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

Abstract

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.

Is the C8ring nonplanar?

“…For smaller carbon aggregates vibrational characteristics of their various isotopomers have been observed [14,15,18,19,34,35]. To assist such possible future experiments table 3 presents the isotopic variations in the most intense vibrational band of the nonplanar C8.…”

“…have received considerable research attention for more than two decades [1]. Nevertheless, the interest has recently become even stronger in both computations [2][3][4][5][6][7][8][9][10][11][12][13] and observations [14][15][16][17][18][19][20][21] in an apparent connection with the large carbon spheroidal molecules [22]. The computational studies [1] mostly predict linear arrangements for odd-numbered small carbon aggregates, while with the even species two isomers are suggested, planar cyclic and linear (triplet electronic state).…”

Electronic structure calculations on the C4 cluster

“…A near-linear bent structure has been proposed as possible origin of the line splitting observed in electron spin resonance experiments. 7,8 Gas phase high resolution spectroscopy confirmed a linear structure. 9 Theoretically, previous works have found the cumulenic structure ͑of 3 ⌺ g − symmetry species͒ as the most stable form of l-C 4 .…”

Comprehensive Survey of the Structures of C4, C4−, and C4+ Clusters

“…The same strong band was subsequently found at 1548.6128 cm −1 in the gas phase, 1547.2 cm −1 in neon, D 2 and H 2 matrices, and 1539.5 cm −1 in a krypton matrix, respectively. A weak band at 1699.8 cm −1 that mimicked the 13 C isotopic spectra of C 4 was also identified in argon . The combination of the results for vibrational bands at 1543.4 cm −1 , 1699.8 cm −1 and prior triplet EPR signals in argon and neon matrices led to the conclusion that the C 4 cluster was not a straight linear structure, but in a slightly cis ‐bent geometry with a bond angle (C 1 ‐C 2 ‐C 3 ) of no more than 177.5° .…”

“…A weak band at 1699.8 cm −1 that mimicked the 13 C isotopic spectra of C 4 was also identified in argon . The combination of the results for vibrational bands at 1543.4 cm −1 , 1699.8 cm −1 and prior triplet EPR signals in argon and neon matrices led to the conclusion that the C 4 cluster was not a straight linear structure, but in a slightly cis ‐bent geometry with a bond angle (C 1 ‐C 2 ‐C 3 ) of no more than 177.5° . Continual study of the matrix‐trapped C 4 reached a contradictory conclusion that the two bands at 1543.4 cm −1 and 1699.8 cm −1 did not arise from the same species, and the band at 1699.8 cm −1 was reassigned to the linear C 4 − anion based on density functional theory (B3LYP/6‐31G* and B3LYP/6‐31+G*) calculations .…”