Electron Localization in Solid Acetonitrile

Abstract

Electron localization in the bulk condensed phases of acetonitrile differs drastically from the corresponding processes that take place in water and aliphatic alcohols. In particular, electron capture following the ionization of liquid and solid acetonitrile yields well-defined radical anions in place of the largely structureless solvated and trapped electrons characteristic of irradiated solutions and rigid glasses. This is remarkable given the many similarities that exist not only between the intrinsic properties of these individual solvent molecules, such as polarity, but also in their cooperative capacity to function as solvents. Here, with the aid of detailed experimental and computational studies, we examine on a fundamental level what gives rise to such a sharp contrast in the behavior of these alkyl cyanides as compared to water and alcohols. EPR and optical spectra, and ab initio calculations on the radical anions are used together with diffraction data for the crystal structures to demonstrate that electron attachment to nitrile molecules in the crystalline state results in a significant bending of the linear CCN moiety. Moreover, in the formation of the dimer anions, which only takes place in certain crystalline phases, it is determined that the intermolecular bonding occurs between the cyanide carbons in an antiparallel arrangement. A staggered structure for the dimer anion in R-acetonitrile is then suggested, in which there is a nearly coplanar arrangement with the two bent CCN fragments. The resulting structure accounts for the observed EPR parameters, the position of the UV-vis absorption band, and the trends observed in the vibronic progressions for the four H/D isotopomers. The dimer radical anion in R-acetonitrile retains the same orientation and symmetry as that of the preexisting neutral {CH 3 CN} 2 pairs in this crystal despite the resulting changes in molecular geometry and in the translational coordinates for the cyanide carbons as the two acetonitrile molecules come together. We suggest that dimer anions of nitriles can form only in the phases where such "antiparallel" pairs occur, including liquid systems; otherwise, the electron remains attached to a single molecule, as in β-acetonitrile. Because electron capture by water or alcohol molecules in the condensed phase appears to require larger molecular clusters, the contrast between the behavior of these solvents and acetonitrile is attributable to the relative ease with which acetonitrile molecules in the crystal or solvent can undergo valence-electron attachment via their accessible low-lying π* orbitals.

Toward Electron Encapsulation: Polynitrile Approach

“…26,33 Reaction (3) shifts equilibrium (2) to the right side which makes it necessary to take it into account in order to determine the equilibrium parameters of reaction (2). For glutaro-and succino-nitrile ( Figures 2 and 3, respectively), the rate constants for dimerization reaction (3) are close to those for diffusion-controlled reactions in n-hexane; for adiponitrile this reaction is ca.…”

“…Radiolysis of solid succinonitrile (n=2), either in cubic (plastic) or monoclinic phases at 77 K results in the formation of a dimer anion that absorbs at 540 nm; 32a irradiation of a room temperature cubic crystal yields a monomer anion that absorbs at 450 nm and decays in 230 ns via the loss of CN -. 32b Such a dissociation can also be stimulated by absorption of visible light, and it was observed for all acetonitrile anions, both in solid 31,33 and in neat liquid. 16 Crystalline adiponitrile is also known to yield dimer radical anion in low-temperature radiolysis.…”

“…32a Shkrob et al 33 and others 26,31 found that acetonitrile molecules in the dimer and the monomer anions have similar geometry, with the C-C-N angle of 130 o . In the centrosymmetrical dimer anion, there is a 3-electron-2-center bond between the two cyanide carbons with the C-C distance of 1.65 Å.…”

“…In the centrosymmetrical dimer anion, there is a 3-electron-2-center bond between the two cyanide carbons with the C-C distance of 1.65 Å. 33 Other nitriles are expected to form anions with the same structural motif. Radiolysis of solid succinonitrile (n=2), either in cubic (plastic) or monoclinic phases at 77 K results in the formation of a dimer anion that absorbs at 540 nm; 32a irradiation of a room temperature cubic crystal yields a monomer anion that absorbs at 450 nm and decays in 230 ns via the loss of CN -.…”

“…The validity of such an approach is suggested by the ability of smallbasis DFT calculations to capture all observable properties of monomer and dimer anion of acetonitrile in crystalline β− and α− acetonitrile, respectively. 33 The resulting optimized structures are shown in Figures 6, 7, and 8; the structural parameters and energetics are summarized in Table 2, and calculated magnetic parameters (isotropic and anisotropic hfcc's) are given in Table 1S in the Supplement. The adiabatic electron affinity (EA) given in the fourth column of Table 2 Table 2S that a large fraction of the spin density is contained in the N and C 2p orbitals of CN groups.…”