The crystal structures of salts of methylated purines and pyrimidines. IV. 9-Methylguanine hydrobromide

The bromine ion on one plane is situated directly above the pyrimidine ring of uracil on the plane below. The distances are as follows: Br-N(1) 3.45 A, Br-C(6) 3.65 J~, Br-C(5) 3.82 ~, Br-C(4) 3.78 /~, Br-N(3) 3.59 A, Br-C(2) 3.43 A. If one considers the half ~hickness of the aromatic molecule to be 1.7 A, and the ionic radius of bromine to be 1.95 J~, it is evident that ~he observed distance of 3.43 Jk must reflect a shortening of the carbon van der Waals radius in the direction of the center of the pyrimidine ring. The Br-N(1) distance of 3.45 _~ is considerably shorter than the other bromine-light atom distances. This may be due to electrostatic interaction between the bromine ion and the positively charged nitrogen N(1) atom associated with resonant form (I). The 0(2) oxygen of an adjacent molecule is roughly equidistant from C(6) and C( The angle subtended by atoms C(4)-0(4)... Br(1) is 115.3 , which is in good agreement with the C-O 0 angle of 114 found in formic acid (Jones & Templeton, 1958).