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.     

Concentration‐ and temperature‐dependent conformation change of cyclohexyl isocyanide on gold nanoparticle surfaces

The conformational changes of cyclohexyl isocyanide (CHNC) on gold nanoparticle surfaces were investigated by means of concentration‐ and temperature‐dependent surface‐enhanced Raman scattering (SERS). The equatorial chair conformer appeared to be dominant at high‐bulk concentrations or low temperatures, whereas both the equatorial and axial chair conformers of CHNC were found to exist at low‐bulk concentrations or high temperatures as in the previous reports of cyclohexanethiol (CHT). Depending on concentrations and temperatures, the spectral changes of the NC stretching vibration on gold nanoparticles appeared to be more conspicuous than those of the cyclohexyl ring modes. A density functional theory (DFT) calculation was performed at the level of B3LYP/6‐31G + + (d,p) to compare the energetic stability of the various conformers of CHT and CHNC. The energy differences between the equatorial and axial chair conformers were predicted to be smaller for CHNC than for CHT by ∼3 kJ mol⁻¹ from the DFT calculation. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical calculations for neighboring group participation in gas‐phase elimination kinetics of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride

Theoretical calculation of the kinetics and mechanisms of gas‐phase elimination of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(d,p), B3PW91/6‐31G(d,p) and CCSD(T) levels of the theory. The two substrates undergo parallel elimination reactions. The first process of elimination appears to proceed through a three‐membered cyclic transition state by the anchimeric assistance of the aromatic ring to produce the corresponding styrene product and HCl. The second process of elimination occurs through a five‐membered cyclic transition state by participation of the oxygen of o‐OH or the o‐OCH₃ to yield in both cases benzohydrofuran. The B3PW91/6‐31G(d,p) method was found to be in good agreement with the experimental kinetic and thermodynamic parameters for both substrates in the two reaction channels. However, some differences in the performance of the different methods are observed. NBO analysis of the pyrolysis of both phenethyl chlorides implies a C? Cl bond polarization, in the sense of C^δ+…Cl^δ?, which is a rate‐determining step for both parallel reactions. Synchronicity parameters imply polar transition states of these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical study of neighboring carbonyl group participation in the elimination kinetics of chloroketones in the gas phase

The gas‐phase elimination of kinetics 4‐chlorobutan‐2‐one, 5‐chloropentan‐2‐one, and 4‐chloro‐1‐phenylbutan‐1‐one has been studied using electronic structure methods: B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW91PW91/6‐31G(d,p), MPW91PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE /6‐31++G(d,p), and MP2/6‐31++G(d,p). The above‐mentioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4‐chlorobutan‐2‐one suggest a non‐synchronous four‐membered cyclic transition state (TS) type of mechanism. However, in the case of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one, the carbonyl group assists anchimerically through a polar five‐membered cyclic TS mechanism. The polarization of the C? Cl bond, in the sense of C^δ+…Cl^δ?, is a rate‐determining step in these elimination reactions. The significant increase in rates in the elimination of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C? Cl bond polarization in the TS. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical calculations of the kinetics and mechanisms of the gas phase elimination of primary,secondary, and tertiary 2‐hydroxyalkylbenzenes

Theoretical study of the elimination kinetics of 2‐phenylethanol, 1‐phenyl‐2‐propanol, and 2‐methyl‐1‐phenyl‐2‐propanol in the gas‐phase has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), and PBEPBE/6‐31++G(d,p) levels of theory. The three substrates undergo two parallel elimination reactions. The first elimination appears to proceed through a six‐membered cyclic transition state to give toluene and the corresponding aldehyde or ketone. The second parallel elimination takes place through a four‐membered cyclic transition state producing water and the corresponding unsaturated aromatic hydrocarbon. Results from MP2/6‐31G(d,p) and MPW1PW91/6‐31++G(d,p) methods were found to be in good agreement with the experimental kinetic and thermodynamic parameters in the formation of toluene and the corresponding carbonyl compound. However, the results for PBEPBE/6‐31G(d,p) were in better agreement with the experimental data for the second parallel reaction yielding water and the corresponding unsaturated aromatic hydrocarbon. The charge distribution differences in the TS related to the substitution by methyl groups in the substrates can account for the observed reaction rate coefficients. The synchronicity parameters imply semi‐polar transition states for these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.     

