M. Szafran

Strong hydrogen bonds in 1:1 and 2:1 complexes of pyridine betaine with strong acids

Abstract The crystal structure of bis(pyridine betaine) hydrochloride- d 1 monohydrate- d 2 has been determined by X-ray analysis. The carboxylate groups of a pair of pyridine betaine molecules are bridged by a deuteron to form a centro-symmetric dimer featuring a very strong hydrogen bond of length 2.444(4) A. The geometric mass effect (Δ R ≈ 0.008 A) is well within the range observed for this type of hydrogen bond. The FT-IR spectra of polycrystalline 1:1 and 2:1 complexes of pyridine betaine with HNO 3 , HCl, HBr, HI, HO 3 SCF 3 , HClO 4 , HBF 4 , and H 2 SO 4 have been investigated in the 4000–200 cm −1 range. In the 1:1 complexes a proton is transferred from the acid to the betaine molecule, C 5 H 5 N + CH 2 COOH · A − , and both the νOH and νCO frequencies vary with the proton acceptor properties of the anion. The spectra of the 2:1 complexes show broad and intense O · H · O stretching absorptions in the 1500–200 cm −1 range which are slightly affected by the anion and are similar to that for type A acid salts of carboxylic acids. The skeletal vibrations of the betaine residue were identified by second derivative spectroscopy. Evidence based on the νCO vibration and deuteration suggests that the hydrogen bonds in [C 5 H 5 NCH 2 COO · H · OOCCH 2 NC 5 H 5 ] + A − are described by single minimum potentials; ν H = 940 cm −1 , ν H /ν D = 1.2. As betaines are widely distributed in plants and animal tissue and form complexes with strong hydrogen bonds, such bonds should be formed in biological systems.

UV, 1H and 13C NMR spectra, and AM1 studies of protonation of aminopyridines

Abstract The effect of single and double protonation on the UV, 1 H and 13 C NMR spectra of 2-, 3-, and 4-aminopyridines and their N -methyl and N,N -dimethyl derivatives has been investigated. All the spectra confirm that aminopyridines in diluted acids form monocations by protonation at the ring nitrogen atom and dications in concentrated acids (e.g. 80% D 2 SO 4 ). The formation of dications decreases in the order: 3 > 4 > 2. Computed values of proton affinity PA(1) ring and PA(2) subst. are in agreement with the above interpretations.

Recent aspects of the proton transfer reaction in H-bonded complexes

Abstract Proton transfer processes cover a very wide range of situations and time scales and they are of great interest from the viewpoint of chemical reactions in solution. These processes can occur via thermally activated crossing or tunneling. This review considers various aspects of this many-faceted field. Spectroscopic, dielectric, colligative and energetic properties and structures of various species with H-bonds are examined. Proton transfer reactions in water and organic solvents, and the contribution of various H-bonded species and ions to these processes are discussed. Among other topics, this survey includes the effects of solvent, acid-base stoichiometry, concentration, temperature and impurity on proton transfer reactions in complexes of phenols and carboxylic acids with amines, pyridines and pyridine N -oxides. The contribution of the nonstoichiometric acid-base complexes and ionic species to the reversible proton transfer mechanism is discussed.

Ab initio and DFT calculations of structure and vibrational spectra of pyridine and its isotopomers

Abstract The molecular parameters and vibrational spectra of pyridine, pyridine-d 5 and partially deuterated pyridines have been computed using the MP2, BLYP, BP86, BVWN, SVWN and B3LYP methodology and the cc-pVDZ, cc-pVTZ and 6-311++G(d,p) basis sets. The results are compared with the available experimental data in the gas phase. Bond distances, bond angles, dipole moment and harmonic frequencies computed by the MP2/cc-pVTZ and B3LYP/cc-pVTZ are in good agreement with the available experimental data. The calculations are found to be valuable in verifying a number of less certain experimental vibrational assignments. The results suggest that the DFT methods incorporate some anharmonic contributions, which improve the agreement with the experimental frequencies.

