Calculated Dipole Moments for Silicon and Phosphorus Compounds of Astrophysical Interest

High-temperature spectra of the PNO molecule based on robust first-principles methods

The PNO molecule is an important species found in the interstellar medium, and its spectroscopic information is helpful for its detection. We present the first line list of PNO (X1Σ+) using robust first-principles methods. The analytical potential energy surface and the dipole moment surface were constructed based on 11 942 ab initio points. The variational nuclear motion calculation was implemented in TROVE to obtain the rovibrational energy levels, Einstein A coefficients and other parameters. The J-dependent Coriolis-decoupled Hamiltonian was adopted with k ≤ 15, and the l-type doubling was considered for the bending vibration of the linear molecule. The line list contained almost 5.87 billion transitions between 3.61 million levels with rotational excitation up to J = 200 and was used to generate the PNO spectrum below 3000 K in the wavenumber range from 0 to 6000 cm-1. The millimetre wave spectrum agrees well with available experimental benchmarks. The Fermi resonance effects in the PNO spectrum are universal and complex, resulting in significant intensity increment of the related weak transition. This line list may be helpful for the spectroscopic characterization and possible astronomical detection of PNO, especially in high-temperature environments.

High-temperature rotation-vibration spectrum of iminosilylene (HNSi)

Iminosilylene (HNSi) has been observed in the laboratory and is expected to occur in the envelopes of carbon-rich stars. However, the lack of spectroscopic information for HNSi has hampered its further astrophysical detection. Using robust ab initio methods, we present the first and comprehensive molecular line list for HNSi (X 1Σ+). The new line list contains almost 3.36 billion transitions between 1.57 million levels with rotational excitation up to J = 160. It is suitable for temperatures up to 3000 K and covers the wavenumber range of 0-9000 cm-1 (wavelengths λ > 1.11 μm). This new line list can be helpful for the future spectroscopic characterization and molecular detection of HNSi in the laboratory and interstellar space.

Phosphine Reactivity and Its Implications for Pyrolysis Experiments and Astrochemistry

Despite the importance of phosphorus-bearing molecules for life and their abundance outside Earth, the chemistry of those compounds still is poorly described. The present study investigates phosphine (PH₃) decomposition and formation pathways. The reactions studied include phosphine thermal dissociation, conversion into PO(²Π), PN(¹Σ₊), and reactions in the presence of H₂O⁺. The thermodynamic and rate coefficients of all reactions are calculated in the range of 50-2000 K considering the CCSD(T)/6-311G(3df,3pd)//ωB97xD/6-311G(3df,3pd) electronic structure data. The rate coefficients were calculated by RRKM and semiclassical transition-state theory (SCTST). According to our results, PH₃ is stable to thermal decomposition at T < 100 K and can be formed promptly by a reaction network involving PH(³Σ_-), PO(²Π), and PN(¹Σ₊). In the presence of radiation or ions, PH₃ is readily decomposed. For this reason, it should be mainly associated with dust grains or icy mantles to be observed under the physical conditions prevailing in the interstellar medium (ISM). The intersystem crossing associated with the dissociation of the isomers PON, NPO, and PNO is accessed by multireference methods, and its importance for the gas-phase PH₃ formation/destruction is discussed. Also, the implications of the present outcomes on phosphorus astrochemistry are highlighted.

Icy Grain Mantle Surface Astrochemistry of MgNC: The Emergence of Metal Ion Catalysis Studied via Model Ice Cluster Calculations