Joint theoretical and experimental study of the gas‐phase elimination kinetics of tert‐butyl ester of carbamic,N, N‐dimethylcarbamic,N‐hydroxycarbamic acids and 1‐(tert‐butoxycarbonyl)‐imidazole

The gas‐phase elimination kinetics of the title compounds were carried out in a static reaction system and seasoned with allyl bromide. The working temperature and pressure ranges were 200–280 °C and 22–201.5 Torr, respectively. The reactions are homogeneous, unimolecular, and follow a first‐order rate law. These substrates produce isobutene and corresponding carbamic acid in the rate‐determining step. The unstable carbamic acid intermediate rapidly decarboxylates through a four‐membered cyclic transition state (TS) to give the corresponding organic nitrogen compound. The temperature dependence of the rate coefficients is expressed by the following Arrhenius equations: for tert‐butyl carbamate logk₁ (s^?1) = (13.02 ± 0.46) – (161.6 ± 4.7) kJ/mol(2.303 RT)^?1, for tert‐butyl N‐hydroxycarbamate logk₁ (s^?1) = (12.52 ± 0.11) – (147.8 ± 1.1) kJ/mol(2.303 RT)^?1, and for 1‐(tert‐butoxycarbonyl)‐imidazole logk₁ (s^?1) = (11.63 ± 0.21)–(134.9 ± 2.0) kJ/mol(2.303 RT)^?1. Theoretical studies of these elimination were performed at Møller–Plesset MP2/6‐31G and DFT B3LYP/6‐31G(d), B3LYP/6‐31G(d,p) levels of theory. The calculated bond orders, NBO charges, and synchronicity (Sy) indicate that these reactions are concerted, slightly asynchronous, and proceed through a six‐membered cyclic TS type. Results for estimated kinetic and thermodynamic parameters are discussed in terms of the proposed reaction mechanism and TS structure. 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.     

Experimental and theoretical study of the mechanism for the kinetic of elimination of methyl carbazate in the gas phase

The elimination kinetic of methyl carbazate in the gas phase was determined in a static system over the temperature range of 340–390 °C and pressure range of 47–118 Torr. The reaction is homogeneous, unimolecular, and obeys a first order rate law. The decomposition products are methyl amine, nitrous acid, and CO gas. The variation of the rate coefficients with temperatures is given by the Arrhenius expression: log k₁ (s^?1) = (11.56 ± 0.34) ? (180.7 ± 4.1) kJ mol^?1(2.303 RT)^?1. The estimated kinetics and thermodynamics parameters are in good agreement to the experimental values using B3LYP/6‐31G (d,p), and MP2/6‐31G (d,p) levels of theory. These calculations imply a molecular mechanism involving a concerted non‐synchronous quasi three‐membered ring cyclic transition state to give an unstable intermediate, 1,2‐oxaziridin‐3‐one. Bond order analysis and natural charges implies that polarization of O (alkyl)? C (alkyl) bond of the ester is rate determining in this reaction. Copyright © 2008 John Wiley & Sons, Ltd.     

Conformational stability from Raman spectra,r0 structural parameters,and vibrational assignment of methylcyclohexane

The Raman spectra (3500–50 cm⁻¹) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm⁻¹ in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 °C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair‐equatorial conformer and the chair‐axial form was determined to be 712 ± 71 cm⁻¹ (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm⁻¹ (8.50 kJ/mol) from the MP2(full)/6‐311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6‐31G(d) ab initio calculations. With two scaling factors of 0.88 for the C‐H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B‐type for A″ modes) a complete vibrational assignment has been made for the chair‐equatorial conformer. Predicted r₀ structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical investigation of the transition states leading to HCI elimination in 2-chloropropene

This paper describes ab initio electronic structure calculations on the planar transition states of 2-chloropropene leading to HCI elimination in the ground electronic state to form either propyne or allene as the cofragment. The calculations provide optimized geometries of the transition states for these two reaction channels, together with vibrational frequencies, barrier heights, and reaction endothermicities. The calculated barrier heights for the two distinct four-centre HCI elimination transition states, one leading to HCI and propyne and the other leading to HCI and allene, are 72.5kcalmol^?1 (77.8kcalmol^?1 without zero-point correction) and 73.2kcalmol^?1 (78.7kcalmol^?1) at the MP2/6-311G(d, p) level, 71.Okcalmol^?1 (76.3kcalmol^?1) and 70.5kcalmol^?1 (76.0kcalmol^?1) at the QCISD(T)/6-311 +G(d, p)//MP2/6-311G(d, p) level, and 66.9kcalmol^?1 (71.7kcalmol^?1) and 67.3kcalmol^?1 (72.1kcalmol¹) at the G3//B3LYP level of theory. Calculated harmonic vibrational frequencies at the B3LYP/6-31G(d) level along with transition state barrier heights from the G3//B3LYP level of theory are used to obtain RRKM reaction rate constants for each transition state, which determine the branching ratio between the two HCI elimination channels. Even at internal energies well above both HCI elimination barriers, the HCI elimination leading to propyne is strongly favoured. The smaller rate constant for the HCI elimination leading to allene can be attributed to the strong hindrance of the methyl rotor in the corresponding transition state.     

