Infrared spectroscopic studies of solvent effects on the conformation of n-alkanes

Abstract

. Can. J. Chem. 64, 1544Chem. 64, (1986. The infrared spectra of specifically deuterated n-tridecane-7,7-d2 pure and mixed with several linear and branched hydrocarbons have been measured as a function of temperature. ' The average gauche fraction at the middle of the n-tridecane chain has been determined from the intensities of conformation-specific CD2 rocking bands. The results indicate that the concentration of gauche rotarners in the centre of the n-CI3 chains varies with the solvent. For example, when n-tridecane is dissolved in other n-hydrocarbons the gauche concentration decreases when the chain length of the solvent is shorter than n-CI3 and increases when the solvent chains are longer than n-C13. However, no simple, direct correlation is found between these solvent-induced changes in gauche concentration and measured thermodynamic quantities of mixing.H. L. CASAL, P. W. YANG et H. H. MANTSCH. Can. J. Chem. 64, 1544Chem. 64, (1986. OpCrant ti diverses temgratures, on a mesurC les spectres infrarouges du n-tridCcane-d2-7,7 soit ti l'ttat pur soit sous forme de mClange avec plusieurs hydrocarbures linCaires ou ramifiCs. En se basant sur les intensitCs des bandes de vibration de rotation du CD2 qui sont spkifiques aux conformations, on a dCterminC la fraction moyenne de conformbres gauche qui existe au centre de la chaine du n-tridCcane. Les rCsultats indiquent que la concentration de rotambres gauche dans le centre des chaines n-CI3 varie avec le solvant. Par exemple, lorsqu'on dissout du n-tridCcane dans d'autres hydrocarbures linCaires, la concentration de conformkres gauche diminue si la longueur de la chaine du solvant est plus courte que celle du n-CI3 alors qu'elle augmente lorsque les chaines du solvant sont plus longues que celle du n-C13. Toutefois, il n'existe aucune corrClation directe entre ces changements, induits par le solvant, dans les concentrations de conformbres gauche et les quantitts thermodynamiques de mClange qui ont Ct C mesurkes.[Traduit par la revue]

Quantitative Correlation of Raman Spectral Indicators in Determining Conformational Order in Alkyl Chains

“…The ν s (CH 2 ) peak frequency is a popular indicator of order not only in bulk alkane systems but also in surface-bound alkane systems as probed by surface vibrational methods such as surface FTIR and vibrational sum-frequency generation. − , This indicator should be used with some caution, however, in light of the fact that the ν s (CH 2 ) frequency shifts by 6−8 cm -1 for neat octadecane in the crystalline state. This shift is a clear indication that this indicator alone does not adequately describe the conformational state of these systems.…”

“…Nonetheless, the IR spectra of alkanes also contain modes that are sensitive to alkane conformational order. Specifically, those modes cited most frequently as containing information about conformational order include the ν s (CH 2 ) and ν a (CH 2 ) peak frequencies (∼2850 and ∼2922 cm -1 ) and the ν(C−C) gauche, trans, and kink modes (∼1000−1300 and ∼620−700 cm -1 ). − These indicators are important in elucidating alkane structure in materials; however, the chain coupling information and numbers of gauche and trans conformers provided by these indicators are only a small portion of the information necessary for a complete molecular understanding of the disorder in these materials. More direct comparison between Raman and IR spectral data for elucidation of conformational order is difficult, because the majority of intense Raman-active modes are either weak in the IR or IR-inactive and vice versa .…”

“…Numerous experimental techniques have been used to study alkane-chain structure and conformation including nuclear magnetic resonance (NMR) spectroscopy, ,, Fourier transform-infrared spectroscopy (FTIR), − and Raman spectroscopy. ,,− ,,− NMR experiments specifically probe alkane chain mobility, allowing information about conformational order to be inferred. Although a distinct correlation between chain mobility and conformational order exists, a direct measure of conformational order is necessary for understanding the subtleties of alkane structure and function.…”

“…Numerous experimental techniques have been used to study alkane-chain structure and conformation including nuclear magnetic resonance (NMR) spectroscopy, 20,26,27 Fourier transform-infrared spectroscopy (FTIR), [28][29][30][31][32][33][34][35][36][37][38][39][40][41] and Raman spectroscopy. 5,10,[15][16][17][18]29,[42][43][44][45][46][47][48][49][50][51] NMR experiments specifically probe alkane chain mobility, allowing information about conformational order to be inferred.…”

Conformation of Alkanes in the Gas Phase and Pure Liquids

“…An electron diffraction study of the alkanes n -butane through n -heptane by Bartell and Kohl 4 revealed that the gas-phase conformations often contain gauche dihedral angles, with relatively high populations of conformers with single gauche bonds for n -pentane, n -hexane, and n -heptane. IR measurements for deuterated n -tridecane were interpreted to indicate significant alteration of gauche populations in different alkane solvents . However, Menger and D'Angelo 6 used 13 C NMR to observe the conformational equilibria of undecane-2,5-di- 13 C in solvents ranging from chloroform to aqueous ethanol; by measuring 3 J CC , they obtained the fraction of trans and gauche for the C3−C4 bond.…”