Eq Conformer Research Articles

The conformational behavior and structure of monosubstituted 1,3,5‐trisilacyclohexanes—Part III: 1‐Methyl‐1,3,5‐trisilacyclohexane

Abstract1‐Methyl‐1,3,5‐trisilacyclohexane was synthesized and its structure and conformational properties have been determined by gas‐phase electron diffraction (GED) and quantum chemical (QC) calculations. The molecule may exist in two forms differing from each other by the substituent's position. QC results show that the equatorial conformer is predicted to be slightly more stable than the axial conformer; note that one method (M06‐2X) with basis set 6‐311G** shows an equal amount of Ax and Eq conformers: ratio (Ax/Eq) = (30–50):(70–50)% (depending on the method and basis set). From the GED data, the molar fractions of the conformers were found to be Ax:Eq = 54(10):46(10) at 280(5) K. A temperature‐dependent Raman experiment resulted in an Ax:Eq ratio of 58(4):42(4). Conformational properties are compared in series of analogous 1‐X‐1‐(hetero)cyclohexanes.

A Computational Study of the Conformational Behavior of 2,5-Dimethyl- 1,4-dithiane-2,5-diol and Analogous S and Se: DFT and NBO Study

Conformational behaviors of 2,5-dimethyl-1,4-dithiane-2,5-diol (compound 1), 2,5- dimethyl-1,4-dithiane-2,5-dithiol (compound 2) and 2,5-dimethyl-1,4-dithiane-2,5-diselenol (compound 3) were investigated by the B3LYP/6-311+G **, the M06-2X/aug-ccpvdz levels of theory and natural bond orbital NBO analysis. The structures and the structural parameters of the mentioned molecules were optimized by the B3LYP and the M06-2X methods. We assessed the roles and contributions of the effective factors in the conformational properties of the mentioned compounds by means of the B3LYP and M06-2X levels of theory and the NBO interpretations. The stereoelectronic effects of the mentioned molecules were studied using the NBO analysis. The results showed that the stereoelectronic effects were in favor of the (ax,ax) conformers (the most stable conformations), from compound 1 to compound 3; therefore, these effects have impacts on the conformational properties of compounds 1-3, and stabilization energies associated with LP2X→ σ*S1-C2 electron delocalization, where [X= O, S, and Se], for 1-ax, ax conformer has the greatest value between all of the other conformers. Therefore, according to the calculated thermodynamic parameters, the stability of the 1-ax, ax compound was justified by the presence of LP2X→σ*S1-C2 electron delocalization. A molecular orbital explanation was conducted to investigate the correlations between the linear combinations of natural bond orbitals in the HOMOs, LUMOs and the molecular reactivity parameters. There is a direct relationship between the stereoelectronic effects, molecular reactivity and thermodynamic parameters of compounds 1 to 3 as the harder ax, ax conformations with the greater stereoelectronic effects and ΔG(eq-ax) values are more stable than their corresponding eq, eq conformers. Besides frontier molecular orbitals (FMOs), mapped molecular electrostatic potential (MEP) surfaces of conformations of compounds 1 to 3 were investigated.

Theoretical Study on Thermally Activated Delayed Fluorescence Emitters in White Organic Light-Emitting Diodes: Emission Mechanism and Molecular Design

Employing dual-emission thermally activated delayed fluorescence (TADF) molecules to fabricate single emitters in highly efficient white organic light-emitting diodes (WOLEDs) is still a challenge. In this work, a systematic study on the luminescence mechanism of three TADF molecules with blue-orange dual emission is performed. It is found that the quasi-axial (ax) conformer is responsible for blue normal fluorescence (NF), while the quasi-equatorial (eq) conformer is responsible for orange TADF. In addition, the upconversion for the TADF molecules happens from the first triplet state (T1) and the second triplet state (T2) to the first singlet state (S1) in the quasi-equatorial conformers. In addition, the transition from T1 to S1 and T2 to S1 also happens in the process of conformation transformation. Besides, our study also indicates that the donor with an asymmetric bond length in the six-membered ring could favor the generation of dual conformations and dual emission. Some molecules with PTZ or PXZ as donors are predicted as potential emitters with white light emission. Our research would favor the design of TADF emitters with dual emission in WOLEDs.

Conformational Study and Chiroptical Properties of Chiral Dimethyl-Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF).

