Faculty of Science

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Browsing Faculty of Science by Subject "ab initio calculations"

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    Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde
    (The Journal of Physical Chemistry A, 2018-02-06) Sun, Wenhao; Lozada, Issiah B.; van Wijngaarden, Jennifer
    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 which 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 (13C, 18O) in natural abundance allowed derivation of the ground state effective (r0) structures and mass dependence (rm) structures for the lowest energy rotamer of 2-FBD (O-trans) and both rotamers of 3-FBD which compare favourably with ab initio estimates of the equilibrium (re) 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.
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    Rotational Spectra and Structures of Phenyl Isocyanate and Phenyl Isothiocyanate
    (The Journal of Physical Chemistry A, 2019-02-28) Sun, Wenhao; Silva, Weslley G. D. P.; van Wijngaarden, Jennifer
    The pure rotational spectra of phenyl isocyanate (PhNCO) and phenyl isothiocyanate (PhNCS) were investigated using Fourier transform microwave (FTMW) spectroscopy in the range from 4 to 26 GHz. For each molecule, rotational transitions due to the parent species and nine minor isotopologues including seven 13C, one 15N and one 18O/34S have been observed in natural abundance. The rm(1) geometries were derived from the resulting sets of rotational constants and are consistent with the equilibrium structures (re) from ab initio calculations performed at the MP2/aug-cc-pVTZ level. NBO and Townes-Dailey analyses were conducted to better understand the electronic structure and geometry of each compound. In the case of PhNCS, the nitrogen atom displays more sp-like character resulting in shorter C-N bonds and a larger CNC angle relative to those of PhNCO.
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    Rotational Spectroscopic and Ab Initio Investigation of the Rotamer Geometries of 2-Fluoroanisole and 3-Fluoroanisole
    (The Journal of Physical Chemistry A, 2020-03-02) Bergmann, Katrina; van Wijingaarden, Jennifer
    The rotational spectra of 2-fluoroanisole (2-FA) and 3-fluoroanisole (3-FA) were investigated using Fourier transform microwave (FTMW) spectroscopy in the 4-26 GHz range. Assigned transitions correspond to the lowest energy rotamer for 2-FA which has the O-CH3 group directed away (anti) from the fluorine substituent whereas for 3-FA, the spectrum is consistent with the presence of two rotamers arising from syn and anti orientations of the methoxy moiety relative to fluorine. Ab initio calculations at the MP2/cc-pVTZ level were used to estimate the equilibrium (re) geometries of the three observed rotamers. Their assignments were confirmed through the observation of the rotational transitions of eight minor isotopologues (13C and 18O) in natural abundance for each species. The mass dependence (rm(1)) structures derived using the experimentally determined rotational constants compare favourably with the ab initio estimates. The resulting sets of geometric parameters suggest that the aromatic ring backbone is distorted by the introduction of the angular methoxy substituent, with a tendency to induce bond length alternation around the ring, and by the electron withdrawing effects of fluorine.
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    Structural elucidation of 2-fluorothiophenol from Fourier transform microwave spectra and ab initio calculations
    (Journal of Molecular Structure, 2017-09-15) SUN, WENHAO; van Wijngaarden, Jennifer
    Pure rotational transitions corresponding to the ground vibrational state of 2-fluorothiophenol (2FTPh) were recorded via Fourier transform microwave (FTMW) spectroscopy in the range of 4–26 GHz. The observed transitions were assigned to two planar conformers in which the SH bond is directed toward (cis-2FTPh) or away from (trans-2FTPh) the fluorine substituent with the former predicted to lie 3.86 kJ/mol lower in energy from ab initio calculations (MP2/6-311++G(2d,2p)). The rotational constants determined from the spectral analysis were used to derive effective ground state (r0) structures of the lower energy cis-2FTPh conformer using spectra of the corresponding 13C and 34S isotopologues which were observed in natural abundance. Geometry optimization at the MP2/6-311++G(2d,2p) level provided the equilibrium (re) structure which is in close agreement with the experimentally-derived geometry. Comparison with results from natural bond orbital (NBO) calculations provide evidence of a weak intramolecular interaction between the lone pair on sulfur and the CF moiety in cis-2FTPh that is not found in trans-2FTPh or thiophenol (TPh). This interaction stabilizes the cis-2FTPh conformer by 2.18 kJ/mol.