UV photodissociation of N-methylpyrrole: The role of πσ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the ¹A₂ state, arising from a 3s/σ^*←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved ‘action’ spectra for forming CH₃ photoproducts, and by velocity map imaging studies of these CH₃ products (in their v = 0 and v₂ = 1 vibrational levels). CH₃ products are observed at all excitation wavelengths within the NMP absorption band. Direct dissociation on the ¹A₂ potential energy surface (PES) yields ‘fast’ CH₃ fragments, with an average total kinetic energy release (TKER) of 6500 cm⁻¹, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH₃ images also show a broad component, peaking at lower TKER (1700 cm⁻¹); this component extends beneath the ‘fast’ feature in images recorded at wavelengths near the origin, and accounts for all of the CH₃ products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the ¹A₂ state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along R_N–CH3/R_N–H), and the possible couplings between these surfaces. The present study confirms that the proposed model of ¹πσ^* state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.     

On the photodissociation of ozone in the range of 5–9 eV

The photoabsorption spectrum of ozone in the UV range (5–9 eV) is calculated from a short-time wave packet propagation using six potential energy surfaces obtained from ab initio electronic structure calculations. It is shown that the (unnamed) band around 7 eV, which is immediately adjacent to the intense Hartley band, is primarily due to excitation of three electronic states: 5 ¹A′ (3 ¹A₁), 6 ¹A′ (4 ¹A₁), and 4 ¹A″ (2 ¹B₁). Excitation of the state 8 ¹A′ (¹B₂) leads to a broad and intense band starting around 8 eV with a maximum near 9.1 eV. In full accord with the recent experimental study of Brouard et al. [M. Brouard, R. Cireasa, A.P. Clark, G.C. Groenenboom, G. Hancock, S.J. Horrocks, F. Quadrini, G.A.D. Ritchie, C. Vallance, J. Chem. Phys. 125 (2006) 133308], the excitation at 193 nm (6.42 eV) involves at least two states (5 ¹A′ and 4 ¹A″) different from the state excited in the Hartley band (3 ¹A′). The dynamics along the dissociation path is discussed in terms of one-dimensional potential curves. Several avoided crossings among the excited ¹A′ as well as the ¹A″ states point to a complicated fragmentation process. Although a quantitative analysis of branching ratios is not possible on the basis of the present calculations, we surmise, that in addition to and O(¹D) + O₂(¹Δ_g), the next higher spin-allowed channel, , also is likely to be a major product channel, in agreement with experimental observations.     

Mechanism of dissociative excitation of BrCN in electron cyclotron resonance plasma flow of He

The dissociative excitation reaction of BrCN induced by the products of the electron cyclotron resonance (ECR) plasma flow of He was studied based on the electrostatic-probe measurements and on the optical emission spectra of the B²Σ⁺ − X²Σ⁺ transition of CN radicals. The partial pressures of He and BrCN were 3 and 1 mTorr, respectively, and the partial pressure of H₂O, P_H2O, was in the range of 0.0–0.6 mTorr. The electron density, n_e, showed a negative dependence on P_H2O as (2.63 ± 0.13) × 10¹² − (0.23 ± 0.10) × 10¹² m⁻³, and the electron temperature, T_e, a positive dependence, (2.38 ± 0.36) − (4.51 ± 0.15) eV. The CN(B²Σ⁺ − X²Σ⁺) emission intensity showed a negative dependence on P_H2O. Based on a kinetic analysis of these P_H2O dependencies, the decomposition of BrCN does not proceed via electron impact; instead, decomposition proceeds via the processes involving He⁺ and/or He metastable atoms.     

Rare gas–iodine complexes in the ion-pair states

Luminescence and luminescence excitation spectra in the vicinity of the optical–optical double resonance transitions to the I₂(, v_f = 8 and 9, J_f ≈ 55) levels have been measured at the bulb conditions for the I₂ + Rg mixtures (Rg = He, Ar, Xe) at the rare gas pressures 2–20 Torr and room temperature. Luminescence attributed to the RgI₂ complexes in the ion-pair states has been observed for the first time. It is argued that the complexes can be formed by direct optical excitation from the complexes or colliding pairs. Besides, the RgI₂ complexes in the ion-pair states can be formed in nonadiabatic internal conversion processes from the one. The complexes have rather long lifetime, especially in the case of Xe, and decay radiatively and nonradiatively forming I₂ molecules in different ion-pair states.     

