Barriers to internal rotation in some dimethylamino substituted azoles

Activation parameters for the hindered rotation in some dimethylamino substituted azoles are reported and the effects of various ring systems and substituents on the barrier are discussed. Possible errors in ΔH≠ and ΔS≠ are investigated.     

Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A 13C NMR band shape study

Two series of neopentylbenzenes with one or two substituents on the benzyl group have been synthesized. In one series the substituents were H, F, Cl, Br, I, OCH₃, OCOCH₃, OSi(CH₃)₃ CH₃ and CH₂CH₃, and in the other OH and R [R ? H, CH₃, CH₂CH₃, (CH₂)₃CH₃, CH(CH₃)₂ and C(CH₃)₃]. Barriers to internal C? C and C? C rotation have been estimated by ¹³C NMR band shape methods. Estimated barriers were found to increase as the size of the substituent increases. The results are discussed in terms of possible initial and transition states, based on summations of results from molecular mechanics (MM) calculations, using the Allinger MMP1 program. Barriers estimated experimentally are compared with results from other systems found in the literature.     

Barriers to internal rotation in phosphoric amide molecules

Internal rotation in phosphoramide and 0,0-dimethyl phosphoramidate molecules in their different stable conformations was studied by different quantum-chemical methods. Rotation of the amide group about the P-N bond is inhibited with barrier values of 27 kJ/mole.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 221–22, March–April, 1985.     

Barriers to rotation and inversion in acyclic alkylhydrazines

Stereodynamics of acyclic hydrazines is reviewed. The energy barriers to the conformational interconversions in acyclic alkyl-hydrazines, which were observed by DNMR spectroscopy, are listed together with their assignment. The following points are emphasized: (1) The existence of two types of nitrogen inversions (the passing and the nonpassing N-inversion) in acyclic hydrazines has been confirmed: (2) the barrier to the single-passing rotation about the NN bond has recently been unambiguously determined in several hydrazines. The SCF-MO ab-initio calculations of tetramethylhydrazine have shown that the transition state of the single-passing rotation about the NN bond is in the vicinity of the conformation in which the torsion angle C-N-N-C is about 140°.     

Barriers to internal rotation in biphenyl radical ions

Molecular orbital theory shows that the conformation of, and barriers to rotation in, radical ions of biphenyl derivatives are greatly different from those of the neutral compounds. The results are consistent with the ESR results on the cation radicals of some substituted biphenyls.     

Restricted rotation in substituted propane epoxides

Dynamic nmr spectroscopy is used to show that 1,1-diaryl-2-methyl-1,2-epoxypropanes display restricted rotation.     

Heterocycles. CII. Barriers to rotation in some N′-Heteroaryl N,N-dimethylformamidines

The free energies of activation about the =CH? NMe₂ bond in N′-heteroaryl N,N-dimethylformamidines have been found in the range from 15.6 kcal/mole to 23 kcal/mole.     

1H NMR studies on thioamides: Barriers to internal rotation

The barrier to the internal rotation of the dimethylamino group in thioamides of structure R? CS? N(CH₃)₂, R being (CH₃)₂,N? CS? , CH₃O₂C? or N?C? , is studied by proton magnetic resonance, using the lineshape analysis method of Nakagawa. In the solvents o-dichlorobenzene, naphthalene and nitrobenzene all ΔG≠ values are in the range of 23 to 24 kcal/mol. In these solvents the E_a and ΔS≠ values of each product are linearly related to the dielectric constants.     

Internal rotation and rotational isomerism in substituted ethanes

The types of potential energy curves of internal rotation in substituted ethanes were studied. Formulas for the calculation of the energy differences in rotational isomers and of the potential barriers to internal rotation in these derivatives were obtained by Bernstein's method. As a rule, the calculated results are in agreement with experiment.     

Barriers to internal rotation in 1,3,5-trineopentylbenzenes—II: Unsymmetrically halogen-substituted compounds

The temperature dependence of the three different methylene AB spectra due to the 1-, 3- and 5-neopentyl groups in 2-chloro-4-iodo, 2-bromo-4-iodo and 2-bromo-4-chloro-1,3,5-trineopentyl-benzene was studied by proton magenetic resonance measurements at 60 MHz. The identification of the lines of the AB quartets at low temperature was carried out on an HA-100 spectrometer by means of the INDOR technique. The activation parameters for all three barriers in each compound were found to be approximately the same, which perhaps reflects the interdependence of the rotations, with the smaller of the two halogens determining the size of the barrier. An explanation in terms of magnetic nonequivalence induced in the 1- and 5-methylenes by the 3-neopentyl group is considered to be most plausible.     

13C nuclear magnetic resonance studies on acetophenones: Barriers to internal rotation

The barrier to internal rotation in a series of p-substituted acetophenones has been determined by means of low temperature carbon-13 n.m.r. and total bandshape analysis, resulting in: ΔG = 5·4 ± 0·1 kcal mol^?1 (22·4 ± 0·4 kJ mol^?1) for the unsubstituted acetophenone. The substituent effects on the barrier are found to be the same as for the corresponding benzaldehydes. The barrier height is discussed in terms of contributions from resonance and steric effects.     

Barriers to the internal rotation and observables of the ground state for hydrogen peroxyde

SCF energy and one electron properties of the H₂O₂ molecule have been calculated by using a basis of 52 STO's. The minimum total energy reached is –150.83188 a.u. By carrying out the calculation for three different geometrical configurations a reasonable agreement with experimental values of the barriers to the internal rotation is obtained.

---

Zusammenfassung Die SCF-Energie und die Einelektroneneigenschaften des H₂O₂-Moleküls werden mit einer Basis von 52 Slaterorbitalen berechnet. Als minimale Gesamtenergie werden der Wert von –150,83188 a.E. erreicht. Die Ergebnisse für 3 verschiedene Geometrien liefern eine befriedigende Übereinstimmung mit dem experimentellen Wert der Barriere für die innere Rotation.

---

Résumé L'énergie SCF et les propriétés monoélectroniques de la molécule H₂O₂ ont été calculées en utilisant une base de 52 orbitales de Slater. L'énergie minimum totale atteinte est –150.83188 u.a. Un accord raisonnable avec les valeurs expérimentales des barrières de rotation est obtenu en effectuant les calculs pour trois configurations géométriques différentes.

---

---

Note. When this paper was completed, we have received the work of Dunning and Winter. Their main results are quoted in Table 4. The agreement of the result for the trans barrier is very good. As in the case of Ref. [4] and [8] the geometry optimization contributes heavily (75% of the total value) to the result for the trans barrier.