Tizanidine and tizanidine hydrochloride: on the correct tautomeric form of tizanidine

A crystallization series of tizanidine hydrochloride, used as a muscle relaxant for spasticity acting centrally as an α₂‐adrenergic agonist, yielded single crystals of the free base and the hydrochloride salt. The crystal structures of tizanidine [systematic name: 5‐chloro‐N‐(imidazolidin‐2‐ylidene)‐2,1,3‐benzothiadiazol‐4‐amine], C₉H₈ClN₅S, (I), and tizanidine hydrochloride {systematic name: 2‐[(5‐chloro‐2,1,3‐benzothiadiazol‐4‐yl)amino]imidazolidinium chloride}, C₉H₉ClN₅S⁺·Cl⁻, (II), have been determined. Tizanidine crystallizes with two almost identical molecules in the asymmetric unit (r.m.s. deviation = 0.179 Å for all non‐H atoms). The molecules are connected by N—H...N hydrogen bonds forming chains running along [21]. The present structure determination corrects the structure determination of tizanidine by John et al. [Acta Cryst. (2011), E 67 , o838–o839], which shows an incorrect tautomeric form. Tizanidine does not crystallize as the usually drawn 2‐amino–imidazoline tautomer, but as the 2‐imino–imidazolidine tautomer. This tautomer is present in solution as well, as shown by ¹H NMR analysis. In tizanidine hydrochloride, cations and anions are connected by N—H...Cl hydrogen bonds to form layers parallel to (100).     

Methyl 3‐para‐anisyl‐4‐(2‐hydroxy­benzoyl)­isoxazole‐5‐carboxyl­ate

The title compound, C₁₉H₁₅NO₆, contains a planar isoxazole ring. An intramolecular hydrogen bond is formed between the OH group attached to a phenyl ring and a carbonyl O atom.     

The addition‐reaction product of 1,1,1,4,4,4‐hexa­chloro‐1,4‐disila­butane with N‐methyl­imidazole

The product of the addition reaction of 1,1,1,4,4,4‐hexa­chloro‐1,4‐disila­butane with N‐methyl­imidazole is μ‐ethyl­ene‐C¹:C²‐bis­[di­chloro­tris(1‐methyl­imidazole‐N³)­silicon(IV)] dichloride, C₂₆H₄₀Cl₄N₁₂Si₂²⁺·2Cl^?. Two of the six Cl atoms are replaced by aromatic nitro­gen bases and the coordination sphere of silicon is extended from four to six. The mol­ecule is located on a crystallographic centre of inversion. The environment around the Si atom can be described as a slightly distorted octahedron with the Cl atoms occupying axial positions and the three N‐methyl­imidazole ligands and the ethyl­ene bridge in the equatorial plane.     

Diethyl 9,10‐di­hydro‐9,10‐ethano­anthracene‐11,12‐trans‐di­carboxyl­ate

The title compound, C₂₂H₂₂O₄, is the product of the Diels–Alder reaction of anthracene with fumaric acid diethyl ester. The molecular C₂ symmetry is nearly fulfilled in the crystal. Only the terminal torsion angles about the O—CH₂ groups show significant differences.     

Packing considerations of bis‐dimedone derivatives

We have determined the crystal structures of 2,2′‐(4‐fluoro­phenyl)­methyl­enebis(3‐hydroxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one), C₂₃H₂₇FO₄, (I), 2,2′‐(4‐chloro­phenyl)­methyl­enebis(3‐hy­droxy‐5,5‐dimethyl‐2‐cyclo­hexen‐1‐one), C₂₃H₂₇ClO₄, (II), 2,2′‐(4‐hydroxy­phenyl)­methyl­enebis(3‐hydroxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one), C₂₃H₂₈O₅, (III), 2,2′‐(4‐methyl­phenyl)­methyl­enebis(3‐hydroxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one), C₂₄H₃₀O₄, (IV), 2,2′‐(4‐methoxy­phenyl)­methyl­enebis(3‐hy­droxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one), C₂₄H₃₀O₅, (V), and 2,2′‐(4‐N,N′‐di­methyl­amino­phenyl)­methyl­enebis(3‐hydroxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one), C₂₅H₃₃NO₄, (VI). Structures (III) to (VI) of these bis‐dimedone derivatives show nearly the same packing pattern irrespective of the different substituent in the para position of the aromatic ring. However, (II) does not fit into this scheme, although the Cl atom is a substituent not too different from the others. The different packing of the fluoro compound, (I), can be explained by the fact that it crystallizes with two mol­ecules in the asymmetric unit, which show a different conformation of the dimedone ring. On the other hand, (I) shows a similar packing pattern to bis(2‐hydroxy‐4,4‐di­methyl‐6‐oxo‐1‐cyclo­hexenyl)­phenyl­methane, a compound containing an aromatic ring without any substituent and with Z′ = 2.     

N‐tert‐Butyl‐N′‐(5,7‐di­methyl‐1,8‐naphthyridin‐2‐yl)­urea

The title compound, C₁₅H₂₀N₄O, has been synthesized as an AADD recognition unit for quadruple hydrogen bonds. All non‐H atoms of the mol­ecule apart from two methyl groups of the tert‐butyl group lie in a common plane. An intramolecular hydrogen bond is formed connecting two N atoms. In the solid state, the title compound crystallizes as a centrosymmetric dimer connected by N—H?O=C interactions with an N?O distance of 2.824 (2) Å.     

Fault Detection in DC Electro Motors Using the Continuous Wavelet Transform

Two time–frequency methods were used to detect typical faults in DC electro motors: the windowed Fourier transform and the continuous wavelet transform. Four groups containing three electro motors each were manufactured with typical faults and examined. These faults included a bearing fault, an increased unbalance, a fragmented brush and a fragmented collector. The velocity of the vibrations at selected points on the electro motors was measured with a laser probe. The parameters of both transforms were selected in order to make both methods comparable. Because of the poor frequency variance, the windowed Fourier transform was, in this case, proven to be inferior to the continuous wavelet transform. Therefore, the continuous wavelet transform was chosen as the primary tool for fault detection.Three criteria were found that successfully discriminated between the typical faults. These were the highest magnitude level, the frequency of the first and second harmonics and the time period between the magnitude pulses in the third (highest) frequency region. If the maximum magnitude levels versus the period of the pulses in the third frequency range are plotted, four distinct regions corresponding to four different faults are obtained. Since the regions do not overlap, linear classifiers can be used with the presented criteria.     

Synthesis and Chloride Affinity of Sterically Demanding Ditopic Lithium Bis(pyrazol‐1‐yl)borates

The synthesis and full characterization of the sterically demanding ditopic lithium bis(pyrazol‐1‐yl)borates Li₂[p‐C₆H₄(B(Ph)pz^R₂)₂] is reported (pz^R = 3‐phenylpyrazol‐1‐yl ( 3 ^Ph), 3‐t‐butylpyrazol‐1‐yl ( 3 ^tBu)). Compound 3 ^Ph crystallizes from THF as THF‐adduct 3 ^Ph(THF)₄ which features a straight conformation with a long Li···Li distance of 12.68(1) Å. Compound 3 ^tBu was found to function as efficient and selective scavenger of chloride ions. In the presence of LiCl it forms anionic complexes [ 3 ^tBuCl]⁻ with a central Li‐Cl‐Li core (Li···Li = 3.75(1) Å).