A computational study of microsolvation effect on ethylene glycol by density functional method

This study focuses on the conformational analysis of ethylene glycol-(water)n (n=1-3) complex by using density functional theory method and the basis set 6-311++G*. Different conformers are reported and the basis set superposition error corrected total energy is -306.767 5171, -383.221 3135, and -459.694 1528 for lowest energy conformer with 1, 2, and 3 water molecules, respectively, with corresponding binding energy -7.75, -15.43, and -36.28 kcal/mol. On applying many-body analysis it has been found that relaxation energy, two-body, three-body energy have significant contribution to the binding energy for ethylene glycol-(water)3 complex whereas four-body energies are negligible. The most stable conformers of ethylene glycol-(water)n complex are the cyclic structures in which water molecules bridge between the two hydroxyl group of ethylene glycol.     

Hydrogen bonding interaction between 1,4‐dioxane and water

This work reports an interaction of 1,4‐dioxane with one, two, and three water molecules using the density functional theory method at B3LYP/6‐311++G* level. Different conformers were studied and the most stable conformer of 1,4‐dioxane‐(water)_n (n = 1–3) complex has total energies ?384.1964038, ?460.6570694, and ?537.1032381 hartrees with one, two, and three water molecules, respectively. Corresponding binding energy (BE) for these three most stable structures is 6.23, 16.73, and 18.11 kcal/mol. The hydrogen bonding results in red shift in O? O stretching and C? C stretching modes of 1,4‐dioxane for the most stable conformer of 1,4‐dioxane with one, two, and three water molecules whereas there was a blue shift in C? O symmetric stretching and C? O asymmetric stretching modes of 1,4‐dioxane. The hydrogen bonding results in large red shift in bending mode of water and large blue shift in symmetric stretching and asymmetric stretching mode of water. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Density functional study of hydrogen‐bonded acetonitrile–water complex

We report the interaction of acetonitrile with one, two, and three water molecules using the Density Functional Theory method and the 6‐31+G* basis set. Different conformers were studied and the most stable conformer of acetonitrile–(water)_n complex has total energies –209.1922504, –285.6224478, and –362.068728 hartrees with one, two, and three water molecules, respectively. The corresponding binding energy for these three structures is 4.52, 8.34, and 22.48 kcal/mol. The hydrogen‐bonding results in blue, blue, and redshift in C?N stretching mode in acetonitrile with one, two, and three water molecules, respectively, whereas there was a redshift in O? H symmetric stretching mode of water. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

吡啶-BH~3相互作用复合物的理论研究

对吡啶-BH~3复合物分别用MP2/6-31+G^*和B3LYP/6-31+G^*进行理论计算以预测该复合物的构型及解离能，得到四种构型，在MP2优化构型基础上作CCSD/6-31+G^*单点能量计算以验证MP2与B3LYP结果的可靠性，然后用B3LYP作振动频率分析，计算了各构型的垂直电离势，最后用更大基组作单点能量计算和自然键轨道(NBO)分析。结果表明，N-B直接相连的构型最稳定，其解离能为141.50kJ/mol,MP2和B3LYP对N-H接近的构型结果相关较大，另外两种构型稳定性介于二者之间，解离能分别为15.18kJ/mol,14.06kJ/mol(MP2/6-31+G^*)。     

Comparison of experimental and density functional study on the molecular structure, infrared and Raman spectra and vibrational assignments of 6-chloronicotinic acid

The experimental and theoretical study on the structures and vibrations of 6-chloronicotinic acid (6-CNA, C(6)H(4)ClNO(2)) are presented. The Fourier transform infrared spectra (4,000-50 cm(-1)) and the Fourier transform Raman spectra (3,500-50 cm(-1)) of the title molecule in solid phase have been recorded, for the first time. The geometrical parameters and energies have been obtained for all four conformers from DFT (B3LYP) with different basis sets calculations. There are four conformers, C1, C2, C3, and C4 for this molecule. The computational results diagnose the most stable conformer of 6-CNA as the C1 form. The vibrations of the two stable and two unstable conformers of 6-CNA are researched with the aid of quantum chemical calculations. The molecular structure, vibrational frequencies, infrared intensities and Raman scattering activities and theoretical vibrational spectra were calculated a pair of molecules linked by the intermolecular OH...O hydrogen bond. The spectroscopic and theoretical results are compared to the corresponding properties for 6-CNA stable monomers and dimer of C1 conformer.     

