Alternation of Singlet and Triplet States in Carbon‐Based Chain Molecules and Its Astrochemical Implications: Results of an Extensive Theoretical Study

Abstract

A variety of homologous carbon chains (HC n H, HC n N, C n S, C n O, and OC n O) are found to exhibit an appealing even-odd effect. Chains containing a number of carbon atoms of a certain parity possess singlet ground states, while members of opposite parity have triplet ground states. From a general perspective, it is important that this even-odd effect confounds straightforward chemical intuition. Whether the most stable form is a triplet or a singlet is neither simply related to the fact that the species in question is a "normal" (closed-shell, nonradical) molecule nor a (di)radical or to the (e.g., cumulene-type) C─C bond succession across the chain. From a computational perspective, the present results are important also because they demonstrate that electron correlations in carbon-based chains are extremely strong. Whether the "gold-standard" CCSD(T) (coupled-cluster expansions with single and double excitations and triple excitations corrections) framework suffices to describe such strongly correlated systems remains an open question that calls for further clarification. Most importantly for astrochemistry, the present results may explain why certain members are not astronomically observed although larger members of the same homologous series are detected; the missing species are exactly those for which the present calculations predict triplet ground states.

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

“…For consistency with previous and ongoing work on related systems [ 20 , 27 , 29 , 41 , 42 , 43 , 44 , 45 , 46 ] and in order to handle shorter and longer molecules on the same footing, all single-point quantum chemical calculations were carried out at the ROCCSD(T) level of theory, wherein restricted open-shell coupled-cluster expansions include single and double excitations as well as perturbative corrections due to triple excitations [ 47 ]. All molecular geometries utilized in these single-point calculations were optimized by means of the three parameter B3LYP hybrid DFT/HF exchange correlation functional [ 48 , 49 , 50 , 51 ] and 6-311++G(3df,3pd) basis sets [ 52 , 53 ]; more precisely, restricted RB3LYP for closed shell and unrestricted UB3LYP for open shell species.…”

“…Possessing permanent dipole, linear HC n N chains can be detected by radio astronomy, which is par excellence the method to observe extraterrestrial molecules that marked the boom in reporting new molecules in space since the early 1960s [ 2 ]. According to existing studies—comprising not only neutral HC n H 0 [ 10 , 20 ] but also cation HC n H + [ 21 , 22 ] and anion HC n H − species [ 23 , 24 , 25 ]—HC n H chains are linear. If they are linear (more precisely, centrosymmetric), they have zero dipole moments.…”

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

“…The results for enthalpies of formation (Table 13 and Figure 10b) indicate again that C 4 Ntriplet chains are more stable than C 4 Nsinglet chains, in contrast to C 3 Nand C 5 N -, for which singlets are more stable than triplets, in agreement with the odd-even singlet-triplet alternation reported earlier for the C n Nhomologous series 19 as well as for its HC n N isoelectronic counterpart. 42,58 So, the above results suggest that neutral C 4 N 0 chains can still be observed in space;…”

“…42,58 that electron correlations (implying by definition that departures from the above MO-based picture are substantial) in carbon-based chains are strong, which represents a challenge for theory. Still, the large values (∼ 1 eV) clearly demonstrate that the neutral quartet lies considerably higher in energy than the doublet.The inspection of Table1and Figures 6 and S8 reveals that the strongest impact of the spin state on the neutral chain is on the moiety opposite to the N atoms.…”

Self-assembled monolayers of oligophenylenes stiffer than steel and silicon, possibly even stiffer than Si$_3$N$_4$

“…A variety of carbon-based molecular families M n [57][58][59][60][61] are known to exhibit even-odd alternation upon adding a repeat unit (phenyl ring in our case) M n → M n+1 . This is a particularly meaningful question because symmetry is found to alternate across the OPDn family; calculations indicate that the odd-number members OPD1 and OPD3 possess C i symmetry while evennumber members OPD2 and OPD4 possess C 2 symmetry.…”