Structural determination of vanillin, isovanillin and ethylvanillin by means of gas electron diffraction and theoretical calculations

The molecular structures of vanillin (4-hydroxy-3-methoxybenzaldehyde), isovanillin (3-hydroxy-4-methoxybenzaldehyde) and ethylvanillin (3-ethoxy-4-hydroxybenzaldehyde) were determined by means of gas electron diffraction. Among them, vanillin and ethylvanillin have a vanilla odor but isovanillin smells differently. The nozzle temperatures were 125, 173 and 146 °C, for vanillin, isovanillin and ethylvanillin, respectively. The results of MP2 and B3LYP calculations with the 6-31G** basis set were used as supporting information. The MP2 calculations predicted that vanillin and isovanillin have two stable conformers and ethylvanillin has four stable conformers. The electron diffraction data were found to be consistent with these conformational compositions. The determined structural parameters (rg and ∠α) of vanillin are as follows: <r(C–C)ring>=1.397(4) Å; r(C1–Caldehyde)=1.471(←) Å; r(C3–OMe)=1.374(9) Å; r(C4–OH)=1.361(←) Å; r(O–CMe)=1.428(←) Å; r(CO)=1.214(8) Å; <r(C–H)>=1.110(11) Å; r(O–H)=0.991(←) Å; ∠C6–C1–C2=120.6(2)°; ∠C1–C2–C3=118.8(←)°; ∠C1–C6–C5=120.1(←)°; ∠C2–C1–Caldehyde=122.7(18)°; ∠C1–CO=119.4(16)°; ∠C4–C3–OMe=112.2(12)°; ∠C3–C4–OH=119.1(←)°; ∠C3–O–C=121.7(29)°. Those of isovanillin are as follows: <r(C–C)ring>=1.402(4) Å; r(C1–Caldehyde)=1.479(←) Å; r(C4–OMe)=1.369(9) Å; r(C3–OH)=1.357(←) Å; r(O–CMe)=1.422(←) Å; r(CO)=1.221(9) Å; <r(C–H)>=1.114(14) Å; r(O–H)=0.995(←) Å; ∠C6–C1–C2=120.2(3)°; ∠C1–C2–C3=119.0(←)°; ∠C1–C6–C5=119.9(←)°; ∠C2–C1–Caldehyde=124.6(25)°; ∠C1–CO=121.3(24)°; ∠C3–C4–OMe=114.4(12)°; ∠C4–C3–OH=121.2(←)°; ∠C4–O–C=123.8(26)°. Those of ethylvanillin are as follows: 〈r(C–C)ring〉=1.397(6) Å; r(C1–Caldehyde)=1.471(←) Å; r(C3–OEt)=1.365(13) Å; r(C4–OH)=1.352(←) Å; r(O–CEt)=1.427(←) Å; r(C–CEt)=1.494(21) Å; r(CO)=1.206(9) Å; <r(C–H)>=1.109(10) Å; r(O–H)=0.990(←) Å; ∠C6–C1–C2=120.2(3)°; ∠C1–C2–C3=118.4(←)°; ∠C1–C6–C5=119.7(←)°; ∠C2–C1–Caldehyde=121.7(21)°; ∠C1–CO=128.8(22)°; ∠C4–C3–OEt=112.8(14)°; ∠C3–C4–OH=119.6(←)°; ∠C3–O–C=115.1(27)°; ∠O–C–CEt=102.7(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrows in the parentheses mean that these parameters are bound to the preceding one.