Tuning Spin-States of Carbynes and Silylynes: A Long Jump with One Leg

Abstract

The challenge motivating this paper is to induce, by chemical substitution, a silylyne, SiR, or a congeneric carbyne, CR, to adopt the high-spin quartet rather than the low-spin doublet as its ground state. The difficulty is seen in the preference for the doublet of the parent SiH (doublet-quartet energy difference ∼39 kcal/mol, favoring the doublet) or CH (∼17 kcal/mol). Strategies for having high-spin ground state parallel those for silylenes and carbenes: greater electropositivity (σ-donation) and π-acceptance of the single substituent favor the high-spin state. The electronegativity trend can be understood from an ions in molecules way of thinking already present in the literature in the works of Boldyrev and Simons, and of Mavridis and Harrison; i.e., the quartet ground state spin of some CR/SiR species is largely determined by the ground state spin of C(-)/Si(-). In this study, we provide a diabatization analysis that solidly confirms the ions in molecules picture and explains the difference in the equilibrium internuclear distances for the two spin states. In general, electronegativity dominates the ordering of spin states. π-Acceptors also help to lower the quartet state energy of the many carbynes (silylynes) examined, whose range of doublet-quartet differences calculated is impressive, 120 (100) kcal/mol. The qualitative understanding gained leads to the prediction of some quartet-ground state carbynes (CMgH, CAlH2, CZnH, CSiH3, CSiF3, etc.) and a smaller number of silylynes (SiMgH, SiMgF, SiBeH, etc.). A beginning is made on the energetics of approach geometries of the fragments in the highly exoergic dimerization of CH to acetylene; it should proceed for the ground state doublet CH through C2h-like trajectories, with no activation energy.

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

“…Chemical substitutions with greater electropositivity (σ‐donation) and π‐acceptance of the single substituent were predicted to favor the high‐spin state. Some carbynes with quartet ground spin state were predicted . Here we report the production and characterization of a multiple radical BeOBeC species with an electronic quintet ground state featuring a quartet carbyne with three unpaired electrons on the carbon center.…”

“…The experimentally known carbynes predominately have doublet electronic ground states . The possibility of tuning the ground state electronic spin from the low‐spin doublet to high spin quartet for carbynes has been theoretically discussed and predicted . Chemical substitutions with greater electropositivity (σ‐donation) and π‐acceptance of the single substituent were predicted to favor the high‐spin state.…”

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

“…Previous theoretical studies have shown that the relative stability of the doublet and quartet states of carbynes RC can be tuned through the electronegativity of substituents R. [17][18][19] Electropositive substituents favor high spin ground state of RC.I n addition, there is another factor that can stabilize the quartet carbyne:w hen substituent Ro fc arbyne RC is a p-acceptor, its orbitals can interact and stabilize the two degenerate p p orbitals of RC to favor aquartet ground state. [19] TheBeOBe molecule is an electropositive substituent. Natural population analysis reveals that the Ca tom carries an egative charge of À0.35 ei nt he quintet state and À0.31 ei nthe triplet state at the B3LYP level, indicating that there is no striking difference in the strength of BeOBe !…”

“…[2,[9][10][11][12][13][14][15][16] Thep ossibility of tuning the ground state electronic spin from the low-spin doublet to high spin quartet for carbynes has been theoretically discussed and predicted. [17][18][19] Chemical substitutions with greater electropositivity (s-donation) and p-acceptance of the single substituent were predicted to favor the high-spin state.S ome carbynes with quartet ground spin state were predicted. [19] Here we report the production and characterization of am ultiple radical BeOBeC species with an electronic quintet ground state featuring aquartet carbyne with three unpaired electrons on the carbon center.T he radical is prepared via carbon monoxide coordination and activation by ab eryllium dimer in solid neon matrix and is further identified by infrared absorption spectroscopy and quantum theoretical calculations.…”

Tuning the Ground State Symmetry of Acetylenyl Radicals

“…26 The configurations involved in these two spectroscopic states are shown in 1 and 2 in Scheme 1, with the σ orbital lying lower than the π orbital (−9.4 vs −1.7 eV of HC 27 ).…”