Investigation of Structural Trends in Mono-, Di-, and Pentafluorobenzonitriles Using Fourier Transform Microwave Spectroscopy

Abstract

The ground state rotational spectra of a series of fluorinated benzonitriles (BN), namely, 2-fluorobenzonitrile (2FBN), 3-fluorobenzonitrile (3FBN), 2,3-difluorobenzonitrile (23DFBN), 2,4-difluorobenzonitrile (24DFBN), and pentafluorobenzonitrile (PFBN), have been investigated between 4 and 24 GHz using Fourier transform microwave (FTMW) spectroscopy. The assigned transitions include those due to the parent as well as the (13)C and (15)N singly substituted isotopologues which were observed in natural abundance. The spectroscopic analysis allowed the derivation of substitution (rs) and effective ground state structures (r0) to investigate the effect of mono-, di-, and pentafluoro substitution on the geometry of the BN backbone and are compared here with ab initio values of the equilibrium parameters (re) obtained from MP2/6-311++G(2d,2p) calculations. Analysis of the (14)N hyperfine structure provides additional information about the electronic structure surrounding the nitrogen atom. The observed geometry changes relative to the reference BN compound are interpreted using natural bond orbital (NBO) analysis to describe differences in the hybridization at various sites and contributions from plausible resonance structures.

Rotational spectra and conformer geometries of 2-fluorophenol and 3-fluorophenol

“…The same effect was noted in 2-and 3-fluorobenzonitrile. 28 Although it is known that fluorine can behave as a π-donor, the second order perturbation analysis within the NBO calculations does not support donation of electron density from the fluorine lone pair into the π-bonding system of the ring. The observed structural changes may instead be explained with a more standard inductive argument in which the electron withdrawing nature of fluorine increases the positive charge at the substituted carbon which strengthens the adjacent C-C bonds.…”

Proton inversion tunneling in the rotational spectrum of acetone cyanohydrin

“…So far, the 14 N nuclear quadrupole coupling of nitrile groups were mainly interpreted for planar molecules like tricyanobenzenes and substituted cyanobenzenes. In those molecules, the CN bond is always expected to be polarized due to the difference in electronegativity of the two elements [124,125], while the mesomeric character changes by interactions with the aromatic system to which they are bound. For acetone cyanhydrin, we assume that the electronic withdrawal is mainly reflected in ionic rather than mesomeric changes of the CN bond.…”

Microwave spectrum, structure and dipole moment of 4-fluorophenylacetylene (4FPA)

“…A list of the frequencies is shown in Table 6 and the resulting standard deviation of the difference between observed and calculated transition frequency values of 2.8 kHz is within the expected precision of measurement. By comparing observed Stark coefficients to calculated values from second order perturbation theory, the experimental dipole moment components of 4FPA were determined to be a = total = 0.8930 (11) and 0.8935(9) D depending on the methods of least squares fitting (Table 6, 7) while b and c components were fixed to zero due to symmetry. The discrepancies in the Stark coefficients are within 0.02-0.6 MHz V -2 cm 2 , and the dipole components from DIPOLE are consistent with those values from QSTARK.…”

“…It indicates that fluorination at ortho-and paraposition would affect electron density of the acetylenic group in the molecule. A recent paper [11] about structural trends in mono-, di-, and pentafluorobenzonitriles also helped us to interpret structural changes in 4FPA because a −C≡N functional group is expected to have structural properties similar to the −C≡CH functional group with a triple bond, although the previous study does not include 4fluorobenzonitrile.…”