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We use a custom built cavity-based Fourier-transform microwave spectrometer to record signals from molecular rotational transitions in the 9 – 22 GHz frequency range. The heart of the spectrometer is a resonant cavity established by two diamond-tip polished aluminum mirrors which housed in a vacuum chamber pumped by a VHS-10 diffusion pump. Gas-phase samples are entrained in argon carrier gas and expanded into the vacuum between the mirrors. Microwave radiation causes transitions between rotational quantum states, and the sample becomes coherently polarized. This polarization of rotationally excited molecules induces an oscillating signal in an antenna mounted on the mirror. The signal is down-converted to a lower frequency using a heterodyne microwave circuit and then digitized. Fourier-transformation of the oscillating signal generates the spectrum in the frequency domain.
The energies of molecular rotational states depend on three rotational constants (A, B, C), which, in turn, are inversely proportional to the moments of inertia about 3 spatial axes (Ia, Ib, and Ic). The moments of inertia are related to molecular structure where
and mi is the mass of each atom and ai is the corresponding a-coordinate. We can determine rotational constants by assigning and fitting observed spectra.
We used this technique to explore the conformational structures of valine methyl ester. Spectra arising from two conformations of the isopropyl side chain were assigned. One of the conformers, with chi1 = -64°, had been observed previously for neutral valine and valinamide, but the conformer with chi1 = 174° had not been detected previously by rotational spectroscopy. We have conducted similar structural investigations of cyclobutenone, guaiacol and guaiacol-argon, 2-phenylethyl methyl ether (and the argon complex), leucinamide, valinamide, alaninamide, and prolinamide.
D. Marasinghe, R. M. Gurusinghe, and M. J. Tubergen, J. Phys. Chem. A 128, 3266 – 3272 (2024). doi: https://doi.org/10.1021/acs.jpca.4c00388.
The frequencies of rotational transitions are very sensitive to molecular structure. Simply replacing a hydrogen atom with a deuterium isotope may shift the frequency of a transition by 100 MHz, which is very large compared to our resolution of 2.5 kHz. Even small structural changes can be detected, and we were able to detect the structural changes that occur when 2-aminoethanol forms a 1:1 complex with water. The water forms a new network of hydrogen bonds, donating a hydrogen bond to the amino nitrogen and receiving one from the hydroxyl group. The N – C – C – O dihedral angle increases from 57° in the monomer to 75° in the complex to accommodate the new hydrogen bonding network. A similar increase in the O – C – C – O dihedral angle of glycidol upon forming new intermolecular hydrogen bonds with water.
M. J. Tubergen, C. R. Torok, R. J. Lavrich. Effect of Solvent on Molecular Conformation: Microwave Spectra and Structures of 2-Aminoethanol van der Waals Complexes. J. Chem. Phys. 119, 8397 - 8403 (2003).
We have recently become interested in the effect of quantum mechanical tunneling on rotational spectra. Internal rotation of methyl groups is associated with a potential energy with three equivalent minima. Wavefunctions within each potential well tunnel through the barriers and combine with each other to form tunneling states with A and E symmetry. These states are slightly separated in energy and give rise to spectral splittings for each transition. We can learn about the height of the potential barrier and the orientation of the rotor by assigning and fitting transitions arising from the A and E states. We investigated the series of methylindoles, with the methyl group attached to each position around the indole ring. Each species exhibited methyl internal rotation splittings which were assigned and fit to barriers heights, which ranged from 121 cm-1 to 426 cm-1. The barrier heights depend on intramolecular steric and hyperconjugative effects, and our data helps lead to a better understanding of the origin of torsional barriers.
R. M. Gurusinghe and M. J. Tubergen. J. Phys. Chem. A 120, 3491 – 3496 (2016).
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