Conformational analysis of n-butanal by Chirped-Pulse Fourier Transform Microwave spectroscopy

Abstract The rotational spectrum of n -butanal was obtained in the 7.5–18.5 GHz range using Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectroscopy. In addition to the previously observed cis/trans and cis/gauche conformers, a new conformer gauche/gauche was assigned; tentative assignments were also made on a second new conformer, gauche/trans. Density functional theory calculations using GAUSSIAN03 allowed us to predict the four most stable geometries (cis/trans, cis/gauche, gauche/trans, and gauche/gauche), which were verified as local minima in vibrational calculations. We also performed relaxed potential energy scans and found internal methyl rotor torsional barriers of V 3  = 8.4 kJ/mol (cis/trans), 11.6 kJ/mol (cis/gauche), 12.5 kJ/mol (gauche/trans), and 11.8 kJ/mol (gauche/gauche). The program XIAM was used to determine a V 3 barrier 6.3 ± 0.2 kJ/mol for the gauche/gauche structure, which is significantly smaller than the lowest V 3 barrier predicted by ab initio calculations of 8.4 kJ/mol. Because of the high signal-to-noise of the CP-FTMW spectrometer, we assigned the 13 C n -butanal isotopologues of cis/trans and cis/gauche in natural abundance. Kraitchman analysis was performed to determine the carbon–carbon bond angles and bond distances in these structures based on our observed data.