Application of 20% silver nanoclusters in polymethacrylic acid on simulated dentin caries; its penetration depth and effect on surface hardness.

The aims of this study were: To evaluate the surface hardness of simulated dentin caries lesions treated with either silver nanoclusters (AgNCls) synthesized in polymethacrylic acid (PMAA) or 38% silver diammine fluoride (SDF), as well as observe the penetration of the treatment solutions into the simulated caries lesions. Dentin blocks 4 mm thick obtained from caries-free third molars were sectioned and then simulated caries lesions on the occlusal dentin surfaces were created. Each specimen (n = 8) was divided into four sections: (A) treated with 20% AgNCls/PMAA; (B) treated with SDF 38% (FAgamin, Tedequim, Cordoba, Argentina); (C) sound tooth protected by nail-varnish during artificial caries generation (positive control); and (D) artificial caries lesion without surface treatment (negative control). AgNCls/PMAA or SDF were applied on the simulated lesions with a microbrush for 10 s, then excess removed. The surface hardness was measured by means of Vickers indentation test. To trace the depth of penetration, up to 400 µm, of silver ions, elemental composition of the samples was observed using EDX, coupled with SEM, and measured every 50 µm from the surface towards the pulp chamber. Laser Induced Breakdown Spectroscopy (LIBS) was also employed to trace silver ion penetration; the atomic silver line 328.06 nm was used with a 60 µm laser spot size to a depth of 240 µm. Student's-t test identified significant differences between treatment groups for each depth and the Bonferroni test was used for statistical analysis of all groups (p < 0.05). Mean surface hardness values obtained were 111.2 MPa, 72.3 MPa, 103.3 MPa and 50.5 MPa for groups A, B, C and D respectively. There was a significant difference between groups A and C compared with groups B and D, the group treated with AgNCls/PMAA achieved the highest surface hardness, similar or higher than the sound dentin control. A constant presence of silver was observed throughout the depth of the sample for group A, while group B showed a peak concentration of silver at the surface with a significant drop beyond 50 µm. The 20% AgNCls/PMAA solution applied to simulated dentin caries lesions achieved the recovery of surface hardness equivalent to sound dentin with the penetration of silver ions throughout the depth of the lesion.

Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid.

The present study reports the first structural characterization of protonated cyameluric acid ([CA + H]+) in the gas phase, which paves the way for prospective bottom-up research on the condensed-phase chemistry of CA in the protonated form. A number of [CA + H]+ keto-enol isomers can a priori be produced as a result of protonation at available N and O positions of precursor neutral CA tautomers, yet ab initio computations predict different reduced [CA + H]+ isomer populations dominating the solution and gas phases that are involved in the ion generation process (i.e., electrospray ionization). Infrared multiple photon dissociation spectra were recorded in the 990-1900 and 3300-3650 cm-1 regions and compared with theoretical [B3LYP/6-311++G(d,p)] IR absorption spectra of several [CA + H]+ isomers, providing a satisfactory agreement for the most stable monohydroxy form in the gas phase, [1358a]+, yet the contribution of its nearly isoenergetic OH rotamer, [1358b]+, cannot be neglected. This is indicative of the occurrence of [CA + H]+ isomer interconversion reactions, assisted by protic solvent molecules, during their transfer into the gas phase. The results suggest that available O positions on neutral CA are energetically favored protonation sites in the gas phase.

On the Interaction between Deprotonated Cytosine [C(-H) ]- and Ba2+ : Infrared Multiphoton Spectroscopy and Dynamics.

Gas-phase interactions between Ba2+ and deprotonated cytosine (C(-H) ) were studied in [C(-H) Ba]+ and [C(-H) BaC]+ complexes by IRMPD spectroscopy coupled to tandem mass-spectrometry in combination with DFT calculations. For the [C(-H) BaC]+ complex only one [C(-H) KAN1O-Ba-Canti ]+ isomer was found, although the presence of another structure cannot be excluded. This isomer features a central tetracoordinated Ba2+ that simultaneously interacts with keto-amino [C(-H) ]- deprotonated on N1 and neutral keto-amino C. Both moieties are in different planes as a consequence of an additional NH…O=C hydrogen bond between C and [C(-H) ]- . A sequential IRMPD dynamics is observed in this complex. For the [C(-H) Ba]+ complex produced by electrospray ionization two isomers ([C(-H) KAN1OBa]+ and [C(-H) KAN3OBa]+ ) were identified, in which Ba2+ interacts simultaneously with the C=O group and the N1 or N3 atom of the keto-amino [C(-H) ]- , respectively. A comparison with the related [C(-H) Pb]+ complex (J. Y. Salpin et al., Chem. Phys. Chem. 2014, 15, 2959-2971) is also presented.

