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The electronic effects of the 5- and 6-membered heterocyclic rings on the C=N-N unit of five different hydrazone derivatives of pyridine-2-, -3- and -4-carbaldehydes, pyrrole-2-carbaldehyde, furan-2- and -3-carbaldehydes and thiophene-2- and -3-carbaldehydes have been studied with the aid of 13C and 15N NMR measurements together with the natural bond orbital (NBO) analysis. As model compounds are used the corresponding substituted benzaldehyde derivatives. The polarization of the C=N unit of the hydrazone functionality of the heteroaryl derivatives occurs in an analogous manner with that of phenyl derivatives. The electron-withdrawing heteroaryl groups destabilize and the electron-donating groups stabilize the positive charge development at the CN carbon while the effect on the negative charge development is opposite. The 15N NMR chemical shift of the C=N and C=N-N nitrogens and the NBO charges at C=N-N unit can be correlated with the replacement substituent constants of the heteroaryl groups. 13C NMR shifts of the C=N carbon of N,N- dialkylhydrazones of the heteroarenecarbaldehydes can be correlated with a dual parameter equation possessing the polar substituent constant ;* of the heteroaryl group and the electronegativity of the heteroatom as variables.
The new tetrathiacrown ethers maleonitrile-tetrathia-12-crown-4 (mn12S(4)) and maleonitrile-tetrathia-13-crown- 4 (mn13S(4)) have been prepared and characterised by X-ray crystallographic analysis. These crown ethers form 2:1, 3:2 and 1: 1 complexes with AgY (Y = BF4, PF6). The crystal structures of [Ag(mn12S(4))(2)]BF4 (3a), [Ag(mn13S(4))(2)]BF4 (4a) and [Ag-2(mn13S(4))(3)](PF6)(2) (6b) have been determined. Compound 3a contains the centrosymmetric sandwich complex cation [Ag(mn12S(4))(2)](+) where each mn12S(4) ligand is coordinated to the Ag centre in an endo manner through all four S atoms. The 2:1 complex [Ag(mn12S(4))(2)](+) is the first sandwich complex with a tetrathiacrown ether and the first complex with an octa(thioether) coordination sphere. The crystal structure of compound 4a also reveals a 2:1 complex. This complex, [Ag(mnl3S(4))(2)](+), exhibits a half-sandwich structure. One mn13S(4) ligand coordinates to Ag+ by all four S donor atoms and the other 13S(4) crown by only one S atom. Compound 6b contains a dinuclear Ag complex. The Ag complexes 3a,b-8a,b were also studied by electrospray ionisation mass spectrometry. Collision-induced dissociation (CID) was used to compare the relative stability of 2:1 complexes [AgL2]+ and 1:1 complexes [AgL](+) (L = mn12S(4), mn13S(4)). The C-13 NMR chemical shifts of 2:1 and 1:1 Ag complexes and their corresponding free ligands were also estimated and compared. The free energy of the barrier of ring inversion (Delta G(double dagger)) for [Ag(mn12S(4))(2)](+) was determined to be 64 kJmol(-1).
The conformations of N-benzylideneani lines p-X-C6H4-CH=N-C6H4 p-Y (X, Y = NO2, CN, CF3, F, Cl, Br, H, Me, OMe, NMe2) have been studied by B3LYP density functional (DFT) hybrid method in combination with the 6-31G* or 6-311G* split valence basis set. The twist of the plane of the aniline ring with respect to the other part of the molecule (tau(2)) is systematically controlled by substituents X and Y, the effect of Y being larger. The value of the dihedral angle tau(2), correlates nicely with equation tau(2) = rho(F)(Y)(x)sigma(F)(Y)+rho(+R)(Y)(x)sigma(+)(R)(Y) + k(x) or tau(2) = rho(F)(X)(y)sigma(F)(X)+rho(-)(R)(X)(y)sigma(+)(R)(X) + k(y), respectively, when aniline or benzylidene substituent is varied. ED substituents X diminish the sensitivity of tau(2) to the aniline substituent Y[rho(F)(Y)(x) and rho(+)(R)(Y)(x)] while ED substituents Y increase the sensitivity Of T2 to the benzylidene substituent X[rho(F)(X)(y) and rho(+)(R)(X)(y)]. There seems to be two competitive conjugative interactions for the aniline ring n electrons: one with the nitrogen lone pair and one with the C=N unit. Substituents X and Y adjust the extent of these interactions and therefore the conformation of the molecule. A good correlation is observed between the dihedral angle tau(2) and the experimental C-13 NMR chemical shift of the C=N carbon of N-benzylideneanilines in CDCl3 (C) 2007 Elsevier B.V. All rights reserved.
