Hydrogen bonding in binary mixtures of tetrahydrofuran and water: a concentration dependent Raman study and ab-initio calculations

Content Provider	Semantic Scholar
Author	Ojha, Animesh K. Srivastava, Sunil Kumar Schlücker, Sebastian Asthana, B. P. Kiefer, Wolfgang
Copyright Year	2004
Abstract	This contribution reports on the wavenumber shifts and linewidth changes observed for the Raman bands of the C-O and C-C stretching vibrations of tetrahydrofuran (THF) in a series of binary mixtures with water. Experimental results are compared with ab-initio calculations on THF and hydrogen-bonded THF-water complexes. The Raman spectroscopic analysis of vibrational relaxation processes such as vibrational dephasing is a powerful approach of investigating dynamical processes in liquids. It is possible to identify two principal mechanisms for interpreting the relaxation process. The first process involves a change in the quantum state of the relevant normal mode, while the second process is related to the phase decay of the vibrational wavefunction [1-2]. Both processes can contribute to the spectral band shape and thereby to the vibrational correlation function obtained from them. In most cases, however, vibrational band profiles are determined predominantly by vibrational dephasing. Raman spectra of neat tetrahydrofuran (THF) and its binary mixtures with water were investigated in the wavenumber intervals 1000 – 800 cm and 1100 – 975 cm; mixtures with mole fractions of 0.1, 0.3, 0.5, 0.7 and 0.9 of the reference system THF were analyzed. Figure 1 shows the Raman spectra of neat THF and two binary mixtures with THF mole fractions of 0.5 and 0.1 in the wavenumber region 1000 – 800 cm. For neat THF, the observed asymmetric band shape is due to the presence of two peaks at ~ 914 cm and ~ 909 cm, which are assigned to the symmetric C-O and C-C stretching vibration, respectively. The peak position of the band at ~ 914 cm shows a downshift (red shift) of ~ 7 cm upon dilution, which can be explained with the help of optimized geometries obtained for THF and hydrogen-bonded THF-water complexes using abinitio calculations at the MP2 level. In the binary mixture (THF + H2O), the oxygen atom of the cyclic ether linkage in THF is the hydrogen bond acceptor. The C-O bond length increases upon formation of the hydrogen bonded complex. The reduced C-O bond order explains the observed shift to lower wavenumbers upon dilution. The linewidth for this mode increases linearly upon dilution by ~ 10 cm, which is basically due to diffusion of the solvent molecule. On the other hand, the other peak at ~909 cm, which is assigned to the C-C stretching mode, shows an upshift (blue shift) of ~ 2 cm upon dilution. In addition, a new band at ~ 896 cm is observed, which is assigned to the hydrogen bonded complexes of THF and water; it also shows a downshift of ~ 9 cm at extreme dilution. This assignment is confirmed by the observation that the Raman intensity ratio of the bands associated with hydrogen-bonded THF and “free” THF shows a sudden rise after C = 0.5. The maximum in the linewidth vs. concentration plot at C = 0.5 and the linear variation in the wavenumber position is explained by Bondarev and Mardaeva’s concentration fluctuation model [3].
Starting Page	334
Ending Page	335
Page Count	2
File Format	PDF HTM / HTML
DOI	10.1071/sa0402151
Volume Number	2
Alternate Webpage(s)	http://www.publish.csiro.au/SA/pdf/SA0402151
Alternate Webpage(s)	http://www.publish.csiro.au/?act=view_file&file_id=SA0402151.pdf
Alternate Webpage(s)	https://doi.org/10.1071/sa0402151
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article