A molecular mechanics study of cyclopropanes within the MM2 and MM3 force fields

Non‐empirical calculations of NMR indirect carbon‐carbon coupling constants. Part 5: Bridged bicycloalkanes

“…Molecular structures, conformations and vibrational spectra of bicyclo[3.1.0]hexane were also investigated elsewhere 29 at HF, MP2 and DFT levels; the calculated structural parameters and ring-puckering potential were in good agreement with the previously reported dipole moments, microwave, electron diffraction, far-IR and Raman spectra 30 and early ab initio 31 and molecular mechanics 2 calculations, all of which demonstrated the preferred boat conformation. According to the most accurate combined analysis of electron diffraction and microwave spectroscopic data, 32 bicyclo[3.1.0]hexane exists in the boat conformation with a puckering angle of 70.6°and 25.2°for the cyclopropane and cyclopentane moieties, respectively, and a bridgehead-bridgehead bond length of only 1.510Å (the shortest in the series of 1-6), in good agreement with the present B3LYP results (Table 1).…”

“…Compound 3, in contrast to bicyclobutane (1) and bicyclopentane (2), is a conformationally flexible molecule giving rise to the two interconverting conformers, namely boat and chair, located in the present B3LYP calculations as true minima stationary points; both structures were found to have C s symmetry ( Fig. 2) with the former being more stable by 12.5 kJ mol 1 .…”

“…As can be seen from Table 6, the bridgehead carbon-hydrogen bond of bicyclobutane is characterized by dramatically increased s-character and is somewhat in between the classical C(sp 2 -H (such as in ethylene) and C(sp)-H (such as in acetylene), while the C-H bonds of bicyclopentane (2) Also, there is a rough correlation between 1 J(C b ,H b ) and the corresponding carbon-hydrogen bond lengths: the coupling constant increases when the corresponding bond length becomes shorter (Table 6). However, this is basically a secondary relationship: both parameters …”

“…Bridged bicycloalkanes consisting of two condensed cycloalkane moieties having two mutual carbons at bridgeheads present a very interesting case of unusual spin-spin coupling due to the remarkable steric strain 2 and multipath coupling mechanism 3,4 involving the carbon framework. The most interesting aspect of their electronic structure is, of course, the nature of chemical bonding between bridgehead carbons, which was the subject of early discussions.…”

Spiroannulated Cyclopropanes

“…The results of nonempirical (4-31G//STO-3G) and molecular mechanics (MM2) calculations were in reasonable agreement with experimental data, while the semiempirical methods resulted in substantial deviations 4 . Even better results were obtained by Aped and Allinger using the MM3 method 5 . Available experimental data on the heats of formation of SPC and matching calculated values are collected in Table 1.…”