Electron collisions with hydrogen-bonded complexes

We investigated elastic collisions of low-energy electrons with the hydrogen-bonded formic-acid dimer, formamide dimer, and formic-acid–formamide complex. We focused on how the ${\pi }^{*}$ shape resonances of the isolated monomers are affected when bonded to another molecule. The scattering cross sections were computed with the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange-plus-polarization approximations, for energies ranging from 1 to 6 eV. The present results support the existence of two low-lying ${\pi }^{*}$ shape resonances for the formic-acid dimer, as suggested in previous theoretical and experimental studies. We also found low-lying ${\pi }^{*}$ shape resonances for the formamide dimer and for the formic-acid–formamide complex. For the dimers, the presence of a center of inversion is key to understanding how these resonances arise from linear combinations of the ${\pi }^{*}$ anion states of the respective monomers. For the formic-acid–formamide complex, the resonances are more localized on each unit, lying at lower energies with respect to the isolated monomers. The present results suggest that if there is no delocalization of the ${\pi }^{*}$ resonances over the pair for hydrogen-bonded molecules, then their positions would lie below those of the units.

Figure 1

Geometric structures of (a) FAD, (b) FD, and (c) FAFC (generated using macmolplt [24]).

Figure 2

Integral cross sections for the $A_u$ and $B_g$ symmetries of FAD in the SE and SEP approximations. The results from Ref. [20] are also shown. Cross sections are in units of $a_0^2$, where $a_0$ is the Bohr radius and $1a_0=0.52918\times {10}^{-10}$ m.

Figure 3

Integral cross sections for the $A_u$ and $B_g$ symmetries of FD in the SE and SEP approximations. Cross sections are in units of $a_0^2$, where $a_0$ is the Bohr radius and $1a_0=0.52918\times {10}^{-10}$ m.

Figure 4

Integral cross sections for the $A^{\prime \prime }$ symmetry of FAFC in the SE and SEP approximations. We also show the results from Refs. [14, 18] for the formic-acid and formamide monomers, respectively. Cross sections are in units of $a_0^2$, where $a_0$ is the Bohr radius and $1a_0=0.52918\times {10}^{-10}$ m.

Figure 5

MVO plots for FAD (top) and FAFC (bottom). Panels (a) and (b) show the two lowest-lying MVOs having $a_u$ and $b_g$ symmetries (FAD). Panels (c) and (d) show the two lowest-lying MVOs having $a^{\prime \prime }$ symmetry (FAFC) (generated using macmolplt [24]). Although not shown here, the low-lying $a_u$ and $b_g$ MVOs of FD are very similar to those of FAD. See text for discussion.