Structure and NMR properties of 6‐substituted‐5,6‐dihydrobenzo[c]phenanthridine alkaloids

We report a preparation of new 6‐substituted‐5,6‐dihydrobenzo[c]phenanthridines by the reaction of azoles with quaternary benzo[c]phenanthridine alkaloids sanguinarine and chelerythrine. The prepared compounds have been characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. Conformational behaviors of carbazole derivatives in solution have been investigated by low‐temperature NMR experiments. Barriers to rotation around newly formed C6–N bonds were determined to be 12–13 kcal/mol. Quantum chemical calculations have been used to reproduce the experimental observations. Large structural effects on several ¹H NMR resonances were observed experimentally, analyzed by Density Functional Theory (DFT) calculations at B3LYP/6‐311+G(d,p)/PCM level, and interpreted by ring‐current effects of the benzo[c]phenanthridine and carbazole units. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental and four‐component relativistic DFT studies of tungsten carbonyl complexes

We present a theoretical and experimental study of the structure and nuclear magnetic resonance (NMR) parameters of the pentacarbonyltungsten complexes of η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine, η²‐norbornene, and imidazolidine‐2‐thione. The three complexes have a pseudo‐octahedral molecular structure with the six ligands bonded to the tungsten atom. The η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine‐pentacarbonyl tungsten complex was synthesized for the first time. For all compounds, we present four‐component relativistic calculations of the NMR parameters at the Dirac–Kohn–Sham density functional level of theory using hybrid functionals. These large‐scale relativistic calculations of NMR chemical shifts and spin–spin coupling constants were compared with available experimental data, either taken from the literature or measured in this work. The inclusion of solvent effects modeled using a conductor‐like screening model was found to improve agreement between the calculated and experimental NMR parameters, and our best estimates for the NMR parameters are generally in good agreement with available experimental results. The present work demonstrates that four‐component relativistic theory has reached a level of maturity that makes it a convenient and accurate tool for modeling and understanding chemical shifts and indirect spin–spin coupling constants of organometallic compounds containing heavy elements, for which conventional non‐relativistic theory breaks down. Copyright © 2015 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.     

Dynamic 1H NMR investigation along with a theoretical study around the C–C and C = C bonds in a particular phosphorus ylide

In the present work, the dynamic ¹H NMR effects were investigated at variable temperatures within a particular phosphorus ylide involving a 2‐benzoxazolinone around the carbon–carbon single bond and also partial carbon–carbon double bond in two Z‐ and E‐rotational isomers. Activation and kinetic parameters including ΔH^≠, ΔG^≠, ΔS^≠ and E_a were determined in accord with the dynamic ¹H NMR data for three rotational processes. In addition, theoretical studies based upon rotation around the same bonds were investigated using ab initio and DFT methods at the HF/6‐31G(d,p) and B3LYP/6‐31G(d,p) levels of theory. Theoretical activation and kinetic parameters including ΔH^≠, ΔG^≠, ΔS^≠ and E_a were calculated at 298 K and experimental temperatures for five rotational processes. These results (experimental and theoretical), taken together, indicate that the rotational energy barrier around the C = C double bond was considerably high and the observation of the two rotational isomers was impossible (seen as a single isomer) at the high temperatures, in this case rotation around the C = C bond was too fast on the NMR time scale. When the temperature was relatively low, the rate of rotation was sufficiently slow; therefore, observation of the two Z‐ and E‐isomers was possible. In addition, calculations at the HF/6‐31G(d,p) level of theory showed very favorable results in agreement with the experimental data on rotation around the C = C bond. While, B3LYP level using the 6‐31G(d,p) basis set was provided the reasonable data for the restricted rotations around the C–C and C–N single bond. Copyright © 2012 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.     

Investigation of cation–anion interaction in 1‐(2‐hydroxyethyl)‐3‐methylimidazolium‐based ion pairs by density functional theory calculations and experiments

Gas‐phase structure, hydrogen bonding, and cation–anion interactions of a series of 1‐(2‐hydroxyethyl)‐3‐methylimidazolium ([HOEMIm]⁺)‐based ionic liquids (hereafter called hydroxyl ILs) with different anions (X = [NTf₂]^–, [PF₆]^–, [ClO₄]^–, [BF₄]^–, [DCA]^–, [NO₃]^–, [AC]^– and [Cl]^–), as well as 1‐ethyl‐3‐methylimizolium ([EMIm]⁺)‐based ionic liquids (hereafter called nonhydroxyl ILs), were investigated by density functional theory calculations and experiments. Electrostatic potential surfaces and optimized structures of isolated ions, and ion pairs of all ILs have been obtained through calculations at the Becke, three‐parameter, Lee–Yang–Parr/6‐31 + G(d,p) level and their hydrogen bonding behavior was further studied by the polarity and Kamlet–Taft Parameters, and ¹H‐NMR analysis. In [EMIm]⁺‐based nonhydroxyl ILs, hydrogen bonding preferred to be formed between anions and C2–H on the imidazolium ring, while in [HOEMIm]⁺‐based hydroxyl ILs, it was replaced by a much stronger one that preferably formed between anions and OH. The O–H···X hydrogen bonding is much more anion‐dependent than the C2–H···X, and it is weakened when the anion is changed from [AC]^– to [NTf₂]^–. The different interaction between [HOEMIm]⁺ and variable anion involving O–H···X hydrogen bonding resulted in significant effect on their bulk phase properties such as ¹H‐NMR shift, polarity and hydrogen‐bond donor ability (acidity, α). Copyright © 2011 John Wiley & Sons, Ltd.     

