Conformations of 2-Cyclohepten-1-one (Tropilene)


Programs: Gromacs (2020) on macOS Mojave (10.14.6); ORCA (4.2.1) on CentOS Linux 7; WebMO (20.0).

Because tropilene is a pseudo-six-membered compound, it can exist as chair, twist-boat (skew-boat), boat, and half-chair forms. The microwave spectrum of the molecule was inconsistent with a chair form.1) Groenewald et al. reported that a twist-half-boat or twist-half-chair (TH) form is the most stable form in the CCSD(T) level calculation and its rotational constants are close to the experimental values.2) 

I generated conformers of tropilene by the simulated annealing method implemented in the Gromacs program with the GAFF2 force field and obtained four enantiomeric pairs: one chair (C) and three twist-boat (S) forms. In the PM7 level of theory, the 6S1 (or 1S6) twist-boat form was converged to the chair form; on the other hand, in the density functional theory (DFT) level, the chair and 6S1 (or 1S6) forms were converged to a half-chair (5H6 and 6H5) form.  Their relative energies at several DFT levels including B97-3c/def2-mTZVP3) along with the reference CCSD(T) level2) were listed in the below table. Albeit not being successful to predict the global minimum, the low-cost B97-3c provided lower errors compared to the B3LYP/6-31G(d) or the B3LYP/6-311+G(d,p)2) level.


 Groenewald’s notationa
ΔG (kcal mol−1)











6H5 and 5H6 S6/TH2 0.0 0.0 0.0 0.0 0.1 0.53 0.46
1S4 and 4S1 TH6 0.8 0.4 0.1 0.1 0.0 0.00 0.00
7S2,3 and 2,3S7 H2/TH5 1.2 0.9 0.6 0.6 0.6 0.52 0.56

a H, half-chair or half-boat (five atoms are approximately in a plane); S, sofa (six atoms are approximately in a plane); T, twist- .2) b Cited from Ref. 2.


ORCA input options for the meta-hybrid GGA M06-2X functional.

! M062X 6-311+G(d,p) 
! NumFreq

! Grid6 NoFinalGrid
%method IntAcc 12 end
%geom Calc_Hess true; NumHess true end



  1. Kitchin, R. W., et al., The microwave spectrum, dipole moment, and molecular conformation of 2-cycloheptene-1-one. J. Mol. Spectrosc. 1979, 75, 429-439. doi: 10.1016/0022-2852(79)90086-9.
  2. Groenewald F., et al., A computational investigation of the effect of the double bond on the conformations of seven membered rings. Struct. Chem. 2013, 24, 243–250. doi: 10.1007/s11224-012-0073-4.
  3. Brandenburg, J. G., et al., B97-3c: A revised low-cost variant of the B97-D density functional method. J. Chem. Phys. 2018, 148, 064104. doi: 10.1063/1.5012601.


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