Experimental Gas Phase 1H NMR Spectra and Basis Set Dependence of ab initio GIAOMO Calculations of 1H and 13C NMR Absolute Shieldings and Chemical Shifts of Small Hydrocarbons

2004 ◽  
Vol 59 (10) ◽  
pp. 1153-1176 ◽  
Author(s):  
Thomas Zuschneid ◽  
Holger Fischer ◽  
Thomas Handel ◽  
Klaus Albert ◽  
Günter Häfelinger
Keyword(s):  
Ab Initio ◽  
Gas Phase ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
Basis Set ◽  
Basis Sets ◽  
Gaussian Basis Sets ◽  
Phase Measurements ◽  
H Nmr ◽  
Set Up

AbstractHigh-resolution gas phase measurements of 1H NMR spectra at 400 MHz and atmospheric pressure of seven small hydrocarbons are presented. The developed new method and the experimental set-up are described. Ab initio GIAO MO calculations of 1H and 13C NMR absolute shieldings on the HF, MP2 and B3LYP levels using 25 standard gaussian basis sets are reported for these hydrocarbons, based on experimental re distances. The measured gas phase 1H chemical shifts have been converted to an absolute σ0 shielding scale by use of the literature shielding of methane. These and gas phase 13C literature values have been transferred with literature ZPV data to estimated σeexp shieldings which are used to evaluate the basis set dependence of the calculated σe shieldings utilizing linear least squares regressions. Exponential extrapolations of Dunning basis set calculations allow the determination of basis set limits for 1H and 13C shieldings. 1H and 13C chemical shifts have been derived from the HF calculated shieldings with shieldings of TMS which has been geometry optimized and GIAO calculated in each basis. Standard deviations (esd) as low as 0.09 ppm for 1H and 0.76 ppm for 13C calculations have been obtained.The statistically best basis set for simultaneous calculation of 1H and 13C absolute shieldings or relative shifts is 6-311G* within the HF and B3LYP methods. Aiming for highest accuracy and precision, 1H and 13C have to be treated separately. In this case, best results are obtained using MP2/6-311G** or higher for 1H shieldings and MP2/cc-pVTZ for 13C shieldings.

The Rotational Barrier About the Disulphide Bridge in Dimethyl Disulphide: An Ab Initio Study

1984 ◽  
Vol 39 (5) ◽  
pp. 495-498
Author(s):  
V. Renugopalakrishnan ◽  
R. Walter
Keyword(s):  
Ab Initio ◽  
Rotational Barrier ◽  
Basis Set ◽  
Basis Sets ◽  
Disulphide Bridge ◽  
Minimal Basis ◽  
Gaussian Basis Sets ◽  
Type Function ◽  
Dimethyl Disulphide ◽  
Extended Basis

An ab initio molecular orbital technique was used to investigate the rotational barrier about the disulphide bridge in dimethyl disulphide. Various minimal and extended basis sets were used in the calculations. The chosen minimal basis set was the STO-3G set, and the extended basis sets were the STO 4-31G set, the Dunning and Hay set consisting of contracted Gaussian basis sets: [2s], [3s, 2p] and [6s, 4p] for H, C, and S atoms, and the Dunning and Hay basis set augmented with a d-type function on S atoms. The total energy was calculated as a function of the torsion angle about the disulphide bond. The barrier to rotation about this bond was found to be two-fold in nature, in accordance with previous findings. The heights of the barriers were observed to depend upon the basis set and input geometry. For our particular choice of basis sets and input geometry, the calculated value of the eis and trans barriers ranged from 12.68 to 16.49 kcal/mol and from 6.23 to 8 kcal/mol, respectively. Inclusion of a d-type function in the basis sets was found to result in better agreement between the calculated and experimental values, thereby emphasizing the need for considering 3d orbitals of sulphur in MO calculations

SYNTHESIS AND GIAO NMR CALCULATIONS FOR TWO DIASTEREOISOMERS OF 2′-ACETYLOXY-2′-PHENYLSPIRO[INDENO[1,2-b]QUINOXALIN-11,1′-CYCLOPROPANE]

