ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
The hydrogen abstraction reaction between cyclopentadienyl radicals [Xcpd, X=H, CH3(m), F, CN] and substituted methanes (CH4, C2H6, CH3CH2OH, CH3Cl, CH2F2, CHF3, and CH3OH) is studied for the isolated one-to-one van der Waals clusters created in a supersonic expansion. Three different types of fluorescence excitation spectra are characterized for these cluster systems: (1) sharp spectra are observed for some clusters, suggesting no cluster chemistry for either the ground or excited electronic states of Xcpd—CNcpd/CH3Cl, CH2F2, CHF3, CH3OH; (2) broad spectra are observed suggesting initiation of cluster chemistry on the excited state cluster potential energy surface—CNcpd–CH4, Fcpd–CHF2Cl, CHF3; and (3) only a greatly reduced bare radical signal is observed, but no cluster emission can be detected—cpd, mcpd/all substituted methanes, Fcpd–CH2F2, CH3Cl, CH3CH2OH, CH3OH, C2H6, and CNcpd/C2H6, CH3CH2OH. These results, taken together, suggest that the Xcpd radicals undergo an excited electronic state electrophilic hydrogen abstraction reaction with substituted methanes. The radical reactivities are in the order mcpd∼cpd〉Fcpd〉CNcpd and the substituted methane reactivities are in the order C2H6〉C2H5OH〉CH4〉CH3Cl∼CH3OH〉CH2F2〉CHF2Cl〉CHF3. All Xcpd radicals show intense, sharp spectra with CF4. This indication of an excited state Xcpd radical hydrogen abstraction reaction with substituted methanes is further explored by ab initio quantum chemistry techniques at the (7×7) CASSCF/6-31G (complete active space self-consistent field) and cc-pVDZ levels for cpd–CH4. Calculations confirm the idea that the ground state cluster has a reaction barrier (approximately +170 kJ/mol) and a positive free energy of reaction (∼80 kJ/mol). The excited cpd radical, however, can react with CH4 along a barrierless path to generate substantial hot ground product states (C5H6 and CH3). Experimental data are consistent with an Xcpd–C2H4 addition reaction, as well. © 1999 American Institute of Physics.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.478413
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