The rotational spectra of NH3–CO and NH3–N2

The Radiation Chemistry of NH3···CO Complex in Cryogenic Media as Studied by Matrix Isolation

The NH₃···CO complex can be considered an important building block for cold synthetic astrochemistry leading to the formation of complex organic molecules, including key prebiotic species. In this work, we have studied the radiation-induced transformations of this complex in Ar, Kr, and Xe matrices using FTIR spectroscopy. On the basis of comparison with the quantum chemical calculations at the CCSD(T)/L2a_3 level of theory, it was found that the initial complex had the configuration with hydrogen bonding through the carbon atom of CO. Irradiation of the matrix isolated complex with X-rays at 6 K leads to the formation of a number of synthetic products, namely, HNCO (in all matrices), formamide NH₂CHO, NH₂CO, and HNCO-H₂ (in argon and krypton). The matrix effect on the product distribution was explained by the involvement of different excited states of the complex in their formation. It was suggested that formamide results from the singlet excited states while other species mainly originate from triplet excited states. The latter states are efficiently populated through ion-electron recombination (in all matrices) and through intersystem crossing (particularly, in xenon). High yield of the recombination triplet states is a feature of the processes induced by high-energy radiation (in contrast to direct photolysis). NCO, CN, and NO were found as minor secondary products at high adsorbed doses. The astrochemical implications of the obtained results are discussed.

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.

Unimolecular photolysis mechanisms of formamide: the effect of thermal energy

The C–N dissociation is the most favorable in S₁ state, and radical products (1) are precursors in other channels.

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.

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).