Theoretical Studies of Two Key Low-Lying Carbenes of C₅H₂Missing in the Laboratory

Abstract

The equilibrium geometries and spectroscopic properties of two key singlet carbenes, buta-1,3-diynylcarbene (6) and 2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (9), which have not been experimentally observed to date, are investigated using high-level coupled-cluster (CC) methods. The current theoretical study necessitates new experimental data on C 5 H 2 isomers considering the relevance of these molecules to interstellar chemistry. Bent-pentadiynylidene (4) has been missing in the laboratory and the prime focus of our earlier theoretical work. The present theoretical study indicates that isomers 6 and 9 are also viable experimental targets. Apart from ethynylcyclopropenylidene (2), pentatetraenylidene (3), ethynylpropadienylidene (5), and 3-(didehydrovinylidene)cyclopropene ( 8), which are identified by Fourier transform microwave spectroscopy, the dipole moments of elusive 4, 6, and 9 are also non-zero (µ = 0). The relative energies of these isomers, calculated at the CCSDT(Q)/CBS level of theory, with respect to linear triplet pentadiynylidene (1) reveal that they all lie within 25.1 kcal mol −1 . Therefore, geometric, energetic, aromatic, and spectroscopic parameters are reported here, which may assist the efforts of molecular spectroscopists in the future. Anharmonic vibrational calculations on isomers 6 and 9 indicate that the former is loosely bound and would be challenging to be detected experimentally. Among the undetected carbenes, 9 may be considered as a potential target molecule considering its higher polarity and aromatic nature.

A chemical dynamics study on the gas-phase formation of triplet and singlet C 5 H 2 carbenes

“…Therefore, our data support that at least ethynylcyclopropenylidene (p1) and/or pentadiynylidene (p2) is/are formed under single-collision conditions. The order of energies for different C 5 H 2 isomers calculated here at the CCSD(T)-F12/cc-pVTZ-f12 level (33,34), p1 (0 kJ•mol −1 ) < p2 (9) < p3 (59) < p4 (81) closely agrees with the most accurate CCSDT(Q)/ CBS (35) results, p1 (0) < p2 (3) < p3 (57) < p4 (84) (12), whereas earlier calculations gave a different result predicting ethynylcyclopropenylidene as the most stable structure: p2 (0) < p1 (8) < p3 (58) < p4 (88) (10,12). Note that our calculations also elucidated a possible molecular hydrogen loss channel leading to pentynylidyne (p5, X 2 Π, Δ r G = −124 ± 4 kJ•mol −1 ); recall that within the sensitivity of our detection system no evidence for a molecular hydrogen loss channel was found.…”

“…reaction dynamics | molecular beams | trajectory calculations | electronic structure theory | carbenes S ince the pioneering discovery of the structural isomers cyclopropenylidene (c-C 3 H 2 , X 1 A 1 , 1), vinylidenecarbene (H 2 CCC, X 1 A 1 , 2), and propargylene (HCCCH, X 3 B, 3) (Fig. 1) in the laboratory and in extraterrestrial environments (1)(2)(3)(4)(5)(6)(7)(8)(9), the homologous series of the isomers C 3 H 2 (1 to 3) and C 5 H 2 (4 to 6) fascinated the physical (organic) and computational chemistry communities from the fundamental points of view of electronic structure and chemical bonding (10)(11)(12) as prototypes of highly unsaturated (partially) 2π-Hückel aromatics (1 and 4), singlet carbenes (2 and 5), and triplet diradicals (3 and 6). Cyclopropenylidene (1)-a three membered ring species of C 2v symmetry depicting an aromatic character and a carbene moiety at the apical carbon atom-represents the thermodynamically most stable C 3 H 2 isomer.…”

Six Low-Lying Isomers of C11H8 Are Unidentified in the Laboratory–A Theoretical Study

“…Since experimental values are unavailable at the moment, percentage errors cannot be calculated. Nevertheless, prior experience of some of us in comparison of experimental results with theoretical spectroscopic parameters in C n H 2 (here n = 5, 77,78 7, [79][80][81] and 9 82,83 ) and SiC 4 H 2 24 isomers suggest that the percentage error for coupled-cluster values lie within 2-3 % of experimental accuracy. The later scenario has been proven for other systems as well elsewhere in the literature.…”

Energetics and Spectroscopic Properties of Low‐lying CaC6H2 Isomers: An Astrochemical Perspective

“…have theoretically studied structures and stabilities of several open‐chain and cyclic magnesium and calcium‐containing carbon clusters using B3LYP method in conjunction with 6‐311G(d) and 6‐311+G(d) basis sets. A substantial amount of theoretical work on silicon‐carbon and carbon clusters has been done by Thimmakondu et al [34–45] . Etim et al [46] .…”

“…Largo et al [29][30][31][32][33] have theoretically studied structures and stabilities of several open-chain and cyclic magnesium and calcium-containing carbon clusters using B3LYP method in conjunction with 6-311G(d) and 6-311 + G(d) basis sets. A substantial amount of theoretical work on silicon-carbon and carbon clusters has been done by Thimmakondu et al [34][35][36][37][38][39][40][41][42][43][44][45] Etim et al [46] performed a systematic theoretical study on the interstellar carbon chain molecules at B3LYP/6-311 + + G(d,p) level of theory and calculated enthalpy of formation and global reactivity indices for these molecules. Dong et al [47] observed the Mg m C n H x and Be m C n H x clusters in their mass spectra and supported their experimental findings by theoretical calculation.…”