Does prop-2-ynylideneamine, HC≡CCH=NH, exist in space? A theoretical and computational investigation

Abstract

MP2, DFT and CCSD methods with 6-311++G** and aug-cc-pvdz basis sets have been used to probe the structural changes and relative energies of E-prop-2-ynylideneamine (I), Z-prop-2-ynylideneamine (II), prop-1,2-diene-1-imine (III) and vinyl cyanide (IV). The energy near-equivalence and provenance of preference of isomers and tautomers were investigated by NBO calculations using HF and B3LYP methods with 6-311++G** and aug-cc-pvdz basis sets. All substrates have Cs symmetry. The optimized geometries were found to be mainly theoretical method dependent. All elected levels of theory have computed I/II total energy of isomerization (ΔE) of 1.707 to 3.707 kJ/mol in favour of II at 298.15 K. MP2 and CCSD methods have indicated clearly the preference of II over III; while the B3LYP functional predicted nearly similar total energies. All tested levels of theory yielded a global II/IV tautomerization total energy (ΔE) of 137.3-148.4 kJ/mol in support of IV at 298.15 K. The negative values of ΔS indicated that IV is favoured at low temperature. At high temperature, a reverse tautomerization becomes spontaneous and II is preferred. The existence of II in space was debated through the interpretation and analysis of the thermodynamic and kinetic studies of this tautomerization reaction and the presence of similar compounds in the Interstellar Medium (ISM).

Figure 1

The numbering of atoms for E-prop-2-ynylideneamine (I), Z-prop-2-ynylideneamine (II) and vinyl cyanide (IV), together with their optimized bond lengths (Å) and angles (degrees) using B3LYP/aug-cc-pvdz level of theory.

Figure 2

The numbering of atoms for the Intermediate (III) and Transition States (TS1 and TS2) together with their optimized bond lengths (Å) and angles (degrees) using B3LYP /aug-cc-pvdz level of theory.

Figure 3

Schematic potential energy profile showing the activation energies (kcal/mol) for the tautomerization of II and IV, which were calculated by using the elected levels of theory.

Figure 4

The NBO charge distribution of (a) Z-prop-2-ynylideneamine (II); (b) TS2; (c) vinyl cyanide (IV) and (d) TS1 which were calculated using B3LYP/aug-cc-pvdz level of theory. The migrating hydrogen atom acquired a positive charge (+0.213e) in II and an intensified charge (+0.317e) in TS2 while it acquired a positive charge (+0.230e) in IV that was condensed (+0.445e) in TS1.