2012 ◽  
Vol 11 (06) ◽  
pp. 1227-1236 ◽  
Author(s):  
MASOUD SHAABANZADEH ◽  
HAMID HASHEMIMOGHADDAM ◽  
MARYAM BIKHOF TORBATI ◽  
TAHEREH SOLEYMANI AHOEE
Keyword(s):  
Nmr Spectra ◽  
Chemical Shifts ◽  
Atomic Orbital ◽  
Basis Sets ◽  
Computational Results ◽  
Nmr Chemical Shifts ◽  
H Nmr ◽  
Chemical Structures ◽  
B3lyp Method ◽  
Experimental Values

Two diastereoisomers of 2′-acetyloxy-2′-phenylspiro[indeno[1,2-b]quinoxalin-11,1′-cyclopropane] were synthesized and their 1 H NMR spectra were recorded. Their chemical structures were fully optimized at B3LYP/6-311+G(d,p) level of theory using the Gaussian 03W program package. The 1 H NMR chemical shifts were calculated for geometry-optimized structures of the diastereoisomers with the gauge independent atomic orbital (GIAO) and B3LYP method with the 6-311+G(d,p), 6-311++G(d), 6-31++G(d,p) and 6-31+G(d) basis sets. The computational results were then compared with the experimental values and the structures associated with each spectrum were assigned.

scholarly journals On the Interactions of Fused Pyrazole Derivative with Selected Amino Acids: DFT Calculations

2017 ◽  
Vol 2017 ◽  
pp. 1-9 ◽  
Author(s):  
Kornelia Czaja ◽  
Jacek Kujawski ◽  
Elżbieta Jodłowska-Siewert ◽  
Paulina Szulc ◽  
Tomasz Ratajczak ◽  
...  
Keyword(s):  
Amino Acids ◽  
Chemical Shifts ◽  
Basis Set ◽  
Basis Sets ◽  
Pyrazole Derivative ◽  
Solvation Model ◽  
Basis Set Superposition Error ◽  
H Nmr ◽  
In Silico Studies ◽  
Cytostatic Compounds

Due to the increasing prevalence of neoplasms, there is a permanent need for new selective cytostatic compounds. Anticancer drugs can act in different ways, affecting protein expression and synthesis, including disruption of signaling pathways within cells. Continuing our previous research aiming at elucidating the mechanism of pyrazole’s anticancer activity, we carried out in silico studies on the interactions of fused pyrazole derivative with alanine, lysine, glutamic acid, and methionine. The objective of the study is to improve our understanding of the possible interactions of pyrazole derivatives with the above-mentioned amino acids. For this purpose, we apply the DFT formalism (optimization using the B3LYP, CAM-B3LYP, PBE0, and M06L functionals) and interaction energy calculations (counterpoise corrected method based on the basis set superposition error, BSSE) together with QTAIM approach and estimation of the 1H NMR chemical shifts of analyzed pyrazole derivative using different basis sets and DFT functionals in CPCM solvation model (and water used as a solvent).

Photoelectron Spectroscopic Studies of Some Substituted Borotrans

1987 ◽  
Vol 40 (5) ◽  
pp. 803 ◽  
Author(s):  
JB Peel ◽  
RG Rothwell ◽  
DX Wang
Keyword(s):  
Thermal Decomposition ◽  
Ab Initio ◽  
Gas Phase ◽  
Molecular Orbital ◽  
Spectroscopic Studies ◽  
Photoelectron Spectra ◽  
Basis Sets ◽  
Molecular Orbital Calculations ◽  
Phase Measurements ◽  
Dative Bond

The HeI photoelectron spectra of a series of borotrans have been measured. Ab initio molecular orbital calculations using minimum basis sets were performed on the six molecules in this study, by using geometries based on the known structure of the parent borotran, a tricyclic system containing a relatively short B←N dative bond. Temperatures up to 312� were required for the gas-phase measurements and considerable thermal decomposition was observed in two cases. The lowest-energy photoelectron bands are assigned to combinations of oxygen orbitals of the BO3 unit, but the B←N bonding electrons cannot be accurately located in the spectra.

