Some INDO/S?SOS calculations of the effects of hydrogen bonding on the nitrogen shielding of some heterocycles

Some INDO/S parameterized SOS shielding calculations are reported for the nitrogen nuclei of some N-heterocycles. Hydrogen bonding with H₂O and CF₃CH₂OH is incorporated in a supermolecule approach. The calculated effects of hydrogen bonding are found to be in satisfactory agreement with observed nitrogen shielding variations upon a change of solvent.     

Solvent effects in the GIAO‐DFT calculations of the 15N NMR chemical shifts of azoles and azines

The calculation of ¹⁵N NMR chemical shifts of 27 azoles and azines in 10 different solvents each has been carried out at the gauge including atomic orbitals density functional theory level in gas phase and applying the integral equation formalism polarizable continuum model (IEF‐PCM) and supermolecule solvation models to account for solvent effects. In the calculation of ¹⁵N NMR, chemical shifts of the nitrogen‐containing heterocycles dissolved in nonpolar and polar aprotic solvents, taking into account solvent effect is sufficient within the IEF‐PCM scheme, whereas for polar protic solvents with large dielectric constants, the use of supermolecule solvation model is recommended. A good agreement between calculated 460 values of ¹⁵N NMR chemical shifts and experiment is found with the IEF‐PCM scheme characterized by MAE of 7.1 ppm in the range of more than 300 ppm (about 2%). The best result is achieved with the supermolecule solvation model performing slightly better (MAE 6.5 ppm). Copyright © 2014 John Wiley & Sons, Ltd.     

Density functional theory study of tautomerization of 2‐aminothiazole in the gas phase and in solution

The amino/imino tautomeric equilibrium in the isolated, mono‐, di‐, and trihydrate forms and dimer of 2‐aminothiazole, and the effects of hydration or self‐assistance on the transition state structures corresponding to proton transfer from the amino to imino form, have been investigated by the B3LYP method in conjunction with 6‐31+G(d,p) and 6‐311+G(3df,2p) basis sets in the gas phase and in solution. The amino form has been found to be the predominant tautomer. The tautomeric barrier heights for water‐ and self‐assisted tautomerization reactions are significantly lower than that from the amino to imino form by the intramolecular proton transfer, showing the catalytic effect of water molecule(s) and the important role of 2‐aminothiazole itself for intermolecular proton transfer. Comparison between the tautomeric barriers demonstrates that the self‐association tautomerization through the dimerization is the most favorable pathway. Bulk solvent effects have been taken into account using the polarizable continuum model (PCM) of water and CCl₄. The polar medium is favorable for the population of the imino form. The amino/imino equilibrium is also analyzed using the aromaticity index nucleus‐independent chemical shift (NICS); the NICS values for the amino form (about ?10 ppm) are more negative than the imino species (about ?8 ppm), showing that the amino form is more stable. The time‐dependent density functional theory (TDDFT) calculations of electronic absorption spectra suggest that the λ_max of dimer is 255 nm. The oscillator strength of the imino forms is less than the amino form, and increases with the polarity of the solvents. All calculations for the tautomerization of 2‐aminothiazole are in reasonable line with the available experiments. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical and experimental study of 15N NMR protonation shifts

A combined theoretical and experimental study revealed that the nature of the upfield (shielding) protonation effect in ¹⁵N NMR originates in the change of the contribution of the sp²‐hybridized nitrogen lone pair on protonation resulting in a marked shielding of nitrogen of about 100 ppm. On the contrary, for amine‐type nitrogen, protonation of the nitrogen lone pair results in the deshielding protonation effect of about 25 ppm, so that the total deshielding protonation effect of about 10 ppm is due to the interplay of the contributions of adjacent natural bond orbitals. A versatile computational scheme for the calculation of ¹⁵N NMR chemical shifts of protonated nitrogen species and their neutral precursors is proposed at the density functional theory level taking into account solvent effects within the supermolecule solvation model. Copyright © 2015 John Wiley & Sons, Ltd.     

