Beta-alanine-oxalic acid (1:1) hemihydrate crystal: structure, 13C NMR and vibrational properties,protonation character

The crystal structure of beta-alanine-oxalic acid (1:1) hemihydrate complex has been reinvestigated by X-ray diffraction method at 293 K. Formation of monoclinic crystal system belonging to C2/c space group and consisting of semi-oxalate chains, diprotonated beta-alanine dimers and water molecules bonded to both these units is confirmed. New results are obtained for distances in the carboxylic groups and hydrogen bonds. These structural observations are used for protonation degree monitoring on the carboxylic oxygen atoms. They are in accordance with our vibrational study. The 13C NMR spectra provide insights into the solid structure of this complex, character of its hydrogen bonds and the beta-alanine protonation.     

Beta-alanine-hydrochloride (2:1) crystal: structure, 13C NMR and vibrational properties,protonation character

The crystal structure of beta-alanine-hydrochloride (2:1) complex (2A-HCl) has been determined by X-ray diffraction method at 298 and 100 K as monoclinic, space group C2/c, Z=4. The crystal comprises chloride anions and protonated beta-alanine dimers: two beta-alanine zwitterions are joined by strong, symmetric (Ci) hydrogen bond with the O...O distance of 2.473 A at room temperature. Powder FT-IR and FT-Raman as well as solid state 13C NMR spectra provide insights into the solid structure of this complex, character of its hydrogen bonds and the beta-alanine protonation.     

(13)C-(1)H and (13)C-(13)C NMR J-couplings in (13)C-labeled N-acetyl-neuraminic acid: correlations with molecular structure

N-acetyl-neuraminic acid (Neu5Ac, 2) was prepared enzymatically containing single sites of (13)C-enrichment at C1, C2, and C3. Aqueous solutions of the three (13)C isotopomers were studied by (1)H and (13)C NMR spectroscopy at p(2)H 2 and pH 8 to obtain J(CH) and J(CC) values involving the labeled carbons. Experimental studies were complemented by DFT calculations of the same set of J-couplings in protonated and ionized structural mimics of 2 to determine how well theoretical predictions match the experimental findings in saccharides bearing ionizable functionality. Results show that: (a) (2)J(C2,H3ax/eq) values in 2 depend on anomeric configuration, thus complementing (3)J(C1,H3ax/eq) behavior, (b) J(CH) and J(CC) values involving C2 depend on anomeric configuration, the C1-C2 bond torsion, and solution pH, and (c) long-range (4)J(C2,H7) is sensitive to glycerol side-chain conformation. Intraring J(HH) and most (2)J(CH), (3)J(CH), (2)J(CC), and (3)J(CC) involving C1-C3 of 2 appear largely unaffected by the ionization state of the carboxyl group. In vacuo and solvated DFT calculations of geminal and vicinal J(CH) and J(CC) values are similar and reproduce the experimental data well, but better agreement with experiment was observed for (1)J(C1,C2) in the solvated calculations. The present work provides new information for future treatments of trans-glycoside couplings involving Neu5Ac residues by (a) providing new standard values of intraring J(CC) for coupling pathways that mimic those for trans-glycoside J(CC), (b) identifying potential effects of solution pH on trans-glycoside couplings inferred through the behavior of related intraring couplings, and (c) providing specific guidelines for more reliable DFT predictions of J(CH) and J(CC) values in ionizable saccharides.     

Differential protonation and dynamic structure of doxylamine succinate in solution using 1H and 13C NMR

A protonation and dynamic structural study of doxylamine succinate, a 1:1 salt of succinic acid with dimethyl-[2-(1-phenyl-1-pyridin-2-yl-ethoxy)ethyl]amine, in solution using one- and two-dimensional 1H and 13C NMR experiments at variable temperature and concentration is presented. The two acidic protons of the salt doxylamine succinate are in 'intermediate' exchange at room temperature, as evidenced by the appearance of a broad signal. This signal evolves into two distinct signals below about -30 degrees C. A two-dimensional 1H-1H double quantum filtered correlation experiment carried out at -55 degrees C shows protonation of one of the acidic protons to the dimethylamine nitrogen. A two-dimensional rotating frame 1H-1H NOE experiment at the same temperature reveals that the other proton remains with the succinate moiety. Comparison of the 1H and 13C chemical shifts and the 13C T1 relaxation times of the salt with those of the free base further substantiate the findings.     

