Matrix isolation and ab initio study of the HXeCCH...CO2 complex

The HXeCCH...CO2 complex is studied experimentally and computationally. The complex is prepared in a low-temperature xenon matrix using UV photolysis of propiolic acid (HCCCOOH) and thermal mobilization of H atoms at 45 K. Photolysis of propiolic acid leads to the HCCH...CO2 complex as one of the photolysis products. The HCCH...CO2 complex is further photolyzed to the HCC...CO2 complex. Thermal annealing leads to the formation of HXeCCH complexed with CO2. The H-Xe stretching absorption of the HXeCCH...CO2 complex is blueshifted (+31.9 and +5.8 cm(-1)) from the value of the HXeCCH monomer in a xenon matrix. In the calculations, three HXeCCH...CO2 structures were found (one parallel and two linear structures) corresponding to the true energy minima on the potential energy surface. For the H-Xe stretching mode, the calculations give blueshifted values of +19.2 or +19.5 cm(-1) depending on the computational level [MP2/6-311++G(2d,2p) and MP2/aug-cc-pVDZ] for the parallel structure and +19.4 or +27.9 cm(-1) for one linear structure. For the second linear structure, the H-Xe stretching frequency is redshifted by -8.6 or -9.4 cm(-1) at these levels of theory. Based on the calculations, the experimental band shifted by +5.8 cm(-1) (1492.2 cm(-1)) most likely corresponds to the HXeCCH...CO2 parallel structure. The band with larger blueshift of +31.9 cm(-1) (1518.3 cm(-1)) can be due to another matrix site of the same structure or to the blueshifting linear structure.     

Rotational spectrum, inversion, and geometry of 2,5-dihydrofuran...ethyne and a generalization about Z...H-C hydrogen bonds

The ground-state rotational spectra of nine isotopomers of a complex formed between 2,5-dihydrofuran and ethyne were recorded with a pulsed-jet, Fourier-transform microwave spectrometer. Rotational and centrifugal distortion constants were obtained for C4H6O...HCCH, C4H6O...DCCH, C4H6O...HCCD, C4H6O...DCCD, [3,4-D2]-C4H6O...HCCH, C4H6O...H13CCH, C4H6O...HC13CH, , and [3(13C]-C4H6O...HCCH. The substituted species were studied in their natural abundances. For the more abundant isotopomers, weak c-type transitions as well as strong a-type transitions were observed. The primary intermolecular binding was shown to consist of a hydrogen bond formed by the ethyne subunit acting as the proton donor and the O atom of 2,5-dihydrofuran as the proton acceptor. The complex has a plane of symmetry that includes the O atom and the ethyne subunit, with a pyramidal configuration at oxygen. A fit of the principal moments of inertia of all nine isotopomers under the assumption of unperturbed 2,5-dihydrofuran and ethyne geometries yielded the values r(O...H)=2.127(8) A, phi=57.8(18) degrees , and theta=16.2(32) degrees, where phi is the angle made by the HCCH subunit at O and theta is the angular deviation of the O...H-C nuclei from collinearity. This geometry is compared with those obtained by ab initio calculations conducted with a range of basis sets and with electron correlation taken into account at the MP2 (M?ller-Plesset second order) level of theory. A small inversion doubling (approximately equal to 20-30 kHz) of c-type transitions, well resolved only for the parent isotopomer and [3HCCH, was attributed to a vibrational motion that inverts the configuration at oxygen. A one-dimensional model for this motion was used with a double minimum potential energy function of the type V(phi)=alphaphi(4)+betaphi(2) to estimate the observed separation DeltaE(01) of the lowest pair (v=0 and v=1) of associated energy levels. The predicted DeltaE(01) had the same magnitude as that deduced from the inversion doubling of the c-type transitions. The geometry of C4H6O...HCCH is compared with those other B...HCCH, where B is vinyl fluoride, oxirane, and thiirane. A rationalization of the angular geometries of various B...HX, where X=F, Cl, Br, or CCH, is presented.     

