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Rotation-Inversion Isomerization of Tertiary Carbamates: Potential Energy Surface Analysis of Multi-Paths
Brian Jameson1, Rainer Glaser1
1Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri, 65401, USA.
Summary
This study analyzes fluorinated carbamates, revealing that transition states feature pyramidal nitrogen, contrasting with planar minima. This finding aids in predicting experimental rotational barriers and understanding molecular behavior.
Area of Science:
- Computational Chemistry
- Organic Chemistry
- Molecular Modeling
Background:
- Tertiary carbamates exhibit rotational barriers around the carbamate bond, influenced by nitrogen lone pair (NLP) hybridization.
- Fluorination can significantly alter molecular properties and reaction pathways.
Purpose of the Study:
- To investigate the rotational barrier of the CN carbamate bond in fluorinated carbamates.
- To analyze the interplay between NLP hybridization and rotational barriers.
- To elucidate the factors stabilizing transition states and their agreement with experimental data.
Main Methods:
- Potential energy surface (PES) analyses using SMD[MP2/6-311++G(d,p)] and higher-level computations.
- 1D relaxed rotational profiles and 2D PES scans to identify rotational and inversion pathways.
- Calculation of NMR properties (chemical shifts and coupling constants) using DFT/basis set combinations.
Main Results:
- Identified four unique chiral minima and eight unique rotational transition state (TS) structures for the fluorinated carbamate VII.
- All TS structures exhibit N-pyramidalization, while minima show sp2-hybridized nitrogen.
- Computed rotational barrier for VII agrees well with experimental data for a similar fluorinated carbamate.
Conclusions:
- The stability of TS structures is governed by a synergy of NLP/CO2 repulsion minimization, negative hyperconjugation, and electrostatic stabilization.
- Boltzmann statistics are crucial for predicting experimental rotational barriers from computed energetics.
- Calculated NMR properties show good agreement with experimental values, validating the theoretical models.
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