Infrared Spectrum of Acetylene*

The infrared absorption spectrum of C2H2 has been studied in the region from 2500 to 4150 cm−1. Nineteen bands were observed, of which 11 were sufficiently intense and well resolved to permit rotational analysis. Five bands involving the ground vibrational state were analyzed giving the rotational constants B0=1.17654±0.00004 and D0=1.51±0.03×10−6 cm−1. The l-doubling constants for the degenerate modes were determined to be q4=5.27, q5=4.63×10−3 cm−1. The rotational constants determined for the various observed states clearly show that additional data will be needed and due account taken of resonances to obtain a satisfactory set of rotational constants. The use of difference bands involving the v4=1-level permits the determination of the vibrational frequency of that level to be 612.88 cm−1.