Samdal, S.; Møllendal, H.; Reine, S.; Guillemin, J.C. J. Phys. Chem. A 2015, 119, 4875–4884
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission
This work is licensed under a Creative Commons AttributionNoDerivs 3.0 Unported License.
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission
A recent reinvestigation of the structure of 2oxazoline demonstrates the difficulties that many computational methods can still have in predicting structure.
Samdal, et al. report the careful examination of the microwave spectrum of 2oxzoline and find that the molecule is puckered in the ground state.^{1} It’s not puckered by much, and the barrier for inversion of the pucker, through a planar transition state is only 49 ± 8 J mol^{1}. The lowest vibrational frequency in the nonplanar ground state, which corresponds to the puckering vibration, has a frequency of 92 ± 15 cm^{1}. This low barrier is a great test case for quantum mechanical methodologies.
And the outcome here is not particularly good. HF/ccpVQZ, M062X/ccpVQZ, and B3LYP/ccpVQZ all predict that 2oxazoline is planar. More concerning is that CCSD and CCSD(T) with either the ccpVTZ or ccpVQZ basis sets also predict a planar structure. CCSD(T)F12 with the ccpVDZ predicts a nonplanar ground state with a barrier of only 8.5 J mol^{1}, but this barrier shrinks to 5.5 J mol^{1} with the larger ccpVTZ basis set.
The only method that has good agreement with experiment is MP2. This method predicts a nonplanar ground state with a pucker barrier of 11 J mol^{1} with ccpVTZ, 39.6 J mol^{1} with ccpVQZ, and 61 J mol^{1}with the ccpV5Z basis set. The nonplanar ground state and the planar transition state of 2oxazoline are shown in Figure 1. The computed puckering vibrational frequency does not reproduce the experiment as well; at MP2/ccpV5Z the predicted frequency is 61 cm^{1} which lies outside of the error range of the experimental value.
Nonplanar

Planar TS

Figure 1. MP2/ccpV5Z optimized geometry of the nonplanar ground state and the planar transition
state of 2oxazoline.
state of 2oxazoline.
References
(1) Samdal, S.; Møllendal, H.; Reine, S.; Guillemin, J.C. "Ring Planarity Problem of 2Oxazoline Revisited Using Microwave Spectroscopy and Quantum Chemical Calculations," J. Phys. Chem. A 2015, 119, 4875–4884, DOI: 10.1021/acs.jpca.5b02528.
InChIs
2oxazoline: InChI=1S/C3H5NO/c125341/h3H,12H2
InChIKey=IMSODMZESSGVBEUHFFFAOYSAN
InChIKey=IMSODMZESSGVBEUHFFFAOYSAN
This work is licensed under a Creative Commons AttributionNoDerivs 3.0 Unported License.