Ab initio studies on amides: triformamide

1981 ◽  
Vol 34 (1) ◽  
pp. 7
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Double Bond ◽  
Transition State ◽  
Ab Initio ◽  
Internal Rotation ◽  
Direct Observation ◽  
Ground State ◽  
Basis Set ◽  
Model Transition ◽  
Bond Character ◽  
Complete Optimization

Complete optimization with the STO-3G basis set of the geometries of the planar ground-state conformers of the (unknown) triformamide molecule predicts that the E,E,E conformer will be more stable than the E,Z,Z conformer by 12.6 kJ mol-1 (or 42.3 kJ mol-1 by energy evaluations with the 4-31G basis set). These differences make direct observation of the E,Z,Z-conformer unlikely. Corresponding optimization of a model transition state for the conversion of the E,Z,Z into the E,E,E conformer suggests that the barrier to internal rotation is low with a calculated height of 33 kJ mol-1 (24 kJ mol-1 when evaluated with the 4-31G basis set). Successive formyl substitution in ammonia, represented by the series of molecules formamide, diformamide (formimide) and triformamide, leads to decreasing double-bond character in the N-C bonds as reflected in increasing N-C bond lengths, decreasing Mulliken π-overlap populations and decreasing barriers to rotation. Extension of the results for these three molecules suggests that the (known) triacetamide molecule has a non-planar E,E,E-type structure as is consistent with the observation of a single signal in its 1H n.m.r. spectrum.

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

1980 ◽  
Vol 33 (11) ◽  
pp. 2337 ◽  
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Hydrogen Atom ◽  
Ab Initio ◽  
Internal Rotation ◽  
Model Systems ◽  
Methyl Groups ◽  
Electrostatic Repulsion ◽  
Conformational Preferences ◽  
Out Of Plane ◽  
Model Transition ◽  
Good Agreement

Diacetamide, like other diacylamines, is capable of existing in three basic conformations about the N-C bonds. Optimization (STO-3G) of model systems in which all first-row atoms and the amido hydrogen atom are held coplanar predicts that the E,Z conformer (3) is of lowest energy, the Z,Z conformer (2) of somewhat higher energy (4.2 kJ mol-1), and the E,E conformer (1) of highest energy (23.6 kJ mol-1); 4-31G evaluation of the energies suggests that (1) and (2) are each of higher energy than (3) by 27-28 kJ mol-1. It is suggested that (2) is destabilized with respect to (3) by electrostatic repulsion of the two negatively charged oxygen atoms whereas destabilization of (1) is due to substantial methyl-methyl steric interactions as reflected in the very wide <CNC (136°); the energy of (1) is, however, raised by out-of-plane or rotational movements of the methyl groups, i.e., the preferred structure (excluding methyl hydrogens) is planar. The calculated height of the barrier to internal rotation of (3) by either of two model transition states is 41-45 kJ mol-1, in good agreement with an experimental value of 45.2 kJ mol-1 in solution at -60°.

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).

THEORETICAL STUDY ON INTERNAL ROTATION OF NITROSOUREAS AND TOXICOLOGICAL ANALYSIS

2007 ◽  
Vol 06 (02) ◽  
pp. 245-253 ◽  
Author(s):  
YA SONG CUI ◽  
LI JIAO ZHAO ◽  
YONG DONG LIU ◽  
RU GANG ZHONG
Keyword(s):  
Transition State ◽  
Internal Rotation ◽  
Ground State ◽  
Energy Barrier ◽  
Theoretical Study ◽  
Rotational Energy ◽  
Carcinogenic Potency ◽  
Toxicological Analysis ◽  
Ground State Structures ◽  
The Relationship

A theoretical study has been carried out for internal rotation of nitrosoureas at the B3LYP/6-311G* level of theory. For each nitrosourea compound, two ground state structures have been found and the E isomer is predicted to be more stable than the Z isomer. Two transition state conformations for the isomerization have also been obtained and the calculated results show that the isomerization through TS1 is easier than that through TS2. The relationship between energy barrier and toxicity has also been investigated. It is concluded that the carcinogenic potency increases along with the decrease of rotational energy barrier.

The influence of basis set on the ab initio prediction of internal rotation barrier heights in ethene thiol

1992 ◽  
Vol 3 (1) ◽  
pp. 3-8 ◽  
Author(s):  
Christopher Plant ◽  
James E. Boggs ◽  
John N. Macdonald ◽  
Gwilym A. Williams
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Basis Set ◽  
Rotation Barrier ◽  
Barrier Heights ◽  
Ab Initio Prediction ◽  
Internal Rotation Barrier

Ab initio study of the structure of the hydrogen maleate anion

1993 ◽  
Vol 71 (3) ◽  
pp. 303-306 ◽  
Author(s):  
Miguel A. Ríos ◽  
Jesús Rodríguez
Keyword(s):  
Hydrogen Bond ◽  
Proton Transfer ◽  
Ab Initio ◽  
Electron Correlation ◽  
Basis Set ◽  
Ab Initio Methods ◽  
Ab Initio Study ◽  
Barrier Heights ◽  
Minimum Potential ◽  
Complete Optimization

The hydrogen maleate ion was studied by ab initio methods with complete optimization at the 3-21G, 6-31G, 6-31G**, and 6-31+G(2d,1p) levels. To study the influence of electron correlation, MP2 calculations have been done for the 6-31G** geometry. All calculations at the HF level predicted an asymmetric hydrogen bond with a double minimum potential governing transfer between the two equivalent structures. Moreover, both asymmetry and proton transfer barrier increase systematically with the power of the basis set used, with calculated barrier heights of 0.12 (3-21G), 1.59 (6-31G), 1.64 (6-31G**), and 2.00 kcal/mol (6-31+G). Only the introduction of the electron correlation at the MP2 level seems to predict a single minimum potential.

