Insertion of Noble Gas Atoms into Cyanoacetylene:  An ab Initio and Matrix Isolation Study

Abstract

A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm(-1), and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.

Matrix-Isolation and ab Initio Study of HNgCCF and HCCNgF Molecules (Ng = Ar, Kr, and Xe)

“…To remind ourselves, HKrCC was reported to be an unstable species. ), which is also observed for the Xe/Kr pairs of other noble-gas hydrides such as HNgCl, 44 HNgCN, 45 HNgC 3 N, 9 HNgCCH, [19][20][21] and HNgC 4 H. 46 The D-Xe stretching band of DXeCCF shifts upon deuteration from 1548 to 1121.5 cm -1 , which features a proper H/D frequency ratio of 1.380. It is worth noting that the absorption of DXeCCF is very close to one of the HXeD bands;…”

“…Similar HCCNgCN molecules (Ng ) Ar, Kr, and Xe) were computationally predicted but did not form in experiments. 9 The difference may be in the higher mobility of F atoms in solid krypton compared to CN radicals.…”

Infrared Photodissociation Spectroscopy of C2n+1N− Anions with n = 1 – 5

“…For C n N − clusters an odd/even effect has been observed in which the anions containing an odd number of carbon atoms are more stable, due to their closed electronic shell [13,14]. Electronic and vibrational absorption spectra of C 2n+1 N − ( = 1-4) have mainly been investigated in raregas matrices [15][16][17][18][19]. Initial matrix studies on C 3 N − generated via cyanoacetylene dissociation were uncertain regarding the assignment of infrared (IR) absorptions to the anion [16,17,20].…”

On theoretical predictions of noble-gas hydrides

“…These molecules can be reliably identified based on their intense H-Ng stretching absorption even though some specific problems can arise from matrix-site structure 3,20 and isomerization. 19 In conclusion, the single-reference MP2 calculations can produce a rather inaccurate energy diagram for the formation of noble-gas hydrides, and this may mislead the theoretical predictions. This is especially important for the present search for additional Ar compounds and possible He and Ne compounds.…”

“…In some cases such as HArC 4 H, HArCN, and HArC 3 N, the computationally ͑MP2͒ stable species did not form in the matrixisolation experiments, even though extensive experimental work has been done. 12,18,19 In all these cases, the computational overestimate of the HY dissociation energy by the single-reference MP2 method is larger than the stability of HNgY with respect to the H + Ng+ Y energy. This leads us to an empirical hypothesis that these HNgY molecules are, in fact, higher in energy than the corresponding H + Ng+ Y fragments.…”