Chemical Physics Letters (v.318, #4-5)

In the present Letter, the reaction pathways for H2NCHO decomposition in the ground and excited states are traced with the use of the MP2 and CASSCF energy gradient techniques. The intersection point of the S1 and T1 surfaces are determined by means of the state-average CASSCF optimization. The n→π excitation leads to the first excited singlet state, where a radical pair of NH2 and CHO is formed. Then a hydrogen atom transfers from the CHO radical to the NH2 radical, producing CO and NH3 in the ground state. This is in good agreement with recent experimental findings. The calculated results also show that H2 and HNCO can be formed along a two-step T1 pathway. A recent experimental study on the photochemistry of the H2NCHO suggested that H2 elimination from H2NCHO takes place on the T1 surface.

Internal rotation of the methyl group in 1- and 2-methylnaphthalenes has been investigated by the ab initio theory. The rotational barriers in the S0 and S1 states calculated by the Hartree–Fock and configuration–interaction with single-excitation operator methods are in reasonable agreement with experimental values. The variations of the rotational barriers by excitation (S0→S1), ionization (S0→C0), and electron attachment (S0→A1) are shown to be directly connected with the stability of the HOMO and/or LUMO by the first-order treatment. In the HOMO and LUMO, a new type of orbital interaction named π*–σ* hyperconjugation appears and determines their stability. The interpretation based on the π*–σ* hyperconjugation can consistently and comprehensively explain the barrier variations.

Guiding paths and time-dependent basis sets for wavefunction propagation by Dmitrii V. Shalashilin; Bret Jackson (305-313).
Two methods are explored for evolving wavefunctions using short-time propagators of Herman–Kluk form [M.F. Herman, E.K. Kluk, Chem. Phys. 91 (1984) 271]. At each time step the wavefunction is expanded in a set of coherent states distributed in phase space about a guiding trajectory. A harmonic approximation for the potential allows the stability analysis to be done analytically. A Monte Carlo `path integral' form for the long-time propagator is derived and tested for a simple harmonic system. Another approach, where the entire wavefunction is evolved one step at time, is applied to wavepacket motion in a Morse potential.

Relativistic and electron correlation effects are investigated for the superheavy element compounds (111)Li, (111)H and (111)F. Relativistic effects are very large for all properties calculated and change bond lengths by more than 0.4 Å. Spin–orbit effects are important for dissociation energies but small for bond distances.

Exact statistical mechanical treatment of benzene adsorption in a zeolite twin-pore one-dimensional lattice model by Zhimei Du; Lawrence J. Dunne; George Manos; Martin F. Chaplin (319-324).
An exact matrix calculation of the statistical mechanics of a lattice model of benzene adsorption in silicalite modelled as two types of quasi one-dimensional pores is presented. The calculation reproduces the experimentally observed two steps in the level of adsorption with rising pressure and also satisfactorily gives the essential features of the loading dependence of the heat of adsorption.

Theory of time-resolved single-molecule fluorescence spectroscopy by Andrzej Molski; Johan Hofkens; Thomas Gensch; Noël Boens; Frans De Schryver (325-332).
The statistics of time-resolved photon detection in single-molecule continuous-excitation fluorescence spectroscopy is examined for three- and two-state models of a dye molecule. A unified description is provided in terms of stationary stochastic point processes. The pair distribution function and the distribution of interdetection times are related to the parameters of the models, and explicit expressions are given for the two-state model. The effect of the detected background is taken into account. Based on an analysis of triplet blinking, we argue that time-resolved photon detection offers an alternative to fluorescence intensity spectroscopy in unravelling the underlying single-molecule processes.

Do Cu2+NH3 and Cu2+OH2 exist?: theory confirms `yes!' by Ahmed M El-Nahas; Nobuo Tajima; Kimihiko Hirao (333-339).
CCSD(T) and MRMP calculations with TZP+ quality basis sets are used to study the interaction of Cu2+ with up to two ligand molecules L (L=H2O, H2S, NH3, and PH3). The dissociation to Cu+ and L+ is much more stable than the formation of Cu2+L adducts (by 76–100 kcal/mol). Nevertheless, it might be possible to obtain a one-ligand complex for H2O and NH3; barrier heights of 7 and 9 kcal/mol, respectively, are assigned for the dissociation process. Two ligands from H2O and NH3 can give more stable complexes with Cu2+ as the barrier heights reach 40 kcal/mol. In order to obtain stable complexes of PH3 or H2S with Cu2+, at least two ligands are required.

