<Emphasis Type="Italic">Ab initio</Emphasis> Study of Structural and Conformational Properties of Five- to Nine-Membered Cyclic 1,2,3-Trienes

Summary.  The structures and relative energies of fundamental conformations of cyclopenta-1,2,3-triene, cyclohexa-1,2,3-triene, cylohepta-1,2,3-triene, cycloocta-1,2,3-triene, and cyclonona-1,2,3-triene were calculated by the HF/6-31G^* and MP2/6-31G^*//HF/6-31G^* methods. Only a C _2v symmetric planar conformation is available to cyclopenta-1,2,3-triene and cyclohexa-1,2,3-triene. The calculated energy barrier for ring inversion of the C _S symmetric puckerd conformation of cyclohepta-1,2,3-triene via the planar geometry is 62.2 kJ·mol⁻¹. The C ₂ symmetric twist conformation of cycloocta-1,2,3-triene was calculated to be the most stable one. Conformational racemization of the twist form takes place via the C _S symmetric half-chair geometry, which is by 60.8 kJ·mol⁻¹ less stable than the twist conformer. The C _S symmetric chair and unsymmetrical twist-boat conformations of cyclonona-1,2,3-triene were calculated to have similar energies; their interconversion takes place via an unsymmetrical low-energy (18.4 kJ·mol⁻¹) transition state. The twist (C ₂) and boat (C _S) geometries of cyclonona-1,2,3-triene are higher in energy by 13.2 and 33.9 kJ·mol⁻¹, respectively. Ring inversion in chair and twist-boat conformations takes place via a twist form as intermediate and requires 33.6 kJ·mol⁻¹. Corresponding author. E-mail: isayavar@yahoo.com Received March 25, 2002; accepted April 4, 2002     

AB INITIO MOLECULAR ORBITAL STUDY OF 1,2-DITHIANE AND 1,2,4,5-TETRATHIANE

Ab initio calculations at HF/6-31+G? level of theory for geometry optimization, and MP2/6-31+G?//HF/6-31+G? and B3LYP/6-31+G?//HF/6-31+G? levels for a single-point total energy calculation, are reported for the chair and twist conformations of 1,2-dithiane (1), 3,3,6,6-tetramethyl-1,2-dithiane (2), 1,2,4,5-tetrathiane (3), and 3,3,6,6-tetramethyl-1,2,4,5-tetrathiane (4). The C₂ symmetric chair conformations of 1 and 2 are calculated to be 21.9 and 8.6 kJ mol^?1 more stable than the corresponding twist forms. The calculated energy barriers for chair-to-twist processes in 1 and 2 are 56.3 and 72.8 kJ mol^?1, respectively. The C_2h symmetric chair conformation of 3 is 10.7 kJ mol^?1 more stable than the twist form. Interconversion of these forms takes place via a C₂ symmetric transition state, which is 67.5 kJ mol^?1 less stable than 3-Chair. The D₂ symmetric twist-boat conformation of 4 is calculated to be 4.0 kJ mol^?1 more stable than the C_2h symmetric chair form. The calculated strain energy for twist to chair process is 61.1 kJ mol^?1.     

Conformational Properties of (Z,Z)-, (E,Z)-, and (E,E)-Cycloocta-1,4-dienes

Summary.  Ab initio calculations at the HF/6-31G^* level of theory for geometry optimization and the MP2/6-31G^*//HF/6-31G^* level for a single point total energy calculation are reported for (Z,Z)-, (E,Z)-, and (E,E)-cycloocta-1,4-dienes. The C ₂-symmetric twist-boat conformation of (Z,Z)-cycloocta-1,4-diene was calculated to be by 3.6 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form; the calculated energy barrier for ring inversion of the twist-boat conformation via the C _S-symmetric boat-boat geometry is 19.1 kJ·mol⁻¹. Interconversion between twist-boat and boat-chair conformations takes place via a half-chair (C ₁) transition state which is 43.5 kJ·mol⁻¹ above the twist-boat form. The unsymmetrical twist-boat-chair conformation of (E,Z)-cycloocta-1,4-diene was calculated to be by 18.7 kJ·mol⁻¹ more stable than the unsymmetrical boat-chair form. The calculated energy barrier for the interconversion of twist-boat-chair and boat-chair is 69.5 kJ·mol⁻¹, whereas the barrier for swiveling of the trans-double bond through the bridge is 172.6 kJ·mol⁻¹. The C _S symmetric crown conformation of the parallel family of (E,E)-cycloocta-1,4-diene was calculated to be by 16.5 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form. Interconversion of crown and boat-chair takes place via a chair (C _S) transition state which is 37.2 kJ·mol⁻¹ above the crown conformation. The axial- symmetrical twist geometry of the crossed family of (E,E)-cycloocta-1,4-diene is 5.9 kJ·mol⁻¹ less stable than the crown conformation. Corresponding author. E-mail: isayavar@yahoo.com Received March 25, 2002; accepted April 3, 2002     

