Double bond overlapping effect

Different from the correction for differences of the number of carbon atoms between individual species of a lipid class, correction for another isotope effect on quantification due to the double-bond overlapping effects (see below) has to be performed for those utilizing unit-resolution mass spectrometers. For those who utilize high-resolution mass spectrometers for quantification, another approach to extract ion peak intensity from a partially overlapped ion peak can be consulted as described [33]. Herein, the double-bond overlapping effect refers to the overlap or partial overlap between the two atom-containing isotopologue of a species M (i.e., M-i-2 isotopologue) and the ion of a species with one less double bond than M. 

The double-bond overlapping effect is common and severe when low-to-moderate resolution mass spectrometers are used for analysis of lipids by either shotgun lipidomics or an LC-MS approach. In the latter case, this effect is present unless resolving individual species of a class can be achieved. Due to the presence of this overlapping effect, the measured monoisotopic peak intensity of a species may not represent the true monoisotopic peak intensity of the species. Therefore, prior to performing de-isotoping as represented by Equation 15.6, correction for the double-bond overlapping effects has to be performed as follows ... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The cis stereoisomer exhibits groups on the same side of the double bond, while the tram stereoisomer exhibits groups on opposite sides of the double bond. The two drawings above represent different compounds with different physical properties, because the double bond does not experience free rotation as single bonds do. Why not Recall that a it bond is formed from the overlap of two p orbitals (Figure 5.2). Rotation about the C—C double bond would effectively destroy the overlap between the p orbitals. Therefore, the C—C double bond does not experience free rotation at room temperature. 

The bonding in conjugated dienes is not so obviously exotic as it is in cumulated dienes, such as the allenes. Nevertheless, there are some subtle effects with far-reaching implications. The source of these effects can be seen from simple, schematic orbital drawings of 1,3-butadiene and 1,4-pentadiene. In a conjugated diene, the orbitals of the two double bonds overlap in an unconjugated diene, they do not (Fig. 12.14). 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

Atoms of the Period 2 elements C, N, and O readily form double bonds with one another, with themselves, and (especially for oxygen) with atoms of elements in later periods. However, double bonds are rarely found between atoms of elements in Period 3 and later periods, because the atoms are so large and bond lengths consequently so great that it is difficult for their p-orbitals to take part in effective side-by-side overlap. 

These results have been interpreted in terms of HOMO-LUMO interactions. As a result of the orbital perturbation, the interaction of the HOMO of the cyclohexene double bond with the LUMO of the developing cation may become effective. At the first stage of this interaction, an overlap of the LUMO of the cyclobutyl cation with the p lobe of the double bond located close to the cation center is probably important. However, when the reaction progresses, the interaction with the p lobe on the remote carbon atom has been assumed to increase significantly. 

When we consider double bonds to oxygen, as in carbonyl groups (C=0) or to nitrogen, as in imine functions (C=N), we find that experimental data are best accommodated by the premise that these atoms are sp hybridized (Figure 2.22). This effectively follows the pattern for carbon-carbon double bonds (see Section 2.6.2). The double bond is again a combination of a ct bond plus a jt bond resulting from overlap of p atomic orbitals. The carbonyl... 

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