Chemical Physics Letters (v.489, #4-6)

Contents (iii-ix).

Homogeneous nucleation of the three gases, NO2, H2O, and NH3, directly leads to the production of ammonium nitrate (NH4NO3) aerosols and nitrous acid (HONO) gas.The homogeneous nucleation of three gases, NO2, H2O, and NH3, to form NH4NO3 aerosols and HONO gas is proposed to be a new significant source of atmospheric HONO. Four reaction pathways are examined by density functional theory calculations of various complexes of reactants and products involved. NH3 is shown to play a critical role in lowering the Gibbs free energy barrier and stabilizing the HNO3 product by forming particulate ammonium nitrate. In conclusion, the system of gaseous NO2, H2O, and NH3 is expected to undergo a spontaneous homogeneous nucleation process, which is likely an important source of nighttime atmospheric HONO concentrations.

Potential hydrogen storage of lithium amidoboranes and derivatives by Saartje Swinnen; Vinh Son Nguyen; Minh Tho Nguyen (148-153).
In the rate-determining step of hydrogen release from LiNH2BH3 and NaNH2BH3, one H(B) is transferred to Li, and afterwards this H undergoes contact with on H(N) resulting in a low energy barrier for H2 formation.Lithium amidoboranes show potential as hydrogen storage materials. The mechanism for production of LiNH2BH3 from LiH and NH3BH3 is studied using quantum chemical methods (MP2. CCSD(T)). The reaction is exothermic with a low energy barrier. Pathways of H2 production from LiNH2BH3, NaNH2NH3 and (LiNH2BH3)2 show that a H-transfer from B to Li or Na constitutes the rate-determining step. Substitution of one borane hydrogen by a more electron-donating group (OH and NH2) tends to lower the energy barriers for H2 release. Calculated results point out that LiNH2BH2OH and LiNH2BH2NH2 can be considered as potentially promising materials for hydrogen storage.

There exist two regimes of hydrogen exchange processes before decomposition of protonated ethanol to form H3O+; the fast hydrogen scrambling in the C2H5 group and the slow hydrogen exchange between C2H5 and OH2 groups.Formation of H3O+ from ethanol clusters induced by intense laser fields (100 fs, ∼1 × 1014  W/cm2, 800 nm) was investigated by time-of-flight mass spectrometry. A positive correlation of H3O+ and (C2H5OH)H+ yields, along with the H3− n D n O+ (n  = 0–3) ratio from deuterium-labeled ethanol, led to the conclusion that H3O+ was efficiently eliminated from (C2H5OH)H+. Ab initio calculation for (C2H5OH)H+ isomers suggests the existence of two timescale regimes of hydrogen exchange processes before decomposition. The rate constant for decomposition was experimentally estimated to be 0.17 of the slower one, corresponding to hydrogen exchange between C2H5 and OH2 sites.

An ab initio investigation of the properties of H2:HX hydrogen-bonded complexes by Ibon Alkorta; José Elguero; Janet E. Del Bene (159-163).
H–H bond stretching frequencies, 1H chemical shieldings, and indirect spin–spin coupling constants of nine H2:HX complexes have been investigated. The toroidal ring of negative charge which surrounds the H2 σ bond acts as the basic site in H2:HX complexes.Eight complexes H2:HX formed with the σ-bond of the H2 molecule as the proton acceptor and proton donors HCCH, HCCLi, HCCF, HCN, HNC, H2O, HF, and HCl have been optimized at MP2/aug-cc-pVTZ. Analyses of the electron densities indicate that these are weakly-bound hydrogen-bonded complexes, in contrast to H2:HH which is a van der Waals complex. H–H bond stretching frequencies of the H2 molecule, 1H chemical shieldings, and indirect spin–spin coupling constants have been computed in order to identify the most promising spectroscopic tool for characterizing these complexes. The H2 stretching vibration is the property which is most sensitive to complex formation.

