Rovibrational analysis of the water bending vibration in the mid-infrared spectrum of atmospherically significant N2–H2O complex

Understanding the high-resolution spectral signature of the N2-H2O van der Waals complex in the 2OH stretch region

We present the observation of the N2-H2O van der Waals complex in the 2OH stretch overtone region. The high-resolution jet cooled spectra were measured using a sensitive continuous wave cavity ringdown spectrometer. Several bands were observed and vibrationally assigned in terms of ν1, ν2, and ν3, the vibrational quantum numbers of the isolated H2O molecule, as (ν1'ν2'ν3')←(ν1″ν2″ν3″)=(200)←(000) and (101) ← (000). A combination band involving the excitation of the in-plane bending motion of N2 and the (101) vibration of water is also reported. The spectra were analyzed using a set of four asymmetric top rotors, each associated with a nuclear spin isomer. Several local perturbations of the (101) vibrational state were observed. These perturbations were assigned to the presence of the nearby (200) vibrational state and to the combination of (200) with intermolecular modes.

Spectroscopic study of the tunneling dynamics in N2-water observed in the O-D stretch region

The O-D stretch rovibrational spectra of N₂-D₂O and N₂-DOH were measured and analyzed. A combination band involving the in-plane N₂ bending vibration was also observed. These bands were recorded using a pulsed-slit supersonic jet expansion and a mid-infrared tunable optical parametric oscillator. The spectra were analyzed by considering the feasible tunneling motions, and transitions were fitted to independent asymmetric rotors for each tunneling state. The rotational constants of the four tunneling components of N₂-D₂O were retrieved for the excited vibrational states. A two order of magnitude increase in the tunneling splittings is observed for the asymmetric O-D stretch (ν₃ in D₂O) excitation compared to the symmetric stretch (ν₁ in D₂O) and to the ground vibrational state. This last finding indicates that the ν₃ vibrational state is likely perturbed by a combination state that includes ν₁. Finally, the observation of a local perturbation in the ν₃ vibrational band, affecting the positions of few rovibrational levels, provides an experimental lower limit of the dissociation energy of the complex, D₀ > 120 cm^-1.

Explicitly correlated ab initio potential energy surface and predicted rovibrational spectra for H2O-N2 and D2O-N2 complexes

An ab initio intermolecular potential energy surface (PES) for the van der Waals complex of H₂O-N₂ that explicitly incorporates the intramolecular Q₂ bending normal mode of the H₂O monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] with an augmented correlation-consistent triple zeta basis set and an additional bond function. Analytic five-dimensional intermolecular PESs for ν₂(H₂O) = 0 and 1 are obtained by fitting to the multi-dimensional Morse/long-range potential function form. These fits to 40 890 points have the root-mean-square (rms) discrepancy of 0.88 cm^-1 for interaction energies less than 2000.0 cm^-1. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for microwave transitions of H₂O-N₂, the rms discrepancy is only 0.0003 cm^-1, and for infrared transitions of the A₁ symmetry of the H₂O(ν₂ = 1 ← 0)-N₂, the rms discrepancy is 0.001 cm^-1. The calculated infrared band origin shifts associated with the ν₂ bending vibration of water are 2.210 cm^-1 and 1.323 cm^-1 for H₂O-N₂ and D₂O-N₂, respectively, in good agreement with the experimental values of 2.254 cm^-1 and 1.266 cm^-1. The benchmark tests and comparisons of the predicted spectral properties are carried out between CCSD(T)-F12a and CCSD(T)-F12b approaches.

Ab initio potential energy surface and microwave spectrum of the NH3-N2 van der Waals complex

We present a five-dimensional intermolecular potential energy surface (PES) of the NH₃-N₂ complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH₃ symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH₃ unit closest to the N₂ unit, which is nearly parallel to the NH₃ symmetry axis. The intermolecular distance is 7.01 a₀, and the binding energy D_e is 250.6 cm^-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH₃ combines with (o/p)-N₂, were calculated on this intermolecular potential surface. The computed dissociation energies D₀ are 144.91 cm^-1, 146.50 cm^-1, 152.29 cm^-1, and 154.64 cm^-1 for (o)-NH₃-(o)-N₂, (o)-NH₃-(p)-N₂, (p)-NH₃-(o)-N₂, and (p)-NH₃-(p)-N₂, respectively. Guided by these calculations, the pure rotational transitions of the NH₃-N₂ van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole ¹⁴N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH₃-N₂ nuclear spin isomers.