X-ray, Fourier-transform infrared, 1H and 13C nuclear magnetic resonance, and PM3 studies of (N—H⋯N)+ and (O—H⋯O)– intramolecular hydrogen bonds in a complex of 1,8-bis(dimethylamino)naphthalene with maleic acid

The crystal structure of the title compound has been determined by X-ray analysis. The intramolecular hydrogen bond lengths are 2.606(3)A for the (NHN)+ bridge in protonated 1,8-bis(dimethylamino)naphthalene cation (DMAN+H) and 2.401(4)A for the (OHO)– bridge in the hydrogen maleate anion (HM–). The H-bonds are asymmetrical and not strictly linear: NHN, 157(3)° and OHO, 170(5)°. The geometries of the N—H⋯N and O—H⋯C bridges of the investigated cation and anion are dominated by the spherical repulsions of their constituent atoms.The overlapping bands in the absorbance IR spectra of potassium hydrogen (deuteron) maleate are separated in the second-derivative spectra. The strong mixing of the in-plane modes with skeletal modes in the hydrogen maleate ion causes a larger separation (Δν≈ 115 cm–1) of the ν(CO) bands in comparison with those in other type A acid salts containing intermolecular hydrogen bonds (Δν≈ 20–35 cm–1). The observed lack of solvent effect on the IR absorption suggests that the hydrogen bonds in tetrabutylammonium hydrogen maleate and 1,8-bis(dimethylamino)naphthalene hydrogen maleate are not extremely polarizable. 1H and 13C NMR chemical shifts of the investigated compound were measured and identified in two-dimensional (2D) experiments. The 1H NMR spectra show two narrow signals at ca. 19.5 and 18.7 ppm due to the OHO and NHN protons, respectively. The structural parameters of the cation and anion were also determined by quantum-mechanical calculations with the semiempirical MNDO-PM3 method.

DFT studies of the structure and vibrational spectra of 8-hydroxyquinoline N-oxide

The geometry, frequency and intensity of the vibrational bands of 8-hydroxyquinoline N-oxide (8-HQNO) and its deuterated derivative (8-DQNO) were obtained by the density functional theory (DFT) with the BLYP and B3LYP functionals and 6-31G(d,p) basis set. The optimized bond lengths and bond angles are in good agreement with the X-ray data. The IR and INS spectra of 8-HQNO and 8-DQNO computed at the DFT level reproduce the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments.

1H and 13C NMR spectra of betaines, >N+(CH2)nCOO−, and their hydrogen halides. Additivity rules for carbon‐13 chemical shifts

The 1H and 13C chemical shifts of N‐methylpiperidine betaines (1), C5H10N+(CH3)(CH2)nCOO−, and their derivatives C5H10N+(CH3)(CH2)nCOOH·X− (2) and C5H10N+(CH3)(CH2)nCOOC2H5·X− (3) with n = 1–5 were identified using two‐dimensional homonuclear (1H) chemical shift correlation (COSY) and heteronuclear (1H, 13C) chemical shift correlation spectroscopy (H,C‐COSY). The effects of the positively charged nitrogen and COO−, COOH, COOEt groups on the methylene protons in the tether groups [N+(CH2)nCOO] and the ring protons in 1, N‐methylpyrrolidine betaines (4), pyridine betaines (6) and their hydrogen halides, were analysed. Empirical additivity rules for the 13C chemical shifts of tether C‐1, C‐2, C‐3, C‐4 and C‐5 atoms for 1, 4 and 6 and their hydrogen halides were derived from the experimental results. The effect of N+ on the β carbon vary with the ring type. Copyright

Conformational analysis of N-methylpiperidine betaine studied by X-ray diffraction, FTIR spectroscopy and ab initio calculations

Abstract N-methylpiperidine betaine crystallized either as anhydrous or monohydrate. The crystal of the anhydrous form are monoclinic, space group C2/c, a =18.512(2) A, b =11.3017(10) A, c =9.2212(10) A, β=118.48(1) o . The molecule in crystals has a conformation with the N + ⋯O intermolecular distance of ca. 2.90 A. Structures of isolated molecule optimized with BLYP and MP2 methods are very similar to this observed in the crystal.