The structures of S 0 and S 1 states of catechol and catechol–Al(III) complex have been calculated in aqueous solution.The electronic structures of the ground and the lowest excited states of catechol and its 1:1 complex with aluminium have been optimized by taking into account the solvent effects. The good agreement between the theoretical absorption and fluorescence emission wavelengths with the experimental ones has allowed the validation of the computed electronic structures. If the structure of the S 0 state of catechol is planar, it is twisted in the S 1 state in water, contrary to the results obtained in vacuum. In opposition, the changes observed for the 1:1 complex structures in the S 0 and S 1 states are negligible.

Vibrational dynamics of metal cyanides by Daniel Weidinger; Gerald M. Sando; Jeffrey C. Owrutsky (169-174).
Time-resolved IR spectroscopy of C–N stretching bands of aqueous molecular cyanides show that relaxation times correlate with spectral properties.Time-resolved IR spectroscopy has been used to characterize vibrational and rotational relaxation dynamics of the C–N stretching bands for aqueous molecular cyanides, Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - , Co ( CN ) 6 3 - , Mn ( CN ) 6 3 - , and Ru ( CN ) 6 4 - . The spectra and dynamics of Ru ( CN ) 6 4 - resemble those previously reported for ferrocyanide with a relatively short (<10 ps) vibrational energy relaxation (VER) time. VER times are longer (>30 ps) for Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - and Co ( CN ) 6 3 - . Mn ( CN ) 6 3 - is an intermediate case with a VER time of 15 ps in water. These VER dynamics extend and reinforce the trends for metal cyanide CN vibrational band frequencies and intensities.

Increasing the efficiency of the ring-opening reaction of photochromic indolylfulgides by optical pre-excitation by Thomas Brust; Simone Draxler; Jonas Eicher; Watson J. Lees; Karola Rück-Braun; Wolfgang Zinth; Markus Braun (175-180).
Strong increase of the ring-opening quantum efficiency of indolylfulgides induced by an immediately preceding ring-closure reaction investigated by a multipulse femtosecond absorption experiment.For three indolylfulgides the quantum efficiency of the ring-opening reaction upon pre-excitation is investigated in a multipulse experiment. The quantum efficiency grows by factor of up to 3.4, when the pre-excitation pulse immediately precedes the excitation process. The change in quantum efficiency after pre-excitation is discussed as a function of reaction time, steady-state quantum efficiency and energetic barriers in the excited electronic state. The observed differences can be explained by the molecular properties of the investigated indolylfulgides.

Zinc and cadmium dioxide anions are produced via electron capture of the corresponding neutral molecules during condensation in excess neon.Zinc and cadmium dioxide anions are produced via electron capture of the corresponding neutral molecules during condensation of laser-ablated zinc and cadmium atoms with O2 in excess neon at 4 K. Photosensitive infrared absorptions at 624.1 and 483.6 cm−1 are assigned to the antisymmetric stretching vibrations (ν 3) of the linear ZnO 2 - and CdO 2 - anions on the basis of isotopic shifts and splittings as well as density functional calculations. The predicted quite low ν 3/ν 1 ratios for both anions with 2Πg ground state are similar with the valence isoelectronic CO 2 + and BO2 molecules.

Synthesis, characterization and microwave absorption of carbon-coated Sn nanorods by Z.H. Wang; Z. Han; D.Y. Geng; Z.D. Zhang (187-190).
Absorption-tubes for materials with μr  = 1. For illustrations, layers with thicknesses of 1.8, 3.5 and 5.5 mm are given; solid triangles are the measured relative complex permittivity of the Sn/C–paraffin composite.Carbon-coated Sn nanorods have been synthesized by an arc-discharge method. The Sn/C–paraffin composite shows excellent electromagnetic (EM) absorption properties. Reflection losses exceeding −20 dB can be realized in any interval within the 2–18 GHz range by choosing an appropriate thickness of the absorbent layer between 1.5 and 9 mm. The EM-wave absorption properties of nanocomposite materials are illustrated in detail by means of an absorption-tube-map. The carbon-coated Sn nanorods are attractive candidates for EM-wave absorption, which significantly enriches the family of EM-wave nano-absorbents.