A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1',2':5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives.
Tria-, penta-, hepta- and nonafulvenes (1-4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding ;C=C orbital of the exocyclic C=C double bond, obtained by NBO analysis, quantitatively proves ;-electron delocalization which can reveal partial 2-, 6- and 10-;-electron aromaticity, and 4-, 8- and 12-;-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic C=C double bond whilst the (anti)aromaticity of the ring moieties of 1-4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).
Together with the nonsubstituted reference compound, para-methoxy- and para-nitro cyclohexyl benzoates have been synthesized and their conformational equilibria studied by low temperature NMR spectroscopy and theoretical DFT calculations. The free energy differences ;G° between axial and equatorial conformers were examined with respect to polar substituent influences on the conformational equilibrium of O-mono-substituted cyclohexane.
Through the cyclization of 1-(;-hydroxynaphthyl)-1,2,3,4-tetrahydroisoquinoline and 1-(;- hydroxynaphthyl)-1,2,3,4-tetrahydroisoquinoline with formaldehyde, phosgene, p-nitrobenzaldehyde or p-chlorophenyl isothiocyanate, 8-substituted 10,11-dihydro-8H,15bH-naphth[1,2-e][1,3]oxazino[4,3-a]isoquinolines (3 and 4) and 10,11- dihydro-8H,15bH-naphth[2,1-e][1,3]oxazino[4,3-a]isoquinolines (15 and 16) were prepared. Conformational analysis of both the piperidine and the 1,3-oxazine moieties of these heterocycles by NMR spectroscopy and an accompanying theoretical study revealed that these two conformationally flexible six-membered ring moieties prefer twisted chair conformers.
Electrospray ionization was employed to study the mass spectrometric behavior of the maleonitrile tetrathiacrown ethers mn12S(4) (1) and mn13S(4) (2) and maleonitrile pentathiacrown ether mn15S(5) (3) and of their complexes with various metal salts (MX2, M=Pd, Pt, Ni, Co, Fe; X=Cl, CrCl3, Ni(BF4)(2), TIPF6 or Cd(NO3)(2)) and Cu(SO3CF3)(2). Both singly charged, [MXL](+) and [MXL2]+, and doubly charged complexes, [MLn](2+) (n = 2-5), were observed. The formation of the different complexes consisting of the transition metal ion, the counterion and the various crown ethers and their subsequent dissociation was also studied by collision-induced dissociation measurements which were also used to evaluate the relative stabilities of the complexes. It was found that the collisional voltages for the dissociation of the complexes were generally greater in the [MXL](+) complexes than in the corresponding [MXL2]+ complexes. Copyright (c) 2006 John Wiley & Sons, Ltd
Variously substituted tolanes were employed to show that the push-pull effect is also active in C equivalent to C triple bonds by the successful correlation of the occupation quotient pi*/pi of the pi orbital in resonance with the substituted phenyl moieties of tolanes versus the bond length of the C equivalent to C triple bond. In addition, the influences of the ortho phenyl ring substituents on the C-13 chemical shifts of the triple bond carbon atoms, which were estimated by Rubin et al.(4) to be "inapplicable for describing triple bond polarization", were re-evaluated, leading to the conclusion that, while anisotropic effects of ortho substituents are negligible, the steric ortho-substituent effects do in fact dominate the deviations obtained. A detailed theoretical NBO/NCS study has been employed to illuminate the facts of this case
