Conformational analysis of 1,2-difluoroethane, an ab-initio investigation

Contrary to expectation, the gauche conformer of 1,2-difluoroethane is more stable than the trans conformer in the gas phase. In order to understand the underlying causes of the “gauche effect”, a complete geometry relaxation was performed for the gauche and trans conformers of 1,2-difluoroethane with the 4-21G, 4-31G and 4-31G** basis sets. The 4-31G** optimized geometry of the gauche conformer compares well with the experimental values obtained from a number of electron-diffraction studies. A correction for the correlation energy, calculated by means of second-order Møller—Plesset perturbation theory with the 6-31G** basis set, proves to be essential to obtain a correct estimate of the energy difference between the gauche and trans conformers 1,2-difluoroethane.     

Conformational study of CH3OP(O)Cl2 and CD3OP(O)Cl2 : Part 2. Gas phase spectra and profile sinulation

The gas phase infrared- and far infrared spectra of the normal and totally deuterated molecules have been recorded and are briefly discussed.

Rigid-rotor asymmetric top profiles for gauche and trans conformers were used to simulate the experimental gas phase infrared absorption profiles of four conformational doublets of CH₃OP(O)Cl₂. From this, a consistent assignment of the more intense component of each doublet to the gauche, and the less intense component to the trans conformer, is deduced.     

The molecular structure and conformational composition of epichlorohydrin as determined by gas phase electron diffraction

The molecular structure of gaseous epichlorohydrin has been investigated using electron diffraction data obtained at 67°C. The conformational composition at this temperature is such that the molecules exist predominantly in a gauche-2 conformer (where the C---Cl bond is 160° away from the C---O) bond). Refinements showed that 33% (σ = 4) of the molecule exist in the gauche-1 form. The important distances (r_g) and angle () with the associated uncertainties are r(C---H) = 1.095(5) Å, r(C---O) = 1.442(3) Å, r(C---C) = 1.475(8) Å, r(C---C_M) = 1.523(7) Å, r(C---Cl) = 1.788(2) Å, CCO = 114° (1), CCC_M = 119°(1), ClCC = 108.9° (7), and Tau(ClCCO) = −150°(10) (gauche-2) and Tau(ClCCO) = 78° (10) (gauche-1).     

Determination of gauche and trans proton vicinal coupling constant Jg and Jt for 1,2-dichloropropane as a model compound for 1,2-disubstituted and 1,2,3-trisubstituted profanes

Variations in temperature and nature of solvent lead to changes in the population of the three rotamers of the title compounds in solution. The medium can be adjusted in this way so that either equality in the population of the three rotamers is reached, or the population of the conformer with two gauche interactions is decreased to near zero. These two limits are used to calculate J_t and J_g, for 1,2-dichloropropane.     

Conformational stability of 1-pentyne from temperature dependent FT-IR spectra of liquid rare gas solutions and ab initio calculations

Variable temperature studies of the infrared spectra (3500–400 cm⁻¹) of 1-pentyne, CH₃CH₂CH₂CCH, dissolved in liquid xenon (−55 to −100°C) and liquid krypton (−105 to −150°C) have been recorded. These data indicate that the anti (methyl group trans to the acetylenic group) and gauche conformers have relative concentrations that vary with the temperature, i.e. enthalpy nonzero. Utilizing seven sets of conformer pairs for the xenon solution and ten sets of conformer pairs for the krypton solution, the enthalpy difference has been determined to be 50±6 cm⁻¹ (0.60±0.07 kJ/mol) and 45±4 cm⁻¹ (0.54±0.05 kJ/mol), respectively, with the anti conformer the more stable form. Because of two equivalent gauche forms, this conformer is estimated to be in higher abundance at 61±1% in the xenon solution and 62±1% in the krypton solution. Optimized geometries and conformational stabilities have been obtained from ab initio calculations with basis sets 6-31G(d), 6-311+G(d,p), 6-311+G(2d,2p) and 6-311+G(2df,2pd) with full electron correlation by the perturbation method to second order (MP2). All of the calculations predict the gauche rotamer to be the more stable form with a high value of 181 cm⁻¹ from the MP2/6-311+G(d,p) calculations and a low value of 107 cm⁻¹ from the MP2/6-311+G(2d,2p) calculation. The r_o adjusted structural parameters have been obtained from a combination of the microwave rotational constants and ab initio predicted parameters. The values are compared to the recently reported values from an electron diffraction study where the value for the CC bond distance appears to be too long. The results are discussed and the conformational stability is compared to those obtained for some similar molecules.     

