A semirigid bender analysis of rinh-puckering in cyclobutane

Quantum chemical study of several monocyclic complex β‐lactam C‐3, C‐4, and N‐derivatives, and β‐ring model molecules

“…The AM1 semiempirical model Hamiltonian fails to describe the cyclobutane ring puckering, giving a planar form. However, by experiment 4, 8–10, 13, 15 and by B3LYP/6‐311++G(3df,pd), it is folded diagonally across the four‐membered ring with a CCCC torsional angle by B3LYP of 17.69°; although the difference of energy between both forms, planar (saddle) and nonplanar, is very small, 0.869 kcal/mol by B3LYP/6‐311++G(3df,pd). Analogously, AM1 fails in azetidine with a NCCC torsional angle of 2.41° (α = 3.47°) when by experiment 13, by HF/6‐31G** with 17.53° (α = 25.47°), by MP2/6‐31G** with 22.29° (α = 32.20°), and by B3LYP/6‐311++G(3df,pd) with 17.94° (α = 25.97°) is nonplanar.…”

“…Trimethyleneimine (also called azetidine) is also a nonplanar structure, slightly more puckered than cyclobutane, with a puckering angle of 35.1° determined by ED 4 or 29.7° by the joined (ED + MW) 13, 15, also studied by ab initio methods 4, 6, 16, and with a barrier to the interconversion between both forms estimated 17 by microwave (MW) and ab initio calculations in the range 1900–2600 cm −1 , much higher than in cyclobutane 10, 513 ± 2 cm −1 . The fundamental vibrational frequencies by IR (in solution and vapor phase 13, 18 and in Ar matrix 16b) and by Raman 18, 19 spectroscopy have been compared with their scaled ab initio force field computations 13, 18.…”

“…Cyclobutane is a well known nonplanar molecule 4 with a puckering angle of about 28° (27.9° by Egawe et al 5), which has been studied by ab initio 6, 7, x‐ray 8, electron diffraction (ED) 9, and by spectroscopic experiments 10, 11. As in cyclopentane 12, the puckered structure is found to be energetically preferred to accommodate angle strain and torsion strain.…”

The ordered and orientationally disordered crystalline phases of the flexible C4F8 molecule

“…Here we propose a simple model molecule, which includes a low torsional barrier for the CCCC ring torsion, τ , and a rocking angle, β, for the CF 2 group (that moves mainly rigidly in a plane perpendicular to the corresponding CCC plane). By comparison to the very well known data for the molecule C 4 H 8 18 , we propose the following intramolecular potential model and parameters:…”

“…4 shows the correlation between the torsional and rocking angles, determined at the lowest temperature in the "cubic " sample. For C 4 H 8 18 a relationship of the form β = 0.21τ − 3.8 * 10 −5 τ 3 .…”