Chemical Physics Letters (v.347, #1-3)

The energy dependence of the photodesorption cross-section in the OCS/Ag(111) system is interpreted in terms of the hot electron attachment model including the electron cascade process and the substrate absorbance. An electron attachment level 3 eV above the Fermi level gives the best fit of the experimental action spectrum. The sensitivity of the fit to a variety of parameters is analysed. The fit is a strong function of attachment level energy. The weight of other parameters (attachment level width, surface–adsorbate barrier, secondary electron cascade profile) is less significant. It is shown that the model may reproduce the reported universal wavelength dependence of photodesorption on different metal substrates only if the attachment level is located a constant distance from the metal Fermi level.

End-group and defect analysis of soluble single-walled carbon nanotubes by M.A. Hamon; H. Hu; P. Bhowmik; S. Niyogi; B. Zhao; M.E. Itkis; R.C. Haddon (8-12).
By use of solution phase mid-IR spectroscopy we are able to obtain an estimate for the ratio of the carbon atoms in the single-walled carbon nanotube (SWNT) backbone to the carbon atoms in the end-groups and at defect sites of the octadecylamido (ODA) functionalized soluble SWNTs (s-SWNT-CONH(CH2)17CH3). This analysis shows that the weight percentage of the octadecylamido functionality in the s-SWNTs is about 50%.

Polymeric fullerene chains in RbC60 and KC60 by Ashfia Huq; P.W. Stephens; Goetz M. Bendele; R.M. Ibberson (13-22).
We present a detailed study of the structure of the polymer phase of AC60 (A=K, Rb) using data obtained from high-resolution neutron powder diffraction. We are able to obtain stable Rietveld refinements of both structures with no constraints on any carbon atom. KC60 has lattice parameters a=9.952 Å, b=9.091 Å, c=14.372 Å in space group Pnmn and RbC60 has a=10.125 Å, b=9.086 Å, c=14.207 Å and β=90.316° in space group I12/m1. We notice significant distortion throughout the ball from its original icosahedral symmetry. Coordinates of the atoms will allow more accurate theoretical calculation of the physical properties of these fullerides and might shed light on the differences among the cations and the dimensionality of their electronic properties.

Lineshapes of radial breathing mode (RBM) Raman scattering from single-wall carbon nanotubes (SWNTs) encasing the fullerenes vary depending on the sizes of fullerenes. The lineshape fit using Lorentzian lines to the spectrum indicates downshift of ω r (RBM frequency) for most lines as compared with those observed in empty-SWNTs, except for one upshifted line associating with the stress to the tube-wall from the inside fullerenes. The smallest limit of tube diameters (d) is determined from this upshifted line, and they are resulting to be ∼1.37 nm for C60, ∼1.45 nm for both C76 and C78, and ∼1.54 nm for C84 when using ω r  (cm −1)=246/d (nm).

Pump-probe photoelectron spectroscopy is used to study excited states in pristine and photopolymerized C60. For the first time in a molecular solid, vibrationally broadened singlet (S1) and triplet (T1) exciton photoemission spectra are temporally isolated and unambiguously assigned. Complex transient behaviors of S1 and T1 populations, including non-Markovian decays and a strongly enhanced production of T1 states at high excitation, provide evidence for interactions among excitons and free carriers. A new rate-equation model reproduces a wide range of both laser- and synchrotron-based data from polymerized films.

Supercapacitors from nanotubes/polypyrrole composites by K Jurewicz; S Delpeux; V Bertagna; F Béguin; E Frackowiak (36-40).
A novel type of composite electrode based on multiwalled carbon nanotubes (MWNTs) coated with polypyrrole (PPy) has been used in supercapacitors. A homogeneous layer of PPy has been deposited on the nanotubular materials by electrochemical polymerization of pyrrole, that in all cases enhances the specific capacitance. A maximum value of 163 F/g has been obtained for MWNTs prepared at 600 °C and modified by a PPy layer of 5 nm, whereas it is only 50 F/g for the pristine nanotubes. The open entangled network of the nanocomposite favors the formation of a three-dimensional electrical double layer allowing a more effective contribution of the pseudofaradaic properties of PPy.

