Temperature Dependence of <Superscript>1</Superscript>H and <Superscript>17</Superscript>O NMR Shifts of Water: Entropy Effect

Density functional theory together with statistical thermodynamics based on the equilibrium constants method and concept of orientational entropy were applied to reproduce the temperature dependences of ¹H and ¹⁷O nuclear magnetic resonance (NMR) chemical shifts in liquid water. Despite a rather simple theoretical model, a satisfactory agreement between calculated NMR quantities and corresponding experimental data was found. By using only a single adjustable parameter of arbitrary directionality, we succeeded to imitate the first-order temperature effect for both (¹H and ¹⁷O) NMR signals in the neat water. ¹H and ¹⁷O magnetic shielding tensors of water molecules in various water clusters have been calculated using the density functional theory. The full geometry optimization was performed using Becke's three-parameter hybrid method and the Lee–Yang–Parr correlation functional (B3LYP) combined with 6-311++G^** basis set. Magnetic shielding tensors have been calculated using the modified hybrid functional of Perdew, Burke and Ernzerhof, and the gauge-including atomic orbital approach was applied to ensure gauge invariance of the results. Solvent effects were taken into account by the polarized continuum model. Authors' address: Vytautas Balevicius, Faculty of Physics, Vilnius University, Sauletekio 9, Vilnius 10222, Lithuania     

Structure of molecules in crystalline lattice obtained by a modified method of molecular mechanics: Calculations of<Superscript>13</Superscript>C chemical shifts

A program for the calculation of the geometric structure of molecular crystals on the basis of the methods of molecular mechanics (MM) has been developed. A standard MM method has been modified by including force fields taking into account the specific (H-bond and van der Waals) interactions and the periodicity of the crystal lattice of an arbitrary form and symmetry. The geometric parameters of the molecule in a crystal calculated by this method are in agreement with the experimental X-ray data within reasonable accuracy. The nuclear magnetic resonance¹³C chemical shifts have been calculated for the molecular geometry obtained by the quantum chemical UB3LYP/6-31G(d, p) method. The results of the calculations have been used to explain some unusual NMR spectral data.     

Calculation of the structure and IR spectrum of methyl-β-D-glucopyranoside by density functional theory

Structural-dynamic models of methyl-β-D-glucopyranoside have been constructed by a density functional method using a B3LYP functional in bases 6-31G(d) and 6-31+G(d,p). Energies have been minimized. Structures, dipole moments, polarizabilities, frequencies of normal modes in the harmonic approximation, and the intensity distribution in the molecular IR spectrum have been calculated. The calculation results have been compared with both the experimental spectra of methyl-β-D-glucopyranoside in the region 400–3700 cm^–1 and data obtained within the framework of an approach that uses the classical valence-force method to calculate normal mode frequencies and the quantum-chemical CNDO/2 technique to calculate the electronic structure.     

Combined experimental and theoretical studies of the elimination kinetic of 2‐methoxytetrahydropyran in the gas phase

The products formed in 2‐methoxytetrahydropyran elimination reaction in the gas phase are 3, 4‐dihydro‐2H‐pyran and methanol. The kinetic study was carried out in a static system, with the vessels deactivated with allyl bromide, and the presence of the free radical suppressor toluene. Temperature and pressure ranges were 400–450 °C and 25–83 Torr, respectively. The process is homogeneous, unimolecular, and follows a first‐order rate law. The observed rate coefficient is represented by the following equation: log k (s^?1) = (13.95 ± 0.15) ? (223.1 ± 2.1) (kJ mol^?1) (2.303RT)^?1. The reactant exists mainly in two low energy chair‐like conformations, with the 2‐methoxy group in axial or equatorial position. However, the transition state (TS) for the elimination of the two conformers is the same. Theoretical calculations of this reaction were carried for two possible mechanisms from these conformations by using DFT functionals B3LYP, MPW1PW91, and PBE with the basis set 6‐31G(d,p) and 6‐31G++(d,p). The calculation results demonstrate that 2‐methoxytetrahydropyran exists mainly in two conformations, with the 2‐methoy group in axial or equatorial position, that are thermal in equilibrium. The average thermodynamic and kinetic parameters, taking into account the populations of the conformers in the equilibrium, are in good agreement with experimental values at B3LYP/6‐31++(d,p) level of theory. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular structure,vibrational spectroscopic studies and natural bond orbital analysis of 7-amino-4-trifluoromethyl coumarin

