Comparison of the Structures of Ammonium Myristate, Palmitate, and Stearate by X-ray Diffraction, Infrared Spectroscopy, and Infrared Hole Burning

Abstract

X-ray diffraction studies of ammonium myristate and palmitate show crystals that belong to space group P2 1 /n with the methylene groups packing in a triclinic subcell. The infrared spectra of the palmitate and stearate are similar as expected, but the bands of the myristate salt show additional splitting in the methylene rocking and twisting bands. A number of other factors of the myristate spectrum and of the infrared hole burning are unique as well. The differences show that the myristate salt has two molecules per spectroscopic unit cell, which are postulated to differ by small rotations of the polar carboxyl groups relative to the plane of the chain. A second form of ammonium myristate is found by crystallizing under different conditions.

Crystal structure determination of the silver carboxylate dimer [Ag(O2C22H43)]2, silver behenate, using powder X-ray diffraction methods

“…This result suggests that there could be some disorder in the eight-membered ring, the long aliphatic side chains, or both. Disorder in carboxylates with long side chains is known, e.g., ammonium myristate structure (Yu et al , 1999). Such disorder is difficult to model in structures determined using powder X-ray diffraction data.…”

Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance along the Backbone

“…For example, adherence to the n-alkane dispersion curve of the progression of infrared methylene rocking and wagging bands has been used successfully to identify the same all-trans packing of methylene sequences of myristate salts that crystallize in two different polymorphs. 21 Trans packing of methylene sequences and conformational disorder has been extracted for other molecules as well from the analysis of progression bands in reference to the n-alkane behavior. 17−21 The effect of the CH 3 ends in n-alkanes or of the acid end group in fatty acids is to reduce the symmetry of the sequence of oscillators and leads to observable, albeit weak, forbidden (even) K modes in their infrared spectra, and to deviations from the predicted frequencies of the n-alkane dispersion curve at the highest K values.…”

“…Taking as reference the unique structural behavior found in PE15Cl, it is likely that other precision polyethylenes may develop more than one solid phase, or display “polytypes” differing from one another only by different stacking arrangements of otherwise identical layers, which are common features observed in n -alkanes and in most fatty acid compounds. − Polytypes are of special significance in evaluating folding or self-assembly of precision polyethylenes with moieties that can be easily incorporated into a crystalline lattice, because the continuity of a chain-like molecule will favor certain stacking arrangements, for example those with higher symmetry, over others that result in more defective crystallites. The latter can be probed evaluating the structure and conformation of other precision polyethylenes with different types and content of co-units.…”

“…Infrared spectroscopy has been highly instrumental in analyzing the conformation of crystalline continuous methylene sequences, based on rocking, twisting, and wagging vibrational bands for a large number of systems, including n -alkanes, , fatty acids, , lipid bilayers, ,, phospholipid gels, intercalated surfactant bilayers, polyamides, , and arylate polyesters, among others. By treating n -alkanes as a system of many oscillators (i.e., methylene groups, CC bonds) and using GF method, Snyder successfully associated the frequencies of methylene rocking, twisting, and wagging, as well as C–C stretching modes to the phase difference (ϕ) in motion between adjacent methylene groups of n -alkanes. , In brief, for a particular vibration of a system of m coupled oscillators, i.e., the methylene sequence of the n -alkane molecule, the solution of the secular equation gives, …”

A new polymorph of ammonium succinate – serendipity in action

“…In the literature, related ammonium salt structures having no functional groups other than carboxylates, show the R 2 4 (8) motif [CSD refcodes AHGLUT (Macdonald & Speakman, 1971), BAMXAR (Churakov et al, 2014), CARVIA (Taka & Kashino, 1999), VOHMOU (Teslya et al, 1990) (Smith, 2014)]. For molecular salts having a long alkyl chain, ammonium palmitate (GUKZOB; Yu et al, 1999) and ammonium myristate (GUKZUH; Yu et al, 1999) show larger hydrogen-bonded ring motifs where the entire carboxylate group is included, giving an R 3 4 (11) motif. This is presumably due to packing factors, were the long alkyl chain prohibits close packing of individual O atoms.…”