EE Isomer Research Articles

Intramolecular hydrogen bonds interactions in the isomers of the bilirubin molecule: DFT and QTAIM analysis.

Bilirubin is an important molecule, used as a marker of some liver diseases, and it can also be toxic and cause jaundice, especially in newborns. The main treatment for neonatal jaundice is phototherapy with blue light, which is still widely studied because the photophysical processes involved are not fully understood. Calculations based on the density functional theory (DFT) at M062X/6-31G(d,p) level were performed in order to evaluate the structural, electronic, and topological properties of bilirubin isomers. It was found that the ZZ conformation can form a greater number of hydrogen bonds, which gives the isomer greater energy stabilization compared to the other ZE, EZ, and EE isomers, and that the EE isomer is the conformer with the lowest energy of stabilization. The hydrogen bonds were characterized by the quantum theory of atoms in molecules (QTAIM) and for the ZZ isomer four hydrogen bonds (HBs) were found classified as intermediate, ∇2ρ(r) > 0, H(r) > 0. The ZE, EZ, and EE isomers show weak HBs, ∇2ρ(r) > 0, H(r) > 0.

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study.

The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of EE (major) and ZE (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections. Two additional but minor isomers are generated by collisional excitation of the electrosprayed styryl 9M ions and are assigned to the EZ and ZZ geometric isomers, with the latter predicted to have a π-stacked configuration. The isomer assignments are supported through calculations of equilibrium structures, collision cross sections, and statistical isomerization rates. Photoexcitation of selected isomers using an IMS-photo-IMS strategy shows that each geometric isomer photoisomerizes following absorption of near-infrared and visible light, with the EE isomer possessing a S1 ← S0 electronic transition with a band maximum near 680 nm and shorter wavelength S2 ← S0 electronic transition with a band maximum near 430 nm. The study demonstrates the utility of the IMS-photo-IMS strategy for providing fundamental gas-phase photochemical information on molecular systems with multiple isomerizable bonds.

Easy coordinate geometry indexes, τ4 and τ5 and HSA study for unsymmetrical Pd(II), Fe(II), Zn(II), Mn(II), Cu(II) and VO(IV) complexes of a tetradentate ligand: Synthesis, characterization, properties, and antioxidant activities

In this study, versatile multifunctional Schiff base derivative and its metal complexes were synthesized and characterized by 1H, 13C NMR, UV–vis, IR, CHN analysis, Mass spectra, m.p. and X-ray diffraction. The tetradentate Schiff base H2L was synthesized using a microwave-assisted method. Metal complexes [M = Pd (1), Zn (2), Cu (3), Fe (4), Ni (5) Mn (6) and V(O) (7)] were prepared using mild conditions. Their antioxidant activities were assessed by four different methods (DPPH, FTC, HRSA, and H2O2 SA). The antioxidant results revealed that these compounds have moderate to good activities against the BHA which was used as a standard or a positive control. The crystal and molecular structures of H2L, 1, 2 and 7 have been determined by X-ray diffraction on single crystals with Hirshfeld surfaces and fingerprint plots analysis facilitating a comparison of intermolecular interaction. H2L possesses intra-molecular hydrogen bonds. In the complexes, the tetra dentate Schiff base ligands (ONNO) forms a heterocyclic base plane of a distorted square-pyramidal coordination geometry in 1, 2 and 7. Interestingly, only the enol-imine tautomeric form was confirmed by various techniques as an EE isomer. Geometric parameter for four and five-coordinate complexes, τ4 and τ4 for quantitatively evaluating the geometry of the complexes are presented.

Synthesis, XRD, HAS, in silico molecular docking studies and biological assessment of novel Schiff base compounds as anti-cancer and antimicrobial agents

In this study, versatile multifunctional Schiff base (SB) derivatives were synthesized. Compounds 1–8 were prepared by a mild condition and were pharmacologically assessed for their role in vitro anti-cancer and their impact on human fibrosarcoma (HT-1080) and cervical cancer cells (HeLa), in addition to their antimicrobial activity regarding fungal strains, gram-positive and gram-negative bacteria. Preliminary in silico study of 1–8 and the standard compound (5-Fluorouracil) was accomplished, using Drug2Way and PASS software. Besides, docking investigations were carried out using Schrödinger software to determine the interaction of p53-MDM2 and pf-DHFR binding affinity for all the compounds. The antimicrobial results exhibited that these novel compounds have modest to good inhibitory action against the tried bacterial and fungal strains. The crystal structures of 2 and 7 have been determined. Hirshfeld Surface Analysis (HSA) is in agreement with the XRD studies. Both compounds have shown enol–imine tautomeric forms as EE isomer.