Solvation of barium atoms and singly charged cations in acetonitrile clusters.

The size distributions of neutral and cationic Ba x (CH3CN) n (x = 0, +1; n ≤ 7) clusters, as produced by a standard laser vaporization-supersonic expansion pick-up source, were determined from molecular beam experiments. The size distribution for cations is in the range of n = 1-7, whereas only the n = 1 complex is observed for neutral clusters, and these two features are unaffected by the variables controlling the performance of the cluster source. The distinct behavior is compatible with the expected charge-dipole interactions in the ionic species, which are stronger than the dipole induced-dipole interactions at play in neutral clusters, and it is corroborated by the relative magnitude of the theoretical successive binding energies (SBEs) for the lowest-lying isomers of cationic and neutral clusters with n = 1-5, as computed at the density functional theory level. The theoretical results also allow for the rationalization of the bimodal Ba+(CH3CN)1-7 size distribution, featuring an apparent minimum at n = 3, in terms of chiefly 6s-5d σ hybridization of the Ba+ ions, which ultimately leads to a relatively small third SBE for the Ba+(CH3CN)3 complex, as compared to those for n = 1, 2, and 4. Additional Born-Oppenheimer molecular dynamics simulations on the Ba+(CH3CN)2-4 clusters suggest that all of the ligands are coordinated to the Ba+ ion and prevent considering completion of the first solvent shell as responsible for the bimodal size distribution.

Fingerprints of Both Watson-Crick and Hoogsteen Isomers of the Isolated (Cytosine-Guanine)H+ Pair.

 Gas phase protonated guanine-cytosine (CGH+) pair was generated using an electrospray ionization source from solutions at two different pH (5.8 and 3.2). Consistent evidence from MS/MS fragmentation patterns and differential ion mobility spectra (DIMS) point toward the presence of two isomers of the CGH+ pair, whose relative populations depend strongly on the pH of the solution. Gas phase infrared multiphoton dissociation (IRMPD) spectroscopy in the 900-1900 cm-1 spectral range further confirms that the Watson-Crick isomer is preferentially produced (91%) at pH = 5.8, while the Hoogsteen isomer predominates (66%) at pH = 3.2). These fingerprint signatures are expected to be useful for the development of new analytical methodologies and to trigger isomer selective photochemical studies of protonated DNA base pairs.

Hydration of barium monohydroxide in (H2O)(1-3) clusters: theory and experiment.

The ionization energies (IEe's) of small BaOH(H2O)m clusters (m = 1-3), as generated in a laser vaporization-supersonic expansion source have been determined by laser photoionization experiments over the 3.65-4.55 eV energy range. Complementary ab initio studies show that the IEe's are in good agreement with computed adiabatic ionization energies and that BaOH(H2O)m structures with a direct coordination of the Ba atom to water molecules are favored over those that are characterized by H-bonded networks involving H2O molecules and the OH group of BaOH. Additional calculations have been performed on the hydration energies for the most stable isomers of the relevant BaOH(H2O)1-3 clusters. A comparison is made between the closed-shell title system and the results of related theoretical studies on the open-shell alkali monohydroxides, which allows for an interpretation of the opposite trends that are found in the cluster size dependence of the vertical ionization energies for both series of systems, and highlights the role of the BaOH unpaired electron in its ionization process. Altogether, the present evidence suggests for the initial steps of the BaOH hydration process to be dominated by electrostatic and polarization interactions between the Ba(+) and OH(-) ion cores, which become both increasingly solvated upon sequential addition of water molecules.

IR spectroscopy of b4 fragment ions of protonated pentapeptides in the X-H (X = C, N, O) region.