The enantiopure dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) donor exists as biaxial (ax, ax) and biequatorial (eq, eq) conformers in equilibrium. DFT calculations combined with free energy surface (FES) analysis establish that the (ax, ax) form is more stable by ∼0.7 kcal·mol-1 than the (eq, eq) one and that the inter-conversion process involves a sequential conformational change through a boat type (ax, eq) conformer with an activation energy of ∼3 kcal.mol-1. TD-DFT calculations show similar, but opposite in sign, CD bands for the two conformers. A Boltzmann type average of the two CD curves, corresponding to a ratio of three (ax, ax) for one (eq, eq) conformers in equilibrium in solution, provides an excellent agreement with the experimental curve.

R0 Structural parameters, conformational, vibrational studies and ab initio calculations of cyanocyclopentane

The infrared and Raman spectra (3100–50cm−1) of the gas, liquid or solution, and solid have been recorded of cyanocyclopentane, c-C5H9CN. Variable temperature (−60 to −100°C) studies of the infrared spectra (3100–400cm−1) of the sample dissolved in liquid xenon have been carried out. From these data, both the envelope-equatorial (Eq) and Ax conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 55±12cm−1 (0.66±0.14kJ/mol) with the Eq conformer the more stable form. The percentage of the Ax conformer is estimated to be 45±1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for both conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The r0 structural parameter values for the Eq[Ax] form are; for the heavy atom distances (Å): CN=1.160 [1.160] (3); Cα–C=1.463 [1.463] (3); Cα–Cβ, Cβ′=1.543 [1.545] (3); Cβ–Cγ, Cγ′=1.540 [1.541] (3); Cγ–Cγ′=1.552 [1.553] (3) and angles (°): ∠Cα–CN=179.0 [178.9] (5); ∠CβCα–C=113.1 [110.1] (5); ∠CβCαCβ′=103.0 [102.1] (5); ∠CαCβCγ=104.1 [104.8] (5); ∠CβCγCγ′=106.3 [106.0] (5). The results are discussed and compared to the corresponding properties of some related molecules.

Microwave, structural, conformational, vibrational studies and ab initio calculations of isocyanocyclopentane

The infrared and Raman spectra (3200–50cm−1) of the gas, liquid or solution, and solid have been recorded of isocyanocyclopentane, c-C5H9NC. FT-microwave studies have also been carried out and 23 transitions were recorded for the envelope-axial (Ax) conformer. Variable temperature (−65 to −100°C) studies of the infrared spectra (3200–400cm−1) dissolved in liquid xenon have been carried out. From these data, both the Ax and envelope-equatorial (Eq) conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 102±10cm−1 (1.21±0.11kJmol−1) with the Ax conformer the more stable form. The percentage of the Eq conformer is estimated to be 38±1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for both conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The structural parameter values for the Ax form are; for the heavy atom distances (Å): CN=1.176 (3); CαN=1.432 (3); CαCβ,Cβ′=1.534 (3); CβCγ,Cγ′=1.542 (3); CγCγ′=1.554 (3) and angles (°):∠CαNC=177.8 (5); ∠CβCαN=110.4 (5); ∠CβCαCβ′=102.9 (5); ∠CαCβCγ=103.6 (5); ∠CβCγCγ′=105.9 (5). The results are discussed and compared to the corresponding properties of some related molecules.

Spectroscopic and Theoretical Studies of Some 3-(4′-Substituted phenylsulfanyl)-1-methyl-2-piperidones

