Ab initio quantum chemical investigation of the ground and excited states of salicylic acid dimer

The structure and stability of different forms of salicylic acid dimer have been examined by Hartree-Fock and density functional theoretic calculations using 6-31G(d,p) and 6-311++g(d,p) basis sets. Vertical excitation energies for the monomer as well as the dimer have been computed using the time-dependent density functional theory using 6-311++G(d,p) basis set. The predicted absorption maxima for the first excited singlet state of salicylic acid monomer and the dimer of the primary form are in reasonable agreement with the experimental result. There is a slight red shift (approximately 6 nm) in the absorption maximum in going from the monomer to the dimer, in accord with the experimental observation. Configuration-interaction calculations including single excitation have been carried out to map the potential-energy profile for the intra- as well as the intermolecular proton transfer in different forms of the dimer. The barrier for proton transfer in the ground state as well as the excited states makes it clear that most of the processes take place in the primary form and largely by intramolecular proton transfer.     

HOCl…HCOCl复合物的结构和电子性质

在DFT-B3LYP/6-311++G**水平上求得HOCl+HCOCl复合物势能面上的四种稳定构型(S1, S2, S3和S4). 其中, 在复合物S1和S3中, HOCl单体的5H原子作为质子供体, 与HCOCl单体中作为质子受体的1O原子相互作用, 形成红移氢键复合物; 在复合物S4中, HOCl单体的7Cl原子作为质子供体, 与HCOCl单体中作为质子受体的1O原子相互作用, 形成红移卤键复合物; 而在复合物S2中, 同时存在2C—3H…6O蓝移氢键和4Cl…5O相互作用. 在MP2/6-311++G**水平上计算的单体间的相互作用能考虑了基组重叠误差(BSSE)和零点振动能(ZPVE)校正, 其值在-5.05与-14.76 kJ·mol-1之间. 采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查, 并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

气相中O3与HSO自由基间的氢键复合物

气相中O3与HSO自由基之间的相互作用及其反应在大气化学中非常重要.在DFT-B3LYP/6-311++G**和MP2/6-311++G**水平上求得O3+HSO复合物势能面上的稳定构型,B3LYP方法得到了三种构型(复合物Ⅰ,Ⅱ和Ⅲ),而MP2方法只能得到一种构犁(复合物Ⅱ).在复合物Ⅰ和Ⅲ中,HSO单元中的1H原子作为质子供体.与O3分子中的端基O原子作为质子受体相互作用,形成红移氢键复合物;而在复合物Ⅱ中,虽与复合物Ⅰ和Ⅲ中具有相间的质子供体和质子受体,却形成了蓝移氢键复合物.B3LYP/6-311++G**水平上计算的单体间相互作用能的计算考虑了基组重甍误差(BSSE)和零点振动能(ZPVE)校正,其值在-3.37到-4.55 kJ·mol-1之间.采用自然键轨道理论(NBO)对单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了三种复合物中氢键的电子密度拓扑性质.     

Structures and Electronic Properties of HOCl···HCOCl Complexes

B3LYP/6-311++G** and MP2/6-311++G** calculations were used to analyze the interaction between hypochlorous acid (HOCl) and formyl chloride (HCOCl). The results showed that there were four equilibrium geometries (S1, S2, S3, and S4) optimized at B3LYP/6-311++G** level, and all the equilibrium geometries were confirmed to be in stable states by analytical frequency calculations. Complexes S1 and S3 use the 5H atom of HOCl as proton donor and the terminal 1O atom of HCOCl as acceptor to form red shift hydrogen bond systems. However, the blue-shifted hydrogen bond (2C-3H···6O) coexists with 4Cl···5O interaction in structures S2. As for S4, it uses the 7Cl atom of HOCl as proton donor and the terminal 1O atom of HCOCl as acceptor to form red shift halogen bond system. Interaction energies between monomers in the four complexes corrected with basis set superposition error (BSSE) and zero-point vibrational energy (ZPVE) lie in the range from −5.05 to −14.76 kJ·mol⁻¹ at MP2/6-311++G** level. The natural bond orbital (NBO) and atoms in molecules (AIM) theories have also been applied to explain the structures and the properties of the complexes.     

