Interplay between steric and electronic factors in determining the strength of intramolecular resonance-assisted NH...O hydrogen bond in a series of [beta]-ketoarylhydrazones

Publisher's version/PDFThe crystal structures of six [beta]-ketoarylhydrazones are reported: 1,(Z)-2-(2-bromophenylhydrazono)-3-oxobutanenitrile; 2, (Z)-2-(2-methylphenylhydrazono)-3-oxobutanenitrile; 3, (E)-methyl-2-(2-methoxyphenylhydrazono)-3-oxobutanoate; 4, E, methyl-2-(2-cyanophenylhydrazono)-3-oxobutanoate; 5, (Z)-methyl-2-(4-cyanophenylhydrazono)-3-oxobutanoate; 6, pentane-2,3,4-trione-3-(2-carboxyphenylhydrazone). All of them form intramolecular hydrogen bonds assisted by resonance (RAHB), with N...O distances in the range 2.541(5)-2.615(3) [Angstrom]. These hydrogen bonds are differently aff ected by the substituents at the heterodienic fragment, being strengthened by electronwithdrawing substituents in position 2 (more by 2-COMe than 2-CN substitution), and weakened in [beta]-esterhydrazones and when the N-H forms a bifurcated hydrogen bond. The role played by the different steric and electronic properties of the substituents in strengthening the H-bond is investigated, besides X-ray crystallography, by IR and [superscript 1]H NMR characterization of the NH proton, and quantum mechanical DFT calculations at the B3LYP/6-31 + G(d,p) level of theory on test molecules

Interplay of hydrogen bonding and other molecular interactions in determining the crystal packing of a series of anti-[beta]-ketoarylhydrazones

Publisher's version/PDFThe crystal structures of six anti-[beta]-ketoarylhydrazones are reported: (a1) (E)-2-(4-cyanophenylhydrazono)-3- oxobutanenitrile; (a2) (E)-2-(4-methylphenylhydrazono)-3-oxobutanenitrile; (a3) (E)-2-(4-acetylphenylhydrazono)-3-oxobutanenitrile; (a4) (E)-2-(2-methoxyphenylhydrazono)-3-oxobutanenitrile; (a5) (E)-2-(2-acetylphenylhydrazono)-3-oxobutanenitrile; (a6) (E)-2-(2-nitrophenylhydrazono)-3-oxobutanenitrile. All compounds contain the [pi]-conjugated heterodienic group HN--N==C--C==O and could form, at least in principle, chains of intermolecular N--H...O hydrogen bonds assisted by resonance (RAHB-inter). Compounds (a1) and (a2) form this kind of hydrogen bond though with rather long N...O distances of 2.948 (3) and 2.980 (2) [Angstrom], and compound (a6) undergoes the same interaction but even more weakened [N...O 3.150 (1) [Angstrom]] by the intramolecular bifurcation of the hydrogen bond donated by the N--H group. The intrinsic weakness of the intermolecular RAHB makes possible the setting up of alternative packing arrangements that are controlled by an antiparallel dipole-dipole (DD) interaction between two C==O groups of the [beta]-ketohydrazone moiety [compounds (a4) and (a5)]. The critical factors that cause the switching between the different packings turn out to be the presence of hydrogen bonding accepting substituents on the phenyl and, most frequently, the intramolecular N--H...O bond with the O atom of the phenyl o-substituent. The crystal packing is widely determined by RAHB-inter (three cases) or DD (two cases) interactions. Only compound (a3) displays a different packing arrangement, where the DD interaction is complemented by a non-resonant hydrogen bond between a p-acetyl phenyl substituent and the hydrazone N--H group [N...O 2.907 (2) [Angstrom]]. Crystal densities range from 1.24 to 1.44 Mg m[superscript -3] and are shown to increase with the number of intermolecular hydrogen bonds and other non-van der Waals interactions

Hydrogen Bond at the Dawn of the XXI Century. New Methods, New Results, New Ideas

Chapter 14 of the Proceedings of the NATO Advanced Research Workshop on Physical-Chemical Properties from Weak Interactions. Erice, Italy, 23-29 May 2001. / Though the hydrogen bond (H-bond) is known since 1920 and in spite of the extraordinary number of books and scientific papers dedicated to it, all attempts to predict its geometry and energetics from the simple knowledge of the chemical structure of the interacting molecules have been so far unsuccessful, a question we have sometimes indicated as the H-bond puzzle. A recent advance in the solution of this problem is represented by the Electrostatic-CovalentH-Bond Model (ECHBM) according to which (i) weak H-bonds are electrostatic in nature but become increasingly covalent with increasing strength, very strong bonds being essentially three-centre-four-electron covalent bonds; (ii) strong and very strong H-bonds may belong only to a limited number of classes which are three for X-H...X homonuclear and four for X-H...Y heteronuclear H-bonds; (iii) within each class, H-bonds are the stronger the smaller is DeltaPA, the difference between the proton affinities of the H-bond donor and acceptor atoms. It is shown that this model leads to an exhaustive classification of all H-bonds in chemical classes which, in turn, becomes a base for the prediction of H-bond strength starting from the chemical structures of the interacting molecules

