Calculation of the nitrogen screening constants of some N-heterocycles

Absolute nitrogen screening constants and chemical shifts of some N-heterocycles are calculated within the CNDO/S level of approximation to Pople's model. Satisfactory agreement between calculated and observed nitrogen chemical shifts is obtained in most cases. The application of the results to the study of tautomeric equilibria is discussed. ‘Pyridine-type’ nitrogen chemical shifts are dominated by n → π* transitions together with significant contributions from π → σ* and σ → π* transitions whereas the latter two are the dominant ones for ‘pyrrole-type’ nitrogen nuclei.     

Non-empirical interpretation of nitrogen 14 nuclear quadrupole coupling constants

The ¹⁴N nuclear quadrupolar coupling constants have been calculated for a series of nitrogen containing molecules from ab initio SCF-LCAO-MO wavefunctions built on basis sets of Gaussian type atomic functions. The various nuclear and electronic (from MO's, atoms, basis functions) contributions to the total field gradient have been analyzed and their relation to molecular and electronic structure is discussed.The effect of the nature of the basis set on the accuracy of the computed field gradients has also been studied.

---

Zusammenfassung Die ¹⁴N-Kernquadrupol-Kopplungskonstanten sind für eine Reihe von N-haltigen Verbindungen aus ab initio-Wellenfunktionen, die auf Gauß-Orbitalen basieren, berechnet worden. Die einzelnen Kern- und Elektronenbeiträge zum Feldgradienten wurden untersucht und diskutiert, desgleichen der Einfluß des Typus der Basis auf die Genauigkeit der Resultate.

---

Résumé Les constantes de couplage quadrupolaire nucléaire de ¹⁴N ont été calculées pour une série de composés azotés à partir de fonctions d'onde ab initio SCF-LCAO-MO (Hartree-Fock) construites sur une base de fonctions atomiques gaussiennes. Les diverses contributions nucléaires et électroniques (provenant des orbitales moléculaires, des atomes, des fonctions atomique de base) au gradient de champ total ont été analysées et reliées aux caractéristiques structurales des molécules considérées.L'effet de la nature de la base de fonctions atomiques sur la précision des gradients de champs calculés a aussi été étudié.

---

---

We thank Drs. H. Basch, J. L. Calais, J. F. Harrison and P. Pyykkö for sending us test calculations for checking our program and Drs. H. Basch and G. Berthier for communicating to us the molecular wavefunctions of some of the nitrogen containing heterocycles.     

Spectral and acidity constants of some heterocyclic nitrogen Schiff bases

The spectral behaviour of some arylidene-2-aminopurines in pure and mixed organic solvents as well as in buffer solutions of varying pH have been studied. It is recognized that the hydroxy derivatives form an intermolecular hydrogen bond solvated molecular complex with DMF solution. The pK values of the purine compound and the aminopurine Schiff bases of some aromatic aldehydes have been determined and discussed.     

Medium effect on acidity constants of some heterocyclic nitrogen azomethines

The effects of different organic solvent-water mixtures on the acidity constants of some azomethines derived from 3-amino-1,2,4-triazol; 2-amino-1,3-pyrimidine and also 2-amino-4-methyl-1,3-pyrimidine have been examined. Two pK(a) values for the o-OH derivatives were determined; one is assigned to the deprotonation of the nitrogen azomethine group and the other one is ascribed to the ionization of the OH group. The p-OH derivatives give one K(a) due to the ionization of the OH group. On the other hand, it was observed that with the increase in the amount of organic solvent in the medium, the pK(a) of the compound studied is decreased. This behaviour can be accounted for in terms of the high stabilization of both the non-protonated and ionic forms of these compounds by dispersion forces rather than by hydrogen bonding. Also, the effects of medium polarizability on the K(a) values and thus on the spectra of the charge transfer band observed have been discussed.     

Some calculations of the local diamagnetic contribution to nuclear screening constants

Some INDO/S parameterized calculations of local diamagnetic contributions to some carbon and nitrogen screening constants are reported. It is shown that Pople's local term model provides data which are in satisfactory agreement with X-ray photoelectron results. The restricted atom-dipole model also provides reasonable agreement. However the complete atom-dipole expression is found to be incompatible with the experimental data due to the presence of a non-local contribution.     

Calculation of some N and C nuclear shielding parameters for some ureas and thioureas

Molecular orbital calculations using CNDO/S parameters are employed to determine bond-orders, charge-densities and the shielding of the ¹⁵N and ¹³C nuclei of a series of ureas and thioureas. For the sterically uncrowded molecules a linear relationship is noticed between the ¹⁵N chemical shifts and C---N bond-orders. When steric crowding occurs this simple relationship is no longer applicable.     

Calculation of nuclear quadrupole coupling constants in pyrazole and imidazole from ab initio wavefunctions

The nuclear quadrupole coupling constants and asymmetry parameters of ¹⁴N and ²H of pyrazole and imidazole have been computed from ab initio SCF wavefunctions of double-zeta quality. The results agree with the recent experimental values obtained from microwave studies and are much better than those of previous theoretical studies.     

