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Abstract The rotational barrier about the CN carbamate bond ofN‐(4‐hydroxybutyl)‐N‐(2,2,2‐trifluoroethyl)tert‐butyl carbamate1was determined by variable temperature (VT)13C and19F NMR spectroscopy. The −CH2CF3 appendage reports on rotational isomerism and allows for the observation of separate signals for the E‐ and Z‐ensembles at low temperature. The activation barrier for E/Z‐isomerization was quantified using Eyring‐Polanyi theory which requires the measurements of the maximum difference in Larmor frequency Δνmax and the convergence temperature Tc. Both Δνmax and Tc were interpolated by analyzing sigmoidal functions fitted to data describing signal separation and the quality of the superposition of the E‐ and Z‐signals, respectively. Methods for generating the quality‐of‐fit parameters for Lorentzian line shape analysis are discussed. Our best experimental value for the rotational barrier ΔGc≠(1)=15.65±0.13 kcal/mol is compared to results of a higher level ab initio study of the modelN‐ethyl‐N‐(2,2,2‐trifluoroethyl) methyl carbamate.
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Rotation‐Inversion Isomerization of Tertiary Carbamates: Potential Energy Surface Analysis of Multi‐Paths Isomerization Using Boltzmann Statistics
Abstract Potential energy surface (PES) analyses at the SMD[MP2/6–311++G(d,p)] level and higher‐level energies up to MP4(fc,SDTQ) are reported for the fluorinated tertiary carbamateN‐ethyl‐N‐(2,2,2‐trifluoroethyl) methyl carbamate (VII) and its parent systemN,N‐dimethyl methyl carbamate (VI). Emphasis is placed on the analysis of the rotational barrier about the CN carbamate bond and its interplay with the hybridization of theN‐lone pair (NLP). All rotational transition state (TS) structures were found by computation of 1D relaxed rotational profiles but only 2D PES scans revealed the rotation‐inversion paths in a compelling fashion. We found four unique chiral minima ofVII, one pair each ofE‐andZ‐rotamers, and we determined theeightunique rotational TS structures associated with every possibleE/Z‐isomerization path. It is a significant finding that all TS structures featureN‐pyramidalization whereas the minima essentially contain sp2‐hybridized nitrogen. We will show that the TS stabilities are affected by the synergetic interplay between NLP/CO2repulsion minimization, NLP→σ*(CO) negative hyperconjugation, and two modes of intramolecular through‐space electrostatic stabilization. We demonstrate how Boltzmann statistics must be applied to determine the predicted experimental rotational barrier based on the energetics of all eight rotamerization pathways. The computed barrier forVIIis in complete agreement with the experimentally measured barrier of the very similar fluorinated carbamateN‐Boc‐N‐(2,2,2‐trifluoroethyl)‐4‐aminobutan‐1‐olII. NMR properties ofVIIwere calculated with a variety of density functional/basis set combinations and Boltzmann averaging over theE‐andZ‐rotamers at our best theoretical level results in good agreement with experimental chemical shifts δ(13C) andJ(13C,19F) coupling constants ofII(within 6 %).
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- Award ID(s):
- 2153206
- PAR ID:
- 10389348
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemPhysChem
- Volume:
- 24
- Issue:
- 1
- ISSN:
- 1439-4235
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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