Saddle-point geometries and barriers to internal rotation of formamide : an ab initio study

1980 ◽  
Vol 33 (2) ◽  
pp. 249 ◽  
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Transition State ◽  
Ground State ◽  
Energy Surface ◽  
Saddle Points ◽  
Geometry Optimization ◽  
Internal Motion ◽  
Basis Set ◽  
Minimal Basis ◽  
Experimental Values ◽  
Extended Basis

By use of a direct transition-state program and the STO-3G minimal basis set, two saddle-points are detected on the energy surface for internal motion of formamide. These correspond mainly to rotation about the C-N bond along with some lengthening of this bond and increased pyramidal distortion at nitrogen as compared with that in the ground state. The STO-3G estimates of the barrier height (34-39 kJ mol-1) are in very poor agreement with experimental values (70-90 kJ mol-1), but 4- 31G energy evaluations for the STO-3G-optimized structures give much better estimates (62-80 kJ mol-1). Contrary to a previous report, use of the 4-31G extended basis set for geometry optimization suggests that only the lower-energy member (NH2 cis to CO) of the above pair is a true transition state for internal motion of formamide; its energy relative to that of the 4-31G-optimized ground state (planar) is 83.5 kJ mol-1, very close to the midpoint of the experimental range. The transition state appears to lie in a region of the 4-31G energy surface that is relatively flat with respect to pyramidal distortion at nitrogen; constraining the amino group to planarity raises the calculated energy by only 6.5kJmol-1.

Ab initio studies on amides: conformational preferences of formimide and barriers to interconversion of the conformers

1980 ◽  
Vol 33 (8) ◽  
pp. 1635 ◽  
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Ground State ◽  
Room Temperature ◽  
Geometry Optimization ◽  
Basis Set ◽  
Partial Geometry ◽  
Electrostatic Repulsion ◽  
Full Geometry Optimization ◽  
Vapour State ◽  
Conformational Preferences ◽  
Model Transition

Formimide (diformamide), the parent of the diacylamines, is capable of existing in three basic ground-state conformations about the N-C bonds. Full geometry optimization with the STO-3G basis set predicts that all three conformers are fully coplanar, that the E,E (1) and E,Z(3) conformers are of similar energy, and that the Z,Z (2) conformer is of somewhat higher energy (by 11 kJ mol-1); 4-31G evaluation of the energies suggests that (2) is by far the least stable and that (1) is of higher energy than (3) by 6.5 kJ mol-1. Analysis of the calculated charge distribution suggests that (2) is destabilized by electrostatic repulsion. These results are consistent with experimental conclusions that planar (3) is strongly preferred in the vapour state at room temperature and that (2) has not been observed in the vapour state or in solution. Partial geometry optimization with the STO-3G basis set of model transition states for internal rotation suggests a barrier height of 52 kJ mol-1 (72 kJ mol-1 when evaluated with the 4-31G basis set) for the conversion (3) → (1).

scholarly journals Accurate global potential energy surface for the ground state of CH2+ by extrapolation to the complete basis set limit

RSC Advances ◽  
2018 ◽  
Vol 8 (25) ◽  
pp. 13635-13642 ◽  
Author(s):  
Lu Guo ◽  
Hongyu Ma ◽  
Lulu Zhang ◽  
Yuzhi Song ◽  
Yongqing Li
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Ground State ◽  
Energy Surface ◽  
Correlation Energy ◽  
Basis Set ◽  
Basis Sets ◽  
Complete Basis ◽  
Complete Basis Set ◽  
Complete Basis Set Limit

A full three-dimensional global potential energy surface is reported for the ground state of CH2+ by fitting accurate multireference configuration interaction energies calculated using aug-cc-pVQZ and aug-cc-pV5Z basis sets with extrapolation of the electron correlation energy to the complete basis set limit.

