Moment data, dipole

Dipole moment data have provided valuable information for the study of the tautomerism of compounds such as isonicotinic acid, pyrid-4-one, and ethyl acetoacetate, However, this method must be used with discretion since it can lead to inconclusive results. Thus, the fact that 4-aminopyridine has a higher dipole moment than the algebraic sum of the dipole moments of pyridine and aniline was originally interpreted as proof that structure 54 exists with a strong contribution from 36, and it was stated that 55 w ould have a very low moment. Later, Angyal and AngyaF pointed out that the... 

The ultraviolet spectra of the pyridinecarboxylic acids (334) were initially interpreted assuming that the proportion of the zwitterion structure 335 was not appreciable,and the early pK work was inconclusive. However, Jaffe s calculations based on the Hammett equation indicated that about 95% of nicotinic and isonicotinic acids existed in the zwitterion form, and ultraviolet spectral data showed that the actual percentages of picolinic, nicotinic, and isonieotinie acids existing in the zwitterion form in aqueous solution are 94, 91, and 96%, respectively.This was later confirmed by Stephenson and Sponer, who further demonstrated that the proportion of the zwitterion form decreases in solvents of low dielectric constant, becoming very low in ethanol. Dipole moment data indicate that isonicotinic acid exists as such in dioxane, and 6-hydroxypyridine-3-carboxyiic acid has been shown to exist in form 336 u.sing pK data. ... 

Tetronic acids exist predominantly in the dioxo form (32) in solvents of low polarity, while the existence of the monoenol form (33) has been established in other solvents by infrared " and ultraviolet spectral comparisons- and from dipole moment data. " Haynes and Pliramer " have recently reviewed the structure of these compounds [see also reference 28(a)], and the tautomerism of vitamin A (34), which has a related structure, has also been surveyed.- Analogous compounds carrying an amino group in the 3-position are also know n. ... 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

Benzothiazole-2-thiones are associated,but the predominance of the thione forms (154, R = H, Me) has been demonstrated by ultraviolet spectral comparisons wdth the methylated derivatives of both possible tautomeric forms (cf. reference 182), comparative dipole moment data, and infrared spectral data. " Ultraviolet spectroscopy has also been used to demonstrate that benzoseIenazole-2-thione exists as such. The predominance of the thione form of nonaromatic 3,4-dihydrothiazole-2-ones (155) has also been established on the basis of ultraviolet spectral evidence. ... 

A comparison of MP2/6-31G structural parameters of 1,2-oxazole 19 (isox-azole) and 1,3-oxazole 20 with microwave data is provided by Kassimi et al. (Scheme 16) [96JPC8752]. The general agreement is excellent. The same authors investigated dipole moments, quadrupole moments, octopole moments, and dipole polarizabilities of 19 and 20 together with several oxadiazoles and oxatriazoles [96JPC8752, 99JPC(A) 10009]. For the mean polarizability of these species, they found the approximative formula... 

The results obtained for the correlation of a and 0 values of sets 2-1 through 2-4 are not significant. Thus, eqs. (21) and (22) are not obeyed in the case of dipole moment data. 

Dipole moments of substituted nitriles were correlated with the a constants by Taft and with the Op constants by Charton (18). In addition to dipole moment data, some information is available on the C=N stretching frequencies of substituted nitriles. The sets studied are reported in Table XXXVI. Results of the correlations with eq. (2) are given in Table XXXVII, and values of Pr are set forth in Table XXXVIII. The correlation of the dipole moments of substituted nitriles with eq. (2) gave significant results, which were very much improved by the exclusion of the value for X=I (set 36-1-2). In contrast to the results obtained for substituted ethylenes, acetylenes, and benzenes, the value of /3 obtained for dipole moments of substituted nitriles is not significant. The value of a obtained for the substituted nitriles is comparable to the value of a observed for the substituted acetylenes. 

Table 1 Ionization potential and dipole moment data for compound 26 <1997JFA2345>...

Both 1,2- and 1,4-pyrones (91 and 92, respectively) should be mentioned. It also seems appropriate to mention oxygen derivatives of this type, because some properties of these molecules are to a large extent determined by their topology. Properties of the C=Y bonds, the heats of combustion, the UV and IR spectra,88 and dipole moment data... 