A novel Zn(4)O-based triazolyl benzoate MOF: synthesis, crystal structure, adsorption properties and solid state 13C NMR investigations

The newly synthesized Zn(4)O-based MOF (3)(∞)[Zn(4)(μ(4)-O){(Metrz-pba)(2)mPh}(3)]·8 DMF (1·8 DMF) of rare tungsten carbide (acs) topology exhibits a porosity of 43% and remarkably high thermal stability up to 430 °C. Single crystal X-ray structure analyses could be performed using as-synthesized as well as desolvated crystals. Besides the solvothermal synthesis of single crystals a scalable synthesis of microcrystalline material of the MOF is reported. Combined TG-MS and solid state NMR measurements reveal the presence of mobile DMF molecules in the pore system of the framework. Adsorption measurements confirm that the pore structure is fully accessible for nitrogen molecules at 77 K. The adsorptive pore volume of 0.41 cm(3) g(-1) correlates well with the pore volume of 0.43 cm(3) g(-1) estimated from the single crystal structure.     

Study of the protonation of 1-methylimidazo (4,5-B)pyrazines by1H and13C NMR methods

Conclusions A study of the¹H and¹³C NMR spectra of the neutral molecules, monocations, and dications of 1-methyl- and 1,5,6-trlmethylimidazo(4,5-b)pyrazLnes has shown that the first protonation of these compounds occurs at the 3-N atom of the imidazole fragment and the second at the 4-N atom of the pyrazine ring.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1060–1068, May, 1977.     

6-Nitro-[1,10]phenanthroline-1-ium nitrate: crystal structure, ab initio calculations and protonation character

The title compound, 6-nitro-[1,10]phenanthroline-1-ium nitrate, has been synthesized and characterized by elemental analysis, electron absorption spectroscopy, IR, ¹H and ¹³C NMR spectroscopy. The X-ray crystal structure study showed that the compound crystallizes in the monoclinic system, space group Cc, with M_r=288.22 (C₁₂H₈N₄O₅), a=13.861(3), b=10.142(2), c=8.7320(17) Å, β=103.70(3)°, V=1192.6(4) Å³, Z=4, D_c=1.605 g/cm³, F(000)=592, μ(Mo Kα)=0.129 mm⁻¹, R=0.0439, wR=0.1125, GOF=1.110. In the crystal lattice, the molecules create a network structure through hydrogen bonds. Ab initio calculations of the structures, charges distribution, natural bond orbitals, topological analysis and thermodynamic functions of 5-nitro-[1,10]phenanthroline and its protonated cation were performed at HF/6-311G** and B3LYP/6-311G** levels of theory. The calculation results are in a good agreement with the X-ray data and show that the protonated structure is stable. The calculation of second order optical nonlinearity was carried out and a higher molecular hyperpolarizability of 24.66×10⁻³⁰ esu was predicted.     

1H and 13C NMR studies of the protonation of isomeric methoxysulmazole analogues

The major protonation sites of six cardiotonic isomeric 2-aryl-n-methoxy-1H-imidazo[4,5-b]- and -[4,5-c]-pyridines (n = 4–7) were determined by ¹H and ¹³C NMR methods. All the 1H-imidazo[4,5-c]pyridines and the 7-methoxy derivative of sulmazole were found to protonate at the pyridyl nitrogen. Protonation occurred at the imidazo nitrogen, however, for the 5- and 6-methoxy derivatives of sulmazole.     

Substituent effects on ionisation and (13)C NMR properties of some monosubstituted phenols A potentiometric, spectrophotometric and (13)C NMR study

The pK(a) values of ionisation of a set of phenols ortho, meta and para substituted are studied by spectrophotometry and (13)C NMR spectroscopy. A dual substituent analysis of equilibrium and NMR results, according to the Swain and Lupton procedure, is presented. The results of this analysis allow the assignment of the contribution of field and resonance contributions, both on equilibrium constants and chemical shifts.     

