Rotation-Vibration Spectra of Diatomic and Simple Polyatomic Molecules with Long Absorbing Paths VII. The Spectrum of Dideuteroacetylene (C2D2) in the Photographic Infrared

Abstract

Twelve absorption bands of C₂D₂ have been observed in the photographic infrared. Eleven of these bands show simple P and R branches with intensity alternation, i.e., they are Σ_u⁺—Σ_g⁺ bands while one band, without intensity alternation, is a difference band of type Π—Π. From the positions of the bands some of the vibrational constants ω_i⁰ and x_ik are determined. In addition, from the rotational fine structure, the rotational constants B_[v] and D_[v] are determined. For B_[0] the value 0.8478₇ cm⁻¹ is obtained. The α_i values are α₁=+0.0057₈, α₂=+0.0030₂, α₃=+0.0045₅, α₄=−0.0012₀, α₅=−0.0026 cm⁻¹. From the α_i and B_[0] one obtains B_e=0.8507₅ cm⁻¹. Combining B_e(C₂D₂) with B_e(C₂H₂) yields, for the internuclear distances in the acetylene molecule, r_e(C≡C)=1.201₀×10⁻⁸ and r_e(C–H)=1.063₇×10⁻⁸ cm in very satisfactory agreement with the values previously obtained from B_e(C₂HD) and B_e(C₂H₂). Evidence for l‐type doubling in C₂D₂ is given and approximate values for the l‐type doubling constants q₄ and q₅ are obtained.