Calculation of indirect nuclear spin-spin coupling constants within the regular approximation for relativistic effects

A new method for calculating the indirect nuclear spin-spin coupling constant within the regular approximation to the exact relativistic Hamiltonian is presented. The method is completely analytic in the sense that it does not employ numeric integration for the evaluation of relativistic corrections to the molecular Hamiltonian. It can be applied at the level of conventional wave function theory or density functional theory. In the latter case, both pure and hybrid density functionals can be used for the calculation of the quasirelativistic spin-spin coupling constants. The new method is used in connection with the infinite-order regular approximation with modified metric (IORAmm) to calculate the spin-spin coupling constants for molecules containing heavy elements. The importance of including exact exchange into the density functional calculations is demonstrated.     

Calculation of anharmonic force constants

Non-linear transformations from internal to cartesian displacements are considered for anharmonic calculation of vibrational frequencies. Force constants in normal coordinates, up to quartic terms, are related to Christoffel symbols. The latter are tabulated for valence angles or, for torsion and wagging out of plane, evaluated through expressions which avoid differentiation of explicit functional forms for each type of internal coordinate. The same symbols are used for cartesian tensors and redundancy coefficients up to the third order.     

Calculation of macromolecular force constants

Force constant refinement may be carried under a system of constraints imposed on well characterized modes as the refinement proceeds. Resulting stability permits the accurate calculation of many independent macromolecular force constants. A useful formalism for this technique is developed. Prediction of initially poorly characterized modes can emerge from this stability. A brief discussion of currently used refinement schemes has been included. We show that the standard least-squares procedure is a special case of our present more general method, the former being useful when the molecule is not too large.     

Calculation of nuclear spin-spin coupling constants by SCF perturbation theory with MINDO/3 approximation

LCAO-MO-SCF-MINDO/3 approximation is used to calculate nuclear spin-spin couplings, ⁿ J ^AB, between magnetic nuclei A and B (A, B ¹H,¹³C,¹⁹F) separated byn bonds in a number of molecules. The theory predicts reasonably good values for directly bonded couplings (except those involving fluorine), but the results for multi-bond couplings are not so encouraging. Reasons for this deficiency of the theory are examined in the text.Based on Ph.D. Thesis of P. K. K. Pandey, Banaras Hindu University, 1977.     

INDO/S parameterized calculations of some 13C nuclear screening data

Nuclear screening tensors, their anisotropies and ¹³C chemical shifts are calculated for a number of carbon environments and compared with the corresponding experimental data. The agreement found with the experimental results is an improvement over that obtained from CNDO/S parameterized calculations. The improvement stems from the inclusion of the one-centre exchange integrals, which results in smaller values for the excitation energies from the INDO/S calculations. It is demonstrated that a linear relationship between ¹³C chemical shift and the lowest energy transition is not to be expected. The average values of the excitation energies of the various transitions contributing to the ¹³C screening are found to have significant variations.     

Calculation of dissociation constants and chemical hardness of some biologically important molecules: A theoretical study

The gas‐phase geometries of neutral, protonated, and deprotonated forms of some biologically important molecules, alanine (Ala), glycine (Gly), phenylalanine (Phe), and tyrosine (Tyr), were optimized using density functional theory at B3LYP/6‐311++G(d) and the ab initio HF/6‐311++G(d) level of theories. The neutral and different stable ionic states of Ala, Gly, Phe, and Tyr have also been solvated in aqueous medium using polarizable continuum model for the determination of solvation free energies in the aqueous solution. The gas‐phase acidity constants of above four molecules have been also calculated at both levels of theories and found that the values calculated at HF/6‐311++G(d) method are in good agreement with experimental results. A thermodynamic cycle was used to determine the solvation free energies for the proton dissociation process in aqueous solution and the corresponding pK_a values of these molecules. The pK_a values calculated at B3LYP/6‐311++G(d) method are well supported by the experimental data with a mean absolute deviation 0.12 pK_a units. Additionally, the chemical hardness and the ionization potential (IP) for these molecules have been also explored at both the level of theories. The Tyr has less value of chemical hardness and IP at both levels of theories compared with other three molecules, Ala, Gly, and Phe. The calculated values of chemical hardness and IP are decreasing gradually with the substitution of the various functional groups in the side chain of the amino acids. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Calculations of the 13C nuclear screening tensors for coumarin and some methoxycoumarins

INDO calculations are reported of the ¹³C nuclear screening tensors in coumarin and some of its methoxy derivatives. Calculations based upon a linear combination of gauge dependent atomic orbitals and those using an uncoupled Hartree–Fock approach are described. Comparison with the results of less sophisticated calculations shows that changes in the excitation energies, bond orders and the 〈r^?3〉_2p term are together responsible for determining the ¹³C chemical shifts.     

Kerr constants of some nitrogen-containing heterocycles

The molecular Kerr constants [ (mK₂)] were measured for pyridine, 2- and 4-methylpyridines, quinoline, and acridine in dioxane. The molecular moments of pyridine and 4-methylpyridine are directed along the axis of maximum polarizability. The B = B₁₂—B₁ values for dilute solutions of azaaromatic compounds in dioxane are proportional to T^–2.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 974–976, July, 1971.