Key indicators: single-crystal X-ray study; T = 293 K; mean (C-C) = 0.003 A ˚ ; R factor = 0.045; wR factor = 0.117; data-to-parameter ratio = 14.3. In the crystal structure of the title compound, C 12 H 10 N 6 , the molecules deviate slightly from planarity. The plane of the central triazole ring makes angles of 6.13 (9) and 3.28 (10) with the pyridyl ring planes. Intramolecular N-HÁ Á ÁN interactions form six-membered closed rings. The crystal packing also shows weak C-HÁ Á Á and C-HÁ Á ÁN interactions. Related literature For related literature, see: Dirtu et al. (2007); Faulmann et al. Experimental Crystal data C 12 H 10 N 6 M r = 238.26 Monoclinic, P2 1 =c a = 6.6191 (2) A ˚ b = 14.7136 (4) A ˚ c = 11.4703 (4) A ˚ = 95.474 (2) V = 1112.01 (6) A ˚ 3 Z = 4 Mo K radiation = 0.09 mm À1 T = 293 (2) K 0.20 Â 0.13 Â 0.12 mm Data collection Bruker APEX CCD area-detector diffractometer Absorption correction: multi-scan (SADABS; Sheldrick, 2000) T min = 0.91, T max = 0.99 23253 measured reflections 2751 independent reflections 1465 reflections with I > 2(I) R int = 0.052
The electronegativity dependence of the torsion angle-independent term in the Karplus equation, i.e. of the 'constant' A in the Fourier expansion A + B cos 4 + C cos 24 + . . . , was investigated. Experimental proton-proton coupling constants of substituted ethanes and isopropanes appeared to be suitable for this purpose. A data set was constructed which contained 70 couplings, newly measured or remeasured at 300 MHz, and 25 couplings taken from the literature. The accuracy of each data point is estimated as GO.02 Hz, with a few exceptions. The actual analysis was carried out on 93 data points, i.e. on J values of 55 mono-and of 38 l,l-di-substituted ethanes, including 22 isopropyl derivatives. A total of 55 chemical groups is represented in the set; some of these were taken together, leaving 50 distinct groups. Regression analysis of the present data versus standard electronegativities did not yield acceptable results. Instead, substituent parameters A ' , valid for 'J(HH) in saturated H -C -C -H fragments, were derived in a least-squares procedure from the data set. The couplings from mono-and 1,l-di-
Chemical shifts of exchangeable and non-exchangeable protons in double-helical B DNA fragments were collected from the literature , subjected to statistical analysis and tabulated. The tables are arranged according to the central residue involved (A, G, C and T) and to the 5 0 -and 3 0 -flanking residues (triplet model). The standard deviations in the mean (90% confidence limits) are of the order of 0.02-0.04 ppm for most protons. In addition, data on terminal residues were collected (terminal doublets) and it is shown that these shifts play a key role in the empirical determination of physically meaningful shielding/deshielding increments induced by the 5 0 -and 3 0 -flanking bases in triplets. The r.m.s. error of the shifts predicted with the aid of our new incremental scheme is 0.01-0.03 ppm. The analysis throws new light on the mechanism involved in the fraying process.
Stereochemical dependences of vicinal fluorine-fluorine nuclear magnetic resonance coupling constants (3JFF) have been studied with the multiconfigurational self-consistent field in the restricted active space approach, with the second-order polarization propagator approximation (SOPPA), and with density functional theory. The SOPPA results show the best overall agreement with experimental couplings. The relationship with the dihedral angle between the coupled fluorines has been studied by Fourier analysis, the result is very different from that of proton-proton couplings. The Fourier coefficients do not resemble those of a typical Karplus equation. The four nonrelativistic contributions to the coupling constants of 1,2-difluoroethane configurations have been studied separately showing that up to six Fourier coefficients are required to reproduce the calculated values satisfactorily. Comparison with Fourier coefficients for matching hydrogen fluoride dimer configurations suggests that the higher order Fourier coefficients (Cn> or =3) originate mainly from through-space Fermi contact interaction. The through-space interaction is the main reason 3JFF do not follow the Karplus equation.
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