Finite perturbation theory for nuclear spin coupling constants

Pettitt²

1979

Can. J. Chem. 57, 1274 (1979). Spin-lattice relaxation times are reported for the cyano, methylene, and ring carbons in liquid benzyl cyanide at selected temperatures between 215 and 406 K. The rotation of the molecule is modelled on the diffusion of a prolate spheroid and the following features are deduced: (I) rotation of the aromatic ring about the long axis of the molecule is about 3 times faster than rotation of this axis, (2) rotation of the -CH,CN group about the long axis is about 1.5 times faster than the aromatic ring, and (3) activation energies associated with the temperature dependence of the 13C-'H dipolar relaxation rates agree with the hydrodynamical prediction of 3.6 kcal/mol associated with the temperature dependence of the ratio shear viscosity -temperature.Linewidths of the cyano carbon resonance are shown to result from scalar coupling with 14N.Spin-spin coupling constants involving the cyano and methylene carbons are reported and compared with those calculated at the INDO level of molecular orbital theory. RODERICK E. WASYLISHEN et BRIAN A. PETTITT. Can. J. Chem. 57, 1274 (1979). On rapporte les temps de relaxation spin-rCseau des carbones du cyano, du rnkthylkne et du cycle du cyanure de benzyle liquide 2 des temperatures determinees entre 215 et 406 K. On peut imaginer le modkle pour la rotation de la molecule sur la base d'une diffusion d'une sphkroi'de allongee et I'on a pu deduire les caracteristiques suivantes: (I) la rotation du cycle aromatique autour de son axe le plus long est environ 3 fois plus rapide que la rotation de cet axe, (2) la rotation du groupe -CH2CN autour de son axe le plus long est environ 1.5 fois plus rapide que celle du noyau aromatique et (3) les energies d'activation associbes avec le fait que les taux de relaxation dipolaire 13C-'H dependent de la temperature sont en bon accord avec la prediction hydrodynamique de 3.6 kcal/mol qui est associ6e avec la relation entre la temperature et le rapport viscosite de cisaillement -temperature.L'klargissement des raies de resonance du carbone du cyano provient d'un couplage scalaire avec le 14N.

Section: C Spin-lattice Relaxation Times and Rotationalmentioning

confidence: 99%

A carbon-13 nuclear magnetic resonance study of benzyl cyanide

Pettitt²

1979

“…Recently Pople and co-workers (18,20,21) have developed a finite perturbation technique which allows the calculation of coupling constants without making some of the approximations inherent in the earlier MO methods. The application of this technique using unrestricted INDO MO wave functions in an approximate SCF framework has been described (22)(23)(24).…”

Section: Introductionmentioning

confidence: 99%

The Geometrical Dependence of Experimental and Theoretical Nuclear Spin–Spin Coupling Constants in the ¹⁵NH₂ Fragment

Schaefer²

1973

Approximate molecular orbital theory at the INDO level of approximation has been used to examine 1J(15N,H) and 2(HNH) in ammonia as a function of the HNH angle. The couplings are very sensitive to the value of the HNH angle in the range from 110 to 120°. The computed trends are compared with those observed in other compounds containing the 15NH2 fragment. The signs and magnitudes of 1J(15N,H) and 2J(HNH) have been measured for 2-aminoacetophenone at temperatures where the amino protons are nonequivalent. The observed and calculated results suggest that the amino group lies in the plane of the benzene ring. In aniline the magnitude of 2J(HNH) lies between 1 and 2 Hz and on the basis of the trends in the calculated and experimental values the sign can be confidently taken as negative.

“…In addition, the long-range coupling constants involving the 15NH, fragment are calculated by the finite perturbation technique (25) in the INDO 'molecular orbital approximation (26). A comparison of calculated with observed couplings leads to inferences about the conformation of the amino gropp in aniline.…”

Section: Introductionmentioning

confidence: 99%

Long-range Spin–Spin Coupling Constants from Amino Protons and ¹⁵N to Ring Protons in Aniline-¹⁵N and Some Derivatives. INDO Molecular Orbital Calculations

Rowbotham²,

Ernst³

et al. 1972

A complete analysis (8-spins) is given of the p.m.r. spectrum of aniline-15N, of the spectra of some haloa n i l i n e~-~~N and of 2-aminoacetophenone-'5N. Intermolecular exchange of the amino protons is slow enough for observation of their spin-spin coupling to the ring protons. The magnitudes of the coupling constants between amino protons and 15N or ring protons are a measure of the geometry of the amino group. This is not true of the couplings between 15N and the ring protons. Long-range couplings computed in the CND0/2 and INDO approximations of molecular orbital theory show points of agreement with experiment. For example, their signs and magnitudes are consistent with a nonplanar but not with a planar conformation of aniline. Couplings from 15N to ring protons are also computed for nitrobenzene.L'analyse complete (8 spins) du spectre de r.m.n. est donnee pour I'aniline 15N, quelques haloanilines 15N et pour l'amino-2 acetophenone 15N. L'khange intermoleculaire des protons de l'amino est suffisamment lente pour pouvoir observer leur couplage spin-spin avec les protons du cycle. Les amplitudes des constantes de couplage entre les protons de l'amino et "N ou les protons du cycle donnent une mesure. de la geomttrie du groupe amino. Cela n'est plus vrai pour les couplages entre 15N et les protons du cycle. Les couplages a longue distance calcultrs avec les approximations du CND0/2 et INDO de la theorie des orbitales moleculaires sont en accord avec les valeurs experimentales. Par exemple, les signes et les amplitudes des couplages ne sont compatibles qu'avec la conformation non plane de I'aniline. Les couplages entre 15N et les protons du cycle ont Cte egalement calcults pour le nitrobenztne.