Geometric Structure of 2-Phenyl-1,3-dithia-5,6-benzocycloheptene 1-Oxide

Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) was used to determine proton-proton distances for the chair conformer of 2-phenyl-1,3-dithia-5,6-benzocycloheptene 1-oxide in CD₂Cl₂ in the system CS₂-CDCl₃. Changing the solvent by a more polar changes the molecular conformation. As a result, the dipole moment may also be affected, and this should be taken into account in analysis of solvent effects on thermodynamic parameters of the conformational equilibrium.     

Computational study of sulfoxides of thiacyclohexane, 4‐silathiacyclohexane, 4‐fluoro‐4‐silathiacyclohexane,and 4,4‐difluoro‐4‐silathiacyclohexane: Relative energies of conformations and sulfinyl oxygen stabilized pentacoordinate silicon in boat and twist structures

Second‐order Møller‐Plesset theory (MP2) has been used to calculate the equilibrium geometries and relative energies of the chair, 1,4‐twist, 2,5‐twist, 1,4‐boat, and 2,5‐boat conformations of thiacyclohexane 1‐oxide (tetrahydro‐2H‐thiopyran 1‐oxide), 4‐silathiacyclohexane 1‐oxide, cis‐ and trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide, and 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide. At the MP2/6‐311+G(d,p) level of theory, the chair conformer of axial thiacyclohexane 1‐oxide is 0.99, 5.61, 5.91, 8.57, and 7.43 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of equatorial thiacyclohexane 1‐oxide is 4.62, 6.31, 7.56, and 7.26 kcal/mol more stable (ΔE) than its respective 1,4‐twist and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of axial 4‐silathiacyclohexane 1‐oxide is 1.79, 4.26, 3.85, and 5.71 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The 2,5‐twist conformer of axial 4‐silathiacyclohexane 1‐oxide is stabilized by a transannular interaction between the sulfinyl oxygen and silicon, to give trigonal bipyramidal geometry at silicon. The chair conformer of equatorial 4‐silathiacyclohexane 1‐oxide is 2.47, 7.90, and 8.09 kcal/mol more stable (ΔE) than its respective 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The chair conformer of axial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 4.18 and 5.70 kcal/mol more stable than its 1,4‐twist conformer and 2,5‐boat transition state and 1.51 kcal/mol more stable than the chair conformer of equatorial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide. The chair conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 5.02 and 6.11 kcal/mol more stable than its respective 1,4‐twist conformer and 2,5‐boat transition state, but is less stable than its 2,5‐twist conformer (ΔE = ?1.77 kcal/mol) and 1,4‐boat transition state (ΔE = ?1.65 kcal/mol). The 2,5‐twist conformer and 1,4‐boat conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide are stabilized by intramolecular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The chair conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is 3.02, 5.16, 0.90, and 6.21 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 1,4‐boat conformers and 2,5‐boat transition state. The 1,4‐boat conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in a trigonal bipyramidal geometry at silicon. The relative energies of the conformers and transition states are discussed in terms of hyperconjugation, orbital interactions, nonbonded interactions, and intramolecular sulfinyl oxygen–silicon coordination. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Photolytic Decarbonylation of Oxalyl Diisothiocyanate in Solid Argon Matrices to syn-anti Carbonyl Diisothiocyanate and Its Isomerization

Oxalyl diisothiocyanate, ((CO)NCS)₂, has been studied in solid argon matrices at 4.2 K with the aid of infrared (IR) spectroscopy. The spectra show mainly signals attributed to the most stable anti-anti conformer, which is corroborated by comparison to computed anharmonic fundamental IR transitions. Upon irradiation with 254 nm UV light, oxalyl diisothiocyanate eliminates carbon monoxide under formation of carbonyl diisothiocyanate, CO(NCS)₂. This reaction is only slightly exothermic by 0.4 kcal mol⁻¹ at the DLPNO-CCSD(T)/def2-QZVPP//B3LYP-D3/def2-TZVPP level of theory. Remarkably, photolysis produces mostly the less stable syn-anti conformer of carbonyl diisothiocyanate. Subsequent annealing at 30 K for two minutes results in a structural relaxation to the 0.7 kcal mol⁻¹ more stable syn-syn conformer confirming a low torsional barrier height between the isomers.     

