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

The 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 ‐Me_ax and 2 ‐Me_eq 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 ‐SiMe_{3 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 & Sons, Ltd.     

Conformational analysis of N‐phenyl‐ and N‐trifyl‐4,4‐dimethyl‐4‐silathiane 1‐sulfimides

N‐Substituted 4,4‐dimethyl‐4‐silathiane 1‐sulfimides [R = Ph ( 1 ), CF₃ ( 2 )] were studied experimentally by variable temperature dynamic NMR spectroscopy. Low temperature ¹³C NMR spectra of the two compounds revealed the frozen ring inversion process and approximately equal content of the axial and equatorial conformers. Calculations of the 4‐silathiane derivatives 1 , 2 and the model compound [R = Me ( 3 )] as well as their carbon analogs, the similarly N‐substituted thiane 1‐sulfimides [R = Ph ( 4 ), CF₃ ( 5 ), Me ( 6 )] at the DFT/B3LYP/6–311G(d,p) level in the gas phase and in chloroform solution using the PCM model at the same level of theory showed a strong dependence of the relative stability of the conformer on the solvent. The electronegative trifluoromethyl group increases the relative stability of the axial conformer. Copyright © 2010 John Wiley & Sons, Ltd.     

Conformational analysis of 4,4‐dimethyl‐4‐silathiane and its S‐oxides

4,4‐Dimethyl‐4‐silathiane and its S‐oxides [n = 0 ( 1 ), 1 ( 2 ), 2 ( 3 )] were studied experimentally by variable temperature dynamic NMR spectroscopy down to 103 K and the frozen ring inversion was revealed for all three compounds. The barriers for the degenerate ring inversion in 1 and 3 were measured to be 4.8 and 5.0 kcal/mol at the coalescence temperatures of 111 and 116 K, respectively, and practically coincide with the calculated barriers of 4.60 kcal/mol in 1 and 4.46 kcal/mol in 3 . The frozen equilibrium mixture 2‐ax/2‐eq contains 37% of the 2‐ax and 63% of the 2‐eq conformer. The ring inversion barrier proved to be ca. 4.8 kcal/mol. Calculations at the B3LYP/6‐311+G(d,p) level of theory showed the 2‐ax conformer to be 0.90 kcal/mol more stable than the 2‐eq conformer in the gas phase whereas in solution the relative stability of the conformers calculated using the PCM model at the same level of theory is inverted to become 0.19 (in CHCl₃) or 0.36 kcal/mol (in DMSO) in favor of the 2‐eq conformer. The chair–chair interconversion mechanism of sulfoxide 2 includes two intermediate energetically equivalent 1,4‐twist forms and the 2,5‐boat transition state: 2‐ax (chair) ? 2 (1,4‐twist) ? [ 2 (2,5‐boat)]^≠ ? 2 (1,4‐twist) ? 2‐eq (chair). The calculated ring inversion barriers are 5.1 ( 2‐ax → 2‐eq ) and 4.2 kcal/mol ( 2‐eq → 2‐ax ) in the gas phase, and 4.03 and 4.22 kcal/mol, respectively, in chloroform. Copyright © 2011 John Wiley & Sons, Ltd.     

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes: synthesis,structure, and conformational analysis

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes RN[CH₂Si(Me)₂]₂O [R = Me ( 1 ), i‐Pr ( 2 )] were synthesized by two methods which provided good yields up to 84%. Low temperature NMR study of compounds ( 1 ) and ( 2 ) revealed a frozen ring inversion with the energy barriers of 8.5 and 7.7 kcal/mol at 163 and 143 K, respectively, which is substantially lower than that for their carbon analog, N‐methylmorpholine. DFT calculations performed on the example of molecule ( 1 ) showed that N? Me_ax conformer to exist in the sofa conformation with the coplanar fragment C? Si? O? Si? C, and its N? Me_eq conformer in a flattened chair conformation. Copyright © 2009 John Wiley & Sons, Ltd.     

