py-OH, 1733.4 cm−1 (5.77 μm) for Py-CHO, 1747.7 cm−1 (5.72 μm) for Py-HO, and 1613.0 cm−1 (6.20 μm) for Py-O2. The IR features of Py-OH and Py-O2 exhibit peaks in the CO stretching region around 6.0 μm, while those of Py-CHO and Py-HO are blue-shifted due to anharmonicity from the additional CH bond. This suggests PAHs with CO group and no additional peripheral CH bond better explain the observed 6.0 μm PAH emission. However, to explain the observed PAH emission bands with the OPAHs, a fully emission-cascade treatment is required in the anharmonic IR spectra. The accurate spectral data presented here are crucial for experimental classification and potential interstellar observations. © 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
Theoretical study of infrared spectra of interstellar PAH molecules with N, NH, and NH2incorporation
(Oxford University Press, 2022) Akant Vats; Amit Pathak; Takashi Onaka; Mridusmita Buragohain; Itsuki Sakon; Izumi Endo
This work presents theoretical calculations of infrared spectra of nitrogen (N)-containing polycyclic aromatic hydrocarbon (PAH) molecules with the incorporation of N, NH, and NH2 using density functional theory (DFT). The properties of their vibrational modes in 2-15 μm are investigated in relation to the Unidentified Infrared (UIR) bands. It is found that neutral PAHs, when incorporated with NH2 and N (at inner positions), produce intense infrared bands at 6.2, 7.7, and 8.6 μm that have been normally attributed to ionized PAHs so far. The present results suggest that strong bands at 6.2 and 11.2 μm can arise from the same charge state of some N-containing PAHs, arguing that there might be some N-abundant astronomical regions where the 6.2 to 11.2 μm band ratio is not a direct indicator of the PAHs' ionization. PAHs with NH2 and N inside the carbon structure show the UIR band features characteristic to star-forming regions as well as reflection nebulae (Class A), whereas PAHs with N at the periphery have similar spectra to the UIR bands seen in planetary nebulae and post-AGB stars (Class B). The presence of N atoms at the periphery of a PAH may attract H or H+ to form N-H and N-H2 bonds, exhibiting features near 2.9-3.0 μm, which are not yet observationally detected. The absence of such features in the observations constrains the contribution of NH and NH2 substituted PAHs that could be better tested with concentrated observations in this range. However, PAHs with N without H either at the periphery or inside the carbon structure do not have the abundance constraint due to the absence of 2.9-3.0 μm features and are relevant in terms of positions of the UIR bands. Extensive theoretical and experimental studies are required to obtain deeper insight. © 2021 The Author(s).
Time-Dependent Density Functional Study of Nitrogen-Substituted Polycyclic Aromatic Hydrocarbons and Diffuse Interstellar Bands
(American Chemical Society, 2022) Nishant Shukla; Akant Vats; Amit Pathak; Gazi A. Ahmed
This work reports theoretical calculations of electronic transitions in nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and cations, using time-dependent density functional theory. The results obtained are compared with the diffuse interstellar bands, a broad group of absorption bands that can be seen mostly in near-ultraviolet and near-infrared wavelengths of the spectrum. It is observed that with nitrogen substituted at the periphery (exoskeletal), these nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and their cation counterparts, similar to their corresponding parent polycyclic aromatic hydrocarbons, absorb in the near-ultraviolet and near-infrared wavelengths, respectively. The analogy then follows a change with nitrogen entering into the structure (endoskeletal) and the nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and cations, unlike their corresponding pure polycyclic aromatic hydrocarbon family, fall in the near-infrared and visible spectral regions, respectively. Based on these and other astrophysical implications, it is concluded that nitrogen-substituted polycyclic aromatic hydrocarbons represent a powerful class of prospective carriers of diffuse interstellar bands. © 2022 American Chemical Society.