Conformational and structural studies of ethyl fluorosilane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3200–30 cm⁻¹) of gaseous and solid ethyl fluorosilane, CH₃CH₂SiH₂F, have been recorded. Additionally, the Raman spectra (3200–30 cm⁻¹) of the liquid and solid have been recorded and quantitative depolarization values obtained. Both the gauche and trans conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 54±16 cm⁻¹ (646±191 J/mol) with the gauche conformer the more stable form. This is consistent with the predictions from ab initio, MP2/6-311+G(2d,2p), calculation as well as those with smaller basis sets with full electron correlations. A complete vibrational assignment is proposed for both the trans and gauche conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies, which are supported by normal-coordinate calculations utilizing the force constants from MP2/6-31G(d) ab initio calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311+G(2d,2p) at levels of restricted Hartree–Fock (RHF) and/or Moller Plesset to the second order (MP2) with full electron correlation. The adjusted r₀ parameters have been obtained for both conformers from a combination of the previously reported rotational constants with ab initio predicted values. All results are compared to similar quantities of some corresponding molecules.     

Is there a general rule for the gauche effect in the conformational isomerism of 1,2-disubstituted ethanes?

The stabilities of the gauche and anti conformations of butane, 1,2-dicyanoethane (DCE), and 1,2-dinitroethane (DNE) have been investigated through theoretical calculations. The gauche effect-the tendency of keeping close vicinal electronegative substituents (thetaX-C-C-X approximately 60 degrees ) in an ethane fragment-is expected to drive the conformational equilibrium of DCE and DNE toward the gauche conformation. It was found that, for butane, where the gauche effect is supposed to be poor/null, the hyperconjugation effect contributes mostly to the anti stabilization in opposition to the traditional sense that the methyl groups repel each other, and this should govern its conformational equilibrium. For DCE the equilibrium was shifted to the anti conformer, essentially due to a gauche repulsion, while for DNE, despite the higher electronic delocalization energies, a predominance of the gauche conformer was obtained, and this was attributed mainly to the attractive dipolar interaction between the two nitro groups. A full orbital energy analysis was performed using the natural bond orbital approach, which showed that bond bending and anti-C-H/C-X* hyperconjugation models, usually applied to explain the origin of the gauche effect in fluorinated derivatives, are not adequate to completely explain the conformational behavior of the titled compounds.     

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

Structure and conformational stability of CH2CHCH2X (X=F, Cl and Br) molecules—post Hartree–Fock and density functional theory methods

The molecular structure and conformational stability of CH₂CHCH₂X (X=F, Cl and Br) molecules were studied using ab initio and density functional theory (DFT) methods. The molecular geometries of 3-fluoropropene were optimized employing BLYP and B3LYP levels of theory of DFT method implementing 6-311+G(d,p) basis set. The MP2/6-31G^*, BLYP and B3LYP levels of theory of ab initio and DFT methods were used to optimize the 3-chloropropene and 3-bromopropene molecules. The structural and physical parameters of the molecules are discussed with the available experimental values. The rotational potential energy surface of the above molecules were obtained at MP2/6-31G^* and B3LYP/6-311+G(d,p) levels of theory. The Fourier decomposition of the rotational potentials were analyzed. The HF/6-31G^* and MP2/6-31G^* levels of theory have predicted the cis conformer as the minimum energy structure for 3-fluoropropene, which is in agreement with the experimental values, whereas the BLYP/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory reverses the order of conformation. The ΔE values calculated for 3-chloropropene at MP2/6-31G^*, BLYP/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory show that the gauche form is more stable than the cis form, which is in agreement with the experimental value. The same levels of theory have also predicted that the gauche form is stable than cis for 3-bromopropene molecule. The maximum hardness principle has been able to predict the stable conformer of 3-fluoropropene at HF/6-31G^* level of theory, but the same level of theory reverses the conformational stability of 3-chloropropene and 3-bromopropene molecules and MP2/6-31G^* level of theory predicted the stable conformer correctly.     

