Characteristic infra-red frequencies and chemical properties of molecules

The integrated intensities, frequencies, and half-band widths of the fundamental sym~netric and asynlmetric NH2 stretching vibrations in 33 ortho-substituted anilines, lneas~~red in dilute carbon tetrachloride solution, have been examined in relation to the corresponding absorption band parameters for 31 meta-and para-substituted anilines, talcing into consideration the electronic effects of the substituents. From an almost tetrahedral configuration in $-phenylenediamine, the calculated s-character of the nitrogen atom gradually increases as the substituent groups become inore electron withdrawing, with a resultant increase ill the apparent HNI-I angle and the KII force constant. Ortho substitution in general leads to enhanced H N H angle opening, probably because of intrarllolecular hydrogen bonding. The decrease in half-bancl width for both vibrational modes in o r t h o -s~~b s t i t~~t e d anilines with respect to corresponding values in meta and para compounds is ascribed to steric hindrance to solvation of thc amino group.The asymmetric intensities in ortho-substituted anilines are generally i~lcreased over corresponding v a l~~e s in meta and para compounds, unlike the behavior of the symmetric mode. These results are consistent with a vibrational nlechanisill taking into acco~lnt the following factors for each mode: (i) the direction of the transition moment, (ii) the estent of nitrogen lone pair and aromatic T-electron participation, and (iii) the direct field effect of the ortho substitucnt. ISTRODUCTION

Section: Istroductionmentioning

confidence: 99%

THE VIBRATIONAL MECHANISM OF THE FUNDAMENTAL NH₂ STRETCHING VIBRATIONS IN ANILINES

Krueger¹

1962

“…Not only are such frequency changes observed, but both the 0-H and C=O stretching frequencies of benzoic acids in carbon tetrachloride have been shown to be linearly related to the Hammett cr-constants, and therefore to the pK's of the acids in water (1)(2)(3)(4)(5)(6)(7). This is somewhat surprising, since ionization constants reflect substituent effects on the free-energy difference between the acid and its anion, both solvated by water.…”

mentioning

confidence: 99%

Infrared spectra of aqueous sodium benzoates and salicylates in the carboxyl-stretching region: chelation in aqueous sodium salicylates

Dunn¹,

McDonald²

1969

Infrared spectra in the frequency region 1300-1760cm-' are reported for 41 substituted sodium benzoates and 10 substituted sodium salicylates in deuterium oxide solution, and for 9 substituted salicylic acids in chloroforn~ and carbon tetrachloride solutions. Carboxylate stretching frequencies of benzoates and salicylates correlate poorly with substituent constants, but the asymmetric frequencies of benzoates and salicylates correlate well with each other, and the asymmetric frequencies of benzoates correlate well with the asymmetric frequencies of the corresponding nitrobenzenes. It is suggested that, among substituted aromatic compounds, group vibrations which couple with the ring vibrations may correlate well with similar vibrations of other groups, but not with coupled vibrations of different symmetry, uncoupled vibrations, or substituent constants. Chelation in chloroform solutions of salicylic acid dimers can be detected by its influence on substituent effects, but infrared spectra provide no good evidence for chelation in aqueous sodium salicylates.

“…A number of empirical relations between molecular vibrational frequencies and some property of a substituent atom or radical in the molecular system have recently been reported (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), the most common properties used being Hammett's o-constant (1-7), electronegativity (10)(11)(12)(13)(14)(15)(16)(17), and ionization potential (18). However, since all these properties decrease down any column of the periodic table, while any vibrational frequency (for example, vco in the system X?C=O) will decrease as a substituent (X) in the n~olecular system is altered down the periodic table, due only to the increasing mass, care must be taken when expressiilg these empirical relations to ensure that they are indeed real.…”

mentioning

confidence: 99%

Sensitive Vibrational Frequencies: Iv. Mass Effects

Wilmshurst¹

1959

The correlation of vibrational frequencies with atomic properties of substituent atoms in the molecular system is critically examined for reality in the light of possible mass effects. The general rule is formulated that only frequencies continuously characteristic of a grouping in the mass–frequency plot can be used in such correlations. Calculated mass–frequency plots are presented for the systems XC≡N, XC≡CX, XC≡CY, X2C=CX2, X2C=O, X2P=O, XCH3, X2CH2, X3CH.