The almost free rotation of the propynyl methyl group

In the first part of this thesis, the computational approach to the theory behind microwave spectroscopy is presented. Firstly, the effect of a linear Operator $\hat{O}$ on any wave function $\Psi$, represented as an element of the Hilbert space, is considered. Then, from the energy terms known from classical physics, the Hamiltonian $\hat{H}$ of a rigid symmetric rotor is derived. In the next steps, an internal rotor with C$_{3}$ rotational symmetry attached to the rigid frame is introduced. It is then shown that the kinetic energy T can be expressed in terms of the angular momentum components P$_{g}$ with g $\in$ {a, b, c, $\alpha$}. Thus the matrix elements of the Hamiltonian matrix can be derived from the known commutator relations of the ladder operators P$_{+}$ and P$_{-}$ and the angular momentum operators. In the second part of this thesis a well-scaling computational concept and Program for Automated Microwave Peak Assignment (PAMPA) is introduced. First of all, the scaling problem of a global approach is explained. Afterwards, a generalized concept for the solution of problems of this kind and this specific case is presented. In this case,the approach is a reduction from the global approach to many single transition fits and the consequential evaluation of the correlation data. Subsequently, the program's structure, written in object oriented python, is presented as pseudo code. Conclusively, the program's functionality is proven on the other molecules researched in this thesis. In the last part, the microwave spectra of five molecules with CH3−C≡C−R structure, 3-pentyn-1-ol, 3-pentyn-2-ol, 4-hexyn-3-ol, methyl-2-butynoate, and ethyl-2-butynoate is described. The C-C-triple bond inserted between the methyl group (−CH3) and the residual group R, acts as a spacer leading to very low barriers to internal rotation of the so called propynyl methyl group. The measurement, assignment and fit process is explained in detail in the 3-pentyn-1-ol chapter (chapter 4). In total, eight conformers could befitted to measurement accuracy. For the three alcohols, as well as for the two esters of 2-butynoic acid, a correlation between the carbon chain length of the residual group Rand the barrier height of the propynyl methyl group's barrier to internal was found. In both groups the barrier decreased with increasing carbon chain length. The fitted values range from 9.4552(94) cm$^{−1}$ for 3-pentyn-1-ol to as low as 0 cm$^{−1}$ for both conformers of ethyl-2-butynoate. Additionally, for 4-hexyn-3-ol, for which three conformers were assigned, it was found that the barrier also decreases from less to more sterically hindered conformers. Both observations contradict common chemical intuition. For methyl-2-butynoate a barrier to internal rotation of 0.4690(36) cm$^{−1}$ was found, which to my knowledge, is of now the lowest determined value greater zero

Conformational effect on the almost free internal rotation in 4-hexyn-3-ol studied by microwave spectroscopy and quantum chemistry

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Low barrier methyl rotation in 3-pentyn-1-ol as observed by microwave spectroscopy

International audienceThe rotational spectrum of 3-pentyn-1-ol, CH3−C≡C−CH2CH2OH, was measured using a molecular beam Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. A two-dimensional potential energy surface was calculated at the MP2/6-311++G(d,p) level of theory for a conformational analysis, yielding five conformers. The most stable conformer exhibits C1 symmetry and was assigned in the spectrum by comparison with the results from quantum chemical calculations. The barrier to internal rotation of the propynyl methyl group CH3−C≡C− was found to be only 9.4552(94) cm−1. Molecular parameters and internal rotation parameters could be accurately determined using the program xiam and belgi-C1. The internal rotation barrier was compared with those of other molecules containing a propynyl methyl group