Carbonates dipole moments

Values are from References 24, 25, 27, 51 and 53. Those in italics are estimates, and / Me parameterize the bond moments of the X-C(sp2) and X-C(sp3) bonds, respectively i values reported as 0/1 take the value 1 when bonded to sp2-hybridized carbon and 0 when bonded to sp3-hybridized carbon. Dipole moments for alternating dienyl and polyenyl groups are assumed to be approximately equal to those for the 1- and 2-(l,3-butadienyl) groups. 

The dipole moment (DM) showed a poor correlation (r < 0.49) for alcohols, all aromatics, and all halogenated aliphatics however, for small molecules (two carbons), dipole moment correlations were significant for halogenated aliphatics and nitriles, as shown in Equation (5.15). 

Figure 24. The direction of the nitrogen-carbon dipole moment.

Table 2.4 NBO charges of metal and interacting carbons, dipole moment (Debye) tmd HOMO- ...

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

Use the same method to calculate the dipole moment of cyclopentadienone,. Assume, for the calculation, that the endocyclic double bonds are parallel and the angle at carbon 2 is the same as in eyelopropenone. 

Calculate the bond length, ioni7 ation potential, and dipole moment of carbon monoxide by MNDO,. AMI, and PM3,... 

Many molecules, such as carbon monoxide, have unique dipole moments. Molecules with a center of inversion, such as carbon dioxide, will have a dipole moment that is zero by symmetry and a unique quadrupole moment. Molecules of Td symmetry, such as methane, have a zero dipole and quadrupole moment and a unique octupole moment. Likewise, molecules of octahedral symmetry will have a unique hexadecapole moment. 

The direction of the dipole moment is toward the more electronegative atom In the listed examples hydrogen and carbon are the positive ends of the dipoles Carbon is the negative end of the dipole associated with the C—H bond... 

We can combine our knowledge of molecular geometry with a feel for the polarity of chemical bonds to predict whether a molecule has a dipole moment or not The molec ular dipole moment is the resultant of all of the individual bond dipole moments of a substance Some molecules such as carbon dioxide have polar bonds but lack a dipole moment because their geometry causes the individual C=0 bond dipoles to cancel... 

Carbon tetrachloride with four polar C—Cl bonds and a tetrahedral shape has no net dipole moment because the result of the four bond dipoles as shown m Figure 1 7 is zero Dichloromethane on the other hand has a dipole moment of 1 62 D The C—H bond dipoles reinforce the C—Cl bond dipoles... 

FIGURE 1 7 Contri bution of individual bond dipole moments to the mo lecular dipole moments of (a) carbon tetrachloride (CCy and (b) dichloro methane (CH2CI2)... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

A particular vibration will give an absorption peak in the IR spectrum only if the dipole moment of the molecule changes dunng the vibration Which vibration of carbon dioxide the sym metric stretch or the antisymmetric stretch is infrared active 2... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

Because carbon is sp hybridized m chlorobenzene it is more electronegative than the sp hybridized carbon of chlorocyclohexane Consequently the withdrawal of electron density away from carbon by chlorine is less pronounced m aryl halides than m alkyl halides and the molecular dipole moment is smaller... 

The carbon-bromine bond is longer than the carbon-chlorine bond therefore although the charge e in the dipole moment expression p, = e d k smaller for the bromine than for the chlo nne compound the distance d is greater... 

Because of point 2, rotational microwave and millimetre wave spectroscopy are powerllil techniques for determining dipole moments. However, the direction of the dipole moment cannot be determined. In the case of 0=C=S, for which /r = 0.715 21 0.000 20 D [(2.3857 0.0007) x 10 ° C m], a simple electronegativity argument leads to the correct conclusion - that the oxygen end of the molecule is the negative end of the dipole. However, in CO, the value of 0.112 D (3.74 x 10 C m) is so small that only accurate electronic stmcture calculations can be relied upon to conclude correctly that the carbon end is the negative one. 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

Electron-density calculations for quinazoline (which has no symmetry) vary markedly with the method used. The diagram (6) has the same bases as that given for pyrimidine above it will be observed that the 2- and 4-positions in quinazoline are comparable with the corresponding positions in pyrimidine and that the aromatic carbon atoms (C-5-C-8) in quinazoline are roughly comparable with C-5 in pyrimidine (67MI21300). The dipole moment of quinazoline does not appear to have been measured, but that of 2-methylquinazo-line is 2.2 D. 

Ethylene, /3-(dimethylamino)-nitro-in pyrrole synthesis, 4, 334 Ethylene, dithienyl-in photochromic processes, 1, 387 Ethylene, furyl-2-nitro-dipole moments, 4, 555 Ethylene, l-(3-indolyl)-2-(pyridyl)-photocyclization, 4, 285 Ethylene, l-(2-methyl-3-indolyl)-l,2-diphenyl-synthesis, 4, 232 Ethylene, (phenylthio)-photocyclization thiophenes from, 4, 880 Ethylene carbonate C NMR, 6, 754 microwave spectroscopy, 6, 751 photochemical chlorination, 6, 769 synthesis, 6, 780 Ethylene oxide as pharmaceutical, 1, 157 thiophene synthesis from, 4, 899 Ethylene sulfate — see 2,2-dioxide under 1,3,2-Dioxathiolane... 

In each case the left-hand atom of the pair as written is the least electronegative. Since dipole moments have direction as well as magnitude it is necessary to add the moments of each bond vertically. For this reason the individual dipole moments cancel each other out in carbon tetrachloride but only partially in chloroform. In other molecules, such as that of water, it is necessary to know the bond angle to calculate the dipole moment. Alternatively since the dipole moment of the molecule is measurable the method may be used to compute the bond angle. 

For most purposes, hydroearbon groups ean be eonsidered to be nonpolar. There are, however, small dipoles associated with C—H bonds and bonds between earbons of different hybridization or substitution pattern. For normal sp earbon, the earbon is found to be slightly negatively charged relative to hydrogen. The electronegativity order for hybridized carbon orbitals is sp > sp > sp. Scheme 1.1 lists the dipole moments of some hydrocarbons and some other organic molecules. 

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