Hydrogen points

Formation of 173 from 171 on hydrogenation pointed to the presence of an allylic alcohol group in 171. This was confirmed by oxidation of 171 to ketone 174. There were strong bands at 1650 and 1580 cm in the IR spectrum of 174 typical for a —CO—C=C—— system. The absence of olefmic protons in the NMR spectrum of 171 suggested the double bond to be at C-11—C-16, while a hydroxyl group is positioned at C-15 or C-17. 

In this case since the molecule is drawn with the hydrogen pointing forward it is best to determine the configuration from the molecule as drawn and then assign the opposite configuration. 

Among the atoms attached to the stereogenic center, the order of decreasing precedence is Br > Cl > F > H. When the molecule is viewed with the hydrogen pointing away from us, the order Br — Cl —> F appears clockwise in the R enantiomer, anticlockwise in the S enantiomer. 

Specifically, two p.-aqua-bridges located in the cleft of the dimer have one hydrogen pointing outward from each side, thus allowing the photo-excited p-benzoquinone (labeled in Scheme 8), but not a bulky 2,5-/-Bu-/ -bcnzoquinone, to enter the cleft and abstract a H radical. This mechanism is consistent with the proposed role of the tyrosyl Yz radical as H abstractor from water [165]. The reaction, however, is not catalytic, because of the irreversible formation of hydroquinone. 

Except for the triple points and one equilibrium hydrogen point (17.042 K), the assigned values of temperature are for equilibrium states at a pressure p = 1 Standard atmosphere (101,325 Pa)... 

Mechanistic investigations with deuterium instead of hydrogen point towards the mechanism already proposed in Scheme 3, but using NHR instead of water. 

With a methyl, an ethyl, and an iaopropyl group, 4 hydrogen pointe in toward the ring. With a nri butyt group, a methyl pointa in. 

The sp3 hydroxy group of the serine must finally spin about bond B which links it to the to the rest of the protein, until its hydrogen points as well as possible towards the probe s lone pair. 

Ha = axial hydrogen (point up and down) He = equatorial hydrogen (point sideways)... 

Point (b) within each cycle corresponds to 1/ = n where to is an integer, and is just the hydrogenic energy. Point (c) occurs at v = to + and is described as a half hydrogenic point. There is a special case B = C to which we return below. 

---