Theoretical Study on Unimolecular Reactions of Acetyl Cyanide and Acetyl Isocyanide

Abstract

The potential energy surfaces of acetyl cyanide and its functional isomer acetyl isocyanide in their electronic ground state have been investigated using ab initio molecular orbital MP2(FU)/6-311G** and QCISD(T)/6-311G** calculations. The molecular eliminations of CH3COX (X = CN,NC) to CH3X + CO, HX + CH2CO, and CH2 + HCOX, the unimolecular rearrangements to CH3C−OX and CH3O−CX carbenes, 1,3-hydrogen migration to CH2C(OH)X, 1,3-methyl migration to CH3NCCO, and radical decompositions to X• + CH3CO and CH3 • + COX have been examined. Also the secondary decomposition processes of COCN and CONC radicals in their ground states and the secondary elimination of HX from CH2C(OH)X have been investigated. Cyanide−isocyanide isomerization is found to be the predominant channel in the unimolecular reactions of acetyl cyanide. Despite being exothermic, the decomposition of acetyl cyanide into CH3CN + CO is found to be kinetically unfavorable. Decomposition into HCN + CH2CO, the reverse reaction of CH3COCN synthesis, is found to be a preferred dissociation pathway and is expected to compete at high temperatures with the 1,3-hydrogen migration, yielding cyanovinyl alcohol. The barriers for the rearrangement processes to carbenes in acetyl isocyanide are quite high in contrast to those of acetaldehyde and fluoroformaldehyde. The large magnitude of the reverse barrier from the carbenes to acetyl cyanide suggests the stability of the carbenes. In the ground state of CH3COX, the CH3−C bond cleavage is found to be more facile compared to the C(O)−X bond cleavage. The secondary dissociation from CONC requires a rather small barrier, and both cyanide and isocyanide forms are found to have very similar potential energy surface.

Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

“…The energy state of ts HCN is consistent with 273 kJ/mol predicted by Sumathi and Nguyen. 13 The ts HCN structure shows a H-CN distance of 1.821 Å, much larger than the bond length 1.066 Å in the HCN product. That is why, HCN is dissociated with vibrationally hot v 3 stretch mode.…”

“…38 Furthermore, the energy barriers to CH 3 and CN were calculated to be 369 (or 378) and 437 (or 484) kJ/mol by QCISD(T) (or MP2) method. 13 These two channels are kinetically less favorable at 308 nm.…”

“…[11][12][13] The related decomposition mechanism and product analysis are worthwhile for investigation, and thus, photodissociation of acetyl cyanide at 308 nm is focused in this work. An effusive beam of CH 3 COCN is irradiated in the presence of Ar gas.…”

Theoretical study on the mechanism for the reaction of pentafulvenone with HNC in singlet and triplet states, interconversions and solvation effect

“…In the theoretical investigation for the isomerisation of acetyl isocyanide to acetyl cyanide, Sumathi and Nguyen [14], and So [15] reported that the barriers of the isomerisation were 26.7 and 28.2 kcal mol À1 , respectively, and they concluded that the isomerisation was possible at low temperatures. In our theoretical study, P b2 0 isomerises to P b2 via TS18 with a barrier of 26.3 kcal mol À1 , and we infer that the interconversions between cyclopentadienyl isocyanides and cyclopentadienyl cyanides are feasible.…”

“…The reactivities of ketene (CH 2 CO) and its derivatives have been widely studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Sumathi and Nguyen [14] and So [15] studied the unimolecular reaction of acetyl cyanide and acetyl isocyanide in theory, and revealed the potential energy profile of the decomposition reactions of acetyl isocyanide to CH 2 CO and HNC and acetyl cyanide to CH 2 CO and HCN.…”