Computer Methods for Investigating Hydrogen-Bonded Equilibria in Alcohols

  • MD. Fokhray Hossain

    Student thesis: Doctoral Thesis


    The general phenomenon of hydrogen bonding is reviewed in relation to O-H groups in alcohols and the use of physical methods, particularly infrared spectroscopy, and computing methods to gain better understanding of the structures arising from the formation of hydrogen bonds.

    The infrared spectra of methanol as a dilute solution in CC14 has been measured. The integrated absorption coefficient of the monomeric O-H stretching mode is calculated as (2.174 ± 0.012)xl0 4 m mol" 1 and the proportion of the components associated with the three principal bands and a fourth weaker band estimated.

    Seven possible components were considered which were monomer, closed cyclic and open chain dimers, trimers and tetramers. Ab initio calculations were carried out on these components using six basis sets up to the restricted Hartree Fock 6-31 l++G(3df,3pd) level. Relevant calculated infrared wavenumber and intensity values, O-H...O bond lengths and hydrogen bonding energies are reported. The cyclic dimer is shown to be a transition state with the open dimer forming a stable minimum energy form. In the case of the trimer and tetramer the hydrogen bonding energy is calculated to be respectively 12 and 32 kJ mol" greater in the cyclic form than in the open form with good agreement at the RHF/6-3 IG(d) and RHF/6-3 l++G(d,p) levels. The experimental and theoretical results for methanol are consistent with an equilibrium involving monomer, open dimer, cyclic trimer and cyclic tetramer.

    Calculations were also carried out on ethanol. Models of the cyclic trimers created from a number of possible chair and boat forms are optimised by ab initio calculations and lead to new proposed structures. These comprise high energy structures in which the alkyl groups are on the same side of a six-membered ring (with approximate C3 symmetry) which are compared with previously described low energy forms in which alkyl groups are on opposite sides (with approximate Q symmetry). Selected spectroscopic and thermodynamic properties together with calculated structures are determined for both forms using three basis sets and the results are reported at the highest basis set used {RHF/6-3 lG(d,p)}. Hydrogen bonding energies are reported for methanol and ethanol trimers as 46.0 and 45.8 kJ mol" 1 for the C3 form and 48.5 and 48.2 kJ mol" 1 for the C l form. These values are susceptible to choice of basis set within the restricted Hartree Fock method, to the energy terms included and to basis set superposition errors. From the four possible structures for the cyclic tetramers that with £4 symmetry is found to be more stable than those with Q, Q or C 4 symmetry. The results are consistent with an equilibrium involving monomer, open dimer, cyclic trimer (two forms) and cyclic tetramer.
    Date of AwardMay 1998
    Original languageEnglish
    SupervisorRhobert Lewis (Supervisor) & Bill George (Supervisor)

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