Molecules in laboratory and in interstellar space?

\begin{wrapfigure}{r}{0.5\textwidth} \vspace{-32pt} \begin{center} \includegraphics[width=0.40\textwidth]{venky-ismsfig.eps} \end{center} \vspace{-32pt} \end{wrapfigure} In this talk, the quantum chemistry of astronomically relevant molecules will be outlined with an emphasis on the structures and energetics of C$_7$H$_2$ isomers, which are yet to be identified in space. Although more than 100's of isomers are possible for C$_7$H$_2$, to date only 6 isomers had been identified in the laboratory.\footnote{Apponi, A. P.McCarthy, M. C.Gottlieb, C. A.Thaddeus, P. Laboratory Detection of Four New Cumulene Carbenes: H$_2$C$_7$, H$_2$C$_8$, H$_2$C$_9$, and D$_2$C$_{10}$, {\emph {Astrophys. J.}} 2000, 530, 357-361.}$^{,}$\footnote{ Ball, C. DMcCarthy, M. C.Thaddeus, P. Cavity Ringdown Spectroscopy of the Linear Carbon Chains HC$_7$H, HC$_9$H, HC$_{11}$H, and HC$_{13}$H. {\emph {J. Chem. Phys.}} 2000, 112, 10149-10155.}$^{,}$\footnote{Dua, S.Blanksby, S. J.Bowie, J. H. Formation of Neutral C$_7$H$_2$ Isomers from Four Isomeric C$_7$H$_2$ Radical Anion Precursors in the Gas Phase. {\emph {J. Phys. Chem. A}}, 2000, 104, 77-85.} The equilibrium geometries of heptatriynylidene ({\bf 1}), cyclohepta-1,2,3,4-tetraen-6-yne ({\bf 2}), and heptahexaenylidene ({\bf 3}), which we had investigated theoretically will be discussed briefly.\footnote{Thimmakondu, V. S. The equilibrium geometries of heptatriynylidene, cyclohepta-1,2,3,4-tetraen-6-yne, and heptahexaenylidene, {\emph {Comput. Theoret. Chem.}} 2016, 1079, 1-10.} While {\bf 1} and {\bf 3} are observed in the laboratory, {\bf 2} is a hypothetical molecule. The theoretical data may be useful for the laboratory detection of {\bf 2} and astronomical detection of {\bf 2} and {\bf 3}

From Molecules with a Planar Tetracoordinate Carbon to an Astronomically Known C5H2 Carbene

Ethynylcyclopropenylidene (2), an isomer of C5H2, is a known molecule in the laboratory and has recently been identified in Taurus Molecular Cloud-1 (TMC-1). Using high-level coupled-cluster methods up to the CCSDT(Q)/ CBS level of theory, it is shown that two isomers of C5H2 with a planar tetracoordinate carbon (ptC) atom, (SP-4)-spiro[2.2]pent-1,4-dien-1,4-diyl (11) and (SP-4)-spiro[2.2]pent-1,4-dien-1,5-diyl (13), serve as the reactive intermediates for the formation of 2. Here, a theoretical connection has been established between molecules containing ptC atoms (11 and 13) and a molecule (2) that is present nearly 430 light years away, thus providing evidence for the existence of ptC species in the interstellar medium. The reaction pathways connecting the transition states and the reactants and products have been confirmed by intrinsic reaction coordinate calculations at the CCSDT(Q)/CBS//B3LYP-D3BJ/cc-pVTZ level. While isomer 11 is non-polar (μ = 0), isomers 2 and 13 are polar, with dipole moment values of 3.52 and 5.17 Debye at the CCSD(T)/cc-pVTZ level. Therefore, 13 is also a suitable candidate for both laboratory and radioastronomical studies

Matrix-isolated infrared absorption spectrum of CH_2BrOO radical

The bromomethylperoxy radical, CH_2BrOO, has been generated in cryogenic matrices. Six fundamental bands for CH_2BrOO have been observed in an argon matrix at 5 K. The experimental frequencies (cm^(−1)) are: ν_4 = 1274.3, ν_5 = 1229.4, ν_6 = 1086.7, ν_7 = 961.8, ν_8 = 879.9, and ν_(10) = 515.4, two of which are detected for the first time. Ab initio calculations have been performed employing coupled-cluster methods. The experimental frequencies are shown to be in good agreement with the computation as well as the four bands (ν_4, ν_6, ν_7 and ν_8) observed by Huang and Lee in the gas phase

Matrix-isolated infrared absorption spectrum of CH_2BrOO radical

The bromomethylperoxy radical, CH_2BrOO, has been generated in cryogenic matrices. Six fundamental bands for CH_2BrOO have been observed in an argon matrix at 5 K. The experimental frequencies (cm^(−1)) are: ν_4 = 1274.3, ν_5 = 1229.4, ν_6 = 1086.7, ν_7 = 961.8, ν_8 = 879.9, and ν_(10) = 515.4, two of which are detected for the first time. Ab initio calculations have been performed employing coupled-cluster methods. The experimental frequencies are shown to be in good agreement with the computation as well as the four bands (ν_4, ν_6, ν_7 and ν_8) observed by Huang and Lee in the gas phase

Flat Crown Ethers with Planar Tetracoordinate Carbon Atoms

Novel flat crown ether molecules have been characterized in silico using DFT hybrid and hybrid-meta functionals. Monomer units of Si2C3 with a planar tetracoordinate carbon atom have been used as building blocks. Alkali (Li+, Na+, K+, Rb+, and Cs+) and alkaline-earth (Ca2+, Sr2+, and Ba2+) metals, and uranyl (UO2+ 2 ) ion selective complexes have also been theoretically identified. The high symmetry and higher structural rigidity of the host molecules may likely to impart higher selectivity in chelation. Theoretical binding energies have been computed and experimental studies are invited

CCSD(T) Rotational Constants for Highly Challenging C₅H₂ Isomers—A Comparison between Theory and Experiment

We evaluate the accuracy of CCSD(T) and density functional theory (DFT) methods for the calculation of equilibrium rotational constants (Ae, Be, and Ce) for four experimentally detected low-lying C5H2 isomers (ethynylcyclopropenylidene (2), pentatetraenylidene (3), ethynylpropadienylidene (5), and 2-cyclopropen-1-ylidenethenylidene (8)). The calculated rotational constants are compared to semi-experimental rotational constants obtained by converting the vibrationally averaged experimental rotational constants (A0, B0, and C0) to equilibrium values by subtracting the vibrational contributions (calculated at the B3LYP/jun-cc-pVTZ level of the theory). The considered isomers are closed-shell carbenes, with cumulene, acetylene, or strained cyclopropene moieties, and are therefore highly challenging from an electronic structure point of view. We consider both frozen-core and all-electron CCSD(T) calculations, as well as a range of DFT methods. We find that calculating the equilibrium rotational constants of these C5H2 isomers is a difficult task, even at the CCSD(T) level. For example, at the all-electron CCSD(T)/cc-pwCVTZ level of the theory, we obtain percentage errors ≤0.4% (Ce of isomer 3, Be and Ce of isomer 5, and Be of isomer 8) and 0.9–1.5% (Be and Ce of isomer 2, Ae of isomer 5, and Ce of isomer 8), whereas for the Ae rotational constant of isomers 2 and 8 and Be rotational constant of isomer 3, high percentage errors above 3% are obtained. These results highlight the challenges associated with calculating accurate rotational constants for isomers with highly challenging electronic structures, which is further complicated by the need to convert vibrationally averaged experimental rotational constants to equilibrium values. We use our best CCSD(T) rotational constants (namely, ae-CCSD(T)/cc-pwCVTZ for isomers 2 and 5, and ae-CCSD(T)/cc-pCVQZ for isomers 3 and 8) to evaluate the performance of DFT methods across the rungs of Jacob’s Ladder. We find that the considered pure functionals (BLYP-D3BJ, PBE-D3BJ, and TPSS-D3BJ) perform significantly better than the global and range-separated hybrid functionals. The double-hybrid DSD-PBEP86-D3BJ method shows the best overall performance, with percentage errors below 0.5% in nearly all cases

CCSDT(Q)/CBS thermochemistry for the D-5h -> D-10h isomerization in the C-10 carbon cluster: Getting the right answer for the right reason

The D5h → D10h isomerization in the C10 carbon cluster is investigated at the relativistic, all-electron CCSDT(Q)/CBS level. Previous high-level studies examined this isomerization at the valence CCSD(T)/CBS level. We show that capturing this isomerization energy requires accurate treatment of the CCSD(T)/CBS, post-CCSD(T), core-valence, scalar relativistic, diagonal Born-Oppenheimer, and zero-point vibrational energy components. Combining these components shows that the two structures are practically isoenergetic at 0 K (i.e., the D5h structure is more stable by merely +0.100 kcal mol-1). We also show that computationally economical composite protocols erroneously predict that the D10h structure is energetically more stable at 0 K

Energetic and spectroscopic properties of the low-lying C7H2 isomers: a high-level ab initio perspective

We use high-level ab initio CCSD(T) and CCSDT(Q) methods to investigate the energetic and spectroscopic properties of nine low-lying isomers of C7H2, which lie within 1 eV. Among these, heptatriynylidene (1), 1-(buta-1,3-diynyl)cyclopropenylidene (2) and heptahexaenylidene (9) have been detected experimentally. The other six isomers, 1,2-(diethynyl)cyclopropenylidene (3), bicyclo[4.1.0]hepta-1,2,4,5-tetraene-7-ylidene (4), cyclohepta-1,2,3,4-tetraen-6-yne (5), bicyclo[4.1.0]hepta-4,6-diene-2-yne-7-ylidene (6), bicyclo[4.1.0]hepta-1,5-diene-3-yne-7-ylidene (7) and 1-(buta-1,3-diynyl)propadienylidene (8), remain hypothetical to date. Except for 1, all of the isomers are associated with a non-zero dipole moment (μ ≠ 0). Although Fourier-transform microwave spectroscopy had detected 2 and 9, our study reveals that six hypothetical isomers (3–8) are thermodynamically sandwiched between the experimentally known and astronomically relevant isomers 2 and 9. The structural parameters, dipole moments, rotational and centrifugal distortion constants, harmonic vibrational frequencies, and infra-red intensities presented here may be useful for the laboratory detection of these previously unidentified isomers (3–8) and also all others (2–9) in astronomical sources

The quest for the carbene bent-pentadiynylidene isomer of C5H2

The equilibrium geometry of the singlet ground electronic state of the bent isomer of C5H2, bent-pentadiynylidene (4; X̃1 A1; C2v ), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C5H2, namely linear-pentadiynylidene (1; X̃3Σḡ; D∞h), ethynylcyclopropenylidene (2; X̃1A'; Cs ), pentatetraenylidene (3; X̃1A1; C2v ), ethynylpropadienylidene (5; X̃1A'; Cs ), and 3-(didehydrovinylidene)cyclopropene (6; X̃1A1; C2v ) had already been identified in the laboratory. With respect to 1, the relative energy difference calculated at the CCSDT(Q)/CBS level of theory including zero-point vibrational energy corrections are: 0.66 (2), 13.53 (3), 14.12 (4), 15.40 (5), and 20.01 (6) kcal mol-1, respectively. Isomers 2-6 are associated with a non-zero dipole moment (µ ≠ 0), however, except 4, all the other four isomers were identified by Fourier Transform Microwave spectroscopy, including 5 and 6 which lie higher in energy. Isomer 4 remains elusive to date. We believe that the theoretical data such as, optimal geometry, dipole moment, rotational and centrifugal distortion constants, harmonic vibrational frequencies, infra-red intensities, and isotopic shifts (12C-13C) in harmonic vibrational frequencies presented here would assist experimentalists in the identification of this elusive molecule (4)