Carbon-13 NMR studies of substituted naphthalenes. II—rotational barriers and conformations in some naphthaldehydes and azulenaldehydes

Proton-coupled and noise-decoupled ¹³C NMR spectra of 1-azulenecarbaldehyde, 1-acetylazulene and 1,3-azulenedicarbaldehyde have been studied, and complete assignments have been made based on the ¹³C? ¹H coupling constants, additivity of substituent effects (SIS), and previous assignment for the parent hydrocarbon. The barriers to rotation of the aldehyde group in the above azulenecarbaldehydes and in some naphthaldehydes have been determined by ¹³C dynamic NMR, (DNMR), resulting in free energies of activation of 42.7, 26.8 and 34.4 kJ mol^?1 for 1-azulenecarbaldehyde, 1-naphthaldehyde and 2-naphthaldehyde respectively. The same order of barriers is obtained by CNDO/2 calculations. A 4-methoxy substituent in 1-naphthaldehyde and a 6-methoxy substituent in 2-naphthaldehyde increases the rotational barrier by 4.6 and 2.9 kJ mol^?1, respectively, whereas a 3-methoxy substituent in 2-naphthaldehyde reduces the barrier by 6.7 kJ mol^?1. The conformations of the dominant rotamers are deduced from ¹³C chemical shifts to be Z for 1-azulenecarbaldehyde and 1-naphthaldehyde, and E for 2-naphthaldehyde.     

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions,Ab Initio Calculations,and r 0 Structural Parameters of the Propenoyl Halides (F,Cl, Br)

Variable temperature (–100 to –150°C) studies of the infrared spectra (3500–400 cm^–1) of propenoyl bromide, CH₂=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm^–1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm^–1, which is in excellent agreement with the experimental results. The r ₀ adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

Conformational Studies of Methyl Vinyl Silane from Temperature-Dependent FT–IR Spectra of Xenon and Krypton Solutions

Variable-temperature (–55 to –155°C) studies of the infrared spectra (3500–400 cm^–1) of methyl vinyl silane, CH₂CHSiH₂CH₃, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of conformer doublets due to the cis and gauche rotamers, the enthalpy difference has been determined to be 133 ± 11 cm^–1 (1.59 ± 0.13 kJ/mol) with the gauche conformer the more stable form in the krypton solution. In the xenon solution, the enthalpy difference could not be determined because the infrared bands become so broad and the overlap was so extensive that meaningful areas could not be determined. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G(2d,2p) from which structural parameters and conformational stabilities have been determined. With the largest basis set, the cis conformer is predicted to be the more stable conformer, which is inconsistent with the experimental results. Utilizing previously reported microwave rotational constants for both conformers along with the ab initio predicted distances and angles, r ₀ parameters have been obtained for both the cis and gauche conformers. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Intramolecular interaction effects and the structure of N2OS and N2O2: An Ab initio study

An ab initio study of O?N? N?S with full geometry optimization has been carried out to corroborate the presence of an interaction between the terminal atoms in this type of structure, which, in O?N? N?O, apparently stabilizes the cis conformer. Using the unscaled 4–31G basis set with a full set of d functions on the sulfur, there is a potential minimun at the trans but not the cis geometry. A gauche conformer with a torsional angle of 77.2° is the most stable. With N₂O₂ this basis set gives potential minima at both the cis and trans geometries, but the trans conformer is slightly more stable, contrary to experiment and the results of (7,3) basis-set calculations reported in the literature in which Gaussian lobe functions were employed. Using a (9,5) basis set there is no longer a potential minimum at the cis geometry, and a gauche structure is more stable than the cis conformer as in the case of N₂OS with the less-extended basis set. Force constants (harmonic and anharmonic), compliance constants, relaxed force constants, and interaction-displacement coordinates for both molecules are compared for key structural elements.     