气相中O3与HSO自由基间的氢键复合物

气相中O3与HSO自由基之间的相互作用及其反应在大气化学中非常重要. 在DFT-B3LYP/6-311++G**和MP2/6-311++G**水平上求得O3+HSO复合物势能面上的稳定构型, B3LYP方法得到了三种构型(复合物I, II和III), 而MP2方法只能得到一种构型(复合物II). 在复合物I和III中, HSO单元中的1H原子作为质子供体, 与O3分子中的端基O原子作为质子受体相互作用, 形成红移氢键复合物; 而在复合物II中, 虽与复合物I和III中具有相同的质子供体和质子受体, 却形成了蓝移氢键复合物. B3LYP/6-311++G**水平上计算的单体间相互作用能的计算考虑了基组重叠误差(BSSE)和零点振动能(ZPVE)校正, 其值在-3.37到-4.55 kJ·mol-1之间. 采用自然键轨道理论(NBO)对单体间相互作用的本质进行了考查, 并通过分子中原子理论(AIM)分析了三种复合物中氢键的电子密度拓扑性质.     

Matrix isolation infrared and ab initio study of the hydrogen bonding between formic acid and water

The infrared spectra of the formic acid-water complexes isolated in argon matrices are reported. Both supersonic jet expansion and a conventional effusive source followed by trapping in solid argon at 10K are used to obtain the matrices. The experimental IR spectra are compared to the data obtained from high level ab initio (MP2) and DFT (B3LYP) calculations with 6-311++G(d,p) and aug-cc-pVTZ basis sets. The complex formation results in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid. The O-H stretching modes of water also exhibit pronounced red shifts. Both the MP2 and B3LYP calculations located three minima corresponding to cyclic HCOOH...H2O complexes with two hydrogen bond interactions. The binding energies are -10.3, -5.1, and -3.5 kcal mol(-1), respectively, for the three complexes at the MP2/ aug-cc-pVTZ level, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. Comparison of the calculated frequencies of the three complexes with the matrix IR spectrum reveals that the lowest energy complex is formed. In addition, a complex of formic acid with two water molecules is observed.     

HOCl…HCOCl复合物的结构和电子性质

在DFT-B3LYP/6-311++G**水平上求得HOCl+HCOCl复合物势能面上的四种稳定构型(S1,S2,S3和S4).其中,在复合物S1和S3中,HOCl单体的5H原子作为质子供体,与HCOCl单体中作为质子受体的10原子相互作用,形成红移氢键复合物;在复合物S4中,HOCl单体的7Cl原子作为质子供体,与HCOCl单体中作为质子受体的IO原子相互作用,形成红移卤键复合物;而在复合物S2中,同时存在2C-3H…6O蓝移氢键和4Cl…5O相互作用.在MP2/6-311++G**水平上计算的单体间的相互作用能考虑了基组重叠误差(BSSE)和零点振动能(ZPVE)校正,其值在-5.05与-14.76 kJ·mol-1之间.采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

Theoretical Studies on Intermolecular Hydrogen-bond Interactions between Hexamethylenetetramine and Nitric Acid

The structures of the complexes generated by hexamethylenetetramine and nitric acid have been fully optimized by B3LYP method at the 6-311++G** and aug-cc-pVTZ levels. The intermolecular hydrogen-bonding interactions have been calculated by the B3LYP/6-311++G**, B3LYP/aug-cc-pVTZ, MP2(full)/6-311++G** and CCSD(T)/6-311++G** methods, respectively. The NBO (nature bond orbital), AIM (atom in molecule), temperature effect and solvation effect have been analyzed to reveal the origin of the interactions. The results indicate that the stable hydrogen-bonded complexes could be generated by hexamethylenetetramine and nitric acid. The interactions follow the order of (a)>>(e)>(b)>(c)>(d)>(f)>(g). The C-N bonds which are adjacent to the methylene involving the hydrogen bonds tend to break in the chemical reaction. Due to the exothermic process, low temperature is conducive to the formation of the composition, which tallies with the experimental result.     