A Hybrid statistical mechanics—quantum chemical model for proton transfer in 5‐azauracil and 6‐azauracil in water solution

A hybrid statistical physics—quantum‐chemical methodology was implemented to study the water‐assisted intramolecular proton‐transfer processes in 5‐ and 6‐azauracils in aqueous solutions. The solvent effects were included in the model by explicit inclusion of two pairs of water molecules, which model the relevant part of the first hydration shell around the solute. The position of these water molecules was initially estimated by carrying out a classical Metropolis of dilute water solutions of the title compounds and subsequently analyzing solute–solvent intermolecular interactions in the Monte Carlo‐generated configurations. Sequentially to the statistical physics simulation, ab initio quantum mechanical (QM) level of theory was implemented. The effects of the water as solvent (at ab initio QM level) were introduced at two different levels—using solute–solvent clusters (four‐water molecules) and using the same clusters embedded in an external continuum. Full geometry optimizations of these complexes were carried out at MP2/6–31 + G(d, p) and conductor‐polarizable continuum model (C‐PCM)/MP2/6–31 + G(d, p). Single point calculations were performed at CCSD(T)/6–31 + G(d, p)//MP2/6–31 + G(d, p) computational level to obtain more accurate energies. According to our calculations hydrated azauracils should exist in three forms: mainly dioxo form and two hydroxy forms. The calculated proton transfer activation energies for tautomeric reactions of 5‐azauracil and 6‐azauracil show different pictures for these two compounds. According to C‐PCM/MP2/6–31 +G(d, p) data, water‐assisted proton transfer in 5‐azauracil realizes through two parallel reactions: 1,3,5‐triazine‐2,4(1H,3H)‐dione → 6‐hydroxy‐1,3,5‐triazin‐2(1H)‐one and 1,3,5‐triazine‐2,4(1H,3H)‐dione → 4‐hydroxy‐1,3,5‐triazin‐2(1H)‐one. Tautomeric equilibrium in 6‐azauracil in water could occur by two contiguous reactions: 1,2,4‐triazine‐3,5(2H,4H)‐dione → 5‐hydroxy‐1,2,4‐triazin‐3(2H)‐one and 5‐hydroxy‐1,2,4‐triazin‐3(2H)‐one → 3‐hydroxy‐1,2,4‐triazin‐5(2H)‐one. The proton transfer investigated reactions in 5‐ and 6‐azauracils involve concerted atomic movement. © 2015 Wiley Periodicals, Inc.     

Comparison of polarizable continuum model and quantum mechanics/molecular mechanics solute electronic polarization: study of the optical and magnetic properties of diazines in water

A combination of the polarizable continuum model (PCM) and the hybrid quantum mechanics/molecular mechanics (QM/MM) methodology, PCM-MM/QM, is used to include the solute electronic polarization and then study the solvent effects on the low-lying n→π(?) excitation energy and the (15)N nuclear magnetic shielding of pyrazine and pyridazine in aqueous environment. The results obtained with PCM-MM/QM are compared with two other procedures, i.e., the conventional PCM and the iterative and sequential QM/MM (I-QM/MM). The QM calculations are made using density functional theory in the three procedures. For the excitation energies, the time-dependent B3LYP/6-311+G(d) model is used. For the magnetic shielding, the B3LYP/aug-pcS2(N)/pcS2(C,O,H) is used with the gauge-including atomic orbitals. In both cases, i.e., PCM-MM/QM and I-QM/MM, that use a discrete model of the solvent, the solute is surrounded by a first shell of explicit water molecules embedded by an electrostatic field of point charges for the outer shells. The best results are obtained including 28 explicit water molecules for the spectral calculations and 9 explicit water molecules for the magnetic shielding. Using the PCM-MM/QM methodology the results for the n→π(?) excitation energies of pyridazine and pyrazine are 32,070 ± 80 cm(-1) and 32,675 ± 60 cm(-1), respectively, in good agreement with the corresponding I-MM/QM results of 32,540 ± 80 cm(-1) and 32,710 ± 60 cm(-1) and the experimental results of 33,450-33,580 cm(-1) and 32,700-33,300 cm(-1). For the (15)N magnetic shielding, the corresponding numbers for the gas-water shifts obtained with PCM-MM/QM are 47.4 ± 1.3 ppm for pyridazine and 19.7 ± 1.1 ppm for pyrazine, compared with the I-QM/MM values of 53.4?±?1.3 ppm and 19.5 ± 1.2 ppm and the experimental results of 42-54 ppm and 17-22 ppm, respectively. The agreement between the two procedures is found to be very good and both are in agreement with the experimental values. PCM-MM/QM approach gives a good solute polarization and could be considered in obtaining reliable results within the expected QM/MM accuracy. With this electronic polarization, the solvent effects on the electronic absorption spectra and the (15)N magnetic shielding of the diazines in water are well described by using only an electrostatic approximation. Finally, it is remarked that the experimental and theoretical results suggest that the (15)N nuclear magnetic shielding of any diazine has a clear dependence with the solvent polarity but not directly with the solute-solvent hydrogen bonds.     