Rationalizing nuclear overhauser effect data for compounds adopting multiple-solution conformations

An algorithm is described for refining the populations of a set of multiple-solution conformers using experimental nuclear Overhauser effects (nOes). The method is based upon representing the effective relaxation matrix for the set of interconverting proposed conformers as a linear combination of relaxation matrices (LCORMs) due to each conformer. The conformer population derivative of the nOe is derived from a Taylor series expression for the calculated nOe. This derivative may then be used in a standard nonlinear least-squares refinement procedure. The LCORM nOe procedure is tested using a monosaccharide system, 1-O-methyl-α-L -iduronate, that is known to exhibit conformational variability. The measured nOes for this system are used to refine the populations of a set of three static conformers, namely, the ¹C₄, ⁴C₁, and ²S₀ ring conformers. The populations thus derived are compared to those previously obtained using nuclear magnetic resonance proton-proton coupling constant information. Two possible extensions to the method are discussed: The first uses combined nOe and coupling constant data while the second removes the restrictions that the conformers used for fitting be rigid entities. © 1994 by John Wiley & Sons, Inc.     

A Computational Study of Chair–Twist Energy Differences and the Chair–Chair Conformational Interconversion in Tetrahydro-2H-thiopyran (Thiacyclohexane,Thiane)

Ab initio molecular orbital theory with the LANL2DZ, 3-21G, 6-31G(d), 6-31+G(d), 6-31+G(d,p), 6-311+G(d,p),6-31G(2d), 6-31G(3d), and 6-311G(d,p) basis sets and density functional theory (B3P86, B3LYP, B3PW91) have been used to calculate the structures, relative energies, enthalpies, entropies, and free energies of the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran (tetrahydrothiopyran, thiacyclohexane, thiane, pentamethylene sulfide). All levels of theory calculated similar energy values and the effect of basis sets on the calculated energies was small. The chair conformer of tetrahydro-2H-thiopyran was 5.27 kcal/mol more stable than the 1,4-twist conformer, which was slightly more stable (0.81 kcal/mol) than the 2,5-twist conformer. The chair–1,4-twist and chair–2,5-twist free energy differences ( G°_{c – t}) were 5.44 and 5.71 kcal/mol, respectively. Intrinsic reaction coordinate [IRC, minimum-energy path (MEP)] calculations connected the transition state between the chair and the 2,5-twist conformers. This transition state is 9.73 kcal/mol higher in energy than the chair conformer and the energy differences between the chair and the 1,4-boat and 2,5-boat transition states were 8.07 and 6.38 kcal/mol, respectively. Stereoelectronic hyperconjugative interactions were observed in the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran. The stereoelectronic hyperconjugative effects in the chair conformer of tetrahydro-2H-thiopyran have been compared to those in the respective chair conformers of tetrahydro-2H-pyran, tetrahydro-2H-selenane, and tetrahydro-2H-tellurane.     

Pyrrolizidine alkaloids necine bases: Ab initio,semiempirical, and molecular mechanics approaches to molecular properties

The structural stabilities of endo and exo conformations of retronecine and heliotridine molecules were analyzed using different ab initio, semiempirical, and molecular mechanics methods. All electron and pseudopotential ab initio calculations at the Hartree-Fock level of theory with 6-31G* and CEP-31G* basis sets provided structures in excellent agreement with available experimental results obtained from X-ray crystal structure and ¹H-NMR (nuclear magnetic resonance) studies in D₂O solutions. The exo conformations showed a greater stability for both molecules. The most significant difference between the calculations was found in the ring planarity of heliotridine, whose distortion was associated with the interaction between the O(11)H group and the C(1)-C(2) double bond as well as with a hydrogen bond between O(11)H and N(4). The discrepancy between pseudopotential and all-electron optimized geometries was reduced after inclusion of the innermost electrons of C(1), C(2), and N(4) in the core potential calculation. The MNDO, AM1, and PM3 semiempirical results showed poor agreement with experimental data. The five-membered rings were observed to be planar for AM1 and MNDO calculations. The PM3 calculations for exo-retronecine showed a greater stability than the endo conformer, in agreement with ab initio results. A good agreement was observed between MM3 and ab initio geometries, with small differences probably due to hydrogen bonds. While exo-retronecine was calculated to be more stable than the endo conformer, the MM3 calculations suggested that endo-heliotridine was slightly more stable than the exo form. © 1996 by John Wiley & Sons, Inc.     

