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Structures and polarized electronic absorption spectra of dimolybdenum tetraformate crystals

Technical Report ·
OSTI ID:6538626

Polarized electronic spectra in the 21,500 to 27,000 cm/sup -1/ region are presented and discussed for the three polymorphs (original ..cap alpha.., new ..beta.. and ..gamma..) and for its KCl complex. The 22,000 cm/sup -1/ band is assigned to the delta..-->..delta* transition. A weak progression near 25,000 cm/sup -1/ is assigned to the delta..--> pi..* transition. A fourth, unidentified polymorph may also exist. ..beta..-Mo/sub 2/(O/sub 2/CH)/sub 4/ crystallizes in monoclinic P2/sub 1//c. Four molecules occupy general positions, while two molecules occupy positions of inversion symmetry. ..gamma..-Mo/sub 2/(O/sub 2/CH)/sub 4/ crystallizes in monoclinic P2/sub 1//a. Evidence suggests the trend ..cap alpha..>..beta..(general positions)>..beta..(special positions)>..gamma.. in axial (intermolecular) Mo-O bond strength. The axial bonding produces effects in intramolecular Mo-O bond lengths, and Mo-Mo-O, Mo-O-C, and O-C-O bond angles. Orbital geometries of Mo and O explain the tilted chain stacking geometry found in most dimolybdenum tetracarboxylates. Weak progressions are evident in the spectra of the polymorphs at approx. 6K. The band origin shifts over 160 cm/sup -1/. Progressions were assigned based on similarity to those in Mo/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/. Mo/sub 2/(O/sub 2/CH/sub 4/.KCl shows a progression of broad peaks separated by 368 cm/sup -1/. The broadness is attributed to mixing of states by the K/sup +/ crystal field. A red shift of approx. 150 cm/sup -1/ is consistent with structural evidence for a weakened Mo-Mo bond relative to ..cap alpha..-Mo/sub 2/(O/sub 2/CH)/sub 4/. Although polarizations were opposite the ideal z-polarized case, the spectra were similar to those of Mo/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/. Thus, the Mo/sub 2/(O/sub 2/CH)/sub 4/ delta..-->..delta* transition moment is likely to be oriented away from the molecular z axis by crystal effects.

Research Organization:
Ames Lab., IA (USA)
DOE Contract Number:
W-7405-ENG-82
OSTI ID:
6538626
Report Number(s):
IS-T-996; ON: DE83004863
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ABSORPTION SPECTRA
ALKALI METAL COMPOUNDS
ALLOTROPY
BEAMS
CARBOXYLIC ACID SALTS
CHLORIDES
CHLORINE COMPOUNDS
CRYSTAL STRUCTURE
FORMATES
HALIDES
HALOGEN COMPOUNDS
MOLECULAR STRUCTURE
MOLYBDENUM COMPOUNDS
POLARIZED BEAMS
POTASSIUM CHLORIDES
POTASSIUM COMPOUNDS
SPECTRA
SPECTRAL SHIFT
TRANSITION ELEMENT COMPOUNDS