The structure of peptide fragments was studied using "action" IR spectroscopy. We report on room temperature IR spectra of b4 fragments of protonated GGGGG, AAAAA, and YGGFL in the X-H (X = C, N, O) stretching region. Experiments were performed with a tandem mass spectrometer combined with a table top tunable laser, and the multiple photon absorption process was assisted using an auxiliary high-power CO2 laser. These experiments provided well-resolved spectra with relatively narrow peaks in the X-H (X = C, N, O) stretching region for the b4 fragments of protonated GGGGG, AAAAA, and YGGFL. The 3200-3700 cm(-1) range of the first two of these spectra are rather similar, and the corresponding peaks can be assigned on the basis of the classical b ion structure that has a linear backbone terminated by the oxazolone ring at the C-terminus and ionizing proton residing on the oxazolone ring nitrogen. The spectrum of the b4 of YGGFL, on the other hand, is different from the two others and is characterized by a band observed near 3238 cm(-1). Similar band positions have recently been reported for one of the four isomers of the b4 of YGGFL studied using double resonance IR/UV technique. As proposed in this study, the IR spectrum of this ion at room temperature can also be assigned to a linear N-terminal amine protonated oxazolone structure. However, an alternative assignment could be proposed because our room temperature IR spectrum of the b4 of YGGFL nicely matches with the predicted IR absorption spectrum of a macrocyclic structure. Because not all experimental IR features are unambiguously assigned on the basis of the available literature structures, further theoretical studies will be required to fully exploit the benefits offered by IR spectroscopy in the X-H (X = C, N, O) stretching region.

New determination of the adiabatic ionization potential of the BaOH radical from laser photoionization-molecular beam experiments and ab initio calculations.

The adiabatic ionization potential of the BaOH radical, as generated in a laser vaporization-supersonic expansion source has been determined by laser photoionization experiments to be (4.55 ± 0.03) eV. This value supports the three lowest out of seven previous experimental estimates, the former ranging from 4.35 to 4.62 eV. The present result is compared to ab initio calculations, as performed using both quantum chemistry at different levels of theory and density functional theory, and trying several effective core potentials and their accompanying basis sets for Ba. The most satisfactory agreement is obtained for either the adiabatic or vertical ionization potentials that derive from post-Hartree-Fock [MP2 and CCSD(T)] treatments of electron correlation, along with consideration of relativistic effects and extensive basis sets for Ba, in both BaOH and BaOH(+). Such conclusions extend to the results of related calculations on the Ba-OH dissociation energies of BaOH and BaOH(+), which were performed to help in calibrating the present computational study. Bonding in BaOH/BaOH(+), as well as possible sources of discrepancy with previous experimental determinations of the BaOH adiabatic ionization potential are discussed.

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters.

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.

Unexpected size distribution of Ba(H2O)n clusters: why is the intensity of the Ba(H2O)1 cluster anomalously low?

An experimental and theoretical study on the reactivity of neutral Ba atoms with water clusters has been conducted to unravel the origin of the irregular intensity pattern observed in one-photon ionization mass spectra of a Ba(H(2)O)(n)/BaOH(H(2)O)(n-1) (n = 1-4) cluster distribution, which was generated in a laser vaporization-supersonic expansion source. The most remarkable irregular feature is the finding for n = 1 of a lower intensity for the Ba(+)(H(2)O)(n) peak with respect to that of BaOH(+)(H(2)O)(n-1), which is opposite to the trend for n = 2-4. Rationalization of the data required consideration of a distinct behavior of ground-state and electronically excited state Ba atoms in inelastic and reactive Ba + (H(2)O)(n) encounters that can occur in the cluster source. Within this picture, the generation of Ba(H(2)O)(n) (n > 1) association products results from stabilizing collisions with atoms of the carrier gas, which are favored by intramolecular vibrational redistribution that operates on the corresponding collision intermediates prior to stabilization; the latter is unlikely to occur for Ba + (H(2)O) encounters. Overall, this interpretation is consistent with additional in-source laser excitation and quenching experiments, which aimed to explore qualitatively the effect of perturbing the Ba atom electronic state population distribution on the observed intensity pattern, as well as with the energetics of various possible reactions for the Ba + H(2)O system that derive from high level ab initio calculations.