Two‐Photon Voltmeter for Measuring a Molecular Electric Field

We present a new approach for determining the strength of the dipolar solute‐induced reaction field, along with the ground‐ and excited‐state electrostatic dipole moments and polarizability of a solvated chromophore, using exclusively one‐photon and two‐photon absorption measurements. We verify the approach on two benchmark chromophores N,N‐dimethyl‐6‐propionyl‐2‐naphthylamine (prodan) and coumarin 153 (C153) in a series of toluene/dimethyl sulfoxide (DMSO) mixtures and find that the experimental values show good quantitative agreement with literature and our quantum‐chemical calculations. Our results indicate that the reaction field varies in a surprisingly broad range, 0–10⁷ V cm^?1, and that at close proximity, on the order of the chromophore radius, the effective dielectric constant of the solute–solvent system displays a unique functional dependence on the bulk dielectric constant, offering new insight into the close‐range molecular interaction.     

Production of N3 upon Photolysis of Solid Nitrogen at 3 K with Synchrotron Radiation

The photodissociation of gaseous molecular nitrogen has been investigated intensively, but the corresponding knowledge in a solid phase is lacking. Irradiation of pure solid nitrogen at 3 K with vacuum‐ultraviolet light from a synchrotron produced infrared absorption lines of product l‐N₃ at 1657.8 and 1652.6 cm^?1. The threshold wavelength to generate l‐N₃ was determined to be (143.7±1.8) nm, corresponding to an energy of (8.63±0.11) eV. Quantum‐chemical calculations support the formation of l‐N₃ from the reaction N₂+N₂, possibly through an activated complex l‐N₄ upon photoexcitation with energy above 8.63 eV. The results provide a possible application to an understanding of the nitrogen cycle in astronomical environments.     

The effect of weak intermolecular interactions on the nuclear magnetic resonance shielding constant in N2

Ab initio calculations are applied to examine the influence of the intermolecular interactions on the shielding constant in gaseous nitrogen. An accurate literature potential energy surface and the nuclear magnetic resonance shielding surface of the N₂–N₂ complex calculated in this work provide results in satisfactory agreement with the available experimental estimates of the effect.     

A DFT study on the microscopic ionization of cysteine in water

The molecular equilibria involved in the second and third macroscopic deprotonation processes of Cys have been theoretically characterized at B3LYP/6-31++G** computing level. The role of solvent was analyzed by using the supermolecule (up to six discrete water molecules), the continuum, and the hybrid supermolecule-continuum models. Also, a novel approach to reveal the solvation effect of the bulk water was employed. Calculations performed with the supermolecule or the Onsager models overestimate absolute pK_as, whereas the PCM continuum model yields data much closer to the experimental values. The supermolecule-PCM approach estimates the pK_a values for the amino group much better than for the thiol group.     

A New Class of Organosuperbases,N‐Alkyl‐ and N‐Aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene Amines: Synthesis,Structure, pKBH+ Measurements,and Properties

A series of stable organosuperbases, N‐alkyl‐ and N‐aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene amines, were efficiently synthesized from N,N′‐dialkylthioureas and 3‐hydroxy‐2‐butanone and their basicities were measured in acetonitrile. The derivatives with tert‐alkyl groups on the imino nitrogen were found to be more basic than the tBu P₁ (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by ¹³C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino‐substituents, including electron‐acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas‐phase basicity becomes more dependent on the field‐inductive effect, polarizability, and resonance effects of the substituent.     

