AbstractThe infrared spectra of methanol as dilute solutions in CCU and in the vapour phase have been measured between 2500 and 4000 cm"1 and between 1000 and 1100 cm"1 in order to better understand the nature of the hydrogen bonding equilibria present. The integrated absorption coefficient of the monomeric O-H stretching mode is calculated as (2.157 ± 0.025)xl04 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-H-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" 1 greater in the cyclic form than in the open form with good agreement at the RHF6-31G(d) and RHF 6-31++G(d,p) levels.
The experimental and theoretical results are consistent with an equilibrium involving monomer, open dimer, cyclic trimer and cyclic tetramer.
Also, ab initio calculations are reported for HC1 complexes of H^CO, CH3HCO, (CH3)2CO, HCN, CH3CN, C2H5CN, HCOCN and CH3COCN. Comparison with experimentally determined values of hydrogen bond energy, HC1 wavenumber shifts and bond lengths for the four smallest complexes suggest a suitable method and basis set is B3LYP/6-311++G(2d,2p). In the case of CH3HCO
there are two possible isomers in which the HC1 is cis and trans to the aldehydic H atom (low and high energy forms respectively).
The two bi-functional complexes each have three possible isomers; HC1 may bond to the nitrile group, to the carbonyl group cis to the nitrile or to the carbonyl group trans to the nitrile . Calculated values of hydrogen bond energy; harmonic HC1, CO and CN stretching modes; hydrogen bond lengths and other associated lengths and angles are reported for all seven mono- and all six bi-functional complexes and compared with experimentally determined values, when known. These properties are predicted in other cases including those of the newly described high energy complex of CH3HCO, and those of three newly described complexes of both HCOCN and CH3COCN.
|Date of Award||Dec 2000|
|Supervisor||Rhobert Lewis (Supervisor)|