Date of Award
Doctor of Philosophy
The crystal structures of two new polymorphs of Mo(,2)(O(,2)CH)(,4) are reported. The original form is designated the (alpha)-polymorph, and the new forms are designated the (beta)- and (gamma)-polymorphs. Polarized electronic spectra in the 21500-27000 cm('-1) region are presented and discussed for the three polymorphs and for Mo(,2)(O(,2)CH)(,4)(.)KCl. The band originating at (TURN)22000 cm('-1) is assigned to the (delta)(--->)(delta)* transition, with comparable intensity provided by the electric dipole and vibronic mechanisms. A weak progression observed near 25000 cm('-1) is assigned to the (delta)(--->)(pi)* transition, which is vibronically-allowed, but electric dipole-forbidden. Spectral evidence suggests the existence of a fourth, unidentified polymorph;(beta)-Mo(,2)(O(,2)CH)(,4) crystallizes in the monoclinic space group P2(,1)/c with a = 5.485(1)(ANGSTROM), b = 12.365(2)(ANGSTROM), c = 19.862(4)(ANGSTROM), (beta) = 90.24(2)(DEGREES), V = 1347.1(5)(ANGSTROM)('3), and z = 6. Four molecules occupy general positions, while two molecules occupy positions of inversion symmetry. (gamma)-Mo(,2)(O(,2)CH)(,4) crystallizes in the monoclinic space group P2(,1)/a with a = 7.939(1)(ANGSTROM), b = 11.193(1)(ANGSTROM), c = 5.5271(9)(ANGSTROM), (beta) = 110.86(2)(DEGREES), V = 458.9(1)(ANGSTROM)('3), and z = 2. The three polymorphs were found to differ slightly in molecular packing. Structural evidence suggests the trend (alpha) > (beta)(general positions) > (beta)(special positions) > (gamma) in axial (intermolecular) Mo-O bond strength. The axial bonding produces observable effects in intramolecular Mo-O bond lengths, and Mo-Mo-O, Mo-O-C, and O-C-O bond angles. The orbital geometries of Mo and O explained the tilted chain stacking geometry found in most dimolybdenum tetracarboxylates;Weak progressions are evident in the spectra of the polymorphs at (TURN)6K, in addition to the three strong ones previously reported. The band origin shifts over a range of 160 cm('-1), due to slight differences in Mo-Mo and axial Mo-O bonding. Progressions were assigned by separations from the band origin, based on similarity to those observed in crystals of Mo(,2)(O(,2)CCH(,3))(,4). Mo(,2)(O(,2)CH)(,4)(.)KCl shows a progression of broad peaks separated by 368 cm('-1). The broadness is attributed to mixing of states by the k('+) crystal field. A red shift of (TURN)150 cm('-1) is consistent with structural evidence for a weakened Mo-Mo bond relative to (alpha)-Mo(,2)(O(,2)CH)(,4);Although polarizations were found to be opposite the ideal(, )z-polarized case, the spectra were shown to be similar to the spectraof Mo(,2)(O(,2)CCH(,3))(,4). Thus, the Mo(,2)(O(,2)CH)(,4) (delta)(--->)(delta)* transition momentis likely to be oriented away from the molecular z axis by crystal effects as found in Mo(,2)(O(,2)CCH(,3))(,4).
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Gary Alan Robbins
Robbins, Gary Alan, "Structures and polarized electronic absorption spectra of dimolybdenum tetraformate crystals " (1982). Retrospective Theses and Dissertations. 7529.