A novel crown ether generation containing different heteroaromatic cations: synthesis, characterization, solid-phase (13)C NMR, X-ray crystal structure, and selective amino acid recognition

The synthesis and structural characterization of a novel generation of crown ethers, 3, 5 and 6 containing pyrilium, thiopyrilium, and pyridinium subunits, respectively, are reported. The crown ether unit is potentially capable of forming host-guest complexes with inorganic and organic cations, while the heteroaromatic cationic unit is suitable to bind with anions. A variety of physicochemical methods including electrospray mass spectrometry, UV-vis spectroscopy, solution and solid-phase NMR, and X-ray crystallography were applied for structural characterization of the new crown ether derivatives. The (1)H and (13)C NMR studies indicate rapid rotation of the B9C3 unit about the C-C bond that connects the two units to each other. Single crystals for 3, 4, and 5 were successfully obtained, and their X-ray crystal structures were resolved. The perchlorate anion in 3 (orthorhombic, space group P2(1)2(1)2(1)) and 5 (orthorhombic, space group P2(1)) is far from O(+) and close to S(+). The solid-phase structure of 3 and 5 show small deviation from planarity for the four aromatic rings, whereas two of the aromatic rings in 4 are out of heteroaromatic ring. Spectrophotometric studies in methanol solution revealed that the ligand 3 can be successfully applied to selective amino acid recognition.     

1H, 13C and 15N NMR observations on the protonation of 1- and 3-deazapurine

Using ¹H, ¹³C and ¹⁵N NMR it has been concluded that 3-deazarpurine protonates exclusively at N-1 with a pK of about 5.6. The base exhibits rapid tautomerism with proportions of 70:30, with the N–7-H tautomer in the majority. The salt exists predominantly as the N-7-H tautomer. 1-Deazapurine protonates essentially in a 1:1 ratio at N-3 and at the imidazole ring, with a pK of about 3.1. This base also exhibits rapid tautomerism with proportions of 30:70, this time with the N-9-H in the majority. The salt also exists in a tautomcric mixture with approximately equal proportions. One form has N-3 and N-9 bearing hydrogens and the other has N-7 and N-9 bearing hydrogens.     

2-(diphenylphosphinylmethoxy)aniline: Synthesis,vibrational spectra,and crystal structure

The synthesis, vibrational spectra, and X-ray diffraction analysis results for 2-(diphenylphosphinylmethoxy) aniline, 2-[(C₆H₅)₂P(O)OCH₂]C₆H₄NH₂(I), are described. The crystals are monoclinic: a = 18.4515(17) Å, b = 10.5421(12) Å, c = 17.897(2) Å, β = 104.479(8)°, V = 3370.7(6) ų, Z = 8, space group P2₁/c, R = 0.0546 for 1770 reflections with I > 2σ(I). The unit cell contains two crystallographically independent molecules Ia and Ib joined by an N-H …O hydrogen bond between a hydrogen atom of the amino group of aniline in molecule Ia (Ib) and the phosphoryl oxygen atom of molecule Ib (Ia) (O…H 2.18 and 2.19 Å, N…O, 2.979(5) and 3.000(5) Å; NHO angle, 154° and 157°).     

Synthesis, complete 1H and 13C NMR assignment and crystal structure of novel epoxide derivatives of cytochalasin B

Five novel epoxide derivatives of cytochalasin B were synthesized. Reaction of cytochalasin B with t-BHP and BuLi led to selective epoxidation of the C-21/22 double bond to give a single monoepoxide, while reaction with m-CPBA yielded two diepoxides. Reaction of the monoepoxide with m-CPBA yielded two triepoxides. The relative configurations of the epoxides were elucidated by analogy with the natural product by means of spectroscopic methods; full assignment of NMR signals was achieved, and the absolute configuration was confirmed by X-ray crystallography.     

Calcium complex of 2,5-dihydroxybenzoic acid (gentisic acid): synthesis,crystal structure,and spectroscopic properties

The synthesis, crystal structure, and spectral characteristics (IR, Raman, and ¹H and ¹³C NMR) of a monomeric hexa(aqua)calcium compound, viz. [Ca(C₇H₅O₄)₂(H₂O)₆]·H₂O (C₇H₅O₄ = 2,5-dihydroxybenzoate), are reported. The central Ca(II) located on a twofold axis is eight coordinate in an approximate square antiprismatic environment, bonded to two monodentate 2,5-dihydroxybenzoate ligands via the carboxylate oxygen, and six waters. The adjacent monomeric units are linked with the aid of several O–H?O hydrogen bonds.     

