TY - JOUR
T1 - Conformational analysis of carboxylic acid anhydrides
T2 - A microwave and computational study
AU - Love, Nathan
AU - Reynolds, Aaron J.
AU - Dvorak, Michael A.
AU - Leopold, Kenneth R.
N1 - Publisher Copyright:
© 2023 Elsevier Inc.
PY - 2023/9/1
Y1 - 2023/9/1
N2 - The microwave spectra of four carboxylic acid anhydrides, RCOOCOR′, (R,R′) = (CH3, CF3), (C(CH3)3, CF3), (C6H5, CF3) and (CH3, C(CH3)3), have been observed in a supersonic jet. Calculations at the M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory predict the lowest energy conformations for all four species to be nonplanar cis structures, i.e., conformations in which the C=O groups point in approximately the same direction, but are twisted out of a coplanar orientation. The observed spectra are consistent with these predictions. In addition, for all but the (R,R′) = (CH3, C(CH3)3) species, higher energy nonplanar trans conformers are also predicted, typically within 1–2 kcal/mole the nonplanar cis form. For (R, R′) = (C(CH3)3, CF3), extensive isotopic substitution has enabled a determination of most of the (non-fluorine) heavy atom structural parameters. Excellent agreement with the DFT and MP2 structures was obtained, thus validating the theoretical methods used. A strong correlation is found between the calculated O=C⋯C=O dihedral angle and the average of the vapor phase C=O stretching frequencies of RCOOH and R′COOH.
AB - The microwave spectra of four carboxylic acid anhydrides, RCOOCOR′, (R,R′) = (CH3, CF3), (C(CH3)3, CF3), (C6H5, CF3) and (CH3, C(CH3)3), have been observed in a supersonic jet. Calculations at the M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory predict the lowest energy conformations for all four species to be nonplanar cis structures, i.e., conformations in which the C=O groups point in approximately the same direction, but are twisted out of a coplanar orientation. The observed spectra are consistent with these predictions. In addition, for all but the (R,R′) = (CH3, C(CH3)3) species, higher energy nonplanar trans conformers are also predicted, typically within 1–2 kcal/mole the nonplanar cis form. For (R, R′) = (C(CH3)3, CF3), extensive isotopic substitution has enabled a determination of most of the (non-fluorine) heavy atom structural parameters. Excellent agreement with the DFT and MP2 structures was obtained, thus validating the theoretical methods used. A strong correlation is found between the calculated O=C⋯C=O dihedral angle and the average of the vapor phase C=O stretching frequencies of RCOOH and R′COOH.
KW - Acid anhydrides
KW - Computational chemistry
KW - Microwave
KW - Molecular conformation
KW - Molecular structure
KW - Supersonic jet
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U2 - 10.1016/j.jms.2023.111844
DO - 10.1016/j.jms.2023.111844
M3 - Article
AN - SCOPUS:85174747066
SN - 0022-2852
VL - 397
JO - Journal of molecular spectroscopy
JF - Journal of molecular spectroscopy
M1 - 111844
ER -