Probing the lignin nanomechanical properties and lignin–lignin interactions using the atomic force microscopy by Miodrag Micic; Ivan Benitez; Melanie Ruano; Melissa Mavers; Milorad Jeremic; Ksenija Radotic; Vincent Moy; Roger M Leblanc (41-45).
By combining atomic force microscopy (AFM) force and environmental scanning electron microscopies (ESEMs), herein we present an evidence for the existence of strong intermolecular forces, which are responsible for holding lignin globules together in higher ordered structures. Based on this observation, we provide a support for the hypothesis that lignin globules consist of at least two individual spherical layers, with space in between filled with solvent or gas.

The size dependent evolution of the electronic structure of isolated gas-phase rhenium clusters Re n (n=2–40) is measured by photoelectron (PE) spectroscopy at 4.99 and 6.42 eV photon energies. For very small clusters the photodetachment threshold energies show a strong size dependence varying between 1.6 eV (Re2) and 2.6 eV (Re13). As for larger clusters, electron affinities (EA) increase smoothly toward bulk work function (WF). Bulk-like electronic features can already be observed around very small cluster sizes.

We have examined the titled dissociation reaction near its 0 K threshold using the pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence methods. This study yields a value of 14.4184±0.0010 eV for the 0 K threshold. The PFI-PE spectrum for CD4 exhibits a sharp step at the 0 K threshold, indicating that the dissociation of excited CD4 in high-n (n⩾100) Rydberg states at energies above the dissociation threshold occurs in ⩽10−7 s. The energetic data for CD 3/CD 3  + and CD 4/CD 4  + are found to be in excellent agreement with those for CH 3/CH 3  + and CH 4/CH 4  + after taking into account the zero-point-vibrational-energy corrections.

The infrared spectrum of the benzene–water cation, C6H6 +–H2O, was recorded in the O–H stretch region to obtain the first experimental information about its geometry and interaction strength. The spectrum is consistent with a charge–dipole structure in which the oxygen atom of H2O approaches the C6H6 + cation in the aromatic plane. The dissociation energy estimated from the spectral shifts is D 0≈14±3 kcal/mol. The interaction in the C6H6 +–H2O cation is rather different from the one in the neutral dimer, demonstrating the dramatic ionization-induced changes of the interaction between an aromatic hydrocarbon and a polar ligand.

Time-resolved and CW EPR were used to study radiolytically generated H atoms in water/ice nanoclusters in NaA, NaX, NaY, and HY zeolites. H atoms dynamic properties and spectroscopy parameters are sensitive to the structural changes of water due to the nanoconfinement. Transient H atoms in HY and NaY zeolites segregate into two different domains: sodalite and super cages. In NaX zeolite only H atoms from super cages were observed. H atoms are created in both silica phase and adsorbed water by radiolytic processes. The decay of H atoms occurs predominantly via reaction with the radiation-induced defects in silica.

Dispersed fluorescence spectrum of the HC 35 Cl A ̃ X ̃ vibronic transition by Chun-Wei Chen; Tsung-Chuan Tsai; Bor-Chen Chang (73-78).
The dispersed fluorescence spectrum following the excitation of HC 35 Cl   A ̃ X ̃ vibronic transition between 570 and 610 nm was successfully obtained in a direct current (DC) discharge supersonic free jet expansion. The analysis of the dispersed fluorescence spectrum results in detailed information of the HC 35 Cl   X ̃ 1 A state vibrational structure. The bending and C–Cl stretching frequencies as well as anharmonicity constants were determined for the first time. In addition, since no perturbation was observed in the vibrational structure of the X ̃ 1 A state up to 2788  cm −1 , a lower limit of triplet–singlet energy gap was established to be roughly 8.0 kcal/mol.

Temperature dependence and deuterium kinetic isotope effects in the HCO  (DCO)+O 2 reaction between 296 and 673 K by John D. DeSain; Leonard E. Jusinski; Andrew D. Ho; Craig A. Taatjes (79-86).
The reactions HCO  (DCO)+O 2 have been measured by the laser photolysis/CW laser-induced fluorescence (LIF) method from 296 to 673 K, probing the ( B ̃ 2 A X ̃ 2 A ) HCO (DCO) system. The HCO  (DCO)+O 2 rate coefficients are 5.63±0.31 and 5.61±0.23×10−12   cm 3   molecule −1   s −1 , respectively, at 296 K; both are nearly independent of temperature between 296 and 673 K. The observed deuterium kinetic isotope effect is within the error estimate of previous measurements but is significantly smaller than recent theoretical predictions.