Ab Initio Study of Conformational Properties of ( Z, Z)-, ( E, Z)-, and ( E, E)-Cyclonona-1,5-dienes

Ab initio calculations at the HF/6-31G^* level of theory for geometry optimization and MP2/6-31G^*//HF/6-31G^* for a single point total energy calculation are reported for the important energy-minimum conformations and transition-state geometries of (Z,Z)-, (E,Z)-, and (E,E)-cyclonona-1,5-dienes. The C₂ symmetric chair conformation of (Z,Z)-cyclonona-1,5-diene is calculated to be the most stable form; the calculated energy barrier for ring inversion of the chair conformation via the C_s symmetric boat-chair geometry is 58.3kJmol^–1. Interconversion between chair and twist-boat-chair (C₁) conformations takes place via the twist (C₁) as intermediate. The unsymmetrical twist conformation of (E,Z)-cyclonona-1,5-diene is the most stable form. Ring inversion of this conformation takes place via the unsymmetrical chair and boat-chair geometries. The calculated strain energy for this process is 63.5kJmol^–1. The interconversion between twist and the boat-chair conformations can take place by swiveling of the trans double bond with respect to the cis double bond and requires 115.6kJmol^–1. The most stable conformation of (E,E)-cyclonona-1,5-diene is the C₂ symmetric twist-boat conformation of the crossed family, which is 5.3kJmol^–1 more stable than the C_s symmetric chair–chair geometry of the parallel family. Interconversion of the crossed and parallel families can take place by swiveling of one of the double bonds and requires 142.0kJmol^–1.     

Ab Initio Study of Conformational Properties of ( Z, Z)-, ( E, Z)-, and ( E, E)-Cyclonona-1,5-dienes

Summary. Ab initio calculations at the HF/6-31G^* level of theory for geometry optimization and MP2/6-31G^*//HF/6-31G^* for a single point total energy calculation are reported for the important energy-minimum conformations and transition-state geometries of (Z,Z)-, (E,Z)-, and (E,E)-cyclonona-1,5-dienes. The C₂ symmetric chair conformation of (Z,Z)-cyclonona-1,5-diene is calculated to be the most stable form; the calculated energy barrier for ring inversion of the chair conformation via the C_s symmetric boat-chair geometry is 58.3kJmol^–1. Interconversion between chair and twist-boat-chair (C₁) conformations takes place via the twist (C₁) as intermediate. The unsymmetrical twist conformation of (E,Z)-cyclonona-1,5-diene is the most stable form. Ring inversion of this conformation takes place via the unsymmetrical chair and boat-chair geometries. The calculated strain energy for this process is 63.5kJmol^–1. The interconversion between twist and the boat-chair conformations can take place by swiveling of the trans double bond with respect to the cis double bond and requires 115.6kJmol^–1. The most stable conformation of (E,E)-cyclonona-1,5-diene is the C₂ symmetric twist-boat conformation of the crossed family, which is 5.3kJmol^–1 more stable than the C_s symmetric chair–chair geometry of the parallel family. Interconversion of the crossed and parallel families can take place by swiveling of one of the double bonds and requires 142.0kJmol^–1.     