Ab initio structures and vibrational analysis of the isoprene conformers

Complete gradient optimizations of the structures and the calculation of the harmonic force fields of the s-trans(anti) and gauche conformers of isoprene (2-methylbuta-1,3-diene) are reported at the RHF/6-31G level. The dihedral angle of the gauche conformer is found to be 41.0° from the planar s-cis(syn) form. The force fields obtained are refined using scale factors transferred from analogous calculations for trans-butadiene-1,3 and ethane. The direct vibrational problems are solved for both conformers of isoprene. A complete assignment of the experimental vibrational frequencies is given.     

Ab initio molecular orbital study of dialuminum-ethylene complexes

The geometries of the various isomeric dialuminum-ethylene complexes Al₂C₂H₄ have been optimized using the STO-3G, 3-21G^(*) and 6-31G^(**) basis sets. Vibrational frequency calculations show that only two out of the six structures examined are thermodynamically stable. Unlike the case of Li₂C₂H₄, 1,2-dialuminoethane is most stable and more ethane-like than 1,2-dilithioethane perhaps due to the larger covalent character of the C---Al bond. Besides, 1,2-dialuminoethane has been found, in contrast to ethane and 1,2-dilithioethane, to have no stable gauche conformer.     

Structure and conformational equilibrium of thiacalix[4]arene by density functional theory

Density functional theory calculations for the structure and conformational equilibrium of thiacalix[4]arene are reported. The conformational equilibrium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms is compared to the conformational equilibrium of calix[4]arene. Thiacalix[4]arene conformational energies relative to the cone conformer (ΔE's) are reduced in comparison with calix[4]arene. This conformational change is in qualitative agreement with recent NMR spectroscopy measurements of the conformational equilibrium for a tetraethylether of thiacalix[4]arene in a CDCl₃ solution which indicates an enhanced chemical exchange of thiacalixarene conformers in comparison with similar methylene bridged structures. Density functional theory results for the structure of thiacalix[4]arene are in good agreement with recent X-ray diffraction measurements. The electrostatic potentials in the cone conformers of thiacalix[4]arene and calix[4]arene suggest that their complexation or recognition abilities can be significantly different. Dipole moments of the four thiacalix[4]arene conformers are in the order: cone>1,2-alternate>partial-cone>1,3-alternate.     

Why Doesn't all-trans-1,2,3,4,5,6-Hexaspiro(THF)cyclohexane complex metal ions?

Despite the inherent preference for placing alkyl substituents, rather than alkoxy substituents, in equatorial positions, all-trans-hexaspiro(THF)cyclohexane strongly favors the all-O-equatorial conformer. Ab initio and density functional calculations on a series of cyclohexane derivatives containing one, two, or three spiro(THF) units demonstrate that this preference results from at least two important factors. First, when oxygen atoms are attached to adjacent carbons, the gauche effect favors the di-O-equatorial arrangement. In trans-1,2-dispiro(THF)cyclohexane, the single gauche interaction overcomes the inherent steric preference for projecting the two oxygen atoms axially. Similarly, in the all-trans-hexaspiro(THF)cyclohexane the six gauche interactions in the all-O-equatorial conformer overpower the inherent conformational biases of the six isolated spiro(THF) moieties. Nevertheless, the gauche effect only partially accounts for the more than 20 kcal/mol conformational bias calculated for this molecule. There is also another factor, the high energetic cost associated with projecting multiple alkoxy substituents axially on the same face of a cyclohexane scaffold. The calculations find the energetic cost of each 1,3-diaxial interaction is about 2 kcal/mol larger between alkoxy substituents than between alkyl substituents.     

