The microwave spectrum of the K=0 states of Ar–NH3

Intermolecular dynamics of NH3-rare gas complexes in the ν2 umbrella region of NH3 investigated by rovibrational laser jet-cooled spectroscopy and ab initio calculations

High resolution jet-cooled spectroscopy experiments have been realized to investigate the intermolecular dynamics of van der Waals (vdW) heterodimers between NH₃ and rare gas (Rg) atoms in the ν₂ umbrella mode region of NH₃. With respect to a previous study dedicated to NH₃-Ar [Asselin et al. Mol. Phys. 116, 3642 (2018)], the sensitivity and spectral resolution of our laser spectrometer coupled to a pulsed supersonic jet have been significantly improved to derive more accurate excited state spectroscopic parameters from rovibrational analyses. In addition, we calculated the ground and ν₂ excited vibration-rotation-tunneling (VRT) states of these complexes on the four-dimensional ab initio potential energy surfaces from Loreau et al. [J. Chem. Phys. 141, 224303 (2014), ibid. 143, 184303 (2015).] Transition frequencies and intensities of the allowed ν₂ = 1 ← 0 transitions obtained from the calculated energy levels and wave functions agree well with the experimental data and are helpful in their analysis. By means of a pseudodiatomic model with the assumption of weak Coriolis coupling, the rovibrational analysis of both the Π_e/f(j = 1,k = 0) ←Σ_f(j = 0,k = 0) and Σ_f(j = 1,k = 0) ←Σ_f(j = 0,k = 0) transitions in ortho NH₃-Rg (Rg = Ne, Ar, Kr, Xe) complexes enabled us to determine reliable excited state parameters and derive accurate values of the effective vdW bond length R_eff, force constant k_s, and vdW stretching frequency ν_s. Comparison between the experimental structural parameters and those from the ab initio calculated VRT levels shows good agreement for NH₃-Ne, NH₃-Ar and NH₃-Kr, and a similar variation of R_eff, k_s, and ν_s with the polarizability of Rg in the ground and ν₂ excited states. Anomalously small values of ν_s and k_s derived for NH₃-Xe in the Π_e/f(j = 1,k = 0) state suggest that the applied model is not valid in this case, due to the presence of another state coupling to the perturbed Π_f state. Such a state could not be found, however.

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.

Dynamics and spectroscopy of van der Waals complexes composed of ammonia and noble gases

In this work, we calculate the rovibrational energies and spectroscopic constants for the systems formed by ammonia (NH₃) and noble gases (Ng=He, Ne, Ar, Kr and Xe). For the spectroscopic constant calculations, we used two different methods: Dunham and another one that use rovibrational energies (here calculated by discrete variable method). In both cases, we used the improved Lennard-Jones potential energy curves (PECs). These PECs, which describe very well van der Waals systems, were built using the dissociation and equilibrium distance obtained from experiments of crossed molecular beams. The spectroscopic constant results, obtained by both methods were in excellent agreement with each other for all NH₃-Ng studied systems. Also in relation to NH₃-He system, we realize that although this system has a relatively small dissociation energy, it has one vibrational level. Finally, the spectroscopic constants and fundamental rovibrational energy results were used to verify the stability of each system through the lifetime decomposition.

Microwave spectra and nuclear quadrupole structure of the NH3-N2 van der Waals complex and its deuterated isotopologues

The microwave spectrum of the NH₃-N₂ van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 - 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH₃-(ortho)-N₂ nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH₃-(para)-N₂ spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole ¹⁴N nuclei of NH₃-N₂ was resolved for both transitions, and the quadrupole coupling constants associated with the NH₃ and N₂ subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C ₃ axis of NH₃ and the N₂ axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH₃-(ortho)-N₂ and (para)-NH₃-(para)-N₂ and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND₃-N₂, NHD₂-N₂, and NH₂D-N₂, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH₃-N₂ complex.

Gas phase complexes of H3N⋯CuF and H3N⋯CuI studied by rotational spectroscopy and ab initio calculations: the effect of X (X = F, Cl, Br, I) in OC⋯CuX and H3N⋯CuX

Complexes of H₃N⋯CuF and H₃N⋯CuI have been synthesised in the gas phase and characterized by microwave spectroscopy.

Potential energy surface and bound states of the NH3-Ar and ND3-Ar complexes

A new, four-dimensional potential energy surface for the interaction of NH3 and ND3 with Ar is computed using the coupled-cluster method with single, double, and perturbative triple excitations and large basis sets. The umbrella motion of the ammonia molecule is explicitly taken into account. The bound states of both NH3-Ar and ND3-Ar are calculated on this potential for total angular momentum values from J = 0 to 10, with the inclusion of Coriolis interactions. The energies and splittings of the rovibrational levels are in excellent agreement with the extensive high-resolution spectroscopic data accumulated over the years in the infrared and microwave regions for both complexes, which demonstrates the quality of the potential energy surface.

Does ammonia hydrogen bond?

