INFRARED-ABSORPTION SPECTROSCOPY OF GAS-PHASE N2O-HF, N2O-HCL, N2O-HBR WEAKLY BONDED COMPLEXES UTILIZING THE N2O NU-3 CHROMOPHORE

Rovibrational absorption spectra of weakly bonded complexes of N 2O with HF, DF, HCl, and HBr were recorded in the ν3 region of N2O by using pulsed, slotted nozzle expansions and tunable diode lasers. A fast-scan technique was used that takes advantage of the rapid tuning capabilities of diode lasers; i.e., 4000 resolution elements were recorded with a single opening of the nozzle. Of the two known NH- and OH-bonded isomers of N2O-HF, we detected only linear ONN-HF; the ground-state rotational constants are in excellent agreement with previous microwave and IR results. Deuteration resulted in ONN-DF linewidths that are much narrower than those of ONN-HF, as observed previously in studies of the analogous CO 2-H(D)F system. Vibrational band origins for ONN-HF and ONN-DF are blue shifted 21.8 and 23.4 cm-1, respectively, relative to uncomplexed N2O. The additional blue shift upon deuteration is attributed to enhanced hydrogen bonding in a highly anharmonic potential. High-resolution spectra of NNO-HCl and NNO-HBr are presented for the first time. The average NNO-HCl geometry is asymmetric, with the separation between the N2O and HCl centers-of-mass Rcm equal to 3.51 Å. The angle between Rom and the NNO principal axis θ1 is 72°-76°. NNO-HBr complexes are also asymmetric (θ1 = 75°-82°) with Rcm = 3.62 Å. Linear ONN-HCl(Br) isomers were not observed. Blue shifts in the NNO-HCl and NNO-HBr band origins are 2.44 and 1.86 cm-1, relative to uncomplexed N2O. The qualitative changes observed in the NNO-HX geometries and force fields are attributed to competing effects arising from hydrogen-bonding and dispersion forces, as were observed with CO2-HF(Cl) and CO2-HBr. The experimental geometries and vibrational frequencies are compared to ab initio calculations; agreement with N2O-HF is good, CO2-HCl less so. Although the H atom position cannot be determined experimentally with NNO-HCl(Br), ab initio estimates suggest it is localized near the O atom. Implications for photoinitiated reactions in weakly bonded complexes are discussed. © 1990 American Institute of Physics.