pKa slide rule predictions against CSD structural results

Bond energies and intramolecular distances of normal chemical bonds are weakly affected by their environment, which has made it possible to produce extended compilations of these quantities. Hydrogen bonds (HBs) have a quite different behavior, their binding energies depending not only on the electronegativities of the HB donor (D) and acceptor (A) atoms, but displaying a large spread of values even for a same donor-acceptor couple. This surprising behavior (the H-bond puzzle) is presently interpreted by saying that the forces determining HB strength are a mixture of electrostatic and covalent contributions, that the covalent part is steeply increasing while the donor-acceptor difference of the proton affinities, DPA= PA(DG) – PA(A), or acidic constants, DpKa=pKAH(D-H) - pKBH+(A-H+), tends to zero, and that, when this limit is achieved, the strong and symmetrical D…H…A bond formed is better classified as a true three-center-four-electron covalent bond. This emphasizes the essential role played by PA/pKa equalization in strengthening the HB. We have undertaken a wide research program intended to verify the validity of such a PA/pKa equalization rule which consists of two steps. The first requires the compilation of pKa table for the most typical HB donor and acceptor molecules. The results so obtained have been organized in an unique graphical table called the pKa slide rule which is a practical tool for the prediction of HB strengths based on the fact that, according to the rule, only the donor-acceptor couples lying on a same horizontal line of the rule can give rise to strong HBs with DpKa=0. In the second part of the project the validity of the pKa equalization rule and the reliability of the pKa slide rule prediction has been carried out by a wide search on the Cambridge Structural Database (CSD) for specific classes of HBs. For each D-H…A bond the values of dD…A, dD-H, dH…A and D-H-A angle were registered and the relative HB strength were evaluated from the donor-acceptor distance corrected for the D-H-A angle. Such a verification is not an easy matter because there are tens of thousands of H-bonded crystal structures and thousands of combining molecules with often uncertain pKa values. We have pointed to two main projects that, though handling only a few thousand structures, still retain large diagnostic capabilities: (i) all strong (short) HBs have small or null DpKa value; (ii) in the selected N-H…O/O-H…N system, HB strengths (lengths) are modulated by DpKa in the full DpKa range

Crystal and Molecular Structures of 2,6-cis-Dimethylpiperidyl-N-phenylacetamidine and 2,6-cis-Dimethylpiperidyl-N-phenyl-2,2-dimethylpropionamidine. An X-ray Crystallographic Investigation of the C(sp2)-N(piperidyl) Bond

The single crystal X-ray analyses of 2, 6-cis-dimethylpiperidyl-N-phenylacetamidine (MA) and 2, 6-cis-dimethylpiperidyl-N-phenyl-2, 2-dimethylpropionamidine (TBA) are described. MA crystallizes in the space group P2 1\c with four molecules in the unit cell of dimensions a = 10.238 (2), b = 10.189 (2), c = 12.875 (3) Å. and β = 95.82 (2)°. The structure was solved and refined from 1401 unique observed reflections collected on an automated four-circle diffractometer to final values of the discrepancy indices of R = 0.046 and Rw = 0.058. TBA crystallizes in the space group P21\c with eight molecules in the unit cell of dimensions a = 8.470 (2), b = 16.095 (3), c = 24.900 (4) Å, and β = 96.29 (2)°. From 2633 unique observed reflections similarly collected the structure was solved and refined to final values of the discrepancy indices of R = 0.061 and Rw = 0.074. The structure analyses show, in agreement with 13C NMR spectroscopic data, that ihe two molecules adopt different conformations around the C(sp2)-N(piperidyl) bond, the amidinic group and the piperidyl ring being approximately coplanar in MA and orthogonal in TBA, respectively. The comparison of the present data with the data in the literature, supported by nonbonded intramolecular potential energy calculations and IN DO calculations, allows clarification of the relationship among the torsion angle around the C-N bond, the bond distances in the amidinic group, the pyramidality of the N (piperidyl) atom and the conformation of the 2, 6-m-methyl groups in the piperidyl ring. © 1979, American Chemical Society. All rights reserved

N-Cyclohexyl-2,3,3-Triphenyl-Thioacrylamide, C27H27NS

--

Third Italian-Israeli Meeting on chemical crystallography: 'Crystal structure and molecular recognition'

Third Italian-Israeli Meeting on chemical crystallography: 'Crystal structure and molecular recognition

Intramolecular O-H...O hydrogen bonds assisted by resonance. Correlation between crystallographic data and 1H NMR chemical shifts

A number of crystal structures of molecules where the pi-conjugated ... O=C-C=C-OH ...beta-diketone enol group is found to form intramolecular O-H ... O hydrogen bonds and for which H-1 NMR spectroscopic data were known are discussed, Five of these structures, determined by X-ray diffraction techniques, are reported and the other 42 were retrieved from the Cambridge Structural Database, It is shown that all the descriptors of hydrogen-bond strength [d(O ... O) shortening, increased enolic H-1 NMR chemical shift, delta(OH), and increased pi-delocalization of the hydrogen-bonded heteroconjugated fragment] are mutually and linearly intercorrelated according to the rules defined by RAHB (resonance-assisted hydrogen bonding), Such a model is found to be of general applicability to all intramolecular O-H ... O bonds observed in a variety of molecules of different complexity embedding the simple beta-diketone enol fragment and to be extensible to other hydrogen-bonded conjugated compounds such as ... O=C-C-C=C-C-OH ...beta-diketone enols and ... O=C-C-N-OH ...alpha-keto-oximes, The proton chemical shifts, delta(OH), measured in CDCl3 solutions are found to depend strongly on the O ... O contact distances going from 8.6-10.1 ppm for weak non-resonant [2.59 less than or equal to d(O ... O) less than or equal to 2.64 Angstrom] to 14.9-19.0 ppm for the strongest resonant hydrogen bonds [2.41 less than or equal to d(O ... O) less than or equal to 2.55 Angstrom], Comparison with H-1 NMR data obtained in the solid-state shows a strictly similar dependence of delta(OH) on d(O ... O), irrespective of the very different experimental conditions and in spite of the fact that solution and solid-state values concern intramolecular and intermolecular hydrogen bonds, respectively