Picosecond time-resolved infrared spectra of photo-excited phenol–(NH3)3 cluster by Shun-ichi Ishiuchi; Makoto Sakai; Kota Daigoku; Tadashi Ueda; Takaya Yamanaka; Kenro Hashimoto; Masaaki Fujii (87-92).
Picosecond time-resolved IR spectra of phenol–(NH3)3 have been measured by UV–IR–UV ion dip spectroscopy for the first time. It was found that the time-evolution of two vibrational bands at 3180 and 3250  cm −1 is different from each other. The results show that two transient species are generated from the photo-excited phenol–(NH3)3 cluster. From ab initio calculation, the transient species are assigned to two isomers of (NH3)2NH4.

Photodissociation of Mg+(NH3) ion by Shinji Yoshida; Nobuhiro Okai; Kiyokazu Fuke (93-100).
Electronically excited states of Mg+(NH3) are studied by photodissociation after mass selection. The dissociation spectrum shows relatively sharp vibronic transitions centered at about 28 000 and 36 000  cm −1 . These absorption bands are assigned to the 2 P–2 S type transitions localized on the Mg+ ion. In photodissociation, a photoinduced charge-transfer process to produce NH3 + is observed in addition to evaporation and intracluster reaction processes to produce Mg+ and MgNH2 + ions, respectively. The mechanism for the production of these ions is discussed in terms of the predissociative and non-adiabatic interactions between the low-lying states.

The method of producing of an intensive secondary pulsed molecular beam of controlled kinetic energy with high-power IR laser is suggested. The experimental results on CO2-laser-induced acceleration of SF6 molecules in a secondary beam are presented. The intensive (more or equal to 1020 molecules/str s) molecular beams of SF6 with kinetic energy of about 1.5 eV without a carrier gas and about 2.5 and 2.7 eV with carrier gases He (SF6/He=1/10) and H2 (SF6/H2=1/10), accordingly, were obtained.

Excitation-wavelength dependence of the ultraviolet fluorescence dynamics of 7-azaindole dimer was examined in solution by femtosecond up-conversion method. It was found that the fluorescence decay of the dimer excited state showed a significant wavelength dependence. It changes from a bi-exponential decay to a single-exponential decay, when we scanned the excitation wavelength from 280 toward 313 nm (the red-edge of the dimer absorption). This result demonstrates that the proton-transfer dynamics itself exhibits a single exponential behavior. The obtained fluorescence data deny the appearance of the intermediate species and strongly support the concerted mechanism of the double proton transfer reaction.

Laser-assisted photochemistry of N,N-dimethyl-4-nitroaniline initiator of polymerization by A Costela; I Garcı́a-Moreno; O Garcı́a; R Sastre (115-120).
Applying time-resolved laser (337 nm) spectroscopy, we have analysed the photochemical and photophysical behaviour of N,N-dimethyl-4-nitroaniline (DMNA) and its dependence on the irradiation time and the presence of N,N-dimethylaniline (DMA), as reducting agent, and lauryl acrylate (LA) monomer, under aerobic and anaerobic conditions. The laser-induced phosphorescence of the initiator is efficiently quenched by DMA inducing the appearance of new emissions as a result of radiationless processes. The spectral and temporal characteristics of these new bands together with their dependence on both the laser intensity and irradiation time allowed their assignment and provided evidence for the photosensitization process of DMA from the excited initiator to be the main photoinitiation mechanism of polymerization.

Dipole moments of 2Σ and 2Π states of CN radical at different internuclear distances are reported using the multi-reference coupled cluster (MRCC) linear response approach. We also report as a by product the potential energy surface, equilibrium geometry and harmonic frequency of CN radical states that agree well with those calculated using multi-configuration quasi-degenerate perturbation theory (MCQDPT). The dipole moment of the ground state of the radical is in excellent agreement with the experimental value. Dipole moments are also calculated using MRCI using bigger complete active space and state specific orbitals and results of both methods agree well.

Structures of uranyl hydroxide monomeric UO2(OH)2 0, and dimeric (UO2)2(OH)2 2+ were studied by hybrid density functional theoretical calculations. It was found out that both the monomeric ortho-UO2(OH)2 0 and dimeric (UO2)2(OH)2 2+ complexes have bent OUO angles (∼170°). This is attributed to significant π donation from the equatorial OH ligands to the uranyl moiety. It was also found that the ortho- and meta-UO2(OH)2(H2O)3 0 complexes lie very close in energy, and that both isomers may exist in normal aqueous media.

