The basicity of a series of sulfonamides and carboxamides with respect to protonation and hydrogen-bonded complex formation with phenol was investigated by calculations using the Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional with the 6-311G** and 6-311++G** basis sets and by infrared spectroscopy. The effect of fluorinated substituent was studied for the two series. The proton affinity of nitrogen in sulfonamides is higher than oxygen, in contrast to carboxamides, which are protonated at oxygen. The phenyl group in benzenesulfonamide increases the basicity of both heteroatoms, but more strongly of the nitrogen, whereas in benzamide the effect on the two heteroatoms is about the same. The CF 3 group equally decreases the basicity of nitrogen and oxygen atoms in sulfonamides and carboxamides. The second fluorinated substituent decreases the basicity of oxygen in (CF 3 CO) 2 NH more strongly than of nitrogen. For sulfonamides, the same effect results in the reverse of the center of basicity from nitrogen in (MeSO 2 ) 2 NH to oxygen in (CF 3 SO 2 ) 2 NH. All studied carboxamides give H-complexes via the carbonyl oxygen, whereas for sulfonamides two types of H-complexes, with the OHÁÁÁN and OHÁÁÁO=S, were found theoretically, the latter being more stable. The exception is bisimide (CF 3 SO 2 ) 2 NH, for which only the OHÁÁÁO=S complex is stable. Experimentally, only the oxygen-bound complexes are observed. Analysis of the natural charges revealed an 'abnormal' increase of the electron density on the NH group by electron-acceptor substituents in CF 3 SO 2 NHR, which was explained using the natural bond orbital analysis by loosening of the S-N bond because of orbital interactions with the s* SÀN orbital.

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The first Si-H-containing azasilaheterocycle, 1,3-dimethyl-3-silapiperidine 1, was synthesized, and its molecular structure and conformational properties were studied by gas-phase electron diffraction (GED), low temperature NMR, IR and Raman spectroscopy and quantum chemical calculations. The compound exists as a mixture of two conformers possessing the chair conformation with the equatorial NMe group and differing by axial or equatorial position of the SiMe group. In the gas phase, the SiMe(ax) conformer predominates (GED: ax/eq = 65(7):35(7)%, ΔG = 0.36(18) kcal/mol; IR: ax/eq = 62(5):38(5)%, ΔG = 0.16(7) kcal/mol). In solution, at 143 K the SiMe(eq) conformer predominates in the frozen equilibrium (NMR: ax/eq = 31.5(1.5):68.5(1.5)%, ΔG = -0.22(2) kcal/mol). Thermodynamic parameters of the ring inversion are determined (ΔG(‡) = 8.9-9.0 kcal/mol, ΔH(‡) = 9.6 kcal/mol, ΔS(‡) = 2.1 eu). High-level quantum chemical calculations (MP2, G2, CCSD(T)) nicely reproduce the experimental geometry and the predominance of the axial conformer in the gas phase.

The structures of pyrrolylmethylidene derivatives of 2,3-dihydro-1H-inden-1-one (3), 3,4-dihydro-naphthalen-1(2H)-one (4), and cycloalkanones (5-7) were studied for the first time in the solid state and solution by NMR, IR, and UV spectroscopies supported by DFT quantum mechanical calculations. It was shown that all studied compounds except cycloheptanone derivative 7 both in crystal and in solution exist in the form of dimers where single E or E,E configuration with respect to the exocyclic C═C bond is stabilized by intermolecular hydrogen bonds N-H···O═C. UV irradiation at a wavelength of 365 nm of MeCN or DMSO solutions of 3-6 results, depending on the exposition time and solvent, partial to complete isomerization to the Z or Z,E isomers (in the case of 6, also the Z,Z isomer). The NMR and IR spectroscopy data show the existence of a strong intramolecular hydrogen bond N-H···O═C in the Z moieties of isomerized compounds. The studied compounds are protonated by trifluoroacetic acid at the carbonyl oxygen, in spite of the reverse order of basicity and nucleophilicity of the carbonyl group and the pyrrole ring. Investigation of the behavior of compound 6 with respect to acetate and fluoride anions allows one to consider it as a potential fluoride sensor.

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3-Methyl-3-silatetrahydropyran 1 was synthesized and its molecular structure and conformational behavior was studied by gas-phase electron diffraction (GED), FTIR, low temperature (1)H and (13)C NMR spectroscopy, and by theoretical calculations (DFT, MP2). Two conformers, 1-ax and 1-eq, were located on the potential energy surface. In the gas phase, a slight predominance of the axial conformer was determined, with the ratio 1-ax:1-eq = 54(9):46(9) (from GED) or 53:47 or 61:39 (from IR). In solution, LT NMR spectroscopy at 103 K gives the ratio 1-ax:1-eq = 35:65 (-ΔG°103 = 0.13 kcal/mol). Simulation of solvent effects using the PCM continuum model or by calculation of the corresponding solvent-solute complexes allowed us to rationalize the experimentally observed opposite conformational predominance of the conformers of 3-methyl-3-silatetrahydropyran in the gas phase and in solution. Comparative analysis of the effect of heteroatom in 1-hetero-3-methyl-3-silacyclohexanes on the structure, stereoelectronic interactions, and relative energies of the conformers is done.

The series of push-pull compounds containing the pyrrole ring as a donor and the 1,3-indandione derived moieties as the acceptor unit were synthesized, and strong intramolecular hydrogen bonding in their molecules was studied. In the presence of fluoride and acetate anions their solutions undergo color changes. It was shown by NMR, UV-vis, and quantum chemical calculations including AIM analysis that all these compounds undergo solvent-assisted rupture of the intramolecular hydrogen bond followed by the formation of a strong intermolecular hydrogen bond with fluoride and acetate anions which finally abstract a proton from the pyrrole ring. The insensitivity of the studied compounds to other anions (Cl, Br, HSO4, PF6) is consequence of the instability of the corresponding hydrogen-bonded complexes.

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