Bohlmann effect

if the effect is general, we expect that the C1-C2 bond will similarly lengthen when anti to the lone pair (the anti conformation), and indeed the MP2 bond length is 

A similar but somewhat smaller Bohlmann effect occurs with ethers and alcohols, where the lone pairs on the oxygen can delocalize into C-H and C-C bonds. And, of course, for oxygen there are two lone pairs, each of which must be separately considered. The spectroscopic shift in this case is of limited practical use, however, since although the bond lengthening of the C-H bond from the Bohlmann effect does occur. 

TABLE 7.2. Bohimann Distortions for Bonds (A) and Bond Angles (deg) in Ethylamine and Ethanol (re) 

There is evidence of analogous hyperconjugative interaction in F-substituted compounds. In particular, the v(C-H) for methyl fluoride is 16 cm red-shifted relative to methane (2976 vs. 2992 cm ). Because hybridization effects imposed by Bent s rule should increase s-character in the C-H bonds and lead to the blue-shift, the experimental red-shift observation indicates the importance of np- -o C-H interactions. Additional fluorine substituents remove most of the red-shift (v(C-H) for CH F and CHFj are 2984 vs. 2990cm respectively) suggesting the increasing importance of rehybridization (progressive s-character increase in the C-H bond(s)) and, possibly, the importance of Up o C-F interactions that partially neutralize the donor capacity of the fluorine lone pairs. Small blue-shifts are observed for the chloro- and bromoderivatives of methane. 

Bohimann effect lower IR-stretching frequency for the antiperiplanar C-H bond 

Two cT-acceptors compete for a lone pair Bohimann effect 

The key important IR-active functionality of the carbonyl family, the C=0 group, responds to structural changes in a more complex way as it reflects the interplay of two orthogonal sets of delocalizing interactions the in-plane n c-x out-of-plane p - interactions (see Chapter 6 for a detailed discnssion). The first of these interactions strengthens the (5 = 0 group with a concomitant blue-shift whereas the other one has the opposite structural and spectroscopic consequences. 

C-Sn and C-Si bonds can stabilize the positive charge so efficiently (see the earlier discussion of p-effects in Chapter 6) that it allows for the isolation and structural characterization of the respective vinyl cations.  

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The mechanical, nonbonded, and electrostatic expressions described above are not sufficient to describe some structural and spectroscopic effects. Three specific structural and spectroscopic phenomena have been incorporated into MM3. They are the electronegativity, anomeric, and Bohlmann effects, which essentially can be traced to molecular orbital origins. These chemical effects are a part of the MM3 program.74,75... 

It has been known for a long time that amines which have a hydrogen on a carbon attached to the nitrogen so that the C—H bond is antiperiplanar to the lone pair, show abnormally low stretching frequencies for those C—H bonds. In order to reproduce this (Bohlmann) effect MM3 corrects the natural bond lengths and force constants of such C—H bonds by31 ... 

C—H length and stretching frequencies accounting for electronegativity, carbonyl, and Bohlmann effect Thomas 1994... 

The better known hyperconjugative interaction that relates to all C-0 torsions, including C-2-0-2 is the Bohlmann effect first identified from characteristic con-formationally dependent shifts in the IR spectra of amines.168 169 This effect is a result of electron donation from lone pairs on N or O to the a orbital of adjacent C-H bonds. This effect is maximal for an anti relationship between the relevant lone pair and the C-H bond. One such interaction is always present in gauche H-C-O-C conformations and is one of the factors that stabilize such conformations. 

To see the relationship between the Bohlmann effect and the anomeric effect we can look at the valence bond stractures for methylamine and dimethoxymethane. Structure 4 ... 

We have suggested " that a Class 3 force field be defined as one which contains chemical effects, in addition to the physical effects so far discussed. Chemical effects would include such things as hyperconjugation, the electronegativity effect, the anomeric and Bohlmann effects, and so on. These effects depend upon exactly which atom occupies a position, that is, they depend not only on ordinary mechanical quantities, but also on specific properties of oxygen, for example, compared with nitrogen, or with carbon. These effects can, of course, be properly represented by suitable cross terms in the force constant matrix. The origins of these terms, however, have a definite chemical basis. 

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