Photoelectron Spectra of Azabenzenes and Azanaphthalenes: I. Pyridine,diazines, s-triazine and s-tetrazine

The experimental and theoretical basis of a recently proposed reassignment of the bands in the PE. spectra of pyridine, pyridazine, pyrimidine and pyrazine is discussed in detail. A characteristic feature of the derived orbital sequence is that it takes the ‘through-space’ and ‘through-bond’ interaction between the ‘lone pair’ basis orbitals explicitly into account. A simple parametrization of the orbital energies, based on HMO-type models for the π-orbitals and for the ‘lone pair’ linear combinations, yields excellent agreement with the observed band positions in the PE. spectra of s-triazine and s-tetrazine. Our new assignment is compared to those proposed previously.     

Three‐Residue Turn in β‐Peptides Nucleated by a 12/10 Helix

A new three‐residue turn in β peptides nucleated by a 12/10‐mixed helix is presented. In this design, β peptides were derived from the 1:1 alternation of C‐linked carbo‐β‐amino acid ester [BocNH‐(R)‐β‐Caa_(r)‐OMe] (Boc=tert‐butyloxycarbonyl), which consisted of a D ‐ribo furanoside side chain, and β‐hGly residues. The hexapeptide with (R)‐β‐Caa_(r) at the N terminus showed the ‘turn’ stabilized by a 14‐membered NH(4) ??? CO(6) hydrogen bond at the C terminus nucleated by a robust 12/10‐mixed helix, thus providing a ‘helix‐turn’ (HT) motif. The turn and the helix were additionally stabilized by intraresidue electrostatic interaction between the furan oxygen in the carbohydrate side chain and NH in the backbone. However, the hexapeptide with a β‐hGly residue at the N terminus demonstrated the presence of a 10/12 helix through its entire length, which again showed the intraresidue interaction between NH and furan oxygen. The intraresidue NH ??? O? Me electrostatic interactions observed in the monomer, however, were absent in the peptides.     

A Quantitative Assessment of „Through-space”︁ and „Through-bond”︁ Interactions. Application to Semi-empirical SCF Models

The scheme of ‘through-space’ and ‘through-bond’ interaction of (semi)localized orbitals, originally proposed by Hoffmann, is reexamined in terms of SCF many-electron treatments. It is shown that the two types of interaction can be characterized by examining the corresponding off-diagonal matrix elements of the Hartree-Fock matrices of the localized or the symmetry adapted localized orbitals and of the partially diagonalized Hartree-Fock matrices referring to ‘precanonical orbitals’. The procedure outlined is applied to three practical examples using the semiempirical many-electron treatments SPINDO, MINDO/2 and CNDO/2:

• a A reassessment of ‘through-space’ and ‘through-bond’ interaction in norbornadiene indicates, that the latter type of interaction is also of importance for the orbital based mainly on the antisymmetric combination of the localized x-orbitals. The differences in the predictions derived from the three models are critically examined.
• b The competition between ‘through-space’ and ‘through-bond’ interaction in the series of bicyclic dienes from norbornadiene to bicyclo[4.2.2]-dcca-7,9-diene and in cyclohexa-1,4-diene, i. e. their dependence on the dihedral angle UI is reexamined. It is found that the rationalization for the orbital crossing near ω = 130° deduccd from PE. spectroscopic data can not be as simple as originally suggested and that the relay’ orbitals responsible for ‘through-bond interaction affecting both the symmetric and the antisymmetric combination of the π-orbitals extend over the whole CC-σ-system of the six membered ring.
• c ‘Through-bond’ interaction of the two lone pair orbitals in 1,4-diazabicyclo[2.2.2]octane is found to be large for their symmetric and the antisymmetric linear combination.

The analysis quoted, draws attention to some of the dangers involved in using semiempirical treatments for the interpretation of PE. data in terms of Koopmans′ theorem, without due caution.     