Atmospheric reaction between the HS radical and chlorine by Stella M Resende; Fernando R Ornellas (340-344).
The mechanism and the kinetics of the reaction between HS and Cl2 leading to HSCl and Cl were studied theoretically. The effect of the complete basis set limit (CBS) was included by extrapolating the results obtained with cc-pVDZ, cc-pVTZ and cc-pVQZ correlation consistent basis sets functions. At the highest level of calculation, CCSD(T), the calculated rate constant is 1.0×10−12 cm3 molecule−1 s−1. The activation Gibbs free energy was calculated to be 7.4 kcal/mol, which is only 0.2 kcal/mol above the value determined experimentally. The enthalpy and Gibbs free energies of the reaction are −6.9 and −6.5 kcal/mol, respectively.

Using the MP2 method and, in some cases, a basis set-up to aug-cc-pVTZ quality, the properties of C–H⋯π bonds have been investigated in model dimers. Their strength goes from 0.55 up to 2.5 kcal/mol, depending on the C–H carbon hybridization and the π system. The presence of these bonds is identified by the presence of a bond critical point linking the H atom and atoms of the π system. The critical point characteristics and the vibrational shift of the C–H⋯π bonded dimers are similar to those present in C–H⋯O and O–H⋯π hydrogen-bonded dimers, thus indicating a hydrogen-bonded nature.

Theoretical prediction of phosphorus nanotubes by G Seifert; E Hernández (355-360).
Using a well-established theoretical model, we have addressed the energetic viability of phosphorus nanotubes, and have found that nanotubes based on the black-phosphorus layered structure are indeed stable, having strain energies lower than 0.1 eV/atom for diameters above 1.25 nm. Further, we have found that, if synthesized, these nanotubes would be uniformly semi-conducting and would have a Young's modulus of ca. 300 GPa.

Alternative reaction mechanisms for C2–C3 bond-breaking and inversion of configuration at C2 within the carboxylation path of Rubisco substrate, d-ribulose 1,5-bisphosphate, are investigated. Hartree–Fock calculations were carried out at 3-21G and 6-31G∗∗ level, and 3- and 5-carbon molecular models used; electron correlation has been included for the smallest model at MP2/6-31G∗∗. The new proposal has been mapped as a set of transition structures with putative intermediary species. The results agree with experiment. However, even if most of the mechanism can be mapped to in vacuo steps, inversion at C2 results in a number of alternatives. As the model offers a number of propensities we conclude that fine tuning the mechanism must be modulated by the enzyme. The C–C bond breaking appears to be related to formation of a carbonyl group at C3 irrespective of the mechanism used to transfer the H-atom from one of the hydroxyl groups initially bound to C3 in the gem-diol species.

Local perturbative triples correction (T) with linear cost scaling by Martin Schütz; Hans-Joachim Werner (370-378).
A new method for the perturbative calculation of the correlation energy due to connected triple excitations (T) in the framework of local coupled cluster theory (LCCSD) is presented, for which all computational resources scale linearly with molecular size. The present implementation, which so far neglects inter-triples couplings via the off-diagonal elements of the occupied–occupied block of the Fock matrix, recovers 80–90% of the conventional triples energy. The present LCCSD(T) method should allow to treat systems with more than 1000 basis functions and several hundred correlated electrons. Test calculations for medium sized molecules already show speedups by factors of 500–1000 as compared to the conventional (T) correction.

A time-resolved study of forward and reverse excitation energy transport in a disordered two-component system of Na-fluorescein (donor) and rhodamine 6G (acceptor) in glycerol–ethanol solution is reported. The experimental results are compared with those of the hopping model (HM), accounting for the effect of reverse energy transfer. Quantitative agreement between the experiment and the theory is found for critical parameters close to those obtained from independent spectral measurements. Simultaneously, disagreement between experimental data and no-reverse-transport HM theory was found for this system.

The NO vibrational state distribution in the reaction O( 1 D )+N2O → 2NO by P.J Pisano; M.S Westley; P.L Houston (385-392).
The vibrational distribution of NO products from the reaction O( 1 D )+N2O → 2NO has been measured from v=0 to v=12. The measurement was performed in a molecular beam apparatus, in which the product NO was rotationally but not vibrationally cooled. The measured NO vibrational population is found to be peaked at v=7, with average vibrational energy between 24 000 and 28 000 cm−1, much greater than previously reported. The vibrational distribution presented in this Letter is inconsistent with the dominance of either a stripping or a statistical mechanism producing a substantial fraction of the product NO.

