Infrared absorption of t -HOCO ⁺ , H ⁺ (CO ₂ ) ₂ , and HCO ₂ ^- produced in electron bombardment of CO ₂ in solid para -H ₂

We have employed electron bombardment during matrix deposition of CO₂ (or ¹³CO₂, C¹⁸O₂) and para-hydrogen (p-H₂) at 3.2 K and recorded infrared (IR) spectra of t-HOCO⁺, H⁺(CO₂)₂, HCO₂^-, CO₂^-, t-HOCO, and other species isolated in solid p-H₂. After the matrix was maintained in darkness for 13 h, intensities of absorption features of t-HOCO⁺ at 2403.5 (ν₁), 2369.9 (ν₂), 1018.1 (ν₄), and 606.5 (ν₆) cm^-1 and those of H⁺(CO₂)₂ at 1341.1, 883.6, and 591.5 cm^-1 decreased. Corresponding lines of isotopologues were observed when ¹³CO₂ or C¹⁸O₂ replaced CO₂. In contrast, lines of HCO₂^- at 2522.4 (ν₁), 1616.1 (ν₅), 1327.9 (ν₂), and 745.6 (ν₃) cm^-1 increased in intensity; corresponding lines of H¹³CO₂^- or HC¹⁸O₂^- were also observed. Lines of t-DOCO⁺ and DCO₂^- were observed in an electron bombarded CO₂ /normal-deuterium (n-D₂) matrix. Data of ν₆ of t-HOCO⁺ and all observed modes of H¹⁸OC¹⁸O⁺ and HC¹⁸O₂^- are new. The assignments were made according to expected chemical behavior, observed isotopic shifts, and comparisons with vibrational wavenumbers and relative intensities of previous reports and calculations with the B3PW91/aug-cc-pVQZ method. The ν₁ line of t-HOCO⁺ in solid p-H₂ (2403.5 cm^-1), similar to the line at 2673 cm^-1 of t-HOCO⁺ tagged with an Ar atom, is significantly red-shifted from that reported for gaseous t-HOCO⁺ (3375.37 cm^-1) due to partial proton sharing between CO₂ and H₂ or Ar. The ν₁ line of HCO₂^- in solid p-H₂ (2522.4 cm^-1) is blue shifted from that reported for HCO₂^- in solid Ne (2455.7 cm^-1) and that of HCO₂^- tagged with Ar (2449 cm^-1); this can be explained by the varied solvation effects by Ne, Ar, or H₂ on the mixing of H⁺ + CO₂ and H + CO₂^- surfaces. Possible formation mechanisms of t-HOCO⁺, H⁺(CO₂)₂, HCO₂^-, CO₂^-, t-HOCO, H₂O, and t-HCOOH are discussed.