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.     

Rotational spectrum of the dimethyl ether-acetylene complex: evidence for an effective C2v geometry

The rotational spectra for five isotopomers of the 1:1 weakly bound complex formed between dimethyl ether (DME) and acetylene (HCCH) have been measured by Fourier transform microwave spectroscopy. The experimental rotational constants, planar moments, and dipole moment components are consistent with a floppy complex possessing an effective C2v structure in which the hydrogen atom of acetylene is hydrogen bonded to the oxygen atom of dimethyl ether with an intermolecular H...O separation of 2.08(3) A. Experimental rotational constants for the normal isotopic species are A = 10382.5(17) MHz, B = 1535.7187(18) MHz, and C = 1328.3990(17) MHz and the dipole moment components are mua= mutotal = 1.91(10) D. Ab initio calculations at the MP2/6-311++G(2d,2p) level indicate that the energy barrier for motion of the HCCH subunit between the lone pairs of the DME, via a C2v intermediate structure, is very low (approximately 0.29 kJ mol(-1)). Inclusion of basis set superposition error and zero point energy corrections to the energies of four stationary points located on the potential energy surface shows that the relative stabilities are particularly sensitive to these corrections. The ab initio optimizations give rotational constants for the C2v structure of A = 10066 MHz, B = 1496 MHz, and C = 1324 MHz, and a dipole moment of mua= mu(total) = 2.12 D, in reasonable agreement with the experimentally determined values. The structural parameters and energetics of the DME-HCCH complex will be discussed and compared to similar complexes such as H2O-HCCH.     

甲醛与甲酰胺相互作用的从头算研究

The optimizations geometries and vibrational frequencies of H2CO,HCONH2 and acquired 3 complexes between H2CO?HCONH2 have been calculated by using the ab initio method at the MP2/6-31G（ d）and MP2 （FC）/6-311++G（d,p）level. The non-minimum structures with negative vibrational frequencies are excluded. The lowest energy conformer of these complexes is a cyclic structure with N - H?O and C - H?O hydrogen bonds in a common plane. No significant changes are observed in the geometries of the monomers in their complexed state. The most characteristic geometrical properties of the complex are the lengthening of the contacting N-H bonds by 0.4-1.1 pm,and the general shortening of the contacting C-H bonds by 0.3-0.4 pm with respect to the monomers. The interaction energies of complexes have been corrected by the basis set superposition error （BSSE）using the full Boys-Bernardi counterpoise correction scheme. The corrected complex interaction energies of 3 structures at MP2/6-311++G（2df,3p）/ / MP2（FC）/6-311++G（d,p）level are -29.94, -16.10 and -18.45 kJ/mol,respectively. The interaction energies indicate that C - H?O is a weak hydrogen bond. The results of natural bond orbital population analysis reveals that there is only a small charge-transfer in the process of forming the complexes. The results of natural bond orbital analysis and atom in the molecule scheme appear quite significant in view of their importance for understanding the mechanisms of intermolecular interaction leading to hydrogen bonding. The results of molecular interaction energy decomposition analysis show that the electrostatic interaction plays an essential role in stabilizing the H2CO?HCONH2 complexes.     

3-羰基吡唑质子转移过程的理论研究

采用密度泛函B3LYP／6—311G^＊＊方法,对3-羰基吡唑几何构型进行了全自由度优化,获得了它们的几何结构和电子结构．计算并考察了3-羰基吡唑的两种构象即syn和anti构象的稳定性以及3-羰基吡唑进行结构互变的质子转移过程的四种可能途径：（a）分子内质子转移;（b）水助质子转移;（C）同种二聚体双质子转移;（d）异种二聚体双质子转移．计算结果表明3-羰基吡唑的syn构象中N2-H型的稳定性大于N1-H型,进行质子转移时途径（C）所需要的活化能最小（52．78kJ／mol）,途径（a）所需要的活化能最大（200．59kJ／mol）;3,羰基吡唑的。anti构象中N1-H型的稳定性大于N2-H型,进行质子转移时途径（d）所需要的活化能最小（61．09kJ／mol）,途径（a）所需要的活化能最大（204．15kJ／mol）．     