Ab initio molecular orbital study of energies and conformers of 3,4-dihydro-1,2-dithiin, 3,6-dihydro-1,2-dithiin, 4H-1,3-dithiin,and 2,3-dihydro-1,4-dithiin

Optimized geometries and energies for 3,4-dihydro-1,2-dithiin ( 1 ), 3,6-dihydro-1,2-dithiin ( 2 ), 4H-1,3-dithiin ( 3 ), and 2,3-dihydro-1,4-dithiin ( 4 ) were calculated using ab initio 6-31G* and MP2/6-31G*//6-31G* methods. At the MP2/6-31G*//6-31G* level, the half-chair conformer of 4 is more stable than those of 1 , 2 , and 3 by 2.5, 3.5, and 3.6 kcal/mol, respectively. The half-chair conformers of 1 , 2 , 3 , and 4 are 2.9, 7.1, 2.0, and 5.6 kcal/mol, respectively, more stable than their boat conformers. The calculated half-chair structures of 1 – 4 are compared with the calculated chair conformer of cyclohexane and the half-chair structures for cyclohexene, 3,4-dihydro-1,2-dioxin ( 5 ), 3,6-dihydro-1,2-dioxin ( 6 ), 4H-1,3-dioxin ( 7 ), and 2,3-dihydro-1,4-dioxin ( 8 ). © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1064–1071, 1998     

Conformational Studies of Cyclopropylmethyl Ketone from Temperature-Dependent FT–IR Spectra of Xenon Solutions

Variable temperature (–60 to –100°C) studies of the infrared spectra (3500–400 cm^–1) of cyclopropylmethyl ketone, c-C₃H₅C(CH₃)O, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the cis and near-trans conformers, the enthalpy difference has been determined to be 269 ± 26 cm^–1 (3.22 ± 0.31 kJ/mol) with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From these data it is estimated that 79 ± 3% of the cis form is present at ambient temperature. Ab initio calculations have been carried out with different basis sets up to 6-311+G(2df,2pd) at the restricted Hartree–Fock and/or with full electron correlation by the perturbation method to second order (MP2) from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions that the cis form is the more stable conformer. A complete vibrational assignment is given for the cis conformer, which is supported from a normal coordinate calculation utilizing ab initio force constants. Several of the fundamentals of the near-trans conformer have been identified and assigned. Adjusted r ₀ structural parameters have been obtained from combined ab initio predicted values and previously reported rotational constants from the microwave investigation. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Structure and Properties of 2,2-Dibromovinyl Trifluoromethyl Ketone

It was established by IR spectroscopy and quantum-chemical calculations along nonempirical DFT method in B3LYP version with the basis set 6-311 G(d,p) that 2,2-dibromovinyl trifluoromethyl ketone consisted of a mixture of s-cis planar conformer and s-trans-form deviating from a plane by 13°, whereas the s-cis-form is more energetically stable than the s-trans one (E -5.07 kcal mol^{- 1}). Also in 2,2-dibromovinyl methyl ketone the planar s-cis conformer is more stable. Chlorine-containing analogs, 2,2-dichlorovinyl trifluoromethyl ketone and 2,2-dichlorovinyl methyl ketone, are more stable in the planar s-trans-conformation. Charge distribution and polarization in the dibromovinyl ketones are analogous to those in dichlorovinyl ketones in agreement with the established reactivity of dibromovinyl trifluoromethyl ketone. By reaction of 2,2-dibromovinyl trifluoromethyl ketone with 2,4-dinitrophenyl-, alkylhydrazines, N,N-dimethylhydrazine, N,N-, N,O-, N,S-binucleophiles were respectively obtained hydrazone, derivatives of pyrazole, imidazole, oxazole, and 1,3-thiazine containing a trifluoromethyl group.     

DFT and neutron diffraction study of 1,6-anhydro-β-D-glucopyranose (levoglucosan)

Geometries of 27 generated conformers of levoglucosan were optimized in vacuo at DFT level of theory combining several functionals with high quality basis sets. For the sake of comparison a reference molecular and crystal geometry obtained from 30 K single crystal neutron diffraction data was used. Analysis of the conformers’ geometries revealed that in all stable conformers intramolecular two-or three centre hydrogen bonds were formed. Relative energy of the conformer, which approximated the molecule in the crystal structure the most, was only ∼3 kcal/mol higher, than the energy of the most stable conformer in vacuo. The largest discrepancies between the geometries calculated in vacuo and experimental geometry concentrated in the vicinity of anomeric C1. These differences were reduced by involving O1 to intermolecular hydrogen bond using a simple model of the respective hydrogen bond in the crystal.      

