Steric effects in conjugated systems: II. 2,3-Dimethylbutadiene

Based on earlier results of conformational analysis by the Wiberg method¹ of monosubstituted methylbutadienes, analogous calculations for 2,3-dimethylbutadiene have been carried out. Contrary to the opinion of Aten et al.² who assumed almost free rotation, the molecule was shown to exhibit a considerable strain which hinders the rotation of methyl groups and lengthens the C_sp²-C_sp² bond compared with the length of the central bond in butadiene. The calculated lengthening agrees qualitatively with that obtained by electron diffraction¹.     

Spectroscopic studies of the monosubstituted amides. VI. Neutron inelastic scattering spectra of N-methyl acetamide

We report the determination of the barriers to methyl group rotation in N-methyl acetamide. These were, for N-methyl V₃ = 590 cm^?1, and for C-methyl V₃ = 670 cm^?1. Some previous spectral assignments of this compound have been confirmed and one new band at about 160 cm^?1 is shown to involve the motion of both methyl groups.     

Infrared and raman spectra,vibrational assignment and barriers to internal rotation for methyldisilylamine

The Raman spectra of gaseous and liquid (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded to within 10 cm^?1 of the exciting line. The IR spectra of (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded from 80 cm^?1 to 3800 cm^?1 in the gaseous state, and from 80 cm^?1 to 450 cm^?1 in the solid state. A vibrational assignment has been made, and from the low-frequency vibrational data, an upper limit of 3.3 kcal mol^?1 was calculated for the barrier to internal rotation of the silyi groups, whereas a barrier of ~450 cal was calculated for internal rotation of the methyl group. It is concluded that there exists a significantly strong d_π—p_π interaction in methyldisilylamine.     

Mid-infrared spectra of methane dispersed in solid neon and argon

We have recorded, in absorption, infrared spectra of samples of methane, CH₄ and CD₄, dispersed at molar fraction 0.0001–0.005 in solid neon, in solid argon and in their equimolar solid mixture in the temperature range 3–21 K and in the spectral domain 5000–500 cm⁻¹ at resolution 0.04–0.2 cm⁻¹. We undertook quantitative fitting of the spectral profiles with components of gaussian and lorentzian shapes. Comparison of our spectra in regions of fundamental modes both ν₃ and ν₄ with published spectra of CH₄ and CD₄ in either crystalline para-dihydrogen or droplets of liquid helium indicates evidence for hindered rotation of CH₄ molecules in Ar but not for rotation of CH₄ or CD₄ in Ne or in a mixture of Ne and Ar. For CD₄ in solid Ar, the evidence for even hindered rotation is ambiguous. We make new assignments of lines to ¹³CH₄ and ¹³CD₄ in their environments in solid Ne and Ar.     

Internal dynamics in 2,4,6, triodophenol

The conformational changes in 2,4,6 triodophenol along the internal rotation itinerary of the OH and OD groups have been studied by infrared and Raman spectroscopy. It is shown that the tilt angle between the CO bond and the aromatic plane undergoes a remarkable variation when the hydroxyl group is deuterated. The hindered rotation potential function associated to the torsional motion has been determined. Tentative assignments for the ψ^as_o → ψ^as₁ and ψ^S_o → ψ^as₁ transitions corresponding to the inversion of the OH and OD groups are also carried out.     

Carbon-13 NMR. The analysis of the relaxation times T1 of benzofuran and of a series of its methyl derivatives. Correlations between molecular motions and structural properties

Carbon-13 relaxation times (T₁) and nuclear Overhauser enhancements (η) have been measured for benzofuran and a series of its methyl derivatives. The contributions of dipolar (T₁ ^DD) and spin rotation (T₁^SR) mechanisms have both been determined. The temperature dependence of T₁ has been studied. The relationships between molecular motions and structural properties have been emphasized. The overall motional anisotropy of the benzofuran molecule is increased by substitution in positions 2 and 5. The internal rotation of a methyl group may change depending on its position in the molecule and on the influence of other methyl groups in its close neighbourhood.     