Spectroscopic characterizations of the stereochemistry of complexes of ammonia (NH(3)) have strongly confirmed some long-held ideas about the weak interactions of NH(3) while casting doubt on others. As expected, NH(3) is observed to be a nearly universal proton acceptor, accepting hydrogen bonds from even some of the weakest proton donors. Surprisingly, no evidence has been found to support the view that NH(3) acts as a proton donor through hydrogen bonding. A critical evaluation of the work that has been done to gather such evidence, as well as of earlier work involving condensed-phase observations, suggests that NH(3) might well be best described as a powerful hydrogen-bond acceptor with little propensity to donate hydrogen bonds.

Study of the Xe-NH3 van der Waals complex: high-resolution microwave spectra and ab initio calculations

An ab initio potential energy surface of the Xe-NH(3) van der Waals complex was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. The small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the Xe atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with midbond functions. Rotational spectra of the Xe-NH(3) van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the Sigma0(0) and Pi1(1) (lower) states, were measured and assigned to the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues. For the deuterated isotopologues, only the Sigma0(0) states were observed. Two inversion components were observed for each state except for the "s" component of the Sigma0(0) state of the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues, which has a spin statistical weight of zero. Nuclear quadrupole hyperfine structures arising from the (14)N (nuclear spin angular momentum quantum number I=1) and (131)Xe (I=32) nuclei were detected and analyzed. The observed spectra suggest that the Pi1(1) (lower) state has lower energy than the unobserved Sigma1(1) state, in contrast to the case of Ar-NH(3).

Spectroscopic study of the Ne-Xe-NH3 van der Waals trimer

Rotational spectra of the Ne-Xe-NH3 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Both a- and b-type transitions of eight isotopologues, namely 20Ne-132Xe-14NH3, 20Ne-129Xe-14NH3, 20Ne-132Xe-15NH3, 20Ne-129Xe-15NH3, 20Ne-131Xe-15NH3, 22Ne-132Xe-15NH3, 22Ne-129Xe-15NH3, and 22Ne-131Xe-15NH3 were measured and assigned. Nuclear quadrupole hyperfine structures arising from the 14N (nuclear spin quantum number I = 1) and 131Xe (I = 3/2) nuclei were detected and analyzed. The determined rotational constants were used to fit structural parameters. A harmonic force field analysis was performed based on centrifugal distortion constants to extract information about vibrational motions of the complex. A comparison of van der Waals bond lengths and stretching force constants between the Ne-Xe-NH3 trimer and the corresponding dimers indicates that non-additive three-body effects are present in the trimer system. Analyses of the 14N and 131Xe nuclear quadrupole coupling constants suggest that the NH3 unit undergoes nearly free internal rotation within the complex and that the presence of Ne has little effect on the orientation of NH3 with respect to the Xe atom.

The Nature of Variations of Ammonia Proton Affinity in an Argon Environment

The presence of solvent molecules, even inert, may significantly influence processes taking place in the gas phase. The reason for the solvent activity may be found in studies of complexes originating from microsolvation. The structure, thermodynamics, and vibrational properties of NH(4)(+)Ar(n) (n = 0-5) and NH(3)Ar(n) (n = 0-4) complexes are presented in this work with the aim to elucidate the effect of microsolvation on protonation/deprotonation processes. The relation between the nature of interactions in cationic and neutral clusters and the proton affinity is studied as a function of the number of ligands in complexes.

Ne3−NH3 van der Waals Tetramer: Rotational Spectra and ab Initio Study of the Microsolvation of NH3 with Rare Gas Atoms

Microwave rotational spectra of eleven isotopomers of the Ne(3)-NH(3) van der Waals tetramer were measured using a pulsed jet, Balle-Flygare type Fourier transform microwave spectrometer. The transitions measured fall between 4 and 17 GHz and correspond to the ground internal rotor state of the weakly bound complex. The (20)Ne(3)- and (22)Ne(3)-containing species are symmetric top molecules while the mixed (20)Ne(2)(22)Ne- and (20)Ne(22)Ne(2)-isotopomers are asymmetric tops. For each of the deuterium-containing isotopomers, a tunneling splitting was observed due to the inversion of NH(3) within the tetramer. The (14)N nuclear quadrupole hyperfine structures were resolved and included in the spectroscopic fits of the various isotopomers. The rotational constants obtained from the fits were used to estimate the van der Waals bond lengths of the tetramer while the (14)N nuclear quadrupole coupling contants and the observed inversion tunneling splittings provided information about the internal dynamics of the NH(3) moiety. The experimental results were complemented by the construction of three ab initio potential energy surfaces [CCSD(T)] for the Ne(3)-NH(3) complex, each corresponding to a different internal geometry of NH(3) ( 90 degree angle HNH = 106.67 degrees, 90 degree angle HNH = 113.34 degrees, and 90 degree angle HNH = 120.00 degrees ). The topologies of the surfaces are related to the structures and dynamics of the tetramer. Extensive comparisons are made between the results obtained for the Ne(3)-NH(3) tetramer in this work and previous experimental and ab initio studies of related Rg(n)-NH(3) van der Waals clusters.