Conformational stability and vibrational study of phenylacetyl chloride

The Fourier transform infrared spectroscopy and Fourier transform Raman spectra of phenylacetyl chloride were recorded and analyzed in the range 3500–400 and 3500–200 cm^?1 at room temperature, respectively. In order to obtain the structural information and conformational stabilities, a potential energy surface scan for internal rotation was carried out at the B3LYP/6‐31G(d) level. The potential energy surface reveals that the title compound has two minimal conformers (A and B). The optimized geometries, structural parameters, stabilities, energies, thermodynamic parameters, vibrational wavenumbers, infrared intensities, and Raman activities for the two conformers (A and B) have been obtained by employing B3LYP and MP2 calculations with 6‐311++G (d, p) basis sets. The conformational energy difference between A and B is very small, indicating that the B conformer coexists with the A conformer. The detailed vibrational assignments of vibrational spectra of each conformer have been made on the basis of the potential energy distributions analysis. The highest occupied molecular orbital –lowest unoccupied molecular orbital energy gap and molecular electrostatic potential of the two conformers have been also calculated for comparison of their chemical activities. Copyright © 2015 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.     

Calculations of <Superscript>31</Superscript>P Magnetic Shielding Constants of Derivatives of Betaine and Phosphine Molecules Dissolved in Different Solvents by Using Supermolecular Model and Combined Methods of Quantum Chemistry and Molecular Mechanics

Spatial structures of molecular clusters modeling a solvate shell around phosphorus-containing methyl- and butyl-derivatives of phosphine and betaine molecules dissolved in different solvents (acetone, toluene, formamide) have been calculated by using different variants of density functional theory (unrestricted Becke three-parameter Lee–Yang–Parr [UB3LYP], Perdew–Burke–Ernzerhof [PBE], optimized exchange functional [OPTX] developed by Handy and Cohen in conjunction with Lee–Yang–Parr [LYP] correlational functional [OLYP]) with 6-31G(d,p) and 6-31G++(d,p) basis sets. The ³¹P magnetic shielding constants for the structures are calculated with the usage of gauge-including atomic orbitals in UB3LYP/6-31G(d,p) and 6-31G++(d,p) methods. The modeling of molecular clusters is done by using the supermolecular model, the molecular mechanics method and the combination of quantum chemistry and molecular mechanics methods (QM/MM). The own N-layered integrated molecular orbital method (ONIOM) has been applied for modeling and calculating of isotropic ³¹P nucleus magnetic shielding of clusters of trimethylphosphine and trimethylbetaine molecules dissolved in acetone using combinations of UB3LYP/6-31G(d,p) (higher level) and unrestricted Hartree–Fock (UHF)/6-31G(d,p) (lower level) methods. Applicability of the ONIOM approach and different ways of modeling to the calculation of ³¹P nucleus magnetic shielding constants is studied. A comparison of the results obtained by the density functional theory, ONIOM and MM methods is given.     

Substituent effects on the reduction of 2–alkylcyclohexanones by LiAlH4: an investigation of conformational equilibria and transition states

Transition state (TS) structures for the reduction of 2‐Me and 2‐i‐Pr‐cyclohexanone by LiAlH₄ were optimized by density functional theory (B3LYP/6‐31G(d,p)). Four TS structures corresponding to axial and equatorial attacks by LiAlH₄ were located for each ketone conformer. Electronic potential maps were used to investigate the substituent electronic effect on the TS stabilization. The uneven carbonyl orbital distribution in the LUMO (π*) was also analyzed. Reduction stereoselectivity was shown to depend on both the ketone conformational ratio and on the reaction TS. Copyright © 2010 John Wiley & Sons, Ltd.     

Complete basis set,hybrid‐DFT study and NBO interpretations of conformational behaviors of trans‐2,3‐ and trans‐2,5‐dihalo‐1,4‐dithianes