Nonempirical quantum chemical calculation for nitramide,its chloro- and methyl-substituted derivatives. II. Structures and energies of transition states for internal rotation and inversion of the amino group

For five N-nitramines (H₂NNO₂, MeNHNO₂, ClNHNO₂, MeNClNO₂, Me₂NNO₂) using the program GAUSSIAN-90 we have carried out quantum chemical calculations by the restricted Hartree—Fock method, taking into account electron correlation by second-order Møller—Plesset perturbation theory in a standard 6–31G^* basis. In this paper, we consider the transition states for inversion of the amine nitrogen atom and rotation about the NN bond. We have obtained data on the changes in the geometric parameters during inversion and rotation. The changes in the NN bond length are especially significant they increase by 0.06–0.08 Å in the transition states for internal rotation compared with the equilibrium forms. We have calculated the barriers to inversion and internal rotation, the height of which strongly depends on the electronegativity of the substituents on the amine nitrogen atom. Estimates of the barriers to inversion lie within the range 0.4–6.0 kcal/mole while estimates of the barriers to rotation lie within the range 6–13 kcal/mole, which are 1.5–2 times lower than in amides and N-nitrosoamines.Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 12–19, January–February, 1993.     

Dicarbonyl allyl derivatives of molybdenum and tungsten containing β-diketonate substituents

A number of complexes of the type M(CO)₂(η³-allyl)(diket)L (where M is Mo or W, diket is a β-diketonate group and L is pyridine, tetrahydofuran or acetonitrile) have been prepared and characterized by elemental analysis and IR and NMR spectroscopy. These complexes apparently adopt an octahedral configuration similar to related neutral species but spectroscopic data indicate an equilibrium between two conformers probably due to rotation of the allyl groups.     

[Pd(Fmes)2(tmeda)]: A Case of Intermittent C?H⋅⋅⋅F?C Hydrogen‐Bond Interaction in Solution

The X‐ray structure of the title compound [Pd(Fmes)₂(tmeda)] (Fmes=2,4,6‐tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′‐tetramethylethylenediamine) shows the existence of uncommon C? H???F? C hydrogen‐bond interactions between methyl groups of the TMEDA ligand and ortho‐CF₃ groups of the Fmes ligand. The ¹⁹F NMR spectra in CD₂Cl₂ at very low temperature (157 K) detect restricted rotation for the two ortho‐CF₃ groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen‐bond energy shows that it is too weak (about 7 kJ mol^?1) to account for the rotational barrier observed (ΔH^≠=26.8 kJ mol^?1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.     

Formation and dissociation mechanism of amide complexes. V. Interconversion of dimeric and monomeric Cu2+-complexes with 1,8-diamino-3,6-diaza-2,7-octanedione: Comparison between the reactivity of terminal and internal amide groups

The protonation and deprotonation rates of the coordinated amide groups in the Cu²⁺-complexes of 1,8-diamino-3,6-diaza-2,7-octanedione (DED = L) have been studied by stopped-flow techniques. Starting at low pH from Cu²⁺ and DED the dimeric Cu₂L₂⁴⁺-complex, fully formed within the mixing time of the stopped-flow instrument, reacts in two consecutive steps to yield the final product CuLH_?2. The rate constants of the forward and backward reactions have been determined and are given in Table 1. The intermediate was identified as Cu₂L₂H_?2²⁺ by measuring its VIS.-absorption spectrum. The rate constants for the interconversion of the amide groups from the O- to the N-coordinated form in the Cu²⁺-complexes of DED, 2,10-dioxo-1, 4, 8, 11 tetraazaundecane (DANA) and triglycine (TRIGLY) are compared with each other. It is shown that these rate constants are similar, no matter whether the amide group is terminal or internal as long as the rotation is easily possible as is the case in the dimeric species Cu₂L₂⁴⁺ and Cu₂L₂H_?2²⁺. However, for CuLH_?2 the inter-conversion only takes place after opening of one of the chelate rings in a rapid protonation preequilibrium.     

