Notes: 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 A(ring). 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 A(ring). 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.

Notes: A high resolution rovibrational absorption spectrum of the weakly bonded CO2–DBr complex has been recorded in the 2350 cm−1 region by exciting the CO2 asymmetric stretch vibration with a tunable diode laser. The CO2–DBr band origin associated with this mode is 2348.2710 cm−1, red-shifted by 0.87 cm−1 from uncomplexed CO2. The position of the hydrogen atom is determined from differences in moments-of-inertia between CO2–DBr and CO2–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO2–H(D)Br has an average geometry that is planar and inertially T-shaped, with essentially parallel HBr and CO2 axes. Average values of intermolecular parameters are: Rcm=3.58 A(ring), θBrCO=79.8°, and θHBrC=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO2–HBr is discussed. The experimental structure is corroborated qualitatively by results from Møller–Plesset second-order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T-shaped complex is planar with structural parameters: RCBr=3.62 A(ring), θBrCO=89°, and θHBrC=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero-point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out-of-plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in-plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with ROH=2.35 A(ring) at equilibrium is calculated to be as stable as the T-shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO2–HX complexes are discussed.

Notes: Infrared absorption spectra associated with the CO2 asymmetric stretch vibration have been recorded for weakly bonded gas-phase complexes of CO2 with HF, DF, HCl, DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15×38 mm2) that provides 〉20 MHz overall resolution. Results obtained with CO2–HF are in agreement with earlier studies, in which the HF-stretch region near 3900 cm−1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO2–DF, the natural linewidths are markedly narrower than with CO2–HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO2–HF. Both CO2–HF and CO2–DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigned to either a slightly asymmetric rotor with Ka=1, or a hot band involving a low-frequency intermolecular bend mode.Results obtained with CO2–HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the vibrationally excited state is almost identical to the lower state. Like CO2–DF, linewidths of CO2–HCl and CO2–DCl are much sharper than those of CO2–HF, and in addition, CO2–HCl and CO2–DCl exhibited weak hot bands, as were also evident with CO2–HF and CO2–DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO2 underwent a blue shift relative to uncomplexed CO2. This can be understood in terms of the nature of the hydrogen bonds, and ab initio calculations are surprisingly good at predicting these shifts. Deuteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero-point energy, and in a highly anharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,CO2–HBr is asymmetric, with the Br–C symmetry line essentially perpendicular to the CO2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive to the location of the H atom in CO2–HBr, Bose–Einstein statistics require that odd K‘a states are missing for C2v symmetry, as is observed with T-shaped CO2–(rare gas) complexes. However, we observe a full complement of odd and even Ka states, indicating that the H atom is not located symmetrically about the C2v axis on the time scale of the measurement. With CO2–HBr, the low gas-phase acidity of HBr and the high Br-atom polarizability encourage a qualitative change in the geometry relative to CO2–HCl and CO2–HF. This has valuable implications for photoinitiated reactions in such complexes.

Notes: The rovibrational absorption spectrum of CO2–Ar has been recorded in the 2376 cm−1 region by using a pulsed, slit nozzle expansion and tunable IR diode laser. The spectrum is ascribed to the 410510 combination band, where ν4 corresponds to the asymmetric stretch of the CO2 moiety and ν5 is the intermolecular bend. This combination band is ∼102 times less intense and 27.82 cm−1 higher in energy than the 410 fundamental. Effective upper-state rotational constants were determined for this nearly prolate rotor, and the ν4+ν5 moments of inertia show a large inertial defect, Δ=9.48 amu A(ring)2. In planar molecules, this indicates large vibrational amplitude, and reflects inadequacy of the rotational Hamiltonian. From the moments of inertia derived using a Watson Hamiltonian, geometric properties of ν4+ν5 can be calculated. The average O–C–Ar angle is 76.9°, while the center-of-mass separation between CO2 and Ar is 3.60 A(ring). These values differ from those of the ground state (83.1° and 3.51 A(ring)) and ν4=1 (83.0° and 3.50 A(ring)). Results are compared with previous work, which used different models and approximations to obtain intermolecular frequencies. In addition, inertial defects were calculated for the ground and vibrationally excited states using a simple normal mode model developed by Herschbach and Laurie, and Oka and Morino. With harmonic force fields, it accounts for most of the inertial defect in the ground and excited states, and yields reasonable R and θ values. However, we conclude that the good agreement between experimental and calculated inertial defects may be fortuitous, since several positive and negative contributions cancel.

Notes: Laser induced fluorescence spectra of expansion-cooled NO2/Ne samples (1 and 2 K) are reported for transitions that originate from the lowest rovibronic levels and terminate on levels near D0. At 1 K, nearly all transitions originate from N‘=0. With the present resolution of 0.02 cm−1, the 1 K spectra are resolved rather well. The high density of transitions is due to couplings between rovibronic levels with different N and K quantum numbers and with electronic characters that borrow oscillator strength from bright B2 vibronic species of the mixed 2A1/2B2 electronic system. Just above reaction threshold, such rovibronic species comprise the manifold of levels sampled by optically prepared wave packets. However, at higher energies we argue that the density of B2 vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.