Boat-shaped structures

The value of q3 = (6) V2R (R is the CC bond length) is 0.63 A. Under pseudorotation the equatorial boat-shaped structures B (0 = 90°, = 0, 60°, 120°,.. . ) turn into a twist-boat structure TB (0 = 90°, = 30°, 90°,.. . ). The transitions between the chair and twist boat structures involve the intermediate formation of half boat (HB) and half chair (HC) structures. Quantum chemical calculations carried out by Dixon and Komornicki [1990] show that the axial structure C with symmetry D3d is stable. The energies of structures B and TB are 7.9 and 6.8kcal/mol higher than C. The barrier for transition from C to TB is 12.2-12.4 kcal/ mol. Because of the high barriers for pseudorotation, only thermally activated conformational transitions occur in cyclohexane. 

N3P3CL2(NHPt )4 (geminal) Free base of hydrochloride where structure reported previously. Distorted boat-shaped ring protonation does not 208... 

Acidic properties of zeolite L were observed to correlate well with its structural disorders. The Si-MAS-NMR spectrum of zeolite L having a Si/AI ratio different from 3 revealed that Al distribution deviated from the ideal and suggested the presence of six different boat-shaped 8-ring patterns. Differential molar heats of adsorption of ammonia changed step-wise with the adsorbed amount, which reflects the difference in the acid strength of protons located in structurally different 8-rings. 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

Fig. 5.45 One might have guessed that the chair cyclohexane conformations 1 and I are connected by a boat-shaped intermediate 2. However, this C2v structure shows an imaginary frequency it is a transition state which wants to twist toward 3 (arrows) or 3 (arrows in opposite directions, not shown), which are the actual intermediates (no imaginary frequencies) between 1 and 1. The chair conformation reaches the twist via a half-chair 4...

Double-layered tetrathiafulvalenes have been prepared but with different face-to-face orientations. The coupling of 12 with triethylphosphine in toluene under reflux gave (26 %) 13, in which the TTF planes are slightly bent into a boat shape and overlapped with each other [95CL523]. Whereas, the related dimeric structure 14, where the two TTF moieties possess a cross-orientation, has recently been reported [95CL579,95TL5045]. 

Two conformations are possible for compound 73 one a compact folded structure (74 R = PMP) and the other a more extended molecule 75. In 74 (R = PMP), the H(8a)-C(8a) bond is in the flagpole position of a boat-shaped six-membered ring and is almost perpendicular to the plane of the fused benzene ring. Evidence from NMR spectroscopy shows that this form is the dominant or exclusive conformer present in solution <1998T15227>. 

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