Molecular beam study of the chemiluminescent reaction of manganese and ozone

• Published in: The Journal of chemical physics 2000-01, Vol.112 (4), p.1721-1732
• Main Authors: Green, K. M., Kampf, R. P., Parson, J. M.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.480737

The electronically chemiluminescent reaction Mn+O3→MnO*+O2 was investigated using a beam-gas configuration. Light from the MnO A 6∑+–X 6∑+ transition was collected by a charge coupled device (CCD) array detector with resolutions of 0.5 and 0.1 nm. The spectrum at lower resolution (500–655 nm) encompassed the Δv=−3 to +2 sequences, while that at higher resolution (555.5–583.5 nm) encompassed only the Δv=0 sequence. These two spectra were separately fitted with a nonlinear least-squares program to obtain vibrational and rotational distributions of the nascent MnO*. The limited vibrational-state coverage of the higher-resolution spectrum made it unrealiable for determining the vibrational state distribution, and it was useful only for characterizing the rotational distribution when v′=0. The best-fit vibrational excitation is somewhat less than for the Prior model, but the rotational excitation is considerably greater. A consideration of the electronic structure of reactants and products indicates that principal changes occurring in the chemiluminescent reaction are σ–electron donation from the sdz2 hybridized Mn orbital to the O3 lowest unoccupied molecular orbital (LUMO) (2b1) and π–electron backdonation from the O–O 4b2 orbital to the Mn 3dπ orbital. Correlation of the orbitals involved indicates that direct access is allowed to the MnO A 6∑+(10σ*18σ1) state. This mechanism favors Mn approach perpendicular to the O3 plane and suggests that the product’s rotational excitation may originate in O2–OMn repulsion arising from removal of electron density from the slightly bonding 4b2 orbital of O3. However, some rotational excitation could also be attributed to conservation of angular momentum arising from a sizable reactive impact parameter. The lack of significant vibrational excitation is a consequence of the short-range nature of the partial charge transfer in this reaction channel.