The regioselectivities of methyl- and phenylhydrazine with acridin-9-yl isothiocyanate (thus yielding thiosemicarbazides with acridine substituted on the urea-type side) were examined. Methythydrazine regioselectivity was high with the alpha-nitrogen atom overwhelmingly more nucleophilic than the beta-nitrogen atom; phenylhydrazine regioselectivity was poor but varied with the solvent and only in the case of ethanol was nucleophilic predominance of the alpha-nitrogen atom pronounced. Of note, whilst both phenyl thiosemicarbazides were present in solution only as spiro forms, the methyl product was present as an equilibrium mixture of open-chain and spiro thiosemicarbazides. Reactions on the NH2 blocked analogue of methyl acridin-9-ylthiosemicarbazide (1-isopropylidene-2- methylthiosemicarbazide) were also examined. Interestingly, present in the starting material itself was a structural motif of novelty wherein a triazolethione represented the major species of an equilibrium between cyclic and open-chain forms
The crystal and molecular structures of sodium and barium complexes of dibenzo-24-crown-8 ether
(2006)
The sodium and barium isothiocyanate complexes of 6,7,9,10,12,13,20,21,23,24,26,27-dodecahydrodibenzo[b,n]- 1,4,7,10,13,16,19,22-octaoxacyclotetracosin (dibenzo-24-crown-8 ether = DB24C8) were synthesized and analyzed by X-ray diffraction. The sodium complex, [Na(DB24C8)(NCS)(H2O)] 1, crystallizes in the orthorhombic space group Fdd2 with 16 molecules in the unit cell. The coordination number of Na is 6 and the central ion is located in a distorted octahedric environment. Only four of the crown ether oxygen atoms are involved. The coordination polyhedron is completed by the isothiocanate anion and by a water molecule, which is stabilized by hydrogen bonds. The barium complex, [Ba(DB24C8)(NCS)(2)] 2, crystallizes in the trigonale space group P3(1)21 with 3 molecules in the unit cell. Crystallographic C-2 symmetry is observed for the complex. The coordination number of Ba is 10. Barium is coordinated with the eight oxygen atoms of the macrocyclic ligand and with two isothiocyanate anions. The absolute structure was estimated using the FLACK parameter
Different chemical and enzymatic methods were applied for the hydrolysis of main stems from Lupinus nootkatensis (harvest November 2002). The whole process (all steps) is based on the lignocellulose-feedstock biorefinery regime. The acid hydrolysis of L. was performed with concentrated hydrochloric acid; advantages in this process are exothermic hydrolysis and the possibility of acid recovery. Enzymatic hydrolysis achieved high yields of fermentable carbohydrates (regarding to input cellulose) with high selectivity. However, this way requires the generation of cellulose from L. by chemical pulping. Monosaccharide derivatives thus obtained were identified by their GC retention times and the corresponding MS fragmentation. Hexamethyldisilazane was used as derivatization reagent to prepare the trimethylsilyl derivatives of the carbohydrates and of the degradations products of cellulose from the different fractions. The glucose content was quantified by GC peak integration with respect to an internal standard.
A new synthetic approach to 2,3-dihydro-4H-1,3-thiazine derivatives based upon reductive rearrangement of 1,2- dithiole-3-ylidene thiones has been developed. In turn, the 1,2-dithiole derivatives were prepared by an efficient ring- opening-closing process of 2-alkylidene-4-oxothiazolidines, induced in the presence of Lawesson's reagent by intramolecular non-bonded 1,5-type S...O interactions in the 4-oxothiazolidine precursors.
