•Automated assignment of broadband rotational spectra.•Structure determination and conformational assignment of 1-hexanal.•Experimental determination of conformational abundances.•Systematic study of quantum chemistry techniques for determination of accurate molecular structures.Broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectrometers have increased the sensitivity for molecular rotational spectroscopy. The measurement dynamic range is often large enough that isotopologues of the molecular species with the highest transition strength will also be detectable. In order to analyze the complex spectra from these broadband measurements, an automated spectral assignment program called AUTOFIT has been developed. The algorithm of AUTOFIT is described and its performance is illustrated by the analysis of the CP-FTMW spectrum of 1-hexanal obtained over the spectral range 6–40 GHz. The rotational spectra of a total of 12 conformers of 1-hexanal have been assigned using AUTOFIT to automatically identify the spectrum of conformers predicted by ab initio calculations. In addition, the rotational spectra of the 13C and 18O isotopologues for the two lowest energy conformers and the 13C isotopologues for the third and fourth lowest energy conformers are assigned. The effect of the quality of the input theoretical estimates of the rotational spectroscopy parameters on AUTOFIT performance is discussed. It is shown that the use of B3LYP-D3/aug-cc-pVTZ optimized structures can improve AUTOFIT search speeds by a factor of 10–60× compared to B3LYP structures.

The pure rotational spectrum of glycolaldehyde has been recorded from 6.5–20 GHz and 25–40 GHz in two pulsed-jet chirped pulse Fourier transform microwave spectrometers. The high phase stability of the spectrometers enables deep signal integration, allowing transitions from the 13C-substituted, 18O-substituted, and deuterium-substituted isotopologues to be observed in natural abundance. Transitions from HCOCH218OH are reported for the first time. Additional transitions from the 13C-substituted, deuterium-substituted, and HC18OCH2OH isotopologues, as well as previously unobserved weak lines from the main isotopologue, have been observed. Transitions from all isotopologues are used with previously reported transitions to refine the spectroscopic parameters for each isotopologue. A Kraitchman analysis was performed using the experimental rotational constants to determine the molecular structure of glycolaldehyde.Graphical abstractHighlights► The rotational spectrum of glycolaldehyde has been recorded up to 40 GHz. ► Transitions from the 13C, 18O, and D substituted isotopologues were observed. ► Transitions from HCOCH218OH are reported for the first time.► A Kraitchman analysis was performed to determine the glycolaldehyde structure.

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl4). Ultrafast excitation of the broad O–H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O–H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm−1) and the chemical identity of the dimer.

The rotational spectra of 3-fluoropropyne in the ground and first excited acetylenic C–H stretch vibrational state have been measured. The pure rotational spectrum of the normal species and the 13C isotopomers were measured using FTMW-cwMW double-resonance spectroscopy based on the Autler–Townes (AC Stark) effect. The lineshape properties of this measurement make it possible to determine the transition strength, ΔJ-selection rules, and the relative energy ordering of the quantum states. The frequency accuracy of this technique is tested against a previous pure rotational study of 3-fluoropropyne. The rotational spectrum of vibrationally excited state was obtained through IR-FTMW-cwMW methods. In this technique a single-longitudinal-mode pulsed infrared laser source vibrationally excites the acetylenic C–H stretch with J-selectivity. The rotational spectrum of the excited state is then obtained by FTMW and FTMW-cwMW double-resonance methods. The excited-state measurements have a signal-to-noise ratio comparable to the pure rotational spectrum. The residuals in the excited-state fit are larger than those obtained in the ground-state fit. This greater deviation from a standard asymmetric top spectrum is most likely due to weak perturbations to the acetylenic C–H spectrum.

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl4). Ultrafast excitation of the broad O–H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O–H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm−1) and the chemical identity of the dimer.