Precision analysis of 12C/13C ratios in Orion IRc2 acetylene isotopologs via χ2 fitting

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Table 4

Rotational constants (B̃) and centrifugal distortion constants (D̃_J) for acetylene (C₂H₂) and its isotopologs, determined using quantum chemical calculations based on the B3LYP method.

Isotopolog ID	FSF	B̃ (cm⁻¹)	D̃_J (×10⁻⁶ cm⁻¹)
0	0.941337	1.1766	12.2538
1	0.941522	1.1485	11.4259
2	0.972094	0.9901	8.4493
3	0.941337	1.1197	10.6241
4	0.972094	0.9659	7.9032
5	0.972094	0.9740	8.0506
6	0.941337	0.8457	8.4405
7	0.972094	0.9491	7.5049
8	0.941337	0.8310	8.0511
9	0.941337	0.8159	7.6643

Notes. FSF values were adjusted to best fit the observed ν₅ band spectra in HITRAN for C₂H₂, ¹³CCH₂, and C₂HD, while FSFs for other isotopologs were estimated based on structural similarities. The B̃ values are expressed in cm⁻¹, and D̃_J values are scaled by 10⁻⁶ for clarity.

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