A computational study of azine,azoethene, and diimine linkages in the poly/oligoazine system

Hartree-Fock (HF) and molecular mechanics calculations were performed on linear azine oligomers and model compounds. The rotational energy curves for the model compounds formaldazine, H₂C = N? N = CH₂, ethenyl diazene, H₂C = CH? N = NH, and ethanediimine, HN = CH? CH = NH were calculated for a variety of basis sets at the HF and MP2 level. In all of these cases the rotational energy barriers are quite different from butadienes or aza-substituted butadienes because of the lone pair–lone pair interaction of the adjacent nitrogen atoms. The results on the model compounds were used to generate a set of molecular mechanics (MM) parameters that are appropriate for linear oligo- and polyazines. Comparison of the geometries of the HF results and MM results for the oligoazines showed that the two methods gave comparable results. © 1994 by John Wiley & Sons, Inc.     

Comparative Infrared,Raman, and Natural‐Bond‐Orbital Analyses of King's Sultam

By means of ¹H‐NOESY‐ and Raman‐spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N? Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone‐pair anomeric stabilization should be revised to 1.5–1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)‐comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five‐membered‐ring sultams do not seem to be particularly well‐stabilized by the S? C σ* orbital in the N‐substituted pseudo‐axial conformation, as opposed to an idealized anti‐periplanar situation for the six‐membered‐ring analogues. In this latter case, the other anti‐periplanar C? C σ* and C(1′)? H/C(2′) σ*orbitals are as important, if not more, when compared to the S? C σ* participation. In the pseudo‐equatorial conformation, γ‐sultams particularly benefit from the N lone‐pair hyperconjugation with the anti‐periplanar S? O₁ σ* and C(2)? H/C or C(1′)? H/C σ* orbitals. This is also the case for δ‐sultams when the steric requirement of the N‐substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N‐atom is prone to sp² hybridization or/and conformational changes (i.e., 12c ). In that case also, the mode of stereoelectronic stabilization differs from γ‐ to δ‐sultams.     

14N and 35Cl NQR studies of organophosphorus compounds

¹⁴N and ³⁵Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N? P bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the N? P bond due to the p_π? d_π interaction and p_π? σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N? P bond populations is observed in X ? PR_n (R₂N)_3-n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P?Se > P?S > P?O > P.     

New insight into the electronic structure of iron(IV)‐oxo porphyrin compound I. A quantum chemical topological analysis

The electronic structure of iron‐oxo porphyrin π‐cation radical complex Por^·+Fe^IV?O (S? H) has been studied for doublet and quartet electronic states by means of two methods of the quantum chemical topology analysis: electron localization function (ELF) η(r) and electron density ρ(r). The formation of this complex leads to essential perturbation of the topological structure of the carbon–carbon bonds in porphyrin moiety. The double C?C bonds in the pyrrole anion subunits, represented by pair of bonding disynaptic basins V_i=1,2(C,C) in isolated porphyrin, are replaced by single attractor V(C,C)_i=1–20 after complexation with the Fe cation. The iron–nitrogen bonds are covalent dative bonds, N→Fe, described by the disynaptic bonding basins V(Fe,N)_i=1–4, where electron density is almost formed by the lone pairs of the N atoms. The nature of the iron–oxygen bond predicted by the ELF topological analysis, shows a main contribution of the electrostatic interaction, Fe^δ+···O^δ?, as long as no attractors between the C(Fe) and C(O) core basins were found, although there are common surfaces between the iron and oxygen basines and coupling between iron and oxygen lone pairs, that could be interpreted as a charge‐shift bond. The Fe? S bond, characterized by the disynaptic bonding basin V(Fe,S), is partially a dative bond with the lone pair donated from sulfur atom. The change of electronic state from the doublet (M = 2) to quartet (M = 4) leads to reorganization of spin polarization, which is observed only for the porphyrin skeleton (?0.43e to 0.50e) and S? H bond (?0.55e to 0.52e). © 2012 Wiley Periodicals, Inc.     

