N interactions

Molecular dynamics (MD) studies the time evolution of N interacting particles via the solution of classical Newton s equations of motion. 

Separation of enantiomers by physical or chemical methods requires the use of a chiral material, reagent, or catalyst. Both natural materials, such as polysaccharides and proteins, and solids that have been synthetically modified to incorporate chiral structures have been developed for use in separation of enantiomers by HPLC. The use of a chiral stationary phase makes the interactions between the two enantiomers with the adsorbent nonidentical and thus establishes a different rate of elution through the column. The interactions typically include hydrogen bonding, dipolar interactions, and n-n interactions. These attractive interactions may be disturbed by steric repulsions, and frequently the basis of enantioselectivity is a better steric fit for one of the two enantiomers. ... 

Fig. 4.8 The n -n interactions in E2N4S2 (E = CNMe2, PR2) and the dithionite dianion [S2O4]"-...

The intramolecular Se N interactions in complexes of the type 15.5 result in a downfield shift in the Se NMR resonances the magnitude of this shift corresponds approximately to the strength of the interaction. ... 

The methodology of heteroatom-directed lithiation has been applied to the synthesis of a variety of organochalcogen compounds, including unstable, low-valent compounds (Section 15.6). ° The Se N interactions in the diselenide 15.4 activate the Se-Se bond toward insertion of a sulfur atom or a methylene group to give the derivatives 15.21a,... 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

Scheme 15.2 Stereochemical conti ol under the influence of an Se N interaction...

Transannular Te N interactions have also been employed to stabilize compounds of the type 15.24 with terminal Te=E (E = S, Se) bonds.The Te=Se bond length in 15.24b is 2.44 A (cf. 2.54 A for a Te-S single bond) and d(Te N) = 2.62 A. Intramolecular coordination was also employed in the isolation of the first aryl-selenenium and -tellurenium cations 15.25a,b as [PEg] salts. 

Chiral organoselenenyl halides may also be stabilized by intramolecular Se N interactions Se NMR chemical shifts indicate that these interactions are maintained in solution. ... 

The partition function Q here describes the whole system consisting of N interacting particles, and the energy states Ei are consequently for all the particles (in Section 12.2 we considered N non-interacting molecules, where the total partition function could be written in terms of the partition function for one molecule, Q — /N[). More correctly... 

The brush-type (Pirkle-type) CSPs have been used predominantly under normal phase conditions in LC. The chiral selector typically incorporates tt-acidic and/or n-basic functionality, and the chiral interactions between the analyte and the CSP include dipole-dipole interactions, n-n interactions, hydrogen bonding, and steric hindrance. The concept of reciprocity has been used to facilitate the rational design of chiral selectors having the desired selectivity [45]. 

Small needle-shaped single crystals were examined by transmission electron microscopy (TEM) and electron diffraction (ED) (see Fig. 16-17). The results show that the crystals are elongated along the b-axis, which is the direction of weak intermolecular n-n interactions, and have a well-developed (ab) top surface. It corresponds to the surface of aliphatic tails (direction of weak intermolecular interactions). There are indications of displacement of successive ( / )-laycrs along the fl-axis, in line with the other signs of disorder in the aliphatic layer. 

The high enantioselectivity observed was interpreted in terms of the face selectivity of the (Z)-enolate 59 (Scheme 1.20). The phenyl moiety is thought to stabilize the enolate through a n-n interaction and effectively shield its Re face such that the incoming ketone approaches preferentially from the Si face. 

Cyclophanes or 7r-spherands have played a central role in the development of supramolecular chemistry forming an important class of organic host molecules for the inclusion of metal ions or organic molecules via n-n interactions. Particular examples are provided by their applications in synthesis [80], in the development of molecular sensors [81], and the development of cavities adequate for molecular reactions with possible applications in catalysis [82]. The classical organic synthesis of cyclophanes can be quite complex [83], so that the preparation of structurally related molecules via coordination or organometallic chemistry might be an interesting alternative. 

Due to the steric requirement of these substituents the formation of a columnar structure with infinite M M interactions is inhibited, and only the association of pairs of molecular units is allowed. The Ni Ni distance is 3.21 A [164]. If the same compound is crystallized in the presence of benzimidazole, the [Ni(dmg-BF2)2]2 dimer units are sandwiched between sheets of benzimidazole molecules due to n-n interactions resulting in an increased Ni Ni separation of 3.358 A [165]. With anthracene the n-n interactions seem to be stronger, because in this case the parent dimer molecule is cleaved. Each monomer now has a conformation of type B (Fig. 32) and is sandwiched by anthracene molecules [166]. Compound 121 has the same configuration [163d]. 

The alkene substituted with the electron accepting group has the LUMO (it ) lowered by the interaction with the vacant orbital of the substituent. The high-lying SOMO interacts with the LUMO of the alkene more effectively than with the HOMO. The interaction is the symmetry-allowed it - n interaction (Scheme 30a). The configuration of the alkene is retained. 

The alkene with the electron donating group has the HOMO (n) raised by the interaction with the occupied orbital of the substiment. The low-lying SOMO (n ) interacts with the HOMO of the alkene more effectively. The frontier orbital interaction is the interaction (Scheme 30b), which is impossible at the four-membered... 

We pointed out that these results can be attributable to the a-n interaction. At the transition state, the o orbital at C5 on the anti side of the dienophile is parallel with the n orbital, the o bond electrons are able to delocalize much more effectively than that on the syn side. Since the electron donating o bond on the anti side stabilizes the transition state, the a-n interaction can contribute to rr-facial selectivity. These results suggested that the bond lengthening cannot necessarily be convincing evidence for the Cieplak effect, but can be explained in terms of the a-n interaction without assuming the incipient a bonds at the transition state (Scheme 30). 

Scheme 30 c-n Interaction at the starting diene and the transition state... 

Another type of dimer is that which consists of two radical molecules stacked on each other in a n-n interaction. Such dimers have been observed e.g., with 9-ethylphenazyl radical, tetramethyl-p-phenylenediamine cation radical (167), 7,7,8,8-tetracyanoquinodimethane radical anion (168), methylviologen cation radical (169), and l-alkyl-4-carbomethoxypyridinyl radicals (170). Attempts have been reported (170, 171) to interpret the electronic spectra of dimers of this kind by MO calculations. 

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