Vibrational Spectra, Assignment, Conformational Stability and Ab Initio/DFT Calculations for 1-Nitropropane

The infrared spectra (4000–400 cm^{– 1}) of solid and the Raman spectra (3500–30 cm^{– 1}) of liquid and solid 1-nitropropane, CH₃CH₂CH₂NO₂, have been registered. Both the trans and gauche conformers have been identified in the fluid phase, while the trans form remains in the stable solid. Temperature dependence (190–230K) of the liquid 1-nitropropane Raman spectra has been carried out. From these data, the enthalpy difference was determined to be 870 ± 105 J-mol^–1, with the gauche conformer being the more stable rotamer. Ab initio and DFT calculations at different levels of approximation (HF, MP2, B3LYP, B3PW91) gave optimized geometries, harmonic force fields, and vibrational frequencies for the trans and gauche conformers. All the calculations (except the B3PW91/6-31G* level) predicted gauche as the low-energy conformer. Theoretical force constants are analyzed for formulating constraints in the molecular force field model of 1-nitropropane.     

Spectra and structure of organophosphorus compounds—XXXVII. Infrared and Raman spectra,conformational stability,vibrational assignment and normal coordinate analysis of ethyldichlorophosphine-d0 and -d5

The infrared (3500 to 40 cm⁻¹) and Raman (3500 to 10 cm⁻¹) spectra have been recorded for the gaseous and solid phases of ethyldichlorophosphine, CH₃CH₂PCl₂, and CD₃CD₂PCl₂. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. In the spectrum of the gas the gauche conformer is predominant with about 65% abundance whereas in the spectrum of the liquid at ambient temperature the amount of gauche conformer is reduced compared to the gas phase and at −100°C the trans conformer predominates. The trans conformer is the more stable species in the solid. A variable temperature study was carried out on the Raman spectrum of the liquid and ΔH and ΔS values of 190 ± 30 cm⁻¹ (543 ± 87 cal/mol) and 2.86 ± 0.3 eu were determined, respectively, with the trans conformer being more stable. Similar variable temperature studies have been carried out on a number of conformer peaks in the infrared spectrum of the gas and a ΔH value of 53 ± 38 cm⁻¹ (152 ± 110 cal/mol) was obtained, again with the trans conformer being more stable. All the fundamental modes of both conformers have been assigned utilizing band contours, depolarization values, isotopic shift factors and group frequencies. A normal coordinate calculation has been carried out utilizing a modified valence force field to calculate the frequencies and potential energy distribution for both conformers. The barriers to methyl rotation of the trans and gauche conformers are 2.2 ± 0.1 and 2.3 ± 0.1 kcal/mol, respectively. These results are compared to similar quantities for some corresponding molecules.     

Structure,electronic, and CV properties of hydroxy-naphthaldehyde Schiff-base copper(II) complexes derived from alkyl amines: X-ray structure of bis(n-i-propyl-3-hydroxy-2-naphthaldiminato)copper(II)

Copper(II) complexes with Schiff bases derived from linear and branched amines with 2-hydroxy-1-naphthaldehyde and 3-hydroxy-2-naphthaldehyde were synthesized and characterized by IR and electronic spectroscopies, magnetic susceptibilities, electrochemistry, cyclic voltammetry (CV), X-ray diffraction, and theoretical calculations. Electronic spectra are a function of the R group on the imine nitrogen of the ligand that determines steric effects. Complexes obtained from 3-hydroxy-2-naphthaldehyde ligand are more distorted from planarity in dimethylformamide solutions than those of 2-hydroxy-1-naphthaldehyde for the same R group. As a result, the former are easier to reduce to the related copper(I) complexes than those of 2-hydroxy-1-naphthaldiminate. The structure of bis(N-i-propyl-3-hydroxy-2-naphthaldiminato)copper(II) was determined by single-crystal X-ray diffraction. Its molecular structure can be described as distorted flat tetrahedral influenced by important packing effects and is flatter than that calculated with density functional theory methods. Additional related flat tetrahedral complexes were studied by theoretical calculations and it has been found that electronic effects also play a significant role.     

