Infrared absorption spectroscopy of gas-phase N2O–HX (X=F, Cl, Br) weakly bonded complexes utilizing the N2O ν3 chromophore

Abstract

Rovibrational absorption spectra of weakly bonded complexes of N2O 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 CO2–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 Rcm 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.