Holo‐ and Hemidirected Coordination Spheres in a Novel Three‐Dimensional Polymeric KIPbII Heteropolynuclear Complex: X‐Ray Crystal Structure of [KPb(AcO)2(SCN)]n

A novel three‐dimensional polymeric K^IPb^II heteropolynuclear complex, [KPb(AcO)₂(SCN)]_n, with mixed acetate and thiocyanate ligands, has been synthesized and characterized. Its single‐crystal X‐ray structure (Fig. 1) shows three types of K⁺ ions with coordination numbers of seven, and three types of Pb²⁺ ions with coordination numbers of eight, eight, and nine, respectively. The Pb centers (Pb(1) and Pb(3); Fig. 1) with coordination numbers of nine and eight, respectively, possess stereochemically ‘inactive’ electron lone pairs, and the coordination sphere is holodirected. However, the arrangement of O‐, N‐, and S‐atoms for the eight‐coordinated Pb(2) suggests a gap or hole in the coordination geometry around this atom. This ‘hole’ is possibly occupied by a stereochemically ‘active’ electron lone pair of Pb(2), and its coordination sphere is, thus, hemidirected.     

NMR study of 2-alkoxy-1,3,2-dioxaphospholanes. II—molecular conformation of alkoxy substituents

The conformation of some 2-σ-1,3,2-dioxaphospholanes (σ=OAlkyl, CI) is studied. The determination of the ²J(POC) and ³J(POCC) coupling constants, which are influenced by the bulk of the alkoxy group, is a means of obtaining information about the rotation around the P? OR bond, which is dependent on steric interactions between the phosphorus lone pair, the alkoxy group and the substituents on the ring. When σ is a tert-butoxy group, a direct ‘through-space’ interaction is found between the phosphorus atom and one of the primary carbons of the OR group. If there is no substituent on the ring, the ³¹P chemical shifts are little affected by changes in OR; a diamagnetic effect is observed, however, in the case of the tert-butoxy group which is enhanced for the 4,4,5,5-tetramethyl derivatives.     

Stereoelectronic Aspects of the Anomeric Effect in Fluoromethylamine

High-level ab initio calculations have been made for fluoromethylamine to study structural and energetic effects of the relative orientation of the N lone pair to the C? F bond. The anti-conformer (N lone pair anti-planar to the C? F bond) corresponds to the global energy minimum. It has the longest C? F distance, the shortest C? N distance, and is 7.5 kcal·mol^?1 more stable than the related perpendicular conformation (lone pair perpendicular to the C? F bond). The syn-conformation also shows hallmarks of the anomeric effect: long C? F bond, short C? N bond, and energetic stability when allowance is made for the two pairs of eclipsed hydrogens. The transition state for N inversion is close to the syn-structure; rotation about the C? N bond is strongly coupled with this inversion process. Small bond distance changes of ca. 0.02 Å between parallel and perpendicular conformations are associated with dissociation energy differences of ca. 30 kcal·mol^?1. Various criteria for assessing the strength of the anomeric effect are discussed.     

On the Correlation between Photoelectron and Electronic Spectra. Part. II: Experimental indications about the shape of lone pair orbitals

For a π-molecular system containing two symmetry-equivalent heteroatoms, a qualitative relationship between the difference in the n-ionization potentials (ΔIP) and the difference between the n → π* excitation energies (ΔΔE) is derived, using semi-localized orbitals as a basis. The comparison between ΔIP and ΔΔE yields information about the energies and/or the shapes of the two lone pair MO's in the model system. The results provide further insight into ‘through space’ and ‘through bond’ interaction concept introduced by Hoffmann.     

Synthetic and Structural Studies of 1,8‐Chalcogen Naphthalene Derivatives

Four novel 1,8‐disubstituted naphthalene derivatives 4 – 7 that contain chalcogen atoms occupying the peri positions have been prepared and fully characterised by using X‐ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular distortion due to noncovalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X‐ray data for 4 – 7 was compared to the series of known 1,8‐bis(phenylchalcogeno)naphthalenes 1 – 3 , which were themselves prepared from novel synthetic routes. A general increase in the E???E′ distance was observed for molecules containing bulkier atoms at the peri positions. The decreased S???S distance from phenyl‐ 1 and ethyl‐ 4 analogues is ascribed to a weaker chalcogen lone pair–lone pair repulsion acting in the ethyl analogue due to the presence of two equatorial S(naphthyl) ring conformations. Two novel peri‐substituted naphthalene sulfoxides of 1 , Nap(O?SPh)(SPh) 8 and Nap(O?SPh)₂ 9 , which contain different valence states of sulfur, were prepared and fully characterised by using X‐ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular structures were analysed by using naphthalene ring torsions, peri‐atom displacement, splay angle magnitude, S???S interactions, aromatic ring orientations and quasi‐linear O?S???S arrangements. The axial S(naphthyl) rings in 8 and 9 are unfavourable for S???S contacts due to stronger chalcogen lone pair–lone pair repulsion. Although quasi‐linear O?S???S alignments suggest attractive interaction is conceivable, analysis of the B3LYP wavefunctions affords no evidence for direct bonding interactions between the S atoms.     

