Surroundings of functional groups

The observed interactions between molecules in a crystal are a consequence of the minimization of the free energy for the entire atomic 

FIGURE 17.2. Directional preferences of surroundings of divalent sulfur (Ref. 6). From an analysis of the environments of C-S-C groups in crystal structures it was concluded that electrophiles approach at an angle of 50-90° from the C-S-C plane, while nucleophiles interact within 0-30° of the C-S-C plane. 

There was no evidence of anisotropy (preferential angle of approach) of C-F- -H-0 interactions, unlike the the anisotropy observed for C-Cl, C-Br, and C-I interactions in a similar type of analysis,for which it was found that electrophiles approach carbon-halogen bonds at an angle of about 100° (nearly perpendicularly), nucleophiles approach at about 165° (almost directly). 

Fluorine is the most electronegative element. Why, then, does the C-F bond form hydrogen bonds so rarely, especially since former when H- -F forms the shortest hydrogen bonds measured (H- F = 1.15 A) The strength of a hydrogen bond [D-H /I) appears to be a complicated balance of various factors which include  

FIGURE 17.4. Examples of C-F H-O and C-F H-N interactions, (a) Chemical formula of a fluorocitrate ester, and (b) a ball-and-stick model of the results of the crystal structure analysis of this ester. Note the different H-O and H F distances (2.0 A and 2.3 A, respectively) (Ref. 13). 

---

Chemical modification is known to be capable to change substantially sorption properties of silica surface [72]. Due to identity of support for a hydroxylated and chemically modified silica changes in sorption properties of different samples are to be connected naturally with corresponding peculiarities of p within the surrounding of functional groups. Thus in case of chlorinated silica (Figure 28.7) it should be noticed that p has substantially... 

---