Easy Synthesis and Crystal Structure of a Novel Ortho Hydroxyl Schiff Base Compound CH2-{CYCLO-C6H10)-NCH-C6H4-OH-2}2

A one-pot and neat reaction procedure is used for the synthesis of CH2-{cyclo-C6H10)-NCH-C6H4-OH-2}2. 1H NMR shows a mixture of ZE and EE isomers in a ratio of 2:0.9. The EE isomer is easily isolated as crystals suitable for the X-ray analysis. 1H,13C NMR and UV-visible spectra of the title compound are studied in different solvents. The crystal structure shows intramolecular H bonds as an EE isomer. The molecule lies about a twofold symmetry axis, passing through the C1 atom. The cyclo-hexyl ring C2-7 shows a chair conformation, with both C2-C1 and C5-N1 bonds in equatorial orientations. The phenolic hydrogen atom H1a forms a good hydrogen bond with the imino nitrogen atom N1 and the N1-C8 bond is short, as expected for a double bond (1.265(4) A).

Easy synthesis and crystal structure of a novel ortho hydroxyl Schiff base compound CH2—{cyclo-C6H10)—NCH—C6H4—OH-2}2

A one-pot and neat reaction procedure is used for the synthesis of CH2—{cyclo-C6H10)—NCH—C6H4—OH-2}2. 1H NMR shows a mixture of ZE and EE isomers in a ratio of 2:0.9. The EE isomer is easily isolated as crystals suitable for the X-ray analysis. 1H,13C NMR and UV-visible spectra of the title compound are studied in different solvents. The crystal structure shows intramolecular H bonds as an EE isomer. The molecule lies about a twofold symmetry axis, passing through the C1 atom. The cyclo-hexyl ring C2—7 shows a chair conformation, with both C2—C1 and C5—N1 bonds in equatorial orientations. The phenolic hydrogen atom H1a forms a good hydrogen bond with the imino nitrogen atom N1 and the N1—C8 bond is short, as expected for a double bond (1.265(4) Å).

Crystal structure, Hirshfeld surface, physicochemical, thermal and DFT studies of (N1E, N2E)-N1,N2-bis((5-bromothiophen-2-yl)methylene)ethane-1,2-diamine N2S2 ligand and its [CuBr(N2S2)]Br complex

A new tetradentate N2S2 Schiff base ligand derived from 5-bromothiophene-2-carbaldehyde and its [CuBr(N2S2)]Br complex were synthesized in good yield. FT-IR was investigated to monitor the condensation reaction during the ligand synthesis process. The ligand, N2S2, and its complex, [CuBr(N2S2)]Br, complex have been characterized with the aid of several spectroscopic techniques such as UV–vis., MS, FT-IR, EA, EDS, NMR (for N2S2), as well as by thermal (TG/DTG) analysis. In addition, cyclic voltammetry has been employed to examine the redox behavior of [CuBr(N2S2)]Br complex in N,N’-dimethylformamide (DMF) containing 0.10 M tetra-n-butylammonium perchlorate (TBAP). The anti-conformation EE isomer of the ligand (N2S2) was confirmed by means of X-ray crystallography; ligand crystallizes in to the monoclinic P21/c space group. N2S2 ligand was subjected to DFT-theoretical calculations and results are consistent with the experimental chemical analysis.