One of a small number of known magnesium-containing astromolecules, magnesium isocyanide (MgNC) was first detected in 1986. MgNC is an intriguing reactant to consider: it is an open-shell radical in which its metal atom forms a bond with CN that is a mixture of ionic and covalent character. While its gas phase astrochemistry has received prior attention, the grain surface chemistry of MgNC has never been studied. Because of its ionic character, MgNC is found to interact far more strongly with an ice surface than molecules with a greater degree of covalency. As a radical, it may react with closed-shell molecules deposited from the gas phase. In this work, cluster calculations treated with density functional theory and correlation consistent basis sets were used to model the deposition of MgNC on clusters containing 17 and 24 water molecules, which were then allowed to react with acetylene (HCCH) and hydrogen cyanide (HCN) as well as with H atoms. The addition of H to MgNC-nH₂O yields hydromagnesium isocyanide (HMgNC), a known astromolecule that may be ejected into the gas phase. HCCH and HCN bind to MgNC-nH₂O to form intermediate radical compounds that may then also react with H atoms. There is enough reaction energy from H addition to eject fragments of the intermediates into the gas phase: the vinyl radical (C₂H₃) for HCCH and the methaniminyl radical (H₂CN) for HCN. That leaves MgNC-nH₂O to perform further catalytic activity. Alternatively, various hydrogenated divalent Mg compounds may also be stabilized and frozen into the ice or potentially ejected into the gas phase. Benchmark coupled cluster theory calculations in limited systems were used to characterize the submerged reaction barriers present when HCCH or HCN add to MgNC in the gas phase.

Theoretical study on the cycloaddition reaction of phosphenium cation and formaldehyde

The mechanism of cycloaddition reaction between phosphenium cation and phosphindene with formaldehyde has been systematically investigated at the B3LYP/6-311++G(d,p) level of theory in order to better understand the reactivity for the valence isoelectronic of carbene. Phosphenium cation acts as an electrophilic reagent and accepts σ electrons of formaldehyde to form a complex in the first addition step. The greater the positive charge on phosphorus atom in phosphenium cation, the more stable the formed complex is. Introduction of substituents will decrease positive charge on phosphorus atom in phosphenium cation. The order of positive charge on phosphorus atom is HP+–F >  HP+–OH >  HP+–NH2, which is consistent with their Lewis acidities. The complex transforms to a three-membered ring product via a transition state in the second cyclization step. The product is less stable than the complex due to its tension of small ring.

Insights into the reaction mechanism between phosphacyclopropenylidene and methyleneimine: A theoretical study

The reaction mechanism between phosphacyclopropenylidene and methyleneimine has been systematically investigated at the M06–2X/6–311++G(d,p) level of theory in order to better understand the reactivity of unsaturated cyclic phosphorus-bearing carbene. Geometry optimizations and vibrational analyses have been conducted for the stationary points on the potential energy surface of the system. Calculations show that the spiro bicyclic intermediate could be produced through the cycloaddition process between phosphacyclopropenylidene and methyleneimine initially. The reaction mechanism is illustrated with frontier molecular orbital theory. Introduction of electron-withdrawing group in phosphacyclopropenylidene will better facilitate the addition process. Through subsequent ring-expanding and hydrogen-migrating process, fuse-ring and allene compounds could be produced, respectively. Furthermore, it’s easy for spiro bicyclic intermediate and another methyleneimine to form a spiro tricyclic compound. This study is helpful to understand the reactivity of phosphacyclopropenylidene, the evolution of phosphorus-bearing molecules in space, and to offer an alternative approach to the formation of phosphorus-bearing heterocyclic compound.

Theoretical study on the reaction between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane): An alternative approach to the formation of heterocyclic silylene

The reaction mechanism between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane) has been systematically investigated at the B3LYP/6-311+G* level of theory in order to better understand the reactivity of unsaturated cyclic silylene. Geometry optimizations and vibrational analyses have been conducted for the stationary points on the potential energy surface of the system. Calculations show that the Si-spiroheterocyclic intermediate and four-membered heterocyclic silylene compound could be produced through the insertion process and subsequent dissociation process between silacyclopropenylidene and three-membered heterocyclic compounds. For the insertion process, it is easier for silacyclopropenylidene to insert into C-N bond of azirane than into C-O bond of oxirane. This study is helpful to understand the reactivity of silacyclopropenylidene, the evolution of silicon-bearing molecules in space, and to offer an alternative approach to the formation of enlarged heterocyclic silylene compound.