AB INITIO SCF MO STUDIES ON THE COMPLEXES BETWEEN CH_3OH AND H_2CO

<正> The interaction between CH3OH and H2CO has been studied by ab ini-tio method at the level of STO-3G and 6-311G basis sets. It has been found that there are two possible complexes; a hydrogen bonded complex CH3OH...CH2O(Ⅰ) and an electron donor-acceptor complex CH3OH.....OCH2(Ⅱ).The stabilization energies of (Ⅰ) and (Ⅱ) are 14. 6 and 3. 6kJ/mol (STO-3G results) or 25. 1 and 17. 1kJ/mol (6-311G results) respectively. The nature of these complexes has been discussed by using the energy decomposition scheme.     

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。     

Ab Initio Calculation of Room Temperature Ionic Liquid 1-Ethyl-3-Methyl-Imidazolium Chlorocuprate （I）

The Hartree-Fock method has been employed to investigate the electronic structures of EMIM^＋（1-ethyl-3-methyl-imidazolium^＋）, CuCl2^-, Cu2Cl3^-, CuCl3^2-, EMIM＋^-CuCl2^-, EMIM^＋-Cu2Cl3^-, and EMIM^＋-CuCl3^2- pairs. Full optimization and frequency analyses of EMIM^＋, CuCl2^-, Cu2Cl3^-, CuC13^-, eight initial EMIM^＋-CuCl2^-, six EMIM^＋-Cu2Cl3^-, and four EMIM^＋-CuCl3^2- geometries have been carried out using Gaussian-94 soft-package at 6-31＋G（d,p） basis set level for hydrogen, carbon, nitrogen, chlorine atoms and Hay-Wadt effective core potential for copper atom. The electronic structures of lowest energy of EMIM^＋-CuCl2^-, EMIM＋-Cu2Cl3^-, EMIM^＋-CuCl3^2-, single EMIM^＋, CuCl2^-, Cu2Cl3^-, and CuCl3^2- have been comparatively studied. The calculated results showed that EMIM^＋-CuCl2^- pair conformer of lowest energy was five ring parallel to Cl-Cu-Cl with 3.2A distance, EMIM^＋-CuCl3^2- pair conformer of lowest energy was five ring parallel to CuCl3^2-plane with 3.4A distance, and the optimized EMIM^＋-Cu2Cl3^- pair conformer of lowest energy was five ring perpendicular to Cl-Cu-Cl-Cu-Cl plane with 3.0A distance between the terminal Cl atoms and the 5-ring of EMIM^＋. The cohesion between cations and anions is brought about by C-H. C1 hydrogen bonds that are reinforced by charge assistance. The frequency analyses suggested that all stationary points are minimum because of no appearing of imaginary frequency. The assigned frequencies were in agreement with the experimental report. The low energy of interaction because of the bulkyasymmetry of EMIM＋ and the charge dispersion of cation and anion leads to the low melting point of the ionic liquids, EMIM^＋-CuCl2^-, EMIM^＋-Cu2Cl3^-, and EMIM^＋-CuCl3^2-. The interaction energy of EMIM^＋-CuCl2^-, EMIM^＋-Cu2Cl3^-, and EMIM^＋-CuCl3^2- is 309.0 kJ/mol, 316.8 kJ/mol, and 320.2 kJ/mol, respectively. The relationship of interaction energy via distance between cations and anions was also investigated by single point energy scan.     

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^*)。     

On the weakly C-H···π hydrogen bonded complexes of sevoflurane and benzene

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1?:?1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.     