Neutron diffraction analysis at 15 K and ab initio molecular orbital calculations for α-cyanoacetohydrazide: A crystal structure with a high energy conformer

The crystal structure of α-cyanoacetohydrazide, C₃H₅N₃O, is refined using single-crystal neutron diffraction data at 15 K. Nuclear equilibrium geometries of the isolated molecules are calculated using GAUSSIAN-82 for the eight possible conformers having C_s symmetry. The conformation observed in the crystal has the eighth highest calculated energy, 36.6 kJ mol⁻¹ above the lowest energy conformer, and the largest calculated dipole moment, 8.6 D, with a HF/6-31G* basis. In the crystal the molecule is distorted from the ideal C_s symmetry. These distortions add another 20.3 kJ mol⁻¹ in energy, calculated with HF/6-31G*. All the hydrogens, including the CH, are involved in intermolecular hydrogen-bonding which is unusual in that it is formed entirely by three-center bonds. These bonds form a network which includes dimer and chain configurations.     

N,N-dimethyl-selenobenzamide: two solid phases and low-temperature phase change

Two polymorphs (I: mp 49.0–50.0°C; II: mp 80.0–82.0°C) of N,N-dimethyl-selenobenzamide, (CH₃)₂NC(Se)Ph, have been observed. Both I and II can be prepared separately using the same reaction under different conditions. The phase change from phase I to phase II occurs at low temperatures and this has been confirmed by solid state NMR (¹³C), powder X-ray, and single-crystal structural studies. The single-crystal X-ray diffraction study reveals that the lower melting point form (phase I) crystallizes in the triclinic space group P-1 with two conformations in the unit cell, while the higher melting point form (phase II) crystallizes in the monoclinic space group P2₁/c with one conformation in the unit cell. Theoretical calculations on model clusters using the Universal Forcefield (UFF) show that the total energy of phase I (triclinic form) is 5.9 kcal per mol molecule higher than that of phase II (monoclinic form). Also, the immersion energy which is due to non-bonding interactions, namely Van der Waals and Coulombic (electrostatic) terms, has been calculated using UFF. The Van der Waals terms were very similar in the two crystalline forms (triclinic: −43.1 kcal mol⁻¹; monoclinic: −44.8 kcal mol⁻¹), but the Coulombic terms were significantly different (triclinic: −14.0 kcal mol⁻¹; monoclinic: −31.5 kcal mol⁻¹) and favor the monoclinic form. The triclinic form (phase I) is a kinetically favored metastable phase and upon cooling it changes to the monoclinic form (phase II), a thermodynamically stable phase.     

A dynamic nuclear magnetic resonance spectroscopic study of conformational properties of 1,3-dioxepane and 4,4,7,7-tetramethyl-1,3-dioxepane

The 251 MHz ¹H and the natural abundance 63.1 MHz ¹³C NMR spectra of 1,3-dioxepane (1) and 4,4,7,7-tetramethyl-1,3-dioxepane (2) have been investigated over the temperature range of 5 to ?180 °C. While the spectra of 1 show no dynamic NMR effect, compound 2 exists in solution as a 1:1 mixture of a symmetrical (C₂) twist-chair and its mirror image conformation. The free energy barrier for the conformational racemization of 2 is 43 kJ mol^?1 (10.3 kcal mol^?1). Interconversion paths between various conformations of 2 are discussed. Compound 1 is suggested to have a symmetrical (C₂) twist-chair conformation which is rapidly pseudorotating via a chair conformation to achieve a time averaged symmetry of C_2v, even at ?180 °C.     

The vibrational spectra,molecular structure and conformation of organic azides—IX. Azidoethane

A safe method for the synthesis of azidoethane from ethylbromide is given and ¹H and ¹³C NMR data are reported.The i.r. and Raman spectra of azidoethane have been recorded in the region 4000-40 cm⁻¹ and interpreted in terms of two conformers, anti and gauche, present in the vapour and in the liquid and of the gauche conformer in the crystalline solid. Matrix isolation studies reveal the gauche conformation to be the more stable in argon and in nitrogen matrices and probably also more stable in the vapour. The enthalpy difference between the conformers is calculated to be ΔH⁰_a→g (N₂ matrix) ≈ ΔH⁰_a→g (vap.) = −0.56(10) kJ mol⁻¹, and the barrier to rotational isomerism (anti → gauche) as 9.0(10) kJ mol⁻¹ in the nitrogen matrix and less than 6 kJ mol⁻¹ in the argon matrix. Careful Raman studies of the liquid at 140–290 K reveal the gauche conformation to be the more stable in the liquid phase as well, the enthalpy difference being ΔH⁰_a→g (liq.) = − 1.15 (5) kJ mol⁻¹.The majority of the fundamentals for both conformers have been assigned with the aid of normal coordinate calculations using previously developed scaled quantum mechanical force fields which are also presented.     

