Matrix Isolation Infrared Spectra of Hydrogen Halide and Halogen Complexes with Nitrosyl Halides

Matrix isolation infrared spectra of nitrosyl halide (XNO) complexes with HX and X2 (X = Cl, Br) are presented. The r elative frequency shifts of the HX mode are modest (ClNO · H-Cl, !':..:v/v = = 0.045; BrNO·H-Br, Av/v = ·-0.026), indicating weak hydrogen bonds 1-3 kcal/mo!. These shifts are accompanied by significant shifts to higher frequencies in the XN-0 stretching mode (CIN-O · HCl, Av/v = +0.016; BrN-O·HBr, Av/v = +0.011). Similar .;hifts were observed for the XN-0 · X2 complexes (ClN-0 Cl2, Aviv = +0.009; BrN-0-Br2, Av/v = +0.013). In all four complexes, the XNO stretching mode relative shift is opposite in sign and about 1.6 times that of the NO stretching mode. These four complexes are considered to be similar in structure and charge distribution. The XN0 frequency shift suggests that complex formation is accompanied by charge withdrawal from the NO bond ranging from about .04 to .07 electron charges. The HX and X2 molecules act as electron acceptors, drawing electr ons out of the antibonding orbital of NO and strengthening the XN-0 bond. The implications of the pattern of vibrational shifts concerning the structure of the complexes are discussed.