DFT and TD‐DFT investigation of IR and UV spectra of solvated molecules: Comparison of two SCRF continuum models

We report the calculation of liquid‐phase infrared (IR) and ultraviolet (UV) spectra in the framework of the solute's response to the reaction field of several solvents. In particular, we compare these two properties for the multipolar expansion model developed in the Nancy continuum model (NCM) and the polarized continuum model (PCM) scheme developed in Pise and Naples. All calculations are carried out at the (TD‐)DFT/6–311G(2d,2p) level of theory. The cavity size used for modeling the solute effects on the IR and UV spectra are examined. To calibrate the solute cavity size, we have investigated the IR spectra of coumarin and of a set of 14 additional solutes of different size and polarity in several dielectrical surroundings. It turns out that: (i) PCM and NCM present an identical behavior when a common cavity is used to calibrate the models; and (ii) for both NCM and PCM models, the IR spectra are highly sensitive to the solute and solvent polarity. The UV/VIS investigation of coumarin derivatives demonstrates that both models provide close estimates of λ_max independent of the solute cavity size. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

How to model solvent effects on molecular properties using quantum chemistry? Insights from polarizable discrete or continuum solvation models

We present a comparative study of solvent effects on the 15N NMR shielding constants and the lowest electronic excitation energy (n --> pi*) in the three diazines (pyrazine, pyrimidine, and pyridazine) in aqueous solution. This solvent is modeled using either a polarizable continuum model (PCM) or a discrete polarizable model (DPM). We analyze the results obtained with the two models in terms of differences/similarities in the reaction field produced at the solute. The PCM reaction field is found to be quite sensitive to the dimension of the cavity and so are the molecular properties. However, constructing the cavity so that the DPM and PCM reaction fields become similar in magnitude leads to quite similar results for the studied molecular properties modeling the solvent using either the PCM or the DPM. Compared to experimental data, the most accurate predicted results are obtained by describing the closest water molecules at the same level of sophistication as that of the solute, whereas the bulk solvent may be described using either PCM or MM. Finally, a comparison with geometry-optimized clusters seems to show that it is important to check potential deficiencies in the force field in order for this to treat hydrogen bonding in a consistent manner.     

Computing the 7Li NMR chemical shielding of hydrated Li+ using cluster calculations and time-averaged configurations from ab initio molecular dynamics simulations

Ab initio molecular dynamics (AIMD) simulations have been used to predict the time-averaged Li NMR chemical shielding for a Li(+) solution. These results are compared to NMR shielding calculations on smaller Li(+)(H(2)O)(n) clusters optimized in either the gas phase or with a polarizable continuum model (PCM) solvent. The trends introduced by the PCM solvent are described and compared to the time-averaged chemical shielding observed in the AIMD simulations where large explicit water clusters hydrating the Li(+) are employed. Different inner- and outer-coordination sphere contributions to the Li NMR shielding are evaluated and discussed. It is demonstrated an implicit PCM solvent is not sufficient to correctly model the Li shielding, and that explicit inner hydration sphere waters are required during the NMR calculations. It is also shown that for hydrated Li(+), the time averaged chemical shielding cannot be simply described by the population-weighted average of coordination environments containing different number of waters.     

A Photosensitive Liquid Crystal Studied by 14N NMR, 2H NMR,and DFT Calculations

An azobenzene derivative, namely diheptylazobenzene, showing the nematic and smectic A liquid crystalline phases, was investigated by means of a combined approach based on NMR and DFT calculations. ¹⁴N NMR quadrupole‐ and chemical‐shift‐perturbed spectra were acquired in the whole mesophasic range, providing both experimental quadrupolar splittings and chemical shift anisotropy values. On the same mesogen, deuterium labelled at the α‐position of the hydrocarbon chain, ²H NMR quadrupole‐perturbed spectra were recorded. The analysis of these NMR data was performed with the help of ab initio calculations, in vacuo and by taking into account the effect of the anisotropic environment typical of liquid crystals, by using the IEF‐PCM model. The geometry optimizations of the azomesogen in the trans and cis configurations were performed by DFT calculations employing the combination of B3LYP functional with the 6‐311G(d) basis set. The analysis of experimental NMR data was performed by considering the trans configuration as the most populated one and the corresponding quadrupolar tensors and chemical shielding tensors were determined at the DFT level of theory. The main result of this work is the determination of a relatively high and temperature‐dependent molecular biaxiality of the trans state of this azomesogen.     

