Vibronic spectra of jet-cooled 2-aminopurine·H2O clusters studied by UV resonant two-photon ionization spectroscopy and quantum chemical calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine·H(2)O monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift δν of the S(1) ← S(0) transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm(-1)), while that of isomer B is much larger (δν = -889 cm(-1)). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H(2)O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy D(e) = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest (1)ππ* states of A and B in excellent agreement with the observed 0(0)(0) bands; also, the relative intensities of the A and B origin bands agree well with the calculated S(0) state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited (1)ππ* state to the lower-lying (1)nπ* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the (1)ππ* state of B is planar and decoupled from the (1)nπ* state. These observations agree with the calculations, which predict the (1)nπ* above the (1)ππ* state for isomer B but below the (1)ππ* for both 9H-2AP and isomer A.     

Argon predissociation infrared spectroscopy of the hydroxide–water complex (OH−·H2O)

We report the first vibrational spectrum of the degenerate proton transfer system OH⁻·H₂O. The complex is cooled by attachment of argon atoms and the spectrum is observed by argon predissociation spectroscopy in the OH stretching region. A strong, sharp transition is observed just below the region usually associated with the free OH stretch, while broader bands appear lower in energy and are weaker than the dominant free OH peak. The latter are assigned with the aid of ab initio calculations to the first overtone of the coupled intramolecular bend and strongly red-shifted H-bonded OH stretching modes.     

Photoelectron spectroscopy of (CO2)n(H2O) m − clusters

Photoelectron spectra of (CO₂)_nH₂O^? (2≤n≤8) and (CO₂)_n(H₂O) ₂ ^? (1≤n≤2) were measured at the photon energy of 3.49 eV. The spectra show unresolved broad features, which are approximated by Gaussians. The vertical detachment energies (VDEs) were determined as a function of the cluster size. For (CO₂)_nH₂O^?, the VDE-n plots exhibit a sharp discontinuity between n=3 and 4; the VDE value is ≈3.5 eV at n=3, while it drops down abruptly to 2.59 eV at n=4. This discontinuity in VDE is ascribed to "core switching" at n=4; a C₂O ₄ ^? dimer anion forms the core of (CO₂)_nH₂O^? for n≤3, while a monomer CO ₂ ^? is the core for n≥4. The (CO₂)₂(H₂O) ₂ ^? ion has a VDE of 2.33 eV, indicating the presence of a CO ₂ ^? monomer core in the binary clusters containing two H₂O molecules.     

Molecular geometries of H2S···ICF3 and H2O···ICF3 characterised by broadband rotational spectroscopy

The rotational spectra of three isotopologues of H(2)S···ICF(3) and four isotopologues of H(2)O···ICF(3) are measured from 7-18 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, B(0), centrifugal distortion constants, D(J) and D(JK), and nuclear quadrupole coupling constant of (127)I, χ(aa)(I), are precisely determined for H(2)S···ICF(3) and H(2)O···ICF(3) by fitting observed transitions to the Hamiltonians appropriate to symmetric tops. The measured rotational constants allow determination of the molecular geometries. The C(2) axis of H(2)O/H(2)S intersects the C(3) axis of the CF(3)I sub-unit at the oxygen atom. The lengths of halogen bonds identified between iodine and sulphur, r(S···I), and iodine and oxygen, r(O···I), are determined to be 3.5589(2) ? and 3.0517(18) ? respectively. The angle, φ, between the local C(2) axis of the H(2)S/H(2)O sub-unit and the C(3) axis of CF(3)I is found to be 93.7(2)° in H(2)S···ICF(3) and 34.4(20)° in H(2)O···ICF(3). The observed symmetric top spectra imply nearly free internal rotation of the C(2) axis of the hydrogen sulphide/water unit about the C(3) axis of CF(3)I in each of these complexes. Additional transitions of H(2)(16)O···ICF(3), D(2)(16)O···ICF(3) and H(2)(18)O···ICF(3) can be assigned only using asymmetric top Hamiltonians, suggesting that the effective rigid-rotor fits employed do not completely represent the internal dynamics of H(2)O···ICF(3).     