Differences between the N·H·O and O·H·O hydrogen bonds in complexes of 2,6-dichloro-4-nitrophenol with pyridines and pyridine N-oxides

Abstract Complexes of five pyridines and nine pyridine N-oxides with 2,6-dichloro-4-nitrophenol (DCNP) in solution and the solid state were studied by Fourier transform IR and UV spectroscopy, by quantum-mechanical calculations with the semiempirical parametric method 3 (PM3) and by X-ray analysis. The crystals of the 1 : 1 complex of 4-methoxy-2,6-dimethylpyridine N-oxide with DCNP are monoclinic, space group P 2 1 n , a = 4.5936(5) A , b = 21.953(3) A , c = 15.664(2) A , β = 92.87(1)°, V = 1577.6(8) A 3 , Z = 4. The molecules of the complex are joined together by an N+OH⋯O− hydrogen bond with an O⋯O distance of 2.425(3) A, a CO− distance of 1.286(3) A and a (N+O)H⋯O− angle of 152.9°. The PM3 method predicts for all the investigated complexes two minima, the deeper one for B⋯HA complexes and the shallower one for the B+H⋯A− forms. For the 4-methylpyridine complex the N+H⋯O− distance is reproduced correctly but for the 4-methoxy-2,6-dimethylpyridine N-oxide complex the N+H⋯O− distance is too long. The predicted hydrogen-bond angles differ from the experimental values by more than 10°. In solid state complexes of pyridines the N⋯O distances and the broad absorption due to a protic vibration are not directly related to ΔpKa. This is due to the crystal packing forces. In solution the broad absorption varies with ΔpKa. A band in the 3500 cm−1 region due to the solvated phenol is present in all investigated complexes in solution. Absorption in the 3000−2000 cm−1 region of pyridine complexes is more intense than that of the pyridine N-oxides, in agreement with the difference in N⋯O and NO⋯O distances. The broad absorption in the spectra of pyridine complexes is more influenced by solvent effects than in the pyridine N-oxide complexes. The UV spectra of the pyridine complexes show two bands due to B⋯HA (305–315 nm) and B+H⋯A− (382–395 nm) forms. The UV spectra of complexes of pyridine N-oxides of intermediate strengths in CH2Cl2 are not combinations of the spectra of phenol and phenolate. The band in the intermediate position denotes that neither species close to phenol nor to phenoxide ion is present. In these complexes the proton is probably localized in a single minimum and the minimum moves from the donor to the acceptor or, what is more probable, reorganization of the solvent molecules around the complex is faster than the time range of UV spectroscopy. In acetonitrile the situation is quite different as two bands are present, in agreement with a prototropic equilibrium. Effects of solvent, concentration and stoichiometry on interactions of DCNP with pyridines and pyridine N-oxides are compared and discussed. An extended mechanism of the proton-transfer reaction is proposed.

X-ray, FT-IR and PM3 studies of hydrogen bonds in complexes of some pyridines with trifluoroacetic acid

Abstract The crystal structures of trifluoroacetic acid complexes with 4-NMe 2 -, 4-Me- and 4-CN-pyridines were determined by X-ray analysis; the NH⋯O bonds are 2.724(3), 2.702(4) and 2.587(5) A respectively. The H-bonds are nearly linear for 4-NMe 2 , 177(4)°; 4-Me, 177(4)°; and 4-CN, 174(6)°. IR spectra (Nujol) show continuous absorption, whose intensity decreases with elongation of the H-bond length. The continuous absorption is not observed in D 2 O spectra. The solid-state spectra in the 1700 cm −1 region are more complex than those in D 2 O; the characteristic overtones of the pyridine rings borrow intensity from the continuous absorption via Fermi resonance. The overtones indicate modified structure in the 1700 cm −1 region. The results of both diffraction and FT-IR experiments are comparable. The structural parameters of the complexes were also determined by quantum-mechanical calculations with the semiempirical MNDO—PM3 method. A solvent effect was taken into account using a self-consistent reaction field theory.