TD-DFT investigation of the UV spectra of pyranone derivatives

The UV absorption spectra of more than 80 substituted coumarins and chromones have been investigated with the PCM-TD-DFT theoretical scheme using three hybrid functionals (O3LYP, B3LYP, and PBE0) and taking into account methanol or ethanol solvation effects. For most of the studied derivatives, there are at least two allowed excited states presenting a strong oscillator strength in the UV region. The first allowed excitation is associated to a HOMO-LUMO transition whereas the second corresponds to a transition from the HOMO-1 to the LUMO. Both involve a charge transfer from the benzenic cycle to the pyranone moiety. Statistically treating the PBE0 results allows a prediction of the lambda(max) with small standard deviations: in methanol, 6 nm (0.07 eV) for the first excitation (lambda(max)(1)) and 5 nm (0.08 eV) for the second one (lambda(max)(2)), whereas in ethanol 6 nm (0.08 eV) for (lambda(max)(1)) and 6 nm (0.13 eV) for (lambda(max)(2)).     

Inverse hydrogen bonds between SiH_4 and hydrides of Na,Mg and Be

The optimized geometries of the three complexes between MeHn (Me=Na,Mg,Be;n=1 or 2) and SiH4 have been calculated at the B3LYP/6-311++g**,MP2/6-311++g(3df,3pd) and MP2/aug-cc-pvtz levels,respectively.The red-shift inverse hydrogen bonds (IHBs) based on Si-H,an electron donor,were reported.The calculated binding energies with basis set super-position error (BSSE) correction of the three complexes are-5.98,-8.65 and-3.96 kJ mol-1 (MP2/6-311++g(3df,3pd)),respectively,which agree with the results obtained via M...     

Hydrogen peroxide clusters: the role of open book motif in cage and helical structures

Hartree-Fock (HF) calculations using 6-31G*, 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets show that hydrogen peroxide molecular clusters tend to form hydrogen-bonded cyclic and cage structures along the lines expected of a molecule which can act as a proton donor as well as an acceptor. These results are reiterated by density functional theoretic (DFT) calculations with B3LYP parametrization and also by second-order M?ller-Plesset perturbation (MP2) theory using 6-31G* and 6-311++G(d,p) basis sets. Trends in stabilization energies and geometrical parameters obtained at the HF level using 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets are similar to those obtained from HF/6-31G* calculation. In addition, the HF calculations suggest the formation of stable helical structures for larger clusters, provided the neighbors form an open book structure.     

Theoretical studies on the intramolecular hydrogen bond and tautomerism of 8-mercaptoquinoline in the gaseous phase and in solution using modern DFT methods

A theoretical quantum chemical study of the intramolecular hydrogen bonding interactions in 8-mercaptoquinoline has been carried out. Special attention has been paid to the rotation of S-H bond and intramolecular proton-transfer reactions. Therewith, the B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p), MPW1K/6-311++G(d,p), MPW1K/6-31+G(2d,2p), BH&HLYP/6-311++G(d,p), and G96LYP/6-311++G(d,p) methods have been used. By means of the Onsager and PCM reaction field methods, the effects of solvent on hydrogen-bond energies, conformational equilibria, rotational barriers, and tautomerism in aqueous solution have been studied. These simulations were done at the MPW1K/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. Natural-bond orbital analysis has been performed to study the intramolecular hydrogen bond (IHB) in the gaseous phase and in aqueous medium. The stability of forms under consideration in solution does not coincide with that in the gaseous phase, underlining a great importance of the electrostatic influence of solvent. Double-proton transfer in the prototropic tautomerization of 8-mercaptoquinoline, one water molecule complex in the gaseous phase and in solution, has been systematically studied. The double-proton transfer occurs concertedly and synchronously. The water-assisted tautomerization is kinetically less, but thermodynamically more favorable, compared to that of the single-proton transfer. As in the case with single-proton transfer, for water-assisted reaction, the tautomerization energies and barrier heights decrease with the increase in dielectric constant, which implies faster and more complete tautomerization of 8-mercaptoquinoline in a polar solvent.     