scholarly journals Potential Energy Surface (PES) Scan of Gas-Phase L-Proline

2014 ◽  
Vol 38 ◽  
pp. 26-34
Author(s):  
Anouar el Guerdaoui ◽  
Yassine el Kahoui ◽  
Malika Bourjila ◽  
Rachida Tijar ◽  
Abderrahman el Gridani
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Gas Phase ◽  
Density Functional ◽  
Energy Surface ◽  
Conformational Stability ◽  
Geometry Optimization ◽  
Basis Set ◽  
Absolute Minimum ◽  
Pes Scan

We performed here a systematic ab initio calculations on neutral gas-phase L-proline. A total of 8 local minima were located by geometry optimization of the trial structures using density functional theory (DFT) with B3LYP three parameter hybrid potential coupled with the 6-31G)d( basis set. The absolute minimum obtained will be subject to a rigid potential energy surface (PES) scan by rotating its carboxylic group using the same method with more accurate basis set B3LYP/6-311++G(d,p), to get a deeper idea about its conformational stability. The main aim of the present work was the study of the rigidity of the L-proline structure and the puckering of its pyrrolidine ring.

Calculation of Magnetic Shielding and Susceptibility of H2O and FOH with Gauge Invariant Gaussian Basis Set

1976 ◽  
Vol 31 (12) ◽  
pp. 1727-1728 ◽  
Author(s):  
Matjaz Žaucer ◽  
Danilo Pumpernik ◽  
Milan Hladnik ◽  
Andrej Ažman
Keyword(s):  
Basis Set ◽  
Magnetic Shielding ◽  
Gauge Invariant ◽  
Finite Perturbation ◽  
Gaussian Basis Set ◽  
Experimental Values ◽  
Extended Basis

AbstractSusceptibility and magnetic shieldings of all nuclei are calculated for water and hypofluorous acid. A finite perturbation SCF method with gauge invariant gaussian basis set is used. Results calculated with a slightly extended basis set agree well with experimental values.

The simplified ab initio method calculation of linear polarizability

1973 ◽  
Vol 26 (5) ◽  
pp. 921 ◽  
Author(s):  
RD Brown ◽  
GR Williams
Keyword(s):  
Ab Initio ◽  
Small Molecules ◽  
Basis Set ◽  
Basis Sets ◽  
Orbital Method ◽  
Polarizability Anisotropy ◽  
Minimal Basis ◽  
Hartree Fock ◽  
Numerical Performance ◽  
Experimental Values

The simplified ab-initio molecular-orbital method described previously is particularly suited to the calculation of polarizabilities by the non-perturbative coupled Hartree-Fock technique. Trial calculations on CO and HF, for which comparison with corresponding ab-initio calculations is possible, show that the method gives an adequate numerical performance. Minimal basis set calculations in general tend to give values that are considerably too low because of inadequate flexibility of the basis and this is the origin of the large discrepancy between theory and experiment, especially for small molecules. ��� Results are also reported for N2O and O3. For these larger systems the SAI results with minimal basis sets are noticeably nearer experimental values. The polarizability anisotropy for N2O is particularly well reproduced by the SAI method. �

An inequality formulation of conformational energy differences

1976 ◽  
Vol 54 (4) ◽  
pp. 526-530 ◽  
Author(s):  
Johannes Pieter Colpa ◽  
H. Bernhard Schlegel ◽  
Saul Wolfe
Keyword(s):  
Theoretical Analysis ◽  
Transition State ◽  
Geometry Optimization ◽  
Energy Difference ◽  
Basis Set ◽  
Lower State ◽  
Energy Minimum ◽  
Numerical Tests ◽  
Pyramidal Inversion ◽  
True Energy

A theoretical analysis has been performed of the effects of geometry optimization upon computed barriers to rotation and pyramidal inversion. It is shown that, where ΔE is the true energy difference between the energy minimum and the transition state for a particular conformational process, (barrier using the optimized geometry of the lower state) > ΔE > (barrier using the optimized geometry of the higher state). Some numerical tests of this inequality are discussed. These reveal certain limitations upon the assumption that an optimized geometry is transferable from one basis set to another.

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