Andrzej Sygula

Inclusion complexes of buckycatcher with C60 and C70

The dispersion corrected B97-D functional studies find a previously overlooked conformer of the buckycatcher C(60)H(28) (2) exhibiting intramolecular pi-pi stacking of its corannulene pincers to represent a global potential energy minimum conformation. B97-D/TZVP calculated geometry of C(60)@2 supramolecular assembly is in excellent agreement with the X-ray structure, slightly better than the previously reported M06-L results. In contrast, our calculated binding energy of C(60)@2 complex is dramatically higher than both M05-2X and M06-2X results and we conclude that the latter numbers are grossly underestimated. The lack of specificity of the buckycatcher for molecular recognition of C(60)vs. C(70) fullerenes is predicted by our calculations and confirmed by the NMR titration experiment which provides the equilibrium constant of 6800 +/- 400 M(-1) for association of C(70) with C(60)H(28) which is only slightly lower than the previously reported association constant of C(60)@2 assembly.

‘Buckybowls’—introducing curvature by solution phase synthesis

Abstract New, non-pyrolytic methods are presented for the synthesis of curved-surface, polynuclear aromatic hydrocarbons related to the fullerenes. Tetrabromocorannulene is conveniently prepared on a large scale and then converted to corannulene, and also used as a synthon for further elaboration.

MNDO study of the tautomers of nucleic bases: Part I. Uracil, thymine and cytosine

Abstract The optimal geometries, relative stabilities, dipole moments and ionization potentials of a series of tautomers of uracil, thymine and cytosine have been calculated by the MNDO method. The results are in good agreement with the available experimental data. The optimal MNDO geometry of the “diketo” tautomer of uracil is very close to the recently published optimal ab initio STO-3G geometry. It is expected that the MNDO geometries of other pyrimidine base tautomers could be used in ab initio calculations. The calculated relative stabilities agree with the results of earlier non-empirical studies performed without geometry optimization, although the differences in energy of the tautomers found by MNDO are significantly smaller than those predicted by ab initio methods.

On the possibility of fluorescence from twisted intramolecular charge transfer states of 2-dimethylamino-6-acylnaphthalenes. A quantum-chemical study☆

Abstract MNDO geometry, CNDO/S CI spectra and dipole moments of 2-dimethylamino-6-propionylnaphthalene (PRODAN), a representative of a class of highly environment-sensitive fluorescent probes, were computed. Only the conformer with a twisted dimethylamino group has an excited state dipole moment high enough (27 D) to explain spectral properties of PRODAN. On the basis of the Amos—Burrows theory, fluorescence from the TICT state is predicted in polar solvents.

Thermodynamics of host-guest interactions between fullerenes and a buckycatcher.

1H NMR and isothermal titration calorimetry (ITC) experiments were employed to obtain reliable thermodynamic data for the formation of the 1:1 inclusion complexes of fullerenes C60 and C70 with the buckycatcher (C60H28). NMR measurements were done in toluene-d8 and chlorobenzene-d5 at 288, 298, and 308 K, while the ITC titrations were performed in toluene, chlorobenzene, o-dichlorobenzene, anisole, and 1,1,2,2-tetrachloroethane at temperatures from 278 to 323 K. The association constants, Ka, obtained with both techniques are in very good agreement. The thermodynamic data obtained by ITC indicate that generally the host–guest association is enthalpy-driven. Interestingly, the entropy contributions are, with rare exceptions, slightly stabilizing or close to zero. Neither ΔH nor ΔS is constant over the temperature range studied, and these thermodynamic functions exhibit classical enthalpy/entropy compensation. The ΔCp values calculated from the temperature dependence of the calorimetric ΔH values are negative for the association of both fullerenes with the buckycatcher in toluene. The negative ΔCp values are consistent with some desolvation of the host-cavity and the guest in the inclusion complexes, C60@C60H28 and C70@C60H28.

