Structural and Thermodynamic Study on Short Metal Alkanoates: Lithium Propanoate and Pentanoate

Abstract

Lithium propanoate and pentanoate were characterized by DSC, single crystal and powder XRD and FTIR and impedance spectroscopies. Lithium propanoate presents a solid-to-solid transition (SII-SI) at T(ss) = (549.1 +/- 0.7) K on first heating that varies on the second and next ones, followed by a fusion at T(f) = (606.1 +/- 0.5) K. For lithium pentanoate, two solid-to-solid transitions (SIII-SII and SII-SI), at T(ss) = (205.5 +/- 0.5) K and T(ss) = (325.2 +/- 0.7) K, respectively, and a melting point at T(f) = (576.5 +/- 0.3) K were found. The crystal structures for both compounds were characterized at 100 and 298 K (and for the lithium propanoate also at 160 K). Single-crystal XRD showed that the SII phase of both compounds has a monoclinic structure with the same symmetry group (P2(1)/c). This is the first time that a single-crystal structure has been reported for any member of the lithium alkanoates series, so far. FTIR and impedance spectroscopies were also carried out to better characterize the solid phases in these compounds.

Thermal and odd–even behaviour in a homologous series of lithium n-alkanoates

“…However, they are in excellent agreement with values reported for other soaps [27,28]. Other studies [11,[29][30][31] have shown that with increasing temperatures the lamellar structure is maintained, though with increased disorder of the alkyl chains, leading to a reduction in interlayer d spacing. Only on melting is the chain conformation destroyed.…”

“…This indicates that these carbons are in a similar chemical environment. Also, no odd-even alternation was observed in the asymmetric stretch of 1.12 ⁄ LiC 10 1.08 1.07 LiC 11 1.12 ⁄ LiC 12 1.06 1.06 LiC 13 1.10 ⁄ LiC 14 1.05 1.05 LiC 15 1.10 ⁄ LiC 16 1.05 1.04 LiC 17 1.09 ⁄ LiC 18 1.04 1.04 LiC 19 1.08 ⁄ ⁄No available data, q exp ± 0.02 Á 10 3 kg Á m À3 . the methyl groups in the IR spectra [9].…”

“…However, an explanation for alternation can be elicited from the model shown in figure 3. According to Cheda and coworkers [11], lithium is tetrahedrally coordinated to four oxygen atoms as shown. The chains in the bi-layer are tilted at an average angle, h of $60°to the lithium basal plane.…”

“…Additionally, the compounds crystallize in a triclinic system with P1 space group. However, for the short chain length compounds, at least up to lithium pentanoate, a monoclinic crystal structure with P2 1 /c symmetry is indicated [11]. Since the strength of the lithium-oxygen bond is reported to be chain length dependent [9,10,12], it is entirely reasonable to expect that the thermal behaviour of these compounds will be governed by chain length, the relative strength of van der Waals interaction between hydrocarbon chains and the arrangement of the chains within the molecular lattice.…”

Thermal decomposition of barium valerate in argon

“…It presents all the characteristic absorption bands of a valerate by comparison with published data for related compounds like Yvalerate [20], Dy-valerate [19], Li-valerate [21], Zn-valerate [22] and Cu-valerate [18]. The absence of the νs absorption band for H2O around 3400cm -1 [23] indicates that the Ba-valerate produced by the present technique is anhydrous.…”