Vibrational spectra,methyl torsional potential functions,internal rotational potential functions,and conformational properties of ethyl vinyl ether

The vibrational spectra of ethyl vinyl ether in both the fluid and solid states have been recorded from 20 to 3500 cm^?1. The 33 fundamental modes of vibration have been assigned. Three rotational isomers have been observed and their structures have been determined. The most stable conformer, s-cis/s-trans, is planar and of C_s symmetry. The two less stable rotamers, skew/s-trans and skew/gauche, are non-planar and of C_i molecular symmetry. The barrier to internal rotation of the methyl rotor has been determined for each conformation; these barriers are 3.43 kcal mol^?1 (s-cis/s-trans), 3.35 kcal mol^?1 (skew/s-trans) and 3.19 kcal mol^?1 (skew/gauche). A potential function for each of the two asymmetric internal rotations has been calculated and barriers to conformer interconversion have been determined. From the asymmetric potential function calculations, ΔH, the enthalpy difference between the conformers, has been determined. The s-cis/s-trans conformer is 1.87 kcal mol^?1 more stable than the skew/s-trans conformer; the skew/s-trans conformer is more stable than the skew/gauche conformer by 1.10 kcal mol^?1. The energetics of conformer interconversion and methyl internal rotation have been described in terms of molecular geometry and non-bonded interactions. These results are compared to those found in other alkyl vinyl and dialkyl ethers.     

Jahn—Teller effects in the mindo approximation: structures of the molecular cations of methane and the chloromethanes

The MINDO/3 technique gives geometries for (CH₄)⁺, (CCl₄)⁺ and the intermediate ions (CH_nCl_{4 ? n})⁺ (n = 1, 2, 3) which have symmetries in precise accord with the predictions of the Jahn—Teller effect. The ground state of (CH₄)⁺ has D_2d symmetry, with a C_3v structure ca. 45.6 kJ mol^?1 higher. (CCl₄)⁺ has a C_2v ground state, with a D_2d structure ca. 144 kJ mol^?1 higher: no bound state of C_3v symmetry could be found. (CH₃Cl)⁺ and (CHCl₃)⁺ both have C_s symmetry, and (CH₂Cl₂)⁺ has C_2v symmetry. The analogous fluoro ions are discussed briefly.     

Structure and Conformational Stability of 1,3-Dihalo-2,2,4,4-tetrachloro-1λ3,3λ3-diphosphetidines

The optimal geometry of isomeric molecules of (XP-CCl₂)₂ with X = F, Cl, Br was determined by RHF/6-31G(d) calculations. With X = F and Cl, the electronic correlation was considered on the MP2/6-31G(d) level. The P₂C₂ ring is nonplanar. With X = Cl and Br, the trans conformation is energetically preferable compared to the two possible cis conformations: by 7.8 and 14.2 kJ mol^{- 1} for X = Cl and by 7.5 and 14.1 kJ mol^{- 1} with X = Br. respectively. With X = F, the calculated energies of the cis and trans forms are very close.     

High-level computational study on the thermochemistry of saturated and unsaturated three- and four-membered nitrogen and phosphorus rings