The conformational behaviors of trans‐2,3‐dihalo‐1,4‐dithiane [halo = F ( 1 ), Cl ( 2 ), Br ( 3 )] and trans‐2,5‐dihalo‐1,4‐dithiane [halo = F ( 4 ), Cl ( 5 ), Br ( 6 )] have been analyzed by means of complete basis set CBS‐4, hybrid‐density functional theory (B3LYP/6‐311 + G**//B3LYP/6‐311 + G**) based methods, and natural bond orbital (NBO) interpretation. Both methods showed that the axial conformations of compounds 1–5 are more stable than their equatorial conformations but CBS‐4 resulted in an equatorial preference for compound 6 . The Gibbs free energy difference (G_eq?G_ax) values (i.e., ΔG_eq–ax) at 298.15 K and 1 atm between the axial and equatorial conformations decrease from compound 1 to compound 2 but increase from compound 2 to compound 3 . Also, the calculated ΔG_eq–ax values decrease from compound 4 to compound 6 . The NBO analysis of donor–acceptor (LP → σ*) interactions showed that the anomeric effect (AE) increase from compound 1 to compound 3 and also from compound 4 to compound 6 . On the other hand, the calculated dipole moment values between the axial and equatorial conformations [Δ(µ_eq?µ_ax)] decrease from compound 1 to compound 3 . The conflict between the increase of AE and the decrease of Δ(µ_eq?µ_ax) values could explain the variation of the calculated ΔG_eq–ax for compounds 1–3 . The Gibbs free energy difference values between the axial and equatorial conformations (i.e., ΔG_ax–ax and ΔG_eq–eq) of compounds 1 and 4 , 2 and 5 and also 3 and 6 have been calculated. The correlations between the AE, bond orders, pairwise steric exchange energies (PSEE), ΔG_eq–ax, ΔG_ax–ax, ΔG_eq–eq, dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–6 have been investigated. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular structure,experimental and theoretical 1H and 13C NMR chemical shift assignment of cyclic and acyclic α,β‐unsaturated esters

The experimental ¹H and ¹³C NMR spectra of 13 phenyl cinnamates and four 4‐methylcoumarins were investigated and their chemical shifts assigned on the basis of the two‐dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6‐311++G(d,p) level of theory. ¹H and ¹³C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6‐311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s‐cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects. Copyright © 2013 John Wiley & Sons, Ltd.     

Theoretical studies on structure and performance of [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine‐based derivatives

Based on energetic compound [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine, a series of functionalized derivatives were designed and first reported. Afterwards, the relationship between their structure and performance was systematically explored by density functional theory at B3LYP/6‐311 g (d, p) level. Results show that the bond dissociation energies of the weakest bond (N–O bond) vary from 157.530 to 189.411 kJ · mol^?1. The bond dissociation energies of these compounds are superior to that of HMX (N–NO_2, 154.905 kJ · mol^?1). In addition, H1, H2, H4, I2, I3, C1, C2, and D1 possess high density (1.818–1.997 g · cm^?3) and good detonation performance (detonation velocities, 8.29–9.46 km · s^?1; detonation pressures, 30.87–42.12 GPa), which may be potential explosives compared with RDX (8.81 km · s^?1, 34.47 GPa ) and HMX (9.19 km · s^?1, 38.45 GPa). Finally, allowing for the explosive performance and molecular stability, three compounds may be suggested as good potential candidates for high‐energy density materials. Copyright © 2016 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.     

Structural and vibrational analyses of 2‐(2‐benzofuranyl)‐2‐imidazoline

We have studied 2‐(2‐benzofuranyl)‐2‐imidazoline (BFI) and characterized it by using infrared and Raman spectroscopies. The density functional theory (DFT) method together with Pople's basis set shows that two conformers exist for the title molecule as have been theoretically determined in the gas phase and that, probably, an average of both conformations is present in the solid phase. The harmonic vibrational wavenumbers for the optimized geometry of the latter conformer were calculated at the B3LYP/6‐31G* level in the proximity of the isolated molecule. For a complete assignment of the IR and Raman spectra in the compound in the solid phase, DFT calculations were combined with Pulay's scaled quantum mechanics force field (SQMFF) methodology in order to fit the theoretical wavenumbers to the experimental ones. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical and Experimental Study of the Conformation and Vibrational Frequencies of N‐Acetyl‐L‐alanine and N‐Acetyl‐L‐alaninate

The vibrational frequencies of N‐acetyl‐L‐alanine (NAAL), its potassium salt (NAALK) and its free anionic form (NAAL^?) are calculated using density functional theory (B3LYP) combined with the 6‐311 + + G(d,p) basis set. The experimental Raman spectrum of solid NAALK and the scaling factors for calculated values are discussed as well. The three species are characterized by intramolecular NH…O hydrogen bonds leading to the formation of a five‐membered ring. As indicated by the intramolecular (N)H…O distances and by the ν(NH) frequencies, the strength of the intramolecular hydrogen bond is ordered as follows: NAAL^? < NAALK < NAAL^?. Owing to their difference in the coupling with other vibrational modes, the in‐plane and out‐of‐plane vibrations do not reflect the strength of the hydrogen bond.