A combination band at 1699.8 cm−1 for C4 trapped in Ar

An absorption has been detected at 1699.8 cm−1 in the midinfrared spectrum of the vacuum ultraviolet photolysis products of 1,3-butadiene (C4H6) trapped in Ar at 10 K, which on the basis of extensive13 C-substitution experiments is shown to belong to the C4 carbon chain. The behavior of the new band correlates closely with the ν2, C■C stretching vibration of linear C4 at 1543.4 cm−1 identified in an earlier paper. It is argued that the 1699.8 cm−1 absorption is a combination band involving the C■C stretching vibration and a bending mode of C4, with a cis-bent structure.