Density shifts

Figure 6.11 A comparison of inductive stabilization for methyl, primary, secondary, and tertiary carbocations. The more alkyl groups there are bonded to the positively charged carbon, the more electron density shifts toward the charge, making the charged carbon less electron-poor (blue in electrostatic potential maps).

C17-0088. Draw Lewis structures of the acids in Problem (b) and (c), and use arrows to show electron density shifts that account for their different acid strengths. 

At the platinum electrode the individual steps of the four-electron reaction cannot be studied separately. Slope b has its usual value of about 0.12 V, but in contrast to what is seen at the mercury electrode, the polarization is practically independent of solution pH (i.e., the potential at a given current density shifts by 0.06 V in the negative direction when the pH is raised by a unit). It follows that the reaction rate depends on hydrogen ion concentration. The step in which an electron and a proton are transferred while the 0-0 bond is broken is probably the ratedetermining step. 

Fig. 1 N-formyltryptophanamide used for QM calculations on a tryptophan trimmed from a protein structure, also showing the two kinds of electron density shifts that control Trp fluorescence wavelength (red) and intensity/lifetime (green)...

In a completely different interpretation Zefirov (242) proposed a new concept of frontier-orbital mixing (243) to explain how conformational and electronic effects in monosubstituted cyclohexanes are transmitted to remote 8-carbon atoms (Scheme 36). The orbitals at C(l) and C(4) in 112 are considered to be equatorial (242). A perturbation at C(l) (H is replaced by X) produces an electron-density shift from H(4) toward C(4) (242), which is associated with an upheld shift of the latter s signal. Although this approach appears to be quite crude and does not account for axial substituents, it deserves fiirther attention. 

Ab initio calculations at the HF/6-31G level have been used to explore energy changes, structural variation, and electron density shifts during jr-face selective addition of substituted acetylide ions to cyclohexanone and cyclohexanethione. Charge polarization of the jr-bond on approach of the nucleophile is such that the carbonyl carbon becomes considerably electron deficient for most of the reaction path (and may... 

The 4-oxo-l,3-oxazinium salts and their benzo derivatives are characterized by a very strong band at 1730-1770 cm attributed to C = 0 bond vibrations. Such an abnormally high frequency was explained by an increase in the C = 0 bond order due to an electron-density shift from the carbonyl oxygen to the ring (80TH1). 

From these data, there is now good evidence that PA is electronically coupled to Pb and is thus involved in P7oo- The decrease in spin density on PB (Table 5) in all threonine mutants can be interpreted as a spin density shift from Pb towards PA. The effect of H-bonding to P7q0 shows the same characteristics as for QA in... 

As the halogen becomes more electronegative, an increasing amount of electron density shifts from the O-H bond toward the halogen, thus weakening the O-H bond and increasing its polarity. As a result, the proton is more easily transferred to a solvent water molecule, and so the acid strength increases. 

Egan, P.O., Hughes, V.W. and Yam, M.H. (1977). Precision determination of the fine-structure interval in the ground state of positronium. IV. Measurement of positronium fine-structure density shifts in noble gases. Phys. Rev. A 15 251-260. 

Every chemist is trained in conceptualizing reactions and rearrangements by pushing arrows to track the movement of electrons in the bonds. This is a depiction that chemists have long found useful. Of course, electron pairs do not really move around like this. In the realm of physical reality, interatomic distances lengthen and shorten, and electron density shifts in the course of a reaction, but no electron pairs hop among the bonds and atoms. Quantum mechanics works. 

TABLE 2. Melting points, boiling points, densities, shifts of the most important chlorosilanes20-22 refractive indices and 29Si NMR chemical... 

Recently, the trimer theory has been used for the interpretation of the optical properties of the. Ymethylthiouronium salt [(MT)2(TCNQ)3 2H2Oj [67,68], The dominant feature of the polarized reflection spectrum cf (MT)2(TCNQ)3 2H20 (Fig. 13) is a broad intensive band of electronic reflection, with a sharp edge and low minimum at the frequency co = 9090 cm The intensive structure observed in the middle IR range (lines lto 8) is attributed to the e-mv coupling. Lines 2 and 4 have a fine structure which could be understood if one takes into account the equilibrium charge density shift pb = 0.25e and 3g = 0.15< in the two halves of TCNQ ( 3). 

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