Crystallography

R. P. Shibaeva, L. O. Atovmyan

Crystal Structure of α-Trichloromethyl-N-methylolethylenimine $\mathrm{C_4H_6ONCl_3}$

(Presented by Academician N. V. Belov, 7 VIII 1964)

α-Trichloromethyl-N-methylolethylenimine

\[ \begin{array}{c} \mathrm{H_2C} \\ \quad \diagdown \ \ \diagup \\ \mathrm{H_2C}\quad \mathrm{N} - \mathrm{C} - \mathrm{OH} \\ \quad\quad\quad\quad | \\ \quad\quad\quad \mathrm{CCl_3} \end{array} \quad \begin{array}{c} \mathrm{H} \\ | \\ \end{array} \]

, one of the most active chemical mutagens, is a derivative of ethylenimine—a three-membered nitrogen-containing heterocycle.*

Very unstable single crystals of $\mathrm{C_4H_6ONCl_3}$ were grown from a saturated solution in ether by slow evaporation at constant temperature. Crystals of $\mathrm{C_4H_6ONCl_3}$ readily sublime in air and dissolve well in all common organic solvents, on the basis of which adhesives are prepared; therefore, to protect them from sublimation and decomposition during X-ray photography, the single crystals were first coated with a thin layer of paraffin and then with a layer of BF adhesive. The experimental X-ray diffraction material, obtained in a Weissenberg camera using Cu radiation, consists of a set of goniometric records of three equatorial layers of the lines $hk0$, $h0l$, $0kl$ and four nonzero layer lines $h1l$—$h4l$ (345 reflections with $F \ne 0$ in all).

The reflection intensities were estimated visually according to the usual $(\sqrt[4]{2})$ blackening scale. From rotation and Weissenberg photographs the unit-cell parameters were established: $a = 10.50 \pm 0.05$ Å; $b = 9.25 \pm 0.03$ Å; $c = 7.75 \pm 0.03$ Å. Systematic absences (among reflections of the type $h00$, $0k0$, $00l$ only those for which $h, k, l = 2n$ were present) fixed for $\mathrm{C_4H_6ONCl_3}$ the enantiomorphic space group $D_2^4 = P2_12_12_1$. With molecular weight $M = 190.47$ for $z = 4$, the calculated density is $d = 1.69\ \mathrm{g/cm^3}$.

The structural study was begun with an analysis of the Patterson projections $P(xz)$ and $P(xy)$. The presence in the composition of the compound $\mathrm{C_4H_6ONCl_3}$ of relatively heavy chlorine atoms ($z = 17$) made it possible to apply the superposition method $(^{2,3})$ for solving the structure. The projection $P(yz)$ was excluded from consideration at the first stage of the study, since it showed a regular arrangement of bands of maxima. The Patterson projection $P(xz)$ had first been modified by the function $M = \sin^2 \vartheta/\lambda^2$. Since in the group $P2_12_12_1$ all projections are centrosymmetric, the analysis of the Patterson projections $P(xz)$ and $P(xy)$ for “centrosymmetric triplets” of peaks $(^4)$ made it possible to isolate peaks for minimization. Minimization functions $M_2(xz)$ and $M_2(xy)$ were constructed, and then, using the sliding-reflection lines, we obtained $M_4(xz)$ and $M_4(xy)$. As a result, the positions of three Cl atoms were roughly localized, after which we proceeded to the three-dimensional distribution of electron density. First, an electron-density synthesis was calculated for 3Cl with factor $R = 44.1\%$; then, by the method of successive approximations, a very rough model of the structure was obtained. Refinement of this model by the least-

* The compound $\mathrm{C_4H_6ONCl_3}$ was synthesized at the Institute of Chemical Physics of the Academy of Sciences of the USSR by P. G. Kostyanovskii and kindly provided to us for the X-ray structural investigation.

...least squares reduced the \(R\)-factor only to 33.7% with extremely unsatisfactory interatomic distances. The next substantial step in determining the structure of \(\alpha\)-trichloromethyl-\(N\)-methylolethyleneimine was made through the use of a procedure for refining structures by the least-squares method with additional conditions, described by Waser \((^5)\) and programmed at the Institute of Chemical Physics of the Academy of Sciences of the USSR by V. I. Andrianov.

