Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde

Abstract

The rotational spectra of 2-fluorobenzaldehyde (2-FBD) and 3-fluorobenzaldehyde (3-FBD) were recorded using Fourier transform microwave (FTMW) spectroscopy from 4 to 26 GHz. Two planar rotamers were observed for each species that correspond to structures in which the carbonyl bond is directed toward (O-cis) or away from (O-trans) the C1-C2 bond. Observation of transitions due to heavy atom isotopes (C, O) in natural abundance allowed derivation of the ground state effective (r) structures and mass dependence (r) structures for the lowest energy rotamer of 2-FBD (O-trans) and both rotamers of 3-FBD, which compare favorably with ab initio estimates of the equilibrium (r) geometries at the MP2/aug-cc-pVTZ level. The resultant parameters are consistent with the introduction of bond length alternation in the benzene ring, which is dependent on the orientation of the aldehyde group. Careful study of the experimental structure and results of natural bond orbital (NBO) analysis do not support the presence of intramolecular hydrogen bonding as the source of its stabilization of O-trans 2-FBD over its cis counterpart. Furthermore, calculations of the interconversion pathways between rotamers suggest that despite being 9.39 kJ/mol higher in energy, the O-cis 2-FBD moiety is metastable in the molecular beam, which has allowed the observation of its microwave spectrum for the first time.

Rotational Spectra and Structures of Phenyl Isocyanate and Phenyl Isothiocyanate

“…In principle, with 10 isotopologues observed for each, the experimental rotational constants can be used to derive substitution structures (r s ) through a Kraitchmann analysis . In practice, as the a -axis passes close to C1 and C7 in both molecules, the positions of these atoms are not well-determined by this method, and instead, mass dependence structures (r m (1) ) as described by Watson were derived as shown for other benzene derivatives. , The 30 rotational constants from the set of 10 isotopologues were used in Kisiel’s STRFIT least-squares fitting routine to estimate key geometric parameters for the heavy atoms while internal parameters involving the H atoms were held fixed at the values obtained from MP2/aug-cc-pVTZ calculations. To preserve the orientation of the H atoms relative to the ring, the difference of the external angles (for example ∠HC2C1 – ∠HC2C3) of each H were also held at the ab initio values following the example of substituted benzonitriles .…”

Analysis of the Complex Quadrupole Hyperfine Patterns for Two Chlorine Nuclei in the Rotational Spectrum of 2,5-Dichlorothiophene

“…Similar structural effects are noted at C 2 in 2CT in comparison to the parent compound. The increase in the ring angle at the site of chlorination is smaller than that has been reported for fluorination on aromatic rings (2–3°). − This was previously explained in terms of the polarity of the C–X bond as a result of the inclusion on an electron-withdrawing substituent (X). This change results in more p-character in the hybrid orbital centered on the carbon directed toward the halogen atom.…”

Rotational Spectroscopic and Ab Initio Investigation of the Rotamer Geometries of 2-Fluoroanisole and 3-Fluoroanisole

“…For comparison, the predicted bond length alternation in benzaldehyde with the angular formyl group is smaller (0.002−0.004 Å) 21 and could not be spectroscopically confirmed as the experimental uncertainties in the bond lengths were larger in magnitude than the expected AGIBA effect. The r m (1) experimental geometries derived for 2-fluoroand 3-fluorobenzaldehyde 20 faced similar limitations but the addition of fluorine to anisole, as in the present study, affords the opportunity to explore whether the electron withdrawing fluorine substituent enhances or reduces the size of the AGIBA effect in comparison to that established for the parent compound. 21 With fluorine substitution in the ortho or meta position relative to the methoxy group, rotational isomerism is introduced with the possibility of the methoxy substituent oriented toward (syn) or away (anti) from fluorine.…”

“…Using this method as incorporated in Kisiel's STRFIT program, 34 we initially found relatively large uncertainties in parameters when all 27 rotational constants were included which mirrored our experience with the fluorobenzaldehydes. 20 While the fluoroanisoles are not strictly planar, the uncertainties of the geometric parameters were reduced when only two rotational constants, for example, A and B, were included in the leastsquares fit, but this approach still yielded unsatisfactorily large uncertainties with several bond lengths determined to only about 0.01 Å. These results are reported in the Supporting Information.…”

“…10,11 In addition to characterizing the overall structures of increasingly complex molecules, modern high resolution Fourier transform microwave (FTMW) spectroscopy has proved invaluable for probing very subtle geometric changes that accompany the addition of a substituent in an organic compound. For example, the fluorinated ring compounds of benzene, [12][13][14] pyridine, 15,16 and cyanobenzene, 17 have served as simple prototypes in building a detailed understanding of the influence of fluorine on the ring geometry while studies of fluorinated phenol, 18 thiophenol, 19 and benzaldehyde 20 have extended this to also include the impact of fluorine on intramolecular interactions and conformer distributions. In these ring compounds, the largest geometric changes induced by fluorination are found in close proximity to the substitution site and typically involve an increase in the ring angle at that site by 3-4 o and a shortening of the adjacent C-C bonds by 0.005-0.010 Å.…”