The electronic spectrum of cyanoacetylene: Part I. Analysis of the 2600-Å system

Abstract

The banded absorption spectrum of cyanoacetylene in the gas phase between 2600 Å and 2300 Å has been measured and analyzed. The main features of the spectrum can be explained on the basis of an electronic transition from the linear ground state with 1Σ + symmetry to a nonlinear, planar, excited state with 1 A″ symmetry under the C 8 point group. The electronic origins are at 38 485 38 519 cm −1 for HCCCN DCCCN . Progressions are observed in two excited state frequencies; 880 680 cm −1 , which is assigned to the CCH bending mode ν 5′( a′) and 160 150 cm −1 , which is assigned to the skeletal bending mode ν 7′( a′). The corresponding π-type ground-state modes ν 5″ and ν 7″ are also active in accordance with the Franck-Condon principle. The K-type substructure of the bands is that expected for a linear-bent perpendicular type transition. Partial rotational analyses of certain bands suggests the following approximate excited state dimensions: r( CH) = 1.08 A ̊ , r( CC) = 1.25 A ̊ , r( CC) = 1.40 A ̊ , r( CN) = 1.16 A ̊ ; angles CCH = 164°, CCC = 143°, CCN = 180°.