Fourier transform infrared studies of 5′-GMP in 2H2O and H2O solutions.

In a previous work (ref. 1) we observed important changes in the 1700–1400 cm⁻¹ region of FTIR spectra in ²H₂O solutions when 5′-GMP concentration increases. These changes can be attributed to the self-association of this mononucleotide. Recently, study of this process has been extended to other regions of the spectrum and to H₂O solution. Fourier deconvolution has been employed in order to resolve the broad band into component bands. Differences have been observed between spectra in H₂O and ²H₂O for the same solute concentration. The possible causes of these differences are indicated.     

Synthesis and crystal structure of the hydrated magnesium diphosphate Mg2P2O7·3.5H2O and its high temperature variant Mg2P2O7·H2O

The synthesis and crystal structure of a novel hydrated magnesium diphosphate and its high temperature variant are described. Both structures were solved from powder X-ray diffraction data. The room temperature variant with composition Mg₂P₂O₇·3.5H₂O crystallises in the monoclinic space group P2₁/c (No. 14) with a=10.9317(1), b=8.05578(9), c=9.2774(1) Å, β=90.201(1)°, V=816.99(2) ų and Z=4. The structure consists of sheets stacked along [100] which are linked through MgO₂(H₂O)₄ pillars into a three-dimensional framework with cavities containing water molecules. Within the sheets there are infinite edge-sharing chains of Mg octahedra along [010] which are cross linked by P₂O₇⁴⁻ groups. A high temperature variant exists around 200°C. The crystal structure of this compound with composition Mg₂P₂O₇·H₂O was solved and refined in the monoclinic space group C2/c (No. 15) with a=18.6596(4), b=7.9769(1), c=8.9757(2) Å, β=107.378(1)°, V=1275.01(4) ų, Z=8. The transformation to Mg₂P₂O₇·H₂O involves removal of the water molecules in the cavities and the water molecules of the Mg octahedral pillars in Mg₂P₂O₇·3.5H₂O. The sheets in Mg₂P₂O₇·3.5H₂O however remain unchanged during the transformation as the water molecule coordinating Mg here is retained. These sheets are linked through tetrahedral MgO₄ pillars into a three-dimensional structure containing infinite 10-membered ring channels along [001]. Both compounds have been further characterised by ³¹P MAS NMR spectroscopy, thermogravimetric analysis and high temperature powder X-ray diffraction.     

Phase transitions in (C7H12N2)2[SnCl6]Cl2·1.5H2O crystal,studied by NMR and infrared spectroscopy

The (C₇H₁₂N₂)₂[SnCl₆]Cl₂·1.5H₂O complex is a new member of the family of hybrid organic–inorganic perovskite compounds. It exhibits two order–disorder phase transitions with changes in the conformation of aromatic cations at the two transition temperatures 360 and 412 K. Differential scanning calorimetry, nuclear magnetic resonance (NMR), and Fourier-transform infrared (FT-IR) spectroscopy were used to investigate these phase transitions. These transition mechanisms were investigated in terms of the spin–lattice relaxation times T1 for ¹H static NMR and the chemical shifts for ¹³C CP–MAS. The temperature dependence of T1(¹H) and ¹³C chemical shifts are changed near T_C1 and T_C2. Furthermore, the splitting for ¹³C NMR signals in Phases (II) and (III) indicated a ferroelastic characteristic of the compound. In addition, FT-IR results indicate that the ordered conformational structure of aromatic cations undergoes a remarkable disorder with increasing temperature. The NMR and FT-IR studies suggest that the phase transition mechanisms are related to the reorientational motion of [C₇H₁₂N₂]²⁺ cations as a whole. Phase transition was examined in light of the interesting optical properties of this material.     