The trans diesters of 1,4-cyclohexanediol with a number of acetic acid analogues, CX3COOH, of varying steric hindrance and polarity (CX3 = Me, Et, iso-Pr, tert-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, CBr3) were synthesized, and the axial, axial/equatorial, equatorial conformational equilibria were studied by low-temperature H-1 NMR spectroscopy in CD2Cl2. The structures and relative energies of the axial, axial and equatorial, equatorial conformers were calculated at both the MP2/6-311G* and the MP2/6-311+G* levels of theory, and it was only by including diffuse functions that a good correlation of Delta G degrees(calcd) vs Delta G(exptl) could be obtained. Both the structures and the energy differences of the axial, axial and equatorial, equatorial conformers are discussed with respect to the established models of conformational analysis, viz., steric 1,3-diaxial and hyperconjugative interactions. Interestingly, the hyperconjugative interactions sigma(C-C)/sigma(C-H)->sigma*(C-O), together with a steric effect which also destabilizes the equatorial, equatorial conformers on increasing bulk of the substituents, proved to dominate the position of the conformational equilibria. In addition, the preference of the axial, axial conformers with respect to their equatorial, equatorial analogues was greater than expected from the conformational energies of the corresponding substituents in the monosubstituted cyclohexyl esters. The reason for this very interesting and unexpected result is also discussed
CAMPHOR: A GOOD MODEL FOR ILLUSTRATING NMR TECHNIQUES. The use of Nuclear Magnetic Resonance spectroscopy to establish the three-dimensional structures of molecules is an important component of modern Chemistry courses. The combination of techniques that can be used for this purpose is conveniently illustrated by their application to the camphor molecule. This paper presents applications of several techniques used in NMR spectral interpretation in an increasing order of complexity. The result of individual experiments is illustrated in order to familiarize the user with the way connectivity through bonds and through space is established from 1D/2D-NMR spectra and molecular stereochemistry is determined from different NMR experiments
Dynamic 1H NMR (500 MHz) investigation of aryl-N-(arylsulfonyl)-N-(triphenylphosphoranylidene)imidocarbamates in CDCl3, CD3COCD3, and CD3OD at the temperature range of 183-298 K is reported. The observed free energy barriers (almost 12 kcal mol;1) are attributed to conformational isomerization about the NùS bond and these barriers show very little solvent dependence.
The dynamic 1H NMR study of some primary carbamates in the solvents CDCl3 and CD3COCD3 between 183 and 298 K is reported. The free energies of activation, thus obtained (12.4 to 14.3 kcal mol-1), were attributed to the conformational isomerization about the N-C bond. These barriers to rotation show solvent dependence in contrast to the tertiary analogues and are lower in free energy by ca. 2-3 kcal mol-1.
The Push-pull character of two series of donor-acceptor triazenes has been quantified by C-13 and N-15 chemical shift differences of the partial N(1)=N(2) and N(3)=C(4) double bonds in the central linking C=N-N=N-C unit and by the quotient of the occupations of both the bonding pi and antibonding orbitals pi* of these partial double bonds. Excellent correlations of the two estimates, to quantify the push-pull effect, with the bond lengths strongly recommend the occupation quotients pi*/pi, the N-15 chemical shift differences Delta delta[N(l),N(2)], and the corresponding bond lengths as reasonable sensors for quantifying charge alternation along the C=N-N=N-C linking unit, for the donor- acceptor quality of the triazenes 1 and 2 and for the molecular hyperpolarizability beta(0) of these compounds. Within this context, certain Substances can be strongly recommended for NLO application.
Molecular modeling calculations using DFT at the B3LYP/6-31G(d,p) level of theory have been performed on diastereomeric complexes formed between chiral carboxylate anions and chiral urea receptors, a combination previously demonstrated to enable enantiodifferentiation by electrochemical sensing. The calculations correctly predicted the stability order of the enantiomers in acetonitrile solution when the distinction between the enantiomers was above the declared threshold reliability value of 1 kcal mol;1 for computations at this level of theory. Thus, the calculations can not only be applied to predict the likely success of undertakings using the analytical method, it can also, provided ;E is sufficient, potentially be used to determine the absolute configuration of chiral analytes with at least the racemate in hand. The previously successful enantiodifferentiations of various amino acids and alcohols using chiral ion mobility spectroscopy (CIMS) with (S)-2-butanol as the chiral selector were also evaluated by DFT calculations. The calculations again correctly predicted the stability order of the enantiomers when the calculated ;E was above the threshold value though cases not providing a value for ;E above the threshold value was problematic for this system. Attempts to address this shortcoming included an expanded conformational evaluation, a broader analytical approach, and an extended basis set.
Multinuclear dynamic NMR spectroscopy of 5-trifluoromethylsulfonyl-1,3,5-dioxaazinane (4) revealed the existence of two close in energy chair conformers with differently oriented CF3 groups with respect to the ring. Of the two alternative routes for their interconversion, the ring inversion path with intermediate formation of the corresponding 2,5-twist-conformer is preferred, with the energy barrier of 11.2 kcal/mol in excellent agreement with the experimental value (11.7 kcal/mol). The Perlin effect is studied experimentally and calculated theoretically for all CH2 groups and found to be subject to the nature of the adjacent heteroatoms O and N, respectively.