Interactions intramoléculaires. XXVI—Constantes de couplage et hétérogénéités conformationnelles dans une série de R-3 et R-5 cyano-4 cyclohexènes deutériés (R=méthyl ou t-butyl)

For trans-3-R- and 5-R-1-acetoxy-4-cyanocyclohexene-6,6-d₂ the molar fractions of diequatorial conformers are 0.83 (3-methyl), 0.68 (5-methyl), 0.57 (3-tert-butyl) and 0.55–0.69 (5-tert-butyl). For the last two compounds the values of the coupling constants are in agreement with the hypothesis of an ee?aa equilibrium. For the cis isomers, the molar fractions of equatorial alkyl conformers are 0.76 (3-methyl and 5-methyl) and 1.0 (3-tert-butyl and 5-tert-butyl). The cis-1-acetoxy-3-tert-butyl-4-methoxycarbonyl-cyclohexene presents a conformational heterogeneity. The conformational free energy of the methyl group in position 4 has been evaluated as ?0.6 kcal mol^?1 (2.5 kJ mol^?1).     

Stereospecific control of peptide gas‐phase ion chemistry with cis and trans cyclo ornithine residues

We report non‐chiral amino acid residues cis‐ and trans‐1,4‐diaminocyclohexane‐1‐carboxylic acid (cyclo‐ornithine, cO) that exhibit unprecedented stereospecific control of backbone dissociations of singly charged peptide cations and hydrogen‐rich cation radicals produced by electron‐transfer dissociation. Upon collision‐induced dissociation (CID) in the slow heating regime, peptide cations containing trans‐cO residues undergo facile backbone cleavages of amide bonds C‐terminal to trans‐cO. By contrast, peptides with cis‐cO residues undergo dissociations at several amide bonds along the peptide ion backbone. Diastereoisomeric cO‐containing peptides thus provide remarkably distinct tandem mass spectra. The stereospecific effect in CID of the trans‐cO residue is explained by syn‐facially directed proton transfer from the 4‐ammonium group at cO to the C‐terminal amide followed by neighboring group participation in the cleavage of the CO―NH bond, analogous to the aspartic acid and ornithine effects. Backbone dissociations of diastereoisomeric cO‐containing peptide ions generate distinct [b_n]⁺‐type fragment ions that were characterized by CID‐MS³ spectra. Stereospecific control is also reported for electron‐transfer dissociation of cis‐ and trans‐cO containing doubly charged peptide ions. The stereospecific effect upon electron transfer is related to the different conformations of doubly charged peptide ions that affect the electron attachment sites and ensuing N―C_α bond dissociations.     

Uranium–Carbene–Imido Metalla‐Allenes: Ancillary‐Ligand‐Controlled cis‐/trans‐Isomerisation and Assessment of trans Influence in the R2C=UIV=NR′ Unit (R=Ph2PNSiMe3; R′=CPh3)

Uranium(IV)–carbene–imido complexes [U(BIPM^TMS)(NCPh₃)(κ²‐N,N′‐BIPY)] ( 2 ; BIPM^TMS=C(PPh₂NSiMe₃)₂; BIPY=2,2‐bipyridine) and [U(BIPM^TMS)(NCPh₃)(DMAP)₂] ( 3 ; DMAP=4‐dimethylamino‐pyridine) that contain unprecedented, discrete R₂C=U=NR′ units are reported. These complexes complete the family of E=U=E (E=CR₂, NR, O) metalla‐allenes with feasible first‐row hetero‐element combinations. Intriguingly, 2 and 3 contain cis‐ and trans‐C=U=N units, respectively, representing rare examples of controllable cis/trans isomerisation in f‐block chemistry. This work reveals a clear‐cut example of the trans influence in a mid‐valent uranium system, and thus a strong preference for the cis isomer, which is computed in a co‐ligand‐free truncated model—to isolate the electronic trans influence from steric contributions—to be more stable than the trans isomer by approximately 12 kJ mol^?1 with an isomerisation barrier of approximately 14 kJ mol^?1.     