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 flexibility of xanthene‐based covalently linked dimers

The conformational flexibility of three covalently linked dimers consisting of two xanthene‐based moieties connected by a diphenyl ether linker was studied using NMR spectroscopy, X‐ray crystallography, and density functional theory (DFT) calculations. The three dimers interconvert as a function of pH: the doubly cationic dimer (Xan⁺)₂ exists in acidic solutions (pH < 0.5), the mono‐alcohol monocation Xan⁺–Xan‐OH at intermediate pH values (pH = 1–3), and the neutral diol at the highest pH‐values (pH > 3). Each dimer exhibits conformational degrees of freedom associated with rotations of either the xanthene moiety or of the diphenyl ether (DPE) linker. The barriers for rotation of the xanthylium moiety were evaluated using DFT calculations, yielding values of 23 kcal/mol for (Xan⁺)₂ and 11 kcal/mol for (Xan‐OH)₂, respectively. The rotational barrier for the diphenyl ether linker in Xan⁺–Xan‐OH (15 kcal/mol) was experimentally determined using variable temperature NMR measurements. The relative orientation of the two –OH groups in (Xan‐OH)₂ diol was investigated in solution and the solid state using NMR spectroscopy and X‐ray crystallography. The conformer observed in the solid state was found to be the In–Out conformer, while free rotation of the xanthenol units is thought to occur on the NMR timescale at room temperature. These studies are relevant for the design of linkers for efficient water oxidation catalysts. Copyright © 2016 John Wiley & Sons, Ltd.     

Anion effect on the conformational equilibrium of sulfamide and its N,N′‐diindolyl derivative: Insights on anion transportation

The relatively high acidity of the sulfamide hydrogens suggests a potential for the development of sulfamide derivatives as novel anion receptors. The interactions of sulfamide with F^?, Cl^?, CH₃COO^?, and H₂PO₄^? anions were spectroscopically (¹H and ¹⁹F NMR) and theoretically (density functional theory) analyzed, and the complexation through hydrogen bonds was confirmed by changes in the NMR signals and theoretical calculations. The replacement of 2 sulfamide hydrogens with indolyl groups yields the N,N′‐diindolylsulfamide ( DIS , N‐1H‐indol‐4‐yl‐N′‐1H‐indol‐7‐ylsulfuric diamide), whose bond rotations allow the interaction of 4 H(N) atoms with anions. The conformational preferences of DIS change upon the presence of anions, but they are practically insensitive to the anion type. According to the quantum theory of atoms in molecules, natural bond orbital analysis, and NMR chemical shifts, as well as to a thermodynamic cycle, the complex with fluoride is the most stable, followed by the oxoanion‐derived models.     

Physical image vs structure relation: part 12 – structure of 2,2,5,5‐tetramethyl‐dihydro‐furan‐3‐one oxime and its protonated forms through isomerization and NMR spectra

The study of an isomeric A / B mixture of the title oxime 1 , by photolytic or thermal E,Z‐isomerization and NMR measurement including ¹H{¹H}‐NOE difference spectra, led to assignment of the E configuration to its predominating form A . The ¹H/¹³C data were interpreted in terms of steric overcrowding of both forms, especially of the thermolabile photoproduct B . Four classical (empirical) NMR methods of elucidating the oxime geometry were critically tested on these results. Unexpected vapor‐phase photoconversion A → B in the window glass‐filtered solar UV and spectroscopic findings on their protonated states were discussed, as well. The kinetically controlled formation of the N‐protonated species (Z)‐ 5 ⁺ was proved experimentally. In addition, some ¹H NMR assignments reported for structurally similar systems were rationalized ( 3 and 4 ) or revised ( 1 and 7–9 ) with the GIAO‐DFT(B3LYP) and/or GIAO‐HF calculational results. Copyright © 2007 John Wiley & Sons, Ltd.     