MODELING THE GROUND STATE GEOMETRY AND ESTIMATING THE CHARGE TRANSFER TRANSITION ENERGY OF THE TOLUENE–ICl MOLECULAR COMPLEX BY AB INITIO AND DFT METHODS

2008 ◽  
Vol 07 (03) ◽  
pp. 331-346 ◽  
Author(s):  
AMIT S. TIWARY ◽  
PARTHA SARATHI SENGUPTA ◽  
ASOK K. MUKHERJEE
Keyword(s):  
Charge Transfer ◽  
Binding Energy ◽  
Ab Initio ◽  
Aromatic Ring ◽  
Ground State ◽  
Density Functional ◽  
Transition Energy ◽  
Basis Set ◽  
Basis Set Superposition Error ◽  
Ab Initio And Dft

Out of several plausible isomeric structures of the toluene–ICl charge transfer (CT) complex, the most feasible one was determined by a detailed ab initio and DFT study at the HF, B3LYP, and mPW1PW91 levels using 6-31++G(d, p) basis set. Potential energy surface scans were performed with six possible structures ( I and Cl facing the o-, m-, and p-carbon atoms of toluene separately); the structures at the local minima of the surfaces were subjected to frequency calculation and the ones having no negative frequency were accepted as the real structure in the ground state. These structures were then subjected to full optimization. It was observed that the I – Cl bond, with its I atom oriented toward the aromatic ring, stands vertically above a C -atom at the ortho or para positions, being inclined at about 9° to the line perpendicular to the aromatic ring. Complexation increases the I – Cl bond length. After correction for basis set superposition error through a counterpoise calculation, we conclude from the binding energy that the preferred structure is the one with ICl above the ortho C atom. The calculated binding energy closely matches the experimental free energy of complexation. The electronic CT transition energy (hν CT ) with this structure in the ground state was calculated in vacuo by the restricted configuration interaction singlets method and in carbontetrachloride medium by the time dependent density functional theory method under the polarizable continuum model. The value of hν CT obtained from the ground-to-excited state transition electric dipole moments of the complex, is close to (somewhat underestimated) the reported experimental value.

Polarization Gaussian p Functions for the Beryllium Atom: Ab initio Calculations on BeH2 and BeH+

1975 ◽  
Vol 53 (22) ◽  
pp. 2512-2516 ◽  
Author(s):  
P. G. Mezey ◽  
I. G. Csizmadia ◽  
O. P. Strausz
Keyword(s):  
Excited State ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Ground State ◽  
Optimization Procedure ◽  
Energy Functional ◽  
Basis Set ◽  
Negative Ion ◽  
Gaussian Basis Set ◽  
Orbital Exponents

A set of Gaussian p orbital exponents was obtained by optimizing a (9s5p) Gaussian basis set for an excited state of the beryllium atom and the ground state of the beryllium negative ion. In the optimization procedure the method of conjugate gradients was applied for the energy functional. The optimum (9s5p) basis set was tested on the BeH2 and BeH+ structures.

DEVELOPMENT OF A THREE-DIMENSIONAL AB INITIO POTENTIAL ENERGY SURFACE FOR THE He–Cl2(X) SYSTEM AND ITS APPLICATION TO SOLVATION STRUCTURES IN THE HenCl2 CLUSTERS

2005 ◽  
Vol 04 (01) ◽  
pp. 197-207 ◽  
Author(s):  
TOSHIYUKI TAKAYANAGI ◽  
MOTOYUKI SHIGA ◽  
TETSUYA TAKETSUGU
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Ab Initio ◽  
Ground State ◽  
Energy Surface ◽  
Three Dimensional ◽  
Electronic Ground State ◽  
Basis Set ◽  
Electronically Excited ◽  
Electronic Ground

High-quality ab initio electronic structure calculations for the van der Waals interaction of He with Cl 2 in the electronic ground state have been carried out to develop a new three-dimensional potential energy surface for this system. The calculations were performed at the single and double excitation coupled-cluster level of theory with non-iterative perturbational treatment of triple excitations [CCSD(T)] with a very large basis set including an additional set of bond functions. The analytical potential surface developed were then used in the path-integral molecular dynamics calculations for the He n Cl 2 cluster, where quantum solvation structures of helium atoms in clusters were investigated. It has been found that the helium solvation structures are quite different between the electronic ground state and the electronically excited B 3Π state.

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