Nd3+-doped LaF3 nanoparticles with a larger emission cross-section by Xiaoxia Cui; Jiangbo She; Kai Cui; Chao Gao; Chaoqi Hou; Wei Wei; Bo Peng (191-194).
Emission properties of LaF3:Nd nanoparticles with different Nd3+ concentrations were investigated. The fluorescent lifetime was decreased with increasing the Nd3+ concentration. A relatively high quantum yield of 88% was obtained for the 0.5 mol% samples. The larger emission cross-section and strong fluorescence intensity demonstrate that these nanoparticles are promising materials for laser applications.A series of Nd3+-doped LaF3 nanoparticles with Nd3+ concentrations from 0.5 to 10 mol% were synthesized. The fluorescence intensity and lifetime of the nanoparticles at various Nd3+ doping concentration were investigated. The nanoparticles displayed strongest fluorescence intensity at 3 mol% Nd3+ concentration. Eighty-eight percentage quantum efficiency was obtained when the Nd3+ concentration was 0.5 mol%. Optical properties of nanoparticles were studied according to Judd–Ofelt theory. A larger emission cross-section, σ em, for 4F3/2  →  4I11/2 transition of the Nd3+ ion was obtained as 3.21 × 10−20  cm2, which was two times of the currently reported value. The larger emission cross-section and strong fluorescence intensity demonstrate that these nanoparticles are promising materials for laser applications.

The existence of second and fifth line in the Mössbauer spectrum recorded at 5 K with an external applied field of 5 T indicates the presence of a spin non-collinearity in the ε-Fe x N (2 <  x  < 3) nanoparticles.ε-Fe x N (x  = 2.5) nanocrystallites synthesized by controlled nitridation and quenching were found to have a disordered arrangement of nitrogen atoms. Mössbauer spectrum at 5 K shows an increased intensity for the second and fifth line indicating a canted spin structure. The in-field spectra at 5 K show a clear evidence for the existence of non-collinear spin structure by the non-vanishing intensity of second and fifth lines. The spin canting can be attributed to the disordered arrangement of non-magnetic nitrogen atoms which creates vacant interstitial sites and thus different kinds of Fe–Fe and Fe–N interactions to various extents.

Unusual luminescence behavior of Dy3+-doped lead borate glass after heat treatment by J. Pisarska; R. Lisiecki; W. Ryba-Romanowski; T. Goryczka; W.A. Pisarski (198-201).
The Letter reports the observation of unusual luminescence behavior related to radiative transitions of Dy3+ ions and lead tungstate PbWO4 crystallites embedded in lead borate glass, which strongly depends on excitation wavelengths.Dysprosium-doped lead borate glasses before and after heat treatment have been studied. Lead tungstate PbWO4 crystallites were identified in lead borate glasses after heat treatment. Luminescence was examined under different excitation wavelengths. Blue luminescence of PbWO4 crystallites embedded in lead borate glass is observed upon 310 nm excitation. Luminescence spectra measured under excitation at 390 nm consists of two characteristic blue and yellow bands, which correspond to 4F9/26H15/2 and 4F9/26H13/2 transitions of Dy3+. Blue band is enhanced and broadened in comparison to yellow 4F9/26H13/2 transition, when Dy3+ and PbWO4 crystallites in lead borate glass after heat treatment are simultaneously excited by 360 nm line.

Electron injection efficiency from N719 dyes to nanocrystalline TiO2 films is suppressed by the addition of tBP (4-tert-butylpyridine) and found that the effect of tBP was more pronounced for films with lower dye loading. This suggests that sensitizer dyes adsorbed densely onto the surface suppress the adsorption of tBP.The effect of dye coverage on the photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films was studied by transient absorption spectroscopy. We evaluated the electron injection efficiency of films with several dye concentrations on the surface and found no significant difference between them. We also studied the suppression of the efficiency by the addition of tBP (4-tert-butylpyridine) and found that the effect of tBP was more pronounced for films with lower dye loading. This suggests that sensitizer dyes adsorbed densely onto the surface suppress the adsorption of tBP.

