Structural study of methyl isonicotinate by gas phase electron diffraction combined with ab initio calculations

The molecular structure of methyl isonicotinate was studied by gas phase electron diffraction combined with ab initio calculations. The molecular skeleton was assumed to be planar. The determined values of principal structure parameters (r_g and ∠_α) are as follows: r(N-C) = 1.343(5) Å, r(C=C)_ring = 1.401(3) Å, r(C_γ-C) = 1.499(9) Å, r(C=O) = 1.205(5) A. r(C(=O)- O) = 1.331(8) Å, r(O-C_Me) = 1.430(8) Å, (r(C-H) = 1.103(10) Å, ∠CNC = 117.6(9)°, ∠C_βC_γC_β = 118.7(9)°, ∠C_β,transC_γ-C(=O) = 118.6(12), ∠C_γO = 121.4(12)°, ∠C_γC-O = 114.2(10)°, ∠COC = 115.4(15)°, where angled brackets denote average values and C_β,transC denotes the carbon atom which is trans to the carbonyl oxygen atom. Values in parentheses are the estimated limits of error (3σ) referring to the last significant digit. The structure of the ring in methyl isonicotinate agrees with that of pyridine within experimental error. In contrast, the structure parameters of the COOCH₃ group are significantly different from those of methyl acrylate and methyl acetate. These differences have been discussed in terms of hyperconjugation and steric effects.