Computational evidence for methyl-donated hydrogen bonds and hydrogen-bond networking in 1,2-ethanediol-dimethyl sulfoxide

The 1:1 complex of 1,2-ethanediol with dimethyl sulfoxide was studied using density functional theory. A network of three hydrogen bonds holds the complex together, including two in which each methyl group donates to the same hydroxyl oxygen. Four lines of evidence support the existence of methyl-donated hydrogen bonds. The interaction energy is 36 +/- 5 kJ/mol using Becke's three parameter hybrid theory with the 1991 nonlocal correlation functional of Perdew and Wang, and a moderately large basis set (B3PW91/6-311++G**//B3PW91/6-31+G**). To determine the energy of each hydrogen bond, a relaxed potential energy scan was performed in a smaller basis set to break the weaker hydrogen bonds by forced systematic rotation of the methyl groups. Two cross-checking analyses show cooperative effects that cause individual hydrogen bond energies in the network to be nonadditive. When one methyl hydrogen bond is broken, the remaining interactions stabilize the complex by storing an additional 2-3 kJ/mol. With all hydrogen bonds intact, the O[bond]H...O[bond]S hydrogen bond contributes 26 +/- 2 kJ/mol stability, and each weak methyl bond stores 5 +/- 2 kJ/mol.     

Carbon-to-metal hydrogen atom transfer: direct observation using time-resolved infrared spectroscopy

We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os*, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os* support this assignment. Continuous photolysis (lambda > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os* in hexane at 23 degrees C is kH = (2.1 +/- 0.2) x 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.     

Reactive intermediates relevant to the carbonylation of CH(3)Mn(CO)(5). Activation parameters for key dynamic processes

Reported is a time-resolved infrared and optical kinetics investigation of the transient species CH(3)C(O)Mn(CO)(4) (I(Mn)) generated by flash photolysis of the acetyl manganese pentacarbonyl complex CH(3)C(O)Mn(CO)(5) (A(Mn)) in cyclohexane and in tetrahydrofuran. Activation parameters were determined for CO trapping of I(Mn) to regenerate A(Mn) (rate = k(CO) [CO][I(Mn)]) as well as the methyl migration pathway to form methylmanganese pentacarbonyl CH(3)Mn(CO)(5) (M(Mn)) (rate = k(M)[I(Mn)]). These values were Delta H(++)(CO) = 31 +/- 1 kJ mol(-1), Delta S(++)(CO) = -64 +/- 3 J mol(-1) K(-1), Delta H(++)(M) = 35 +/- 1 kJ mol(-1), and Delta S(++)(M) = -111 +/- 3 J mol(-1) K(-1). Substantially different activation parameters were found for the methyl migration kinetics of I(Mn) in THF solutions where Delta H(++)(M) = 68 +/- 4 kJ mol(-1) and Delta S(++)(M) = 10 +/- 10 J mol(-1) K(-1), consistent with the earlier conclusion (Boese, W. T.; Ford, P. C. J. Am. Chem. Soc. 1995, 117, 8381-8391) that the composition of I(Mn) is different in these two media. The possible isotope effect on k(M) was also evaluated by studying the intermediates generated from flash photolysis of CD(3)C(O)Mn(CO)(5) in cyclohexane, but this was found to be nearly negligible (k(M)(h)/k(M)(d) (298 K) = 0.97 +/- 0.05, Delta H(++)(M)(d) = 37 +/- 4 kJ mol(-1), and Delta S(++)(M)(d) = -104 +/- 12 J mol(-1) K(-1)). The relevance to the migratory insertion mechanism of CH(3)Mn(CO)(5), a model for catalytic carbonylations, is discussed.     

On the directed gas phase synthesis of the imidoborane molecule (HNBH)--an isoelectronic molecule of acetylene (HCCH)

The elementary reaction of ground state boron atoms, (B((2)P(j))), with ammonia (NH(3)(X(1)A(1))) was conducted under single collision conditions at a collision energy of 20.5 ± 0.4 kJ mol(-1) in a crossed molecular beams machine. Combined with electronic structure calculations, our experimental results suggested that the reaction was initiated by a barrier-less addition of the boron atom to the nonbonding electron pair of the nitrogen atom forming a weakly bound BNH(3) collision complex. This intermediate underwent a hydrogen shift to a doublet HBNH(2) radical that decomposed via atomic hydrogen loss to at least the imidoborane (HBNH(X(1)Σ(+)) molecule, an isoelectronic species of acetylene (HCCH(X(1)Σ(g)(+))). Our studies are also discussed in light of the isoelectronic C(2)H(3) potential energy surface accessed via the isoelectronic carbon-methyl system.     

