Strong-field coherence breaking as a tool for identifying methyl rotor states in microwave spectra: 2-hexanone

Abstract

The rotational spectrum of 2-hexanone was recorded over the 8–18 GHz region using a chirped pulse Fourier transform microwave spectrometer. Strong field coherence breaking (SFCB) was utilized to selectively modulate the intensities of rotational transitions belonging to the two lowest energy conformers of 2-hexanone, aiding the assignment. In addition, the SFCB method was applied for the first time to selectively identify rotational transitions built off the two lowest energy hindered methyl rotor states of each conformer, 0a1 and 1e. Since these two states have rotational energy levels with different nuclear spin symmetries, their intensities could be selectively modulated by the resonant monochromatic pulses used in the SFCB method. The difference spectra, final fit, and structural parameters are discussed for the three assigned conformers of 2-hexanone.

Microwave Spectrum and Internal Rotations of Heptan-2-one: A Pheromone in the Gas Phase

“…Although such a calibration has led to convincing results while going from pentan-2-one ( 2 ) to hexan-2-one ( 3 ) and on to heptan-2-one ( 4 ) (this work), the application is very limited, since every different set of substance class, conformer, rotational constant and level of theory in use would need its own calibration. An alternative way to circumvent this problem is the modulation and analysis of line intensities using the strong field coherence breaking (SFCB) method, as has been applied by Fritz et al for the chirped-pulse microwave spectrum of hexan-2-one ( 3 ) . There, it has led to the assignment of three conformers, similar to our own results investigating hexan-2-one ( 3 ) by using the calibration method …”

“…An alternative way to circumvent this problem is the modulation and analysis of line intensities using the strong field coherence breaking (SFCB) method, as has been applied by Fritz et al for the chirped-pulse microwave spectrum of hexan-2-one (3). 47 There, it has led to the assignment of three conformers, similar to our own results investigating hexan-2-one (3) by using the calibration method. 18 The experimental spectra of the C s and the C 1 conformer are very similar in intensity.…”

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan‐2‐one

“…Alternatively, intensity patterns can be modulated and analyzed using strong-field coherence breaking (SFCB), as has been done for methyl butyrate [16] and hexan-2-one. [33] Other approaches aim at improving the quantum chemical calculations. One example is how basis set superposition errors of the MP2 method can be estimated by the socalled atomic counterpoise method.…”