Microwave Spectrum of Benzaldehyde

Abstract

The microwave absorption spectrum of C6H5CHO has been investigated in the region 17.5–40.0 GHz and that for C6H5CDO in the region 26.5–40.0 GHz. Rotational transitions in the ground and first three excited torsional states in both molecular species have been identified. There is evidence for interaction of the second torsional state of C6H5CHO with another mode of vibration of the molecule. Accurate relative intensity measurements give the first torsional frequency of 113.8 ± 5.0 cm−1 for C6H5CHO and of 108.4 ± 4.25 cm−1 for C6H5CDO. The barriers to internal rotation of the aldehyde group calculated from these frequencies are 4.90 ± 0.43 and 5.28 ± 0.42 kcal/mole for C6H5CHO and C6H5CDO, respectively. The discrepancy between the observed and calculated values of inertial coefficients suggests the importance of vibration–rotation interaction in the theory of internal rotors with twofold potential barriers.

A PCILO Study of Conformation and Internal Rotation in Mono‐substituted Benzenes

“…Benzaldehyde (Fig. 4 ) is known to be planar in the ground state and has a twofold barrier with a height of 4.7-4.9 kcaVmol [30] [31]. This planar conformation is also confirmed from previous and the present PCILO studies as the most stable one (TableZ), though the calculated barrier seems to be sensitive to the employed geometry.…”

Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

“…20 The structures of the electronic states of benzaldehyde are largely obtained through theoretical means [29][30][31] with some experimental exceptions. The ground state has been studied by both microwave spectroscopy 32 and electron diffraction 33 in the gas phase, and the barrier to formyl group torsion is found to be 1715 and Ͼ1700 cm −1 , respectively. Additionally, analysis of the phosphorescence spectra and S 1 excitation spectra shows the C v O stretch as the most intense band with prominent progression, confirming the n * nature of both states as well as their structural similarity to the ground state ͑except for the C v O length͒.…”

Microwave spectrum of trans 3-fluorophenol in excited torsional states

“…This shift from the harmonic behavior indicates that the second torsional excited state may be experiencing a perturbation from another nearby vibrational mode of this molecule which could be the 'other intramolecular mode' as stated above. It may be noted here that quite a similar effect has been observed in case of benzaldehyde [11], where the second excited torsional state was termed as 'problem level', although the existence of the suspected perturbing mode was not conclusively proved because of the interference from Stark lobes in the expected frequency region. The observed inertial defect values for the ground and torsional excited states of trans 3-fluorophenol and benzaldehyde are compared in Table 3.…”