Theoretical Characterization of C₃H and C₅H and Their Anions

Abstract

Highly correlated ab initio calculations are employed for the structural and spectroscopic characterization of small odd chains of type C 2n+1 H, considering neutral forms, cations, and giving special attention to the anions. This work confirms the stability of the linear carbon chains and carbon clusters containing three-body rings. The smallest species, C 3 H, displays three stable structures, whereas C 5 H possesses at least 8 neutral isomers and 11 and 10 isomers with a negative or a positive charge. The equilibrium geometries, which can be candidates for laboratory and astrophysical detection, are studied using the RCCSD(T)-F12 and MRCI/CASSCF levels of theory, specifying properties for various electronic states. Four different stable isomers are confirmed for the C 5 H − anion. They are two rings and two chains, all showing singlet ground electronic states. The viability of the triplet linear form of C 5 H − (¥ C v (X 3 Σ −)) postulated in previous works, is not confirmed because it appears to be really dependent on the electron correlation energy denoting instability. A quasi-linear singlet (C s (X 1 A′)) represents a secondary minimum. Electronic state crossing occurs close to the linear structure where spin-orbit effects are negligible. The most stable structure of C 5 H − is a three-carbon cycle in which rotational constants have been determined to be A 0 =35479.86 MHz, B 0 =3618.29 MHz, and C 0 =3280.10 MHz. Its dipole moment is relatively large (6.4086 D).

Discovery of C5H+ and detection of C3H+ in TMC-1 with the QUIJOTE line survey

“…Two-color photodetachment studies by Tulej et al (2011) suggested that the ground state is 3 Σ − with a linear structure and a rotational constant of 2476.3 ± 6 MHz. In addition, the reported experimental value for the C 5 H − ( 3 Σ − ) species differs by more than 100 MHz from the predicted values obtained by Bennedjai et al (2019) and our scaled value (Table 2). Moreover, if that assignment were correct, C 5 H − could not be the carrier of the observed lines as significant hyperfine structure is expected due to the spin 1/2 of H and the low spin-spin interaction constant derived, λ = 7200 ± 4500 MHz.…”

“…Hence, our ab initio calculations provide conclusive arguments for the spectroscopic identification of C 5 H + using our sensitive QUIJOTE survey of TMC-1. Using the B e value calculated by Bennedjai et al (2019) for C 5 H, and correcting B e of C 5 H + by their ratio B obs (C 5 H)/B e (C 5 H), we found B 0 (C 5 H + ) ∼ 2408.3 MHz, which also agrees well with our experimental rotational constant.…”

“…A value of B e for this species of ∼2420 MHz was obtained by Aoki (2014). More recent calculations by Bennedjai et al (2019) provided a value for B e of 2391.7 MHz. To obtain accurate spectroscopic parameters, we performed ab initio calculations for C 5 H + and also for the C 5 H species, for which the rotational parameters were experimentally determined.…”

“…However, for the molecule observed by Tulej et al (2011), the situation is the opposite, B(anion)> B(neutral). Additional controversy arises from the recent calculations of Bennedjai et al (2019). They concluded that the ground electronic state of C 5 H − contains a C 3 ring with very different rotational constants than those observed.…”

Discovery of C5H+ and detection of C3H+ in TMC-1 with the QUIJOTE line survey

“…clusive arguments for the spectroscopic identification of C 5 H + using our sensitive QUIJOTE survey of TMC-1. Using the B e value calculated by Bennedjai et al (2019) for C 5 H, and correcting B e of C 5 H + by their ratio B obs (C 5 H)/B e (C 5 H), we found B 0 (C 5 H + ) ∼ 2408.3 MHz, which also agrees well with our experimental rotational constant.…”