Microwave investigation of sulfuric acid monohydrate

The complex H2SO4-H2O has been observed by rotational spectroscopy in a supersonic jet. A-type spectra for 18 isotopic forms have been analyzed, and the vibrationally averaged structure of the system has been determined. The complex forms a distorted, six-membered ring with the water unit acting as both a hydrogen bond donor and a hydrogen bond acceptor toward the sulfuric acid. One of the H2SO4 protons forms a short, direct hydrogen bond to the water oxygen, with an H...O distance of 1.645(5) A and an O-H...O angle of 165.2(4) degrees. Additionally, the orientation of the water suggests a weaker, secondary hydrogen bond between one of the H2O hydrogens and a nearby S=O oxygen on the sulfuric acid, with an O...H distance of 2.05(1) A and an O-H...O angle of 130.3(5) degrees. The experimentally determined structure is in excellent agreement with previously published DFT studies. Experiments with HOD in the jet reveal the formation of only isotopomers involving deuterium in the secondary hydrogen bond, providing direct experimental evidence for the secondary H...O interaction. Extensive isotopic substitution has also permitted a re-determination of the structure of the H2SO4 unit within the complex. The hydrogen-bonding OH bond of the sulfuric acid elongates by 0.07(2) A relative to that in free H2SO4, and the S=O bond involved in the secondary interaction stretches by 0.04(1) A. These changes reflect substantial distortion of the H2SO4 moiety in response to only a single water molecule, and prior to the proton transfer event. Spectral data indicate that the complex undergoes at least one, and probably more than one type of internal motion. Although the sulfuric acid in this work was produced from direct reaction of SO3 and water in the jet, experiments with H2(18)O indicate that about 2-3% of the acid is formed via processes not normally associated with the gas-phase hydration of SO3.     

Ab initio and density functional theory reinvestigation of gas-phase sulfuric acid monohydrate and ammonium hydrogen sulfate

We have calculated the thermochemical parameters for the reactions H(2)SO(4) + H(2)O <--> H(2)SO(4).H(2)O and H(2)SO(4) + NH(3) <--> H(2)SO(4).NH(3) using the B3LYP and PW91 functionals, MP2 perturbation theory and four different basis sets. Different methods and basis sets yield very different results with respect to, for example, the reaction free energies. A large part, but not all, of these differences are caused by basis set superposition error (BSSE), which is on the order of 1-3 kcal mol(-1) for most method/basis set combinations used in previous studies. Complete basis set extrapolation (CBS) calculations using the cc-pV(X+d)Z and aug-cc-pV(X+d)Z basis sets (with X = D, T, Q) at the B3LYP level indicate that if BSSE errors of less than 0.2 kcal mol(-1) are desired in uncorrected calculations, basis sets of at least aug-cc-pV(T+d)Z quality should be used. The use of additional augmented basis functions is also shown to be important, as the BSSE error is significant for the nonaugmented basis sets even at the quadruple-zeta level. The effect of anharmonic corrections to the zero-point energies and thermal contributions to the free energy are shown to be around 0.4 kcal mol(-1) for the H(2)SO(4).H(2)O cluster at 298 K. Single-point CCSD(T) calculations for the H(2)SO(4).H(2)O cluster also indicate that B3LYP and MP2 calculations reproduce the CCSD(T) energies well, whereas the PW91 results are significantly overbinding. However, basis-set limit extrapolations at the CCSD(T) level indicate that the B3LYP binding energies are too low by ca. 1-2 kcal/mol. This probably explains the difference of about 2 kcal mol(-1) for the free energy of the H(2)SO(4) + H(2)O <--> H(2)SO(4).H(2)O reaction between the counterpoise-corrected B3LYP calculations with large basis sets and the diffusion-based experimental values of S. M. Ball, D. R. Hanson, F. L Eisele and P. H. McMurry (J. Phys. Chem. A. 2000, 104, 1715). Topological analysis of the electronic charge density based on the quantum theory of atoms in molecules (QTAIM) shows that different method/basis set combinations lead to qualitatively different bonding patterns for the H(2)SO(4).NH(3) cluster. Using QTAIM analysis, we have also defined a proton transfer degree parameter which may be useful in further studies.     

Ab initio investigation of phloroglucinol

All-electron ab initio Hartree–Fock (RHF ) calculations have been carried out to investigate the keto/enol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5-benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-1-on, in the phenol system. © 1993 John Wiley & Sons, Inc.     

Ab initio structural investigation of methyl and ethyl carbamate, and carbamyl choline

The molecular structures of four conformations of methylcarbamate, three forms of ethylcarbamate, four forms of ethylacetate, and of the trans-form of carbamylcholine, were determined by ab initio gradient geometry refinement on the 4-21G level, and the results are compared with the geometries of homologous systems. Significant changes in bond distances and angles are observed with torsional changes, but, barring long-range non-bonded interactions, they are to a large extent localized in that part of the system which is directly involved with the torsional transition; i.e., through-bond effects in a bond distance chain begin to be insignificant after a sequence of three bonds.     

