3-(2-Chloroethyl)-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one

In the title molecule, C11H11ClN2O, the pyrido[1,2-a]pyrimidine ring system is planar (maximum deviation = 0.0148 Å) and the methyl C and carbonyl O atoms are nearly coplanar to it. The chloroethyl side chain is in a synclinal conformation, nearly orthogonal to the pyrimidine ring, with a dihedral angle between the chloroethyl side chain and the pyrimidine ring of 88.5 (1)°. Weak intermolecular C—H⋯N and C—H⋯Cl hydrogen bonds along with π–π interactions between the pyrimidine and pyridine rings [centroid–centroid distance is 3.538 (2) Å] form a three-dimensional network. The crystal is a racemic twin with a 0.68 (12):0.32 (12) domain ratio. MOPAC AM1 and density functional theory (DFT) theoretical calculations at the B3-LYP/6–311+G(d,p) level support these observations.

In the title molecule, C 11 H 11 ClN 2 O, the pyrido[1,2-a]pyrimidine ring system is planar (maximum deviation = 0.0148 Å ) and the methyl C and carbonyl O atoms are nearly coplanar to it. The chloroethyl side chain is in a synclinal conformation, nearly orthogonal to the pyrimidine ring, with a dihedral angle between the chloroethyl side chain and the pyrimidine ring of 88.5 (1) . Weak intermolecular C-HÁ Á ÁN and C-HÁ Á ÁCl hydrogen bonds along withinteractions between the pyrimidine and pyridine rings [centroid-centroid distance is 3.538 (2) Å ] form a three-dimensional network. The crystal is a racemic twin with a 0.68 (12):0.32 (12) domain ratio. MOPAC AM1 and density functional theory (DFT) theoretical calculations at the B3-LYP/6-311+G(d,p) level support these observations.
QNMHA thanks the University of Mysore for use of its research facilities. RJB acknowledges the NSF MRI program (grant No. CHE-0619278) for funds to purchase an X-ray diffractometer.
The title compound, (I), is an intermediate in the synthesis of risperidone, which is a potent antipsychotic agent, especially useful for treating schizophrenia (Gabbert & Giannini, 1997). In view of the importance of (I), the present paper describes its crystal structure.
While no classic hydrogen bonds are observed, a weak intermolecular hydrogen bond interaction exists between atom C5 from the pyridine ring and N2 from a nearby pyrimidine ring (Table 1 and Fig. 2). In addition, a weak intermolecular interaction between atom C2 from the pyrimidine ring and Cl from the substituted pyrimidine group also occurs, each influencing crystal packing and, therefore, resulting in a three-dimensional network (Fig. 2). In addition, π-π interactions between N1/C1/C9/C7/N2/C6 (centroid Cg1) and N1/C2-C6 (centroid Cg2) rings of molecules at (x, y, z) and (1+x, y, z), supplementary materials sup-2 with a Cg1···Cg2 distance of 3.538 (2) Å, provide additional stability to the crystal packing. The crystal is a racemic twin with domains of 0.68 (12) and 0.32 (12).
In summary, it is clear that the collection of weak intermolecular hydrogen bond interactions and π-π intermolecular interactions do play a role in stabilizing crystal packing of (I).

Refinement
All of the H atoms were placed in their calculated positions and then refined using the riding model with C-H = 0.95-0.99 Å, and with U iso (H) = 1.18-1.50U eq (C). Fig. 1. Molecular structure of (I), showing the atom labeling scheme and 50% probability displacement ellipsoids.