Marcin Gronowski

Computational Chemistry


Posts Tagged ‘ab initio’

An ab initio study of structure, stability, and spectroscopic parameters of 5-atomic [C, C, H, N, S] isomers

This theoretical study is focused on predicting structures, energetics, and selected spectroscopic constants for a range of 5-atomic chemical species sharing the [C, C, H, N, S] stoichiometry, including thioformyl cyanide and iminoethenethione. An in-depth study carried out on the four most stable isomers have yielded (i) structures and energy for the lowest singlet and triplet excited electronic states; (ii) anharmonic vibrational frequencies and IR absorption intensities for the fundamental, overtone and combination modes; (iii) molecular parameters of interest to microwave spectroscopy: rotational constants, electric dipole moments, and quadrupole coupling constants.

Full text: Journal of Molecular Structure, Volume 1090, 15 June 2015, Pages 76–85

Quantum Chemical Study on HKrC5N, HXeC5N, and Related Rare Gas Compounds

The recent identification of HRgC5N (Rg = Kr, Xe) in a cryogenic matrix calls for an in-depth theoretical study on these compounds. Here we present the results of CCSD(T), MP2, and DFT calculations concerning the molecular structure, stability, and vibrational spectroscopy. The procedure combining CCSD(T) calculations for variable H–Rg distances with the anharmonic description of the corresponding stretching vibration, based on a Morse-type potential energy function, was proposed and has led to good agreement between computational and experimental values for H–Rg stretching frequencies, at relatively low computational costs. High Raman scattering activity of HRgC5N and of its isomers, predicted at the DFT level, gives some prospects for the detection of these molecules with a method alternative to the IR absorption spectroscopy.

Full Text: J. Phys. Chem. A, 2015, 119, pp 2672–2682

A Theoretical Study on the Interstellar Synthesis of H2NCS+ and HNCSH+ Cations

HNCS and NCSH molecules, recently discovered in the interstellar medium, are likely formed via the dissociative recombination of H2NCS+ or HNCSH+ isomeric ions. Interstellar synthesis of the latter is discussed on theoretical grounds. The analysis of relevant potential energy surfaces suggests a key role for chemical processes in which CSH+ or HCS+ cations (most likely formed in CS + H3+ collisions) react with NH2 or NH3. The astrochemical kinetic database (kida.uva.2011), appended with 7 sulfur-bearing molecules and 48 corresponding reactions, has been applied to model the evolution of HNCS, NCSH, and their cationic precursors in a quiescent molecular cloud. Based on the model and on spectroscopic predictions, for an object like TMC-1, we expect the total intensity of H2NCS+ microwave lines to be comparable to that observed for HSCN. Theoretically derived molecular parameters, of interest for radio spectroscopy, are given for the most stable cations sharing the H2NCS+ stoichiometry.

Full text:  ApJ 792 89 doi:10.1088/0004-637X/792/2/89

Ab Initio Studies of the Structure and Spectroscopy of CHNMg Stoichiometry Molecules and van der Waals Complexes

A high-level ab initio study was conducted over the range of tetraatomic molecules containing H, C, N, and Mg. Potential energy surfaces were analyzed, leading, for selected molecules, to the optimization of their geometry in the lowest singlet and triplet excited states. Reliable ground state rotational constants are given for the most stable species, namely, HMgNC and HMgCN, together with respective anharmonic vibrational frequencies of fundamental, overtone, and combination bands. In addition, potential energy surfaces describing the interaction of HCN or HNC with a single magnesium atom have been investigated.

Full text: J. Phys. Chem. A, 2013, 117 (21), pp 4455–4461


Prospects for the presence and detection of interstellar cyanovinylidene, CC(H)CN, a Y-shaped isomer of cyanoacetylene, are discussed. It is proposed that CC(H)CN can arise in interstellar clouds as one of the HC3NH+ + e– dissociative recombination products, by rearrangements of the neutral chain radical HC3NH into branched species HCCC(H)N, CC(H)C(H)N, and/or HCC(H)CN, and by the subsequent elimination of a hydrogen atom. It is deduced that the abundance of cyanovinylidene in molecular clouds should be confined between the abundances of its chain isomers HNCCC and HCNCC. Quantum chemical predictions regarding cyanovinylidene geometry, ground-state rotational constants, centrifugal distortion constants, spin-orbit coupling, IR absorption spectroscopy, and electric dipole moment are given. The spectroscopically observed molecules formyl cyanide, NC2(H)O, and propynal, HC3(H)O, with structures qualitatively resembling cyanovinylidene, served to prove the adequacy of the calculational procedures employed.
Full text: Astrophys. J. 701 (2009) 488

