Dionisia Sanz

Substituent effects on the 15N NMR Parameters of Azoles

The 15N chemical shifts and a large collection of coupling constants pertaining to azoles have been gathered from the literature. To complete this collection and to check some anomalies, the spectra of 14 compounds in several solvents were recorded again and 31 compounds were studied for the first time; in all, data for 420 compounds (pyrroles, imidazoles, pyrazoles, triazoles, tetrazoles, indoles, benzimidazoles, indazoles, benzotriazoles and carbazoles) are reported. Additive models are used to discuss the substituent chemical shifts.

Determining hydrogen-bond strengths in the solid state by NMR: the quantitative measurement of homonuclear J couplings

Hydrogen-bonding strengths in the solid state are quantitatively determined by the accurate measurement of 15N-15N J couplings using a straightforward 2D MAS NMR spinecho approach.

Substituent and solvent effects on the proton transfer equilibrium in anils and azo derivatives of naphthol. Multinuclear NMR study and theoretical calculations

Abstract The tautomeric equilibrium due to proton transfer is compared in a series of anils of 2-hydroxynaphthalene-1-carbaldehyde and azo derivatives of 2-naphthol. Although structurally similar, these systems suffer different substituent effects on the solution-state proton transfer properties. The comparative study was performed using 1 H, 13 C and 15 N NMR spectroscopy in a variety of solvents. Hartree–Fock ab initio calculations involving relative stability of tautomers and full geometry optimization for the ground state are in agreement with the experimental observations.

Photophysics of the 2-(2′-hydroxyphenyl)perimidine: On the fluorescence of the enol form

Abstract The photophysical behaviour of 2-(2′-hydroxyphenyl)perimidine (6), which possesses a strong intramolecular hydrogen bond is evidenced as due to the non-transferred enol form. This compound is highly stable to light, with a photodegradative quantum yield in degassed cyclohexane of 7 × 10 −6 , suggesting that the involvement of a lightinduced excited state intramolecular proton transfer (ESIPT) process in which the phenolic proton is transferred from the enol form to the neighbouring nitrogen atom is less important for conferring photostability than has hitherto been supposed for such type of systems.

Structure of bis-, tris- and tetrakispyrazolylborates in the solid state (sodium and potassium salts of tetrakispyrazolylborate by X-ray crystallography) and in solution (1H, 11B, 13C and 15N NMR)

Abstract The knowledge of the properties of pyrazolylborates has been considerably improved both in the solid state and in solution. The hydrates of the sodium and the potassium salts of tetrakis(l-pyrazolyl)borate crystallize in the space group C2/c, both salts being isomorphous. Regular pyrazole rings conform with a pseudosymmetry plane around a distorted tetrahedral boron atom. Chains along the b axis of two different octahedra of coordination, sharing water oxygens, are linked by H bonds. In solution, a careful 1H, 13C, 11B and 15N NMR study was carried out. The applicability of the rules used to assign H(3) versus H(5) and C(3) vcrsus C(5) in the case of pyrazolylborates was reevaluated. Only for tetrakis(1- pyrazolyl)borate were couplings observed with 1B in 1H, 3C and 5N resonances. The regularity of the properties of bis-, tris- and tetrakispyrazolylborates, allows the calculation of the spectral parameters of the still unknown tris-hydrido(1-pyrazolyl)borate, pzBH3−. Finally, the electronic properties, as measured by σp, of the substituents BH3−, pzBH2−, pz2BH− and pz3B− were estimated.

Complete energy profile of a chiral propeller compound: Tris-(2′-methylbenzimidazol-1′-yl) Methane (TMBM). Chromatographic resolution on triacetyl cellulose, x-ray structures of the racemic and one enantiomer, and dynamic NMR study

Abstract Tris-(2′-methylbenzimidazol-1 ′-y) methane (TMBM) presents an interesting example of propeller-like chirality, which is discussed according to Mislows and Dunitzs descriptions. Fortunately, the two most stable isomers (the three methyl groups “up”, i . e . on the same side of the methine proton, and two methyl groups “up” and the third one “down”) were present in the solid state, thus allowing the determination of their molecular structure by x-ray crystallography. The activation barrier which separates both isomers (9.8 kcal.mol −1 ) was determined by dynamic 1 H n.m.r., whereas that corresponding to enantiomers (28.5 kcal. mol −1 ) was determined kinetically by racemization, after pure enantiomers were resolved by chromatography on microcrystalline triacetyl cellulose. Both the racemic TMBM and its enantiomers crystallize with a larger number of water molecules, six and seven respectively, forming cyclic structures.