Quantum mechanical calculations of energies, geometries and vibrational wave numbers of 7-amino-4-trifluoromethyl coumarin (7A4TFMC) were carried out using Hartree-Fock (HF) and density functional theory (DFT) using hybrid functional BLYP and B3LYP with 6–31G(d,p) as basis set. The optimized geometrical parameters obtained by HF and DFT calculations are in good agreement with the experimental X-ray data. The best method to reproduce the experimental wave numbers is B3LYP method with the 6–31G(d,p) basis set. The difference between the observed and scaled wave number values of most of the fundamentals is very small. A detailed interpretation of the infrared spectra of 7A4TFMC was also reported. The entropy of the title compound was also performed at HF using the hybrid functional BLYP and B3LYP with 6–31 G(d,p) as basis set levels of theory. Natural bond orbital (NBO) analysis of the title molecule is also carried out. The theoretical spectrogram for FTIR spectra of the title molecule has been constructed.     

Structural dynamic models and anharmonic analysis of vibrational states of polychloro-substituted dibenzo-<Emphasis Type="Italic">p</Emphasis>-dioxin

We have analyzed the vibrational spectra of β-tetrachloro-substituted and ortho-chlorosubstituted dibenzo-p-dioxin using the DFT/B3LYP/6-31G(d) method, in the anharmonic approximation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 21–24, January–February, 2007.     

The mechanism of the homogeneous,unimolecular gas‐phase elimination kinetic of 1,1‐dimethoxycyclohexane: experimental and theoretical studies

The gas‐phase elimination of 1,1‐dimethoxycyclohexane yielded 1‐methoxy‐1‐cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310–360 °C and the pressure was 25–85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first‐order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s^?1) = [(13.82 ± 0.07) – (193.9 ± 1.0)(kJ mol^?1)](2.303RT)^?1; r = 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6‐31G(d,p) and 6‐31G++(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6‐31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four‐membered cyclic transition state. The transition state structure of methanol elimination from 1,1‐dimethoxycyclohexane is characterized by a significantly elongated C? O bond, while the C_β? H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of chlorine atoms in chlorinated ethylene on the rate and mechanism of its reaction with ozone

The UQCISD, UB3LYP, UMP2, and MRMP2 methods in conjunction with the 6-31+G**/6-311+G** and aug-cc-PVDZ basis sets are used to study the primary reaction of ozone with chlorinated ethylene derivatives: tetrachloroethylene, trichloroethylene, 1,2-trans-dichloroethylene, 1,2-cis-dichloroethylene, 1,1-dichloroethylene, and chloroethylene. The reaction is studied for both concerted and nonconcerted ozone addition. The UB3LYP DFT method in conjunction with the 6-31+G** basis set is used to examine various modes of addition of ozone to these chlorinated ethylenes by the Criegee and DeMore mechanisms. The geometry and energy of the transition states, the enthalpy and entropy, and the rate constants and ratios thereof for all the reactions are calculated. The UB3LYP method generally satisfactorily describes the two reaction pathways and, largely correctly predicts the rate constants, in agreement with the available experimental data. At the same time, this method appears to be inapplicable to modeling the interaction of ozone with 1,1-dichloroethylene. In this case, the single-determinant approximation turns out to be unsuitable, and, therefore, MCSCF methods should be used. The MRMP2 method yields reasonable values of the rate constants for the DeMore mechanism, whereas in the case of the Criegee mechanism, the MP2 method does well. The UB3LYP/6-31+G** and UQCISD/aug-cc-PVDZ methods give similar values of the ratio between the rate constants for the two pathways, a result that demonstrates the versatility of the first one.     

