Figure 2: (a) Molecular chain formed by melamine and uracil molecules, (b) the aperture formed by the triply hydrogen-bonded melamine-uracil pairs.

Mentions: The crystal structure determination gave the asymmetric unit as shown in Figure 1a. The molecular complex crystallizes in C2/c space group with single molecules of melamine and uracil in the asymmetric unit. In this 1:1 complex, both melamine and uracil offer several donor-acceptor sites for hydrogen bonding and each melamine molecule engages in hydrogen bonding via N-H...O and N-H...N bonds with three uracil and two melamine molecules, as shown in Figure 1b (see Table 2). It is clear that the amino groups of the melamine molecule act as hydrogen bond donors and ring nitrogens as acceptors. Unlike with adenine, uracil forms two N-H...O bonds (H9....O2, 2.08Å; H7...O1, 2.03Å) on either side of the N-H...N bond (H2...N6, 1.98Å). All the three hydrogen bonds are strong, with the hydrogen bond angles in the range of 172 – 179°. These hydrogen-bonded pairs are interconnected through a N-H...O hydrogen bond to form a molecular chain as shown in the Figure 2a. The triply bonded melamine-uracil pairs are stacked parallely and the chains run along both a and b axes. Such an arrangement of the chains gives rise to an aperture structure as shown in Figure 2b. Four melamine and two uracil molecules thus enclose the aperture. The apertures are connected through N-H...O (H10...O1, 2.02Å) bonds giving rise to infinite channels along both a and b directions. We show one perspective in Figure 3, where the channels run along the a-direction. It is interesting to note that melamine engages in hydrogen bonding utilizing all the donor-acceptor sites, much the same way it does in the cyanuric acid adduct. [1] However, the channel structure is somewhat different, in shape and dimension. This clearly indicates the role of the uracil molecule in directing the shape of the channel structure.

Thomas R, Kulkarni GU - Beilstein J Org Chem (2007)

Bottom Line: The structure of a 1:1 complex of uracil and melamine obtained by cocrystallization has been investigated.The structure involves hydrogen bonded layers with apertures stacked along the a and b directions giving rise to channels, unlike the complex of cyanuric acid with melamine.

Affiliation: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore - 560064, India. reji@jncasr.ac.in

Abstract: The structure of a 1:1 complex of uracil and melamine obtained by cocrystallization has been investigated. The structure involves hydrogen bonded layers with apertures stacked along the a and b directions giving rise to channels, unlike the complex of cyanuric acid with melamine.