Hydroxymethyl (CH2OH) radicals were produced in a molecular beam via the photoinitiated reaction between Cl atoms and methanol. The rovibronic spectrum obtained following excitation to the 3p Rydberg state was measured by (1+1) and (2+1) REMPI. Analyses of rotational contours of selected vibronic bands obtained in the molecular beam and in a flow cell led to the assignment of the excited Rydberg state as 2 A″(3p z ). The 11±2 cm−1 homogeneous linewidth that best fits the observed contours implies a lifetime of the excited state of 0.5±0.1 ps, consistent with predissociative decay.

The composition of the Xe system changed from mainly atomic at low pressure to molecular at high pressure. The energy levels of this system varied also. By using the fluorescence of the first and second continuum measured near the resonant line of Xe at 147 nm at various pressure conditions, we observed the formation of Xe2 and the energy levels for the excited states A1u, B0u + and B′1u states. In a potential energy diagram, this corresponds to the change of energy levels from R≈∞ to finite R.

Raman spectroscopy of the high- and low-spin states of the spin crossover complex Fe(phen)2(NCS)2: an initial approach to estimation of vibrational contributions to the associated entropy change by Azzedine Bousseksou; John J McGarvey; Francois Varret; José Antonio Real; Jean-Pierre Tuchagues; Andrew C Dennis; Marie Laure Boillot (409-416).
Raman spectra of the spin-crossover complex Fe(phen)2(NCS)2 in the solid state have been recorded at 785 nm as a function of temperature to investigate the contribution of intramolecular vibrations to the entropy change, ΔS, associated with spin crossover. The modes of major interest for estimating the contribution lie in the range 100–500 cm−1, where the largest qualitative changes with temperature in the Raman spectra were observed. Analysis of these data, with the working assumption of an average frequency in this range as representative of the 15 distortion modes of an idealised FeN6 octahedron, leads to the conclusion that the intramolecular vibrations represent a primary contribution to the total ΔS.

The low-lying electronic states of AlP2, Al2P and their anions and cations are investigated using a complete active space multi-configuration self-consistent field technique followed by multi-reference singles+doubles configuration interaction (MRSDCI) calculations. Potential energy surfaces, geometries, energy separations, adiabatic ionization energies, electron affinities, dipole moments vibrational frequencies, zero-point and dissociation energies are computed. While AlP2 exhibits similarity to the heavier analogs, Al2P was found to exhibit a different ground state compared to Ga2P and In2P.

Proton dynamics in NH+–N hydrogen bonds is characterized by high-pressure dielectric and calorimetric studies of ferroelectric/paraelectric phase transitions in perchlorate and tetrafluoroborate mono-salts of 1,4-diazabicyclo[2.2.2]octane (DABCO), [C6H13N2]+·ClO4 and [C6H13N2]+·BF4 . The pT phase diagrams of these ferroelectrics have been determined and described. The positive pressure dependences of T c testify to the strong coupling of the ionic dynamics with the proton disordering in the NH+–N hydrogen bonds. Close isostructurality of the crystals provides a unique opportunity for analysing the proton disordering at varied crystal environments.

Study on dynamic optical nonlinearities of excited state proton transfer in ethanol solution of 7-hydroxyquinoline by Xiao Dong; Zhang Guilan; Wang Haiyan; Tang Guoqing; Chen Wenju (433-439).
The dynamic process of 7-hydroxyquinoline with excited state proton transfer has been studied by the pump–probe method in which a three-dimensional degenerate four-wave mixing technique is employed for the probe. The molecular hyperpolarizability and lifetime of the excited state involved in the excited state proton transfer process of 7-hydroxyquinoline are obtained in our experiment. The decay time of the cation excited state is 35 ps and the decay time of the anion excited state is 250 ps. The molecular hyperpolarizability of 7-hydroxyquinoline in the enol form ground state, cation excited state and anion excited state are 1.3±0.2×10−31, 8.0±1.0×10−31 and 5.8±0.8×10−30 esu, respectively.

The hydrogen-bonded clusters Coumarin 151/(H2O)1 and Coumarin 152A/(H2O)1 have been studied via IR–UV double-resonance spectroscopy. Each cluster exists in two structural forms having distinct electronic properties. Evidence is presented to show that the structures involve water hydrogen bonded to groups at opposite ends of the molecular dipole. These results were supported by semiempirical structure calculations and, for C151, by infrared measurements in the S1 state.