Hydrogen-bonding interactions in acetic acid monohydrates and dihydrates by density-functional theory calculations

Equilibrium structures and the respective binding energies of acetic acid monohydrates and dihydrates have been determined by density-functional theory calculations with different basis sets, including 6-31+G(3d,p), 6-311++G(d,p), and 6-311++G(3df,3pd). Given that the C=O and OH groups in acetic acid provide the predominant hydrogen-bonding interactions with water, six stable conformer structures have been found each for the monohydrate and syn-dihydrate. Of the three syn- and three anti-conformers of acetic acid with water, the most stable monohydrate structure is found to be that of the syn-conformer bonding with water in a cyclic double H-bonded geometry. Similarly, the syn-conformer bonding with two water molecules in a cyclic double H-bonded geometry has also been determined to be the most stable among the six plausible structures for the syn-dihydrate. Frequency analysis of the stable conformers has been performed and the vibrational spectra of the most stable monohydrate and dihydrate structures are compared with the experimental gas-phase and matrix data. Furthermore, the calculated binding energies between an acetic acid and a water molecule for both monohydrate and dihydrate are larger than that between two water molecules, which supports our recent experimental observation of coevaporation of acetic acid with water upon annealing acetic acid on ice.     

Conformers of gaseous proline

Accurate geometries, relative energies, rotational and quartic centrifugal distortion constants, dipole moments, harmonic vibrational frequencies, and infrared intensities were determined from ab initio electronic structure calculations for eighteen conformers of the neutral form of the amino acid L-proline. Only four conformers have notable population at low and moderate temperature. The second most stable conformer is only 2+/-2 kJ mol(-1) above the global minimum, while the third and fourth conformers are nearly degenerate and have an excess energy of 7+/-2 kJ mol(-1) relative to the global minimum. All four conformers have one hydrogen bond: N.HO in the lower energy pair of conformers, and NH.O in the higher energy pair of conformers. The conformer pairs differ only in their ring puckering. The relative energies of the conformers include corrections for valence electron correlation, extrapolated to the complete basis set limit, as well as core correlation and relativistic effects. Structural features of the pyrrolidine ring of proline are discussed by using the concept of pseudorotation. The accurate rotational and quartic centrifugal distortion constants as well as the vibrational frequencies and infrared intensities should aid identification and characterization of the conformers of L-proline by rotational and vibrational spectroscopy, respectively. Bonding features of L-proline, especially intramolecular hydrogen bonds, were investigated by the atoms-in-molecules (AIM) technique.     

1,2,3-三氮杂苯－(水)3复合物多体相互作用

The interaction between 1,2,3-triazine and three water molecules was studied using density functional theory B3LYP method at 6-31-t＋＋G^＊＊ basis set. Various structures for 1,2,3-triazine-（water）n （n= 1, 2, 3） complex were investigated and the different lower energy structures were reported. Many-body analysis was also carded out to obtain relaxation energy and many-body interaction energy （two, three, and four-body）, and the most stable conformer has the basis set superposition error corrected interaction energy of -- 102.61 kJ/mol. The relaxation energy, two- and three-body interactions have significant contribution to the total interaction energy whereas four-body interaction was very small for 1,2,3-triazine-（water）3 complex.     