Unfortunately, although the equilibria for these substituted derivatives were interpreted correctly, for the parent 1,2-dimethylhexahydropyridazine the use of dipole-moment data led to erroneous results. It was first thought... 

Fig. 12. Plots of -7sem against percentage axial lone-pair conformer in 3-alkyltetrahydro-1,3-oxazines. , values for percentage axial lone pair from dipole-moment data.44 A, values for percentage axial lone pair from l3C-NMR measurements.96 J>tm for the iPr compound is now96 given as -8.5 Hz, but no figure for percentage axial lone pair based on C-NMR measurements is available an estimate of 90% has been taken in the figure.

The difference in conformational free energy of the N-Me group in N-methylpiperidine (2.7 kcal mol-1, favoring N-Meeq) and in N-methyltetra-hydro-l,3-thiazine (0.7 kcal mol-1, favoring lV-Meax at -120°C) suggests the predominance of the lV-Meax conformer 463 of 5-methyldihydro-1,3,5-dithiazine (462 R = Me). Indeed, H-NMR spectra and dipole-moment data show that NR axial is the predominant conformer for a series of AT-alkyl-dihydro-l,3,5-dithiazines (462 R = Me, Et, iPr, -Bu).356 This even applies to the Af-tert-butyl compound, which possesses an unconstrained axial tert-butyl group.357... 

By analogy with the tetrahydro-l,3,5-oxadiazine system, the ae conformer 466 of 3,5-dialkyltetrahydro-l,3,5-thiadiazines (465) should predominate in the conformational equilibrium. Investigation of a series of 3,5-dialkyl derivatives by H-NMR disclosed ring-inversion barriers that decrease with increasing size of the substituent (465 R = Me, AG 12.1 + 0.3 R = Et, AG 12.0 + 0.2 R = iPr, AG 10.5 + 0.4 kcal mol ). No further changes in the H-NMR spectra on cooling were found. Dipole-moment data indicated that all the compounds existed predominantly in the ae conformation.356 It is difficult to draw conclusions for Jtem in this series. 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

Chemical Class Dipole Moment Data Points Ehomo Data Points Elumo Data Points Log P Data Points Hammett Constant Data Points... 

The gas-phase dipole moment of arsabenzene was found to be 1.10D 45). In cyclohexane solution it was measured as 1.02D27). These values are typical of those found for tertiary arsines for trimethylarsine p = 0.86D76), for triethylarsine p = 1.04D77) and for triphenylarsine p = 1.23D78). No dipole moment data are available for stibabenzene and bismabenzene. However, based on trends shown in acyclic compounds 79), it is expected that they have smaller moments. 

The formulation of a mesoionic structure for these types of compounds is strongly supported by spectral data, dipole moments and X-ray crystallography. 

Although a number of octahedral complexes have been isolated in both cis and trans modifications, confirmations of configurations using dipole moment data have not been carried out. A large majority of such com-... 

The ratio of yHF to yDF was assumed to be 21/4 in deriving Eqn. (10). For yHF — 20-30°,Eqn.(10)givesadipolemomentdifferenceof0.03-0.08 D, as observed. Thus, the dipole moment data and the hyperfine interactions support a nearly linear equilibrium hydrogen bond, with large amplitude vibrational displacements from equilibrium. It must be emphasized that none of the experimental evidence discussed so far, including this, precludes the hydrogen bond from having a small departure, on the order of 5°, from linearity at equilibrium. 

To simplify the Hartree-Fock problem, Pople introduced CNDO/1 (1965), then CNDO/2 (1967), and then INDO (1967) to yield computer programs that mimic ab initio programs with a minimum of fuss. Jaffe66 improved CNDO to fit spectroscopic absorptions (with a minimum of Cl) this was CNDO/S (1968). Later, Dewar67 introduced MINDO/3 (1975), then MNDO (1977), AMI (1985), and PM3 (1989). For transition metals, Zerner68 introduced ZINDO (1984) these were progressive improvements on INDO, but parameterized to fit thermochemical data, dipole moments, absorption spectra, and so on, to the fitful extent that they are available from experiment. 

Dipole moment data also indicate160 that the line of action of the moment in imidazole makes an angle of 13° with the A axis (where the proposed A axis makes an angle of 28° with the N-H bond and intersects the N-C bond joining the 1- and 2-positions). Rotation constants and moments of inertia determined by the same workers169 indicate that the molecule is planar. 

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