The Correlation of 81^Br NQR Frequency with p_K_b Value and with the N-H…Br Hydrogen Bond in Some Ammonium Bromide Salts

Summary 81 Br NQR of guanidinium bromide C(NH2)3Br(Ⅰ), hydrazinium bromide N2H5Br(Ⅱ), phenylenediammonium bromide H3NC6H5NH3Br2 (Ⅲ), an dd iphenylammonium bromide (C6H5)2NH2 Br (Ⅳ) are observed and these frequencies are well correlated with pKb value so f original amines. Thi st endency could b ee xplained by the strength of N-H…Br hydrogen bonds.The crystal structure of (Ⅰ) is orthorhombic (Pnma, Z = 4, a = 866.2(1), b = 680.9(1), and c = 833.3(1) pm).The crystal structure of (Ⅲ) i st riclinic (P1 with Z = 1, a = 610. 5(2), b = 872. 4(2), c = 448. 6(2) pm, a = 104. 77(2), b = 100. 98(3), g = 70. 23(2)° ).The observed distance so fN-H…Br hydrogen bonds in (Ⅰ) and (Ⅲ) are also correlated with 81 Br NQR frequencies.The results of EFG calculation of Br atoms i nt he giant molecule so f(Ⅰ) and (Ⅲ) cut off from each crystal structure could reproduce the difference of NQR frequency between (Ⅰ) and (Ⅲ).The temperatur ed ependence of 81 Br NQR frequencie so f(Ⅱ)(Ⅳ) showed normal feature which could b ee xplained b yl attice vibration (Baye rt heory), but no unique feature caused by hydrogen bond.