The analysis of the IR carbonyl bands of some 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones 1–6 bearing substituents: NO2 (compound 1), Br (compound 2), Cl (compound 3), H (compound 4) Me (compound 5) and OMe (compound 6) supported by B3LYP/6-31+G(d,p) and PCM calculations along with NBO analysis (for compound 4) and X-ray diffraction (for 2) indicated the existence of two stable conformations, i.e., axial (ax) and equatorial (eq), the former corresponding to the most stable and the least polar one in the gas phase calculations. The sum of the energy contributions of the orbital interactions (NBO analysis) and the electrostatic interactions correlate well with the populations and the νCO frequencies of the ax and eq conformers found in the gas phase. Unusually, in solution of the non-polar solvents n-C6H14 and CCl4, the more intense higher IR carbonyl frequency can be ascribed to the ax conformer, while the less intense lower IR doublet component to the eq one. The same νCO frequency trend also holds in polar solvents, that is νCO (eq) < νCO (ax). However, a reversal of the ax/eq intensity ratio occurs going from non-polar to polar solvents, with the ax conformer component that progressively decreases with respect to the eq one in CHCl3 and CH2Cl2, and is no longer detectable in the most polar solvent CH3CN. The PCM method applied to compound 4 supports these findings. In fact, it predicts the progressive increase of the eq/ax population ratio as the relative permittivity of the solvent increases. Moreover, it indicates that the computed νCO frequencies of the ax and eq conformers do not change in the non–polar solvents n-C6H14 and CCl4, while the νCO frequencies of the eq conformer become progressively lower than that of the ax one going from CHCl3 to CH2Cl2 and to CH3CN, in agreement with the experimental IR values. The analysis of the geometries of the ax and eq conformers shows that the carbonyl oxygen atom of the eq conformer is free for solvation, while the O[CO]…H[o-Ph] hydrogen bond that takes place in the ax conformer partially hinders the approach of the solvent molecules to the carbonyl oxygen atom. Therefore, the larger solvation that occurs in the carbonyl oxygen atom of the eq conformer is responsible for the observed and calculated decrease of the corresponding frequency. The X-ray single crystal analysis of 2 indicates that this compound adopts the most polar eq geometry in the solid. In fact, in order to obtain the largest energy gain, the molecules are arranged in the crystal in a helical fashion due to dipole moment coupling along with C-H…O and C-H…πPh hydrogen bonds.

Conformational and structural studies of ethynylcyclopentane from temperature dependent Raman spectra of xenon solutions, infrared spectra, and ab initio calculations

Variable temperature (−50 to −100°C) Raman spectra (3500–136cm−1) were recorded of ethynylcyclopentane, c-C5H9CCH in liquid xenon. The envelope-equatorial (Eq) conformer was determined more stable than envelope-axial (Ax) form with enthalpy difference of 94±9cm−1 (1.12±0.11kJ/mol) and 39±2% Ax conformer present at ambient temperature. The conformational stabilities have been predicted from ab initio calculations with basis sets up to aug-cc-pVTZ. From previously reported microwave rotational constants and ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters are reported Eq [Ax] distances (Å) rCC=1.211(3) [1.211(3)], rCαCC=1.461(3) [1.467(3)], rCαCβ=1.542(3) [1.542(3)], rCβCγ=1.541(3) [1.542(3)], rCγCγ′=1.556(3) [1.555(3)] and angles (°) ∠CαCC=179.4(5) [179.9(5)], ∠CβCαCC=113.7(5) [111.5(5)], ∠CβCαCβ′=102.6(5) [102.1(5)], ∠CαCβCγ=103.7(5) [103.7(5)], ∠CβCγCγ′=106.0(5) [105.9(5)] and τCβCαCβ′Cγ′=40.8(5) [41.6(5)]. Vibrational assignments are reported, supported by ab initio predictions and results are discussed.

Conformational analysis of 3,3‐dimethyl‐3‐silathiane, 2,3,3‐trimethyl‐3‐silathiane and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane—preferred conformers, barriers to ring inversion and substituent effects

AbstractThe first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1, 2,3,3‐trimethyl‐3‐silathiane 2, and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ΔG≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (−ΔG° = 0.35 kcal/mol) derived from low‐temperature 13C NMR data is fairly consistent with the calculated value. For compound 2, theoretical calculations give ΔE value close to zero for the equilibrium between the 2‐Meax and 2‐Meeq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (−ΔG° = 1.8 kcal/mol) and the predominance of the 3‐SiMe3 eq conformer at room temperature was confirmed by the simulated 1H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The effect of spiroadamantane substitution on the conformational preferences of N-Me pyrrolidine and N-Me piperidine: a description based on dynamic NMR spectroscopy and ab initio correlated calculations