Importance of Long-Range Noncovalent Interactions in the Regioselectivity of Rhodium-Xantphos-Catalyzed Hydroformylation

M06-L-based quantum chemical calculations were performed to examine two key elementary steps in rhodium (Rh)-xantphos-catalyzed hydroformylation: carbonyl ligand (CO) dissociation and the olefin insertion into the Rh–H bond. For the resting state of the Rh-xantphos catalyst, HRh(xantphos)(CO)2, our M06-L calculations were able to qualitatively reproduce the correct ordering of the equatorial–equatorial (ee) and equatorial–axial (ea) conformers of the phosphorus ligands for 16 derivatives of the xantphos ligand, implying that the method is sufficiently accurate for capturing the subtle energy differences associated with various conformers involved in Rh-catalyzed hydroformylation. The calculated CO dissociation energy from the ea conformer (ΔE = 21–25 kcal/mol) was 10–12 kcal/mol lower than that from the ee conformer (ΔE = 31–34 kcal/mol), which is consistent with prior experimental and theoretical studies. The calculated regioselectivities for propene insertion into the Rh–H bond of the ee-HRh(xantphos)(propene)(CO) complexes were in good agreement with the experimental l:b ratios. The comparative analysis of the regioselectivities for the pathways originating from the ee-HRh(xantphos)(propene)(CO) complexes with and without diphenyl substituents yielded useful mechanistic insight into the interactions that play a key role in regioselectivity. Complementary computations featuring xantphos ligands lacking diphenyl substituents implied that the long-range noncovalent ligand–ligand and ligand–substrate interactions, but not the bite angles per se, control the regioselectivity of Rh-diphosphine-catalyzed hydroformylation of simple terminal olefins for the ee isomer. Additional calculations with longer chain olefins and the simplified structural models, in which the phenyl rings of the xantphos ligands were selectively removed to eliminate either substrate–ligand or ligand–ligand noncovalent interactions, suggested that ligand–substrate π-HC interactions play a more dominant role in the regioselectivity of Rh-catalyzed hydroformylation than ligand–ligand π–π interactions. The present calculations may provide foundational knowledge for the rational design of ligands aimed at optimizing hydroformylation regioselectivity.

Catalysts for enantioselective Biginelli reaction based on the composite silica-zirconia xerogels prepared using different zirconium sources

The composite silica-zirconia xerogels have been prepared via sol–gel method using zirconium oxychloride, oxynitrate, acetate and sodium silicate as the precursors. The prepared materials have been characterized using FTIR, TG/DTA, EDX and surface area analyses. It has been established that surface area of silica-zirconia xerogels significantly depends on the zirconium source, but phase transitions and structural features of the xerogels seem to be unaffected by the choice of the zirconia precursor. Prepared xerogels increase the activity of chiral inductor in the asymmetric Biginelli reaction. The anion of the zirconium salt adsorbed on the surface of the synthesized material and the presence of Si–O–Zr heterolinkages determines the reaction yield. The highest chemo- and enantioselectivity towards formation of ee isomer have been provided by the catalyst prepared from zirconium oxychloride.

Synthesis and X-ray crystal structure of isomeric pyridine-based leuco-TAM dyes, 2,2-(2-(pyridinyl)propane-1,3-diylidene)bis(5-chloro-1,3,3-trimethyl indoline) derivatives and unusual stability of 4-pyridinyl compound

The solid-state structures and unequivocal stereochemistries of isomeric pyridine-based leuco-triarylmethane (LTAM) dyes 2,2′-(2-(pyridin-4 or 3-yl)propane-1,3-diylidene)bis(5-chloro-1,3,3-trimethylindoline) derivatives were established by X-ray crystallography. Surprisingly, the EE isomer was formed for the 4-pyridinyl compound, whereas the 3-pyridinyl compound formed ZE isomers. In addition, the latter have a so-called three-bladed propeller conformation, whereas the former possess a Y-shaped conformation. These pyridine-based LTAM compounds stack to form a dimer, adopting either an orthorhombic, with Pcmn space group, or monoclinic crystal system with P21/n space group in the crystal unit cell.