Optical limiting study of double wall carbon nanotube–Fullerene hybrids by Kang-Shyang Liao; Jun Wang; Daniel Früchtl; Nigel J. Alley; Enrico Andreoli; Eoghan P. Dillon; Andrew R. Barron; Hansoo Kim; Hugh J. Byrne; Werner J. Blau; Seamus A. Curran (207-211).
The double wall carbon nanotube–Fullerene hybrids exhibit superior optical limiting performance to those of Fullerenes and CNTs.In order to merge complementary temporal and spatial nonlinear optical characteristics of Fullerene and carbon nanotubes, synthesis of double wall carbon nanotube–Fullerene hybrid was performed by covalently linking DWNT and C60 by amination reaction with polyethylenimine. DWNT–Fullerene hybrids were characterized by thermogravimetric analysis, UV–vis spectroscopy and transmission electron microscopy. Optical limiting performance of DWNT–Fullerene hybrids is superior to those of Fullerenes and SWNTs at the same level (∼80%) of transmission. Whereas nonlinear scattering is an evident mechanism, reverse saturable absorption from Fullerene moieties has significant contribution. Charge transfer between the DWNT and Fullerene moieties may play an important role of optical limiting.

Signature of multiradical character in second hyperpolarizabilities of rectangular graphene nanoflakes by Hiroshi Nagai; Masayoshi Nakano; Kyohei Yoneda; Ryohei Kishi; Hideaki Takahashi; Akihiro Shimizu; Takashi Kubo; Kenji Kamada; Koji Ohta; Edith Botek; Benoît Champagne (212-218).
Signature of multiradical character in open-shell singlet rectangular graphene nanoflakes is observed in the size dependence of second hyperpolarizability (γ).Using spin-unrestricted density functional theory methods, the second hyperpolarizabilities, γ, of rectangular graphene nanoflakes of different sizes have been investigated for their singlet ground state in conjunction with their open-shell character. It is shown that their multiradical nature leads to a unique dependence of the γ component along the armchair edges and that this behavior can be rationalized not only in terms of the diradical character (y 0) derived from the HOMO and LUMO occupation numbers, but also in terms of the one (y 1) derived from the HOMO−1 and LUMO+1 ones: the γ components are enhanced when the systems take intermediate y 1 values in addition to intermediate y 0 values.

Poly(3-hexylthiophene) nanotubes with superior electronic and optical properties by Dipanwita Majumdar; Shyamal Kumar Saha (219-224).
Synthesis of P3HT nanotubes containing regio-regular polymer chains which show remarkably wide absorption spectra as well as superior charge transport.Major disadvantages of organic solar cell materials are limited absorption and weak charge transport. Here, we have synthesized poly(3-hexylthiophene) nanotubes with a tailored combination of regio-random and regio-regular chains by oxidative polymerization technique, to achieve wide range of absorption (400–720 nm) in visible region. Due to regio-regularity, charge transport follows power law behavior instead of Mott’s variable range hopping and the mobility is found to be 2.296 × 10−2  cm2  V−1  s−1, which is comparable to the value reported in the literature. Wide range of absorption and superior charge transport make these nanotubes as important materials in organic photovoltaic applications.

Periodic dinucleotides AA, TT and GC are constructed by one internal nucleotide and one surface nucleotide.As the fundamental repeating units in eukaryotic chromatin, nucleosomes play an important role in many biological processes. For this reason, the study of the structure of nucleosomes may help to reveal some of the crucial principals of these processes. In our research, we have used alpha shapes to model nucleosome structure and discovered that the periodic DNA dinucleotides AA, TT and GC occupy special positions in nucleosome structure with one nucleotide inside and the other outside the nucleosome surface. This structural feature and other dinucleotide characteristics can provide useful information for the study of nucleosome positioning.

Electrostatics of graphene: Charge distribution and capacitance by Zhao Wang; Robert W. Scharstein (229-236).
Representative atomic diagram of net electric charges density in a graphene sheet with induced electric fields.The distribution of net electric charge in graphene is investigated, using both a constitutive atomic charge–dipole interaction model and an approximate analytical solution to Laplace’s equation. These approximations, unlike first-principle calculations, are applicable to graphene sizes that are consistent with experiments. It is found that the analytical model gives a good description of the charge distribution, except for the very edge atoms because of the atomic edge state, which can instead be treated by the numerical model.