DME构象的ADF研究

用ＡＤＦ方法对游离状态下的１,２－二甲氧基乙烷（ＤＭＥ）的构象进行了研究。结果表明,能量最低的３种构象ｔｇｇ,’ｔｇｔ,ｔｔｔ的能量相近,其中ｔｔｔ不是能量最低的构象,而且说明ＤＭＥ在气态、液态、固态和游离状态下分别采用哪种构象为最优构象,除了与构象的能量有关外,还受分子间相互作用及分子的对称性是否适合于紧密堆积的影响。     

Hydrogen abstraction from n-butanol by the methyl radical: high level ab initio study of abstraction pathways and the importance of low energy rotational conformers

Hydrogen abstraction reactions by the methyl radical from n-butanol have been investigated at the ROCBS-QB3 level of theory. Reaction energies and product geometries for the most stable conformer of n-butanol (ROH) have been computed, the reaction energies order α < γ < β < δ < OH. The preference for n-butane to favour H-abstraction at C(β) and C(γ) while, in contrast, n-butanol favours radical reactions at the C(α) carbon is rationalised. Transition state (TS) barriers and geometries for the most stable conformer of n-butanol are presented, and discussed with respect to the Hammond postulate. The reaction barriers order as α < OH < γ < β < δ. This relative ordering is not consistent with product radical stability, C-H bond dissociation energies or previous studies using O[combining dot above]H and HO[combining dot above](2) radicals. We provide a molecular orbital based rationalisation for this ordering and answer two related questions: Why is the γ-channel more stable than the β-channel? Why do the two C(γ)-H H-abstraction TS differ in energy? The method and basis set dependence of the TS barriers is investigated. The Boltzmann probability distribution for the n-butanol conformers suggests that low energy conformers are present in approximately equal proportions to the most stable conformer at combustion temperatures where ?H(3) radicals are present. Thus, the relative significance of the various H-abstraction channels has been assessed for a selection of higher energy conformers (ROH'). Key results include finding that higher energy n-butanol conformers (E(ROH') > E(ROH)) can generate lower energy product radicals, E(ROH') < E(ROH). Moreover, higher energy conformers can also have a globally competitive TS energy for H-abstraction.     

NMR, UV, FT-IR, FT-Raman spectra and molecular structure (monomeric and dimeric structures) investigation of nicotinic acid N-oxide: A combined experimental and theoretical study

In this work, the experimental and theoretical UV, NMR, and vibrational features of nicotinic acid N-oxide (abbreviated as NANO, C(6)H(5)NO(3)) were studied. The ultraviolet (UV) absorption spectrum of studied compound that dissolved in water was examined in the range of 200-800nm. FT-IR and FT-Raman spectra in solid state were observed in the region 4000-400cm(-1) and 3500-50cm(-1), respectively. The (1)H and (13)C NMR spectra in DMSO were recorded. The geometrical parameters, energies and the spectroscopic properties of NANO were obtained for all four conformers from density functional theory (DFT) B3LYP/6-311++G(d,p) basis set calculations. There are four conformers, C(n), n=1-4 for this molecule. The computational results identified the most stable conformer of title molecule as the C1 form. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. (13)C and (1)H nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by using the gauge-invariant atomic orbital (GIAO) method. The electronic properties, such as excitation energies, absorption wavelengths, HOMO and LUMO energies, were performed by CIS approach. Finally the calculation results were applied to simulate infrared, Raman, and UV spectra of the title compound which show good agreement with observed spectra.     

A theoretical investigation of the low energy conformers of the isomers glycine and methylcarbamic acid and their role in the interstellar medium

We have theoretically investigated the low energy conformers of neutral glycine (NH(2)CH(2)COOH) and its isomer methylcarbamic acid (CH(3)NHCOOH) in the gas phase. A total of 16 different levels of the theory, including CCSD(T), MP2 and B3LYP methods with various Pople and Dunning type basis sets with and without polarization and diffuse functions were used. We found eight low energy glycine conformers, where the heavy atoms in three have a planar backbone, and four low energy methylcarbamic acid conformers all with non-planar backbones. Interestingly at all levels of theory, we found that the most stable methylcarbamic acid conformer is significantly lower in energy than the lowest energy glycine conformer. The MP2 level and single point CCSD(T) calculations show the lowest energy methylcarbamic acid conformer to be between 31 to 37 kJ mol(-1) lower in energy than the lowest energy glycine conformer. These calculations suggest that methylcarbamic acid might serve as a precursor to glycine formation in the Interstellar Medium (ISM). We also report the theoretical harmonic vibrational frequencies, infrared intensities, moment of inertia, rotational constants and dipole moments for all of the conformers. In order to understand how glycine or methylcarbamic acid might be formed in the ISM, larger calculations which model glycine or its isomer interacting with several surrounding molecules, such as water, are needed. We demonstrate that B3LYP method should provide a reliable and computationally practical approach to modeling these larger systems.     

