Spectroscopic studies on N,N-dimethylamides—III Amide-rotational barriers of N,N-dimethylbenzamides and -cinnamamides. A comparison of the NMR intensity ratio- and iterative total line shape method

Amide-rotational barriers of a series of para-substituted N,N-dimethylbenzamides and -cinnamamides have been obtained by both the intensity ratio method, according to Rogers and Woodbrey,² and an iterative total line shape analysis. From a comparison of the results, it is concluded that a discussion of rotational barriers obtained with the intensity ratio method should preferably be based on ΔG values, which are nearly equal for both methods, rather than on ΔG^? at other temperatures.     

Spectroscopic studies on N,N-dimethylamides—IV An NMR total line shape study of substituent effects and medium effects on amide-rotational barriers of N,N-dimethylbenzamides and -cinnamamides

Amide-rotational barriers obtained with a total line shape analysis of a series of para-substituted N,N-dimethylbenzamides and -cinnamamides are correlated with the result of an H. M. O. (Ω = 1·4) calculation. There is no difference in the activation parameters at a concentration of 0·25 M and 1 M in CDCI₃ as a solvent. Despite the lower solvent polarity of chloroform (ε = 4·7) compared with acetonitrile (ε = 37·5) the rotational barriers in both solvents are about equal, probably due to hydrogen bonding in chloroform. The amide rotation rate appears to be very sensitive to traces of hydrochloric acid in a non-basic solvent like chloroform.     

Spectroscopic studies on N,N-dimethylamides—I: Structure-reactivity correlation for rotational barriers from a nuclear magnetic resonance study of p- and m-substituted N,N-dimethylbenzamides

Rotational barriers in 21 p- and m-substituted N,N-dimethylbenzamides in CDCl₃ as solvent have been determined using the intensity ratio method of Woodbrey. These data compare very well with values from the literature¹ obtained with a total line shape analysis. From structure-reactivity correlations evidence has been obtained for a relatively strong resonance interaction between the substituent in the aromatic ring and the partially double amide bond.     

Nuclear magnetic resonance studies of internal rotation—III Rotational barriers in META- and PARA-substituted N,N-dimethylcinnamamides

The rotational barriers about the C? N bond of eight m- and p-substituted N, N-dimethyl cinnamamides have been determined by the iterative total line shape NMR method. The ΔG values have been correlated with the substituent constants σ, σⁿ and σ⁺. By comparison of the results with literature data, some conclusions about the accuracy of the barrier determination as well as the transmittance of polar effects in conjugated amides have been drawn.     

Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted N,N-dimethylbenzamides

The barriers to rotation about the C? N bond in eighteen substituted N,N-dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N,N-dimethyl protons. The barriers have been correlated with the substituent constants σ and σ⁺. It has been shown that polar solvents increase the barrier in N,N-dimethylbenzamide. Acid catalysis of rotation about the amide C? N bond in N-(p-N,N-dimethylcarboxamidobenzyl)-pyridinium bromide has been investigated. ¹⁸O exchange studies show that catalysis is due to N-protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H₀, and the water activity, and it is shown that proton exchange between the N- and O-protonated species involves the intermediacy of a water molecule. The differences in chemical shifts for the non-equivalent N, N-dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.     

Spectroscopic studies on olefins—III: NMR of cis- and trans- disubstituted olefins

The proton NMR spectra of four cis/trans pairs of 1,2-disubstituted olefins have been analysed. The observed trends of the olefinic, vicinal and allylic proton-proton coupling constants are discussed in terms of rehybridization at the sp² carbon atoms. It is found that in these substances the trans-allylic couplings are generally more negative than the cis-allylic couplings contrary to Barfield's¹ theoretical prediction but in agreement with some earlier experimental data. Steric hindrance between cisoid substituents is cited as the probable origin of this end related trends.     

Nuclear magnetic resonance studies of internal rotation. V—Rotational barriers of acetylenic amides

The rotational barriers about the C? N bond of the N,N-dimethylamides of propiolic, methylpropiolic and phenylpropiolic acids have been determined by the iterative total line shape nuclear magnetic resonance method.     

Spectroscopic and antimicrobial studies of La, Pr, Nd and Gd complexes of a dipodal [N,N,N] chelating ligand

The lanthanide complexes of bis(benzimidazole-2′-yl-methyl)amine (BImz) having molecular formula [M(BImz)X₃H₂O]·nH₂O (M = La, Pr, Nd, or Gd; X = Cl or ClO₄ and n = 1, 2 or 3) were prepared and characterized spectroscopically through IR, ¹H and ¹³C NMR, FAB-mass, UV–visible and luminescence spectroscopy. TGA data suggested presence of coordinated and the lattice water. The oscillator strengths of the f–f transitions and the covalency parameters (β, b^1/2 and δ) have been evaluated from the electronic spectral data. The proposed hepta-coordinate geometry for the complexes has been ascertained from the molecular model computations. CV studies indicate formation of a stable quasi-reversible redox couple Gd^III/IV in the solution. The in vivo antimicrobial activities of the complexes have been evaluated against gram +ve and gram −ve bacteria and fungi.     