The C3N− anion: First detection of its electronic luminescence in rare gas solids

The 193 nm laser irradiation of cyanoacetylene HCCCN that was isolated in rare gas solids led to a long-lived luminescence origin at 3.58 eV, which was assigned to the a 3Σ+-X 1Σ+ system of cyanoacetylide CCCN−. The identification, which involved 15N and 2H isotopic substitution studies, is based on vibronic spacings in the phosphorescence spectrum compared to previous infrared absorption measurements and to theoretical results regarding CCCN− vibrational frequencies, as well as on a BD(T)/cc-pVTZ prediction for the singlet-triplet energy gap in this anion 3.61 eV. The same emission was also generated from Kr/HC3N mixtures subjected to a glow electric discharge immediately before the solidification cold-window-radial-discharge technique.
Full text: J. Chem. Phys. 128 (2008) 164304

C5N− anion and new carbenic isomers of cyanodiacetylene: A matrix isolation IR study

Products of the vacuum-UV photolysis of cyanodiacetylene HC5N in solid argon —the anion C5N−, imine HNC5, and the branched carbene C4(H)CN —have been identified by IR absorption spectroscopy, in addition to the already discovered isonitrile HC4NC. Spectral assignments were assisted by deuterium substitution experiments, by BD(T) calculations, and by the results of a recent density functional theory study.
Full text: J. Chem. Phys. 128 (2008) 154303

Matrix isolation IR spectroscopic and ab initio studies of C3N− and related species

Coupled cluster calculations were carried out for C3N−, CCNC−, C3N, CCNC, C3N+, and C3O. They support the experimental identification of the C3N− ion by means of matrix isolation infrared IR spectroscopy. The anion was generated in electric discharges through the cyanoacetylene isotopomers HC314N, HC315N, and 2HC3N, trapped in cryogenic rare gas matrices Ne, Ar, Kr, anddetected via its two most intense IR absorption bands, assigned to the 1 and 2 stretching vibrations. C3N− appears to be quite a stable anion, with a vertical detachment energy predicted to be as high as 4.42 eV. A large equilibrium electric dipole moment of 3.10 D facilitates the investigation of C3N− by microwave spectroscopy and radio astronomy. Various structural parameters and spectroscopic properties have been calculated for all tetra-atomic species considered.
Full text: J. Chem. Phys 128 (2008) 154305

Isomers of cyanodiacetylene: Predictions for the rotational, infrared and Raman spectroscopy

Recent ab initio (coupled-clusters) and density functional theory studies on cyanodiacetylene isomers are extended here to yield the quantities of direct interest to future spectroscopic investigations. A bond lengths scaling procedure was developed to obtain the corrected molecular geometries. These, together with calculated vibration–rotation coupling constants, yielded the ground-level rotational constants for seven most stable isomeric species of the HC5N stoichiometry. Former calculations regarding the vibrational transitions of these molecules are complemented here with predictions on Raman scattering activities, and on isotopic (2H, 15N) effects in IR absorption and Raman spectra.
Full text: J. Mol. Struct. 834-836 (2007) 102

Isomers of cyanodiacetylene: Theoretical structures and IR spectra

Coupled-clusters CCSD(T)/cc-pVTZ calculations have been carried out on several lowest energy isomers of cyanodiacetylene H–CC–CC–CN, previously selected in an extensive DFT study. Apart from the izonitrile H–CC–CC–NC, branched species CC(H)CCCN and CCCC(H)CN are found to be of particular importance, less stable than cyanodiacetylene by 48.5 and 51.0 kcal/mol, respectively. Harmonic frequencies and IR absorption intensities of vibrational fundamentals were predicted at the B3LYP/aug-cc-pVTZ level.
Full text: Chem. Phys. Lett. 428 (2006) 245