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of 2-hexyne

The infrared spectra (3500–50 cm⁻¹) of the gas and solid and the Raman spectra (3500–50 cm⁻¹) of the liquid and solid have been recorded for 2-hexyne, CH₃–CC–CH₂CH₂CH₃. Variable temperature studies of the infrared spectrum (3500–400 cm⁻¹) of 2-hexyne dissolved in liquid krypton have also been recorded. Utilizing four anti/gauche conformer pairs, the anti(trans) conformer is found to be the lower energy form with an enthalpy difference of 74±8 cm⁻¹ (0.88±0.10 kJ/mol) determined from krypton solutions over the temperature range −105 to −150 °C. At room temperature it is estimated that there is 42% of the anti conformer present. Equilibrium geometries and energies of the two conformers have been determined by ab initio (HF and MP2) and hybrid DFT (B3LYP) methods using a number of basis sets. Only the HF and DFT methods predict the anti conformer as the more stable form as found experimentally. A vibrational assignment is proposed based on the force constants, relative intensities, depolarization ratios from the ab initio and DFT calculations and on rotational band contours obtained using the calculated equilibrium geometries. From calculated energies it is shown that the CH₃ group exhibits almost completely free rotation which is in agreement with the observation of sub-band structure for the degenerate methyl vibrations from which values of the Coriolis coupling constants, ζ, have been determined. The results are compared to similar properties of some corresponding molecules.     

Conformational Plasticity in Glycomimetics: Fluorocarbamethyl‐L‐idopyranosides Mimic the Intrinsic Dynamic Behaviour of Natural Idose Rings

Sugar function, structure and dynamics are intricately correlated. Ring flexibility is intrinsically related to biological activity; actually plasticity in L ‐iduronic rings modulates their interactions with biological receptors. However, the access to the experimental values of the energy barriers and free‐energy difference for conformer interconversion in water solution has been elusive. Here, a new generation of fluorine‐containing glycomimetics is presented. We have applied a combination of organic synthesis, NMR spectroscopy and computational methods to investigate the conformational behaviour of idose‐ and glucose‐like rings. We have used low‐temperature NMR spectroscopic experiments to slow down the conformational exchange of the idose‐like rings. Under these conditions, the exchange rate becomes slow in the ¹⁹F NMR spectroscopic chemical shift timescale and allows shedding light on the thermodynamic and kinetic features of the equilibrium. Despite the minimal structural differences between these compounds, a remarkable difference in their dynamic behaviour indeed occurs. The importance of introducing fluorine atoms in these sugars mimics is also highlighted. Only the use of ¹⁹F NMR spectroscopic experiments has permitted the unveiling of key features of the conformational equilibrium that would have otherwise remained unobserved.     

Nonlinear dielectric effect (NDE) and molecular orbital study of the conformational equilibrium in 1,4-dimethoxybenzene in benzene solution

The conformational equilibrium in 1,4-dimethoxybenzene (1,4-DMB) in benzene solutions has been studied. On the basis of experimental values of the nonlinear dielectric effect (NDE) parameter, electric permittivity and density, determined in this work, and applying the general statistical theory of NDE, the contributions of the syn-anti and syn-syn conformers and the electric dipole moment of the polar syn-syn conformer were calculated. The molecular orbital method (PM3) has also been applied for calculation of the dipole moments and energies of particular conformers. The results of the NDE study and PM3 calculations are consistent and they reveal the existence of two conformers (syn-anti and syn-syn) of comparable energy values, but different values of dipole moments, and the predominance of the polar form (syn-syn) of the mole fraction in benzene. Moreover, the energies of intermolecular interactions have been determined from the concentration dependence of linear and nonlinear polarisability.     

