In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown?
Posts Tagged ‘conformational analysis’
Conformational analysis of cyclotriborazane
Sunday, February 14th, 2010The conformational analysis of cyclo-octane
Sunday, January 31st, 2010In the previous post, I suggested that inspecting the imaginary modes of planar cyclohexane might be a fruitful and systematic way in which at least parts of the conformational surface of this ring might be probed. Here, the same process is conducted for cyclo-octane. The ring starts with planar D8h symmetry, and at this geometry (B3LYP/6-311G(d,p), DOI: 10042/to-3742) five negative force constants (corresponding to imaginary modes) are calculated. The most negative is non-degenerate, and gives directly the crown conformation of D4d symmetry (DOI: 10042/to-3738). The remaining four modes comprise two degenerate pairs. Following either of the E2u eigenvectors downhill leads to another conformation, D2d (DOI: 10042/to-3741), with a geometry which is noteworthy for exhibiting a pair of unusually close non-bonded H…H contacts (1.908Å). This value is about 0.3Å shorter than the sum of the Wan der Waals radii (DOI: 10.1021/jp8111556). We can debate whether such a close approach or inter-penetration of two hydrogens is a bond or not (an AIM analysis appears at the bottom of this post). (more…)
The conformation of cyclohexane
Thursday, January 28th, 2010Like benzene, its fully saturated version cyclohexane represents an icon of organic chemistry. By 1890, the structure of planar benzene was pretty much understood, but organic chemistry was still struggling somewhat to fully embrace three rather than two dimensions. A grand-old-man of organic chemistry at the time, Adolf von Baeyer, believed that cyclohexane too was flat, and what he said went. So when a young upstart named Hermann Sachse suggested it was not flat, and furthermore could exist in two forms, which we now call chair and boat, no-one believed him. His was a trigonometric proof, deriving from the tetrahedral angle of 109.47 at carbon, and producing what he termed strainless rings.
Spotting the unexpected: Anomeric effects
Friday, September 18th, 2009Chemistry can be very focussed nowadays. This especially applies to target-driven synthesis, where the objective is to make a specified molecule, in perhaps as an original manner as possible. A welcome, but not always essential aspect of such syntheses is the discovery of new chemistry. In this blog, I will suggest that the focus on the target can mean that interesting chemistry can get over-looked (or if observed, not fully exploited in subsequent publications). Taking a synthesis-oriented publication at (almost) random entitled Synthesis of 1-Oxadecalins from Anisole Promoted by Tungsten (DOI: 10.1021/ja803605m) which appeared in 2008, the following molecule appears as one of the (many) intermediates.