ONIOM方法计算胺和二芳基碳正离子的亲核反应的速率常数

利用第一性原理计算了胺和二芳基碳正离子的亲核反应的速率常数. 研究不同的溶剂化模型(PCM、CPCM和COSMORS)、不同类型的原子半径(UA0、UAKS、UAHF、Bondi和UFF)、以及一些单点能计算方法(B3LYP、B3P86、B3PW91、BHANDH、BMKPBEPBE、M06、MP2和ONIOM)在计算这类速率常数时的表现.通过比较速率常数的实验值和计算值,发现ONIOM(CCSD(T)/6-311++G(2df,2p):B3LYP/6-311++G(2df,2p))//B3LYP/6-31G(d)/PCM/UFF方法表现最好. 该方法随后被用于计算更多的胺和二芳基碳正离子的亲核反应的速率常数. 65个反应的速率常数的实验值和计算值之间表现出了相当好的相关性,这表明该方法适用于计算胺二芳基碳正离子的亲核反应的速率常数.     

Electrocyclic [1,5] hydrogen shift in the thermal elimination kinetics of phenyl acetate and p-tolyl acetate in the gas phase: a density functional theory study

The kinetics and mechanisms of thermal decomposition of phenyl acetate and p-tolyl acetate in the gas phase were studied by means of electronic structure calculations using density functional theory methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), B3PW91/6-31G(d,p), B3PW91/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBE/6-31G(d,p) and PBE/6-31++G(d,p). Two possible mechanisms have been considered: mechanism A is a stepwise process involving electrocyclic [1,5] hydrogen shift to eliminate ketene through concerted six-membered cyclic transition-state structure, followed by tautomerisation of cyclohexadienone or by 4-methyl cyclohexadienone intermediate to give the corresponding phenol. Mechanism B is a one-step concerted [1,3] hydrogen shift through a four-membered cyclic transition-state geometry, to produce ketene and phenol or p-cresol. Theoretical calculations showed reasonable agreement with experimental activation parameters when using the Perdew, Burke and Ernserhof (PBE)functional, through the stepwise [1,5] hydrogen-shift mechanism. For mechanism B, large deviation for the entropy of activation was observed. No experimental data were available for p-tolyl acetate; however, theoretical calculations showed similar results to phenyl acetate, thus supporting the stepwise mechanism for both phenyl acetate and p-tolyl acetate.     

Computational studies of vibrational spectra and molecular properties of 6-methyluracil using HF, DFT and MP2 methods

Theoretical investigations of atomic charges, conformers, frontier molecular orbitals, molecular geometries, thermodynamic properties, hyperpolarizabilities and harmonic vibrational frequencies of 6-methyluracil (6MU) have been carried out using ab initio Hartree-Fock (HF), density functional theory (DFT) and second order M?ller-Plesset (MP2) methods. All calculations were performed using the GAMESS-US program package with the basis sets 6-31G(d,p) and 6-311G(d,p). FT-IR and Raman spectra of 6MU were recorded in the regions 50–4000 cm⁻¹ and 60–4000 cm⁻¹ respectively. Optimized geometries were obtained using the global optimization procedure. The calculated structural parameters for two conformers of 6MU have been compared with experimentally observed values. The energy barrier (ΔE=ELUMO-EHOMO) between the HOMO and LUMO is predicted on the basis of theoretical calculations. The simulated TD-DFT spectrum has been compared with experimental electronic spectrum for 6MU. The calculated potential energy distribution (PED) values have been utilized to perform vibrational assignment of the infrared and Raman spectra.     

Analysis of vibrational spectra of methyl-substituted uracils in the anharmonic approximation

We have analyzed the vibrational spectra of uracil and its methyl-substituted derivatives using the DFT/B3LYP/6-31G(p,d) method, in the anharmonic approximation. We have shown that it is possible to use second-order anharmonic theory of vibrational spectra in predictive calculations for substituted uracils. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 431–436, July–August, 2006.     

