Redetermination of 3-methylisoquinoline at 150 K

The structure of the title compound, C19H9O, has been redetermined at 150 K. The redetermination is of significantly higher precision than a previous room-temperature structure [Ribar et al. (1974 ▶). Cryst. Struct. Commun. 3, 323–325]. The C—N bond lengths for this redetermination are much closer to those observed in comparable structures, and the orientation of the methyl group with respect to the isoquinoline plane is clarified. Intermolecular weak C—H⋯N contacts are present in the crystal.

The structure of the title compound, C 19 H 9 O, has been redetermined at 150 K. The redetermination is of significantly higher precision than a previous room-temperature structure [Ribar et al. (1974). Cryst. Struct. Commun. 3, 323-325]. The C-N bond lengths for this redetermination are much closer to those observed in comparable structures, and the orientation of the methyl group with respect to the isoquinoline plane is clarified. Intermolecular weak C-HÁ Á ÁN contacts are present in the crystal.

Related literature
For the structure at room temperature, see: Ribar et al. (1974). For the structure of the parent compound isoquinoline, see: Hensen et al. (1999). The C-N bond length in the structure of Ribar et al. (1974) clearly lies outside of the main distribution for 19 relevant structural fragments in the Cambridge Structural Database, being the second shortest bond in the sample [one shorter bond exists for refcode SAKCIQ, but this structure has R1 = 14.2% (Trumpp-Kallmeyer et al., 1998)]. The corresponding C-N bond length in this redetermination lies exactly at the mean of the CSD sample.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 ; (ii) x; Ày þ 3 2 ; z À 1 2 .

Comment
The structure of 3-iso-methylquinoline at room temperature has been reported by Ribar et al. (1974). This redetermination at 150 K provides significantly improved precision, and more regular positions for the H atoms.
Concerning the H atoms, the orientation of the methyl group in particular is clarified: in the structure of Ribar et al., the H-C(methyl)-H angles are irregular (range 94.8-112.8 °) and the orientation of the group is such that one C-H bond is twisted from the isoquinoline plane with a C-C-C(methyl)-H torsion angle ca 22 °. In the redetermination, the refined orientation of the methyl group places one C-H bond much more clearly in the isoquinoline plane (torsion angle 5.8 (1)°) . This also has an influence on the geometry observed for the intermolecular contact between the methyl group and a neighbouring isoquinoline molecule. In the redetermination, atom H11B lies over the centroid of the C5-C10 ring with H11B···Cg = 2.95 Å and C11-H11B···Cg = 131.9 Å.

Experimental
The colourless block of (I) used for structure determination was taken directly from the sample as supplied by Aldrich Chemical Company.

Refinement
H atoms bound to C(sp 2 ) were positioned geometrically with C-H = 0.95 Å and refined as riding with U iso (H) = 1.2 U eq (C).
The H atoms of the methyl group were positioned with C-H = 0.98 Å and refined as riding with U iso (H) = 1.5 U eq (C), and with rotation about the local 3-fold axis. Fig. 1 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 Rfactors(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.

Figures
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )