Towards a more detailed understanding of the OH-stretching bands of H-bonded liquids

New Method to Analyse IR-Studies of Monomer-Dimer Self-Association: 2,2-Dimethyl-3-Ethyl-3-Pentanol in Carbon Disulfide as an Example

Two new equations have been derived to treat the IR data from the monomer-dimer self-association. Equation (9), in the text, is to regress the integrated absorbances of the monomer bands to determine the molar absorptivity of the monomer band, ε m, and the dimerization constant, K. Equation (13), in the text, is to regress those of dimer band to obtain the molar absorptivity of the dimer band, ε d, and dimerization constant K. The self-association in the dilute solution of 2,2-dimethyl-3-ethyl-3-pentanol in carbon disulfide at different temperatures has been used as an example to demonstrate the usage of these two equations. The discrepancy between the values of K at the same temperature determined either from the monomer band or from the dimer band can be used as a criterion for the goodness of determination. The standard enthalpy and entropy of dimerization have also been obtained via a van’t Hoff plot from the data of K at different temperatures. The advantages of this new approach have also been discussed.

New approach to IR study of monomer-dimer self-association: 2,2-dimethyl-3-ethyl-3-pentanol in tetrachloroethylene as an example

The dimerization of 2,2-dimethyl-3-ethyl-3-pentanol in tetrachloroethylene in the diluted region has been studied at four temperatures by IR spectroscopy. The aforementioned solute compound is chosen because self-association beyond dimerization is hampered by the steric hindrance generated by the bulky sidechains. The integrated absorbances of the monomer bands were treated based on Eq. (9) to obtain its molar absorptivity and dimerization constant. The same dimerization constant as well as the molar absorptivity of dimer band can be obtained based on Eq. (13) from the data treatment of the integrated absorbances of the dimer band. The disparity between two values of dimerization constant determined by two independent sources offers an opportunity to check the consistency of the determination. The standard enthalpy and entropy of dimerization have also been calculated by means of van't Hoff plot, respectively, from the data of temperature-dependent dimerization constants obtained from the monomer bands and dimer bands.

Vibrational relaxation and coupling of two OH-stretch oscillators with an intramolecular hydrogen bond

We studied the vibrational dynamics of the OH-stretch oscillators of an alcohol with two vicinal OH groups using femtosecond midinfrared pump-probe spectroscopy. The absorption spectrum of pinacol (2,3-dimethyl-2,3-butanediol) in CDCl3 shows two OH-stretch peaks belonging to hydrogen bonded and free OH groups. The anharmonicities of the hydrogen-bonded and free OH-stretch vibrations are 180 and 160 cm(-1), respectively. The lifetime T1 of the OH-stretch vibration is found to be 3.5 +/- 0.4 ps for the hydrogen bonded and 7.4 +/- 0.5 ps for the free OH group. We observed sidebands in the transient spectra after excitation of the bonded OH group, which we attribute to a progression in a low-frequency hydrogen-bond mode. The sideband is redshifted 60 cm(-1) with respect to the 0 --> 1 transition. Due to the coupling between the two OH groups and the presence of the sidebands, simultaneous excitation of both OH-stretch vibrations leads to oscillations on the pump-probe signal with frequencies of 40 and 60 cm(-1).