Vibrational spectra and carbon–hydrogen stretching mode assignments for a series of n-alkyl carboxylic acids

Abstract

The Raman and infrared spectra for the C 4 ,C S ,C 12 and C 16 carboxylic acid series were obtained at liquid nitrogen temperatures, in the 2900 cm ~ 1 spectral region and were compared to spectra for selected branched and straight chain alkane systems. For hexadecanoic acid-d J the Fermi resonance interaction between the CD J symmetric stretching mode and the overtone level of the CD 3 asymmetric deformation vibration was examined as a function of physical state. In this example of a negative Fermi resonance perturbation, in which the lower frequency overtone component is more intense than the higher frequency fundamental, the coupling increases as the system passes from either the matrix isolated species or solution phase to the polycrystalline state. The spectra of the carboxylic acid series, as well as the spectra of 2,2,4-trimethyl pentane and n -octane, suggest that in the 2900 cm ~ 1 region an analogous Fermi resonance coupling exists between the CH 3 symmetric stretching vibration at -2938 cm~l and the 2871 cm~l overtone transition of the CH J asymmetric deformation. Unfortunately, the 2930-2940 cm-1 region for the solid systems is further complicated by transitions attributed to crystal interactions. Although the Fermi interaction for the methyl group has been recognized in the simpler halomethane system, for example, it has not been previously discussed in the more general context of significantly larger branched and straight chain molecules. Comparisons of the carboxylic acid spectra in the 2900 cm~l region have also led to assignments for the a-CH 2 (adjacent to the carbonyl group) and the w-CH 2 (adjacent to the methyl group) C-H stretching modes.

Tracking heavy water (D 2 O) incorporation for identifying and sorting active microbial cells

“…1 A and B). C-D X bonds of various types, including C-D 2 and symmetrical and asymmetrical C-D 3 bonds originating from various biomolecules, reportedly produce Raman bands between 2,013 and 2,300 cm −1 (38)(39)(40)(41)(42)(43)(44). Furthermore, modeling of the Raman peaks induced by the C-D X bond using deuterated palmitic acid as the model compound also predicted peaks in this region (SI Appendix, Fig.…”

Raman spectra and conformations of n-hexadecynoic fatty acids and their potassium salts

“…t,1 is the symmetric stretch mode and v6 the anti-or asymmetric vibration [23] of the CH2 groups in a polymethylene chain. The asymmetric stretch of the terminal methyl groups pa(C-H), the in-skeletal plane mode, was found between 2960-3010 cm 1 [7,[23][24][25] and the Z'a(C-H), the out-of-skeletal plane mode, stretch vibration could be assigned to the lines near 2950 cm -a. The antisymmetric stretch v~ (C-H) of the c~-CH2 group, located near the carboxylic head group, was not observed.…”

“…The antisymmetric stretch, va(C-H) of the central CH2 groups, was observed around 2915-2880 cm 1. The ~,a(C-H) vibration of the terminal CH2 group might lie in the region 2900-2890 cm -~, but the assignment is uncertain [7]. The symmetric stretch u,(C-H) of the methyl end-group was observed near 2870 cm -1, while a similar vibration around 2845 cm -1 could be assigned to the CH2 groups.…”

“…Nevertheless assignments could be made for the hexadecynoic acids and the potassium salts following published data on related compounds [7,23,24]. The frequency branches v6 and vl are found in this spectral region [1].…”

“…The bands can be attributed to four different groups present in the compounds: the carboxylic head group, the polymethylene chains, the central acetylene segment and the methyl end group. For the assignment of the Raman lines the spectral data of polyethylene [6], n-hexadecanoic acid [1,7], cis-and trans-9-hexadecanoic acid [2], adipic acid [8], 2-butyne, 2-pentyne and 3-hexyne [9], and several other alkyl-acetylenes [10,11] together with those for anionic soaps [12,13] were used. The assignments were also checked using normal coordinate calculations of polymethylene chains [14--16] and adipic acid crystals [8].…”