Huge dielectric response and molecular motions in paddle-wheel [Cu(II)2(adamantylcarboxylate)4(DMF)2]⋅(DMF)2

The temperature‐dependent dynamic properties of [Cu^II₂(ADCOO)₄(DMF)₂]?(DMF)₂ ( 1 ) and [Cu^II₂(ADCOO)₄(AcOEt)₂] ( 2 ) crystals were examined by X‐ray crystallography, ¹H NMR spectroscopy, and measurements of the dielectric constants and magnetic susceptibilities (ADCOO=adamantane carboxylate, DMF=N,N‐dimethylformamide, and AcOEt=ethyl acetate). In both crystals, four ADCOO groups bridged a binuclear Cu^II? Cu^II bond, forming a paddle‐wheel [Cu^II₂(ADCOO)₄] structure. The oxygen atoms of two DMF molecules in crystal 1 and two AcOEt molecules in crystal 2 were coordinated at axial positions of the [Cu^II₂(ADCOO)₄] moiety, forming [Cu^II₂(ADCOO)₄(DMF)₂] and [Cu^II₂(ADCOO)₄(AcOEt)₂], respectively. Two additional DMF molecules were included in the unit cell of crystal 1 , whereas AcOEt was not included in the unit cell of crystal 2 . The structural analyses of crystal 1 at 300 K showed three‐fold rotation of the adamantyl groups, whereas rotation of the adamantyl groups of crystal 2 at 300 K was not observed. Thermogravimetric measurements of crystal 1 indicated a gradual elimination of DMF upon increasing the temperature above 300 K. The dynamic behavior of the crystallized DMF yielded significant temperature‐dependent dielectric responses in crystal 1 , which showed a huge dielectric peak at 358 K in the heating process. In contrast, only small frequency‐dependent dielectric responses were observed in crystal 2 because of the freezing of the molecular rotation of the adamantyl groups. The magnetic behavior was dominated by the strong antiferromagnetic coupling between the two S=1/2 spins of the Cu^II? Cu^II site, with magnetic exchange energies (J) of ?265 K (crystal 1 ) and ?277 K (crystal 2 ).     

Computational and dynamic NMR investigation of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane

The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by ¹H- and ¹³C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C₂ symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a C_s transition state with a calculated relative free energy of 11.0 kcal mol⁻¹. The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol⁻¹ at 298 K.     

Vibrational spectra and structure of gaseous and solid dimethylboric anhydride

The Raman spectra of gaseous, liquid and solid dimethylboric anhydride (CH₃)₂BOB(CH₃)₂ have been recorded from 10–3500 cm^?1. The IR spectra from 4000–30 cm^?1 have also been recorded. The spectra of the gaseous phase have been interpreted in terms of C₂ symmetry implying a bent B-O-B skeleton with the B(CH₃)₂ groups twisted and consistent with a rather larger barrier to internal rotation about the B-O bonds. The spectra of the crystalline state, however, suggest that the molecular symmetry is altered upon solidification. Isotopic substitution of the oxygen atom by ¹⁸O confirmed that the B-O-B skeleton is linear in the solid state, and the spectra have been interpreted in terms of D_2h molecular symmetry.     

Dynamic stereochemistry at sp2?sp3 bonds part 1—the use of heterocyclic models for quantitative studies of steric effects of alkyl groups

The barriers to rotation of the 3-substituent in six 3-RCH₂-4-Me-δ⁴-thiazoline-2-thiones (R ? Me, Et, Pr, Isobu, Isopr,t-Bu) and in two analogous methiodides have been measured by the ¹H DNMR technique. The barriers are discussed in relation to the possible conformations of the transition states, and comparisons are made with barriers to rotation of the same substituents in other frameworks. It is concluded that the relative steric effects are strongly dependent on the framework and that, in general, a conformational analysis of the individual cases is necessary to understand the steric effects of alkyl groups.     

