Extensive Quantum Chemistry Study of Neutral and Charged C₄N Chains: An Attempt To Aid Astronomical Observations

Abstract

Many molecular species can presumably still be observed in space if they are adequately characterized chemically. In this paper, we suggest that this could be the case of the neutral (C 4 N 0 ) and anion (C 4 N -) cyanopropynylidene chains, which were not yet identified in space although both the neutral (C 3 N 0 and C 5 N 0 ) and anion (C 3 Nand C 5 N -) neighboring members of the homologous series were observed. Extensive data obtained from quantum chemical calculations using density functional theory (DFT), coupled cluster (CC), and quadratic configuration interaction (QCI) methods for all charge and spin states of interest for space science (doublet and quartet neutrals, triplet and singlet anions, and singlet and triplet cations) are reported: e.g., bond metric and natural bond order data, enthalpies of formation, dissociation and reaction energies, spin gaps, rotational constants, vibrational properties, dipole and quadrupole momenta, electron attachment energies (EA) and ionization potentials (IP ). The fact 1 that (not only for C 4 N but also for C 2 N and C 6 N) the quantum chemical methods utilized here are able to excellently reproduce the experimental EA value -which is often a challenge for theory -is particularly encouraging, since this indicates that theoretical estimates of chemical reactivity indices (which are key input parameters for modeling astrochemical evolution) can be trusted. The presently calculated enthalpies of formation and dissociation energies do not substantiate any reason to assume that C 4 N is absent in space. To further support this idea, we analyze potential chemical pathways of formation of both C 4 N 0 and C 4 N -, which include association and exchange reactions.In view of the substantially larger dipole moment (D anion ≫ D neutral ), we suggest that astronomical detection should first focus on C 4 Nchains rather than on neutral C 4 N 0 chains.

HCnH− Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation

“…Insight gained in conjunction with our recent investigations of astrochemically relevant carbon chain anions [ 26 , 27 , 28 , 29 ] made us skeptical that shorter HC n H − anion chains possess a stable linear geometry, as claimed earlier [ 23 ]. Our extensive attempts to optimize HC n H − anions imposing strict linear conformation confirmed previous results reported for sizes [ 24 , 25 ]; we also found that irrespective whether n is odd or even, such sufficiently long HC n H − anions, linear and invariant under spatial inversion, are stable against molecular vibrations, i.e., all calculated frequencies were real.…”

“…Still, are all HC n H − chains with molecular sizes of astrochemical interest really linear and centrosymmetric? This was the fundamental question that triggered the investigation whose results will be presented below, and emphasizing anions is part of our recent [ 26 , 27 , 28 , 29 , 30 ] and ongoing effort to understand their role in astrochemistry, which is claimed to even compete with that of the parent neutrals [ 29 , 31 , 32 , 33 ].…”

Comprehensive Quantum Chemical Characterization of the Astrochemically Relevant HCnH Chain Family: An Attempt to Aid Astronomical Observations

“…[ 48–51 ] A variety of carbon‐based anion chains were investigated in the past [ 26,49,50,52–57 ] also using elaborate ab initio quantum chemical methods beyond the DFT. [ 51,58–60 ] As far as the HC n H family is concerned, previous studies only reported values of EA estimated via Δ‐DFT [ 46,61,62 ] for even members HC 2 k H with $$ 6 \le k \le 12$$ [ 37 ] and $$ 2 \le k \le 6$$, [ 45 ] and for odd members HC$$_{2k+1}$$H with $$ 4 \le k \le 11$$. [ 38 ] However, one may wonder whether DFT‐based EA values are accurate enough for reliable modeling the chemical evolution in extraterrestrial environments.…”

Why Ortho- and Para-Hydroxy Metabolites Can Scavenge Free Radicals That the Parent Atorvastatin Cannot? Important Pharmacologic Insight from Quantum Chemistry

“…In addition, our study emphasize that, while important, e.g., for modeling the temporal evolution of various molecular species interacting among themselves in a given chemical environment [ 65 , 67 ], the global chemical reactivity indices have no direct relevance for antioxidation. Recall that we saw in Section 3.2 that quantitative differences of ATV’s o-ATV’s, and p-ATV’s global chemical reactivity indices are minor.…”