2950 Acta Cryst. (1978). B34, 2950-2953 The Structure of N-Methylpyddiniurn Iodide BY R. A. LALANCETTE, W. FUREV, J. N. COSTANZO, P. R. HEMMES AND F. JORDAN CarlA. Olson Chemistry Laboratories, Rutgers, The State University of New Jersey, Newark, New Jersey 07102, USA (Received 20 January 1978; accepted 9 May 1978) Abstract. C6HslN, orthorhombic, P2~2~2~, a = 7.744(3), b = 8.541(3), e = 12.045 (4) A, V = 796.7 (5) A a. 1057 unique reflections were measured on an automatic diffractometer. The crystal structure was solved by the heavy-atom technique and Fourier maps, and refined by full-matrix least-squares methods to R = 0.066. The N-methylpyridinium ring is very planar with a N-I distance of 3.76 A (out of plane) and C-I distances of 3.90 and 3.98/k (in plane). Introduction. N-Methylpyridinium iodide (NMPI) is a compound of which the long-wavelength UV spectral behavior in solution has been interpreted as arising from the formation of a charge-transfer complex (Kosower, 1955; Kosower & Burbach, 1956). In these laboratories we have conducted both experimental (Hemmes, Costanzo & Jordan, 1973, 1978) and theoretical studies (Jordan, 1975) aimed at the elucida- tion of the types of forces responsible for the complex formed. We have interpreted the experimental data (both thermodynamic and ultrasonic relaxation kinetic) to mean that there are two types of ion pairs present in solution: I- + N+ H20 N+ I- I solvent-separated and intimate ion pairs. The thermo- dynamic data collected on ionic association in this system as a function of solvent dielectric constant, as well as the results of the theoretical calculations, strongly suggested that the association process is driven predominantly by electrostatic forces. In addition, however, the theory employed (both STO-3G minimal basis set ab initio and CNDO/2) suggested that in the intimate ion-pair structure the C1 ion (employed instead of the I- on account of the intractability of the ab initio approach on I-) is not located on the twofold symmetry axis of the N-methylpyridinium ion, rather it is nearer to a C atom (the one adjacent to the N) with a very short C-CI nonbonding contact of 2.3-2.5/k. In order to better understand the system we resorted to a single- crystal X-ray determination of NMPI and here report the results of our studies. A slightly elongated colorless crystal was selected from a batch of NPMI prepared by mixing equimolar amounts of methyl iodide and dry pyridine in an excess of dry acetone. Both the pyridine and acetone were dried over molecular sieves, as the NPMI is extremely hygroscopic. To prevent later absorption of water the crystal was immediately sealed in a glass capillary with epoxy cement. Preliminary Weissenberg and precession photograhs indicated an orthorhombic system. Exam- ination of the hOl and the Okl layers taken with Cu Ka radiation (Ni-filtered) produced the following sys- tematic absences: in h00, h = 2n + 1; in 0k0, k = 2n + 1; in 00l, l = 2n + 1. These conditions uniquely deter- mine the space group to be P2~2~21 and successful solution and refinement of the structure confirm this assignment. A new crystal of dimensions 0.45 x 0.25 x 0.20 mm was sealed in a capillary with epoxy cement and was placed on a Syntex P21 computer-controlled automatic diffractometer where the cell parameters were obtained using 15 accurately centered reflections and Mo Ka radiation (graphite-monochromatized). The temperature was 24(1)°C. The calculated density assuming four molecules of molecular weight 221.04 per unit cell is 1.85 gcm -3. Because of the extremely hygroscopic nature of the crystal an observed density was not obtained; however, a final difference map after' refinement of the structure based on the four molecules described above indicated that all scattering matter in the cell had been accounted for. Assuming the crystal to be a cylinder of radius 0.011 cm and # = 39.9 cm -~ the absorption factor for 0 < 0 < 30 ° ranges from 1.95 to 1-92, a change of 1.5%; thus absorption corrections were considered un- necessary and were not applied. Intensity data were collected at 24 °C by the 0--20 scan technique out to a maximum 20 value of 60 ° . Three standard reflections were collected every 47 reflections throughout the data collection and showed variations of +2.3% but no significant trends. All intensities collected were correc- ted for any variations by applying decay factors calculated from the standard-reflection intensities. A