Dynamic NMR spectroscopy and ab initio correlated calculations revealed that the attachment of a spiroadamantane entity at the C-2 position of N-methylpyrrolidine or N-methylpiperidine induces a severe steric crowding around nitrogen, which changes the conformational space of the heterocycle resulting in: (a) the complete destabilization of the N-Me(eq) conformer in spiranic structures; in contrast the N-Me(eq) conformer corresponds to the global minimum in N-methylpyrrolidine or N-methylpiperidine. The spiroadamantane structure raises the energy of the equatorial conformer because of the severe van der Waals repulsion between the N-Me(eq) group and adamantane C–H bonds. (b) The interconversion between the only populated enantiomeric N-Me(ax) conformers ax→[eq]→ax′; the interconversion eq→ax between N-Me(eq) and N-Me(ax) conformers, which are both populated, is observed in N-methylpyrrolidine or N-methylpiperidine. (c) The raising of ring and nitrogen inversion barriers ax→ts by ∼4–6 kcal mol −1. The dynamic NMR study provides evidence that the most important process required for the enantiomerization between the axial N-Me conformers in spiropiperidine 4 and spiropyrrolidine 5 are different, i.e., a nitrogen inversion in 5 (9.10 kcal mol −1) and a ring inversion in 4 (15.2 kcal mol −1). While an enantiomerization interconverts N-Me axial conformers in spiropiperidine 5 and spiropyrrolidine 4, substitution of the pyrrolidine ring of 5 with a C-Me group effects a diastereomerization between two N-Me axial conformers and reduces effectively the nitrogen inversion barrier according to the protonation experiments and the calculations. In general, all the calculations levels used, i.e., the MM3, B3LYP/6-31+G ∗∗ and MP2/6-311++G ∗∗//B3LYP/6-31+G ∗∗, predict correctly the different stability of the local minima; however only MP2/6-311++G ∗∗//B3LYP/6-31+G ∗∗ was found to be reliable for the calculation of the nitrogen inversion barriers.

Concentration and solvent effects on the conformational equilibrium of cis-3-ethoxycyclohexanol by 1H NMR and IR spectroscopy

NMR data, in CCl 4, show that an increase in the concentration of cis-3-ethoxycyclohexanol ( cis-3-ECH) shifts the conformational equilibrium from the ax–ax conformer (X ax–ax=51% at 0.01 mol L −1), stabilized by an intramolecular hydrogen bond (IAHB), to the eq–eq conformer (X eq–eq=67% at 0.40 mol L −1), which forms intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The Δν values, obtained by IR spectra, indicated that cis-3-ECH presents a stronger IAHB than the cis-3-methoxycyclohexanol, due to the increase in substituent steric and group electronegativity effects. The percentage of eq–eq conformer also increases with the solvent basicity, from 51% (ΔG eqeq– axax=−0.03 kcal mol −1 in CCl 4 to 97% (ΔG eqeq– axax=−2.05 kcal mol −1) in DMSO. Values of 4.58 and 5.39 kcal mol −1, for the IAHB strength in cis-3-ECH, were obtained from the theoretical data at CBS-4M and B3LYP/6-311+G** levels, respectively.

Conformer dipole moment and syn-1,3-diaxial steric effect on the conformational equilibrium of the cis isomer of some 1,3-disubstituted cyclohexanes

The theoretical values of the dipole moment for the diaxial (ax–ax) conformer are larger than for the eq–eq conformer of some cis-3-halocyclohexanols (halo=Cl, Br and I) and cis−3-halo-1-methoxycyclohexanes (halo=F, Cl, Br and I). The experimental 3 J HH coupling constants are rather large (>10 Hz) indicating that the diequatorial (eq–eq) conformer is predominant for all compounds in the solvents studied. Dipole moments and NMR data lead to the conclusion that the conformational equilibra of these 1,3-derivatives are not controlled by the solvent polarity, as is observed for the 1,2-disubstituted cyclohexanes, but are dictated by the syn-1,3-diaxial steric effect, which predominates over stabilization of the ax–ax conformer by an intramolecular hydrogen bond (IAHB).

Conformational analysis of trans‐2‐halocyclohexanols and their methyl ethers: a 1H NMR, theoretical and solvation approach

