Structures and conformations of carbonyl isocyanates and carbonyl azides. An experimental and theoretical investigation

Abstract

The geometric structures and conformational compositions of fluorocarbonyl isocyanate, FC(O)NCO, and fluorocarbonyl azide, FC(O)N 3, were studied by gas electron diffraction, vibrational spectroscopy and ab initio calculations. Both compounds exist in the gas phase as a mixture of two planar conformers ( C s symmetry), with the cis form (carbonyl CO double bond in the cis position with respect to the NC or NN bond) being lower in energy than the trans rotamer. From the experimental scattering intensities the following geometric parameters ( r a distances and ∠ α, angles with 3σ uncertainties) were derived for the predominating cis structures. FC(O)NCO, 75(12)% cis: CO (isocyanate) = 1.154(8) Å, CO (carbonyl) = 1.192(8) Å, NC = 1.215(9) Å, Cz.sbnd;F = 1.320 Å (not refined), NC = 1.388(4) Å, CNC = 125.9(11)°, NCO = 128.7(6)°, NCF = 107.8(8)° and NCO = 174.6(25)°. FC(O)N 3, 90(10)% cis: N βN ω = 1.124(5) Å, N α = N β = 1.246(5) Å, CO = 1.192 Å (not refined), CF = 1.324(3) Å, N αC = 1.390(4) Å, CN α = N β = 110.9(8)°, N αCO = 129.3(4)°, N αCF = 107.4(15)° and N α = N βN ω = 170.7(27)°. The experimentally obtained energy differences between the two conformers of FC(O)NCO (Δ E(ED) = E trans − E cis = 0.7(3) kcal mol −1, Δ E(IR) = 0.4(2) kcal mol −1, Δ E (Raman) = 0.8(3) kcal mol −1) and of FC(O)N 3 (Δ E(ED) = 1.3(6) kcal mol −1, Δ E(IR) = 1.5(4) kcal mol −1, Δ E(Raman) = 1.3(4) kcal mol −1) can be reproduced within their error limits by MP2/6–31G *//HF/6–3G * and MP4SDTQ/6–31G *//HF/6–31G * calculations. HF/6–31G * and MP2/6–31G * approximations overestimate these values. Corresponding ab initio calculations were performed for the parent compounds HC(O)NCO and HC(O)N 3, which are not stable and cannot be studied in the gas phase.