Thermochemistry of two lead borates; Pb(BO2)2·H2O and PbB4O7·4H2O

Two pure hydrated lead borates, Pb(BO₂)₂·H₂O and PbB₄O₇·4H₂O, have been characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Pb(BO₂)₂·H₂O and PbB₄O₇·4H₂O in 1 mol dm^?3 HNO₃(aq) were measured to be (?35.00 ± 0.18) kJ mol^?1 and (35.37 ± 0.14) kJ mol^?1, respectively. The molar enthalpy of solution of H₃BO₃(s) in 1 mol dm^?3 HNO₃(aq) was measured to be (21.19 ± 0.18) kJ mol^?1. The molar enthalpy of solution of PbO(s) in (HNO₃ + H₃BO₃)(aq) was measured to be ?(61.84 ± 0.10) kJ mol^?1. From these data and with incorporation of the enthalpies of formation of PbO(s), H₃BO₃(s) and H₂O(l), the standard molar enthalpies of formation of ?(1820.5 ± 1.8) kJ mol^?1 for Pb(BO₂)₂·H₂O and ?(4038.1 ± 3.4) kJ mol^?1 for PbB₄O₇·4H₂O were obtained on the basis of the appropriate thermochemical cycles.     

Thermal Dehydration and Infrared Studies of Mg 2 P 2 O 7 · 6H 2 O Diphosphate

The thermal dehydration of Mg 2 P 2 O 7 ·; 6H 2 O were studied, in the range 25-800°;C, by thermogravimetric analysis (TG-DSC), x-ray diffraction, and infrared spectroscopy. According to the TG-DSC curves, the dehydration of this salt takes place in two stages. The results of thermal analysis, x-ray patterns, and infrared spectra of this compound heated at different temperatures showed that, after dehydration, Mg 2 P 2 O 7 ·; 6H 2 O decomposes into dihydrate Mg 2 P 2 O 7 ·; 2H 2 O diphosphate then to an amorphous Mg 2 P 2 O 7 product which crystallises at 665°;C to give anhydrous diphosphate f Mg 2 P 2 O 7 . The j H enthalpy of the dehydration of Mg 2 P 2 O 7 ·; 6H 2 O and of the formation of f Mg 2 P 2 O 7 have been calculated from thermogravimetric data. The infrared spectroscopic study of Mg 2 P 2 O 7 ·; 6H 2 O and of its heated products, reveals the existence of the characteristic bands of the P 2 O 7 group ( x as POP and x s POP) and showed that the POP angle is bent in these salts. In these compounds, the POP angle values are estimated using the Lazarev's relationship.     

Polarized infrared reflection spectra of AlCl3·6H2O single crystals

Polarized i.r. reflection spectra of AlCl₃·6H₂O single crystals are presented in the range 40–4000 cm⁻¹. The oscillator parameters and the transverse and longitudinal optical phonon frequencies are calculated by means of the classical oscillator fit method. The TO/LO splittings and frequency shifts of the water bands in A_2u and E_u species are discussed in terms of hydrogen bonds, the metal—water interaction and the orientation of the H₂O molecules in the lattice.     

Spectral signatures of four-coordinated sites in water clusters: infrared spectroscopy of phenol-(H2O)n (∼20 ≤ n ≤ ∼50)

We report infrared spectra of phenol-(H(2)O)(n) (～20 ≤ n ≤ ～50) in the OH stretching vibrational region. Phenol-(H(2)O)(n) forms essentially the same hydrogen bond (H-bond) network as that of the neat water cluster, (H(2)O)(n+1). The phenyl group enables us to apply the scheme of infrared-ultraviolet double resonance spectroscopy combined with mass spectrometry, achieving the moderate size selectivity (0 ≤ Δn ≤ ～6). The observed spectra show clear decrease of the free OH stretch band intensity relative to that of the H-bonded OH band with increasing cluster size n. This indicates increase of the relative weight of four-coordinated water sites, which have no free OH. Corresponding to the suppression of the free OH band, the absorption peak of the H-bonded OH stretch band rises at ～3350 cm(-1). This spectral change is interpreted in terms of a signature of four-coordinated water sites in the clusters.     

