Lone pair effects

Keywords tr-Relaxation o-Relaxation Geminal interaction Inverted bond Lone pair effect Orbital phase continuity Ring strain... 

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

Tetraphosphabicyclo[1.1.0]butane 58 (Scheme 24) has been reported [60, 61]. The SE is much higher than cyclotriphosphane 14 or P (see Sect. 3.3.4), but stiU lower than bicyclo[1.1.0]butane 45 [62, 63], in agreement with the lone pair effect (see Sect. 2.1.4). Interestingly, the bridgehead bonds have been observed to be shorter than a normal P-P bond (2.218 A in diphosphine) [64]. The related... 

We designed less strained analogs of 61a-c by applying the lone pair effects (Sect. 2.1.4) [68]. Substitution of carbons with phosphorus atoms relaxes the strain (Scheme 28). Notably, substitution at the 3- and 4-positions in tricy-clo[2.1.0.0 ]pentane resulted in greater relaxation than substitution at the 2- and 5-positions. 

Scheme 28 The lone pair effect for the designing of less strained molecules...

P3H314 and PjH 77 molecules are less strained than the nitrogen congeners. As mentioned earlier (Sect. 2.1.4), the lone pair on phosphorus atoms is richer in s-character. The lone pair effect on o-relaxation is more remarkable. 

This effect of the position of the lone pair is relevant not only in the case of phthalocyaninato ligands, but also can be a clue to the intriguing behaviour of the [Dy(DOTA)] complex, where the rotation of a water molecule changes the magnetic properties [13]. A PCE or an REC model cannot account for the effect of such a rotation, but an LPEC model would predict a dramatic effect, since the change in the position of the lone pair effectively means a completely different geometry. 

Carbon-hydrogen spin-spin coupling constants are included in a review by Hansen and discussed in terms of ring strain, steric effects, electronegativity, lone pair effects and electric field effects. Additivity of these effects in a few systems is commented on. 

One-bond carbon-carbon spin-spin coupling constants are included in a review by Krivdin and KalabinR". They are analyzed in terms of hybridization, substitution effects, lone pair effects and steric effects as well as respective applications to structural determination. The carbon-carbon spin-spin coupling constants between carbons that are separated by more than one bond were reviewed by Krivdin and DeUa and are discussed in terms of experimental techniques, the effects of hybridization, substituent effects, steric effects and respective additivity patterns. 

Nitrogen lone pair effects on /HC values indicate aldimine and oxime configuration (see Section 4.1.1.2.). 

Reinterpretation of apparent kinetic antiperiplanar lone pair effects in terms of the principle of... 

Figure 22. Lone-pair effects of TBs 95-98 on methylene chemical shifts...

---