Correlated rotations

Similar correlated rotations are observed in 1,1-diarylalkenes [5 Z = CH2] (Kaftory et al., 1990) and benzophenones [5 Z = O)] (Rappoport et al., 1990). A key feature of the pathway for conformational change in the benzophenones is that maximum v overlap is maintained between the carbonyl group and one of the aromatic rings, so that a one-ring flip pathway, [5]<— [5 ], is strongly preferred. The observed structure set... 

Correlation times and activation energy parameters obtained from different techniques may or may not agree with one another. Comparison of these data enables one to check the applicability of the model employed and examine whether any particular basic molecular process is reflected by the measurement or whether the method of analysis employed is correct. In order to characterize rotational motion in plastic crystals properly it may indeed be necessary to compare correlation times obtained by several methods. Thus, values from NMR spectroscopy and Rayleigh scattering enable us to distinguish uncorrelated and correlated rotations. Molecular disorder is not reflected in NMR measurements to this end, diffraction studies would be essential. 

NMR studies of 4,6-bis- and 2,4,6-tris-(MA-dialkylamino)-l, 3,5-triazines show correlated rotations of the alkyl groups in the dialkylamino substituents. Unsymmetrical 2-chloro-, 2-alkoxy- and 2-aryloxy-4,6-bis-(di-/i-alkylamino)-l,3,5-triazines show two non-equivalent n-alkyl groups due to restricted rotation [95JCS(P2)785]. Calculations of the favoured conformations of 2-[A-(hydroxymethyl)-ALmethyl]-4,6-bis-dimethylamino-l,3,5-triazine using MNDO and PM3 methods have also been made [95JCS(P2)469]. 

Figure 1. Schematic of a conformaticmal transition in a polymer chain. Top, rotation arourtd a single bond bottom, correlated rotations around two bonds in a crankshaft-like motion.

Molecules in LCPs may develop correlated rotations about their chain axes, or correlated side chain rotations may develop, over optically resolvable distances. The optical properties of a correlated domain depend both on the configurational and conformational symmetry of the individual molecules and on the extent to which this symmetry is preserved on a larger scale by the correlations. 

The possibility of optical biaxiality should be considered whenever molecules (or side chains) have a lath-like or "sanidic" conformation that might support correlated rotations about their chain axes. If the suggested tests involving optical microscopy are impractical or inconclusive, one must resort to characterization by complementary techniques. For example, optical biaxiality can be inferred if there is no preferred orientation of optical extinction directions in a specimen that consists of globally aligned molecules as demonstrated by x-ray or electron diffraction (181-... 

Dynamic two-dimensional and NMR studies of the conformationally restricted amide iV-(l-naphthoyl)-d5-2,6-dimethylpiperidine reveal correlated rotation about the aryl-CO and amide bonds. 9-(l-Adamantylamino-methyl)-9,10-dihydroanthracene was prepared via condensation of 1-adaman-tylnitrile with 9-anthryllithium and its hydride reduction. An NMR study confirmed a boat conformation for the central ring, with the substituent of C9 in a pseudo-axial position. The structure of bis[(l,3)2-oxaadamantano]-18-crown-6 (I) and its potassium picrato complex were studied using NMR, X-ray structure analysis and molecular mechanics calculations. In the gas phase and in CDCI3 solution, in both I and the corresponding complex cation the most stable conformation of C2h symmetry is found. 

Figure 1. Sctiematic represeiitat ion of conformatioiia] transitions in a flexible chain molecule. (a) A single hindered rotation.

Preceding the first 29Si ENDOR measurement22,94, its feasibility had to be estimated for the correlation rotation time, the most essential condition in the multiparameter optimization experiment, a irot range 0.2 to 3 ns22,94 was correctly predicted both from molecular and from theoretical considerations. 

Rotation Methods An optimal loading matrix is obtained by rotation of factors. One distinguishes orthogonal and oblique (correlated) rotations. In the case of an orthogonal rotation, the coordinate system is rotated. The aim is that the new coordinate axis cut the swarm of points in an optimal way. This can be often better achieved by an oblique rotation. If the data can be described by an orthogonal rotation in an optimal way, then an oblique method will also lead to coordinate axes that are perpendicular to each other. 

Mislow, K. M. "Stereochemical Consequences of Correlated Rotation in Molecular Propellers."... 

In solution the bridging fluorines are rapidly transfOTed between adjacent silicons, such that the Si spectrum features a triplet due to two equivalent fluorines ( 7si-F = 127 Hz). This triplet did not change to a doublet of doublets at the low-temperature limit in toluene-dg solution, and hence freezing out of the fluorine transfer between vicinal silicons could not be achieved. When the temperature was raised, all six fluorines became equivalent, causing the silicon resonance to be a septet ( 7si-F = 43 Hz), presumably due to exchange by correlated rotation of the silicon substituents about the Si-benzene bonds. This rotational process was stopped when a molybdenum complex was formed (152), presumably as a result of increased steric hindrance for rotation. 

Contrasting with the temperature dependent relaxation process, both correlated rotations and librations are located at almost invariable frequencies, which is the signature of a resonant process. In the complex plane representations of e (v) versus e (v) (Cole-Cole plots) and of a (v) versus a (v) (Scaife polarizability plots (49) the high-frequency lines get still more clearly separated from relaxation (figure 11 and (46)). 

The next stage is the recognition of the molecular origin of correlated rotations, whether from shorty-range or long-range... 

Going back to the interpretation of correlated rotations, the issues resumed above give us a picture of short-range order where a central molecule is surrounded by those of its first coordination shell, reminiscent of molecular aggregates that are known to occur about electrons, radicals or ions, and thus refe-red by analogy as a "dipolar solvation" process. At present, the way the dynamics would be affected by dipolar self-solvation deserves further research concerning mechanisms that entail rotational coherence. 

Through a cross-couphng reaction, five peripheric ferrocenyl fragments were introduced in [penta(4-halogenophenyl)cyclopentadienyl][hydrotris (indazolyl)borato] ruthenium complexes (Scheme 46). Both hgands coordinated to Ru undergo a correlated rotation also estabhshed by DFT... 

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