Synthesis,crystal structure,and catalytic performance of two 3-D supramolecular compounds: (C7N2H7)3(C7N2H6) · PMo12O40 · 2H2O and (C7N2H7)3(C7N2H6)2 · AsMo12O40 · 3H2O

Two organic–inorganic compounds based on Keggin building blocks have been synthesized by hydrothermal methods, (C₇N₂H₇)₃(C₇N₂H₆)?·?PMo₁₂O_40?·?2H₂O (1) and (C₇N₂H₇)₃(C₇N₂H₆)_2?·?AsMo₁₂O_40?·?3H₂O (2) (C₇N₂H₆?=?benzimidazole). Single-crystal X-ray analysis revealed that 1 crystallized in the triclinic system, P-1 space group with a?=?9.8980(4)?Å, b?=?11.2893(4)?Å, c?=?25.8933(9)?Å, α?=?93.307(2)°, β?=?90.630(2)°, γ?=?108.330(2)°, V?=?2740.68(18)?ų, Z?=?2, R ₁(F)?=?0.0740, ωR ₂(F ²)?=?0.1511, and S?=?1.037; 2 crystallized in the triclinic system, P-1 space group with a?=?12.3353(4)?Å, b?=?13.2649(4)?Å, c?=?20.2878(6)?Å, α?=?95.6630(10)°, β?=?100.1720(10)°, γ?=?99.3940(10)°, V?=?3195.72(17)?ų, Z?=?2, R ₁(F)?= 0.0329, ωR₂ (F ²)?=?0.1236, and S?=?1.088. The two compounds show a layer framework constructed from Keggin-polyoxoanion clusters and benzimidazole via hydrogen bonds and π–π stacking interactions, resulting in a 3-D supramolecular network. Both have high catalytic activity for oxidation of methanol. When the initial concentration of the methanol is 5.37?g?m^?3 in air and the flow velocity is 4.51?mL?min^?1, methanol is completely eliminated at 150°C for 1 (160°C for 2).     

Synthesis and crystal structure of KBi(C6H4O7) · 3.5H2O

A novel compound, KBi(C₆H₄O₇) · 3.5H₂O (I), has been synthesized in the Bi(NO₃)₂-K₃(HCit) system (HCit^3? is an anion of citric acid C₆H₈O₇) at a component ratio (n) of 8 in a water-glycerol mixture, and its crystal structure has been determined. The crystals are unstable in air. The crystals are triclinic: a = 7.462 Å, b = 10.064 Å, c = 17.582 Å, α = 100.27°, β = 99.31°, γ = 105.48°, V = 1221.2 ų, Z = 2, space group $P\bar 1$ . In the structure of I, asymmetric binuclear fragments [Bi₂(Cit^4?)₂(H₂O)₂]^2? are linked through inversion centers into polymeric chain anions. Ions K⁺ and crystal water molecules are arranged in channels between the chains. The Bi(1)...Bi(2) distances in the binuclear fragment are 4.62 Å, and the Bi(1)...Bi(1) and Bi(2)...Bi(2) distances between bismuth atoms in the chain are 5.83 and 5.95 Å, respectively. The chains are linked through bridging oxygen atoms of the ligands Cit to form layers. In the centrosymmetric four-membered chelate ring Bi₂O₂ formed through Bi-O(Cit) bonds, the distances Bi(1)-Bi(1) are equal to 4.55 Å, and Bi(1)-O are 2.66 and 2.84 Å. The Bi-O bond lengths in I are in the range 2.12–3.16 Å. The Cit ligands act as hexadentate chelating/bridging ligands.     