Generation and structural characterization of Ge carbides GeCn (n = 4, 5, 6) by laser ablation, broadband rotational spectroscopy, and quantum chemistry

Following the recent discovery of T-shaped GeC₂, rotational spectra of three larger Ge carbides, linear GeC₄, GeC₅, and GeC₆ have been observed using chirped pulse and cavity Fourier transform microwave spectroscopy and a laser ablation molecule source, guided by new high-level quantum chemical calculations of their molecular structure. Like their isovalent Si-bearing counterparts, Ge carbides with an even number of carbon atoms beyond GeC₂ are predicted to possess ¹Σ ground electronic states, while odd-numbered carbon chains are generally ³Σ; all are predicted to be highly polar. For the three new molecules detected in this work, rotational lines of four of the five naturally occurring Ge isotopic variants have been observed between 6 and 22 GHz. Combining these measurements with ab initio force fields, the Ge-C bond lengths have been determined to high precision: the derived values of 1.776 Å for GeC₄, 1.818 Å for GeC₅, and 1.782 Å for GeC₆ indicate a double bond between these two atoms. Somewhat surprisingly, the spectrum of GeC₅ very closely resembles that of a ¹Σ molecule, implying a spin-spin coupling constant λ in excess of 770 GHz for this radical, a likely consequence of the large spin-orbit constant of atomic Ge (∼1000 cm^-1). A systematic comparison between the production of SiC_n and GeC_n chains by laser ablation has also been undertaken. The present work suggests that other large metal-bearing molecules may be amenable to detection by similar means.

Probing the coupling of a dipole-bound electron with a molecular core

A dipolar molecule can weakly bind an electron in a diffuse orbital. However, the spin-orbit coupling between this weakly bound electron and the electrons in the molecular core is not known. Here we probe this coupling using the linear C₂P^- anion with the ³Σ⁺ ground state, which possesses dipole-bound excited states because neutral C₂P (²Π) has a sufficiently large dipole moment. Photodetachment spectroscopy and resonant photoelectron spectroscopy are used to probe the nature of the dipole-bound states. Two dipole-bound excited states are observed with a binding energy of 37 cm^-1, corresponding to the two spin-orbit states of neutral C₂P (²Π_1/2 and ²Π_3/2). The current study demonstrates that the weakly bound electron in the dipole-bound excited states of C₂P^- is not spin-coupled to the electrons in the C₂P core and can be considered as a quasi-free electron.

Photochemistry of OPN: Formation of Cyclic PON and Reversible Combination with Carbon Monoxide

Cyclic PON, a first 16-electron triatomic ring consisting of two first-row elements, has been generated through 193 nm laser photolysis of OPN in cryogenic matrices. When the photolysis (365 nm) was performed in the presence of CO, OPN combines CO and furnishes an exotic acyclic molecule OPNCO. Upon the 193 nm laser irradiation, OPNCO dissociates mainly to OPN and CO with traces of PN and CO₂ . The identification of cyclic PON and OPNCO with IR spectroscopy is supported by ¹⁵ N-labeling experiments and quantum chemical calculations.

Accurate Frequency Determination of Vibration-Rotation and Rotational Transitions of SiH. THE ASTROPHYSICAL JOURNAL 2017;849:60. [PMID: 29142330 PMCID: PMC5683345 DOI: 10.3847/1538-4357/aa8fca]

The fundamental 28SiH+ ion has been characterized in a collaborative work, utilizing a hollow-cathode-discharge laser-spectrometer and a cryogenic ion trap spectrometer. Twenty-three vibration-rotation transitions around 4.75 μm have been detected with high accuracy. This has facilitated the first direct measurement of the pure rotational transition J = 1 ← 0 at 453056.3632(4) MHz in the trap spectrometer. The measured and accurately predicted transitions enable the search for this ion in space with IR and sub-mm telescopes.