Conformational analysis of 4,4‐dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane

4,4‐Dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane 1 and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane 2 were studied by variable temperature dynamic ¹H, ¹³C, ¹⁹F NMR spectroscopy and theoretical calculations at the DFT (density functional theory) and MP2 (Møller‐Plesset 2) levels of theory. Both kinetic (barriers to ring inversion) and thermodynamic data (frozen conformational equilibria) could be obtained for the two compounds. The computations revealed two minima on the potential energy surface for molecules 1 and 2 corresponding to the rotamers with the CF₃SO₂ group directed ‘inward’ and ‘outward’ the ring, the latter being 0.2–0.4 kcal/mol (for 1 ) and 1.1 kcal/mol (for 2 ) more stable than the former. The vibrational calculations at the DFT and MP2 levels of theory give the values of the free energy difference ΔG^o for the ‘inward’ ‘outward’ equilibrium consistent with those determined from the experimentally measured ratio of the rotamers. The structure of crystalline compound 2 was ascertained by X‐ray diffraction analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and rotation barriers in 2, 6‐Di‐(o‐anisyl) anisole

Variable temperature ¹H NMR spectroscopic studies of 2, 6‐di(o‐anisyl) anisole show syn and anti atropisomers at low temperature. The barrier for interconverting these isomers by rotation about the aryl‐aryl bond, found by fitting the experimental data, is 41.2 kJ/mol. Copyright © 2012 John Wiley & Sons, Ltd.     

1,1,1‐Trichloro‐3‐(1‐phenethylamino‐ethylidene)‐ pentane‐2,4‐dione—synthesis,spectroscopic, theoretical and structural elucidation

1,1,1‐Trichloro‐3‐(1‐phenethylamino‐ethylidene)‐pentane‐2,4‐dione is spectroscopically and structurally elucidated by means of linear‐polarized IR spectroscopy (IR‐LD) of oriented solids as a colloidal suspension in nematic liquid crystal. Structural information and IR‐spectroscopic assignment are supported by quantum chemical calculations at MP2 and B3LYP level of theory and 6‐311++G** basis set. The geometry is characterized with an inramolecular hydrogen bond of NH^…O?C with length of 2.526 Å and a NHO angle of 140.5(1)°. The NH? C(CH₃)C?C? C?O(CH₃) fragment is nearly flat with a maximal deviation of total planarity of 10.4°. Copyright © 2007 John Wiley & Sons, Ltd.     

Systematic conformational search analysis of the SRR and RRR epimers of 7‐hydroxymatairesinol

An extensive and systematic conformational search was performed on the two epimers of the natural lignan 7‐hydroxymatairesinol (HMR), by means of a home‐made Systematic Conformational Search Analysis (SCSA) code, designed to select more and more stable conformers through sequential geometry optimization of trial structures at increasing levels of calculation theory. In the present case, the starting molecular structures were selected by the semi‐empirical AM1 method and filtered – i.e. decreased in number by choosing the more stable species – on the basis of their energy calculated by the HF method and the 6‐31G(d) basis set. The geometries obtained were further refined by performing density functional theory (DFT) optimizations, using the B3LYP functional and the 6‐31G(d,p) basis set, both in vacuo and in ethanol solution. This procedure allowed us to isolate, at a high level of theory, three groups of epimer conformers characterized by open, semi‐folded, and folded conformations. Moreover, the SCSA allowed us to describe a conformational space made‐up by about 20 species for each of the two epimers. The corresponding energy content of these species was within 27 kJ mol^?1 from the absolute minimum found, both in vacuo and in ethanol solution. The conformational analysis, followed by the inspection of the stereochemistry of the two most stable conformers of both epimers, provides support in rationalizing the proposed reaction mechanism of the catalytic hydrogenolysis of the HMR to matairesinol (MAT). Copyright © 2009 John Wiley & Sons, Ltd.     