5‐Fluorouracil solutions: NMR study of acid–base equilibrium in water and DMSO

A comprehensive analysis of solutions of 5‐fluorouracil (5FU) in water and an organic medium (dimethylsulfoxide, DMSO) was carried out using quantum chemical methods and nuclear magnetic resonance (NMR) spectroscopy. The details of anionic form generation in the solution of 5FU with an equimolar amount of potassium hydroxide were studied by ¹³С, ¹Н, ¹⁵N and ¹⁹F NMR. Interpretation of NMR spectral data was carried out using quantum chemical calculations at the TPSSTPSS/6‐311+G(d,p) level of theory. Specific solvation of 5FU and 1THF‐5FU was modeled in approximation using the five‐water cluster model and solvate complex including two DMSO molecules. It was established that in an alkaline medium in DMSO 5FU occurred mainly as a type of an anion with a deprotonation on N(1) position of a pyrimidine ring whereas in water alkaline solution—as a mixture of two anions with a deprotonation on N(1) and N(3) positions with a predominant content of the latter form. For the quantitative definition of the deprotonation forms of 5FU the technique based on the data of theoretical and experimental NMR ¹³C spectroscopy, tested on a model compound 1‐(tetrahydrofuranyl‐2)‐5‐fluoro‐pyrimidinedione‐2,4 (tegafur, THF‐5FU), was offered. The N(3) anion was found from spectral data to be more thermodynamically stable than the N(1) anion by 2.40 kJ mol^?1 (calculated value 2.24 kJ mol^?1) in an alkaline–water solution. Both alkaline–water and alkaline–DMSO solutions of THF‐5FU (THF‐5FU/KOH = 1/1) were characterized by the ratio of the equilibrium concentrations of the anion and diketo‐tautomer as 9:1 and 4.3:1, respectively. Copyright © 2014 John Wiley & Sons, Ltd.     

Complexation and DFT studies of lower rim hexahomotrioxacalix[3]arene derivatives bearing pyridyl groups with transition and heavy metal cations. Cone versus partial cone conformation

The binding of representative alkali, alkaline earth, transition and heavy metal cations by 2‐pyridylmethoxy derivatives (1b, in cone and partial cone conformations) of p‐tert‐butylhexahomotrioxacalix[3]arene was studied. Binding was assessed by extraction studies of the metal picrates from water into dichloromethane and by stability constant measurements in acetonitrile and methanol, using spectrophotometric and potentiometric techniques. Microcalorimetric studies of some selected complexes in acetonitrile were performed, as well as proton NMR titrations. Computational methods (density functional theory calculations) were also employed to complement the NMR data. The results are compared with those obtained with the dihomooxacalix[4]arene 2b and the calix[4]arene 3b derivative analogues. Partial cone‐1b is the best extractant for transition and heavy metal cations. Both conformers of 1b exhibit very high stability constants for soft and intermediate cations Pb²⁺, Cd²⁺, Hg²⁺, Zn²⁺ and Ni²⁺, with cone‐1b the strongest binder (ML, log β ≥ 7) and partial cone‐1b the most selective. Both derivatives show a slight preference for Na⁺. Besides the formation of ML complexes, ML₂ and M₂L species were also observed. The former complexes were, in general, formed with the transition and heavy metal cations, whereas the latter were obtained with Ag⁺ and Hg²⁺ and partial cone‐1b. In most cases, these species were corroborated by the proton NMR and density functional theory studies. Copyright © 2013 John Wiley & Sons, Ltd.     

Substituent effect investigation of 3‐(2, 4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one. Part 1. Correlation analysis of 13C NMR chemical shifts

A series of substituted chlorinated chalcones namely, 3‐(2,4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one, have been synthesized, X being H, NH₂, OMe, Me, F, Cl, CO₂Et, CN, and NO₂. Dual substituent parameter (DSP) models of ¹³C NMR chemical shift (CS) have revealed that π‐polarization concept could be utilized to explain the reverse field effect at CO, the enhanced substituent field effect at CO, C‐2, and C‐5, and the decreased sensitivity of substituent field effect at C‐6. Chlorine atoms dipole direction at the benzylidene ring either enhances or reduces substituent effect depending on how they couple with the substituent dipole at the probe site. The correlation of ¹³C NMR CS of C‐2, C‐5, and C‐6 with σ and σ indicates that chlorine atoms in the benzylidine ring deplete the ring from charges. Both MSP of Hammett and DSP of Taft ¹³C NMR CS models give similar trends of substituent effects at C‐2, C‐5, and C‐6. However, the former fail to give a significant correlation for CO and C‐6 ¹³C NMR CS. MSP of σ_q and DSP of Taft and Reynolds models significantly correlated ¹³C NMR CS of C_β. MSP of σ_q fails to correlate C‐1′ ¹³C NMR CS. Investigation of ¹³C NMR CS of non‐chlorinated chalcones series: 3‐phenyl‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one has revealed similar trends of substituent effects as in the chlorinated chalcones series for C‐1′, CO, C_α, and C_β. In contrast, the substituent effect of the non‐chlorinated chalcone series at C‐2, C‐5, and C‐6 did not correlate with any substituent constant. Copyright © 2010 John Wiley & Sons, Ltd.     

