Structure determination of methyl nicotinate and methyl picolinate by gas electron diffraction combined with ab initio calculations

The molecular structures of isomers, methyl nicotinate and methyl picolinate, have been studied by gas electron diffraction. The samples were heated to 341-343 K by using a high-temperature nozzle to get enough vapor pressure. HF/6-31G* ab initio calculations were carried out, and structural constraints were taken from their results. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coordinate analyses. The principal structural parameters of the s-trans conformer of methyl nicotinate are as follows (angle brackets mean averaged values): 〈r_g(N-C)〉 = 1.337(7) Å; 〈r_g(C-C)_ring〉 = 1.402(3) Å; r_g(C_ring-C(=O)) = 1.480(12) Å; r_g(C=O) = 1.199(7) Å; 〈r_g(C-O)〉 = 1.380(11) Å; ∠_αCNC = 119.0(14)°; 〈∠_αNCC) = 122.8(11)°; ∠_α(N=)C-C_ring-C(=O) = 118.3(12)°; ∠_αC_ring-C=O = 121.5(12)°; ∠_αC_ring-C-O = 115.6(10)°. Those of the s-trans conformer of methyl picolinate are as follows: 〈r_g(N-C)〉 = 1.344(7) Å; 〈r_g(C-C)_ring〉 = 1.398(4) Å; r_g(C_ring-C(=O)) = 1.497(11) Å; r_g(C=O) = 1.209(7) Å; 〈r_g(C-O)〉 = 1.380(11) Å; ∠_αCNC = 117.2(12)°; 〈∠_αNCC〉 = 123.9(12)°; ∠_αN-C-C(=O) = 115.1(10)°; ∠_αC_ring-C=O = 121.0(12)°; ∠_αC_ring-C-O = 115.1(12)°. The structural parameters of the two isomers and methyl isonicotinate have been compared and their similarity and differences have been discussed.