Electronic Spectroscopy of Methylcyanodiacetylene (CH₃C₅N)

Abstract

The results of a study devoted to the electronic spectroscopy of gaseous, solid, and cryogenic matrix-isolated methylcyanodiacetylene (CH C N) are reported. UV absorption and optical phosphorescence spectra of the compound are described here for the first time, and the corresponding vibronic assignments are proposed. UV absorption, studied directly or through the excitation of phosphorescence, revealed the B˜ E--X˜ A system, very weak A˜ A -X˜ A bands, and a strong, broad absorption feature, tentatively identified as D˜ E-X˜ A . Spectral measurements were assisted by quantum chemical calculations at the DFT and ab initio (coupled cluster) levels of theory.

Low Temperature Synthesis and Phosphorescence of Methylcyanotriacetylene

“…‫−ܣ‬ ܺ transitions (either Σ --Σ + or A 2 -A 1 ) are formally not allowed for both molecules, while ‫ܤ‬ − ܺ, forbidden for HC 7 N (∆-Σ) and formally allowed for CH 3 C 7 N (E-A 1 ), is in fact effectively forbidden also for this latter molecule. One may nevertheless expect to see the ‫ܤ‬ − ܺ absorption of CH 3 C 7 N due to the 9 Herzberg-Teller vibronic coupling of electronic excitation with one of the vibrational bending modes, a phenomenon reported for shorter cyano- 22,54,55 and methylcyanopolyacetylenes 24,29 Molecular geometries for the states ܺ, ‫,ܤ‬ ‫,ܧ‬ and ܽ, involved in the experimentally explored electronic transitions of HC 7 N and CH 3 C 7 N, are depicted in Figure 1 (predictions for the other states of Table 1 can be found in Figure S1). Ground-state HC 7 N and CH 3 C 7 N possess conjugated carbon-nitrogen backbones of closely similar geometries.…”

“…500 cm -1 (see Table 1) between the ‫ܤ‬ − ܺ origins of HC 7 N and CH 3 C 7 N. A very similar value of 475 cm -1 was experimentally found for this shift in the analogous case of the HC 5 N, CH 3 C 5 N pair of molecules. 29,55 Therefore, the 29 330 cm -1 and 31 400 cm -1 features can be related to the HC 7 N doublets around 29 750 and 31 860 cm -1 , respectively. The detected bands are presumably due to the coupling of ‫ܤ‬ − ܺ with a 'zig-zag' distortion of the carbon-nitrogen chain, combined with a chain stretching mode.…”

“…One may therefore expect the main vibronic progressions in ‫ܤ‬ − ܺ absorption and in phosphorescence spectra to be shaped by vibrational modes of similar nature, just as observed for the HC 5 N/CH 3 C 5 N pair. 29 Conversely, the geometry change occurring upon the transition from ܺ to ‫ܧ‬ is much different from those characterizing ܽ − ܺ or ‫ܤ‬ − ܺ.…”

“…Geometry optimizations carried out for excited singlet states of CH 3 C 7 N led to the structures (genuine potential energy minima) very slightly deviating from the C 3v symmetry. Based on our former CH 3 C 5 N study, 29 we suppose this effect to be of a purely numerical nature.…”

“…The smallest of methylcyanopolyacetylenes, CH 3 C 3 N, has been the subject of numerous spectroscopic investigations. [17][18][19][20][21][22][23][24] For CH 3 C 5 N, the next member of the series, gas-phase rotational transitions 25,26 and infrared absorption 27 were reported, as well as vibrational 28 and electronic 29 spectra of pure solid and of rare gas matrix-isolated compound. It was reported that photolysis of CH 3 C≡CH (propyne) mixed with cyanoacetylene leads to CH 3 C 5 N. 16 Kerisit et al 30…”