Quantum chemical and microwave spectroscopic investigations on phenyl ring containing molecules
- Quantenchemische und mikrowellenspektroskopische Untersuchungen an Phenylverbindungen
Ferres, Lynn; Stahl, Wolfgang (Thesis advisor); Lüchow, Arne (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2019
This dissertation reports on the results found in phenyl ring containing molecules obtained via a combination of quantum chemical calculations and microwave spectroscopy. The focus of this work is the influence of slight changes in the molecular structure on the torsional barrier height of methyl groups. The thirteen investigated molecules can be divided by their structures into four groups: 1) Phenetole, representing a rigid-rotor case, 2) Three methylanisoles forming a series of isomers, showing the influence of the substitution site on the barrier to internal rotation of the methyl group, 3) Six constitutional isomers of dimethylanisole, enabling the study of the couplings of internal rotation, and 4) Three molecules performing tunneling motions of the benzene moiety: phenyl formate, phenyl acetate, and S-phenyl thioacetate.In the first part of the dissertation, results from quantum chemical calculations agreed with the experimentally determined molecular parameters of phenetole. The next part comprises the investigations on ortho-, meta-, and para-methylanisole, for which three different barriers to internal rotation were found, showing that the position of the substituent plays a crucial role in the intra-molecular dynamics. Hence, the barrier of 444.05(41) cm−1 found in o-methylanisole is the largest, due to steric hindrance of the methoxy group. The barrier heights of 55.7693(90) cm−1 and 36.6342(84) cm−1 determined for cis-m- and trans-m-methylanisole, respectively, are of the same order of magnitude as the barrier found in p-methylanisole of 48.7400(93) cm−1. Some fitting problems occurred while using the program XIAM, as the barriers to internal rotation are low. Therefore, higher order terms in the Hamiltonian are needed to describe the spectra of these molecules more accurately. Two other programs namely BELGI-Cs and aixPAM were used for this purpose, yielding fits with standard deviations lying within the measurement accuracy. The next step is the analysis of a system with two internal rotors. For an adequate comparison, a system similar to the methylanisoles was chosen: the dimethylanisoles. Six different isomers exist, presenting different study cases. There are isomers with two high barriers to internal rotation as for example 3,4-dimethylanisole (424.16(41) cm−1 and 455.58(19) cm−1 for the cis-conformer and 491.29(35) cm−1 and 519.68(28) cm−1 for the trans-conformer, respectively) and the non-planar 2,6 dimethylanisole (199.0778(11) cm−1 and 457.440(31) cm−1). A case of two low barriers to internal rotation (36.10945(15) cm−1 and 58.57733(11) cm−1) is 3,5-dimethylanisole. A combination of a high and a low barrier to internal rotation occurs in 2,5-dimethylanisole (466.428(73) cm−1 and 65.71452(11) cm−1), in 2,4-dimethylanisole (435.649(20) cm−1 and 48.192(25) cm−1), and also in 2,3 dimethylanisole (518.7(1.2) cm−1 and 26.9047(5) cm−1). Finally, it is concluded that adjacently located rotors exhibit greatly influenced torsional potentials compared to the values found for the mono-methylanisoles mentioned in the previous paragraph. Therefore, further fitting parameters become necessary to decrease the standard deviation. However, these trends cannot yet be assumed with absolute certainty, as the literature shows a lack of comparable studies.In the final part, the tunneling motions of larger molecular moieties are studied, as for example, the formate group in phenyl formate. The complexity is increased by addition of an internal rotor (phenyl acetate) and by substitution of the oxygen atom by a sulfur atom (S-phenyl thioacetate). Hence, very large standard deviations were found by applying the program XIAM. This could be partially resolved for the A species, but in general, no suitable program currently exists to describe the large amplitude motions in these molecules. In S-phenyl thioacetate, two simultaneous inversions occur, which complicates the assignment and fitting processes tremendously. However, the barrier to internal rotation of both, phenyl acetate and S-phenyl thioacetate, could be determined, which validates the already known decreasing trend of the barrier to internal rotation by substituting the oxygen by a sulfur atom.