Abstract
We use high-level ab initio CCSD(T) and CCSDT(Q) methods to investigate the energetic and spectroscopic properties of nine low-lying isomers of C7H2, which lie within 1 eV. Among these, heptatriynylidene (1), 1-(buta-1,3-diynyl)cyclopropenylidene (2) and heptahexaenylidene (9) have been detected experimentally. The other six isomers, 1,2-(diethynyl)cyclopropenylidene (3), bicyclo[4.1.0]hepta-1,2,4,5-tetraene-7-ylidene (4), cyclohepta-1,2,3,4-tetraen-6-yne (5), bicyclo[4.1.0]hepta-4,6-diene-2-yne-7-ylidene (6), bicyclo[4.1.0]hepta-1,5-diene-3-yne-7-ylidene (7) and 1-(buta-1,3-diynyl)propadienylidene (8), remain hypothetical to date. Except for 1, all of the isomers are associated with a non-zero dipole moment (μ≠ 0). Although Fourier-transform microwave spectroscopy had detected 2 and 9, our study reveals that six hypothetical isomers (3-8) are thermodynamically sandwiched between the experimentally known and astronomically relevant isomers 2 and 9. The structural parameters, dipole moments, rotational and centrifugal distortion constants, harmonic vibrational frequencies, and infra-red intensities presented here may be useful for the laboratory detection of these previously unidentified isomers (3-8) and also all others (2-9) in astronomical sources.
Highlights
Fourier-transform microwave spectroscopy had detected 2 and 9, our study reveals that six hypothetical isomers [3,4,5,6,7,8] are thermodynamically sandwiched between the experimentally known and astronomically relevant isomers 2 and 9
More than hundreds of structural isomers are theoretically possible for C7H2,58 to date merely six isomers have been detected experimentally43,59,60–63 and none have been detected in space
The harmonic vibrational frequencies of isomers 1–3, 4–6, and 7–9 calculated at the CC with single and double excitations (CCSD)(T)/cc-pVTZ level of theory are documented in Tables 9, 10, and 11, respectively
Summary
More than hundreds of structural isomers are theoretically possible for C7H2,58 to date merely six isomers (see Fig. 1 and 2) have been detected experimentally43,59,60–63 and none have been detected in space. The harmonic vibrational frequencies of isomers 1–3, 4–6, and 7–9 calculated at the CCSD(T)/cc-pVTZ level of theory are documented in Tables 9, 10, and 11, respectively.
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