Density functional theory of water-gas shift reaction on molybdenum carbide

The density functional theory (DFT) of the water-gas shift (WGS) reaction over molybdenum carbide was studied with the aim of understanding the dissociation of H(2)O, the OH group, and CO to determine on what sections of molybdenum carbide CO(2) and H(2) formed and whether they played a role in the reaction. The energy diagram of each elementary step, the reaction of the hydrogen and oxygen atoms with CO, and the transition state for this elementary step were also studied. The IR spectra of the CO adsorption was experimentally analyzed for the identification of several candidates of the CO adsorption modes. The adsorptions of the threefold Mo site (a) with and (b) without the underlying C atom of the second layer have the second and highest adsorption energies of -281.59 and -321.00 kJ/mol, respectively. The IR data showed that the bands at 1626 cm(-1) from the IR experiments are (a) the nearest adsorption of the threefold Mo site with the underlying C atom at the calculated/corrected band of 1621 cm(-1). The calculated/corrected threefold adsorption (b) had the highest adsorption energy but exhibited an IR band at 1147 cm(-1) which was not observed in the experimental data. The C-O bond length increased to 1.49 from 1.36 after the H(2)O adsorption (b), suggesting the dissociation of C-O after the H(2)O coadsorption. The WGS reaction on the beta-Mo(2)C(001) slab carbide was calculated and took place as follows: H(2)O was dissociated into OH and H on the Mo(2)C surface and the OH subsequently dissociated into H and O atoms. CO approached the O atom to form CO(2).     

HCN消除反应机理的理论研究

采用密度泛函理论方法从HCN氧化和水解两个方面研究了HCN消除反应机理,并考虑了HCN的直接消除反应(途径Ⅰ和途径Ⅱ)和CuO上的HCN消除反应(途径Ⅲ和途径Ⅳ)。途径Ⅰ为HCN与2个O2分子生成CO2、NO和H原子;途径Ⅱ为HCN与1个O2分子和1个H2O分子生成 CO2和NH3;途径Ⅲ为CuO上HNCO水解为CO2和NH3;途径Ⅳ为CuO上HCN水解为CO和NH3。研究发现,途径III速控步骤的活化自由能垒为157.32 kJ/mol,比途径Ⅱ中HNCO水解降低12.34 kJ/mol;比途径Ⅳ降低了63.8 kJ/mol。可见,HNCO是HCN净化过程中的重要中间体,CuO的加入降低了反应能垒,促进了HCN消除。     

Dissociation of carbonic acid: gas phase energetics and mechanism from ab initio metadynamics simulations

A comprehensive metadynamics study of the energetics, stability, conformational changes, and mechanism of dissociation of gas phase carbonic acid, H2CO3, yields significant new insight into these reactions. The equilibrium geometries, vibrational frequencies, and conformer energies calculated using the density functional theory are in good agreement with the previous theoretical predictions. At 315 K, the cis-cis conformer has a very short life time and transforms easily to the cis-trans conformer through a change in the O=C-O-H dihedral angle. The energy difference between the trans-trans and cis-trans conformers is very small (approximately 1 kcal/mol), but the trans-trans conformer is resistant to dissociation to carbon dioxide and water. The cis-trans conformer has a relatively short path for one of its hydroxyl groups to accept the proton from the other end of the molecule, resulting in a lower activation barrier for dissociation. Comparison of the free and potential energies of dissociation shows that the entropic contribution to the dissociation energy is less than 10%. The potential energy barrier for dissociation of H2CO3 to CO2 and H2O from the metadynamics calculations is 5-6 kcal/mol lower than in previous 0 K studies, possibly due to a combination of a finite temperature and more efficient sampling of the energy landscape in the metadynamics calculations. Gas phase carbonic acid dissociation is triggered by the dehydroxylation of one of the hydroxyl groups, which reorients as it approaches the proton on the other end of the molecule, thus facilitating a favorable H-O-H angle for the formation of a product H2O molecule. The major atomic reorganization of the other part of the molecule is a gradual straightening of the O=C=O bond. The metadynamics results provide a basis for future simulation of the more challenging carbonic acid-water system.     