Phase flow deformations and coupled electrochemical oscillators by Antonis Karantonis; Seiichiro Nakabayashi (133-137).
A simple two-dimensional model of coupled electrochemical oscillators is studied, where linear diffusive coupling of the potential causes dephasing of the oscillators. It is shown that coherence breaks down due to a strong deformation of the phase flow near the limit cycle. When a large number of such oscillators is coupled, the temporal response is bursting oscillations.

The problem of the selection of the perturbation operator for a time-dependent perturbation description of excitation-energy transfer between two molecules is raised in this Letter. Then we attempt to determine the best representation of the perturbation through analysis of the nature of initial and final states in the process of the excitation-energy transfer. We concluded that not intermolecular interaction but nonadiabatic interaction must be regarded as the perturbation inducing the excitation-energy transfer.

Optimal RF flip angles of the NMR CRAZED pulse sequence for multiple spin-echoes (MSEs) and intermolecular multiple quantum coherences (iMQCs) of different orders were investigated theoretically and experimentally. An analytical expression for the dependence of signal intensities on RF pulse flip angles was derived for the cases when higher-order expansion terms of Bessel functions can be neglected from a combination of dipolar demagnetizing field treatment and product operator formalism. Theoretical predictions for both the relative signal intensities and the optimal flip angles at which the signal intensities are maximized are found to be in excellent agreement with experimental results.

In this publication we consider electronically multi-fold degeneracy with the aim of revealing the connection between the adiabatic-to-diabatic transformation (ADT) matrices and Wigner's irreducible representation of the rotation group. To form the connection we constructed simplified models of two-, three- and four-states all (electronically) degenerate at a single point and employed the relevant non-adiabatic coupling matrices (NACMs). We found that once these matrices were properly quantized [Chem. Phys. 259, (2000) 123] the ADT matrices and Wigner's d j -rotation matrices are related via a similarity transformation.

The influence of J-coupling on heteronuclear nonlinear (or multiple) spin echoes by Elke Kossel; Rainer Kimmich; Ioan Ardelean (157-162).
Heteronuclear nonlinear spin echoes were investigated with respect to the influence of indirect spin–spin coupling in a two-spin 1/2 system consisting of 1 H and 13 C spins. The heteronuclear nonlinear spin echo phenomenon is essentially based on the 13 C coherence evolution in the presence of the modulated demagnetizing field originating from the 1 H magnetization. It is shown that the first nonlinear 13 C echo amplitude oscillates as a function of the modulation period of the demagnetizing field (or the pitch of the proton magnetization helix). The Fourier transform reveals a doublet with a splitting equal to the heteronuclear spin–spin coupling constant J.

Ultrafast dynamics of mesoionic liquid solutions studied with incoherent light by L.de S. Menezes; Cid B. de Araújo; M.A.R.C. Alencar; P.F. Athayde-Filho; J. Miller; A.M. Simas (163-166).
Optical Kerr shutter (OKS) measurements were performed in a solution of a mesoionic compound (MIC) in dimethylsulfoxide using 10 ns pulses of broadband noisy light with a coherence time of 160 fs. The results show a light induced birefringence with two main components: an instantaneous one, which follows the laser coherence time, and a sub-picosecond component, with a characteristic time of 560 fs, which is attributed to the birefringence decay due to molecular interactions.

Orientational relaxation in polymer and dye solutions and its consequence for the laser threshold by Steven A. van den Berg; Gert W. 't Hooft; Eric R. Eliel (167-172).
We compare orientational relaxation phenomena of a light-emitting polymer and a, spectroscopically similar, conventional laser dye in solutions of comparable viscosity. The orientational relaxation time of the laser dye is found to be very much smaller than that of the polymer. This result is attributed to the large difference in shape between the two light-emitting materials. We demonstrate that these very different reorientation times have a major influence on the operation of a gain-switched laser based on these gain media, in particular on its threshold.

The sense of circular polarization of the fluorescence from dyes dissolved in chiral nematic liquid crystals (CNLCs) with a photonic stopband overlapping the dye emission (`resonance regime') displays a peculiar reversal as a function of wavelength, which so far is not satisfactorily explained. We systematically study this phenomenon and show that theories based on the guest/host alignment of the fluorescent dye in the CNLC matrix are not adequate in the resonance regime. We instead propose a consistent explanation of sign reversal based on the description of CNLCs as one-dimensional (1D) photonic crystals.