Stereoelectronic control of the retro diels–alder reaction in 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene isomers

The cis- and trans-annulated isomers of 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene show different propensities for the retro Diels–Alder fragmentation following electron impact ionization. Molecular ions of the cis-annulated isomer decompose predominantly via the retro Diels–Alder reaction to give [C₉H₁₃N] ⁺· fragments of the appearance energy (AE)=8.45±0.05eV and critical energy E_c=133±8kJ mol^?1. The trans-annulated isomer gives abundant [M–H]⁺ (AE=9.34±0.08eV) and [M–C₆H₆]⁺· fragments, in addition to [C₉H₁₃N]⁺· ions of AE=8.98±0.05eV and E_c=181±8kJ mol^?1. The ionization energies (IE) were determined as IE_cis=7.07±0.05 eV and IE_trans=7.10±0.06eV. The stereochemical information is much less pronounced in unimolecular decompositions of long-lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Diels–Alder fragmentation; T_0.5=3.8±0.3 and 4.8±0.2 kJ mol^?1 for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Diels–Alder reaction prefers a two-step path, with a subsequent cleavage of the C(5)? C(6) and C(1)? C(2) bonds. The open-ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(1)? C(2) bond is the rate-determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 kJ mol^?1. The cleavage of the C(5)? C(6) bond becomes the rate-determining step in the trans-annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (ΔE=95k Jmol^?1) provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.     

Conformational Landscape,Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro-2-Furoic Acid⋅⋅⋅Propylene Oxide Conformers

A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using rotational spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I , II , and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol⁻¹. A rich broadband rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag rotational spectroscopy, an exciting new analytical application of rotational spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.     

13C NMR and force field investigations of hydrindane conformations

¹³C NMR shifts of trans- and cis-annelated bicyclo[4.3.0]nonanes with substituents R in position 8 (R ? H, OH, Cl, Br) and 1-hydroxy derivatives were analysed on the basis of force field calculated torsional angles using Allinger's MM1 program. Shielding increments for the 6 membered ring agree with corresponding cyclohexane values within ± 0.8 ppm maximal deviation. ¹³C NMR line shape analysis with cis-hydrindane between 148 and 180 K yielded ΔH* = 37.0 ± 0.4 kJ mol^?1 and ΔS* = 28 J mol^?1 K^?1 for the topomerization. The force field calculated reaction profile showed ΔH* = 37 kJ mol^?1, in close agreement.     

Structural characterization of isomeric 2,3,5‐substituted tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones prepared by cycloaddition of N‐methyl‐C‐arylnitrones to N‐phenyl‐ or N‐methylmaleimide

1,3‐Dipolar cycloaddition reactions of N‐methyl‐C‐arylnitrones with N‐phenyl‐ or N‐methylmaleimide were studied. The reaction of p‐dimethylamino‐, 4‐benzyloxy‐3‐methoxy‐, p‐nitro‐ and p‐chloro‐substituted phenylnitrones with N‐phenylmaleimide gave cis and trans cycloadducts but that of the corresponding phenylnitrones with N‐methylmaleimides only the cis adducts in the case of p‐dimethylamino and 4‐benzyloxy‐3‐methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by ¹H NMR at 233 K and ¹³C NMR at 208 K) to be mixtures of two invertomers, namely o‐(N‐lone pair antiperiplanar to 3H; minor) and i‐conformations (3H‐C‐C‐3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6–31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.     