Systematic sequences of well-balanced Gaussian basis sets

1994 ◽  
Vol 72 (7) ◽  
pp. 1741-1752 ◽  
Author(s):  
Mariusz Klobukowski
Keyword(s):  
Molecular Structure ◽  
Ab Initio ◽  
Basis Set ◽  
Basis Sets ◽  
Structure And Properties ◽  
Gaussian Basis Sets ◽  
Molecular Calculations

Systematic sequences of Gaussian basis sets with controlled energy errors for the studies of convergence of the basis set effects on the computed results of molecular calculations are presented. The basis sets were prepared for atoms H–Ne and Al–Ar and may be used in systematic ab initio ad limitem studies of molecular structure and properties.

scholarly journals Ab initio studies on complexes formed by melamine and cyclotrione

2007 ◽  
Vol 72 (4) ◽  
pp. 375-382 ◽  
Author(s):  
Liangiang Zhu ◽  
Qiwen Teng ◽  
Shi Wu
Keyword(s):  
Ir Spectra ◽  
Ab Initio ◽  
Electronic Properties ◽  
Electronic Spectra ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
Binding Energies ◽  
Basis Sets ◽  
Ab Initio Methods ◽  
Ir And Raman

Ab initio methods were used to study the binding energies of non-bonded complexes formed by melamine and cyclotrione, with the STO-3G, 3-21G and 6-31G (d) basis sets. The electronic spectra were computed using the INDO/CIS method, and the IR spectra, Raman and NMR spectra with the RHF/6-31G (d)method. It was demonstrated that the complexes could be formed because of the negative binding energies, which were changed with the change in the electronic properties of the monomers. A red-shift of the first absorptions in the electronic spectra and the vibrational frequencies of the N-H bonds in the IR and Raman spectra of the complexes, compared with those of the monomers, occurred; simultaneously, the 1Hand 13C chemical shifts were altered, due to the non-bonded interaction. .

A Systematic DFT Study of Two Elementary Steps Involving Hydrogen Peroxide and the Hydroxyl Radical in the Fenton Reaction

2018 ◽  
Author(s):  
Danilo Carmona ◽  
David Contreras ◽  
Oscar A. Douglas-Gallardo ◽  
Stefan Vogt-Geisse ◽  
Pablo Jaque ◽  
...  
Keyword(s):  
Hydrogen Peroxide ◽  
Hydroxyl Radical ◽  
Ab Initio ◽  
Fenton Reaction ◽  
Basis Set ◽  
Basis Sets ◽  
Dft Methods ◽  
Elementary Steps ◽  
Oxygen Oxygen ◽  
Complete Basis Set

The Fenton reaction plays a central role in many chemical and biological processes and has various applications as e.g. water remediation. The reaction consists of the iron-catalyzed homolytic cleavage of the oxygen-oxygen bond in the hydrogen peroxide molecule and the reduction of the hydroxyl radical. Here, we study these two elementary steps with high-level ab-initio calculations at the complete basis set limit and address the performance of different DFT methods following a specific classification based on the Jacob´s ladder in combination with various Pople's basis sets. Ab-initio calculations at the complete basis set limit are in agreement to experimental reference data and identified a significant contribution of the electron correlation energy to the bond dissociation energy (BDE) of the oxygen-oxygen bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. The studied DFT methods were able to reproduce the ab-initio reference values, although no functional was particularly better for both reactions. The inclusion of HF exchange in the DFT functionals lead in most cases to larger deviations, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF and post--HF methods with an almost MP2 or CCSD level of accuracy. However, no systematic general prediction of the error based on the employed functional could be established and no systematic improvement with increasing the size in the Pople's basis set was found, although for BDE values certain systematic basis set dependence was observed. Moreover, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the error in the angles were between one and two degrees with some systematic trend with the basis sets. From our results we conclude that DFT methods present a computationally less expensive alternative to describe the two elementary steps of the Fenton reaction. However, choice of approximated functionals and basis sets must be carefully done and the provided benchmark allows a systematic validation of the electronic structure method to be employed

scholarly journals 13C and 1H NMR Spectra of Synthetic (25R)-5α-Spirostanes

1999 ◽  
Vol 23 (1) ◽  
pp. 48-49
Author(s):  
Martín A. Iglesias Arteaga ◽  
Carlos S. Pérez Martinez ◽  
Roxana Pérez Gil ◽  
Francisco Coll Manchado
Keyword(s):  
1H Nmr ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
1H Nmr Spectra ◽  
H Nmr ◽  
C 23 ◽  
Main Effects ◽  
Nmr Signals

The assignment of 13C and 1H NMR signals of synthetic (25 R)-5α-spirostanes is presented; the main effects on chemical shifts due to substitution at C-23 are briefly discussed.

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