Initiating molecular growth in the interstellar medium via dimeric complexes of observed ions and molecules

Abstract

A feasible initiation step for particle growth in the interstellar medium (ISM) is simulated by means of ab initio quantum chemistry methods. The systems studied are dimer ions formed by pairing nitrogen containing small molecules known to exist in the ISM with ions of unsaturated hydrocarbons or vice versa. Complexation energies, structures of ensuing complexes and electronic excitation spectra of the encounter complexes are estimated using various quantum chemistry methods. Møller-Plesset perturbation theory (MP2), Z-averaged perturbation theory (ZAPT2), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)), and density functional theory (DFT) methods (B3LYP) were employed along with the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Two types of complexes are predicted. One type of complex has electrostatic binding with moderate (7−20 kcal mol −1 ) binding energies, that are nonetheless significantly stronger than typical van der Waals interactions between molecules of this size. The other type of complex develops strong covalent bonds between the fragments. Cyclic isomers of the nitrogen containing complexes are produced very easily by ion-molecule reactions. Some of these complexes show intense ultravioletvisible spectra for electronic transitions with large oscillator strengths at the B3LYP, ωB97, and equations of motion coupled cluster (EOM-CCSD) levels. The open shell nitrogen containing carbonaceous complexes especially exhibit a large oscillator strength electronic transition in the visible region of the electromagnetic spectrum.

Characterization of the Azirinyl Cation and Its Isomers

“…In 1998, Mayer et al 22 reported the thermodynamic properties of the singlet C 2 H 2 N + isomers, and found the same ordering of the three lowest energy structures. They used Gaussian 2 (G2) theory as well as methods that include extrapolation to the one-particle basis set limit and found relative energies similar to those reported by Harland et al 23 In 1999, Lau et al 24 used G2 theory to examine the stability of C 2 H 2 N + isomers, including the lowest triplet structure (Figure 4, C s , 3 A"), and found the same order of stability for the singlet isomers, though the relative energies of the singlet isomers were somewhat different compared to the findings of Mayer et al, 22 with the lowest triplet isomer being slightly higher in energy than isocyanomethylene cation. In 2003, Di Stefano et al 25 examined the production of cations containing C-N bonds both experimentally and theoretically, including C 2 H 2 N + isomers.…”

Structural and spectroscopic properties of complexes formed between HNCS and SO 2 in low temperature matrices

“…The smaller molecules are pervasive and their interactions may help to understand how larger molecules or grains are formed in ISM [11]. Among important small species found in ISM are isothiocyanic acid HNCS (Sgr B2, 1979 [12]) and sulfur dioxide (Orion A, 1975 [13]).…”

Relative energies, structures, vibrational frequencies, and electronic spectra of pyrylium cation, an oxygen-containing carbocyclic ring isoelectronic with benzene, and its isomers

“…Optimized structure of the next 6 lowest energy isomers of C 6 H 5 O + , at the CCSD(T)/cc-pVTZ level of theory. In ascending energy order, these isomers are 2methylene furanium ion or furfuryl cation (2), 3-methyl-furanium (3), hydroxycyclopentadiene (4), cyclopentenone ion (5), methyl-cyclopropyl ketone ion (6), and meta-cyclopentenone ion (7). Bond lengths are in Angstroms and bond angles are in degrees.…”

“…The vibrational frequencies of the C s symmetry non-planar (two hydrogen atoms out-of-plane) isomers (5,6,7) are presented in Table 6. These vibrational frequencies should be useful for identification of the corresponding isomers in future laboratory experiments and in the interstellar medium.…”

“…A few other experimental and theoretical studies 6,7 have investigated heteroatom insertion in carbocyclic rings in general, and oxygen insertion in the rings in particular, and proved that these species can form in the gas phase. Solano et al predicted, based on electron impact mass spectra, that the furfuryl ion, 2, is synthesized during the dissociation of N-(2-furylmethyl) anilines.…”