The interstellar gas-phase formation of CO2 – Assisted or not by water molecules?

Using state of the art methods of quantum chemistry, potential energy surfaces for the formation of and CO₂ (³B₂) from CO + O (¹D) and CO + O (³P), respectively, have been studied. At the MRSDCI level, we show that the formation of from O (³P) is strongly connected with the height of the barrier localized on the CO + O (³P) entrance channel. At the CCSD(T) level with a large basis set we calculate this barrier to be 5.9 kcal/mol. Consequently, we confirm that the gas-phase formation of CO₂ in interstellar molecular clouds is inefficient. To mimic the formation of CO₂, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, we have extended our study to consider the formation of CO₂ in the presence of water molecules. We show, using density functional and CCSD(T) methods, that the barrier located on the CO + O (³P) reaction entrance channel is hardly affected by the presence of water molecules. We therefore suggest that CO₂ formation, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, should be inefficient too.     

Electronic states of PN obtained by configuration-interaction studies

Potential curves were calculated for eighteen low-lying doublet and quartet states of PN⁺, using configuration-interaction methods and double-zeta plus polarization and diffuse basis sets. Spectroscopic constants were evaluated for fourteen stable states. The X ²Σ⁺ ground state lies very close to A ²Π (0.34 eV calculated). The 2 ²Σ⁺ state has two shallow minima of similar energy, being due to σ* → σ at smaller R, and π → π* at larger R. For N₂⁺, σ* → σ is much lower in energy than π → π*, whereas the opposite situation applies to P₂⁺.     

MRD CI study on the low-lying states of AlP

Large-scale MRD CI calculations assign to AlP the ground state X ³Σ⁻ (9σ²3π²) and a close-lying state 1 ³Π (9σ3π³) (T_e = 0.08 eV). Up to transition energies of 2.0 eV, other states are described by the configurations 9σ3π³ (1¹Π), 8σ²3π⁴ (1 ¹Σ⁺), 9σ²3π² (1 ¹Δ and 2 ¹Σ⁺) and 9σ3π²4π (1 ⁵Π). The 2 ³Π state, located at ≈ 2.30 eV, shows a shallow double minimum. Numerous perturbations are expected to induce predissociation upon 2 ³Π. Multiplets arising from the occupation 8σ²3π³4π are clustered in the 3.25–3.50 eV region. Quintet states with the configuration 8σ9σ3π³4π are bound, with T_e values (in eV) of 3.80 (1 ⁵Σ⁺), 4.44 (1 ⁵Δ) and 4.88 (3 ⁵Σ⁻), respectively. The 9σ → 4s Rydberg members ⁵Σ⁻ and ³Σ⁻ lie in the 4.58–4.72 eV energy region. The first ionization potential (ionization to X⁴Σ⁻ of AlP⁺, 9σ → ∞) is estimated to be 7.65 eV. Ionization to the 1 ²Σ⁻ and 1 ²Π states of AlP⁺ is suggested to occur between 8.0 and 8.8 eV. The dipole moments of X ³Σ⁻, 1 ¹Δ and 2 ¹Σ⁺ are close to 1.0 D, whereas the 1 ¹Σ⁺ state has μ = 3.49 D; 1 ³Π and 1 ¹Π have dipole moments from 2.45 to 2.91 D. All low-lying states show a polarity Al⁺P⁻. Finally, the electronic structure and transition energies of AlP are compared with those of the isoelectronic species BN, AIN, and SiP⁺.     