Molecular beam study of possible CVD intermediates from Group-14 organometallic precursors by Mark Sulkes; Lawrence C. Baldwin; Mark J. Fink (448-453).
Neutral products were examined via photoionization mass spectrometry following high-voltage discharge on a variety of volatile organometallic compounds containing Group-14 elements in helium. HV discharge on organosilicon precursors in the presence of nitrogen produced Si2N as the nearly exclusive product; M2N analogs were observed for M=Ge, Sn precursors but did not show the same remarkable propensity for formation. With an organogermane precursor, GeOH, GeNO2, and GeNO3 also showed significant propensities to form. Finally, toluene – the all carbon analog of phenylsilane – reacted with oxygen copresent in the discharge to give C2O2 + as the only observed cationic product following photoionization.

The first spectroscopic observation of YbS has been made using visible laser excitation spectroscopy. Gas-phase YbS was produced in a Broida oven by the reaction of Yb metal vapour with either CS2 or OCS. Dispersed fluorescence spectra of an Ω=0–Ω=0 electronic transition were recorded with a CCD array detector. A least-squares fit of 34 band-heads leads to the following molecular parameters for the lower state: ω e″=365.6(2) cm−1 and ω ex e″=1.14(2) cm−1. Determination of the parameters for the upper state of the transition was precluded by the irregular vibrational spacings, which increased with increasing v′.

Gain dynamics in oriented thin films of an oligo(para-phenylene vinylene) by T.-A Pham; T Barisien; V Grayer; L Guidoni; G Hadziioannou; J.-Y Bigot (459-465).
The dynamics of the photo-excited states in crystalline thin films of a model oligo(para-phenylene vinylene) is studied with femtosecond pump–probe spectroscopy. The process of amplified spontaneous emission and the corresponding optical gain are shown to depend drastically on the crystallographic order present in the films. The results stress the important role of the molecular orientation in order to obtain efficient light-emitting conjugated organic devices.

Gas-phase photodissociation of AuCH2 +: the dissociation threshold of jet-cooled and rotationally thermalized ions by Fernando Aguirre; John Husband; Christopher J Thompson; Ricardo B Metz (466-470).
The photofragment spectra of jet-cooled and rotationally thermalized AuCH2 + are reported. Two channels are observed: loss of H2 and loss of CH2 with a branching ratio of 1.4:1 over the region studied. The presence of a threshold at 322 nm for the dissociation of jet-cooled AuCH2 + to Au++CH2 implies the upper limit D 0 o(Au+–CH2)≤372±3 kJ mol−1. The dissociation threshold of ions rotationally thermalized in an ion trap shifts to lower energy by the amount of parent rotational energy.

A carbon film was deposited on Si substrate from an ethanol liquid phase by electrolysis at low temperature (60°C) in ambient atmosphere, and various carbon phases in the film were investigated by micro-Raman spectroscopy. Raman spectroscopy analysis confirmed the crystalline diamond structure by the presence of a sharp peak at 1331 cm−1. Meanwhile, short-chain polyethylene, hydrogenated amorphous diamond-like and glassy carbon phases were found in the films.

Ultraviolet reflection–absorption spectroscopy of C6H6 physisorbed on gold by J.R Peck; X.L Peng; L.W Anderson; J.E Lawler (476-480).
Reflection–absorption spectroscopy in the ultraviolet (UV) of C6H6 molecules physisorbed on a gold surface has been observed in a single-pass experiment for the electronic transition 1 A1g  →  1 B2u near 250 nm. The linewidths of transitions in this electronic band are 160 cm−1. This technique permits the detection of a very low density of molecules physisorbed on a surface.

Depth-resolved elemental analysis of perovskite thin films based on lead zirconate–titanate (PZT) is performed by a relatively new technique; radio-frequency glow discharge atomic emission spectroscopy (rf-GD-AES). The technique provides in-depth composition information for the PZT layer, and underlying layers within the device architecture. Total film thicknesses of ∼1 μm are profiled in <30 s. Differences in pyrolysis conditions for two sol–gel produced films are easily identified based on the profiles of residual C, H, and O species. The technique is projected to be a valuable aid in the development of new electronic devices.

Optical limiting behaviour of the water-soluble C60/γ-cyclodextrin complex by M Konstantaki; E Koudoumas; S Couris; J.M Janot; H Eddaoudi; A Deratani; P Seta; S Leach (488-495).
The third-order nonlinear optical response of the water-soluble inclusion complex consisting of C60 incorporated in γ-cyclodextrin was examined employing the z-scan technique. Using 532 nm, 10 ns laser pulses, significant optical limiting action was observed, the effect being slightly lower than that of pure C60 diluted in toluene. Ageing of the C60/γ-cyclodextrin–water solution results in the formation of aggregates that enhance the optical limiting action by a factor of almost two.

Erratum (496).

Erratum (497).