Dražen Vikić-Topić

Investigation of hydrogen bond structure in benzoic acid solutions

Abstract Significant changes in 13 C chemical shifts of benzoic acid (BA) were observed for the carboxyl and quarternary carbon atoms on going from non-polar benzene to solvents with different hydrogen bond donor or acceptor abilities. Shielding effects of 6.145 and 5.413 ppm at the carboxyl carbon in dimethyl sulfoxide (DMSO) and acetone, respectively, are due to the redistribution of electron density upon the formation of new intermolecular hydrogen bonds with solvent molecules. 1 H NMR upfield shifts of the carboxyl proton in DMSO and chloroform solutions in comparison with benzene solution, and shifts in the CO stretching frequency, ν CO , observed in infrared spectra of BA complexes are consistent with these results. Structural parameters and interaction energies of the hydrogen-bonded complexes determined by quantum-chemical calculations substantiate the experimental observations. Using the semiempirical PM3 hamiltonian, hydrogen bonding was treated as the donor-acceptor mechanism within the natural bond orbital (NBO) formalism. Two correlations, which reflect hydrogen bonding in different solvents, are proposed: one between the 13 C chemical shifts and the change in p-character of the carboxyl carbon atom and the other between the carboxyl CO stretching frequency and the calculated interaction energy of the hydrogen-bonded complexes.

Conformational analysis of azithromycin by nuclear magnetic resonance spectroscopy and molecular modelling

Abstract The conformation of azithromycin 1 in the solution was determined by NMR spectroscopy and molecular mechanics calculations and compared with its crystal structure and with some erythromycin derivatives. In solution 1 exists predominantly in a “folded-in” conformation in the C-3 to C-5 region, whereas its crystal state conformation is “folded-out”.

The Novel 6–(N-Pyrrolyl)purine Acyclic Nucleosides: 1H and 13C NMR and X-ray Structural Study†

Abstract Synthesis of the novel nucleoside analogues containing exocyclic pyrrolo moiety and acyclic side chains attached to the purine ring at N-9 and N-7 is described. The site of alkylation was determined by 1H and 13C NMR on the basis of chemical shifts, C-H coupling constants and connectivity in NOESY and HETCOR spectra. The N-9 substitution of 7 was proved by its X-ray crystallographic analysis. † Part of the paper was presented at the Ninth International Course and Conference on the Interfaces Among Mathematics. Chemistry and Computer Sciences. Dubrovnik, Croatia (1994).

Isotope effects in 13C NMR spectra of monodeuteriated trans-N-bezylideneanilines

Abstract Deuterium isotope effects on 13 C chemical shifts have been determined in a series of monodeuteriated trans -N-benzylideneanilines. The effects over two bonds ( 2 Δ) observed at C-1 and over three bonds ( 3 Δ) observed at C-2,6 are sensitive to steric interactions involving nitrogen lone-pair electrons and C-Ph ortho -protons. Like in similar π-electron systems, the effect over six bonds ( 6 Δ) is related to molecular conformation.

13C NMR spectra of some symmetrically 2,2'-disubstituted stilbenes : conjugational and conformational effects

Abstract In the 13 C NMR spectra of symmetrically 2,2′-disubstituted cis -stilbenes the decrease of α-effects and increase of γ-effects at ortho -carbons with respect to tarns -isomers were observed. However the most significant differences in substituent effects between trans- and cis -isomers were found for γ-effects, at unsubstituted ortho -C-6 and C-4 atoms. The one bond C-H splitting of olefinic carbons is greater in cis -than trans -stilbenes. The differences in substituent effects and apparent coupling constants are assumed to originate mainly from the different conjugational interactions in cis -and trans -stilbenes. the conformational and configurational differences of these molecules were substantiated by molecular mechanics calculations.

Long-range deuterium-induced isotope effects on 1H chemical shifts in cis- and trans-stilbene

Deuterium isotope effects on olefinic proton chemical shifts in a series of deuterated cis- and trans-stilbene isotopomers have been determined and analysed. The effects exhibit conformational and solvent dependence. The sign and magnitude of the effects are governed by the additivity rule. An opposite trend between the effects at the olefinic proton and those previously observed at olefinic carbon has been found and discussed for closely related compounds.