Spectroscopic studies of new model compounds for poly[N,N′-bis(phenoxyphenyl)pyromellitimide]

New model compounds for poly[N,N′-bis(phenoxyphenyl)pyromellitimide] have been synthesized in order to investigate the formation of imine bonds which are proposed to form during the curing process and lead to crosslinking in the bulk polymer. Raman studies show that terminal amines can react with imide carbonyls during curing to form C?N bonds. The Raman band due to C?N appears at 1656 cm^?1 and the band due to C?O closest to the imine bond is observed at 1742 cm^?1. These results are in agreement with previously published results on vapor deposited polyimide films.     

ToBaD: A method for the estimation of torsion barriers from crystal structure data; conformational analysis of N,N-dimethylaniline and derivatives

A new method for the estimation of torsion barriers and its application to conformational analysis is presented. This method, the ToBaD method (method of the torsion barrier derivative), makes use of crystal structure data. It is based on the assumption that the conformation of a compound in the crystalline phase must be very close to a (local) minimum energy conformation of this compound in the gas phase. The ToBaD method is demonstrated for the rotation of the phenyl-N bond in N,N-dimethylaniline. Two geometries of this compound are handled separately: one in which the nitrogen substituents are in a pyramidal or sp³ geometry, and the other in which the nitrogen atom and its substituents are coplanar (the sp² geometry). It is predicted, by means of the ToBaD method, that for both geometries the conformation in which the nitrogen lone pair or p orbital is perpendicular to the aromatic ring is the lowest energy conformation. © 1994 by John Wiley & Sons, Inc.     

Spectroscopic studies on carbenium ions derived from aromatic olefins—V [1–4] Experimental and theoretical studies of the cations derived from acenaphthylene

The identification of the cations formed from acenaphthylene is important in the elucidation of the mechanism of its polymerisation. By improved experimental techniques and by MO calculations, some previous assignments of u.v.-vis absorptions to different ions have been corrected and new correlations are suggested. In particular, the absorption of the propagating acenaphthenium ion and of the ion formed by hydride abstraction from dead polymer have been identified.     

Nuclear magnetic resonance studies of internal rotation: IV—rotational barriers and conformation of phthalic amides

The rotational barriers about the C? N bond of the N,N,N′,N′-tetramethyldiamides of phthalic, isophthalic and terephthalic acids have been determined by the iterative total line shape NMR method. Some evidence about the conformation of these compounds in solution has also been obtained by infrared and dipole moment studies.     

NMR studies on benzaldehydes. III—Rotational isomerism and torsional barriers in stable protonated o- and m-substituted benzaldehydes

Proton and carbon-13 NMR spectroscopy have been used to obtain the torsional barrier in some protonated ortho and meta substituted benzaldehydes, and the relative populations of the O-cis and O-trans rotamers have been determined. The results are compared with earlier data on free benzaldehydes.     

Exciton dynamics in molecular crystals from line shape analysis the effect of crystal strain

Experimental data are presented showing effects on the polarized reflection spectrum, from the (001) face of crystalline anthracene which arise upon cooling from room temperature to 80°K. Different crystal mounting methods and different crystal thicknesses are considered. The band profile of only the b-polarized (0—0) region is distorted and a new resonance detected near 25290 cm?¹ at crystal temperatures below 120°K with an intensity dependent on the method of crystal mounting and rate of cooling. This resonance disappears with time. The results ate explained using a theory of submicroscopic voids in the crystal if the crystal has 10^?4 of its volume in the form of extremely thin planar dislocations almost to the direction of light propagation in the crystal.     

NMR studies in the heterocyclic series XXIV—1H, 13C and 15N study of 15N labelled indazoles

The ¹⁵N chemical shifts and ¹H? ¹⁵N and ¹³C? ¹⁵N coupling constants of nine monolabelled indazoles were measured and assigned. The experimental values are discussed in terms of the indazolic and iso-indazolic structures, and compared with literature data for other related heterocycles. All the results are consistent with an N-1(H) tautomeric structure for indazole in DMSO-d₆.     

Photopolymerization V. Unsensitized solution photocyclopolymerization of N,N ′-alkylenebismaleimides

The term, “photopolymerization,” is defined as a polymerization process in which every chain-propagating step involves a photo-chemical reaction. This type of polymerization is exemplified by the unsensitized solution photocyclopolymerization of several N,N ′-alkylenebismaleimides, substituted at the double bond. N,N ′-Alkylenebismaleimides substituted with a bromine atom or two methyl groups at their double bond produce high polymeric products upon ultraviolet irradiation in solution. A general reaction scheme for the photopolymerization is proposed. Kinetic measurements show that the solution photopolymerization of substituted bismaleimides is a multistep reaction.     

Spectroscopic studies of Lewis acid-base complexes: Part V. Raman and infrared spectra,assignments and normal coordinate analysis for difluorophosphine borane

Raman spectra of five isotopic species of HPF₂BH₃ have been obtained in the liquid and solid states together with infrared spectra of the same species in the gaseous and solid states. Vibrational assignments have been made with the aid of a normal coordinate analysis utilizing compliance constants. Both compliance and force constants are reported. The force field is quite similar to that of PF₃BH₃ despite the marked difference in stability of the two compounds.