An 1H and 13C NMR spectroscopic study of the structure of potassium and thallium salts of tris- and tetrakis-(pyrazol-1-yl) borates in solution. Some 13C11B and 13C205Tl residual coupling constants

Abstract The 1 H and 13 C NMR spectra of 18 tris- and tetrakis-pyrazolylborates in the form of potassium or thallium salts have been recorded. Four of these compounds are new. The results are discussed with regard to two main aspects: coupling constants with 205 Tl and positional isomerism of the pyrazole substituents. Most borates are symmetric compounds (all the pyrazole rings being identical isomers) corresponding to less hindered structures (all 3-aryl, 3-i-propyl-4-bromo, 3-t-butyl-5-methyl or 3-t-butyl-5-i-propyl pyrazoles). Only one derivative, potassium hydrotris(2,4,6-trimethylphenylpyrazol-1-yl)borate 5 , does not fit this pattern, being either a 3/3/3/ or a 3/3/5 isomer.

6‐Aminofulvene‐1‐aldimine: A Model Molecule for the Study of Intramolecular Hydrogen Bonds

An unusually substantial coupling is observed across the hydrogen bond of fully 15 N-labeled compound 1 when it is studied by 1 H and 15 N NMR spectroscopy. The structure was determined by X-ray diffraction and shown to correspond to tautomer 1 a (both in the solid state and in solution). These results open up a new field of hydrogen-bond research by NMR spectroscopic methods.

N-aminoazoles. Part 2. Basicity and protonation site of N-aminoazoles: an experimental (pKa, 13C and 15N NMR spectroscopy and crystallography) and theoretical study

The 13C and 15N NMR spectra of 1-aminoimidazole 2, 1-aminopyrazole 3, 4-amino-1,2,4-triazole 4, 1-amino-1,2,4-triazole 5, 1-aminobenzimidazole 12, 2-aminoindazole 13, 1-aminoindazole 14, 2-aminobenzotriazole 15 and 1-aminobenzotriazole 16 have been recorded in neutral (CDCl3 or [2H6]DMSO) and acid (CF3CO2H and SO4H2) conditions. The X-ray crystal structures of two polymorphic forms of N-aminobenzimidazolium picrate 12d have been determined. The main differences between the forms are due to the twist of an ortho-nitro group of the picrate anion up to 53° leading to a different hydrogen bond network. In the two crystals, the relative disposition of both ions is similar being held together by N+–H ⋯ O–/O2N three centre hydrogen bonds. The basic pKas of 1-aminoindole 11, compounds 12 and 13 and 9-aminocarbazole 17 have been measured. Finally, all monoprotonated cations on N-aminoazoles (from pyrrole 1 to carbazole 17) have been calculated. When there is a pyridine-like nitrogen atom on the ring, this is the preferred site of protonation although in sulfuric acid the amino group is also protonated, only 1-aminoindole and 9-aminocarbazole protonate on the amino group.

A Multinuclear NMR Spectral Study of Parent Azoles and Benzazoles: Experimental Results and GIAO Ab initio Calculations

The 13 C and 15 N chemical shifts of twelve azoles and benzazoles have been determined in the solid state at room temperature and in methanol solution at 178 K. The experimental values were compared with the absolute shieldings calculated at the GIAO/B3LYP/6-31G* level. These comparisons show that some signals, 13 C NMR but especially 15 N NMR, have to be corrected for hydrogen bonds present in certain solvents and in the solid state. The linear regressions are good enough to be used to predict some missing values.