Conformational Distributions of N‐Acetyl‐L‐cysteine in Aqueous Solutions: A Combined Implicit and Explicit Solvation Treatment of VA and VCD Spectra

The conformational distributions of N‐acetyl‐L ‐cysteine (NALC) in aqueous solutions at several representative pH values are investigated using vibrational absorption (VA), UV/Vis, and vibrational circular dichroism (VCD) spectroscopy, together with DFT and molecular dynamics (MD) simulations. The experimental VA and UV/Vis spectra of NALC in water are obtained under strongly acid, neutral, and strongly basic conditions, as well as the VCD spectrum at pH 7 in D₂O. Extensive searches are carried out to locate the most stable conformers of the protonated, neutral, deprotonated, and doubly deprotonated NALC species at the B3LYP/6‐311++G(d,p) level. The inclusion of the polarizable continuum model (PCM) modifies the geometries and the relative stabilities of the conformers noticeably. The simulated PCM VA spectra show significantly better agreement with the experimental data than the gas‐phase ones, thus allowing assignment of the conformational distributions and dominant species under each experimental condition. To further properly account for the discrepancies noted between the experimental and simulated VCD spectra, PCM and the explicit solvent model are utilized. MD simulations are used to aid the modelling of the NALC–(water)_N clusters. The geometry optimization, harmonic frequency calculations, and VA and VCD intensities are computed for the NALC–(water)_3,4 clusters at the B3LYP/6‐311++G(d,p) level without and with the PCM. The inclusion of both explicit and implicit solvation models at the same time provides a decisively better agreement between theory and experiment and therefore conclusive information about the conformational distributions of NALC in water and hydrogen‐bonding interactions between NALC and water molecules.     

Polar aspects of intramolecular interactions in 2‐amino‐5‐nitro‐4‐methylpyridines and 2‐amino‐3‐nitro‐4‐methylpyridines

The dipole moments of twelve 2‐N‐substituted amino‐5‐nitro‐4‐methylpyridines ( I‐XII ) and three 2‐N‐substituted amino‐3‐nitro‐4‐methylpyridines ( XIII‐XV ) were determined in benzene. The polar aspects of intramolecular charge‐transfer and intramolecular hydrogen bonding were discussed. The interaction dipole moments, μ_int, were calculated for 2‐N‐alkyl(or aryl)amino‐5‐nitro‐4‐methylpyridines. Increased alkylation of amino nitrogen brought about an intensified push‐pull interaction between the amino and nitro groups. The solvent effects on the dipole moments of 2‐N‐methylamino‐5‐nitro‐4‐methyl‐( I ), 2‐N,N‐dimethylamino‐5‐nitro‐4‐methyl‐ ( II ) and 2‐N‐methylamino‐3‐nitro‐4‐methylpyridines ( XIII ) were different. Specific hydrogen bond solute‐solvent interactions increased the charge‐transfer effect in I , but it did not disrupt the intramolecular hydrogen bond in XIII.     

Density functional theory study of nitrogen‐14 nuclear quadrupole coupling parameters of L‐histidine: hydrogen‐bonded system

The calculations of nitrogen‐14 nuclear quadrupole parameters, nuclear quadrupole coupling constant, χ, and asymmetry parameter, η, of L‐His were done in two distinct environments: one as a free fully optimized molecule, an isolated molecule with the geometrical parameters taken from X‐ray, and the other in the orthorhombic and monoclinic solid states. The most probable interacting molecules with the central molecule in the crystalline phase were considered in the hexameric clusters to include hydrogen‐bonding effects in the calculations. The computations were performed with PW91P86/6‐31++G** and B3LYP6‐31++G** methods using the Gaussian 98 program. The good agreement between the nitrogen‐14 quadrupole parameters of the free His and imidazole molecules with their microwave available data demonstrates that the applied level of theory and the 6‐31++G** basis set are suitable to obtain reliable electric field gradient values. In the solid state, the shifts of quadrupole coupling parameters from the monomer to the solid phase are reasonably well reproduced for the amino and imino sites of imidazole ring in a hexameric cluster. That implies the fact that the hexameric cluster worked effectively to generate the results which are compatible with the experiment. The quadrupole coupling constant values of –N⁺H₃ group are in fair agreement with the experiment. This discrepancy is due to the absences of vibrational effects and the rotation of –N⁺H₃ group around N–C(α) bond. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIII. Accuracy of the calculation of 15N NMR chemical shifts of azines with account taken of solvation effects

Effects of solvation on the accuracy of the calculation of ¹⁵N chemical shifts in the azine series have been analyzed at the DFT/GIAO level of theory. The best results are obtained with the use of the Keal-Tozer KT2 functional in combination with the Dunning aug-cc-pVTZ basis set with inclusion of solvent effects according to the Tomasi polarizable continuum model (PCM). If specific solvation is strong, additional consideration of solvent effects in the supermolecule approximation is necessary with explicit inclusion of solvent molecules.