Unusual conformational preferences of 1,3‐dimethyl‐3‐isopropoxy‐3‐silapiperidine

The conformational analysis of the first representative of the Si‐alkoxy substituted six‐membered Si,N‐heterocycles, 1,3‐dimethyl‐3‐isopropoxy‐3‐silapiperidine, was performed by low‐temperature ¹H and ¹³C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of methyl and alkoxy substituents, the Me_axi‐PrO_eq conformer was found to predominate in the conformational equilibrium in the ratio Me_axi‐PrO_eq : Me_eqi‐PrO_ax of ca. 2 : 1 as from the ¹H and ¹³C NMR study. The thermodynamic parameters obtained by the complete line shape analysis showed that the main contribution to the barrier to ring inversion originates from the entropy term of the free energy of activation. Copyright © 2012 John Wiley & Sons, Ltd.     

Ionic liquid structure: the conformational isomerism in 1‐butyl‐3‐methyl‐imidazolium tetrafluoroborate ([bmim][BF4])

As a probe of local structure, the vibrational properties of the 1‐butyl‐3‐methylimidazolium tetrafluoroborate [bmim][BF₄] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim]⁺ conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim]⁺ conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim]⁺ conformers and the [bmim]⁺− [BF₄]⁻ interactions to the vibrational spectra are discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Vibrational spectra,X‐ray and molecular structure of 1H‐ and 3H‐imidazo[4,5‐b]pyridine and their methyl derivatives: DFT quantum chemical calculations

Molecular structure, vibrational energy levels and potential energy distribution of 1H‐imidazo[4,5‐b]pyridine, 3H‐imidazo[4,5‐b]pyridine, 5‐methyl‐1H‐imidazo[4,5‐b]pyridine, 6‐methyl‐1H‐imidazo[4,5‐b]pyridine and 7‐methyl‐3H‐imidazo[4,5‐b]pyridine were determined using density functional theory (DFT) at the B3LYP/6‐31G(d,p) level. The optimised bond lengths and bond angles are in good agreement with the X‐ray data of 5‐methyl‐1H‐imidazo[4,5‐b]pyridine obtained in the present work (Pbca space group; a = 8.660(2), b = 11.078(2), c = 11.078(3) Å, Z = 8). The N⁺H group plays the role of a proton donor in a medium strong hydrogen bond of the type N H…N, linking the N‐atom of the pyridine with the adjacent molecule related by the symmetry operation: 1/2 − x, y − 1/2, z(N…N = 2.869(25) Å). The presence of hydrogen bond is confirmed by appearance in the IR spectra of a very broad and strong contour in the 2000–3100 cm⁻¹ range. The place of substitution of the methyl group at the pyridine ring influences the proton position of the NH group at the imidazole unit. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis and structural study of 2′‐ and 2′,6′‐positioned methyl‐ and nitro‐substituted 3‐(arylhydrazono)pentane‐2,4‐diones

A systematic series of ortho‐methyl‐ and nitro‐substituted arylhydrazones 2–6 formed by Japp–Klingemann reaction between pentane‐2,4‐dione and the respective aryldiazonium salts have been synthesized and studied by X‐ray crystal structure analysis, with added quantum chemical calculations. The optimized molecular geometries based on DFT calculations, enabling determination of relevant rotational barriers, and the calculated bond and ring critical points, using the method of ‘atoms in molecules’, were found to correspond with the experimental data, involving specific molecular conformations and hydrogen‐bonded ring structure dependent on the ortho‐substitution, thus making possible reliable structural prediction of this compound class. Copyright © 2007 John Wiley & Sons, Ltd.     