4-羟甲基吡啶－水红移氢键的密度泛函理论研究

使用密度泛函理论B3LYP方法和6-311++G**基函数对4-羟甲基吡啶与水形成的1:1和1:2（摩尔比）氢键复合物进行了理论计算研究,分别得到稳定的4-羟甲基吡啶－H₂O和4-羟甲基吡啶－(H₂O)₂氢键复合物3个和8个。经基组重叠误差和零点振动能校正后,最稳定的1:1和1:2氢键复合物的相互作用能分别为-20.536和-44.246 kJ/mol。振动分析显示O-H···N(O)氢键的形成使复合物中O-H键对称伸缩振动频率红移(减小)。自然键轨道分析表明,4-羟甲基吡啶与水形成最稳定的1:1和1:2氢键复合物时,分子间电荷转移分别为0.02642 e 和0.03813 e 。含时密度泛函理论TD-B3LYP/ 6-311++G**计算显示,相对于4-羟甲基吡啶单体分子,氢键H-OH···N和H-OH···OH的形成分别使最大吸收光谱波长兰移8~16纳米和红移4~11纳米。     

Specific hydration effects on oxo-thio triazepine derivatives

The specific hydration of 2,7-dimethyl-1,2,4-triazepine oxo-thio derivatives by one water molecule has been investigated at the B3LYP/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The existence of different hydrogen bond (HB) donor and acceptor centers in these molecules led to different kinds of hydrogen bonds (CH-O, OH-S, NH-O, OH-N, and OH-O) and different kinds of complexes. Among them, the most stable structures correspond to complexes where the heteroatom X or Y at positions 3 and 5 behaves as HB acceptor and the hydrogen atom associated with the nitrogen atom at position 4 as HB donor. In accordance with previous studies, it has been shown that the thiocarbonyl group forms stronger HBs than the carbonyl group because the sulfur atom is a better HB acceptor than the oxygen one. With the help of the AIM (atoms in molecules) theory and ELF (electron localization function) analysis, it has been shown that, in the case of 3O5O, 3S5O, and 3S5S, the most basic site is the heteroatom at position 3, while in 3O5S species the most basic center is the sulfur atom.     

Theoretical model for a tetrad of hydrogen bonds and its application to interpretation of infrared spectra of salicylic acid

Theoretical model of vibrational interactions in hydrogen-bonded salicylic acid dimer is presented which takes into account the adiabatic couplings between high- and low-frequency O-H and O...O stretching vibrations, resonance interactions between both intermolecular hydrogen bonds and between inter- and intramolecular hydrogen bonds, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The model is used for theoretical simulation of the nu(s) stretching bands of salicylic acid and its OD derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far infrared, Raman, and low-frequency Raman spectra of the polycrystalline salicylic acid and its deuterated derivative have been measured. The geometry and experimental frequencies are compared with the results of density-functional theory calculations performed at the B3LYP6-31 ++ G**, B3LYP/cc-pVTZ, B3PW916-31 ++ G**, and B3PW91/cc-pVTZ levels. O-H, O-D, and O...O stretching frequencies are used in theoretical simulation of the nu(s) stretching bands.     

Overtone-induced decarboxylation: a potential sink for atmospheric diacids

Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO(2)NO(2)) and inducing intramolecular rearrangement followed by reaction (in H(2)SO(4)). Here, we propose another potentially important process: the decarboxylation of organic acids. To demonstrate this possibility, we have calculated the decarboxylation pathways for malonic acid and its monohydrate. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results. The transition state is a six-membered ring structure which is accessed via concerted O-H and C-C stretches; excitation of v(OH) > or = 3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low-energy isomer of the malonic acid-water complex forms an eight-membered, multiply hydrogen bonded structure, bound by 3-6 kcal/mol, somewhat less stable than the lowest energy, six-membered ring isomer. Decarboxylation of such complexes uses water as a catalyst; the water accepts an acidic proton from one malonic acid group and transfers a proton to the carbonyl of the other acid group. The barrier for this process is 20-22 kcal/mol, suggesting that complexes excited to v(OH) > or = 2 possess sufficient energy to react. Using estimated absorption cross sections for the OH overtone transitions, we suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas-phase reaction with OH for removal of the acid.     