Preparation,crystal structure and infrared spectroscopy of the new compound rubidium beryllium sulfate dihydrate,Rb2Be(SO4)2·2H2O

The solubility in the three-component system Rb₂SO₄–BeSO₄–H₂O at 25 °C was studied by the method of isothermal decrease of supersaturation. A new compound, Rb₂Be(SO₄)₂·2H₂O, is formed in a wide concentration range (from solutions containing 27.49 mass% beryllium sulfate and 20.16 mass% rubidium sulfate up to solutions containing 15.08 mass% beryllium sulfate and 39.07 mass% rubidium sulfate).Rb₂Be(SO₄)₂·2H₂O crystallizes in the monoclinic space group P2₁/c (a = 11.371(2) Å, b = 11.858(2) Å, c = 7.431(1) Å, β = 96.33(1), V = 996.0 Å³, Z = 4, R1 = 0.039 for 2672F_o > 4σ(F_o) and 153 variables). The crystal structure is characterized by three-membered chain fragments, composed of a central BeO₂(H₂O)₂ polyhedron sharing corners with two SO₄ tetrahedra. These bent [Be(SO₄)₂(H₂O)₂]²⁻ units are linked by rubidium ions and hydrogen bonds to double layers and further to a three-dimensional framework structure. Rb₂Be(SO₄)₂·2H₂O is isotypic to the respective potassium sulfate and selenate compounds.The strengths of the hydrogen bonds in the title compound as deduced from the infrared wavenumbers of the uncoupled OD stretches of matrix-isolated HDO molecules (isotopically dilute sample) are discussed in terms of the O_w⋯O hydrogen bond distances, the different hydrogen bond acceptor capabilities of the sulfate oxygen atoms and the strong BeOH₂ interactions (synergetic effect). The intramolecular OH bond lengths are derived from the ν_OD versus r_OH correlation curve [H.D. Lutz, C. Jung, J. Mol. Struct. 404 (1997) 63].     

Synthesis and thermochemical characteristics of Na2O · Al2O3 · 2.5H2O

A pure phase of monosodium aluminate hydrate Na₂O · Al₂O₃ · 2.5H₂O (MAH) is synthesized and characterized by means of XRD, IR, SEM, TGA, and DSC. The heat capacity of the compound is measured in the temperature range of ?100 to 100°C, and the thermal contributions to enthalpy and entropy are calculated. The standard entropy, enthalpy, and Gibbs energy of formation of MAH at 298 K are estimated.     

Structure of (C3H5NH3)2H2P2O7·H2O

The bis(cyclopropylammonium)dihydrogenodiphosphate monohydrate is a new diphosphate associated with the organic molecule C₃H₅NH₂. We report the chemical preparation and the crystal structure of this organic cation diphosphate. (C₃H₅NH₃)₂H₂P₂O₇.H₂O is orthorhombic (S.G. : P2₁2₁2₁), with Z = 4 and the following unit-cell parameters : a = 4.828(1) Å, b = 11.011(1) Å, c = 25.645(2) Å. The P₂O₇ groups and H₂O water molecules form a succession of bidimensional layers perpendicular to the c axis. The organic cations ensure the three-dimensional cohesion by NH-O hydrogen bonds.     

Infrared and infrared emission spectroscopy of nesquehonite Mg(OH)(HCO3)·2H2O-implications for the formula of nesquehonite

The mineral nesquehonite Mg(OH)(HCO(3))·2H(2)O has been analysed by a combination of infrared (IR) and infrared emission spectroscopy (IES). Both techniques show OH vibrations, both stretching and deformation modes. IES proves the OH units are stable up to 450°C. The strong IR band at 934 cm(-1) is evidence for MgOH deformation modes supporting the concept of HCO(3)(-) units in the molecular structure. Infrared bands at 1027, 1052 and 1098 cm(-1) are attributed to the symmetric stretching modes of HCO(3)(-) and CO(3)(2-) units. Infrared bands at 1419, 1439, 1511, and 1528 cm(-1) are assigned to the antisymmetric stretching modes of CO(3)(2-) and HCO(3)(-) units. IES supported by thermoanalytical results defines the thermal stability of nesquehonite. IES defines the changes in the molecular structure of nesquehonite with temperature. The results of IR and IES supports the concept that the formula of nesquehonite is better defined as Mg(OH)(HCO(3))·2H(2)O.