Accurate predictions of water cluster formation, (H₂O)(n=2-10)

An efficient mixed molecular dynamics/quantum mechanics model has been applied to the water cluster system. The use of the MP2 method and correlation consistent basis sets, with appropriate correction for BSSE, allows for the accurate calculation of electronic and free energies for the formation of clusters of 2-10 water molecules. This approach reveals new low energy conformers for (H(2)O)(n=7,9,10). The water heptamer conformers comprise five different structural motifs ranging from a three-dimensional prism to a quasi-planar book structure. A prism-like structure is favored energetically at low temperatures, but a chair-like structure is the global Gibbs free energy minimum past 200 K. The water nonamers exhibit less complexity with all the low energy structures shaped like a prism. The decamer has 30 conformers that are within 2 kcal/mol of the Gibbs free energy minimum structure at 298 K. These structures are categorized into four conformer classes, and a pentagonal prism is the most stable structure from 0 to 320 K. Results can be used as benchmark values for empirical water models and density functionals, and the method can be applied to larger water clusters.     

Experimental and theoretical study of the structures and binding energies of eugenol (H2O)n, n=0-2

Eugenol (4-Allyl-2-methoxyphenol), a phenol-derivative with an intramolecular -OH ...OCH(3) hydrogen bond (H bond), has been studied in a supersonic expansion using a number of complementary laser spectroscopic techniques. The mass-resolved excitation spectrum of eugenol and its water complexes are reported for the first time. The most intense set of bands on the resonantly enhanced multiphoton ionization (REMPI) spectrum of eugenol originate in a conformer whose S(1)<--S(0) transition is at 35 202 cm(-1) and the ionization threshold at (I(0)<--S(0)) 62 544+/-150 cm(-1) (7.755+/-0.019 eV). In addition, two low intensity features redshifted with respect to the 0(0) (0) transition have been identified as due to a second, less stable conformer. Ab initio calculations show that the potential energy landscape depicts at least three minima associated with one folded and two extended conformers, one of which is the most stable. Clusters of eugenol/water were prepared in a supersonic expansion by seeding eugenol and water in noble gas He and examined by two-color REMPI (R2PI) and IR-UV double resonance spectroscopies. Only one single isomer was observed for both 1:1 and 1:2 complexes, in contrast with the several stable conformers provided by the computations. The dissociation energies of the 1:1 and 1:2 complexes have been determined by the fragmentation threshold method and the results compared with those from ab initio calculations conducted at the B3LYP and MP2 levels with a variety of basis sets.     

Study of metal ions (Na+, K+) interaction with different conformations of glycine molecule

Interaction of metal ions (Na⁺, K⁺) with different binding sites, such as amino nitrogen, hydroxyl oxygen, and carbonyl oxygen for all gaseous conformers of glycine molecule were investigated using Density Functional Theory (B3LYP/6‐311++G**, B3PW91/6‐311++G**) methods. It was found that the order of stability of the conformers was changed due to the binding of the metal ion. The relative energy values show that the 7p conformer is more stable than the 1p conformer when a metal ion binds with the carbonyl oxygen. The intensity of interaction on hydroxyl oxygen is very low due to the low basicity of hydroxyl oxygen. The binding affinities of the complexes were calculated using the thermochemical properties. The relative energy and chemical hardness values predicted the most stable complex. The calculated condensed Fukui functions predict the favorable reactive site among the three binding sites. It is concluded that the reactivity of each binding site varies for each conformation due to the presence of intramolecular hydrogen bonding. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Sarin and Soman: Structure and Properties

