Small esters, ketones, and amines with large amplitude motions

Five esters, two ketones, and three amines showing two different types of large amplitude motions, internal rotation and nitrogen proton tunneling, have been investigated by a combination of molecular beam Fourier transform microwave spectroscopy and quantum chemical calculations. The molecular structure was characterized in terms of experimental rotational constants which have been used to validate the results from quantum chemical calculations. The molecular dynamics has been described by the barriers to internal rotation or inversion. These experimental data were supplemented by potential curves and two-dimensional potential energy surfaces obtained by quantum chemical calculations. The esters and ketones were selected for different frame symmetries and different numbers of internal rotors. This large variety of molecules was important since they were also considered as test systems for the three computer programs XIAM, BELGI, and Erham which treat the internal rotation problem with different methods. In comparative studies the range of potential barriers, for which reliable results can be expected, were investigated. The programs BELGI and XIAM were compared for three molecules, ethyl acetate (one rotor, CS), allyl acetate (one rotor, C1), and methyl propionate (two rotors, CS). XIAM is an user-friendly and extremely fast program. It can be used to fit rotational spectra of molecules with up to three internal rotors of an intermediate or high barrier to almost experimental accuracy. However, the program code needs to be improved for the low barrier case. For ethyl acetate and allyl acetate, where the barrier to internal rotation is on the order of 100 cm-1, XIAM can fit the A species to experimental accuracy but the predictive power for the E species is not satisfactory. BELGI is also easy to use and has no problem with fitting spectra of molecules with low-barrier internal rotation. In contrast to XIAM, in the case of methyl propionate with a rather high barrier to internal rotation of 800 cm-1, BELGI has shown problems with correlations between some internal rotation parameters. The XIAM code was also compared with the Erham code in the case of vinyl acetate (one rotor, CS), isopropenyl acetate (two rotors, C1), and acetone (two equivalent rotors, C2v). Erham is very fast, the transitions frequencies can be usually fitted close to experimental accuracy even for the low barrier case. Especially in the case of isopropenyl acetate, where the splitting due to the propenyl methyl group is in the same order of magnitude as the standard deviation obtained from the XIAM fit, Erham was a good choice to check the assignment. However, since Erham is based on an effective Hamiltonian, it is difficult to extract the rotational barrier. The physical meaning of the fitted parameters is less clear than in the other two programs. Methyl acetate has been the only acetate investigated in the microwave region so far. Investigations of four new acetates within this thesis have shown that typically a low barrier on the order of 100 cm-1 can be expected for the acetyl methyl group. Some higher barriers were found for vinyl acetate (158 cm-1) and isopropenyl acetate (135 cm-1). This is probably due to electronic effects which extends from the vinyl double bond to the ester group. Comparison of quantum chemical calculations using different methods and basis sets with the experimental results (rotational constants, angles between the internal axes and the principal axes) have shown, that currently no general purpose method and basis set for esters and ketones exists. This became especially clear in the case of diethyl ketone and methyl propionate. Therefore, the results of this thesis are also considered as reference data for future developments in quantum chemistry. A combination of three interesting effects, nitrogen inversion tunneling, 14N quadrupole coupling, and high barrier internal rotation was found in two amines, diethyl amine and methyl tert-butyl amine. Nitrogen inversion tunneling in secondary amines has been studied only for dimethyl amine and ethyl methyl amine before. In contrast to previous work, the inversion splittings were described with molecular parameters using the programs spfit and spcat by Pickett in a global fit instead of reporting the splitting of each single rotational transition. For diethyl amine, it was the first time that the complete 14N nuclear quadrupole coupling tensor could be determined just from a perturbation due to a near degeneracy of two energy levels without additional information from isotopic substitution. 14N quadrupole coupling was also studied in triethyl amine, an oblate top molecule with a propeller-like structure. Also in this case, quantum chemical calculations turned out to be very helpful to predict the stable conformers and to give reasonable start values for spectrum assignment, since many possible conformers can be generated by rotating the three ethyl groups