Ab Initio and NBO Studies of the Decomposition ([2+2] and [2+4]Elimination) Mechanisms of Alkylisothiocyanates (alkyl = ethyl-, iso-propyl- and tert-butyl)

Ab initio Molecular Orbital (MO), Density Functional Theory (DFT), and Natural Bond Orbital (NBO) population analysis were used to investigate the mechanism of the decomposition of ethylisothiocyanate (1), iso-propylisothiocyanate (2), and tert-buthylisothicyanate (3). HF/6-31G**//HF/6-31G** and B3LYP/6-31G**//HF/6-31G** results indicate that for the decomposition of compounds 1– 3, the six-membered transition state structures via concerted [2+4]elimination mechanisms have lower energies than the four-membered transition state geometries via [2+2]eliminations. The results at B3LYP/6-31G**//HF/6-31G** and HF/6-31G**//HF/6-31G** levels of theory revealed that the barrier height for the decomposition of compound 1, through the six-membered transition state structure via the concerted [2+4] elimination mechanism, is 46.31 kcal mol ⁻¹ and 59.51 kcal mol ⁻¹ , while the barrier height for the four-membered transition state structure via [2+2] elimination is 71.06 kcal mol ⁻¹ and 72.09 kcal mol ⁻¹ , respectively. [2+4]elimination mechanism by B3LYP/6-31G**//HF/6-31G** method (46.31 kcal mol⁻¹) is in a good agreement with the reported experimental results (45.35 kcal mol⁻¹). The obtained barrier heights via the [2+4]elimination mechanism at the B3LYP/6-31G**//HF/6-31G** and HF/6-31G**//HF/6-31G** levels of theory show also that the decomposition energy barriers via the [2+4]elimination mechanism for compound 1–3 are in the following order: 1 > 2 > 3. The obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the C _sp3 atom (which is attached to the N atom). NBO analysis shows that the occupancies of σ _Csp3−N bonds decrease in the following order: 3 < 2 < 1 and the occupancies of σ * _Csp3−N bonds increase in the opposite order. This fact illustrates a comparatively easier unsymmetrical dissociation of the σ _Csp3−N bond in compound 3 compared to compound 2, and in compound 2 compared to compound 1.

Keywords:

• Isothiocyanate
• decomposition
• molecular modeling
• ab initio
• natural bond orbital

We Would like to acknowledge the helpful discussion with Dr. D. Tahmassebi.

Notes

^a Relative to the ground state structure.

^b Numbers in parenthesis are the corresponding Δ E ₀values in kcal mol⁻¹.

^c Experimental data [see ref. 1].