Electronic states of CsLi and CsLi<Superscript>+</Superscript> molecules

Configuration interaction calculations have been carried out on electronic states of the CsLi molecule and the CsLi⁺ cation. Adiabatic potential energy, spectroscopic constants, dipole moments, and vibrational levels are presented for the lowest states of ^1,3Σ⁺, ^1,3Π, and ^1,3Δ symmetries of the alkali dimer CsLi molecule dissociating into Cs (6s, 6p, 5d, 7s, and 7p) + Li (2s, 2p, 3s, 3p, and 3d) as well as for the lowest ²Σ⁺, ²Π, and ²Δ electronic states of the CsLi⁺ cation dissociating into Li (2s, 2p, 3s, 3p, and 3d) + Cs⁺ and Li⁺ + Cs (6s, 6p, 5d, 7s, and 7p). The results of the present many-electron configuration interaction calculations on the cation support the previous core-polarization effective potential calculations. The present calculations on the CsLi molecule are complementary to previous theoretical work on this system, including recently observed electronic states that had not been calculated previously. We have used an ab initio approach involving a nonempirical pseudopotential for the Li (1s²) and Cs cores and a core-valence correlation correction. A very good agreement of data from spectroscopic constants for some of the lowest states of the CsLi and CsLi⁺ molecules with those available in recent theoretical works has been obtained. The existence of numerous avoided crossings between electronic states of ²Σ⁺ and ²Π symmetries is related to a charge transfer process between the two ionic CsLi⁺ and LiCs⁺ systems.     

The Use of Raman Spectroscopy and Methods of Quantum Chemistry for Assessing the Relative Concentration of Triglycerides of Oleic and Linoleic Acids in a Mixture of Olive Oil and Sunflower Seed Oil

The Raman spectra of five samples of sunflower seed oil and five samples of cold-pressed olive oil of various brands are recorded in the range of 500–2000 cm^–1. Within the framework of the B3LYP/6-31G(d)/6-31G(d,p)/6-31+G(d,p)/6-311G(d)/6-311G(d, p)/6-311+G(d,p) methods, the structural models of eight fatty acids (oleic, linoleic, palmitic, stearic, α-linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic) are constructed, and also within the framework of the B3LYP/6-31G(d) method, the structural models of triglycerides of the first four of the above acids are obtained. The vibrational wavenumbers and intensities in the IR and Raman spectra are calculated. The Raman spectra of olive oil and sunflower seed oil were simulated by using the supermolecular approach. We investigated the dependence of the relative intensity of the vibrational bands ν_exp = 1660 and 1445 cm^–1 on the concentration of triglycerides in oils of oleic and linoleic acids and the dependence of the intensity of these bands on the degree of saturation of fatty acids. Experimental and empirical dependences are constructed to estimate the relative concentration of triglycerides of oleic and linoleic acids in a mixture of olive oil and sunflower seed oil. The applicability of the density functional theory together with the vibrational spectroscopy for the identification of mixtures of vegetable oils is discussed.     

Emission of Ionic Molecules (KrXe)<Superscript>+</Superscript> on Excitation by a Hard Ionizer

The molecular luminescence band appearing in the 445–510-nm region on excitation of a Kr-Xe mixture by the α particles of polonium 210 as well as by the products of nuclear reaction ³He(n,p)T has been investigated. Radiation at the transition of the heteronuclear ionic molecule (KrXe)⁺ has a high excitation efficiency and is observed in a temperature range from −100 to +600°C. The kinetics of the processes in the Kr-Xe mixture and the possibility of using radiation with λ ∼ 490 nm in radioactive light sources are considered. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 301–304, May–June, 2005.     

