The integration of computational and spectroscopic information for hydrogen bonded systems

The infrared spectra of methanol as dilute solutions in CC1₄ 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)x10⁴m 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 6-311++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.<br/><br/>Also, <i>ab initio </i>calculations are reported for HC1 complexes of H₂CO, 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 <i>cis</i> and <i>trans</i> 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 <i>cis</i> to the nitrile or to the carbonyl group <i>trans</i> 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.<br/>

Name dc:type.qualificationname

Doctoral Thesis

Level dc:type.qualificationlevel

Student thesis

Year dc:date.issued

2000

Author dc:creator

• Price, Jason

Language dc:language

eng

Identifier

oai:pure.atira.dk:studenttheses/00cccfcf-1261-4713-89cc-010cbcafdbb6

OAI identifier oai:identifier

oai:pure.atira.dk:studenttheses/00cccfcf-1261-4713-89cc-010cbcafdbb6