Sensitive Carbohydrate Detection using Surface Enhanced Raman Tagging

Glycomic analysis is an increasingly important field in biological and biomedical research as glycosylation is one of the most important protein post-translational modifications. We have developed a new technique to detect carbohydrates using surface enhanced Raman spectroscopy (SERS) by designing and applying a Rhodamine B derivative as the SERS tag. Using a reductive amination reaction, the Rhodamine-based tag (RT) was successfully conjugated to three model carbohydrates (glucose, lactose, and glucuronic acid). SERS detection limits obtained with a 633 nm HeNe laser were ∼1 nM in concentration for all the RT-carbohydrate conjugates and ∼10 fmol in total sample consumption. The dynamic range of the SERS method is about 4 orders of magnitude, spanning from 1 nM to 5 μM. Ratiometric SERS quantification using isotope-substituted SERS internal references allows comparative quantifications of carbohydrates labeled with RT and deuterium/hydrogen substituted RT tags, respectively. In addition to enhancing the SERS detection of the tagged carbohydrates, the Rhodamine tagging facilitates fluorescence and mass spectrometric detection of carbohydrates. Current fluorescence sensitivity of RT-carbohydrates is ∼3 nM in concentration while the mass spectrometry (MS) sensitivity is about 1 fmol, achieved with a linear ion trap electrospray ionization (ESI)-MS instrument. Potential applications that take advantage of the high SERS, fluorescence, and MS sensitivity of this SERS tagging strategy are discussed for practical glycomic analysis where carbohydrates may be quantified with a fluorescence and SERS technique and then identified with ESI-MS techniques.

STRUCTURE AND INVERSION BARRIERS OF CORANNULENE, ITS DIANION AND TETRAANION. AN AB INITIO STUDY

Abstract Ab initio calculations were performed for neutral corannulene ( 1 ) and its dianion and tetraanion. The minimum energy conformations, as well as the transition states for bowl-to-bowl inversion, were located and characterized by calculation of vibrational frequencies. For some charged species, the Hartree-Fock method predicts critical point symmetries that are lower than those deduced from Huckel theory. However, inclusion of electron correlation effects shows this to be an artifact of the HF method. Consideration of electron correlation is also crucial for the estimation of the inversion barriers, while the quality of the basis set has only moderate impact on these results. The “best theoretical estimate” of the barriers decreases from 14.2 kcal mol −1 for neutral 1 , to 7.9–9.2 kcal mol −1 for the dianions, and to 3.2 kcal mol −1 for the tetraanion. While the introduction of negative charges into corannulene causes overall flattening of its curvature, even in the most extreme case — the tetraanion — preference for a bowl-shaped geometry remains. Within C 5 v symmetry, the 3 A 2 triplet state is clearly favored over the 1 A 1 and 1 E 2 singlet states of the isolated dianion. However, no significant preference of the triplet vs. singlet electronic state emerges if the symmetry of the molecular framework of the dianion is allowed to distort. Finally, analysis of the charge distribution in the tetraanion 5 does not support the “anion within a trianion” model for the tetraanion of corannulene.

Chemistry on the rim of buckybowls: derivatization of 1,2,5,6-tetrabromocorannulene

Abstract Synthesis of a number of substituted corannulenes and dibenzo[ a , g ]corannulenes starting from 1,2,5,6-tetrabromocorannulene 3 is described. Since 3 is conveniently obtained by a large scale, non-pyrolytic route, it may serve as a synthon for the further elaboration of this bowl-shaped system.

MNDO study on relative stabilities of monosubstituted pyridine tautomers

Abstract Relative stabilities of monosubstituted hydroxy-, mercapto- and aminopyridine tautomers have been calculated using the MNDO procedure with a full geometry optimization. In all cases considered the “lactim” tautomers proved to be more stable, in full qualitative agreement with the gas-phase experimental data.

Concerning the structure of the corannulene tetraanion.

Abstract The question of an anion within an anion structure for corannulane tetraanion is examined by ab initio and semi-empirical calculations, as well as by the behavior of corannulene anionic intermediates in anhydrous ammonia. Calculations do not lend much support to a central cyclopentadienyl anion in the tetraanion, and a tetraanion is either not formed or does not resist protonation in liquid ammonia.