The Implications of (2S,4S)‐Hydroxyproline 4‐O‐Glycosylation for Prolyl Amide Isomerization

The conformations of peptides and proteins are often influenced by glycans O‐linked to serine (Ser) or threonine (Thr). (2S,4R)‐4‐Hydroxyproline (Hyp), together with L ‐proline (Pro), are interesting targets for O‐glycosylation because they have a unique influence on peptide and protein conformation. In previous work we found that glycosylation of Hyp does not affect the N‐terminal amide trans/cis ratios (K_trans/cis) or the rates of amide isomerization in model amides. The stereoisomer of Hyp—(2S,4S)‐4‐hydroxyproline (hyp)—is rarely found in nature, and has a different influence both on the conformation of the pyrrolidine ring and on K_trans/cis. Glycans attached to hyp would be expected to be projected from the opposite face of the prolyl side chain relative to Hyp; the impact this would have on K_trans/cis was unknown. Measurements of ³J coupling constants indicate that the glycan has little impact on the C^γ‐endo conformation produced by hyp. As a result, it was found that the D ‐galactose residue extending from a C^γ‐endo pucker affects both K_trans/cis and the rate of isomerization, which is not found to occur when it is projected from a C^γ‐exo pucker; this reflects the different environments delineated by the proline side chain. The enthalpic contributions to the stabilization of the trans amide isomer may be due to disruption of intramolecular interactions present in hyp; the change in enthalpy is balanced by a decrease in entropy incurred upon glycosylation. Because the different stereoisomers—Hyp and hyp—project the O‐linked carbohydrates in opposite spatial orientations, these glycosylated amino acids may be useful for understanding of how the projection of a glycan from the peptide or protein backbone exerts its influence.     

An ab initio study of some structural features and the rotational barriers in performic acid,formic acid and hydrogen peroxide

Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol^?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol^?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results.     

Dinuclear Tin(II) Complex of a Bulky cis‐Oxamide: Synthesis,Characterization, Crystal Structure,and DFT Studies

The reaction of the di‐lithiated oxamide of 1 with two equivalents of SnCl₂ provided the tin trans‐oxamide 3 . In solution, spectroscopic analysis suggests exclusively the formation of a trans‐oxamide (trans‐ 3 ). However, the solid state shows an atypical cis‐oxamide (cis‐ 3 ), where the oxamide fragment acts as an anti‐Janus head ligand. An ¹¹⁹Sn‐NMR variable temperature experiment ([D₈]THF) of the trans‐oxamide (trans‐ 3 ) was performed however, at lower temperature no additional signal was observed, which confirmed the absence of a dynamic equilibrium. Dispersion‐corrected density functional calculations revealed that the cis conformation of this tin(II) oxamide complex is more stable than the trans isomer by 1.4 kcal · mol^–1.     

Theoretical studies on the conformation of saccharides. V. Hydration of the acetal segment in glycosides

Hydration sites of the acetal segment were studied in five of the most stable conformers of 2-methoxy-tetrahydropyran (MTHP) as the first step in the determination of the hydration scheme of glycosides. The intramolecular geometries of a supermolecule formed with MTHP and water were calculated by a PCILO quantum-chemical method. The hydration sites determined can be classified into two groups: (a) individual sites, in which water interacts with one oxygen only, and (b) bridging sites, in which water interacts with both oxygens. The interaction energies of the individual sites are approximately 22 kJ mol^?1, and 26 and 29 kJ mol^?1 in the bridging sites. An increase of the number of water molecules in the hydration shell of MTHP showed that monohydration of the glycosidic linkage oxygens was most advantageous. Despite of the fact that the hydration shell have various structures in the individual MTHP conformers, the obtained results indicate that the hydration does not operate against the anomeric or exoanomeric effects, i.e., it does not influence the equilibrium of the MTHP conformers in favor of the trans arrangements of a glycosidic bond. Therefore, the experimentally observed stabilization of the trans positions in aqueous solutions should be considered as a result of influence of water being a dielectric continuum.     

Dynamic proton magnetic resonance studies on complex spin systems. Non-mutual three-spin exchange in N-(trideuteriomethyl)-2-cyanoaziridine

At room temperature and below, the proton NMR spectrum of N-(trideuteriomethyl)-2-cyanoaziridine consists of two superimposed ABC patterns assignable to two N-invertomers; a single time-averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high-temperature spectrum permit the calculation of the thermodynamic quantities ΔH⁰ = ?475±20 cal mol^?1 (?1.987 ± 0.084 kJ mol^?1) and ΔS⁰ = 0.43±0.08 cal mol^?1 K^?1 (1.80±0.33 J mol^?1 K^?1) for the cis ? trans equilibrium. Bandshape analysis of the spectra broadened by non-mutual three-spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔH_tc^≠ = 17.52±0.18 kcal mol^?1 (73.30±0.75 kJ mol^?1), ΔS_tc^≠ = ?2.08±0.50 cal mol^?1 K^?1 (?8.70±2.09 J mol^?1 K^?1) and ΔG_tc^≠ (300 K) = 18.14±0.03 kcal mol^?1 (75.90±0.13 kJ mol^?1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.     