Stereochemical Elucidation of Norbornene Derivatives Synthesized as Leukotriene D4 Receptor Antagonists

The complete ¹H and ¹³C NMR assignment and stereochemical elucidation of four norbornene derivatives (1–4) are presented. These derivatives were obtained by an asymmetric Diels–Alder reaction of a chiral dienophile, from D‐glucose, as leukotriene D4 receptor antagonists. NOE measurements of the four products between H‐7b and H‐5 readily allowed discrimination between exo‐ (2, 4) and endo‐ (1, 3) diastereomers with respect to methyl carboxylate and tetrahydrofuro[1,3]dioxolyl groups. Further discrimination and subsequent absolute configuration determination of the two endo‐ and two exo‐ products were performed by NOE peak measurements and molecular modeling calculations of relative interproton distances of the most stable conformations of each isomer.     

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.     

Conformational analysis of 3‐methyl‐3‐silathiane and 3‐fluoro‐3‐methyl‐3‐silathiane

The conformational equilibria of 3‐methyl‐3‐silathiane 5 , 3‐fluoro‐3‐methyl‐3‐silathiane 6 and 1‐fluoro‐1‐methyl‐1‐silacyclohexane 7 have been studied using low temperature ¹³C NMR spectroscopy and theoretical calculations. The conformer ratio at 103 K was measured to be about 5 _ax: 5 _eq = 15:85, 6 _ax: 6 _eq = 50:50 and 7 _ax: 7 _eq = 25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol^?1) is much less than in 3‐methylthiane 9 (1.40 kcal mol^?1) but somewhat greater than in 1‐methyl‐1‐silacyclohexane 1 (0.23 kcal mol^?1). Compounds 5–7 have low barriers to ring inversion: 5.65 (ax → eq) and 6.0 (eq → ax) kcal mol^?1 ( 5 ), 4.6 ( 6 ), 5.1 (Me_ax → Me_eq) and 5.4 (Me_eq → Me_ax) kcal mol^?1 ( 7 ). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6 , or different conformer ratio for 5 and 7 . Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si–X and adjacent C–H, C–S, and C–C bonds proved responsible. Copyright © 2010 John Wiley & Sons, Ltd.     

XRD studies,vibrational spectra,and molecular structure of 1H‐imidazo [4,5‐b]pyridine based on DFT quantum chemical calculations

The molecular structures and vibrational properties of 1H‐imidazo[4,5‐b]pyridine in its monomeric and dimeric forms are analyzed and compared to the experimental results derived from the X‐ray diffraction (XRD), infrared (IR), and Raman studies. The theoretical data are discussed on the basis of density functional theory (DFT) quantum chemical calculations using Lee–Yang–Parr correlation functional (B3LYP) and 6‐31G(d,p) basis. This compound crystallizes in orthorhombic structure, space group Pna2₁(C_2v⁹) and Z = 4. The planar conformation of the skeleton and presence of the N H···N hydrogen bond was found to be characteristic for the studied system. The temperature dependence of IR and Raman modes was studied in the range 4–294 K and 8–295 K, respectively. The normal modes, which are unique for the imidazopyridine derivatives are identified. Copyright © 2009 John Wiley & Sons, Ltd.     

The mechanism of the homogeneous,unimolecular gas‐phase elimination kinetic of 1,1‐dimethoxycyclohexane: experimental and theoretical studies

The gas‐phase elimination of 1,1‐dimethoxycyclohexane yielded 1‐methoxy‐1‐cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310–360 °C and the pressure was 25–85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first‐order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s^?1) = [(13.82 ± 0.07) – (193.9 ± 1.0)(kJ mol^?1)](2.303RT)^?1; r = 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6‐31G(d,p) and 6‐31G++(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6‐31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four‐membered cyclic transition state. The transition state structure of methanol elimination from 1,1‐dimethoxycyclohexane is characterized by a significantly elongated C? O bond, while the C_β? H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS. Copyright © 2010 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.     

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

The Raman and infrared spectra (4000 to 50 cm^–1) of the gas, liquid or solution, and solid have been recorded of n‐propylamine, CH₃CH₂CH₂NH₂. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm^–1) and far infrared (600 to 10 cm^–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm^–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm^–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm^–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm^–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ from both second‐order Møller–Plesset (MP2, full) and density functional theory calculations by the Becke, three‐parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.