Study of Perfluoroalkyl Chain-Specific Band Shift in Infrared Spectra on the Chain Length

Abstract

The CF symmetric stretching vibration (ν(CF)) band of a perfluoroalkyl (Rf) group in an infrared (IR) spectrum exhibits a unique character, that is, an apparent high wavenumber shift with increasing the chain length, which is an opposite character to that of the CH stretching vibration band of a normal alkyl chain. To reveal the mechanism of the unusual IR band shift, two vibrational characters of an Rf chain are focused: (1) a helical conformation of an Rf chain, (2) the carbon (C) atoms having a smaller mass than the fluorine (F) atom dominantly vibrate as a coupled oscillator leaving F atoms stay relatively unmoved. These indicate that a "coupled oscillation of the skeletal C atoms" of an Rf chain should be investigated considering the helical structure. In the present study, therefore, the coupled oscillation of the Rf chain dependent on the chain length is investigated by Raman spectroscopy, which is suitable for investigating a skeletal vibration. The Raman-active ν(CF) band is found to be split into two bands, the splitting is readily explained by considering the helical structure and length with respect to group theory, and the unusual peak shift is concluded to be explained by the helical length.

Infrared active surface modes found in thin films of perfluoroalkanes reveal the dipole–dipole interaction and surface morphology

“…Selected time-dependent IR ATR spectra of C 6 F 14 deposited on Ge during the evaporation process at ambient temperature. 1148 cm −1 have already been confirmed to be the chain stretching vibration and the symmetric CF 2 stretching vibration (νsCF 2 ) band, respectively, 10,11 whereas the assignment of the most intensive band at 1234 cm −1 has long been under discussion. This unconfirmed band is sometimes temporarily assigned to the anti-symmetric CF 2 stretching vibration [νa(CF 2 )] band, but this assignment should be very inaccurate because it yields an inconsistent result with those using other established bands especially when molecular orientation analysis is performed in a thin film.…”

“…For example, the band of the symmetric CF 2 stretching vibration [νs(CF 2 )] mode appears at ∼1150 cm −1 in IR spectra, whereas it appears at ∼720 cm −1 in Raman spectra. 10,11 R f compounds also exhibit a unique higherwavenumber shift of the νs(CF 2 ) band with an increase in the R f…”

“…The Journal of Chemical Physics ARTICLE scitation.org/journal/jcp chain length, whereas hydrocarbons exhibit a lower-wavenumber shift. 11 In addition, R f molecules are spontaneously aggregated because of the helical conformation cooperated with the D-D interaction, and therefore, the aggregate exhibits chirality, which can be detected by Raman optical activity, 12 which never happens in a normal hydrocarbon. Furthermore, R f compounds are known to give an "extra band" that has a very similar shape and intensity to those of the normalmode bands, since the C− −F bond has a fairly large permanent dipole moment.…”

Grafting of amphiphilic block copolymers on lignocellulosic materials via SI‐AGET‐ATRP

“…In the spectrum of the W‐PMETAC sample (purple), the N + (CH 3 ) 3 asymmetric bending band at 955 cm −1 and CN vibration band at 720 cm −1 are observed . Both spectra of the W‐POFPMA samples show a CF vibration band and CF 2 twist band at 820 and 300 cm −1 , respectively …”

Fluorous Property of a Short Perfluoroalkyl-Containing Compound Realized by Self-Assembled Monolayer Technique on a Silicon Substrate

“…Above this temperature, the helical pitch about the Rf chain is a little bit loosen [24][25][26][27] from the point group of D13 to that of D15. 27,28 As a result, temperature above 19ºC is inappropriate for the molecular spontaneous aggregation, which is not good for exhibiting a fluorous property. In fact, the fluorous property of the Rf5-containing PFA is drastically lost and the contact angle is less than 50º at a temperature above 19ºC (cf.…”