First- and second-order diatomic-to-monatomic phase transitions in a model crystal by Daniel J Lacks; Matthew Kottemann; Dong Yuan (178-182).
The pressure-induced diatomic-to-monatomic phase transition is investigated in a model crystal based on a Tersoff-type interatomic potential. The phase transition is first-order when the many-body interaction is long-ranged, but second-order when the many-body interaction is short-ranged. The change in the order of the phase transition is related to a crossing of the stability limits of the diatomic structure with respect to fluctuations of volume and with respect to fluctuations of the internal parameter.

Photochemistry of Zn+(CH4) by W.-Y. Lu; T.-H. Wong; P.D. Kleiber (183-188).
We report on the photodissociation spectroscopy of Zn+(CH4). Two overlapping continuum absorption bands are observed in the spectral range 227–263 nm. The bands are assigned to transitions correlating with Zn+-based (4p←4s) excitation and photoinduced charge transfer, respectively. The reactive product ZnCH3 + is the major dissociation product showing that the dominant quenching pathway involves C–H bond insertion. Additional dissociation products include CH3 +,Zn+, and ZnH+ with a relative branching that depends on excitation energy. Results are compared to previous studies of the chemical quenching of light metal atoms and ions by CH4.

The di-tert-butyl nitroxide (DTBN) hyperfine tensors are computed in a reasonable time frame using the UB1LYP method and Barone's electron paramagnetic resonance EPR-III basis sets. The effects of six solvents on the 14N hyperfine splittings are calculated using the polarizable continuum method and reproduce the experimental trends. Further improvement in the accuracy is obtained when one extra water molecule is made to interact with the NO moiety. All the DTBN atoms are not magnetically equivalent since the molecule has C1 symmetry. This must be taken into consideration when simulating the EPR line shapes of randomly oriented DTBN molecules isolated in an argon matrix.

Rotational constants for CN(X)–H2/D2(j 2=1), derived from electronic spectra, exhibit an unusually small dependence on H2/D2 isotopic substitution. In the present study, variational bound state calculations were carried out to examine the origin of this effect. A four-dimensional potential energy surface [J. Chem. Phys. 110 (1999) 10380] was used for this model. The results show that the lowest energy rotational manifolds of CN(X)–H2/D2(j 2=1) are significantly influenced by Coriolis state mixing. This perturbation is most pronounced for CN–H2, and is identified as the cause of the anomalous isotope effect.

Excited-state polarizability of J-aggregates by Andrzej Eilmes (205-210).
The difference between the static polarizabilities of the first excited and the ground state of J-aggregates has been derived using the sum-over-states approach. For a simple model analytical expressions as well as numerical calculations show that the polarizability enhancement upon excitation should depend only marginally on the aggregate size. Charge-transfer states have been suggested as the possible origin of the observed large excited-state polarizabilities of J-aggregates.

We compare the performance of two different low-storage filter diagonalisation (LSFD) strategies in the calculation of complex resonance energies of the HO2 radical. The first is carried out within a complex-symmetric Lanczos subspace representation [H. Zhang, S.C. Smith, Phys. Chem. Chem. Phys. 3 (2001) 2281]. The second involves harmonic inversion of a real autocorrelation function obtained via a damped Chebychev recursion [V.A. Mandelshtam, H.S. Taylor, J. Chem. Phys. 107 (1997) 6756]. We find that while the Chebychev approach has the advantage of utilizing real algebra in the time-consuming process of generating the vector recursion, the Lanczos method (using complex vectors) requires fewer iterations, especially for low-energy part of the spectrum. The overall efficiency in calculating resonances for these two methods is comparable for this challenging system.

The dissociation pathway of D 2h   N 4 has been investigated at the CASSCF level of theory. A new C2v transition state to dissociation was found and characterized on the potential energy surface. The effective barrier of dissociation from D 2h   N 4 to 2N2 is estimated to 6.5 kcal/mol from MR-AQCC/VTZ//CAS(12, 12)/VTZ calculations. Vertical excitation energies and oscillator strengths for the lowest 20 singlet states of D 2h   N 4 have been calculated using the EOM-CCSD method. The geometry of the first excited state 1 B 3u , considered as the lowest optically accessible state, has been optimized and characterized at various computational levels. This state might be useful for detection of D 2h   N 4 by means of laser-induced fluorescence (LIF) spectroscopy.

