Knowledge Bank

\maketitle We have investigated two butanols, normal (1-butanol) and iso (2-methyl-1-propanol), by Fourier transform microwave spectroscopy, focusing attention mainly to internal motion and rotational isomerism. We anticipate that $n$-butanol exists in fourteen rotational isomers with the combination of the following conformations: $trans$ ($T$) and $gauche$ ($G$) with respect to the CH$3$CH$2$-CH$2$CH$2$OH bond, $trans$ ($t$) and two $gauche$'s ($g$, $g$') to the CH$3$CH$2$CH$2$-CH$2$OH bond, and two or three potential minima in the internal rotation of OH, and that isobutanol in five. We scanned the frequency region from 7 to 25 GHz using a heated nozzle, and detected six and three isomers for $n$-butanol and isobutanol, respectively: three with Ar as a buffer gas and three with Ne instead of Ar for $n$-butanol, whereas Ar was employed for isobutanol. The observed spectral lines were assigned by comparing the experimental moments of inertia and the observed spectral intensities with those calculated by $ab initio$ MO at the MP2/6-311++G(d,p) level. We thus concluded that five conformers detected for $n$-butanol are $T$-form and one $G$-form and that three observed isomers of isobutanol are $gauche$ with respect to the H(CH$3$)$2$C-CH$2$OH bond and one isomer lacking $a$-type spectra to $trans$ to the CH$2$-OH bond.