Molecular structure of trans-cinnamaldehyde as determined by gas electron diffraction aided by DFT calculations

The molecular structure of trans-cinnamaldehyde ((E)-3-phenyl-2-propenal) was determined by means of gas electron diffraction. The nozzle temperature was 165 °C. The results of B3LYP calculations with the 6-31G∗∗ basis set were used as Supporting information. It was found that this molecule has two stable conformers, s-cis and s-trans, which differ in the orientation of the CHO group. Their abundances at 165 °C were determined to be 25 ± 19% and 75%, for the s-cis and s-trans, respectively. This conformational composition is consistent with the prediction by the theoretical calculations. The determined structural parameters (rg and ∠α) of the more abundant conformer, s-trans, of trans-cinnamaldehyde are as follows: <r(CC)ring> = 1.398(1) Å; r(CC) = 1.348 (←) Å; r(C1C) = 1.470(8) Å; r(CC(O)) = 1.473(←) Å; r(CO) = 1.225(6) Å; <r(CH)> = 1.116(6) Å; ∠C6C1C2 = 118.6(3)°; ∠C1C2C = 121.0(←)°; ∠CC6C1 = 121.4(←)°; ∠C2C1C(C) = 122.0(26)°; ∠C1CC = 128.3(26)°; ∠CCCO = 115.3(27)°; ∠CCO = 126.6(19)°. The C1, C2 and C6 atoms are on the ring with the C1 attached to the CHCHCHO group, and the C2 and C6 are on the cis and trans sides to the CC bond, respectively. 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 the differences to the preceding parameters are fixed.