Chemiluminescence from the Ba((3)P)+N(2)O-->BaO(A (1)Sigma(+))+N(2) reaction: Collision energy effects on the product rotational alignment and energy release.

Both fully dispersed unpolarized and polarized chemiluminescence spectra from the Ba((3)P)+N(2)O reaction have been recorded under hyperthermal laser-ablated atomic beam-Maxwellian gas conditions at three specific average collision energies E(c) in the range of 4.82-7.47 eV. A comprehensive analysis of the whole data series suggests that the A (1)Sigma(+)-->X (1)Sigma(+) band system dominates the chemiluminescence. The polarization results revealed that the BaO(A (1)Sigma(+)) product rotational alignment is insensitive to its vibrational state upsilon(') at E(c)=4.82 eV but develops into an strong negative correlation between product rotational alignment and upsilon(') at 7.47 eV. The results are interpreted in terms of a direct mechanism involving a short-range, partial electron transfer from Ba((3)P) to N(2)O which is constrained by the duration of the collision, so that the reaction has a larger probability to occur when the collision time is larger than the time needed for N(2)O bending. The latter in turn determines that, at any given E(c), collinear reactive intermediates are preferentially involved when the highest velocity components of the corresponding collision energy distributions are sampled. Moreover, the data at 4.82 eV suggest that a potential barrier to reaction which favors charge transfer to bent N(2)O at chiefly coplanar geometries is operative for most of the reactive trajectories that sample the lowest velocity components. Such a barrier would arise from the relevant ionic-covalent curve crossings occurring in the repulsive region of the covalent potential Ba((3)P)cdots, three dots, centeredN(2)O((1)Sigma(+)); from this crossing the BaO(A (1)Sigma(+)) product may be reached through mixings in the exit channel with potential energy surfaces leading most likely to the spin-allowed b (3)Pi and a (3)Sigma(+) products. The variation with increasing E(c) of both the magnitude of the average BaO(A (1)Sigma(+)) rotational alignment and the BaO(A (1)Sigma(+)) rovibrational excitation, as obtained from spectral simulations of the unpolarized chemiluminescence spectra, consistently points to additional dynamic factors, most likely the development of induced repulsive energy release as the major responsible for the angular momentum and energy disposal at the two higher E(c) studied. The results of a simplified version of the direct interaction with product repulsion-distributed as in photodissociation model do not agree with the observed average product rotational alignments, showing that a more realistic potential energy surface model will be necessary to explain the present results.

Chemiluminescent reaction of Ba(3P) with N2O at hyperthermal collision energies: rotational alignment of the BaO(A 1Sigma+) product.

The chemiluminescent reaction Ba(6s6p (3)P)+N(2)O was studied at an average collision energy of 1.56 eV in a beam-gas arrangement. Ba((3)P) was produced by laser ablation of barium, which resulted in a broad collision energy distribution extending up to approximately 5.7 eV. A series of experiments was made to extract the Ba((3)P) contribution to chemiluminescence from that corresponding to Ba 6s(2) (1)S0 and 6s5d (3)D, which are the other two most populated states in the atomic beam. The fully dispersed polarized chemiluminescence spectra at 400-600 nm from the title reaction were recorded and assigned to a BaO molecule excited in the A (1)Sigma+ level. In addition, the average and wavelength-resolved degrees of polarization associated to the parallel BaO(A (1)Sigma+-->X (1)Sigma+) emission are reported. The analysis of the average polarization degree show that the BaO(A (1)Sigma+) product is significantly aligned, suggesting that the reaction mechanism is predominantly direct. The product rotational alignment was found to depend markedly on the emission wavelength, which revealed a negative correlation with the BaO(A (1)Sigma+) product vibrational state. On the basis of experimental and theoretical investigations on the reactions of N(2)O with both the (1)S0, (3)D, and (1)P1 states of Ba and the lighter group 2 atoms, it is suggested that the Ba((3)P) reaction involves a charge transfer at relatively short reagent separations and that restricted collision geometries at the highest velocity components of the broad distribution are necessary to rationalize the data.