The heats of formation and strain energies for saturated and unsaturated three- and four-membered nitrogen and phosphorus rings have been calculated using G2 theory. G2 heats of formation (ΔH_f298) of triaziridine [(NH)₃], triazirine (N₃H), tetrazetidine [(NH)₄], and tetrazetine (N₄H₂) are 405.0, 453.7, 522.5, and 514.1 kJ mol⁻¹, respectively. Tetrazetidine is unstable (121.5 kJ mol⁻¹ at 298 K) with respect to its dissociation into two trans-diazene (N₂H₂) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (ΔH₂₉₈ = −308.3 kJ mol⁻¹ calculated using G2 theory). G2 heats of formation (ΔH_f298) of cyclotriphosphane [(PH)₃], cyclotriphosphene (P₃H), cyclotetraphosphane [(PH)₄], and cyclotetraphosphene (P₄H₂) are 80.7, 167.2, 102.7, and 170.7 kJ mol⁻¹, respectively. Cyclotetraphosphane and cyclotetraphosphene are stabilized by 145.8 and 101.2 kJ mol⁻¹ relative to their dissociations into two diphosphene molecules or into diphosphene (HP(DOUBLE BOND)PH) and diphosphorus (P₂), respectively. The strain energies of triaziridine [(NH)₃], triazirine (N₃H), tetrazetidine [(NH)₄], and tetrazetine (N₄H₂) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol⁻¹, respectively (at 298 K). While the strain energies of the nitrogen three-membered rings in triaziridine and triazirine are smaller than the strain energies of cyclopropane (117.4 kJ mol⁻¹) and cyclopropene (232.2 kJ mol⁻¹), the strain energies of the nitrogen four-membered rings in tetrazetidine and tetrazetine are larger than those of cyclobutane (110.2 kJ mol⁻¹) and cyclobutene (132.0 kJ mol⁻¹). In contrast to higher strain in cyclopropane as compared with cyclobutane, triaziridine is less strained than tetrazetidine. The strain energies of cyclotriphosphane [(PH)₃, 21.8 kJ mol⁻¹], cyclotriphosphene (P₃H, 34.6 kJ mol⁻¹), cyclotetraphosphane [(PH)₄, 24.1 kJ mol⁻¹], and cyclotetraphosphene (P₄H₂, 18.5 kJ mol⁻¹), calculated at the G2 level are considerably smaller than those of their carbon and nitrogen analog. Cyclotetraphosphene containing the P(DOUBLE BOND)P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the strain energies for the analogous unsaturated and saturated carbon and nitrogen rings. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 373–384, 1997     

Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a Sulfonyl Group in Intramolecular Self-Inclusion

The synthesis of tetramethoxyresorcinarene podands bearing p-toluene arms connected by -SO₃- ( 1 ) and -CH₂O- ( 2 ) linkers is presented herein. In the solid state, the resorcinarene podand 1 forms an intramolecular self-inclusion complex with the pendant p-toluene group of a podand arm, whereas the resorcinarene podand 2 does not show self-inclusion. The conformations of the flexible resorcinarene podands in solution were investigated by variable-temperature experiments using 1D and 2D NMR spectroscopic techniques as well as by computational methods, including a conformational search and subsequent DFT optimisation of representative structures. The ¹H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C_4v symmetry. At low temperatures, the molecules exist as a mixture of boat conformations featuring slow exchange on the NMR timescale. Energy barriers (ΔG^≠₂₉₈) of 55.5 and 52.0 kJ mol⁻¹ were calculated for the boat-to-boat exchange of 1 and 2 , respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand 1 adopts a similar conformation to that present in its crystal structure, whereas podand 2 populates a more versatile range of conformations in solution.     

C-Glycosylidene derivatives (exo-glycals): their synthesis by reaction of protected sugar lactones with tributylphosphonium ylids, conformational analysis and stereoselective reduction

Stabilised tributylphosphonium ylids Bu₃PCHCH(EWG), where EWG is CO₂Me, CO₂^tBu or CN, react with protected sugar lactones under mild conditions to give high yields of glycosylidene derivatives (4 and 5) with good Z/E selectivity. X-Ray crystallography shows that in the solid state the tetra-O-benzyl protected (Z)-glucosylideneacetonitrile (Z)-4c adopts a conformation intermediate between a boat and a twist-boat, whereas the isomeric galactose derivative (Z)-5c exists as a distorted chair. NMR data suggest that in solution chair-like conformations are again more favoured for galactosylidene derivatives than for their glucosylidene analogues. Solution phase NMR studies and molecular modelling show that the (E)-double bond geometry disfavours the chair-like geometry of the ring, even in the galactose series; this is consistent with the avoidance of allylic 1,3-strain. Reduction of the glycosylidene double bond to give stereoselective formation of β-C-glycoside derivatives may be achieved by using Et₃SiH-CF₃CO₂H or Et₃SiH-BF₃·Et₂O.     