On the 6Πu state of Na2

The 6¹Π_u state of sodium dimer has been observed up to v = 53 in excitation spectra of the system, recorded by polarisation labelling spectroscopy technique. The Dunham coefficients are derived and the potential energy curve constructed by the inverted perturbation approach method. Equilibrium constants for the 6¹Π_u state of Na₂ are: T_e = 35446.06 ± 0.04 cm⁻¹ (with respect to the minimum of the electronic ground state), Y₁₀ = 111.388 ± 0.019 cm⁻¹, Y₀₁ = 0.112122 ± 0.000017 cm⁻¹.     

Analysis of the interactions between human serum albumin/amphiphilic penicillin in different aqueous media: an isothermal titration calorimetry and dynamic light scattering study

The complexation process of the amphiphilic penicillins sodium cloxacillin and sodium dicloxacillin with the protein human serum albumin (HSA) in aqueous buffered solutions of pH 4.5 and 7.4 at 25 °C was investigated through isothermal titration calorimetry (ITC) and dynamic light scattering. ITC experiments were carried out in the very dilute regime and showed that although hydrophobic interactions are the leading forces for complexation, electrostatic interactions also play an important role. The possibility of the formation of hydrogen bonds is also deduced from experimental data. The thermodynamic quantities of the binding mechanism, i.e, the enthalpy, , entropy, , Gibbs energy, , binding constant, and the number of binding sites, n_i, were obtained. The binding was saturable and is characterised by Langmuir adsorption isotherms. From ITC data and following a theoretical model, the number of bound and free penicillin molecules was calculated. From Scatchard plots, and n_i were obtained and compared with those from ITC data. The interaction potential between the HSA–penicillin complexes and their stability were determined at pH 7.4 from the dependence of the diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug–protein complexes with increase in the concentration of added drug.     

All electron ab initio investigations of the electronic states of the MoN molecule

The low lying electronic states of the molecule MoN were investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction were determined in perturbation calculations. The electronic ground state is confirmed as being ⁴∑⁻. The chemical bond of MoN has a triple bond character because of the approximately fully occupied delocalized bonding π and σ orbitals. The spectroscopic constants for the ground state and ten excited states were derived. The excited doublet states ²∑⁻, ²Γ, ²Δ, and ²∑⁺ are found to be lower lying than the ⁴Π state that was investigated experimentally. Elaborate multi-configuration configuration-interaction (MRCI) calculations were carried out for the states ⁴∑⁻ and ⁴∏ using various basis sets. The spectroscopic constants for the ⁴∑⁻ ground state were determined as r_e=1.636 Å and ω_e=1109 cm⁻¹, and for the ⁴∏ state as r_e=1.662 Å and ω_e=941 cm⁻¹. The values for the ground state are in excellent agreement with available experimental data. The MoN molecule is polar with a charge transfer from Mo to N. The dipole moment was determined as 2.11 D in the ⁴∑⁻ state and as 4.60 D in the ⁴∏ state. These values agree well with the revised experimental values determined from molecular Stark spectroscopic measurements. The dissociation energy, D_e, is determined as 5.17 eV, and D₀ as 5.10 eV.     

Photochemical reactions of electron-deficient olefins with N,N,N′,N′-tetramethylbenzidine via photoinduced electron-transfer

Photoinduced electron transfer reactions of several electron-deficient olefins with N,N,N′,N′-tetramethylbenzidine (TMB) in acetonitrile solution have been studied by using laser flash photolysis technique and steady-state fluorescence quenching method. Laser pulse excitation of TMB yields ³TMB* after rapid intersystem crossing from ¹TMB*. The triplet which located at 480 nm is found to undergo fast quenching with the electron acceptors fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), cinnamonitrile (CN), -acetoxyacrylonitrile (AAN), crotononitrile (CrN) and 3-methoxyacrylonitrile (MAN). Substituents binding to olefin molecule own different electron-donating/withdrawing powers, which determine the electron-deficient property (π-cloud density) of olefin molecule as well as control the electron transfer rate constant directly. The detection of ion radical intermediates in the photolysis reactions confirms the proposed electron transfer mechanism, as expected from thermodynamics. The quenching rate constants of triplet TMB by these olefins have been determined at 510 nm to avoid the disturbance of formed TMB cation radical around 475 nm. All the values approach or reach to the diffusion-controlled limit. In addition, fluorescence quenching rate constants have been also obtained by calculating with Stern–Volmer equation. A correlation between experimental electron transfer rate constants and free energy changes has been explained by Marcus theory of adiabatic outer-sphere electron transfer. Disharmonic k_q values for CN and CrN in endergonic region may be the disturbance of exciplexs formation.     