Comprehensive 1H and 13C NMR assignment of 13 protobassic acid saponins

This study aimed to carry out complete ¹H and ¹³C NMR assignment of 13 protobassic acid saponins, including arganins A–C ( 1 – 3 ) and F ( 4 ), butyrosides B–D ( 5 – 7 ), tieghemelin ( 8 ), 3′-O-glucosyl-arganin C ( 9 ), Mi-saponins A–C ( 10 – 12 ), and mimusopsin ( 13 ), recorded in methanol-d₄. This was accomplished by the analysis of high-resolution one-dimensional (1D) NMR (¹H and ¹³C), two-dimensional (2D) NMR (¹H–¹H COSY, HSQC, and HMBC), and selectively excited 1D TOCSY spectra. Before this study, ¹H and ¹³C NMR data of arganins A–C ( 1 – 3 ) and F ( 4 ) were partially assigned. Our effort leads to their complete assignment, especially the glycon residue, and revises some reported data. Some revisions of the ¹H and ¹³C NMR data in the glycon part of butyroside C ( 6 ), tieghemelin ( 8 ), Mi-saponin A ( 10 ), and mimusopsin ( 13 ) were made. Those data of butyrosides B and D ( 5 & 7 ) and Mi-saponin B ( 11 ), which had not been recorded in methanol-d₄, are provided. In addition, the ¹H and ¹³C NMR data of Mi-saponin C ( 12 ) are reported for the first time. These data, being recorded in methanol-d₄, should be more friendly for use as a reference for identifying the related triterpenoid saponins.     

Phase transition of L-Ser monohydrate crystal studied by (13)C solid-state NMR

We used gravimetric analysis (GA) and (13)C solid-state nuclear magnetic resonance (NMR) to study solid-phase transition from the transparent single crystal of L-serine (L-Ser) monohydrate to a turbid powder. We found that L-Ser monohydrate loses water molecules and transforms into an anhydrate, thus experimentally demonstrating Frey's assumption. Application of a handmade cross-polarization (CP) NMR probe with a saddle-type coil to the oriented crystal of the L-Ser monohydrate revealed the dehydration mechanism. Furthermore, the chemical shift tensor components of the carboxyl carbon in L-Ser monohydrate were determined. The difference in the tensor component of delta(22) between the monohydrate and anhydrate forms was more than 7 ppm, probably owing to differences in the hydrogen-bonding structure of each form.     

Conformational analysis of 2,2'-bithiophene: a (1)H liquid crystal NMR study using the (13)C satellite spectra

We have obtained a very large data set of spectral parameters from the analysis of (1)H NMR and (13)C satellite spectra of 2,2'-bithiophene dissolved in anisotropic, partially orienting mesophases. In particular, this parameter set includes 33 dipolar couplings, which are directly related to the interatomic distances, the dihedral angle phi between the two thiophenic rings, and the anisotropic solute-solvent interaction potential. This allows an exhaustive investigation of the conformational equilibrium of 2,2'-bithiophene in a liquidlike phase. Comparison with the predictions of high-level theoretical calculations for the isolated molecule provides evidence of a strong flattening as well as the sharpening effect of the medium on the conformer population. The approximations needed to apply vibrational corrections to flexible molecules are discussed in detail and some general conclusions concerning their effect on structure and conformational equilibria are proposed.     

Order parameters based on (13)C(1)H, (13)C(1)H(2) and (13)C(1)H(3) heteronuclear dipolar powder patterns: a comparison of MAS-based solid-state NMR sequences

Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments. Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated, which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield-Rhim-Elleman-Vaughn (T-MREV) and Lee-Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled (13)C or (1)H nuclei, as well as due to B(1) inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B(1) field is highly homogeneous. The T-MREV experiment is far better compensated for B(1) inhomogeneity, and it also performs better in situations with limited signal.