Stereospecificity of1H,13C and15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on1H chemical shifts

In the (13) C NMR spectra of methylglyoxal bisdimethylhydrazone, the (13) C-5 signal is shifted to higher frequencies, while the (13) C-6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the (1) H-6 chemical shift and (1) J(C-6,H-6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the -CH═N- bond does not change. This paradox can be rationalized by the C-H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum-chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ((1) H-6) and (1) J(C-6,H-6) parameters. The effect of the C-H⋯N hydrogen bond on the (1) H shielding and one-bond (13) C-(1) H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The (1) H, (13) C and (15) N chemical shifts of the 2- and 8-(CH(3) )(2) N groups attached to the -C(CH(3) )═N- and -CH═N- moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8-(CH(3) )(2) N group conjugate effectively with the π-framework, and the 2-(CH(3) )(2) N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N-2- and N-8- nitrogen lone pairs to the π-framework varies, which affects the (1) H, (13) C and (15) N shieldings.

Study of stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the configurational isomers of the selenophene‐2‐carbaldehyde azine by NMR spectroscopy and MP2‐GIAO calculations

In the (1)H and (13)C NMR spectra of selenophene-2-carbaldehyde azine, the (1)H-5, (13)C-3 and (13)C-5 signals of the selenophene ring are shifted to higher frequencies, whereas those of the (1)H-1, (13)C-1, (13)C-2 and (13)C-4 are shifted to lower frequencies on going from the EE to ZZ isomer or from the E moiety to the Z moiety of EZ isomer. The (15)N chemical shift is significantly larger in the EE isomer relative to the ZZ isomer and in the E moiety relative to the Z moiety of EZ isomer. A very pronounced difference (60-65 mg/g) between the (77)Se resonance positions is revealed in the studied azine isomers, the (77)Se peak being shifted to higher frequencies in the ZZ isomer and in the Z moiety of EZ isomer. The trends in the changes of the measured chemical shifts are reasonably reproduced by the GIAO calculations at the MP2 level of the (1)H, (13)C, (15)N and (77)Se shielding constants in the energy-favorable conformation with the syn orientation of both selenophene rings relative to the C = N groups. The NBO analysis suggests that such an arrangement of the selenophene rings may take place because of a higher energy of some intramolecular interactions.

Single-Crystal-to-Single-Crystal Transformation of Di(isopropylammonium) (Z,Z)-Muconate into the (E,E)-Muconate during One-Way Photoisomerization in the Solid State

The solid-state EZ photoisomerization of di(isopropylammonium) (Z,Z)-muconate (ZZ-1a) into the corresponding EE isomer (EE-1a) via a single-crystal-to-single-crystal reaction process was investigated. The molecular motion based on a bicycle-pedal model was directly observed during the photoisomerization in the crystals by an X-ray single crystal structure analysis. The photoirradiation of ZZ-1a provided EE-1a in a quantitative yield via a one-way isomerization mechanism, being different from the solution products in the photostationary state. The isomerization reactivity was discussed for the ammonium and ester derivatives of muconic acid.

Crystal Phase Transition and Solid-State Photoisomerization of Benzyl (Z,Z)-Muconate Polymorphs Studied by Direct Observation of Crystal Structure Change

We investigated the polymorph structures of benzyl (Z,Z)-muconate crystals (ZZ-Bn-α, β, and γ) and their EZ photoisomerization behavior in order to clarify the EZ isomerization mechanism of diene compounds in the solid state. First, the thermodynamics and crystal phase transition of the polymorphs were investigated by differential scanning calorimetry and X-ray diffraction. A single-crystal-to-single-crystal transition was observed during the change in the polymorph structures from the ZZ-Bn-β to the α form. We also observed in situ the migration of the boundary between the different polymorph domains during the phase transition of the ZZ-Bn-γ to the α form by optical microscopy. We then investigated the change in the crystal structure of ZZ-Bn during the EZ photoisomerization in the solid state. The photoisomerization of ZZ-Bn to the corresponding EE isomer (EE-Bn) proceeded according to a bicycle-pedal model via a single-crystal-to-single-crystal topochemical reaction mechanism, which was directly confi...