AbstractThe conformational equilibria of trans‐1‐methoxy‐2‐chloro‐ (1), trans‐1‐methoxy‐2‐bromo‐ (2) and trans‐1‐methoxy‐2‐iodocyclohexane (3), and their corresponding alcohols (4–6), were studied through a combined method of NMR, theoretical calculations and solvation theory. They can be described in terms of the axial–axial and equatorial–equatorial conformations, taking into account the main rotamers of each of these conformations. From the NMR experiments at 183 K in CD2Cl2–CS2, it was possible to observe proton H2 in the ax–ax and eq–eq conformers separately for 1 and 2, but not for 3, which gave directly their populations and conformer energies. In the alcohols the proportion of the ax–ax conformer was too low to be detected by NMR under these conditions. Those HH couplings together with the values at room temperature, in a variety of solvents allowed the determination of the solvent dependence of the conformer energies and hence the vapor state energy difference. The ΔE (Eax–Eeq) values in the vapor state for 1, 2 and 3 are −0.05, 0.20 and 0.55 kcal mol−1, respectively, increasing to 1.10, 1.22 and 1.41 kcal mol−1 in CD3CN solution (1 kcal = 4.184 kJ). For 4–6 the eq–eq conformation is always much more stable in both non‐polar and polar solvents, with energy differences ranging from 1.78, 1.94 and 1.86 kcal mol−1 (in CCl4) to 1.27, 1.49 and 1.54 kcal mol−1 (in DMSO), respectively. Comparison of the hydroxy and methoxy compounds gives the intramolecular hydrogen bonding energy for the alcohols as 1.40, 1.36 and 1.00 kcal mol−1 (in CCl4) for 4, 5 and 6, respectively. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Solution-Phase Conformations of N-Acetyl-N‘-methyl-l-alaninamide from Vibrational Raman Optical Activity

Experimental and ab initio theoretical vibrational Raman optical activity (VROA) spectra are presented for N-acetyl-N‘-methyl-l-alaninamide (NANMLA), in order to determine the predominant conformations of this molecule in the solution phase. The experimental spectra were obtained in three different solvents, CHCl3, H2O, and D2O. The ab initio VROA spectra were predicted for nine different conformations of NANMLA optimized with the 6-31G* basis set. The vibrational frequencies and normal modes for all nine conformers were also obtained with the 6-31G* basis set. From a comparision of the predicted and observed VROA spectral patterns, it is suggested that there is reasonable support for the presence of C7,eq−C5, C7,eq, and αR conformers in aqueous solution and for C7,eq−C5 and αR conformers in chloroform solutions.

Conformational analysis. Part 28. OH⋯F hydrogen bonding and the conformation of trans-2-fluorocyclohexanol

The 1H and 13C NMR spectra of trans-2-fluorocyclohexanol 1 and the methyl ether 2 have been obtained in a variety of solvents and temperatures. From the low-temperature spectra the proportions of the ax–ax and eq–eq conformers were obtained by direct integration together with the vicinal HH couplings of the 2-proton in the dominant eq–eq conformer. From these results the conformer populations and energies in a variety of solvents are given.In 1 ΔG(aa–ee) varies from 1.5 kcal mol–1(1 cal = 4.184 J) in non polar solvents (e.g. CCl4) to ca. 1.2 kcal mol–1 in very polar solvents (acetone), whereas in 2 the corresponding values are 0.4 and 1.0 kcal mol–1. The proton donor solvents CDCl3 and CD2Cl2 are exceptions due to preferential CH⋯O hydrogen bonding in the eq–eq form.These figures are explained by solvation theory, which also provides the vapour state free energy differences of 1.6 kcal mol–1 1 and 0.1 kcal mol–1 2. In 1 ΔS is zero and ΔH equals ΔG but in 2 values of ΔS of 2.0 cal mol–1 K–1 and of ΔH of 1.1 kcal mol–1 in non polar media and 0.7 kcal mol–1 for the vapour are obtained. These values may be compared with those calculated by ab initio theory at the 6-31G*(MP2) level of 1.1 kcal mol–1 1 and –0.75 kcal mol–1 2. In both cases the eq–eq conformer is more stable than predicted, by 0.5 and 1.5 kcal mol–1.Comparison of the conformer energies with those obtained from the ΔG values for the monosubstituted cyclohexanes gives the OH⋯F hydrogen bonding attraction in the eq–eq conformer as 1.6 kcal mol–1 whilst the gauche OMe⋯F interaction is neutral, neither repulsive nor attractive. These figures support previous theoretical interpretations that the gauche form of 2-fluoroethanol is predominant due to OH⋯F hydrogen bonding and show also that the previous discrepancy between experimental measurements in the condensed phase and theory is due to solvation.

The conformational analysis of saturated heterocycles. Part 92. Conformational equilibria of 1,2-dioxa-4,5-diazacyclohexanes

Proton and 13C n.m.r. spectra indicate that 4,5-dimethyl- and 4,5-diethyl-1,2-dioxa-4,5-diazacyclohexanes exist very predominantly in the equilibrating ax–eq and/or ax–ax conformations: for the di-isopropyl analogue this co-exists with ca. 20% of the eq–eq conformer.