Density functional theory calculation and vibrational spectral analysis of 4‐hydroxy‐3‐(3‐oxo‐1‐phenylbutyl)‐2H‐1‐benzopyran‐2‐one

The Fourier transform infrared (4000–400 cm⁻¹) and Fourier transform Raman (3500–500 cm⁻¹) spectra of 4‐hydroxy‐3‐(3‐oxo‐1‐phenylbutyl)‐2H‐1‐benzopyran‐2‐one (Warfarin) have been measured and calculated. The structure optimization has been made using density functional theory (DFT) calculations. Complete vibrational assignments of the observed spectra have been compared with theoretical wavenumbers. The wavenumber increasing in the methyl group shows the electronic hyperconjugation effect. The natural bond orbital (NBO) analysis reveals the hyperconjugation interaction and the intramolecular hydrogen bonding. The first‐order hyperpolarizability has been calculated. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental and theoretical FT‐IR and FT‐Raman spectroscopic analysis of N1‐methyl‐2‐chloroaniline

In this work, the experimental and theoretical vibrational spectra of N1‐methyl‐2‐chloroaniline (C₇H₈NCl) were studied. FT‐IR and FT‐Raman spectra of the title molecule in the liquid phase were recorded in the region 4000–400 cm^?1 and 3500–50 cm^?1, respectively. The structural and spectroscopic data of the molecule in the ground state were calculated by using density functional method (B3LYP) with the 6‐311++G(d,p) basis set. The vibrational frequencies were calculated and scaled values were compared with experimental FT‐IR and FT‐Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. ¹³C and ¹H NMR chemical shifts results were compared with the experimental values. The optimized geometric parameters (bond lengths and bond angles) were given and are in agreement with the corresponding experimental values of aniline and p‐methyl aniline. Copyright © 2008 John Wiley & Sons, Ltd.     

Crystal and molecular structure of 2‐aminopyridinium‐ 4‐hydroxybenzenosulfonate—IR and Raman spectra,DFT calculations and physicochemical properties

A new organic–organic salt, 2‐aminopyridinium‐4‐hydroxybenzenosulfonate, has been synthesised and characterised by means of FT‐IR and FT‐Raman spectroscopies, differential scanning calorimetry (DSC) and single crystal X‐ray crystallography. Its vibrational spectra have been discussed on the basis of quantum chemical density functional theory (DFT) calculations using the B3LYP/6‐31G(d,p) approach. The role of the intermolecular interactions in this crystal is analysed. The N HċċċO interactions between the hydrogen atoms of the pyridinium cation and oxygen atoms of hydroxybenzenosulfonate anion built the supramolecular arrangement in three‐dimensional space. Copyright © 2008 John Wiley & Sons, Ltd.     

Conformational pathways of simple six‐membered rings

The conformational equilibria of cyclohexane (as well as its fluoro‐, chloro‐, methyl‐, hydroxy‐, and t‐butyl derivatives), cyclohexanone, piperidine, tetrahydropyran (and its 2‐hydroxy derivative) were studied by ab initio and DFT procedures. The transition states were calculated at HF/6‐31G, B3LYP/6‐31+G*, and B3LYP/6‐311+G** levels, whereas the intrinsic reaction coordinates (IRCs) were evaluated at the B3LYP/6‐31+G* level. The degree of puckering and energy data was nearly not basis set‐dependent (using B3LYP) in most of the cases. However, DFT methods gave better agreement with experimental data than HF methods, as expected from electron correlation inclusion. Fluorocyclohexane and 2‐hydroxytetrahydropyran showed the largest basis set‐energy dependence. It was found that the conversion from chair to skew is direct in some cases, whereas in others it goes through the pseudorotational (skew/boat) pathway. The case of t‐butylcyclohexane, with a skew form as stable as one of the chairs, is especially interesting. In this compound, as well as in cyclohexanone and 2‐hydroxytetrahydropyran, large deviations from the known pseudorotation/inversion scheme are observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanism of 4‐methyl‐1,2,4‐triazol‐3‐thiole reaction with formaldehyde. A DFT study

Contrary to the typical nucleophilic substitution, occurring on the sulfur atom of 4‐methyl‐1,2,4‐ triazol‐3‐thiole, the reaction with formaldehyde leads to the formation of the N? C bond rather than the S? C bond. The mechanism of this reaction has been characterized theoretically. Calculations indicate that the reaction proceeds via a cyclic transition state involving one solvent molecule with the Gibbs free activation energy of only 2 kcal/mol. The alternative pathway that leads to the S? C bond formation is about 5 kcal/mol more energetically demanding. Copyright © 2007 John Wiley & Sons, Ltd.     