Insight into global reaction mechanism of [C2, H4, O] system from ab initio calculations by the scaled hypersphere search method

A detailed computational study is performed on the singlet potential energy surface (PES) for possible isomerization and dissociation reactions of CH(3)CHO at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) levels. The pathways around the equilibrium structures can be discovered by the scaled hypersphere search (SHS) method, which enables us to make a global analysis of the PES for a given chemical composition. Fourteen isomers inclusive of 11 single-molecules and three "non-stabilized" oxygen-based ylides, 5 energetically favored complexes, and 79 interconversion transition states have been found on the singlet PES. Four lowest lying isomers with thermodynamic stability are also kinetically stable with respect to metastable intermediates. It was revealed that vinyl alcohols, which could be generated by the tautomerization of acetaldehyde, could undergo dissociation to form acetylene and water. In addition, recombination channels between some fragments, such as H(2)CO + (1)CH(2) and (1)CHOH + (1)CH(2), are energetically accessible via collision complex or oxygen-based ylides. Most of available unimolecular decompositions are found to be responsible for favorable hydrogen abstraction processes.     

N2O…HOCl复合物结构和性质的理论研究

分别在DFT-B3LYP和MP2/6-311++G**水平上求得HOCl + N2O复合物势能面上的六种(S1, S2, S3, S4, S5和S6)和四种(S1, S2, S4和S5)构型. 频率分析表明,其中的S1和S3为过渡态,其它为稳定构型. 在复合物S3, S5 和S6中,HOCl 单体的σ*(5O-6H)作为质子供体,与N2O单体中作为质子受体的3O原子相互作用,形成氢键结构,而在氢键复合物S2中, 质子受体为N2O单体中的端1N原子;复合物S1中,HOCl分子的σ*(5O-4Cl)作为质子供体与N2O分子中的端1N原子(质子受体)相互作用,形成卤键结构,而复合物S4中的卤键结构的质子受体为N2O分子中的端3O原子. 经B3LYP/6-311++G**水平上的计算,考虑了基组重叠误差(BSSE)校正的单体间相互作用能在-1.56 ~ -8.73 kJ·mol-1之间. 采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

Understanding microsolvation of Li+: structural and energetical analyses

A stochastic exploration of the quantum conformational space for the (H(2)O)(n)Li(+), n = 3, 4, 5 complexes produced 32 molecular clusters at the B3LYP/6-311++G** and MP2/6-311++G** levels. The first solvation shell is predicted to comprise a maximum of 4 water molecules. Energy decomposition analyses were performed to determine the relationship between the geometrical features of the complexes and the types of interactions responsible for their stabilization. Our findings reveal that electrostatic interactions are major players determining the structures and relative stabilities of the clusters. The formal charge on the Li atom leads to two distinct types of hydrogen bonds, scattered in a wide range of distances (1.61-2.32 ?), in many cases affording H-bonds that are considerably larger and considerably shorter than those in pure water clusters (typically ～1.97 ?).     

Carbocationic rearrangement of pivaloyl cation and protonated pivalaldehyde in superacid medium: A novel solution equivalent of the McLafferty rearrangement

Both pivaloyl cation in the presence of hydride donors and protonated pivalaldehyde in superacid media (both aprotic and protic) rearrange to protonated methyl isopropyl ketone involving gitionic dicationic intermediates. In our earlier studies we have found that the rearrangement of pivaladehyde to methyl isopropyl ketone occurs quantitatively in the presence of various superacidic media such as anhydrous HF, triflic acid, boron trifluoride-2,2,2-trifluoroethanol complex (BF(3).2CF(3)CH(2)OH) etc. Our present study with environmentally more benign and stable amine:HF complexes, namely pyridinium poly(hydrogen fluoride) (PPHF) (5), poly(4-vinylpyridinium) poly(hydrogen fluoride) (6), and poly(ethyleniminium) poly(hydrogen fluoride) (PEIHF) (7) shows that these modified HF equivalents can carry sufficient amount of immobilized HF and provide ample acidity for complete isomerization of pivalaldehyde to methyl isopropyl ketone. Calculations on protioformyl, acetyl and pivaloyl dications at the B3LYP/6-311 ++ G(d,p) and CCSD(T)/6-311 ++ G(d,p)//B3LYP/6-311 ++ G(d,p) levels have been performed to compare the nature of protosolvation of formyl, acetyl, pivaloyl cations and protonated pivaladehyde in superacid media. These studies further suggest protosolvation of protonated pivalaldehyde leading to gitionic dications at high acidities resulting in the carbocatioinic rearrangement. The reported carbocationic rearrangement under superacidic activation represents a novel solution chemistry equivalent of the well known gas-phase McLafferty rearrangement.