A photoionization study of [C4H7]+ ions in the gas phase

The ionization and [C₄H₇]⁺ appearance energies for a series of C₄H₇CI and C₄H₇Br isomers have been measured by dissociative photoionization mass spectrometry. Cationic heats of formation, based on the stationary electron convention, are derived. No threshold ion is observed with a heat of formation corresponding to the trans-1-methylallyl cation, although there is evidence for formation of the less stable cis isomer. A 298 K heat of formation of 871 kJ mol^?1 is obtained for the cyclopropylcarbinyl cation, with the cyclobutyl cation having a higher value of 886 kJ mol^?1. At the HF/6-31G^** level, ab initio molecular orbital calculations show the 2-butenyl, isobutenyl and homoallyl cations to be stable forms of [C₄H₇]⁺, being less stable than the trans-1-methylallyl cation by 101 kJ mol^?1, 159 kJ mol^?1 and 164 kJ mol^?1, respectively. However, threshold formation is not observed for any of these ions, the fragmentation of appropriate precursor molecules producing [C₄H₇]⁺ ions with lower energy structures.     

Conformational analysis. Part 40: a theoretical and NMR investigation of the conformations of cis‐ and trans‐cyclopentane‐1,3‐diol

The conformations of cis‐ ( 1 ) and trans‐cyclopentane‐1,3‐diol ( 2 ) have been studied by ab initio (Gaussian 98) and molecular mechanics (PCMODEL) calculations and by NMR spectroscopy. The calculations gave two low‐energy conformations for ( 1 ), 1A and 1B , both with axial hydroxyl groups. Two conformations with equatorial hydroxyl groups ( 1C and 1D ) were found but with much higher energy (ca 4.0 kcal mol^?1). Five low‐energy conformers were found for 2 . Four were envelope conformations and one a half‐chair. The complete analysis of the 400 MHz ¹H NMR spectra of 1 in a variety of solvents and 2 in chloroform was performed by extensive decoupling experiments, iterative computer analysis and spectral simulation. This gave all the H,H couplings in the molecule, including in 1 a long‐range ⁴J(H,H) coupling between H‐2cis and H‐4,5cis. The ³J(H,H) couplings were used to determine the conformer populations in these molecules. This was initially achieved using the Haasnoot, de Leeuw and Altona equation. to obtain the conformer couplings. It was found that this equation was not accurate for the C·CH₂·CH₂·C fragment in these molecules and the following equation was derived for this fragment from five‐ and six‐ membered cyclic compounds in fixed conformations: (1) The conformer populations were obtained by calculating the conformer couplings which were then compared with the observed couplings. Compound 1 in benzene solution is an approximately equal mixture of conformers 1A and 1B with small (<4%) amounts of 1C and 1D . In the polar solvents acetone and acetonitrile the populations of 1A and 1B are again equal, with 20% of 1C and <2% of 1D . In 2 the major conformers are 2B and 2D with small amounts of 2C , 2E and 2A . These novel findings are considered with previous data on cyclopentanol and cis‐ and trans‐cyclopentane‐1,2‐diol and it is shown that the axial hydroxyl substituent at the fold of the envelope appears to be a major factor in determining the conformational energies of these compounds. Copyright © 2003 John Wiley & Sons, Ltd.     