The most stable conformers of sarin (isopropyl methylphosphonoflouridate) and soman (pinacolyl methylphosphonofluoridate) are determined in high-level-correlated calculations with extended Gaussian basis sets. The two molecules are found to have three low-energy conformers each. For both molecules two of the lowest energy conformers have almost the same energies with a very small barrier separating the corresponding minima. The third conformer of sarin is found to lie about 1 kcal/mol above the lowest energy form. For soman the corresponding value is equal to about 4 kcal/mol. The significance of these data for the mechanism of the toxic action of sarin and soman is discussed. According to our investigations sarin and soman are highly similar electronically and differences in their features arise mostly from the size and spatial arrangement of the alkoxy substituent at phosphorus. Also the influence of solvents on the conformations and solvation energies of sarin and soman is investigated.     

Energetics during proton transfer process in hydrated zündel ion complex

Zündel ion (H₅O) is one of the two important structures formed during the proton transfer process in aqueous system. This work reports microsolvation of Zündel ion using density functional theory based B3LYP method with aug‐cc‐pVTZ basis set. Interaction of Zündel ion with four water molecules in its first solvation shell is studied using many‐body analysis approach. A change in many‐body energies and their contribution to the binding energy of a complex during the proton transfer process from donor to acceptor water molecule in Zündel ion‐4H₂O complex is obtained. For the hydrated Zündel ion complex, the contribution from total two‐body, three‐body, four‐body, five‐body, and relaxation energy to the binding energy is 84.7, 14, 6.87, 1.6, and 4%, respectively, at B3LYP/aug‐cc‐pVTZ level. Relaxation energy and total five‐body energy have repulsive contribution to the binding energy of a hydrated Zündel ion complex. It is found that the relaxation energy and binding energy of a Zündel‐4H₂O complex is the maximum and minimum, respectively, when a shared proton is at equal distance from oxygen atom of donor and acceptor water molecules. A significant change in two‐body, three‐body, and four‐body energies for which Zündel ion is one of the many‐body terms is observed during the proton transfer process. A change in total two‐body, total three‐body, total four‐body, and relaxation energy is about 2.6, 1.8, 0.4, and 1.1%, respectively, during the proton transfer process. A change in two‐body, three‐body, and four‐body interaction energies between water molecules is very small during the proton transfer process. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Microwave spectrum, conformational composition, and intramolecular hydrogen bonding of (2-chloroethyl)amine (ClCH2CH2NH2)

The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts.     

Identification of conformational structures of 2-phenylethanol and its singly hydrated complex by mass selective high-resolution spectroscopy and ab initio calculations

The flexible prototype molecule 2-phenylethanol (2-PE) and its singly hydrated complex have been investigated in a cold supersonic beam by a combination of high-resolution two-color R2PI spectroscopy and quantum chemistry ab initio calculations. The existence of two monomer structures separated by a high potential energy barrier, gauche and anti ones, was proven. Higher energy conformers are supposed to relax to the observed ones during the jet expansion process. We have identified the conformational structure of the complex between 2-PE and water, which corresponds to water binding to the most stable gauche conformer. No detectable structural changes of the host 2-PE molecule have been observed upon attachment of a single water molecule. A conformational relaxation mechanism is suggested also for the 2-PE x H2O complex.     

A microwave and quantum chemical study of the conformational properties and intramolecular hydrogen bonding of 1-fluorocyclopropanecarboxylic acid

The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four "stable" conformers, denoted conformers I-IV, were found in the quantum chemical calculations, three of which (conformers I -III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18-62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by an intramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylic acid group. Conformer III has a synperiplanar orientation for the F-C-C=O and H-O-C=O chains of atoms. Its dipole moment is: mua = 3.4(10), mub = 10.1(13), and muc = 0.0 (assumed) and mu(tot) = 10.6(14) x 10(-30) C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and III were also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III by relative intensity measurements. Absolute intensity measurements were used to show that the unassigned conformer I is the most abundant form present at a concentration of roughly 65% at room temperature. Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformer II) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations is found in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7 kJ/mol more stable than II.