Pi interactions

SDS solutionsThese cation-pi interactions have been observed in many different fields in... 

The inability of a single value to describe the behavior of the SMe substituent may be the consequence of two pi interaction mechanisms at sulfur (14,21) ... 

We infer from the results of our statistical analysis that the four scales of Table V are definitive and characteristic of the substituent interacting within a given pi framework. The pi interaction frameworks suggested are as follows... 

Fig. 36. Spatial fit between host and guest in 47 benzene (1 1) (see Ref. 64) (a) Electron density in the mean plane of a benzene revealing the encasing hexagonal environment aroung the guest and (b) and (c) van der Waals surfaces of the dimeric benzene units as seen in Fig. 27 (indicated dimensions were calculated by the aid of the CHEM X program system, see Ref. 139). The lack of extensive-enough overlapping to yield in pi-pi interactions is visible from this drawing...

Fig. 51 Phase diagram for PS-PI diblock copolymer (Mn = 33 kg/mol, 31vol% PS) as function of temperature, T, and polymer volume fraction, cp, for solutions in dioctyl ph-thalate (DOP), di-n-butyl phthalate (DBP), diethyl phthalate (DEP) and M-tetradecane (C14). ( ) ODT (o) OOT ( ) dilute solution critical micelle temperature, cmt. Subscript 1 identifies phase as normal (PS chains reside in minor domains) subscript 2 indicates inverted phases (PS chains located in major domains). Phase boundaries are drawn as guide to eye, except for DOP in which OOT and ODT phase boundaries (solid lines) show previously determined scaling of PS-PI interaction parameter (xodt

Comparison with (3.156) shows that F(NBO) is intrinsically of significantly higher accuracy than h(HMO) for describing the actual pi interactions of benzene. Because F(nbo) js tjje fundamental starting point for localized NBO analysis of conjugafive interactions, we can conclude that the NBO donor-acceptor picture is inherently more accurate than that based on the Hiickel tight-binding approximation. 

The final class of inhibitor to be described contains no electrophilic warhead to interact with the sulfhydryl group of the active site cysteine. The binding affinity of these non-covalent, competitive inhibitors is partly achieved through lipophilic PI interactions of an aminoethylaniline moiety [68]. Electron-donating substituents on the aniline are required for potency against Cat K [7]. 

We first consider the case of conformational isomerism of 1,3-butadiene, the simplest 4 N pi electron molecule. We will discuss the relative stabilities of three important points on the rotational surface the cis isomer (6 = 0°),thegauche isomer (6 = 45°) and the trans isomer (6 = 180°). We shall focus our attention entirely on the effect of pi interactions on conformational preference. 

We focus our attention entirely upon the influence of pi interactions on conformational preference. 

As a general rule, pi bond orders, pi overlap populations and total pi overlap populations allow a diagnosis of the type of pi interactions which obtain between two AO s. The situation is much more uncertain in the case of sigma AO interactions, and one has to rely upon bond orders and/or overlap populations. 

On the basis of a previous discussion, we predicted that pi nonbonded interactions will produce an order of stability of gauche > tram > cis, assuming appreciable Ci—C4 pi overlap in 1,3-butadiene. However, if sigma nonbonded interactions dominate pi interactions the order of stability will be cis > gauche > tram. Finally, in the event that steric effects are the controlling factor the order of stability will become trans > gauche > cis. 

We have already discussed the structural effects of n—n interactions in Part. II. The reader is referred to Section 3.15 for a discussion of pi interactions in butadiene which represents a model system for these interactions. 

Rahman, M.M., Takafuji, M., Ansarian, H.R., and Ihara, H., Molecular shape selectivity through multiple carbonyl-pi interactions with noncrystalline solid phase for RP-HPLC, Anal. Chem., 11, 6671, 2005. 

Takai A, Chkounda M, Eggenspiller A et al (2010) Efficient photoinduced electron transfer in a porphyrin tripod-fullerene supramolecular complex via pi-pi interactions in nonpolar media. J Am Chem Soc 132 4477-4489... 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

Ikeda, A., Tsuzuki, H., Shinkai, S. 1994. X-ray crystallographic studies of a 1,3-al-temate-calix[4]arene Na+ complex - Is the cation-pi interaction operative between the benzene rings and Na+ Tetrahedron Lett. 35 8417-8420. 

Gokel, G. W., Barbour, L. J., Ferdani, R., Hu, J. X., Lariat ether receptor systems show experimental evidence for alkali metal cation-pi interactions. Acc. Chem. Res. 2002, 35, 878-886. 

Meadows, E. S. DeWall, S. L. Barbour, L. J., Gokel, G. W., (2001) Alkali metal cation-pi interactions observed by using a lariat ether model system J. Am. Chem. Soc. 123, 3092-3107. 

Beene DL, Brandt GS, Zhong W, Zacharias NM, Lester HA, Dougherty DA. Cation-pi interactions in ligand recognition by serotonergic (5-HT(3A)) and nicotinic acetylcholine receptors the anomalous binding properties of nicotine. Biochemistry 2002 41(32) 10,262-10,269. 

The essential realization in this spontaneous ordering process is the importance of noncovalent bonding interaction between molecules, that is, supramolecular chemistry. These conformation-specific interactions are governed by weak forces including hydrogen bonding, metal coordination, van der Waals forces, pi-pi interactions, and electrostatic Coulombic effects. The cooperative action of multiple noncovalent interaction forces is precisely the path nature takes to produce shape and form. 

The optimum binding energies for each of the five positions described above are shown in Table II. It is clear that the formation of C-O-AI complex (position 5, a = 0°) is favored. The AI-0 distance is found to be 1.80 A. (We should stress here that the C-0 distance is not reoptimized after bonding to Al.) In Table 2, we also show the vibrational frequency calculated, using the harmonic oscillator approximation, for the AI-0 streching in position 5. Its value is 890 cm 1. Pireaux et al (15), using HREELS techniques to study the AI/PI interaction, found that the vibration at 1720 cm 1 associated with C = 0... 

Crown Ethers as Tools to Explore Cation-Pi Interactions 809... 

Heteromacrocycles Used to Study Double and Triple Bond Cation-Pi Interactions 813... 

The two center structures show the complex that forms between iV,iV -bis(2-phenylethyl)-4,13-diaza-18-crown-6 and KI <2000PNAS6271>. The K+ ion is bound in the center of the macroring, as expected for any 18-crown-6 macrocycle. The twin sidearms of the bibracchial lariat ether turn inward in this complex and the arenes serve as apical donors. The top center structure shows the symmetrically bound cation and illustrates that the iodide anion is excluded from the cation s solvation sphere. The bottom center structure shows the superimposition of the two benzene rings upon each other and upon the K+ ion. Note in the bottom center structure that the iodide anion is not illustrated. The ideal sandwich of arene-cation-arene confirms the cation-pi interaction between benzene and K+. 

In tryptophan, the indole residue is attached to the amino acid s side chain at indole s position 3. When the point of attachment was moved to indole position 5, however, macrocycle 13 complexed NaBPli4 in the pi-fashion but benzene rather than pyrrole was clearly the donor group <2002JA10940>. One may conclude that 12 is a less reasonable model than is 13. In fact, both unequivocally confirm the cation-pi interaction. Further, the fact that cation-pi interactions are prominent in both cases means that in the natural environment, the rarest of the 20 common amino acids is a truly versatile pi donor. 

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