Conformational equilibria in EE-1,3-di-(3′-thienylethenyl)benzene: One-way adiabatic interconversion of rotamers in S1 and their excited state properties

A refined procedure of the kinetic fluorescence analysis (KFA) (accounting for the lack of isoemissive points in the fluorescence spectra) has been applied to study the rotamerism of the EE isomer of 1,3-di-(3′-thienylethenyl)benzene [(3TE)2B]. The effect of the excitation wavelength and temperature on the fluorescence spectra, quantum yields and decay profiles allowed the excited state properties (radiative and reactive) of the two most abundant conformers (rotamers), implying the simultaneous, restricted rotation around the quasi-single bonds of the ethenic bridges with both the central ring and the 3′-thienyl groups, to be obtained. At high temperature (above 320K), another activated process of the lowest excited singlet state, implying the interconversion of the short-lived rotamer into the longer-lived one, is operative. The EE→EZ photoisomerization was also investigated as a function of temperature.The present results lead to a complete and quantitative description of the conformational equilibrium including the radiative and radiationless relaxation rate parameters of the lowest excited states of the two rotamers. A simultaneous singlet and triplet mechanism for the geometrical photoisomerization, with an involvement of the former above room temperature only, seems to reasonably account for the experimental results. The role played by the rotamers in the photoreactive behaviour of (3TE)2B is also discussed.

Conformer-specific and two-fold adiabatic photoisomerization of ZZ-1,4-di-(2-quinolylethenyl)benzene.

Irradiation of the ZZ stereoisomer of 1,4-di-(2'-quinolylethenyl)-benzene was found to cause direct adiabatic (one photon-two bond) isomerization to a product having the same lifetime as the EE isomer but a rather different spectrum with respect to that obtained by direct excitation of the EE one. To clarify this unexpected behaviour, the conformational equilibria of the EE stereoisomer have been studied in non-polar solvent by fluorimetry. The most abundant conformers, formed by the hindered rotation of the condensed-ring groups around the quasi-single bond with the ethenic carbons, have been characterized by the selective effect of the excitation energy on the fluorescence spectrum. The combined application of the principal component analysis allowed the separation of the spectral properties of three conformers to be achieved. Information on their structures was obtained by theoretical calculations. The results of the present conformational study clearly indicated that the fluorescence spectrum of the photoproduct of ZZ belongs to a specific component of the conformer mixture of the EE isomer.

Reversible Photochemical Phase Transition Behavior of Alkoxy-Cyano-Substituted Diphenylbutadiene Liquid Crystals

Reversible photochemical phase transition behavior of some alkoxy-cyano-substituted diphenylbutadiene liquid crystals has been studied. The EE isomers of these derivatives were found to undergo photoisomerization to yield thermally stable EZ and ZE isomers, which could be transformed back to the EE isomer using light of a different wavelength. On photolysis, the liquid-crystalline EE isomers of these derivatives exhibited isothermal phase transition to the isotropic phase due to the formation of the non-liquid-crystalline EZ and ZE isomers. The thermal stability of the EZ and ZE isomers ensures that reverse switching of these materials to the liquid-crystalline phase remains purely a photon-controlled process.

Synthesis, characterization and a theoretical investigation of the formation of silicon, germanium and tin azapentadiene compounds from lithium azapentadienyls