Low temperature 13C NMR study of 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkyl- aminocyclohexanes

Cyclohexane and piperidine ring reversal in 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkylaminocyclohexanes was investigated by ¹³C NMR. An unusually low conformational energy ΔG = 0.59 kJ mol^?1 and activation parameters ΔG^≠₂₁₈ = 43.8 ± 0.4 kJ mol^?1, ΔH^≠ = 48.9 ± 2.5 kJ mol^?1 and ΔS^≠ = 23 ± 9 J mol^?1 K^?1 were found for the diequatorial to diaxial transition of the cyclohexane ring in the trans-pyrrolidinyl derivative. In the trans-piperidinyl derivative, ΔG₂₂₂ = 44.7 ± 0.5 KJ mol^?1, ΔH^≠ = 55.7 ± 6.3 kJ mol^?1 and ΔS^≠ = 51 ± 21 J mol^?1 K^?1 was found for the piperidine ring reversal from the non-equivalence of the α-carbons.     

Peptide cation-radicals. A computational study of the competition between peptide N-Calpha bond cleavage and loss of the side chain in the [GlyPhe-NH2 + 2H]+. cation-radical

Cation‐radicals and dications corresponding to hydrogen atom adducts to N‐terminus‐protonated N_α‐glycylphenylalanine amide (Gly‐Phe‐NH₂) are studied by combined density functional theory and Møller‐Plesset perturbational computations (B3‐MP2) as models for electron‐capture dissociation of peptide bonds and elimination of side‐chain groups in gas‐phase peptide ions. Several structures are identified as local energy minima including isomeric aminoketyl cation‐radicals, and hydrogen‐bonded ion‐radicals, and ylid‐cation‐radical complexes. The hydrogen‐bonded complexes are substantially more stable than the classical aminoketyl structures. Dissociations of the peptide N? C_α bonds in aminoketyl cation‐radicals are 18–47 kJ mol^?1 exothermic and require low activation energies to produce ion‐radical complexes as stable intermediates. Loss of the side‐chain benzyl group is calculated to be 44 kJ mol^?1 endothermic and requires 68 kJ mol^?1 activation energy. Rice‐Ramsperger‐Kassel‐Marcus (RRKM) and transition‐state theory (TST) calculations of unimolecular rate constants predict fast preferential N? C_α bond cleavage resulting in isomerization to ion‐molecule complexes, while dissociation of the C_α? CH₂C₆H₅ bond is much slower. Because of the very low activation energies, the peptide bond dissociations are predicted to be fast in peptide cation‐radicals that have thermal (298 K) energies and thus behave ergodically. Copyright © 2003 John Wiley & Sons, Ltd.     

On the Effect of Tether Composition on cis/trans Selectivity in Intramolecular Diels–Alder Reactions

Intramolecular Diels–Alder (IMDA) transition structures (TSs) and energies have been computed at the B3LYP/6‐31+G(d) and CBS‐QB3 levels of theory for a series of 1,3,8‐nonatrienes, H₂C?CH? CH?CH? CH₂? X? Z? CH?CH₂ [? X? Z? =? CH₂? CH₂? ( 1 ); ? O? C(?O)? ( 2 ); ? CH₂? C(?O)? ( 3 ); ? O? CH₂? ( 4 ); ? NH? C(?O)? ( 5 ); ? S? C(?O)? ( 6 ); ? O? C(?S)? ( 7 ); ? NH? C(?S)? ( 8 ); ? S? C(?S)? ( 9 )]. For each system studied ( 1 – 9 ), cis‐ and trans‐TS isomers, corresponding, respectively, to endo‐ and exo‐positioning of the ? C? X? Z? tether with respect to the diene, have been located and their relative energies (E_rel^TS) employed to predict the cis/trans IMDA product ratio. Although the E_rel^TS values are modest (typically <3 kJ mol^?1), they follow a clear and systematic trend. Specifically, as the electronegativity of the tether group X is reduced (X?O→NH or S), the IMDA cis stereoselectivity diminishes. The predicted stereochemical reaction preferences are explained in terms of two opposing effects operating in the cis‐TS, namely (1) unfavorable torsional (eclipsing) strain about the C4? C5 bond, that is caused by the ? C? X? C(?Y)? group’s strong tendency to maintain local planarity; and (2) attractive electrostatic and secondary orbital interactions between the endo‐(thio)carbonyl group, C?Y, and the diene. The former interaction predominates when X is weakly electronegative (X?N, S), while the latter is dominant when X is more strongly electronegative (X?O), or a methylene group (X?CH₂) which increases tether flexibility. These predictions hold up to experimental scrutiny, with synthetic IMDA reactions of 1 , 2 , 3 , and 4 (published work) and 5 , 6 , and 8 (this work) delivering ratios close to those calculated. The reactions of thiolacrylate 5 and thioamide 8 represent the first examples of IMDA reactions with tethers of these types. Our results point to strategies for designing tethers, which lead to improved cis/trans‐selectivities in IMDAs that are normally only weakly selective. Experimental verification of the validity of this claim comes in the form of fumaramide 14 , which undergoes a more trans‐selective IMDA reaction than the corresponding ester tethered precursor 13 .     

