Aeta Cryst. (1984). C40, 413-415 Structure of Trimethylamine, C3HgN , at 118 K BY ALEXANDERJ. BLAKE, E. A. V. EBSWORa'I-I AND ALAN J. WELCH Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3J J, Scotland (Received 19 August 1983; accepted 24 October 1983) 413 Abstract. M r=59.1, P3, a=6.1366(29), c= 6.8516 (28) A, U= 223.5 (3)/k 3, Z= 2, Dx= 0.878 Mg m -a, Mo Ka, 2 = 0.71069 ,/k, /t = 0.058 mm -~, F(000)= 68, R = 0.0559 for 384 reflec- tions. The structure consists of discrete molecules arranged head-to-tail in columns which run alternately parallel and antiparallel to the c crystallographic axis. Individual molecules have effective Car symmetry, with a pyramidal geometry at N [C-N-C = 110.40 (7)°], and the C-N bond length is 1.4535 (11)/~. The conformation about the C-N bond is such that the symmetry-unique H atom is anti to the N lone pair of electrons. Introduction. Although the crystal structures of adducts of trimethylamine with iodine (Stromme, 1959), iodine chloride (Hassel & Hope, 1960), and sulphur dioxide (van der Helm, Childs & Christian, 1969), as well as the 10.25H20 clathrate (Panke, 1968), have been known for several years, molecular parameters were not accurately determined. Moreover, no solid-state struc- ture of NMe 3 itself has been published, although a gas-phase electron diffraction study (Beagley & Hewitt, 1968), and a microwave determination (Wollrab & Laurie, 1969) have been reported. As part of continuing work in this Department on low-melting inorganic compounds, we are engaged in study of members of the homologous series N(CH3)3_ x- (SiH3) x (x = 0-3) (Barrow & Ebsworth, 1984; Blake, Ebsworth & Welch, 1984). Apart, therefore, from its intrinsic interest, the accurate crystal structure of trimethylamine serves as a basis for comparisons with the silylamines of this series. Experimental. Colourless, cylindrical crystal, 0.05 × 0.04 x 0.04cm, grown in situ on low-temperature- equipped Weissenberg goniometer from sample sealed in Pyrex capillary. Space group identified by com- bination of oscillation and Weissenberg photography, E statistics, and successful refinement. Crystal transferred without melting as described previously (Blake, Cradock, Ebsworth, Rankin & Welch, 1984) to similarly equipped CAD-4 diffractometer, 118 K, 25 reflections (13 < 0 < 15 °) centred, graphite-mono- chromated Mo Ktx radiation. For data collection 0max 0108-2701/84/030413-03501.50 = 30 °, 09-20 scan in 96 steps, a,,-scan width (0.8 + 0.35 tan0) °. Rapid prescan after which reflections with I > 0.5 tr(/) remeasured such that final net intensity had I > 33tr(/) subject to maximum measuring time of 90 s. No significant crystal decay or movement. 440 unique reflections over 26 X-ray hours. No absorption cor- rection. For structure solution and refinement 384 amplitudes [F _> 2.0tr(F)]. Automatic centrosymmetric direct methods (Sheldrick, 1976). Full-matrix least- squares (F), w-1= [o2(F) + 0.002064(F)2], aniso- tropic thermal parameters for C and N, isotropic for H, R 0.0559, wR 0.0681, data:variable ratio 15:1. (A/tr)max in final cycle <0.03. Max. peak and min. trough in final AF synthesis 0.43 and -0.25 e A -a respectively. No correction for secondary extinction. Neutral-atom scattering factors for C and N (Cromer & Liberman, 1970) and H (Stewart, Davidson & Simpson, 1965). Computer programs SHELX76 (Shel- drick, 1976). XANADU (Roberts & Sheldrick, 1976), XRA Y76 (Stewart, Machin, Dickinson, Ammon, Heck & Flack, 1976)and ORTEPII (Johnson, 1976). Discussion. Table 1 lists derived fractional coordinates and thermal parameters.* Fig. 1 is a projection of a single molecule in a direction normal to the crystal- lographically imposed C 3 molecular axis, and clearly demonstrates that the effective molecular symmetry is C3v. Within this symmetry there are two possible molecular conformations, corresponding to syn and anti orientations of the unique H atoms relative to the N lone pair of electrons. The former would be severely crowded intramolecularly, and the determined structure clearly has the latter stereochemistry. Table 2 lists interatomic separations and interbond angles. Tri- methylamine crystallizes head-to-tail in loosely packed (as evidenced by the relative U11 and U33 values) columns running alternately parallel and antiparallel to the c crystallographic axis, as shown in Fig. 2 (deposited). The structure of trimethylamine in the crystal is very similar to that determined in the vapour phase by * Lists of structure factors, Fig. 2, and Table 3 have been deposited with the British Library Lending Division as Supplemen- tary Publication No. SUP 38977 (6 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystal- lography, 5 Abbey Square, Chester CH 1 2HU, England. © 1984 International Union of Crystallography