Infrared absorption of t-HOCO(+), H(+)(CO2)2, and HCO2 (-) produced in electron bombardment of CO2 in solid para-H2.

Abstract

We have employed electron bombardment during matrix deposition of CO2 (or (13)CO2, C(18)O2) 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 (13)CO2 or C(18)O2 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 H(13)CO2 (-) or HC(18)O2 (-) 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 H(18)OC(18)O(+) and HC(18)O2 (-) 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.