Conformational analysis,part 43. A theoretical and LIS/NMR investigation of the conformations of substituted benzamides

A refined Lanthanide‐Induced‐Shift Analysis (LISA) is used with molecular mechanics and ab initio calculations to investigate the conformations of benzamide ( 1 ), N‐methylbenzamide ( 2 ), N,N‐dimethylbenzamide ( 3 ) and the conformational equilibria of 2‐fluoro ( 4 ), 2‐chloro ( 5 ) and N‐methyl‐2‐methoxy benzamide ( 6 ). The amino group in 1 is planar in the crystal but is calculated to be pyramidal with the CO/phenyl torsional angle (ω) of 20–25°. The LISA analysis gave acceptable agreement factors (Rcryst ≤ 1%) for the ab initio geometries when ω was decreased to 0°, the other geometries were not as good. In 2 , the N‐methyl is coplanar with the carbonyl group in all the geometries. Good agreement was obtained for the RHF geometries, with ω 25°, the other geometries were only acceptable with increased values of ω. In 3 , good agreement for the RHF and PCModel geometries was found when ω was changed from the calculated values of 40° (RHF) and 90° (PCModel) to ca. 60°, the X‐ray and B3LYP geometries were not as good. The two substituted compounds 4 , 5 and 6 are interconverting between the cis (O,X) and trans (O,X) conformers. The more stable trans conformer is planar in 4 and 6 but the cis form non‐planar. Both the cis and trans conformers of 5 are non‐planar. There is an additional degree of freedom in 6 due to the 2‐methoxy group, which can be either planar or orthogonal to the phenyl ring in both conformers. The conformer ratios were obtained from the LISA analysis to give Ecis‐Etrans in 4 > 2.3 kcal/mol (CDCl₃) and 1.7 kcal/mol (CD₃CN), in 5 0.0 kcal/mol (CD₃CN) and in 6 > 2.5 kcal/mol (CDCl₃) and 2.0 kcal/mol (CD₃CN). These values were used with the observed versus calculated ¹H shifts to determine the conformer ratios and energies in DMSO solvent to give Ecis‐Etrans 1.1, ?0.1 and 1.8 kcal/mol for ( 4 ), ( 5 ) and ( 6 ). Comparison of the observed versus calculated conformer energies show that both the MM and ab initio calculations overestimate the NH..F hydrogen bond in ( 4 ) by ca. 2 kcal/mol. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MI_ax) and equatorial (2MI_eq) conformers of 2MI have been observed. A 2MI_eq/2MI_ax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, \Updelta E = E _{2 \textMI\textax} - E _{2 \textMI\texteq} \Updelta E = E_{{ 2 {\text{MI}}_{\text{ax}} }} - E_{{ 2 {\text{MI}}_{\text{eq}} }} , of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: two equatorial conformers (2PI_eq0 and 2PI_eq90), and one axial conformer (2PI_ax). Only the axial conformer of 2PI (2PI_ax) was experimentally observed. The indan ring of 2PI_ax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular C_sp2–H/π interactions in 2PI_ax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial–equatorial interconversion. The barrier (2.33 kcal/mol) to axial–equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C–H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the C_sp2–H bond than that of C_sp3–H bond.     

Conformational studies of aliphatic secondary ozonides (propene, 1-butene and 1-heptene) by means of FTIR spectroscopy

Ozonization reaction of simple alkenes was studied by means of FT infrared absorption gas spectroscopy. The reaction was performed at 95 K in neat films of the reactants. IR absorption spectra of the gaseous products were recorded. The spectra were analyzed combining experimental results with theoretical calculations performed at B3LYP 6-311++G (3df, 3pd) level. We found that among all theoretically predicted conformers of propene secondary ozonide, only one which has the O-O half-chair configuration for the five membered ring and the radical attached in the equatorial position was present in the sample. Samples of 1-butene and 1-heptene secondary ozonides consist from two conformers of very similar energy (ΔH=0.3 kJ/mol). The most stable conformer for both ozonides is the one with O-O half-chair configuration of the five membered ring and the radical attached in equatorial position and the aliphatic chain in gauche position. The second stable conformer has the aliphatic chain in anti position.