The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent ³¹P nuclei. Additional couplings with two equivalent ¹³C are observed with Ar–P=¹³C=¹³C=P–Ar. The ³¹P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]^{•–} and [H–P=CH–CH=P–H]^{•–}. Although the optimized geometries of these two species are clearly different, neither of them leads to¹³C/³¹P hyperfine tensors in conflict with the experimental results. The absence of any ¹H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]^{•–} more probable. 

The repartition of molecular hydrogen in space, and its depletion on solid particles in particular, is an important question of modern astrophysics. In this paper, we report a theoretical study of the physisorption of molecular hydrogen, H₂, on a major component of the interstellar dust known as polycyclic aromatic hydrocarbons (PAHs). Two different density functional theory approaches were used:  (i) the conventional Kohn−Sham theory and (ii) the subsystem-based approach (Kohn−Sham equations with constrained electron density, KSCED) developed in our group. The approximate exchange-correlation energy functional applied in all calculations and the nonadditive kinetic-energy functional needed in KSCED have a generalized gradient approximation form and were chosen on the basis of our previous studies. The results of both approaches show similar trends:  weak dependence of the calculated interaction energies on the size of the PAH and negligible effect of the complexation of two PAH molecules on the adsorption of molecular hydrogen. The KSCED interaction energy calculated for the largest considered PAH (ovalene), amounting to 1.27 kcal/mol, is in excellent agreement with experimental estimates ranging from 1.1 to 1.2 kcal/mol, whereas the one derived from supermolecular Kohn−Sham calculations is underestimated by more than 50%. This result is in line with our previous studies, which showed that the generalized gradient approximation applied within the KSCED framework leads to interaction energies of weakly bound complexes that are superior to the corresponding results of supermolecular Kohn−Sham calculations.

Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The ³¹P and ¹³C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]√^− and [HP---CH=Cp]√ have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]√^− is readily protonated and that the EPR spectra are due to the phosphaallylic radical.

n view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the “freeze-and-thawâ€� cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C₆H₆⋯X(X=O₂, N₂, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [WesoÅ‚owski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.

Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP•âˆ’CHCH2) ↔ (HPCH−•CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.

Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.

IR spectra of anthracene and pyrene derivatives, serving as models for isolated, linear and isolated, compact PAHs, respectively, have been calculated using ab-initio quantum mechanical methods. The separate and combined effects of ionization and multiple dehydrogenation have been studied. This study confirms and refines the trends of our preliminary paper on the smallest possible PAH, naphthalene. If small PAHs are responsible for any UIR bands, they should be ionized and partially dehydrogenated, with a few triple bonds at the periphery of the carbon skeleton. In the appendix are given the complete IR spectra of all the isomers of the derivatives of anthracene and pyrene calculated for the purpose of this study. Tables I are for anthracene and Tables II for pyrene. Positions of the the missing hydrogens in the dehydrogenated species are referred as in Figures 1 and 2 of the original publication.

In order to provide a possible explanation for the lack of detection of both HSiN and HNSi in the interstellar medium, an ab initio study of the Si+ + NH3 reaction is presented: it includes accurate energetic considerations and sketches dynamics discussions as well. It is unambiguously concluded that the X1A1 ground state of the SiNH2+ cation is the only exit channel of this reaction assuming interstellar conditions. The rotational and vibrational constants of this species are reported to stimulate its experimental and astrophysical searches. Upon dissociative recombination, it is likely that SiNH2+ can evolve toward HNSi: unfortunately, the dramatic weakness of the dipole moment of the latter species (0.05 D) makes it an unlikely candidate for today's radiotelescopes. At variance with HNSi, the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysical searches, but under interstellar conditions, we show that it can derive neither from the unimolecular HNSi ↔ HSiN equilibration nor from the Si+ + NH3, N + SiH3+ or N+ + SiH3 reactions as sometimes incorrectly stated in the astrophysical models that deduce interstellar silicon chemistry from that of carbon. Throughout this study, the very hazardous character of conclusions deduced from isoelectronic considerations should be considered as the leading feature: the finishing stroke to such isoelectronic analogies is given by our study of the H+ + HNSi ↔ HSiN + H+ reactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes.

Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C₆H₆···X (X = O₂, N₂, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.

The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results.

In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble.

The hydrogen exchange reaction H+CNH->HCN+H may be a key step in gas-phase interstellar nitrogen chemistry. It is one of the reactions supposed to cause increasing HNC depletion with increasing temperature in dense interstellar clouds. In this paper we report the results of extensive ab-initio calculations on the H+CNH<->H+HCN system that partially confirm this hypothesis. It is shown that both forward and reverse reactions possess activation barriers. However, the activation energy of the H+CNH channel (4.2+/-1.0kcal/mol.) is four times smaller than for the endothermic HCN+H path. Calculations on the rate of the forward reaction show that tunneling under the entrance channel barrier allows a small rate coefficient at the temperatures under 100K and that the rate coefficient increases steadily with increasing temperature for T>100K.

For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNHright harpoon over leftHCNHright harpoon over leftHCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2±1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions.

The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN⁺ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH^ + ₂. On many points, the differences relative to the SiNH^ + ₂ isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH^ + ₂. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN⁺ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH₂, H₂SiNH, and SiNH^ + ₂ species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm^–1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 eV

The experimental and the theoretical interests for the silicon chemistry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH2 and HNSiH2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH+ cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G** atomic orbital expansion. Accurate geometries and Be rotational constants have been determined at the Möller-Plesset MPn(n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH2, HNSiH2, and finally HSiNH2 in its floppy linear singlet ground state and in its lowest cis-bent a3A′ state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH+ entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species.

The electroaffinity of the O2 molecule is revisited using density functional theory (DFT) and perturbation treatments built on a MCSCF wavefunction that includes most of the non-dynamic correlation effects (MC/P approach). Using a standard 6–31 + G* basis set, DFT treatments based on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/perturbation treatments using an Epstein-Nesbet partition of the molecular Hamiltonian give a more accurate value of +0.492 eV in excellent agreement with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far less tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the dynamic correlation of the electron pair describing the σO---:O bond.

The structure of the ground and lowest two excited states of H₂NO have been determined in large scale configuration interaction calculations using a multiconfiguration self-consistent description of the molecular orbitals. These treatments are based on a systematic building of the correlation contribution which has been designed to account for the characteristics of the nitroxide group. This approach shows that the aminoxyl functional group is more than a three electron group shared by two atoms, but, in fact, a nine electron entity. Our best estimate of the geometry of the ground electronic state, obtained after second-order configuration interaction using a large basis of atomic natural orbitals, is pyramidal. However, since the potential depth between 0° and 40° is lower or of the same order of magnitude as the estimated inversion frequency, the conclusion that this molecule behaves like a planar system is totally justified. The structure of the excited (η−π*) and (π−π*) states have been determined and the transitions energies are in accordance with the experimental results on the highly substituted stable nitroxide radicals.