Density functional and molecular orbital study of physical process of inversion of nitrogen trifluoride (NF3) molecule

The physical process of the umbrella inversion of the nitrogen trifluoride molecule has been studied invoking the formalisms of the density functional theory, the frontier orbital theory, and the molecular orbital theory. An intuitive structure and dynamics of evolution of the transition state for the event of inversion is suggested. The physical process of dynamic evolution of the molecular conformations between the equilibrium (C_3v) shape and the planar (D_3h) transition state has been followed by a number of molecular orbital and density functional parameters like the total energy, the eigenvalues of the frontier orbitals, the highest occupied molecular orbital and lowest unoccupied molecular orbital, the (HOMO–LUMO) gap, the global hardness and softness, and the chemical potential. The molecular conformations are generated by deforming the ∠FNF angle through steps of 2° from its equilibrium value, and the cycle is continued till the planar transition state is reached, and the geometry of each conformation is optimized with respect to the length of the N? F bond. The geometry optimization demonstrates that the structural evolution entails an associated slow decrease in the length of the N? F bond. The dipole moment at the equilibrium form is small and that at the transition state is zero and shows a strange behavior with the evolution of conformations. As the molecular structure begins to distort from its equilibrium shape by opening of the ∠FNF angle, the dipole moment starts increasing very sharply, and the trend continues very near to the transition state but abruptly vanishes at the transition state. A rationale of the strange variation of dipole moment as a function of evolution of conformations could be obtained in terms of quantum mechanical hybridization of the lone pair on the N atom. The pattern of charge density reorganization as a function of geometry evolution is a continuous depletion of charge from the F center and piling up of charge on the N center. The continuous shortening of bond length and the pattern of variation of net charge densities on atomic sites with evolution of molecular conformations predicts that the bond moment would decrease continuously. The quantum mechanical hybridization of the lone pair of the central N atom shows that the percentage of s character of the lone‐pair hybrid on the N atom decreases at a very accelerated rate, and the lone pair at the transition state is accommodated in a pure p orbital. The result of the continued destruction of asymmetry of charge distribution in the lone pair on the central N atom due to the elimination of contribution of the s orbital with evolution of molecular conformations is the sharp decrease in lone‐pair moment. The decrease in bond moment is overcompensated by the sharp fall of its offsetting component, the lone‐pair moment, resulting in a net gain in dipole moment with the evolution of molecular geometry. Since the offsetting component decreases very sharply, the net effect is a sharp rise of dipole moment with the evolution of molecular conformations just before the transition state. The lone‐pair moment is zero by virtue of the symmetry of the pure p orbital, the lone pair of the central atom in the transition state, and the sum of the bond moments is zero by symmetry of the geometry. The barrier height is quite high at ～65.45 kcal/mol, which is close to values computed through more sophisticated methods. It is argued that an earlier suggestion regarding the development of high barrier value of NF₃ system seems to be misleading and confronting with the conclusions of the density functional theory. An analysis and a comparative study of the physical components of the one‐ and two‐center energy terms reveals that the pattern of the charge density reorganization has the principal role in deciding the origin and the magnitude of barrier of inversion of the molecule and the barrier originates not from a particular energetic effect localized in a particular region of the molecule, rather the barrier originates from a subtle interplay of one‐ and two‐center components of the total energy. The decomposed energy components show that the F?F nonbonded interaction and N? F bonded interaction favor the formation of transition state, while the one‐center energy terms prohibit the formation of the transition state. The barrier principally develops from the one‐center energy components. The profile of the HOMO is isomorphic and that of the LUMO is homomorphic with the potential energy curve for the physical process of the event of umbrella inversion of the molecule. The variation of the HOMO–LUMO gap, ?ε, the global hardness, η, and the softness, S, as a function of the reaction coordinates of angular deformation of NF₃ molecule are quite consistent with the predictions of the molecular orbital and the density functional theories in connection with the deformation of molecular geometry. The profiles of ?ε, η, and S, as a function of reaction coordinates, mimic the potential energy curve of the molecule. The eigenvalues of the frontier orbitals, and the ?ε, η, S parameters are found to be equally effective theoretical parameters, like the total energy, to monitor the physical process of the inversion of pyramidal molecules. The nature of the variation of the global hardness parameter between the equilibrium shape and the transition state form for the inversion is in accordance with the principle of maximum hardness (PMH). © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

