Nitrogen Inversion Barriers Research Articles

Investigation of ring and nitrogen inversion in N-Methyl-1,2,3,4-tetrahydroisoquinoline by dynamic N.M.R. methods

ΔG? for the overall ring and nitrogen inversion barriers in N-methyl-1,2,3,4-tetrahydroisoquinoline has been found to be 35.2 kJ/mol in vinyl chloride, and 50.2 kJ/mol in aqueous solution. Both measurements are believed to reflect the height of the nitrogen inversion barrier. The difference is attributed to hydrogen bonding in the aqueous solution. At low pH anomalous inversion rates were obtained.

Calculation of the inversion barrier of ammonia with small optimized GTF basis sets

Two small GTF contracted basis sets including polarization functions on nitrogen have been partially optimized on the ammonia molecule. Basis set A (7,3,1;4/2,1,1;1) is found to yield an energy value less than 0.2 a.u. above the Hartree—Fock limit and a nitrogen inversion barrier fairly close to the experimental height. These results may make A an acceptable basis set for calculations on larger systems containing nitrogen. Basis set B (5,2,1;3/2,1,1;1) does not simultaneously yield acceptable values for the energy and the inversion barrier and should not be considered seriously for further calculations.

The barrier to nitrogen inversion in six-membered rings

N-Methyl inversion barriers of 13·5–13·8 kcal mol–1 in dihydro-1,2-oxazines and an empirical least-squares correlation of nitrogen inversion barriers in 20 six-membered rings, show that the process observed in N-methyltetrahydro-1,2-oxazine of 13·7 kcal mol–1 is nitrogen and not ring inversion.

The effect of the phenyl substituent on the nitrogen inversion barrier in a 2-phenyloxaziridine

The effect of the phenyl substituent on the nitrogen inversion barrier in a 2-phenyloxaziridine

Configurational stability of pyramidal nitrogen in spiro-oxaziridines

Several optically active spiro-oxaziridines, whose stabilities were sufficient to obtain nitrogen inversion barriers without decomposition, have been synthesized. The magnitude of these barriers is markedly dependent on the nature of both nitrogen and carbon ring substituents. The relatively small solvent effect on the rate of racemization is attributed to the low basicity of the nitrogen atom and also to steric inhibition of solvation. Nitrogen inversion at ambient temperature in one oxaziridine was observed.

The relationship between rotational barriers in amides and nitrogen inversion barriers in related amines

Certain bicyclic amines have high nitrogen inversion barriers but their N-acetyl and N-benzoyl derivatives have normal amide rotational barriers.

Nitrogen inversion barrier and nature of the Si–N bond in silylamine: an ab initio SCF–LCAO–MO study

The electronic structure (nature of the N–Si bond, d-orbital participation) and the energy barrier to nitrogen inversion in silylamine have been studied using extended Hartree–Fock SCF–LCAO–MO calculations.

A variable temperature nuclear magnetic resonance spectroscopy study of nitrogen inversion in medium-ring monocyclic and bicyclic systems

Nitrogen inversion barriers and structural effects on the barriers have been studied by variable temperature n.m.r. spectroscopy in monocyclic (six- and seven-membered ring) and bicyclic (bicyclo[2,2,2]octane and bicyclo[3,2,2]nonane) amines.

Etudes cinétiques et conformationnelles par résonance magnétique nucléaire—XV : Fixation conformationnelle et rigidité de systèmes bicycliques du type [2.2.n

The method of “ conformational fixation” 2,5 using the geometrical and energetical properties inherent to the diurethane fragment N(COOMe)N(COOMe) to lock conformations and to slow down conformational equilibria, has been applied to the study of bicyclic systems of type [2.2.n] (n = 1–4). The introduction of the diurethane fragment into these systems permits the observation of a twisted conformation of the bicyclic molecules and of an equilibrium B ⇌ C between such conformations. Approximate values of the barriers hindering the interconversion B ⇌ C have been measured; they may be considered as an indirect estimate of the flexibility of the different bicyclic systems. A “ flexibility index” may be defined. Attributing a value of 1 to an acyclic reference molecule (V), leads to the following indices: ⩽0·6; 0·75 (0·85); 0·85; 0·95 respectively for systems of the type bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, bicyclo[3.2.2]nonane, bicyclo[4.2.2]decane. Some nitrogen inversion barriers have also been measured for the corresponding N.N′-dimethyl-diaza-bicyclic systems. The potential usefulness of the “ conformational fixation” method is briefly discussed and the general concept involved is delineated.

Etudes cinétiques et conformationnelles par résonance magnétique nucléaire-XIV : Dérivés substitués du diaza-2,3bicyclo[2.2.1]hepténe-5 propriétés spectrales et equilibres conformationnels

A number of substituted derivatives of 2,3-dimethyl-2,3-diazabicyclo[2.2.1]-5-heptène, Ib, have been prepared. The study of the NMR spectra of these compounds allowed us: 1. to identify the signals and the coupling constants due to the various protons and to study the effect of the orientation of an electronic lone-pair of an heteroatom linked to a HCCH groups on the vicinal 3J HH coupling; 2. to determine the steric effects of various groups X on the relative stability of the two invertomers A and B; 3. to give a clear illustration of the effects of specific solvation of form A in water on the conformational equilibrium A ⇌ B and on the effective ( or apparent) size of the substituents X. Some substituent effects on the nitrogen inversion barrier in derivatives of Ib are described. Furthermore the presence of an n → π* absorption band in the UV spectra of various compounds of type Ib points to the existence of an homoconjugative interaction between an endo lone-pair on nitrogen and the carbon-carbon double bond.

An ab initio SCF-LCAO-MO study of the nitrogen inversion barriers in ammonia and in ethylenimine

An ab initio SCF-LCAO-MO study of the nitrogen inversion barriers in ammonia and in ethylenimine