Theoretical study of the electronic structure of LiNa and LiNa<Superscript>+</Superscript> molecules

Adiabatic potential energy, spectroscopic constants, dipole moments, and vibrational levels of the lowest electronic states of the alkali dimer LiNa molecule dissociating into Na (3s, 3p, 4s, 3d, and 4p) + Li (2s, 2p, 3s, and 3p) in ^1,3Σ, ^1,3Π, and ^1,3Δ symmetries are presented. Adiabatic results are also reported for ²Σ, ²Π, and ²Δ electronic states of the molecular ion LiNa⁺ dissociating into Li (2s, 2p, 3s, and 3p) + Na⁺ and Li⁺  + Na(3s, 3p, 4s, 3d, and 4p). We use an ab initio approach involving a non-empirical pseudopotential for the Li (1s²) and Na (1s²2s²2p⁶) cores and core valence correlation correction. A very good agreement is obtained for some lowest states of the LiNa and LiNa⁺ molecules for spectroscopic constants with the available theoretical works. The existence of numerous avoided crossings between electronic states of ²Σ and ²Π symmetries is related to the charge transfer process between the two ionic systems Li⁺Na and LiNa⁺.     

Theoretical study of the <Superscript>1,3</Superscript>Σ<Superscript>+</Superscript> states of the NaH molecule in the adiabatic and diabatic representations

Adiabatic and diabatic study for all the states dissociating below the ionic limit [i.e., Na (3s, 3p, 4s, 3d, 4p, 5s, 4d, and 4f) + H (1s)] in ¹Σ⁺ and ³Σ⁺ symmetries are presented. Adiabatic results are also reported for ^1,3Π and ^1,3Δ symmetries. Pseudo-potential, operatorial core-valence correlation, and full valence CI approaches combined with an efficient diabatization procedure are used in these ab initio calculations. Our vibrational-level spacings and spectroscopic constants are in good agreement with the available experimental data for the low-lying states. Diabatic potentials and dipole moments are analyzed, revealing the strong imprint of the ionic state in the ¹Σ⁺ adiabatic states. The hydrogen electron affinity correction was taken into account by the use of the efficient diabatization method. This leads to a better agreement with the available experimental data. Experimental suggestions are also given for the higher excited states based on their unusual behavior.     

Theoretical calculations for neighboring group participation in gas‐phase elimination kinetics of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride

Theoretical calculation of the kinetics and mechanisms of gas‐phase elimination of 2‐hydroxyphenethyl chloride and 2‐methoxyphenethyl chloride has been carried out at the MP2/6‐31G(d,p), B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(d,p), B3PW91/6‐31G(d,p) and CCSD(T) levels of the theory. The two substrates undergo parallel elimination reactions. The first process of elimination appears to proceed through a three‐membered cyclic transition state by the anchimeric assistance of the aromatic ring to produce the corresponding styrene product and HCl. The second process of elimination occurs through a five‐membered cyclic transition state by participation of the oxygen of o‐OH or the o‐OCH₃ to yield in both cases benzohydrofuran. The B3PW91/6‐31G(d,p) method was found to be in good agreement with the experimental kinetic and thermodynamic parameters for both substrates in the two reaction channels. However, some differences in the performance of the different methods are observed. NBO analysis of the pyrolysis of both phenethyl chlorides implies a C? Cl bond polarization, in the sense of C^δ+…Cl^δ?, which is a rate‐determining step for both parallel reactions. Synchronicity parameters imply polar transition states of these elimination reactions. Copyright © 2009 John Wiley & Sons, Ltd.     

Scaled DFT Force Constants and Vibrational Spectrum of Cyclopropylamine

Abstract

The molecular structure and harmonic vibrational frequencies of cyclopropylamine have been calculated using the B3LYP density functional method with the 6‐31G(2d,2p) basis set. The scaled DFT force field gives very good reproduction of the experimental vibrational frequencies. Several of the vibrational fundamental modes assigned previously are reassigned on the basis of the B3LYP/6‐31G(2d,2p) method and the scaled force field calculations. The optimized scaling factors were used to scale the B3LYP/6‐31G(2d,2p) force field of cyclopropane and cyclopropylamine (–ND₂) molecules.