Stereoelectronic Properties of Tetrahedral Species Derived from Carbonyl Groups. Ab initio study of aminodihydroxymethane,CH(OH)2NH2, a model tetrahedral intermediate in amide hydrolysis

An ab initio theoretical study of all fifteen fully staggered conformations of aminodihydroxymethane, CH(OH)₂NH₂ has been performed. Optimization of the C? O and C? N bond lengths, population analyses and orbital localisation reveal the presence of marked conformation dependent stereoelectronic effects which influence bond lengths and overlap populations. These effects may be parametrized as a function of number and nature of antiperiplanar (app) oriented electronic lone pairs (1p) and polar bonds. In a Y? C? X fragment an app orientation between a lone pair on Y and the C? X bond increases the length and weakens the C? X bond, shortens and strengthens the C? Y bond. Thus a C? X (X ? O, N) bond of CH(OH)₂NH₂ is longest and weakest when: (i) it is app to two vicinal lp's; (ii) the X 1p's are not app to a vicinal polar bond; (iii) the conformation of the molecule has as many axially oriented lp's as possible. Results (i) and (ii) agree with a simple hyperconjugation picture involving interaction between an electronic 1p and an app oriented antibonding bond orbital σ* (C? X). Bond properties, relative energies and effects on reactivity of the tetrahedral species are discussed, as well as their relations to experimental results on the cleavage of tetrahedral intermediates and to enzyme catalysis.     

Nitrogen-15 nuclear magnetic resonance spectroscopy. 15N?19F coupling constants in fluoropyridines and fluoroanilines

Two-to five-bond ¹⁵N? ¹⁹F coupling constants have been determined for fluoropyridines, 8-fluoroquinoline, fluoroanilines and fluoroaniline derivatives. Only in 2-fluoropyridine is J(NF) large, and this is associated with a lone pair mediated enhancement of the coupling. Values in fluoroanilines and derivatives are <2Hz. Except for the fluoroanilines themselves, J(NF) decreases inversely with the number of intervening bonds. In the anilines, only ⁵J(NF) is observable. Possible influences of lone pair interaction and hydrogen bonding are discussed.     

Glycosylidene Carbenes,Part 30 , Crystal Structure of a Glucose‐Derived N,N‐Unsubstituted Alkoxydiaziridine

The crystal structure of 1,5‐anhydro‐2,3,4,6‐tetra‐O‐benzyl‐1‐hydrazi‐D ‐glucitol ( 2 ) is reported and compared with the structures of other diaziridines. It is the first crystal structure of an N,N‐unsubstituted diaziridine, noncoordinated at the N‐atom, and the first crystal structure of a C‐alkoxy‐diaziridine. Although there is considerable shortening of the C(5)O−C(1) bond, there is no asymmetry in the C(1)−N bond length, the C(5)O, C(1), C(2) plane bisecting the N−N bond. The C(1)−N bonds appear to be slightly shorter and the N−N bond longer than the average for diaziridines, although the structural data for diaziridines do not lend themselves to unequivocal interpretation.     