Additional conditions are understood to mean a priori known information on interatomic distances and bond angles in the structure being refined. (In the present article only the results obtained are reported, without a detailed description of the application of the least-squares method with additional conditions for refining the structure of \(\mathrm{C}_4\mathrm{H}_6\mathrm{ONCl}_3\).)

Fig. 1. Interatomic distances and bond angles in the structure of \(\mathrm{C}_4\mathrm{H}_6\mathrm{ONCl}_3\) (projection of the molecule along the \(b\) axis)

The study of this program for the structure described gave striking success. In the course of refinement the coordinates of all atoms changed substantially (for the Cl atoms along the \(a\) axis the maximum displacement proved to be 0.536 Å). The \(R\)-factor, calculated only for the three Cl atoms with the new coordinates, was 34.1%, i.e., 10.0% lower than the previously calculated \(R\)-factor, also for 3Cl. Therefore a three-dimensional electron-density distribution was constructed from the three Cl atoms. From this synthesis we practically obtained a reasonable model of the structure with quite satisfactory interatomic distances. Subsequently, for refinement of the structure, a combination of the ordinary least-squares method and the least-squares method with additional conditions was used. The final atomic coordinates, individual isotropic temperature corrections \(B_j\), and heights of the electron-density maxima \(\rho\) are given in Table 1.

Table 1

The reliability factor, calculated from the final coordinates for all nonzero reflections, is equal to 15.7%. The errors in the determination of the atomic coordinates, calculated according to the formulas of B. K. Vainshtein \((^6)\) and M. A. Porai-Koshits \((^7)\), are: \(\varepsilon(\mathrm{Cl}) = \pm 0.004\) Å; \(\varepsilon(\mathrm{O}) = \pm 0.011\) Å; \(\varepsilon(\mathrm{N}) = \pm 0.013\) Å; \(\varepsilon(\mathrm{C}) = \pm 0.016\) Å.

The interatomic distances and bond angles in the structure of \(\alpha\)-trichloromethyl-\(N\)-methylolethyleneimine are shown in Fig. 1. The accuracy of determination of the bond angles, calculated according to Darlow’s formulas \((^8)\), is \(\pm 0^\circ 50'\),

and the maximum standard deviation in the determination of interatomic distances for light atoms is \(\Delta r = \pm 0.023\) Å. The structural motif of \(\mathrm{C_4H_6ONCl_3}\) is clearly seen from its projection onto the \(yz\) plane (Fig. 2). The \(\mathrm{C_4H_6ONCl_3}\) molecules are connected with one another by hydrogen bridges \(\mathrm{O—H\ldots N}\) into chains parallel to the \(b\) axis, with two such chains per period \(c\). The length of the hydrogen bond \(\mathrm{O—H\ldots N}\) is 2.75 Å; the angle that it makes with the \(\mathrm{C—O}\) bond is \(114^\circ 12'\). Within the molecule itself, the angle between the \(\mathrm{C—N}\) bond and the plane of the three-membered ring is \(126^\circ 18'\).

Fig. 2. Projection of the structure of \(\alpha\)-trichloromethyl-\(\mathrm{N}\)-methylol-ethyleneimine \(\mathrm{C_4H_6ONCl_3}\) onto the \(yz\) plane

We express our gratitude to the members of the X-ray structural group of the Mathematical Department of the Institute of Chemical Physics, Academy of Sciences of the USSR, for the computational work.

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
5 VIII 1964

REFERENCES

1. R. G. Kostyanovskii, DAN, 139, No. 4, 879 (1961).
2. M. Buerger, Crystal Structure and Vector Space, IL, 1961.
3. V. I. Simonov, Crystallography, 4, 3 (1959).
4. Kh. S. Mamedov, N. V. Belov, DAN, 106, 462 (1956).
5. J. Waser, Acta Cryst., 16, 11, 1091 (1963).
6. B. K. Vainshtein, ZhETF, 27, 1 (1954).
7. M. A. Porai-Koshits, Practical Course in X-ray Structural Analysis, Moscow, 1961.
8. C. F. Darlow, Acta Cryst., 13, 6, 683 (1960).