Intramolecular electronic effect

Some of the more approximate or specialized FFs encountered in the literature include CFF, CHEM-X, COSMIC, CYFF, DREIDING, MMX, SHAPES, TRIPOS, VALBOND, and UFF. Well-known FFs for modeling proteins and nucleic acids include AMBER, CHARMM, ECEPP, GROMOS, OPLS and their variants. Some of these latter FFs compromise the quality of reproducing subtle intramolecular electronic effects for the sake of being fast enough to treat biomacromolecules in long molecular simulations. 

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

The systematic studies of the effect of substituents on the 35C1 NQR frequencies of substituted chloromethanes discussed in I have been extended to the alkyl iodides19. For the series CH3(CH2) I there occurs the same oscillation in resonance frequency with chain lengths as was noted in I for the alkyl chlorides. These effects are certainly a reflection of intermolecular effects due to crystal packing and not an intramolecular electronic effect. 

Isomerization of diastereomeric alkylzirconiums formed from reaction of (jj -Cp)2ZrHCl and a-pinene is implied in one instance , but such epimerizations are not confirmed in other systems . A vicinal cyclopropyl group influences the stereoiselec-tivity of hydrozirconation by intramolecular electronic effects (Eq. e). 

Relative Adsorption of Meta Versus Para Aromatic Isomers Intramolecular Electronic Effects... 

Fig. 11-1. Intramolecular electronic effects in the adsorption of meta- and para-% ih-stituted pyridines on (a) alumina and (b) silica (15,24). 1, 4-amino 2, 3, 4-dimethyl 3, 4-methyl 4, 4-ethyl 5, 3-methyl 6, pyridine 7, 3-hydroxy 8, 4-chloro 9. 4-aldehyde 10,3-acetyl 11,3-iodo 12,3-aldehyde 13, 3-chloro 14, 3-bromo 15. 4-cyano 16, 3-cyano 17, 3,5-dichloro. [The plots do not include points for the compound 3-aminopyridine see discussion of Ref (/5).]...

Initial analysis of the data In Table lA was carried out graphically, by plotting log (1/C) values for the nineteen compounds against measures of their physicochemical differences, specifically measures of size (molar refractlvlty), affinity for polar solvents (ir), and Intramolecular electronic effect (o). 

The various processes occurring in the stilbene molecule after its excitation exhibit different sensitivity to intramolecular donor-acceptor effects of substituents. This sensitivity was quantitatively characterized by p-constant of the linear Hammett-like relationships [54]. It has been shown that the Stokes shift in nonpolar cyclohexane was not dependent on the structure of the stilbene molecule (Section 3.4.1) [12, 31). Therefore, the substituent effects on vibrational relaxation in the nonpolar solvent can be neglected. Nevertheless, these intramolecular electronic effects on the excitation energy of the substituted stilbenes were found to be very essential even in the nonpolar media (the excitation energy difference between stilbenes substituted with weak and strong donor-acceptor groups can reach 20kcal/mol). 

Cerium forms stable COT complexes only when tetravalent. The first one synthesized, the dicyclooctatetraenyl Ce(COT)2 was pyrophoric, but stable in water. Self-consistent field calculations established that the ground state in cerocene is almost entirely 4f corresponding to the formulation Ce (COT )2 rather than to Ce (COT )2 (Dolg et al. 1991). The methyl substituent in the COT ring stabilizes the structure of bis(methylCOT)cerium(IV) (Boussie et al. 1991) which presents an unusual non-linear Cg-Ce-Cg angle of 176° that may arise from intramolecular electronic effects . The COT substituents are not fully staggered and the two COT are equidistant from the Ce. 

Generalizing, two different approaches — the first based on analysis of contributions of the LCAO composite dipole terms, and the second based on studies of properties of the electron charge-density function reveal a complex picture of intramolecular electronic effects determining band intensities in infiared q>ectia. The application of a bond-moment model in describing this particular molecular property is fru- from straightforward. 

The involvement of a chalcogen atom in an intramolecular interaction may influence the reactivity of the chalcogen centre as a result of steric or electronic effects. This feature may lead to unique reaction patterns. Eor example, the sulfur(II) centre involved in an intramolecular contact... 

However, in more recent years it has become usual to employ ar or crR-type constants, either together in the dual substituent-parameter equation or individually in special linear regression equations which hold for particular infrared magnitudes. In this connection a long series of papers by Katritzky, Topsom and their colleagues on Infrared intensities as a quantitative measure of intramolecular interactions is of particular importance. We will sample this series of papers, insofar as they help to elucidate the electronic effects of sulfinyl and sulfonyl groups. 

In addition to the geometric, steric effects which flexibility directly modifles, all intramolecular electronic properties of a flexible molecule are also coupled to molecular structure. In a real material, such properties as molecular dipole. 

The present approach has been applied to the experiment done by Nelsen et ah, [112], which is a measurement of the intramolecular electron transfer of 2,7-dinitronaphthalene in three kinds of solvents. Since the solvent dynamics effect is supposed to be unimportant in these cases, we can use the present theory within the effective ID model approach. The basic parameters are taken from the above reference except for the effective frequency. The results are shown in Fig. 26, which shows an excellent agreement with the experiment. The electronic couphng is quite strong and the perturbative treatment cannot work. The effective frequencies used are 1200, 950, and 800 cm for CH3CN, dimethylformamide (DMF), and PrCN [113]. 

Yonemura, H., Noda, M., Hayashi, K, Tokudome, H., Moribe, S. andYamada, S. (2002) Photoinduced intramolecular electron transfer reactions in fiillerene-phenothiazine linked compounds effects of magnetic field and spacer chain length. Mol. Phys., 100, 1395-1403. 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

Demchenko AP, Sytnik AI (1991) Solvent reorganizational red-edge effect in intramolecular electron transfer. Proc Natl Acad Sci USA 88 9311-9314... 

These TR results demonstrate that the localized model of Ru(bpy) + is valid on the timescales of electronic motions and molecular vibrations. It is virtually certain that delocalization (via, for example, intramolecular electron transfer or dynamic Jahn-Teller effects) occurs on some longer timescale. 

Patoux C, Coudret C, Launay J-P, Joachim C, Gourdon A (1997) Topological effects on intramolecular electron transfer via quantum interference. Inorg Chem 36(22) 5037-5049... 

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