Cyanides, Isocyanides, and Hydrides of Zn, Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations

Abstract Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H‐atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to 13C and 15N isotopic substitution for comparison with experimental data.


SUPPORTING INFORMATION
TOC graphic: Outline of the reactions that occur with laser ablated Hg atoms and HCN molecules during condensation in excess argon at 5K and subsequent annealing to allow diffusion and reaction of H atoms with the HgCN and HgNC primary reaction products to form their hydrides.
Picture P-1 Mercury amalgam target photographs Model: CCSD(T) with the aug-cc-pVTZ basis sets are used for H, C, N, and Zn and the augcc-pVTZ-pp pseudo potentials and basis sets for Cd and Hg.
MOLPRO electronic-structure calculations for geometry and harmonic vibrations

Computational methods
Density functional theory (DFT) calculations were performed using the TURBOMOLE 7.0.1 program package [1] employing the hybrid exchange-correlation density functional B3LYP [2] with the polarized triple-ξ basis set def 2-TZVP [3] which applies the Stuttgart-Dresden effective core potential for cadmium and mercury [4] . All Coupled Cluster Single Double and perturbative Triple excitations (CCSD(T)) were carried out using the Dunning's augmented correlation consistent polarized triple-ξ basis sets aug-cc-pVTZ for hydrogen, carbon and nitrogen, [5] as well as aug-cc-pVTZ-PP [6] combined with the effective core potentials [7] ECP10MDF for zinc, ECP28MDF for cadmium and ECP60MDF for mercury. The open and closed shell CCSD(T) calculations were carried out using Molpro 2019.2 in the spin unrestricted RHF-UCCSD(T) open-shell coupled cluster formalism using default frozen core settings. [8] Harmonic vibrational frequency calculations were carried out with all optimized structures analytically (B3LYP) or numerically (CCSD(T)).

Computational results
All MCN, MNC, HMCN and HMNC (M = Zn, Cd and Hg) species converged to linear structures with electronic ground states of 2 Σ + (MCN, MNC) and 1 Σg + (HMCN, HMNC). The obtained structural parameters and harmonic frequencies are given in the supporting information and summarized in Tables S2 and S3.         Ratios are defined by frequencies (rightmost column). Note when the 12/13 ratio decreases, the 14/15 ratio increases Figures S1-S4 Table S8 Molecule Metal  Figure S1: Infrared spectra using Nicolet iS50 FTIR for the products of laser ablated zinc atom reactions with cyanogen in excess argon at 4 K. Bottom spectrum recorded following deposition of Zn with 1% (CN)2 for one hour: The arrow points to the ZnCN product absorption measured at 2162.4 cm -1 , P denotes the major (CN)2 absorption: the sharp 2127.6 cm -1 band is from N 12 C 13 CN in natural abundance. The numbers 5, 6, 7 are for major products listed in the observed frequency table. The * indicates the strongest CNCN precursor isomer absorption. The next spectrum going up follows annealing to 25 K, and the next was recorded after full mercury arc irradiation for 10 min: note the substantial growth in the major products labeled 5, 6, and 7. The last two scans going up were recorded after annealing to 30 and 40 K where 6 and 7 increase at the expense of 5. Figure S2. Infrared spectra of the reaction products from laser ablated Zn co-deposited with 1% HCN argon at 5 K. Spectrum after (a) deposition for 120 min with regular laser energy (50% of maximum energy), (b) annealing to 20 K and cooling back to 5 K, (c) full arc irradiation for 20 min for black spectra (d) deposition for 120 min with 20% higher laser energy (60% of maximum energy), (e) annealing to 20 K and cooling back to 5 K, (f) full arc irradiation for 20 min for blue spectra. Figure S4. Infrared spectra between 2360 and 2260 cm -1 for the natural isotopic carbon dioxide trace impurity and from the reaction of C atoms with trace O2 impurity and the O 13 CO reaction product (lower frequency doublet) from laser ablated Hg co-deposited with 0.2% HCN (a-d) or with 0.2% H 13 CN (e-h) in argon at 5 K, respectively. The HOO radical was detected at 1388 cm -1 so we know that trace air impurity contributed to the production of O 12 CO and O 13 CO. Spectrum after (a) and (e) deposition for 120 min, (b) and (f) annealing to 20 K and cooling back to 5 K, (c) and (g) full Hg arc Photolysis for 20 min, (d) and (h) after annealing to 35 K and cooling back to 5 K. Numbers are given for matrix site splittings in the antisymmetric stretching mode for O 12 CO and O 13 CO.
(Information used for calculations of product bond lengths, energies and vibrational frequencies given in Supporting Information Section)