Application of DNMR,group theory and molecular mechanics in the elucidation of the inversion process in the flexible molecule perhydrotetra-azapyrene

The inversion process in cis-10b,10c-perhydro-1H,6H-3a,5a,8a,10a-tetraazapyrene (1) has been investigated by ¹H and ¹³C NMR. The free energy of activation is found to be 14.95 ± 0.2 kcal mole^?1 at 45°. Application of group theoretical techniques led to a graph representing the essential symmetry properties of the potential energy surface for conformational change. The energies of intermediates on this graph were then estimated using molecular mechanics calculations. This combined approach suggests that the total inversion proceeds via a conformation of C_2v symmetry with two non-chair piperazine rings, calculated to be 6.8 kcal mole^?1 less stable than the ground state conformation (C₂ symmetry).     

Conformational analysis—CXIII : The isomeric cyclooctadienes

1,3-Cyclooctadiene, and 1,4- and 1,5-isomers were studied by the force field method. The former is calculated to be a mixture of two conformations, one of which has a C₂ axis, and one is irregular (C₁). There are for the 1,4-isomer two conformations, a boat-boat and a boat-chair, both of which have C_s symmetry. The 1,4-isomer is substantially all in the boat-chair conformation, while the 1,5-isomer is known to be preferentially a twist-boat (C₂) conformation. The calculations indicate the 1,3-isomer is much more stable, followed in turn by the 1,4- and the 1,5. An experimental measurement of the equilibrium between the isomers was also made, and is in semi-quantitative agreement with the calculations.     

Far-IR spectrum,conformation stability,barriers to internal rotation,vibrational assignment,and ab initio calculations of 3-chloro-2-methylpropene

The far-IR spectrum from 375 to 30 cm⁻¹ of gaseous 3-chloro-2-methylpropene, CH₂=C(CH₃)CH₂Cl, has been recorded at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm⁻¹ with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm⁻¹ with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): V₁=189±12, V₂=−358±11, V₃=886±2 and V₄=−12±2 with an enthalpy difference between the more stable gauche and s-cis conformers of 150 ±25 cm⁻¹ (430 ± 71 cal mol⁻¹). This function gives values of 661 cm⁻¹ (1.89 kcal mol⁻¹), 1226 cm⁻¹ (3.51 kcal mol⁻¹) and 812 cm⁻¹ (2.32 kcal mol⁻¹), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm⁻¹ for the gauche conformer, the threefold barrier of 678 cm⁻¹ (1.94 kcal mol⁻¹) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G^* and 6–31G^* basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G^* basis set. These data are compared with the corresponding data for some similar molecules.     

Strain energy minimization study of the mechanism of,and the barrier to,conformational interconversion in five-membered diamine chelate rings

The method of Lagrangian multipliers is used to constrain torsion angles during molecular mechanics refinement for the purpose of plotting strain energy against a reaction coordinate. A complete two-dimensional analysis of the conformational interconversion from δ- to λ-[Co(ethane-1,2-diamine) (NH₃)₄]³⁺ reveals a mechanism in which the transition state geometry has an envelope conformation and an inversion barrier of 15.7 kJ mol^?1. Substitution at the carbon atoms, variation of the metal-nitrogen distance, and replacement of the amine ligands with bidentate amines only slightly alters the inversion barrier. Substitution at the nitrogen atoms of the bidentate ligand increases the inversion barrier significantly to 24.6 kJ mol^?1 for (N,N,N′,N′-tetramethylethane-1,2-diamine) [(NH₃)₄]³⁺.     

NMR studies of conformations and dynamic processes—II : [26]Bicyclophanes with ethylene bridges

The conformations and dynamic processes in two bicyclophanes have been analysed on the basis of temperature-dependent ¹H NMR spectra. Both bicyclophanes are suggested to have a lowest-energy conformation of D₃ symmetry in which the substituents at all ethylene bridges are gauche⁺ (or gauche^?) oriented. The interconversion of the mirror image conformers of each bicyclophane equilibrates the two hydrogens in each methylene group, the barriers being ca 36 and 37 kJ mol^?1, respectively, as determined by line-shape analysis.