A One‐Dimensional Thallium(I) Coordination Polymer Involving Polyhapto‐Aromatic Interactions

A one‐dimensional coordination polymer involving Tl ??? C interactions, [Tl(μ₄‐dpa)]_n(Hdpa = diphenylacetic acid), was synthesized and characterized. The single‐crystal X‐ray data of the compound show that the coordination number of the Tl^I ions is five and that Tl centers have an irregular coordination sphere containing a ‘configurationally active’ lone pair/hexahapto (η⁶) interaction, thus resulting in a total hapticity of eleven for a TlC₆O₅ environment.     

Interactions between Functional Groups. Part I. Crystal Structure of 2-Phenylethynyl-N,N-dimthlaniline at 98 K: A Remarkably Short C?H …︁ C Distance

In the crystal structure of the title compound at 98 K the dimethylamino group has pyramidal geometry. The nitrogenlone pair is not directed towards the triple bond; instead, one N? CH₃ bond lies almost in the plane of the disubstituted ring, directed away from the triple bond, while the other N? CH₃ bond and the lone pair are directed to opposite sides of the plane, nearer to the acetylene. There is a remarkably short intramolecular contact (2.39 Å) between a methyl H- and an acetylenic C-atom. The Taft σ_I parameter of the arylethynyl substituent appears to be similar to that of a carboxylic ester, judging from the bond-angle deformation at the ipso-C-atom.     

Density functional theory study on hydrogen‐bonded complexes of adenine with polyformamide molecules

The structures of hydrogen‐bonded complexes A–F_n (n = 2–7) of adenine with polyformamide molecules have been fully optimized at B3LYP/6‐31G(d) basis set level. All the formamide molecules prefer to be N? H proton donor rather than C? H proton donor and are favorably bound to the five‐numbered moiety of adenine. A displacement of formamide molecules to one side of adenine mean plane has happened with an increasing number of formamide molecules. An obvious effect of hydrogen‐bonding cooperativity can be seen during the complex process. The most interesting geometrical change of adenine upon the complex is the shortening of the bond C4? N6 resulting from the strengthening of the conjugation between the π system of the adenine ring and the lone pair of the nitrogen atom. An existence of weak N? H···π bonding interaction between the π system of adenine and N? H bond of F7 is found and further conformed by an natural bond orbital analysis specially carried out on A–F₇. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity

The helicopodand (PM)- 2 is prepared following the photocyclodehydrogenation route to helicenes (Scheme). At the ends of a [7]helicene backbone, this acyclic receptor (‘podand’) possesses a H-bonding recognition site shaped by two convergent N-(pyridin-2-yl)carboxamide (CONH(py)) units. In the crystal of diethyl [7]helicene-2,17-dicarboxylate ((PM)- 3 ), a direct synthetic precursor of 2 , molecules of the same chirality form stacks, and two stacks of opposite chirality are interlocked in a pair having average face-to-face aromatic contacts of 3.82 Å between benzene rings of different enantiomers (Fig. 2). In contrast, two conformations are observed in the crystal structure of 2 , one with both CONH(py) residues pointing with their H-bonding centers NH/N away from the binding site (‘out-out’) and a second (‘in-out’) with one of the two CONH(py) residues pointing towards the binding site (‘in’; Fig. 4). While no H-bonding network propagates throughout the crystal, enantiomers of 2 in the different conformations ‘out-out’ and ‘in-out’ form H-bonded pairs that are further stabilized by a H-bond to one molecule of CHCl₃. In the productive ‘in-in’ conformation, 2 forms stable 1:1 complexes with α,ω-dicarboxylic acids in CHCl₃, and a diastereoselectivity in complexation of Δ(ΔG°) = 1.4 kcal mol^?1 is measured for two substrates differing only in the (E)/(Z)-configuration at their double bond (see Table 2). A comprehensive force-field molecular-modeling study suggests that only the (E)-derivative possesses the correct geometry for a ditopic four-fold H-bonding interaction between its two COOH residues and the two CONH(py) groups in 2 (Fig. 5). With N,N′-bis [(benzyloxy)carbonyl]-L -cystine, the formation of diastereoisomeric complexes with (PM)- 2 is observed (Fig. 7).