Atomic substituents

Those of the readers who are already quite familiar with proton NMR spectroscopy are aware that, as the most electronegative element, fluorine substituents deshield proximate hydrogens more than any other atomic substituent because of their unique inductive influence. This fact is exemplified below in Scheme 2.11. 

Spin orbit coupling is most effectively increased by using higher Z metals. Heavy atom substituents on the organic ligand have proved of minimal use and may actually make the ligands more photochemically unstable. 

Wanzlick et al. realized in the 1960s that the stability of carbenes should be increased by a special substitution pattern of the disubstituted carbon atom. Substituents in the vicinal position that provide r-donor/CT-acceptor character to fill the p-orbital of the carbene carbon and stabilize the carbene lone pair by a negative inductive effect should reduce the electrophilicity of the singlet carbene and consequently reduce its reactivity (Schem 1). 

When there is only one hydrogen on a carbon atom, group interactions must be taken into account. In this case, the best overall fit to the data is obtained by applying a multiplier to the contribution from the third group. Atomic substituents are always taken as the first and second groups, and larger substituents as the third group. 

The ESR spectra of the stable radical anions of the dibenzometalle-pins (XI M = Si, R2 = Ph2, PhMe, and Me2 and M = Ge, R2 = Ph), show little sensitivity to the metal atom or metal atom substituent that lies in a node of the occupied molecular orbital (117a). In these radical anions, only d orbitals on the metal have the appropriate symmetry to interact with the orbital occupied by the unpaired electron. 

Since the seminal work of Bartlett [363] there have been relatively few reports of useful cycloadditions of fluoroalkenes. Recent work by Haufe showed that fluorine atom substituents depress the reactivity of styrene derivatives in the Diels-Alder reaction with 1,3-diphenylisobenzofuran. A single fluorine atom in the /1-position lowered the rate of the [4 + 2] reaction by a factor of ten when the fluorine atom occupied the -position (Eq. 142), the reaction was over 30 times slower than that of the unsubstituted styrene. 

These Es parameters estimated by Eq. 4 for hetero atom substituents can be combined with those originally developed for various alkyl groups as a set of steric constants for QSAR studies of aromatic systems 6). Thus, apart from the original definition for the intramolecular steric effect, the combined set of Es parameters is able to represent intermolecular steric effects as well. The original Taft E, values for unsymmetrical alkyl groups seem to represent effective steric dimension of the groups which is scaled on the same standard as those for symmetric monoatomic substituents where the effective dimension coincides with the van der Waals radius. 

Since the activity seemed highly dependent on the positions of hetero-atom substituents, the compounds were stereochemically classified into two groups, meso- and (//-analogs, according to their hetero-substituent positions as shown in Fig. 12. QSAR analyses for insecticidal activity of these groups were performed separately to give Eq. 52 and 53 74). 

Equatorial substituents on the C atoms of coordinated 1,3-diamino-propane should be stabilized in the same manner as the 1,2-diaminopropane system and axial substituents on the 1 and 3 C atoms should interact strongly with the axial substituents on the Co atom. Substituents on the N atoms... 

Substituents have considerable influence on emission characteristics of aromatic compounds. Heavy atom substituents tend to reduce the fluorescence quantum yield 4>f in favour of phosphorescence emission f. In halogen series the effect increases in the order F < Cl < Br < I. In Table 5.1 are recorded experimental data for halogen substituted Naphthalenes. 

The first family is represented by peptides based on a, 3-dehydro a-amino adds in which a C=C bond confers rigidity and electronic Y-conjugation on the system (Section 11.1). Peptides rich in these amino acids are widespread in nature, particularly as antibiotics, and possess interesting conformational properties. 1-11 In addition to the E and Z configurational isomers, occurring when two different atoms (substituents) on CP are present, fully-extended, characteristically flat conformations or p-tums and 310/a-helices may be induced by u,p-dehydro a-amino acids. 

Aromatic fluorine bonds can be activated or deactivated by substituents on the ring. Experimental results show that the aromatic C-F bond is activated for reduction if there is an electron-withdrawing group (e. g., ester and carbonitrile groups) or electron-releasing groups (c.g., hydroxy and amino groups) ortho or para to the fluorine atom. Substituents in meta positions usually decrease the reactivity. 

Johnson and Amel (5J) studied the pKa values of a series of phenacylonium salts and found that the acidifying effects of the onium group increased in the following order arsonium < phosphonium < sul-fonium. The acidifying effects of the onium atom substituents increase in the order methyl < n-butyl < phenyl. 

The reactivity of C=C bonds depends on the number and nature of substituents attached to the sp2 carbon atoms. Substituents affect the reactivity by affecting the rate constant of reaction and the adsorption properties. 

The OH radical reaction occurs at the >C=C< bond, and H-atom abstraction from the vinyl C-H bond is neglected. There are three Cl atom substituents around the >C=C< bond, and the "base" structural unit is the -CH=C< group (Table 14.2). The effects of the three Cl atom substituents are taken into account using the C(-Cl) value from Table 14.3. The rate constant, ktotal, for trichloroethene is given by ... 

Fluorine is, of course, the most electronegative atom on the periodic table. Tp-Values and F-values (the pure field inductive effect) provide indications of the electron-withdrawing influence of substituents, and it can be seen that fluorine itself has the largest F value of an atomic substituent. The values for aP and F for various other fluorinated (and nonfluorinated) substituents provide insight into the relative electron-withdrawing power of fluorinated substituents (Table 1.2).25... 

In some cases, room temperature phosphorescence (RTP) may be achieved in the absence of a well-defined ternary complex. A heavy atom substituent on... 

Nitrogen-containing macrocycles are highly complementary for first row transition metals as in the examples shown in Section 1.6. Common azamacrocycles include cyclen ([12]ane-N4) and cyclam (3.45) and many have a history that significantly pre-dates the crown ethers. Unlike the crown ethers which do not have donor atom substituents, the binding constants of azamacrocycles such as cyclam are greatly affected by N-alkylation. Alkylated amines are significantly more basic than ammonia, for... 

The primary supposition of any toxicological QSAR is that the potency of a compound is dependent upon its molecular structure, which is typically quantified by chemical properties (Schultz et al., 2002). Chemical descriptors include a variety of types, including atom, substituent, and molecular parameters. The most transparent of these are the molecular-based empirical and quantum chemical descriptors. Empirical descriptors are measured descriptors and include physicochemical properties such as hydrophobicity (Dearden, 1990). Quantum chemical properties are theoretical descriptors and include charge and energy values (Karelson et al., 1996). Physicochemical and quantum chemical descriptors are for the most part easily interpretable with regard to how that property may be related to toxicity. The classic example of this, the partitioning of a toxicant between aqueous and lipid phases, has been used as a measure of hydrophobicity for over a century (Livingstone, 2000). 

Figure 4.1 Enantiomers are mirror images, not superimposable, and dependent upon the tetrahedral arrangement of carbon atom substituents.

---