Ab initio derivative calculation of vibrational circular dichroism

The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.     

Ab initio spectrograms of the molecular vibrational spectrum

Theoretical spectrograms of the vibrational spectrum of 3,3-dimethylcyclopropene were constructed and juxtaposed with the experimental Raman and IR spectrograms. The theoretical spectrograms are represented as sets of vertical lines starting from the points corresponding to the values of the vibrational frequencies calculated from the scaled quantum-mechanical (QM) force field obtained at the HF/6-31G*//HF/6-31G* level. Two theoretical Raman spectrograms were constructed. In the first case, the heights of the vertical lines correspond to the QM values of the Raman scattering activities. In the second case they represent the relative differential Raman cross-sections calculated using the QM values of Raman scattering activities. The initial vibrational mode matrix remains virtually unchanged upon scaling of the QM force constant matrix because the dispersion of the scale factor values is low. Therefore, the heights of the theoretical lines for the IR spectrogram represent the QM intensities directly. The theoretical spectrogram based on the relative differential Raman cross-sections was shown to depict the experimental Raman spectrum more adequately. This makes it possible to use the results of the corresponding QM calculations more completely and obtain well-substantiated assignments of the vibrational frequencies.     

Ab initio investigation of the vibrational hyper-Raman spectra of ethylene, ethane, and dimethyl ether

The time-dependent Hartree–Fock approach is employed with the aug-cc-pVDZ or 6-311++G** basis set to simulate, in the harmonic approximation, the hyper-Raman spectra of ethylene, ethane, and dimethyl ether. Comparison with the experiment of Verdieck etal. [(1970) Chem. Phys. Lett. 7:219] is performed for ethylene and ethane. Effects of the polarization of the incident light and the detected light are studied for ethylene. Special focus is given on these vibrational normal modes that cannot be detected in IR and Raman spectroscopies.Contribution to the Jacopo Tomasi Honorary Issue     

Ab initio molecular dynamics simulations of the liquid/vapor interface of sulfuric acid solutions

Ab initio molecular dynamics simulations of the liquid-vapor interface are presented for thin slabs of 72 water molecules containing a single molecule of sulfuric acid. Trajectories in the 306-330 K range are calculated for two functionals with double- and triple-ζ quality basis sets. Comparisons are made between BLYP and HCTH/120 results for the slab simulations and for bulk simulations of one H(2)SO(4) in a periodic box with 63 waters. Good agreement is found with the available experimental data and the results of other relevant AIMD studies with respect to ionization of the acid, size of the coordination shells, partitioning of the ions with the hydronium exhibiting a surface preference and the anions in the interior, and the orientational distributions for the hydronium ions and for the surface/subsurface water molecules. The major differences in the performance of the two functionals are attributable to the greater basicity of the anion oxygen atoms with the HCTH functional and the more structured aqueous solution with BLYP. The enhanced basicity results in larger aqueous coordination shells for the anion oxygens. The structuring of the BLYP aqueous solution is observed in the corrugation of the water density profile, the higher first peak in g(OO)(r), and a smaller water self-diffusion constant. This structuring with the BLYP functional yields anion hydrogen bonds that endure longer and where the dissociated ions more rapidly and directly segregate in the slab. The simulations indicate that aqueous surfaces containing ionizable diprotic acids can be modeled with rather modest sized systems and be informative.     

Ab initio study of the vibrational spectra of the hydrogen-bonded nitric acid dimer.

The changes in the vibrational characteristics characterizing the dimerization of nitric acid have been investigated by ab initio calculations at the MP2 level, with 6-31G(d,p) and 6-31 + G(d,p) basis sets, and B3LYP/6-31G(d,p) calculations. The most consistent agreement between the computed values of the frequency shifts for the planar fully symmetric structure (2A) and those experimentally observed suggests that this structure is preferred. It was established that the most sensitive to the complexation is the stretching O-H vibration. The values of the frequency shift (-306 cm(-1)) is indicative for the formation of the relatively strong hydrogen bonds. The calculations predict an increase of the infrared intensity of the stretching O-H vibration in the nitric acid dimer more than 26 times.     

Ab initio structures and vibrational analysis of the isoprene conformers

Complete gradient optimizations of the structures and the calculation of the harmonic force fields of the s-trans(anti) and gauche conformers of isoprene (2-methylbuta-1,3-diene) are reported at the RHF/6-31G level. The dihedral angle of the gauche conformer is found to be 41.0° from the planar s-cis(syn) form. The force fields obtained are refined using scale factors transferred from analogous calculations for trans-butadiene-1,3 and ethane. The direct vibrational problems are solved for both conformers of isoprene. A complete assignment of the experimental vibrational frequencies is given.     

