4-Ethynyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine N-oxide

The six-membered ring of the title compound, C11H16NO, has a distorted envelope conformation. The piperidine N atom deviates by 0.128 (1) Å from the plane through its three neighbouring atoms. In the crystal structure, molecules are connected by intermolecular Cethynyl—H⋯O contacts to form chains extending in the [10] direction.

The six-membered ring of the title compound, C 11 H 16 NO, has a distorted envelope conformation.The piperidine N atom deviates by 0.128 (1) A ˚from the plane through its three neighbouring atoms.In the crystal structure, molecules are connected by intermolecular C ethynyl -HÁ Á ÁO contacts to form chains extending in the [101] direction.

S1. Comment
For EPR measurements of RNA, DNA or proteins, the occurrence of paramagnetic species is required.The title compound is a nitroxide spin label compound.Its synthesis and application for DNA labeling have been reported by Gannett et al. (2001).Frolow et al. (2007) reported an improved synthesis of the compound and its coupling to uridine.
Here we report on the crystal structure of the title compound.
The molecular structure of the title compound is shown in Fig. 1.The geometrical parameters in the title compound are very similar to those in the 2,2,6,6-tetramethyl-1-oxyl-3,4-dehydropiperidine fragment of closely related molecules (Igonin et al., 1990;Wiley et al., 1991;Shklover et al., 1990).The six-membered ring has a distorted envelope conformation with atoms N1 and C5 deviating by 0.186 (1) and 0.725 (2) Å, respectively, in the same direction from the mean plane through atoms C1-C4 [planar to within 0.005 (1) Å].Atom N1 shows a small degree of pyramidalization.The sum of the three valence angles about N1 is 357.6 (1)° and it deviates by 0.128 (1) Å from the plane through the three neighbouring atoms, O1, C1 and C5.
In the crystal structure molecules are connected by intermolecular C ethynyl -H•••O contacts to form chains extending in the [1 0 -1] direction (Fig. 2 and Table 1).

S2. Experimental
The synthesis of the title compound has been reported by Frolow et al. (2007).Crystals were obtained by sublimation at atmospheric pressure.

Special details
Geometry.All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.planes.Refinement.Refinement of F 2 against ALL reflections.The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 .The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Figure 1
Figure 1The molecular structure of the title compound, shown with 50% probability displacement ellipsoids.H atoms are drawn as small spheres of arbitrary radiius.

Figure 2 F
Figure 2 The crystal packing of the title compound, viewed down the a axis.Intermolecular C ethynyl -H•••O contacts are shown as dashed lines.
Symmetry code: