Measuring the Rotational Constants of 4-Fluorophenol Using Fourier-Transform Microwave Spectroscopy

We sought to determine the structure of the chemical 4-fluorophenol using microwave spectroscopy. When a photon in the microwave frequency range interacts with a molecule, there is a transition between rotational energy levels. By sending photons of varying frequencies, we were able to measure a spectrum of transitions. Furthermore, molecules rotate differently based on how their mass is spread apart in the x, y, and z planes. This distribution of mass, called the moment of inertia, determines three rotational constants, each of which correspond to how the molecule rotates. Lastly, we determined the centrifugal distortion constant to account for the bond stretching caused by the molecular rotation. Our method consisted of measuring the 8-18 GHz spectrum using chirped microwave pulses. The chirped microwave pulse is broadcast into a low-pressure vacuum chamber, where it interacts with the molecule, and the energy transitions are measured. After measuring an experimental spectrum, we used the program Gaussian to perform ab initio calculations and create a theoretical spectrum for 4-fluorophenol. We then used fitting software to assign lines from the theoretical transitions to the experimental ones until a low overall error was achieved. We concluded that the rotational and distortion constants were the following: A = 5625.610(36) MHz, B = 1454.67600(30) MHz, C = 1155.99820(49) MHz, DJ = 0.0340(42) kHz with an overall RMS error of 0.0017 MHz