Degree Type

Dissertation

Date of Award

1982

Degree Name

Doctor of Philosophy

Department

Chemistry

Abstract

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).

DOI

https://doi.org/10.31274/rtd-180813-5299

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Gary Alan Robbins

Language

en

Proquest ID

AAI8224247

File Format

application/pdf

File Size

163 pages

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