Vibronic dynamics in the low-lying coupled electronic states of methyl cyanide radical cation

Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the (E + A)  e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed.     

Predissociation of chlorobenzene, beyond the pseudo-diatomic model

Dissociation of chlorobenzene via the lowest singlet excited state has been investigated by means of pump–probe femtosecond spectroscopy and spin–orbit corrected ab initio quantum chemistry. We have found that the so far accepted model with a ¹ππ^* → ³π/nσ^* reaction mechanism has to be amended. We suggest that the mechanism goes via a transition from ¹ππ^* to a πσ^* state that is to 90% a singlet. Further, three nuclear degrees of freedom required to describe the dissociation have been defined.     

A study on copper(II)-Schiff base-azide coordination complexes: Synthesis, X-ray structure and luminescence properties of [Cu(L)(N3)]X (L = Schiff bases; X = ClO4, PF6)

Two Schiff bases N,N′-(bis(pyridin-2-yl)benzylidene)propane-1,3-diamine (pbpd) and N,N′-(bis(pyridin-2-yl)formylidene)butane-1,4-diamine (pfbd) have been prepared and used to synthesize copper(II) complexes. Four complexes of the type [Cu(L)(N₃)]X (1–4) [L = pbpd; X = ClO₄ (1); L = pbpd; X = PF₆ (2); L = pfbd; X = ClO₄ (3); L = pfbd; X = PF₆ (4)] have been synthesized and characterized on the basis of microanalytical, spectroscopic, magnetic, electrochemical, luminescence and other physicochemical properties. Two representative complexes of the series, 2 and 3, have been characterized by single crystal X-ray diffraction measurements which reveal that in each complex the copper(II) ion assumes a distorted trigonal bipyramidal environment through coordination of the metal centre by two pyridine N atoms and two imine N atoms of the Schiff base with the fifth position occupied by a N atom of a terminal . They display intraligand ¹(π–π^*) fluorescence at room temperature and intraligand ³(π–π^*) phosphorescence in glassy solutions (MeOH at 77 K). A band (492 nm) observed for the complexes in their solid-state emission spectra is an excimeric emission arising due to an aromatic π–π interaction. Electrochemical electron transfer study reveals Cu^II–Cu^I reduction in methanolic solutions.     

Theoretical studies on the aromaticity of η-cyclopentadienyl cobalt dithiolene complexes

Some η⁵-cyclopentadienyl cobalt dithiolene complexes CpCoS₂C₂R₂ have been optimized at B3LYP/6-311++G(d) level. The optimized geometries agree well with experiment. The analyses of nature bond orbital and nucleus-independent chemical shift (NICS) at B3LYP/6-311++G(d) and GIAO-B3LYP/6-311++G(d) levels reveal the aromatic character of the η⁵-cyclopentadienyl cobalt dithiolene complexes. However, their aromaticity is weaker than that of the isolated . There are two reasons for the change of heterocyclic aromaticity of the metal dithiolene in the η⁵-cyclopentadienyl cobalt dithiolene complexes with respect to that of the isolated . The better equalization of bond lengths in the isolated cation is the first reason. The other reason is that the contribution to the NICS from the metallic cobalt atom is much larger in the isolated cation . The planar character of cyclopentadienyl is destroyed slightly in the complexes. At the same time, the size of cyclopentadienyl (Cp) becomes bigger than the isolated Cp⁻¹ and this is caused by the cobalt atom in the pentagon. The π-electron delocalization causes stronger aromaticity of the Cp in the complexes than that of the isolated Cp⁻¹.     