Ab initio vibrational transition dipole moments and intensities of formaldehyde

Vibrational transition dipole moments and absorption band intensities for the ground state of formaldehyde, including the deuterated isotopic forms, are calculated. The analysis is based on ab initio SCF and CI potential energy and dipole moment surfaces. The formalism derives from second-order perturbation theory and involves the expansion of the dipole moment in terms of normal coordinates, as well as the incorporation of point group symmetry in the selection of the dipole moment components for the allowed transitions. Dipole moment expansion coefficients for the three molecule-fixed Cartesian coordinates of formaldehyde are calculated for internal and normal coordinate representations. Transition dipole moments and absorption band intensities of the fundamental, first overtone, combination, and second overtone transitions are reported. The calculated intensities and dipole moment derivatives are compared to experiment and discussed in the context of molecular orbital and bond polarization theory.     

Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers

Molecular orbital calculations at HF and MP2 levels have been performed using the 6-31G⁷ basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy.     

Ab initio prediction of vibrational spectra: A database approach

Efforts to develop a database of quadratic force fields for organic molecules are described. The database is based on systematic ab initio calculations, scaled to reproduce the experimentally observed frequencies. The choice of the theoretical method, the basis sets, geometries, internal coordinates and the scaling procedure are discussed. A key point in the procedure is the automatic generation of the internal valence coordinates. This is also very advantageous for geometry optimization. The database should permit the prediction of vibrational frequencies for most organic molecules to 10–20 cm⁻¹, together with semiquantitative intensities. The accuracy is sufficient to identify unknown compounds from a list of reasonable candidates.     

Ab initio study of hydrazoic acid

SCF and CI calculations were carried out on the ground^1A state of HN₃. The equilibrium geometry and vibration frequencies were computed. The results point to a planar structure (groupC _s) but to a non-linear (170 °) N-N-N conformation. The calculated vibration frequencies are in fair agreement with experimental assignments.The dissociation path of the molecule to NH and N₂ products was investigated and compared to the isoelectronic reaction of diazomethane. The dissociation energy of hydrazoic acid is estimated to be about –8 kcal/mole, with a potential barrier to dissociation of about 30 kcal/mole.Boursier IRSIA     

Ab initio vibrational analysis of hexafluoroethane C2F6

Quantum mechanical geometry optimizations and the calculation of vibrational frequencies of hexafluoroethane have been performed at the HF/6-31G*, MP2/6-31G*, CCSD/cc-pVDZ, and B3LYP/6-31G* levels. The force fields obtained were scaled. The necessity is stressed of carrying out the detailed analysis of the vibrational spectra of small reference molecules to determine sets of scale factors which are transferable to quantum mechanical force fields of large molecules for the purpose of predicting their vibrational spectra.     

DNA oligonucleotide-cis-platin binding: Ab initio interpretation of the vibrational spectra

The cis-platin binding to the d(CCTGGTCC)*d(GGACCAGG) model DNA octamer was monitored with infrared absorption (IR) and vibrational circular dichroism (VCD) spectroscopies. The spectra were modeled with the aid of density functional computations and a Cartesian coordinate-based transfer of molecular property tensors from smaller DNA fragments. Because of the fragmentation, the tensors could be calculated with a higher precision. Environmental effects, such as the presence of the solvent or the cis-platin ligand, could be included in the modeling. The solvent was modeled by an explicit inclusion of hydrogen-bound water molecules, positions of which were estimated from a molecular dynamics simulation, or by the polarized continuum COSMO model. The B3LYP and BPW91 functionals used for the calculations of the spectral parameters were combined with the relativistic LANL2DZ platinum pseudo-potentials. The simulations reproduced the main IR and VCD DNA spectral features and explained most of the changes observed experimentally upon metal binding. The results confirmed that the influence of the ligand on DNA vibrational properties is quite complex; it originates in the geometry deformation and normal mode coupling pattern changes of the platinated octamer, as well as in local perturbations of the electronic structure and force field of the GC base pairs to which the platinum is bound. Many of the local effects could be accounted for by a point charge used in place of the metal in the GC complex.     

Ab initio structure and vibrational frequencies of lithium aromatic sulfonyl imide salts

The structure and vibrational frequencies of an aromatic lithium sulfonyl imide, i.e., lithium bis(4-nitrophenylsulfonyl)imide (LiNPSI) has been studied using self-consistent ab initio Hartree–Fock and hybrid density functional methods. These calculations engender two linkage isomers, which correspond to the local minima on the potential-energy surface. In the lowest-energy isomer, the ligand binds to the metal ion through two oxygens, one from each of the different SO₂ groups on the central nitrogen and forms a six-membered ring. Another LiNPSI isomer, wherein the anion coordinates through oxygen and nitrogen atoms and which is 55.9 kJmol⁻¹ higher in energy, has also been obtained. The S–N–S bond angle in the free anion as well as in the LiNPSI complex turns out to be nearly 121°. A comparison of the vibrational spectra of the free NPSI anion and that of the LiNPSI complex reveals that the SO₂ stretching vibrations at 1,239 and 1,205 cm⁻¹ can be used to differentiate between the two linkage isomers of the complex. The stronger complexation ability of the NPSI anion, compared to that for (CF₃SO₂)₂N⁻ has been explained in terms of the charge density within the molecular electrostatic potential isosurface encompassing both SO₂ groups of the anion. Received: 20 February 2002 / Accepted: 25 March 2002 / Published online: 3 June 2002