Multiple nonpolar bonding

NMR studies showed that in polar solvents (DMSO, DMF) catenane 45 adopts a supramolecular conformation (II) that allows the amide protons to interact via multiple hydrogen bonds with solvent molecules and at the same time buries the lipophilic chains in the center of the molecule (Figure 18). In contrast, NMR spectra in nonpolar solvents indicate that the aliphatic chains are situated on the outer sphere of the catenane, whereas the isophthaloyl units are arranged in a way that ensures optimal intramolecular hydrogen-bonding (I). 

Another factor that affects bond length is electronegativity. Bonds lend to be shortened, relative to the expectations for nonpolar bonds, in proportion to the electronegativity difference of the component atoms. Thus the experimental bond length in HF is 91.8 pm versus an expected value of 108 pm. The quantitative shortening of bonds because of electronegativity differences and multiple bonding in elements other than carbon will be discussed in Chapter 8. 

Nonpolar molecule (Section 1.12) A molecule that has no net dipole. A nonpolar molecule has either no polar bonds or multiple polar bonds whose dipoles cancel. 

We have already acknowledged our intent to use relevant estimation approaches to enthalpies of vaporization and sublimation to maximize the usefulness of the data available. That dienes and polyenes have multiple double bonds that are potentially hydrogenatable to the totally saturated aliphatic or alicyclic hydrocarbons allows the employment of two other assumptions. The first assumption argues that the enthalpy of hydrogenation, AHn measured in a nonpolar solvent is essentially equal to that which would be obtained in the gas phase. The second assumption, implicitly employing the first, legitimizes the use of estimation techniques and even molecular mechanics to derive the enthalpy of formation of the totally saturated species. From this last number, the enthalpy of formation of the unsaturated diene or polyene of interest can be derived by equation 3 and simple arithmetic. 

AG and AH can be expressed as a multiplicative function of hydrogen bonding in different polar and nonpolar solvents by means of enthalpy acceptor factors E - enthalpy donor factors free energy acceptor factors Q, and free energy donor factors Q (Eqs. (32) and (33), where kj, 2- 3 [kcal/mol] are regression coefficients). 

Aziridines can add to carbon—carbon multiple bonds. Elevated temperature and alkali metal catalysis are required in the case of nonpolarized double bonds (193—195). On the other hand, the addition of aziridines onto the conjugated polarized double or triple bonds of a,p-unsaturated nitriles (196—199), ketones (197,200), esters (201—205), amides (197), sulfones (206—209), or quinones (210—212) in a Michael addition-type reaction frequendy proceeds even at room temperature without a catalyst. The adducts obtained from the reaction of aziridines with a,p-unsaturated ketones, eg, 4-aziridinyl-2-butanone [503-12-8] from 3-buten-2-one, can be converted to 1,3-substituted pyrrolidines by subsequent ring opening with acyl chlorides and alkaline cyclization (213). 

In Mb, heme is located in the heme pocket via multiple noncovalent interactions such as Fe-His coordination, hydrophobic contacts with several nonpolar amino acid residues, and hydrogen bonding between heme propionates and polar amino acids (69). Therefore, the hemin can be easily removed from the heme pocket under acidic conditions to give apomyoglobin (apoMb) (70, 71). Over the past three decades, a variety of artificial iron porphyrins and porphyrinoids have been incorporated into the apoprotein to reconstitute the... 

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