Restriction bonds

Peptide Bonds Restrict Possible Secondary Conformations... 

In sulfenamides (R S—NR R ), the cation-radicals keep an nnpaired electron occupying a n orbital. This orbital is localized between the sulfur and nitrogen atoms. As a matter of fact, a slight S=N double bond character exists in the neutral sulfenamides. A consequence of this double bond character is an increase in the energy barrier, which restricts rotation around the S—N bond. Restricted rotation about the S—N bond is known in neutral sulfenamides (Kost and Raban 1990). Notably, the energy barrier to this rotation is greater for the derived cation-radicals when compared to the parent compounds (Bassindale and Iley 1990). 

Using the double bonds, we conclude that the twisted configuration shown in Figure 6-17 should not be very stable. Here the p orbitals are not in position to overlap effectively in the tt manner. The favored configuration is expected to have the axes of the p-tt orbitals parallel. Because considerable energy would have to be expended to break the p-tr double bond and to permit rotation about the remaining sp2-cr bond, restricted rotation and stable cis-trans isomers are expected. Similar conclusions can be reached on the basis of the r model of the double bond. 

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

Interconversion Around a "Partial Double Bond" (Restricted Rotation) At room temperature, a neat sample of dimethylformamide shows two CH3 peaks because the rate of rotation around the hindered partial double bond is slow. At... 

In analogy to aromatic bonds, restrictions exist concerning the applicability of the functions. Here also a bond system is affected. Therefore, the number of bonds per DE-system (it will be three) must be taken into account. The proper adjustment of the delocalized electrons (DE-entries of the xr-matrix) must be ensured. 

Amino acids are combined (linked together) through peptide bonds (-C-N-) (Figure 8.1) the peptide bond formed is planar (flat), due to the delocalisation of electrons that form the partial double bond, restricting rotation about the bond. The rigid peptide dihedral angle, co (the bond between C and N), is always close to 180°. The dihedral angles phi (the bond between N and Ca) and psi (the bond between Ca and C) can only have a number of possible values, and so effectively control the protein s three-dimensional structure. 

The concept of cIs and trans Isomers was first introduced for disubstituted cycloalkanes in Chapter 4. In both cases, a ring or a double bond restricts motion, preventing the rotation of a group from one side of the ring or double bond to the other. 

D. New Vibrational Modes The formation of a H bond restricts certain rotational and translational degrees, and forms an equal number of new vibrational degrees of freedom. 

An index of the molecular polarity due to C-H bonds restricted to halogenated hydrocarbons [Di Paolo et al, 1979]. It is calculated as the sum of the contributions to the polarity of all the C-H bonds in a molecule. For each C-H bond are considered three different contributions due to halogens linked to the same carbon atom of the C-H bond, halogens in a-position and halogens in -position with respect to the considered C-H bond ... 

S)-Metolachlor is the active ingredient of the herbicides Dual and Bicep, which are widely applied in the United States and were developed by the Swiss company Ciba-Geigy (now Novartis). The steric hindrance near the aryl-N bond restricts rotation about that bond. This restricted rotation results in a type of stereoisomerism called atropisomerism, resulting in aR and a,S configurations about the chiral aryl-N bond axis.113 There is also a stereogenic center (position /) on a carbon next to the N-atom in the amino side chain. All four possible stereoisomers of metolachlor are shown in Figure 9-3. 

Our design addressed two specific features desirable for selective blocking of one face of the C=N bond, restricted rotamer populations and Leivis acid activation. N,N Dialkylhydrazone A (Figure 2.4) offers little restriction to the rotation of C N and... 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

However, p orbitals must be parallel to engage in side-to-side overlap, so a TT bond restricts rotation around it. Rotating one CH2 group in ethylene with respect to the other must decrease the side-to-side overlap and break the tt bond (see Figure 11.10). (In Chapter 15, you ll see that restricted rotation leads to another type of isomerism.)... 

Groups joined by single bonds can rotate, so a branch pointing down is the same as one pointing up. (Recall that a double bond restricts rotation.)... 

The double bond restricts rotation. Thus, in addition to the form shown in part (b), in which the H atoms are on the same side of the C=C bond, another possibility is the form in which the H atoms are on opposite sides ... 

The C=C Bond and Geometric (cis-trans) Isomerism There are two major structural differences between alkenes and alkanes. First, alkanes have a tetrahedral geometry (bond angles of —109.5°) around each C atom, whereas the double-bonded C atoms in alkenes are. trigonal planar (—120°). Second, the C—C bond allows rotation of bonded groups, so the atoms in an alkane continually change their relative positions. In contrast, the -n bond of the C=C bond restricts rotation, which fixes the relative positions of the atoms bonded to it. 

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