Conformational equilibrium of 1,2-dichloroethane in water: comparison of PCM and RISM-SCF methods

The RISM-SCF and polarizable continuum model (PCM) approaches have been applied to study the conformational equilibrium of 1,2-dichloroethane (DCE) in water. Both the electron correlation effect and basis sets play an important role in the relative energies of the gauche and trans conformers in gas and solution phases. Both PCM and RISM-MP2 methods resulted in a consistent trend with the previous experimental and theoretical studies that the population of the gauche conformer increases in going from the gas phase to the aqueous solution. However, the PCM treatment could not describe the solvent effect completely in that the sign of the relative free energy of the gauche and trans forms is opposite to the most recent experimental and theoretical data, while the RISM-MP2 gives the right sign in the free energy difference. We found that the larger excess chemical potential gain (by ca. -4.1 kcal/mol) for the gauche conformer is large enough to result in the gauche preference of DCE in water, though it has to compensate for more solute reorganization energy (approximately 1.6 kcal/mol) and overcome the energy difference (approximately 1.6 kcal/mol) in the gas phase. The radial distribution functions between DCE and the nearest water shows that the electrostatic repulsion between chlorine and oxygen atoms is higher in the trans conformer than in the gauche one, while the attractive interaction between chlorine and hydrogen of water is higher in the gauche conformer.     

Interaction in trans-2-halocyclohexanols — an infrared and theoretical study

The O–H and C–O stretching frequencies of trans-2-halocyclohexanols in CCl₄ solutions have been measured and theoretical calculations have been performed to elucidate the main interactions, which are responsible for the conformational equilibria in these systems. It can be concluded that hydrogen bonding is predominant for trans-2-fluorocyclohexanol, leading to a stabilization of the eq–eq conformation, while for the chlorine, bromine and iodine derivatives, besides hydrogen bonding, gauche and steric interactions are also present.     

3-Bromo-2-methyl-1-propene: molecular structure and conformation in the gas phase as determined by electron diffraction and molecular mechanics calculation

A gas phase electron diffraction study of 3-bromo-2-methyl-1-propene shows that there is predominantly a gauche conformer present. Data recorded at 20 and 180°C show 4(8) and 5(4)% respectively of a second confomer with a planar heavy atom skeleton. The gauche structural results in terms of r_a distances and angles at 20°C were found to be: r(C---C) = 1.331(9) Å, r(C---CH₂Br) = 1.484(6) Å, r(C---CH₃) — r(C---CH₂Br) = 0.017 Å, (assumed), r(C---Br) = 1.965(6) Å, C=C---CH₂Br = 121.5(0.7)°, C=C---CH₂Br — C=C---CH₃ = 0.7° (constraint from molecular mechanics calculation), C---C---Br = 112.2(0.5)°, torsional ANGLE = 112.5(2.2)°. Uncertainties are given as 2σ, where σ includes uncertainties due to correlation among observations, electron wavelength and other parameters used in the data reduction. The results obtained from the 180°C data agree very well with those given above. The molecular mechanics calculations yield information consistent with the experimental results.     

Conformational stability of 1-bromo-2-fluoroethane from temperature dependent FT-IR spectra of rare gas solutions

Variable temperature (−55 to −135°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-bromo-2-fluoroethane, BrCH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. From these data, the enthalpy difference has been determined to be 108±9 cm⁻¹ (1.296±0.113 kJ/mol) and 112±8 cm⁻¹ (1.346±0.098 kJ/mol) from the krypton and xenon solutions, respectively, with the trans conformer the more stable rotamer. Complete vibrational assignments are presented for both conformers which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G^* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G^* and/or MP2/6-31G^* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate. Structural parameters and conformational stability have also been obtained from MP2/6-311+G^** calculations. Combining the ab initio predicted structural parameters with the microwave rotational constants, r_o parameters have been obtained for the gauche conformer.