Negative rotation

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

A compound represented by XVIII would yield 1,3,4-trimethyI-D-fructose, a crystalline substance with [aJD20 = — 51.4° (water), and 1,3,6-trimethyl-D-fructose (now unknown), upon hydrolysis of its hexa-methyl derivative. No evidence has been obtained to indicate the presence of this negatively rotating trimethyl-D-fructose and it is established that the mixture of trimethyl-D-fructoses from hexamethyl-di-D-fructose anhydride II rotates in the positive range of 25 to 30°. Unless the 1,3,6-trimethyl-D-fructose should prove to possess the unusually high rotation of about + 100°, which seems at least improbable, structure XVIII can be excluded. 

The strongly negative rotation of d-glucoseapigenin is compatible only with the preceding conclusion that it is a beta rather than an a-D-glucoside. 

It should be noted that the enantiomeric excess in the product was determined in most cases by measuring the specific rotation of the neat liquid11, although GC methods are now available13. It is known that this rotation value is temperature sensitive ( + 0,18/deg) so that the temperature of measurement must be measured carefully and specified a value for enantiomer-ically pure 3-phenyl-l-butene of [a] 2 +5.92 (neat) is recommended (the / -enantiomer has a negative rotation). 

A clockwise rotation, as the observer looks towards the beam, defines the substance as dextrorotatory (i.e., rotates to the right) and the angle a is taken as a positive (+) rotation. If the rotation is counterclockwise the substance is described as levorotatory (i.e., rotates to the left) and the angle a is taken as a negative (—) rotation. 

Figure 2.2. (a) The unit vector a bisects the angle between x andy and thus has components 2 A [1 10]. (b) The curved arrow shows the direction of a positive rotation about x. (c) The curved arrow shows the direction of a negative rotation about OZ (Exercise 2.1-l(a)). (d) The product of two symmetry operators R2 R is equivalent to a single operator i 3 e, r, and r2 are three indistinguishable configurations of the system. 

Exercise 2.1-1 (a) Figure 2.2(c) shows that the arrow has the opposite direction to the rotation of a right-handed screw as it moves along OZ from O. Also, on looking down the OZ axis towards O, the rotation appears to be in a clockwise direction. It is therefore a negative rotation with —n < < 0. 

Parameters that satisfy eq. (12) are referred to as standard quaternion (or Euler-Rodrigues) parameters. Note that A = sin(j(/>)n belongs to the positive hemisphere h for positive rotations and to —h for negative rotations. For binary rotations, n, and so X = 0, A = 1, and A belongs to h because there is no rotation R( n n). Therefore for any point group, h must be defined so as to contain the poles of all positive rotations, including binary rotations. Due to the range of , standard quaternion parameters must satisfy either... 

This alkaloid (490), CjgH NjOg, amorphous, [a]D -105° (c 0.001, CHC13), was obtained from Aristolochia elegans Mast. (Aristolochiaceae). MS data established that the OH is in the left-hand head unit final placement was by NMR correlations with published data. The strongly negative rotation is in accord with the indicated (R,R) configuration (560). 

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