Integral reaction cross-section for H  + HD and D  + HD was calculate on a new potential energy surface.A quasi-classical trajectory (QCT) method has been used to calculate integral reaction cross-section for H  + HD and D  + HD. The influence of rotation of the reagent on the integral reaction cross-section and the product branching ratios of the title reactions are discussed. The results indicate that the reactive cross-section of H(D)  + HD → HH(D) + D decreases with an increase of the j for Etran  ⩽ 1.5 eV. The results also show that the reactive cross-section of D(H)  + HD → DD(H) + H decreases with an increase of the j for Etran  ⩽ 1.0 eV and that the integral cross-sections of title reactions are sensitive to the reagent rotation.

Converged quantum dynamics with modified Shepard interpolation and Gaussian wave packets by Terry J. Frankcombe; Michael A. Collins; Graham A. Worth (242-247).
Combining grow and vMCG yields efficient converged quantum dynamics from ab initio calculations.A new hybrid method is presented in which modified Shepard interpolation of a potential energy surface is combined with time-dependent quantum dynamics calculations. The propagation of a wave packet composed of fixed-width Gaussian functions allows a one-to-one mapping between the quantum dynamics results and a small number of quantum trajectories, allowing electronic structure theory calculations to be performed preferentially in dynamically-important regions. The method is designed for demonstrable convergence of the quantum dynamics results from ab initio calculations. The photodissociation of NOCl is used as a test case.

The 19F CSA tensor can be estimated from the CSA-DD cross-correlated spin relaxation rate which shows up as a differential relaxation of the transitions in the fluorine spin multiplet, after a spin-selective pulse that inverts the fluorine magnetization.The 19 F chemical shift anisotropy (CSA) tensor is an indispensable structure estimation tool in the NMR investigations of flourinated biomolecules. This work focuses on the characterization of the 19 F CSA tensor in small molecules, through the combined use of quantum chemical methods and liquid-state NMR cross-correlated spin relaxation experiments. The effect of different basis sets and quantum computational methods on the magnitude and orientation of the 19 F CSA tensor are discussed. The results from ab initio methods and the liquid-state relaxation experiments match well and are comparable to values of the CSA tensor obtained from previous solid-state studies and from theoretical investigations of similar molecules.

Erroneous behaviour of the widely used MP2(full)/aug-cc-pVXZ (X = D,T) level of theory for evaluating the BSSE in ion–π complexes by Carolina Estarellas; Xavier Lucas; Antonio Frontera; David Quiñonero; Pere M. Deyà (254-258).
The calculations of several ion–π complexes have been optimized using the MP2 method and several double-ζ and triple-ζ basis sets. The BSSE (basis set superposition error) counterpoise correction on the ion–π complexes was computed for Pople’s and Dunning’s basis sets using both frozen core and full core MP2 methods. An anomalous behaviour of the widely used MP2(full)/aug-cc-pVXZ (X = D,T) level of theory was observed, specially for sodium complexes.Several complexes of benzene with alkaline cations and s-triazine with halide anions have been optimized using the MP2 method and several double-ζ and triple-ζ basis sets. The BSSE (basis set superposition error) counterpoise correction on the ion–π complexes was computed for Pople’s and Dunning’s basis sets using both frozen core and full core MP2 methods. An anomalous behaviour of the widely used MP2(full)/aug-cc-pVXZ (X = D,T) level of theory was observed, specially for sodium complexes. It is solved using the MP2(full)/aug-cc-pCVXZ (X = D,T) level of theory.

The study of excited state absorptions induced solute migration using two-beam pump–probe Z-scan methods by X. Jin; G. Shi; M. Shui; C.W. Li; J.Y. Yang; X.R. Zhang; Y.X. Wang; K. Yang; Y.L. Song (259-262).
In this Letter we have presented a two-beam pump–probe method to investigate the excited state absorptions induced mass transport.A two-beam pump–probe Z-scan method is proposed to investigate nonlinear absorption induced solute migration in CuPcTs/DMSO solution. The ‘pure’ mass transport phenomenon is observed by this method. Moreover, the population density distribution of the solute molecules can be deduced by measuring the transmittance of the sample. Different input pulse energy at different repetition rates are used to investigate the excited state absorptions induced solute migration. Additionally, an energy-gradient induced mass transport model is used to explain the experimental results.

Author Index (263-267).