Quantum Chemical Calculation of Conformations and Rotation Barriers of Functional Groups in Arylsulfonyl Halides

The semiempirical PM3 method is used to calculate the potential functions of internal rotation of the functional groups –SO₂Cl, –NO₂, –CH₃, –OCH₃, and –NH₂ of benzenesulfonyl halide molecules (PhSO₂Hal, Hal = F, Cl, Br, I) and twelve substituted derivatives of benzenesulfonyl chloride. Molecular conformations have been determined and internal rotation barriers of the functional groups have been calculated. For meta- and para-substituted benzenesulfonyl chlorides, the projection of the S–Hal bond is perpendicular to the plane of the benzene ring. The rotation barriers of the –SO₂Hal group of benzenesulfonyl halides increase in the series Hal = F, Cl, Br, I. The rotation barriers of the –SO₂Cl group of benzenesulfonyl chloride with meta- and para-substituents slightly increase with the electron-donor properties of the substituent. The rotation barriers of the functional groups of ortho-substituted benzenesulfonyl chlorides are 3 or 4 times as high as those of the meta- and para-isomers. For para-substituted benzenesulfonyl chlorides, the rotation barriers of the functional groups increase in the order –CH₃, –NO₂, –SO₂Cl, –OCH₃, –NH₂.     

A 1H and 13C NMR study of intramolecular rotations in syn- and anti-o-tolyl-di-tert-butyl-carbinols

The syn and anti rotamers of o-tolyl-di-tert-butylcarbinol, 2a and 2b, respectively, have been studied by ¹H NMR at 200 MHz and by natural abundance ¹³C NMR at 50 MHz. ¹H-{¹H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time, T₁, of the 2-Me protons in 2b is unexpectedly higher than that for 2a. The ¹³C relaxation times of the 2-Me and the quaternary carbon of the tert-butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies for t-Bu rotation, measured by ¹H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2-Me and t-Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG? values from this work and from the literature with MM2-calculated ΔH? values indicates that this force field systematically underestimates rotation barriers in open-chain systems by a factor of approximately 0.64.     

Dynamic modulation of fluorine isotropic hyperfine interaction in the 2-nitro-1,4-bis(trifluoromethyl)benzene radical anion in acetonitrile

We have performed measurements and numerical reconstruction of the temperature dependence of the EPR spectrum of the 2-nitro-1,4-bis(trifluoromethyl)benzene radical anion in anhydrous acetonitrile, caused by dynamic modulation of the fluorine isotropic hyperfine interaction by hindered internal rotation of the CF₃ group ortho to the nitro group. The activation energy of rotation is 34.2±0.76 kJ·mol^?1.     

Probing the Rotational Dynamics of meso‐(2‐Substituted)aryl Substituents in A2B‐Type Subporphyrins

A₂B‐type B‐methoxy subporphyrins 3 a – g and B‐phenyl subporphyrins 7 a – c , e , g bearing meso‐(2‐substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable‐temperature (VT) ¹H NMR spectroscopy. In these subporphyrins, the rotation of meso‐aryl substituents is hindered by a rationally installed 2‐substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH^≠ and ΔS^≠ in ΔG^≠. 2‐Methyl and 2‐ethyl groups of the meso‐aryl substituents in subporphyrins 3 e , 3 f , and 7 e induce larger rotational barriers than 2‐alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4‐dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B‐phenyl subporphyrins than for B‐methoxy subporphyrins indicate a negligible contribution of S_N1‐type heterolysis in the rotation of meso‐aryl substituents.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

NMR studies of conformations and dynamic processes—1 : [24]cyclophanes with ethylene bridges

The conformations and dynamic processes in a series of relatively unstrained [2₄]paracyclophanes (with one, two or four -CH₂-CH₂-bridges) and some closely related compounds have been analysed. Their ¹NMR spectra have been recorded at low temperatures and the temperature dependence rationalised as being due to essentially two types of dynamic process-the torsional motion around the sp³-sp³ C-C bonds in the bridges, and the rotation around the sp²-sp³ C-C bonds adjacent to the benzene rings. The barriers to the former process are similar for the series of cyclophanes 1–6 and are due to steric and electronic interactions in the syn-oriented transition states. In cyclophanes 7–9, in which anti-orientations of the aromatic rings are possible, the barriers are lower. The latter process, involving the rotation of the benzene rings, becomes important at temperatures below 150 K and has not been further analysed.