The thermodynamics of nucleation of a pure vapor is investigated in an isolated system where the heat of condensation causes an increase of the temperature. The entropy of nucleation is calculated in the system composed of a number of clusters having identical size. In recent work only the influence of temperature on the internal energy and the entropy is considered in a simple manner. In this Letter the dependence of surface tension on temperature is introduced. No principal differences could be observed compared to the use of constant surface tension but the numerical values of the entropy of nucleation differ.

The recently developed equation-of-motion coupled-cluster (EOMCC) method with singles, doubles, and a selected set of triples defined through active orbitals (EOMCCSDt) is applied to the excited-state potential energy curves of the CH+ ion. The results are compared with the EOMCCSD (EOMCC singles and doubles), EOMCCSDT (EOMCC singles, doubles, and triples), and full configuration interaction results. It is demonstrated that the EOMCCSDt method provides the excited-state potentials of the EOMCCSDT quality and that the EOMCCSDt and EOMCCSDT methods restore the asymptotic degeneracy of excited states, which is broken by the EOMCCSD and other EOMCC doubles models.

Strange kinetics and complex energy landscapes in a lattice model of protein folding by Hironori K. Nakamura; Takeshi N. Sasaki; Masaki Sasai (247-254).
Non-exponential relaxation in a simplified lattice model of folding is studied with Monte Carlo (MC) calculation. As folding proceeds, population of the native conformation approaches its equilibrium value with the stretched exponential form. As temperature increases, relaxation becomes less stretched, and for 2 sequences out of 5 tested ones, the relaxation becomes faster than exponential at high temperature. Energy landscape of the model is analyzed and flow of trajectories is followed to explain temperature dependence of kinetics. Measurement of stretched or shrunken kinetics of folding should help to understand nature of intermediates and ruggedness of the landscape.

Ab initio benchmark calculations are reported for the diatomic hydrides of Al, Si, Ga, and Ge and their negative ions using correlation consistent basis sets and several correlated methods. Adiabatic electron affinities for the 0–0 transition were found to be 0.193, 1.256, 0.131, and 1.251 eV for AlH, SiH, GaH, and GeH, respectively, at the RCCSD(T)/aug-cc-pVQZ level of theory. Excitation energies (T 0) of the a1Δ and b1Σ+ bound excited states were found to be 0.560 and 1.017 eV for SiH and 0.595 and 1.026 eV for GeH at the MRCI + Q/aug-cc-pVQZ level. Selected spectroscopic constants are reported for the eight negative ion states.

Calculation of the differential photoionization cross-section of formaldehyde by Ivo Cacelli; Roberto Moccia; Raffaele Montuoro (261-267).
The integral and the differential photoionization cross-sections of formaldehyde are computed in the random phase approximation (RPA) coupled with a K-matrix (KM) based technique. The molecular orbitals in the continuum are projected on suitable L2 special polynomial Gaussian functions. Ionization channels originating from the valence shell occupied orbitals are considered, both in separated and interacting channel (IC) approaches up to a photon energy of 45 eV. It is found that the channel coupling is important in order to reproduce some features in the absorption and ionization spectra.

The ozone–acetylene reaction: concerted or non-concerted reaction mechanism? A quantum chemical investigation by Dieter Cremer; Elfi Kraka; Ramon Crehuet; Josep Anglada; Jürgen Gräfenstein (268-276).
The ozone–acetylene reaction is found to proceed via an intermediate van der Waals complex (rather than a biradical), which is the precursor for a concerted symmetry-allowed [4+2] cycloaddition reaction leading to 1,2,3-trioxolene. CCSD(T)/6-311G+(2d, 2p) and CCSD(T)/CBS (complete basis set) calculations predict the ozone–acetylene van der Waals complex to be stable by 2.2 kcal mol −1 , the calculated activation enthalpy for the cycloaddition reaction is 9.6 kcal mol −1 and the reaction enthalpy −55.5 kcal mol −1 . Calculated kinetic data for the overall reaction (k=0.8  l mol −1   s −1 , A=1.71×106   l mol −1   s −1 , E a =8.6  kcal mol −1 ) suggest that there is a need for refined kinetic measurements.