1H, 13C, 17O NMR and quantum‐chemical study of the stereochemistry of the sulfoxide and sulfone derivatives of 3‐arylidene‐1‐thioflavan‐4‐one epoxides

The oxidation of the trans,cis‐( 2 ) and trans,trans‐epoxides ( 3 ) of differently substituted (Z)‐3‐arylidene‐1‐thioflavan‐4‐ones ( 1 ) with dimethyldioxirane (DMD) yielded the appropriate sulfoxides ( 4, 5 ) and sulfones ( 6, 7 ). The structures were elucidated by the extensive application of one‐ and two‐dimensional ¹H, ¹³C and ¹⁷O NMR spectroscopy. The conformational analysis was achieved by the application of ³J(C,H) coupling constants, NOESY responses and ab initio calculations. The preferred ground‐state conformers (twisted envelope‐A, twisted envelope‐B for 6 and twisted envelope‐A, envelope‐B for 7 ) were obtained as global minima of the theoretical ab initio MO study and also the examination of the ¹⁷O and ¹³C chemical shifts, calculated for the global minima structures of the sulfone isomers by the GIAO method. Analogous results, obtained for the sulfoxide isomers ( 4, 5 ), not only led to the preferred conformers but also gave evidence for the trans arrangement of the 2‐Ph group and the oxygen atom of the S?O group. Chemical shift differences between the isomers, sulfoxides and sulfones were corroborated by ab initio calculations of the anisotropic effects of the oxirane ring and the S?O and SO₂ groups. Copyright © 2003 John Wiley & Sons, Ltd.     

The vibrational and NMR spectra,conformations and ab initio calculations of aminomethylene,propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H₂N-CH=C(CN)₂], (methylamino)methylene propanedinitrile (MAM) [CH₃NH-CH=C(CN)₂] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH₃)₂N-CH=C(CN)₂] as solids and solutes in various solvents have been recorded in the region 4000-50 cm^–1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference H ⁰ between the conformers was measured to be 3.7±1.4 kJ mol^–1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol^–1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol^–1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor^–1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.     

The vibrational analysis and torsional study of trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane

The infrared spectra of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane have been recorded between 4000 and 200 cm^?1 in the polycrystalline solid phase, and 4000 to 80 cm^?1 in the gas phase. The Raman spectra of these two compounds in the gaseous and liquid phases were also recorded between 3100 and 10 cm^?1. An assignment of the thirty-nine fundamental vibrations for both cis- and trans-1,2-dimethylcyclopropane is proposed, and comparisons are made with the vibrations of other similar molecules. Additionally, ten torsional transitions were observed in the far infrared and Raman spectra of cis-1,2-dimethylcyclopropane, and four transitions were observed in the spectra of the trans compound. From these spectral data, torsional barriers were determined. The effective barriers to methyl torsion are 2.92 kcal mol^?1 (12.20 kJ mol^?1) for cis-1,2-dimethylcyclopropane and 2.61 kcal mol^?1 (11.14 kJ mol^?1) for trans-1,2-dimethylcyclopronane.