A series of lithium-1-azapentadienyl compounds Li[ t-BuN⋯CH⋯C(R)⋯C(R′)⋯CHR′′] ( 1′– 5′) has been synthesized by metallation of imines t-BuNCHC(R)C(R′)CH 2R′′ [R=R′=R′′=H ( 1); R=Me, R′=R′′=H ( 2); R=R′′=H, R′=Me ( 3); R=R′=H, R′′=Me ( 4); R=R′′=Me, R′=H ( 5)] with lithium di- iso-propyl amide in a mixture of hexane–THF at low temperature. Preference for the exo W-shaped isomer is proposed for compounds 1′– 4′, based on their chemical derivation products with electrophiles, such as EMe 3Cl (E=Si, Ge, Sn). Compound 5′ shows both W- [ 5′( W)] and U-shaped [ 5′( U)] structures in a 7:1 ratio. Ab initio calculations predict that the 5′( W) compound is more favorable than the 5′( U) by 4.7 kcal mol −1. We have been examining the regio- and stereoselective azapentadienyl compounds using organometallic electrophiles such as EMe 3 + (E=Si, Ge, Sn). The reactions of the preferential EE isomers 1′– 3′ with the electrophiles generally give the addition at the terminal (C5) carbon of the azapentadienyl moiety, affording the corresponding complexes t-BuNCHCHCHCH 2EMe 3 [E=Si ( 6), Ge ( 7), Sn ( 8)]; t-BuNCHC(Me)CHCH 2EMe 3 [E=Si ( 9), Ge ( 10), Sn ( 11)] and the corresponding EE and ZE isomers: t-BuNCHCHC(Me)CH 2EMe 3 [ESi ( 12), ( 12′); Ge ( 13), ( 13′); Sn ( 14), ( 14′)], respectively. Complex 4′ reacts highly regioselectively with SiMe 3Cl affording the EE isomer t-BuNCHCHCH(Me)CHSiMe 3 ( 15), but diminished regioselectivity is observed with the corresponding Ge and Sn analogues. In contrast, attack at the central (C3) carbon is observed for 5′, giving t-BuNCH(Me)C(EMe 3)CHCHCH 3 [E=Si, ( 18); Ge, ( 19)]; although the regioselectivity remains for Si and Ge derivatives, a thermodynamic rearrangement occurs for the Sn complex t-BuNCHC(Me)CH(Me)CHSnMe 3 ( 20′), showing the addition of SnMe 3 fragment at the terminal carbon. The kinetic and thermodynamic products were established through 1H-, 13C- and 119Sn-NMR data, along with thermal isomerization studies. Based on the computed values of density-functional theory, condensed Fukui functions derived from ab initio electronic structure calculations were computed. These are shown to be useful reactivity indexes (better than conventional atomic charges) to rationalize the observed 1′– 5′ chemical derivation products with electrophiles.

Configurational properties of sulfur diimide, methyl sulfur diimide and dimethyl sulfur diimide

MNDO semiempirical SCF MO calculations are used to investigate the configurational properties of sulfur diimide ( 1), dimethyl sulfur diimide ( 2) and methyl sulfur diimide ( 3). For compounds 1 and 2, the EZ configuration is found to be the most stable form, whereas the corresponding ZZ configurations are calculated to be 8.1 and 13.0 kJ mol −1 higher in energy. The EE configurations of 1 and 2 are 24.7 and 28.7 kJ mol −1 less stable than the EZ isomer, respectively. The unsymmetrical diimide ( 3), has two EZ isomers with the E CZ H being 3.0 kJ mol −1 more stable than the E HZ C configuration. The ZZ isomer of 3 is only 8.7 kJ mol −1 above E CZ H, whereas the EE isomer is calculated to be 28.1 kJ mol −1 less stable. The isomerization pathway for interconversion between isomers of each compound is found to be through the S–N bond rotation rather than the nitrogen inversion. Thus the best pathway for EZ to ZE interconversion involves the ZZ configuration as an intermediate and the calculated energy barrier for this process is 69.2 kJ mol −1 for 1,50.8 kJ mol −1 for 2 and 68.3 kJ mol −1 for 3.

Configurational properties of sulfur diimide, methyl sulfur diimide and dimethyl sulfur diimide

MNDO semiempirical SCF MO calculations are used to investigate the configurational properties of sulfur diimide ( 1), dimethyl sulfur diimide ( 2) and methyl sulfur diimide ( 3). For compounds 1 and 2, the EZ configuration is found to be the most stable form, whereas the corresponding ZZ configurations are calculated to be 8.1 and 13.0 kJ mol −1 higher in energy. The EE configurations of 1 and 2 are 24.7 and 28.7 kJ mol −1 less stable than the EZ isomer, respectively. The unsymmetrical diimide ( 3), has two EZ isomers with the E CZ H being 3.0 kJ mol −1 more stable than the E HZ C configuration. The ZZ isomer of 3 is only 8.7 kJ mol −1 above E CZ H, whereas the EE isomer is calculated to be 28.1 kJ mol −1 less stable. The isomerization pathway for interconversion between isomers of each compound is found to be through the S-N bond rotation rather than the nitrogen inversion. Thus the best pathway for EZ to ZE interconversion involves the ZZ configuration as an intermediate and the calculated energy barrier for this process is 69.2 kJ mol −1 for 1, 50.8 kJ mol −1 for 2 and 68.3 kJ mol −1 for 3.