Rotational disorder in 2‐piperidyl‐5‐nitro‐6 ‐methylpyridine: structural phase transition and its vibrational characteristics

X‐ray diffraction (XRD) studies have shown that 2‐piperidyl‐5‐nitro‐6‐methylpyridine, C₁₁H₁₅N₃O₂, undergoes a structural phase transition at T = 240 K. The room temperature structure is tetragonal, space group I4₁/a, with the unit‐cell dimensions a = 13.993(2) and c = 23.585(5) Å. The pyridine ring takes trans conformation with respect to the piperidine unit. While pyridine is well ordered, the piperidine moiety shows apparent disorder resulting from a libration about the linking N C bond. The low‐temperature phase is monoclinic, space group I2/a. Contraction of the unit‐cell volume by 2.3% at 170 K enables the C H···O linkage between the molecules of the neighbouring stacks. As result, the asymmetric unit becomes bi‐molecular. The thermal librations of the piperidine and methyl groups become considerably reduced at 170 K and nearly fully reduced at about 100 K. The IR spectra and polarised Raman spectra agree with the X‐ray structure and confirm the disorder effect on the piperidine ring. The assignment of the bands observed was made on the basis of DFT chemical quantum calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Spectroscopic and thermal studies on the inclusion of trans‐cinnamic acid and a number of its hydroxyl‐derivatives with α, β and γ‐cyclodextrins molecules

The inclusion compounds of α‐, β‐ and γ‐cyclodextrins (α‐CD, β‐CD and γ‐CD) with trans‐cinnamic acid (t‐CIA), 3‐hydroxy‐trans‐cinnamic acid (t‐3OHCIA), 4‐hydroxy‐trans‐cinnamic acid (t‐4OHCIA) and 3,4‐dihydroxy‐trans‐cinnamic acid (t‐3,4OHCIA) were prepared and characterized, in the solid state, by means of thermogravimetry and Raman spectroscopy. The effects of the inclusion process on the guest molecules and on the hydrogen bond interactions of the guest were studied by monitoring sensitive vibrational modes, such as CO, CC and ring C H stretching modes. By combining Raman and TG data with ab initio calculations and information from CSD database on similar compounds, inclusion geometries for the different compounds are proposed. The size of the host cavity and the maximization of host/guest hydrogen‐bonding contacts appear to be the main factors determining the inclusion geometries. Copyright © 2009 John Wiley & Sons, Ltd.     

Merocyanines based on 1,3‐indanedione: electronic structure and solvatochromism

A series of merocyanines derived from 1,3‐indanedione and heterocycles of various electron‐donating properties was studied in detail. Their solvatochromic properties were explored in a wide range of solvent polarities to reveal the dependences of their chromacity and electronic structure on the key structural parameters – the properties of a donor heterocycle and the polymethine chain length. Also the dyes were studied by NMR spectroscopy and by quantum chemical calculations, both with the semiempirical AM1 and the non‐empirical density functional theory/B3LYP method. The solvatochromic properties of the explored dyes are rather close to those of merocyanines derived from malononitrile as acceptor group. Appreciable distinctions were observed only in protic ethanol; obviously, they are connected with the formation of solvent–solute H‐bonds in the case of 1,3‐indanedione derivatives. The electron‐acceptor properties of 1,3‐indanedione were found to be somewhat stronger in comparison with those of malononitrile even in aprotic solvents, contrary to the known literature data. Analysis of the merocyanines' molecular orbitals and simulation of their electronic spectra were carried out both in vacuum and in the solvent matrix, and the absorption electronic transitions were analyzed. Static nonlinear optical properties were calculated for both the new merocyanines and the corresponding cationic and anionic cyanine dyes. Copyright © 2010 John Wiley & Sons, Ltd.