Existence of both blue-shifting hydrogen bond and Lewis acid-base interaction in the complexes of carbonyls and thiocarbonyls with carbon dioxide

In this study, 16 gas phase complexes of the pairs of XCHZ and CO(2) (X = F, Cl, Br; Z = O, S) have been identified. Interaction energies calculated at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level including both BSSE and ZPE corrections range from -5.6 to -10.5 kJ mol(-1) for XCHOCO(2) and from -5.7 to -9.1 kJ mol(-1) for XCHS···CO(2). Substitution of one H atom by one halogen in formaldehyde and thioformaldehyde reduces the interaction energy of XCHZ···CO(2), while a CH(3) substitution increases the interaction energy of both CH(3)CHO···CO(2) and CH(3)CHS···CO(2). NBO and AIM analyses also point out that the strength of Lewis acid-base interactions decreases going from >C1=S3···C6 to >C1=O3C6 and to >C1-X4···C6. This result suggests the higher capacity of solubility of thiocarbonyl compounds in scCO(2), providing an enormous potential application for designing CO(2)-philic materials based on the >C=S functional group in competition with >C=O. The Lewis acid-base interaction of the types >C=S···C, >C-Cl···C and >C-Br···C is demonstrated for the first time. The contribution of the hydrogen bonding interaction to the total interaction energy is larger for XCHS···CO(2) than for XCHO···CO(2). Upon complexation, a contraction of the C1-H2 bond length and a blue shift of its stretching frequency have been observed, as compared to the isolated monomer, indicating the existence of a blue-shifting hydrogen bond in all complexes examined. Calculated results also lend further support for the viewpoint that when acting as proton donor, a C-H bond having a weaker polarization will induce a stronger distance contraction and frequency blue shift upon complexation, and vice versa.     

Photodissociation of CO2 - in water clusters via Renner-Teller and conical interactions

The photochemistry of mass selected CO(2) (-)(H2O)(m), m=2-40 cluster anions is investigated using 266 nm photofragment spectroscopy and theoretical calculations. Similar to the previous 355 nm experiment [Habteyes et al., Chem. Phys. Lett. 424, 268 (2006)], the fragmentation at 266 nm yields two types of anionic products: O(-)(H2O)(m-k) (core-dissociation products) and CO(2) (-)(H2O)(m-k) (solvent-evaporation products). Despite the same product types, different electronic transitions and dissociation mechanisms are implicated at 355 and 266 nm. The 355 nm dissociation is initiated by excitation to the first excited electronic state of the CO(2) (-) cluster core, the 1 (2)B(1)(2A") state, and proceeds via a glancing Renner-Teller intersection with the ground electronic state at a linear geometry. The 266 nm dissociation involves the second excited electronic state of CO(2) (-), the 2 (2)A(1)(2A') state, which exhibits a conical intersection with the 3 (2)B(2)(A') state at a bent geometry. The asymptotic O(-) based products are believed to be formed via this 3 (2)B(2)(A') state. By analyzing the fragmentation results, the bond dissociation energy of CO(2) (-) to O(-)+CO in hydrated clusters (m> or =20) is estimated as 2.49 eV, compared to 3.46 eV for bare CO(2) (-). The enthalpy of evaporation of one water molecule from asymptotically large CO(2) (-)(H(2)O)(m) clusters is determined to be 0.466+/-0.001 eV (45.0+/-0.1 kJ/mol). This result compares very favorably with the heat of evaporation of bulk water, 0.456 eV (43.98 kJ/mol).