Singlet-triplet splittings and ground- and excited-state electron affinities of selected cyanosilylenes, XSiCN (X = H, F, Cl, CH3, SiH3, CN)

Relatives of cyanomethylene: replacement of the divalent carbon by B-, N+, Al-, Si, P+, Ga-, Ge, and As. Phys Chem Chem Phys 2019;21:26438-26452. [PMID: 31774089 DOI: 10.1039/c9cp05777c]

The lowest lying singlet and triplet states of HBCN-, HCCN, HNCN+, HAlCN-, HSiCN, HPCN+, HGaCN-, HGeCN, and HAsCN+ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet-triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet-triplet gap (ΔEST = Esinglet - Etriplet) of 3.5 kcal mol-1, 11.9 kcal mol-1, and 22.6 kcal mol-1, respectively, for HBCN-, HCCN, and HNCN+. The second row increasing favors the singlet state, with singlet-triplet gaps of -20.4 kcal mol-1 (HAlCN-), -26.6 kcal mol-1 (HSiCN), and -26.8 kcal mol-1 (HPCN+). The third row also favors the singlet state, with singlet-triplet gaps of -26.8 kcal mol-1 (HGaCN-), -33.5 kcal mol-1 (HGeCN), and -33.1 kcal mol-1 (HAsCN+). The HXCN species have larger absolute singlet-triplet energy gaps compared to their parent species XH2 except for the case of X = N+. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO.

Dicyanogermylenes: a tale of isomers and interconversions

A systematic investigation is carried out using the B3LYP, BLYP, and BHLYP functionals and MP2 level of theory to characterize the low-lying electronic singlet and triplet GeC2N2 isomers. The basis sets used are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, DZP++. Three bent isomers Ge(CN)2, CNGeCN, and Ge(NC)2 are located on the singlet and triplet potential energy surfaces. In visualizing the reaction pathways for the singlet isomerization of the bent isomers, two three-membered [Ge, C, N] cyclic systems, with exocyclic -C-C≡N and -C-N≡C bonding, appear on the energy surface. Four types of electron affinities reported are: the adiabatic electron affinity, the zero-point vibrationally corrected electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The ionization energies and singlet-triplet gaps for all isomers are also reported. The energetic ordering (kcal mol(-1)) (B3LYP) with zero-point vibrational energy corrections for the singlet ground state isomers follows: Ge(CN)2 (global minimum) < CNGeCN (2.3) < Ge(NC)2 (3.3) < Cyc_exo_CCN (15.3) < Cyc_exo_CNC (30.6). All the bent and cyclic isomers are found to be below the dissociation limit to Ge ((3)P) + C2N2 ((1)Σg). The rate constants for all interconversions are evaluated using transition state theory.

Electron binding capabilities of some silylenes having small singlet-triplet splittings or triplet ground states

Several silyl and alkaline metal substituted silylenes have been investigated using the CAS-ACPF method in conjunction with the aug-cc-pVTZ basis sets. Silylsilylene and disilylsilylene are found to have singlet ground states with DeltaEST(-) values of 0.676 and 0.319 eV, respectively. The adiabatic ground state electron affinities are found to be 1.572 and 2.361 eV for HSiSiH(3) and Si(SiH(3))(2). respectively. Both silylenes possesses a stable 2A1 excited negative ion state, with respective adiabatic EA values of 0.037 and 1.000 eV. In contrast, all silylenes with at least one akaline metal substituent exhibit triplet neutral ground states. The metalated silylenes HSiLi, HSiNa, LiSiSiH(3), NaSiLi, SiLi(2), and SiNa(2) have adiabatic ground state EAs somewhat below 1 eV, but each of these negatively charged system possesses up to three bound excited negative ion states, some of which are dipole-bound states.