Excess molar enthalpies for binary mixtures of trimethyl phosphate with alkanols {CH3(CH2)nOH, n = 0–3} at 298.15 K

The excess molar enthalpies () for the binary mixtures of trimethyl phosphate (TMP) with alkanols {CH₃(CH₂)_nOH, n = 0–3} have been measured with an isothermal calorimeter at 298.15 K and atmospheric pressure. The values are positive for all the mixtures over the whole composition range. The values increase in the order methanol < ethanol < 1-propanol < 1-butanol. The experimental results have been correlated with the Redlich–Kister equation.     

Enthalpies of combustion and formation of 3-formylchromones

The enthalpies of combustion of 3-formylchromone (3F), 3-formyl-6-methylchromone (3F6M) and 3-formyl-6-isopropylchromone (3F6I) were determined by combustion calorimetry. The molar combustion energies () of the 3F, 3F6M and 3F6I are: −(4452.4 ± 1.8), −(5115.6 ± 2.7) and −(6411.4 ± 2.5) kJ mol⁻¹, respectively. The formation enthalpies in the crystalline state () are: −(340.2 ± 2.2), −(355.1 ± 3.1) and −(415.5 ± 3.0) kJ mol⁻¹, respectively.s     

Computational study of glycine–(water)3 complex by density functional method

Glycine–(water)₃ complexes have been studied by means of B3LYP density functional method using 6-311++G* basis set. In the complex considered here, the three water molecule are either attached to the carboxylic group or bridge between the amino group and carboxylic group of glycine. Four such complexes are studied. Relaxation energies, two-, three- and four-body interaction energies are obtained by applying many-body analysis to know their role in binding energy of the complex. The results are compared with recent work on glycine–(water)₃ complex with group as proton donor [A. Chaudhari, P.K. Sahu, S.L. Lee, J. Chem. Phys. 120 (2004) 170]. In the most stable structure of glycine–(water)₃ complex, the three water molecules are attached to the carboxylic group of glycine and it is 5.3 kcal/mol lower in energy than that of the most stable structure reported earlier. The three-body term from water–water–water interaction in the most stable in this work and that reported earlier is unique since the distances between the water molecules are almost same. The two-body term from water–water interaction has significant contribution to the total two-body term when the distance between water molecules is less than 3 Å.     

New regularities and an equation of state for liquids

Three regularities have been introduced for liquids (T < T_C and ρ > ρ_C) based on average potential energy. The experimental data have been used to show the validity of the regularities. First, there exists near-linearity relation between and ρ for all isotherms of a liquid, where P_i and ρ are internal pressure and density, respectively. Second, changes linearly with ρ for each isotherm of any liquid, where Z and V_m are compressibility factor and molar volume, respectively. Third, a new regularity using the definition of bulk modulus and our new equation of state between reduced bulk modulus and density has been introduced, that is versus ρ must be linear for all isotherms of a liquid where B_r is the reduced bulk modulus.

A new equation of state has been also derived. The density of some liquids in the extensive ranges of temperature and pressure has been calculated using the new equation of state. The densities calculated from this equation agree with experiment to better than 0.3%. The new equation of state can predict internal pressure, thermal expansion coefficient, and isothermal compressibility of liquids within experimental error.     

An accurate expression for radial distribution function of the Lennard-Jones fluid

A simple and accurate expression for radial distribution function (RDF) of the Lennard-Jones fluid is presented. The expression explicitly states the RDF as a continuous function of reduced interparticle distance, temperature, and density. It satisfies the limiting conditions of zero density and infinite distance imposed by statistical thermodynamics. The distance dependence of this expression is expressed by an equation which contains 11 adjustable parameters. These parameters are fitted to 353 RDF data, obtained by molecular dynamics calculations, and then expressed as functions of reduced distance, temperature and density. This expression, having a total of 65 constants, reproduces the RDF data with an average root-mean-squared deviation of 0.0152 for the range of state variables of 0.5  T^*  5.1 and 0.35  ρ^*  1.1 (T^*=kT/ε and ρ^* = ρσ³ are reduced temperature and density, respectively). The expression predicts the pressure and the internal energy of the Lennard-Jones fluid with an uncertainty that is comparable to that obtained directly from the molecular dynamics simulations.